nanomaterials Article Formation of Nickel Oxide Nanocuboids in Ferromagnetic La2Ni1−xMn1+xO6 Monica Bernal-Salamanca 1, *, Zorica Konstantinović 2, * , Carlos Frontera 1 , Víctor Fuentes 1 , Alberto Pomar 1 , Lluis Balcells 1, * and Benjamín Martínez 1 1 Institut de Ciència de Materials de Barcelona-Consejo Superior de Investigaciones Científicas (ICMAB-CSIC), Campus de Universitat Autonoma de Barcelona, 08193 Bellaterra, Spain; frontera@icmab.es (C.F.); vfuentes@icmab.es (V.F.); apomar@icmab.es (A.P.); benjamin@icmab.es (B.M.) 2 Center for Solid State Physics and New Materials, Institute of Physics Belgrade, University of Belgrade, Pregrevica 118, 11080 Belgrade, Serbia * Correspondence: mbernal@icmab.es (M.B.-S.); zorica@ipb.ac.rs (Z.K.); balcells@icmab.es (L.B.) Abstract: The control of the spontaneous formation of nanostructures at the surface of thin films is of strong interest in many different fields, from catalysts to microelectronics, because surface and interfacial properties may be substantially enhanced. Here, we analyze the formation of nickel oxide nanocuboids on top of La2 Ni1−x Mn1+x O6 double perovskite ferromagnetic thin films, epitaxially grown on SrTiO3 (001) substrates by radio-frequency (RF) magnetron sputtering. We show that, by annealing the films at high temperature under high oxygen partial pressure, the spontaneous segregation of nanocuboids is enhanced. The evolution of the structural and magnetic properties of the films is studied as a function of the annealing treatments at different temperatures. It is shown   that the formation of NiOx nanocuboids leads to a nanostructured film surface with regions of locally Citation: Bernal-Salamanca, M.; different electrical transport characteristics. Konstantinović, Z.; Frontera, C.; Fuentes, V.; Pomar, A.; Balcells, L.; Keywords: ferromagnetic double perovskite; spontaneous formation of NiOx nanocuboids; local Martínez, B. Formation of Nickel transport properties Oxide Nanocuboids in Ferromagnetic La2 Ni1−x Mn1+x O6 . Nanomaterials 2021, 11, 804. https://doi.org/ 10.3390/nano11030804 1. Introduction Magnetic insulators have gained renewed interest due to their promising properties Academic Editor: João Pedro Araujo as material platforms with efficient magnetization dynamics [1,2]. In this regard, double perovskite La2 NiMnO6 (LNMO) is one of the most widely studied materials due to its Received: 24 February 2021 large magnetodielectric properties and high Curie temperature Tc ~280 K [3–5]. For these Accepted: 18 March 2021 reasons, LNMO has been considered as a promising candidate for the development of Published: 21 March 2021 magnetoelectronic and spintronic devices and thus, has attracted much attention lately [6,7]. LNMO has a double perovskite structure (A2 BB’O6 ), where Ni and Mn ions ideally occupy Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in alternatively the B site positions giving place to a rocksalt-type lattice [5]. However, in thin published maps and institutional affil- film form, as occurs in other double perovskite systems, full B-site Ni-Mn cationic ordering iations. is difficult to achieve [8–11]. In particular, in LNMO system, phase segregation often occurs and the coexistence of two structural phases is detected at room temperature [12,13], leading to the observation of two Curie temperatures [14]. To improve the magnetic properties of LNMO samples post-growth annealing pro- cesses are commonly employed [15–17]. Nonetheless, recent studies report the formation Copyright: © 2021 by the authors. of the NiO phase impurities in LNMO thin films grown by MBE technique with the post- Licensee MDPI, Basel, Switzerland. This article is an open access article growth annealing, with apparently defect-free growth [18,19]. Detailed cross-sectional distributed under the terms and high-angle annular dark-field studies revealed an inverted “pyramid-like” shape morphol- conditions of the Creative Commons ogy of the NiO precipitate, which progresses from a 2–3 nm base to a ∼10 nm wide mouth Attribution (CC BY) license (https:// at the film surface [19]. The coexistence of double perovskite and NiO secondary phase creativecommons.org/licenses/by/ is supported by first principles modeling of growth in oxygen deficient conditions with 4.0/). Nanomaterials 2021, 11, 804. https://doi.org/10.3390/nano11030804 https://www.mdpi.com/journal/nanomaterials Nanomaterials 2021, 11, 804 2 of 13 expected dissolution of inclusions during annealing [18]. Nanometer size nickel-oxide pre- cipitates were also detected during the growth of LaNiO3 -LaAlO3 superlattices on SrTiO3 substrate, changing their electronic properties [20]. Similar phase segregation processes are also observed in other manganite systems with cationic deficiency, strongly affecting magnetic and transport properties of the materials [21]. In this work, we report on the spontaneous formation of nickel-oxide nanocuboids at the surface of La2 Ni1−x Mn1+x O6 thin films, deposited by RF magnetron sputtering technique. Annealing treatments in oxygen at high temperature (in-situ or ex-situ) after thin film growth promote the spontaneous formation of NiO nanocuboids giving place to the formation of a nanostructured surface. The evolution of the structural and magnetic properties of the films as a function of the annealing temperatures has also been studied. As expected, the post-annealing processes improve magnetic properties as well as structural properties, while modifying significantly the local transport properties at the film surface. 2. Materials and Methods La2 Ni1−x Mn1+x O6 (LNMO) thin films were fabricated by RF magnetron sputtering under a total oxygen pressure of 19 Pa at high temperatures (900 ◦ C) on top of SrTiO3 (001) substrates using a stoichiometric La2 NiMnO6 target. The mini-MAK from US-INC mag- netron was 33 mm (1.3 inch) in diameter. The RF power used during thin film deposition was 40 W. The background pressure of the home-made sputtering chamber is in order of ~10−5 Pa. Electron probe microanalysis revealed Ni-deficiency (x~0.47) in the films [11]. Before deposition, substrates were cleaned in an ultrasonic bath with Milli-Q water and then annealed at 1000 ◦ C in air for 2 h to obtain a clean and smooth TiO2 terminated surface [22]. Samples with thicknesses, t, of 40 nm < t < 50 nm, as determined by X-ray reflectivity, were prepared. After film deposition, as-grown samples were annealed (in-situ or ex-situ) for 1 h under high oxygen partial pressure (PO2 ~5 × 104 Pa) to optimize the magnetic properties and oxygen stoichiometry. The evolution of the physical and structural properties was studied after successive annealing processes at 800 ◦ C (Annealed-800 ◦ C) and 900 ◦ C (Annealed-900 ◦ C) for 1 h of the same LNMO thin film. The surface morphology of the LNMO film was characterized by scanning electron microscopy (SEM, QUANTA FEI 200 FEG-ESEM) and atomic force microscopy (AFM, MFP- 3D AFM Asylum Research, Gole, CA, USA) in tapping mode. Structural characterization was made by means of X-ray diffraction (XRD) and reflectivity techniques (X’Pert MRD- Panalytical and a Siemens D5000, Malvern Panalytical, Malvern, UK). Reciprocal space mappings (RSM) were recorded using a Bruker-AXS General Detector Diffraction System (GADDS) model D8 Advance with a 2D detector. Energy dispersive X-ray analysis (EDX) has been performed on a FEI Magellan 400 L XHR SEM using an X-Max Ultim Extreme EDX detector (Oxford Instruments, Abingdon, UK). Transmission electron microscope (TEM) measurements were made by means of a field emission gun FEI Tecnai F20 microscope at 200 kV with a point-to-point resolution of 0.19 nm. Magnetization measurements were done using a superconducting quantum interference device (SQUID, Quantum Design, San Diego, CA, USA). The local electrical response of the films surface was explored by conductive atomic force microscopy (C-AFM), measurements were performed employing a MFP-3D microscope from Asylum Research with an Optimized Resistance Conductance Amplifier (ORCA) module. Diamond doped coated probes (DDESP-FM-V2 from Bruker, Billerica, MA, USA) were used in both the current maps and the I-V curves to ensure the stability of the tip during data acquisition. Moreover, these measures were carried out in a closed chamber with nitrogen atmosphere with the aim of reducing the ambient humidity and, therefore, avoid possible anodic oxidation effects. 3. Results and Discussion As evidenced in Figure 1, the surface morphology of a LNMO film, after usual annealing in-situ process at 900 ◦ C, reveals the existence of a nanostructured surface. The SEM image (Figure 1a) detects uniformly distributed nanocuboids throughout the film Nanomaterials 2021, 11, x FOR PEER REVIEW 3 of 13 Nanomaterials 2021, 11, 804 3 of 13 SEM image (Figure 1a) detects uniformly distributed nanocuboids throughout the film surface, surface,with withnearly nearlysquare-based, square-based, sub-100 sub-100nmnm in size. AFM in size. topography AFM in Figure topography 1b con- in Figure 1b firms the presence of nanocuboids, distributed on top of a flat LNMO confirms the presence of nanocuboids, distributed on top of a flat LNMO surface with surface with low roughness (rms~0.3 low roughness nm). nm). (rms~0.3 The estimated occupation The estimated of theofsurface occupation is below the surface 15%.15%. is below The dis- The tribution of nanocuboids distribution of nanocuboids sizesize is well described is well described bybythe thelog-normal log-normalfunction functionwith withthe themost most probable probablesize size of of around around SS00~58 ~58nm nmandandthe thedistribution distributionwidth widthof of σ~0.23 σ~0.23(see (seeFigure FigureA4A4 inin Appendix B). The inset of Figure 1b reveals a nanocuboid height of ~12 nm. Appendix B). The inset of Figure 1b reveals a nanocuboid height of ~12 nm. An interesting An interesting observation observationisisthat thatnanocuboids’ nanocuboids’edgesedgesare aremainly mainlyoriented oriented along along the the[100] [100]direction direction of of the the substrate substrate(note (notethat thatSEM SEMandandAFMAFMimages imagesarearerotated rotated around around 45° ◦ 45 respect respecttotothe thesubstrate substrate edge edgeduring duringobservation). observation). Figure Figure1.1.(a) (a)SEM SEMmicrographs micrographsofofLNMO-900 LNMO-900°C ◦ Cwith within-situ in-situannealing annealing(thickness, (thickness,tt==42 42nm). nm).Inset Inset shows the θ/2θ XRD pattern around 002 shows the θ/2θ XRD pattern around 002 bragg bragg peak of of peak thethe substrate. (b) (b) substrate. AFM image AFM of LNMO- image of LNMO- 900 900°C ◦ Cfilm filmwith withthe theheight heightprofile profileofofone onenanocuboid nanocuboid inin the inset. the (c,d) inset. Small (c,d) area Small SEM area SEM andand EDXEDX map at energy of E = 15 eV with the corresponding profile spectra of different elements by X-ray map at energy of E = 15 eV with the corresponding profile spectra of different elements by X-ray spectroscopy analysis in (e). spectroscopy analysis in (e). AAdeeper deeperinsight insightininthe thefilms’ films’structure structurewas wasobtained obtainedby byX-ray X-raydiffraction diffractionand andreflec- reflec- tivity tivity techniques. For simplicity and comparison between the film and the substrate, the techniques. For simplicity and comparison between the film and the substrate, the LNMO LNMOlattice latticeparameter parameter isis considered considered inin pseudocubic pseudocubic notation notation (a(abulk = 3.876 Å) [23,24]. bulk = 3.876 Å) [23,24]. LNMO LNMOfilms films on on top top of of STO STO (a(aSTO = 3.905 Å) substrates grow under small tensile strain STO = 3.905 Å) substrates grow under small tensile strain (a LNMO-aSTO)/aSTO × 100% = −0.74%. The in-plane lattice parameter of the film deter- (aLNMO -aSTO )/aSTO ×100% = −0.74%. The in-plane lattice parameter of the film deter- mined minedfromfromthe the reciprocal reciprocal space space maps, maps, around around (103) (103)STO reflection, perfectly matches with STO reflection, perfectly matches with that that of the STO substrate (see Figure A1a in Appendix A).The of the STO substrate (see Figure A1a in Appendix A). Thehigh highresolution resolution θ/2θ θ/2θ X-ray X-ray diffraction diffractionallows allowsidentifying identifyingthetheLNMO LNMOpeak peakin in the the proximity proximity of of the the intense intense substrate substrate one (see inset of Figure 1a) with an out-of-plane lattice parameter of a⊥ = 3.877 ± 0.003 Å. Nanomaterials 2021, 11, x FOR PEER REVIEW 4 of 13 Nanomaterials 2021, 11, 804 4 of 13 one (see inset of Figure 1a) with an out-of-plane lattice parameter of a⊥ = 3.877 ± 0.003 Å. The estimated LNMO unit cell volume (~59.12 Å3) is very similar to the unit cell volume The estimated LNMO unit cell volume (~59.12 Å3 ) is very similar to the unit cell volume of Ni-deficient bulk counterpart (LaNi0.25Mn0.75O3+δ) [12]. A planar view high resolution of Ni-deficient bulk counterpart (LaNi0.25 Mn0.75 O3+δ ) [12]. A planar view high resolution TEM (HRTEM) images supports XRD measurements, with a coexistence of domains with TEM (HRTEM) images supports XRD measurements, with a coexistence of domains with different orientations (Figure A2 in Appendix A). different orientations (Figure A2 in Appendix A). Information regarding the chemical composition of the nanometric cuboids was ob- Information regarding the chemical composition of the nanometric cuboids was tained from energy dispersive x-ray (EDX) analysis. A small area SEM image of a single obtained from energy dispersive x-ray (EDX) analysis. A small area SEM image of a nanocuboid and the corresponding EDX map obtained at 15 KeV are shown in Figure single nanocuboid and the corresponding EDX map obtained at 15 KeV are shown in 1c,d. Figure The profile 1c,d. spectra The profileof constituent elements (La spectra of constituent Lα1, Ni, (La elements MnLand, Ni, O KMnα1) in a single and O Kα1 ) in a α1 nanocuboid are shown in Figure 1e, indicating the presence single nanocuboid are shown in Figure 1e, indicating the presence of nickel of nickel and oxygen.andThe oxygen. presence of La and Mn have been excluded due to the very low detection The presence of La and Mn have been excluded due to the very low detection percentage percentage (no difference observedobserved (no difference along the baseline along in Figure the baseline 1d). Therefore, in Figure semi-quantitative 1d). Therefore, semi-quantitativeEDX EDX analysis indicate that the nanometric cuboids formed on top of the analysis indicate that the nanometric cuboids formed on top of the LNMO films surface LNMO films surface are Ni-rich phase,phase, are Ni-rich most mostprobably NiOxNiOx probably phases, in agreement phases, in agreementwith with recentrecent reports of theof the reports formation of NiOx segregates in LNMO double perovskites [18,19]. formation of NiOx segregates in LNMO double perovskites [18,19]. In those works, In those works, it is it is proposed that oxygen deficiency in LNMO films could favors the proposed that oxygen deficiency in LNMO films could favors the formation of antisite formation of antisite defects [23] and defects [23]NiOx phasephase and NiOx segregations [18,19]. segregations [18,19]. To gain a deeper insight into the formation To gain a deeper insight into the formation of NiOx nanocuboids, of NiOx nanocuboids, successive successiveanneal- annealing ing processes processesatatdifferent differenttemperatures temperaturesare areapplied appliedex-situ ex-situto to aa non-annealed non-annealed as-grown as-grown film. film. The The evolution evolution of of the thephysical physicaland andstructural structuralproperties propertieswas was followed followed ononthethe samesameLNMO LNMO thinthin filmfilm afterafter eacheach annealing annealing process process and theandchanges the changes on theon the morphology morphology are in are presented presented Figurein2a–c. FigureSEM 2a–c. SEM (left-hand (left-hand image) andimage) and AFM (right-hand AFM (right-hand image) micro- image) micrographs in Figure 2a graphsof the as-grown LNMO film without annealing process show a flat surfaceshow in Figure 2a of the as-grown LNMO film without annealing process with an a flat average surface with an average roughness of rms~0.3 roughness nm. In this of rms~0.3 case, no nm. In this signals case, no signals of segregation were of segregation detected. were detected. Figure 2. SEM micrographs (left-hand image) and AFM images (right-hand image) with the typical line profile of the same LNMO thin film (thickness, t = 48 nm) after different annealing processes: Figure(a)2. as-grown SEM micrographs (left-handprocess), film (no-annealing image) and (b) AFM images annealing ◦ C and (c) (right-hand at 800 image) with at annealing the900 ◦ C. typi- cal line profile of the same LNMO thin film (thickness, t = 48 nm) after different annealing pro- However, cesses: (a) as-grown after film an ex-situ process), (no-annealing annealing 800 ◦ C, segregated (b)atannealing nanostructures at 800 °C and (c) annealing atappears 900 dis- °C. tributed through the whole surface as shown in Figure 2b. SEM micrographs revealed the presence of uniformly distributed small nanoparticles. The corresponding AFM confirms the presence of these nanoparticles, reflected in a higher value of global surface roughness (rms~0.45 nm). Log-normal distribution indicates the most probable nanoparticle size of Nanomaterials 2021, 11, 804 5 of 13 around S0 ~13.5 nm with the distribution width of σ~0.13 (see Figure A5a in Appendix B). However, the mean-size of nanostructures remains rather small and with a height of few nanometers ~2 nm (see the line profile in Figure 2b). Performing further ex-situ annealing process at higher temperatures (900 ◦ C) on the very same LNMO film resulted in an enhancement of the growth of the nanoparticles. Nanoparticles became larger in-size as shown in Figure 2c). Log-normal distribution reveals the most probable nanoparticle size of S0 ~18.5 nm with the distribution width of σ~0.17 (see Figure A5b in Appendix B). The average nanoparticle height increases to around ~5 nm which is also reflected in an increase of the roughness (rms~0.7 nm). Although the density of nanoparticles appears to decrease by increasing the annealing temperature (from 5.34 × 10−4 nanoparticles/nm2 for Annealed-800 ◦ C, to 3.70 × 10−4 nanoparticles/nm2 for Annealed-900 ◦ C) the average volume of NiOx at the surface has been estimated to increase around three times (see Table A2 in Appendix B), thus confirming that annealing process enhances the segregation. The effect of different annealing processes on the structural properties of the same LNMO thin film has been also followed by high resolution XRD scan of the (002) reflection and the asymmetric RSM around (103) reflection (Figure 3). In Figure 3a, a shoulder could be distinguished in the proximity of the prominent substrate (0 0 2) peak, which can be associated to the LNMO peak with an out-of-plane parameter of a⊥ = 3.939 ± 0.002 Å. At the same time, RSM around (103) reflection of the same film, indicates the position of Nanomaterials 2021, 11, x FOR PEER REVIEW 6 of 13 the LNMO film just below the substrate peak (right-hand image in Figure 3a), confirming that the as-grown film is fully strained with the in-plane parameter that mimics the lattice parameter of underlying substrate (aSTO = 3.905 Å). Figure Figure3.3.High-resolution High-resolution θ/2θ XRD θ/2θ XRDscans scansof the of (002) reflections the (002) (left)(left) reflections and RSM around and RSM (103) re- around (103) flection (right) reflection of of (right) the same the sameLNMO LNMOthin thinfilm film(thickness, (thickness,tt ==48 48 nm) nm) after different differentannealing annealingprocesses: pro- cesses: (a) as-grown (a) as-grown film, (b)film, (b) annealing annealing at 800 ◦atC800 and°C(c)and (c) annealing annealing at 900 ◦atC.900 The°C. The arrow arrow indicates indicates position position of LNMO peak and the red dotted line indicates the position of LNMO peak and the red dotted line indicates the position of the STO peak. of the STO peak. The The first first annealing annealing process process at at 800 ◦ C (Figure 800 °C (Figure 3b) 3b) does does not not significantly significantly change change the the structural structuralparameters. parameters.From Fromθ/2θ θ/2θXRDXRDscans scans(left-hand (left-handimage imageininFigure Figure3b), 3b),the theout-of- out-of- plane lattice parameter is found to decrease to a⊥ = 3.929 ± 0.002 Å, remaining elongated respect to the corresponding pseudocubic perovskite lattice parameter of bulk LNMO (abulk = 3.876 Å) [23]. However, under annealing process at 900 °C in Figure 3c, the shoulder in θ/2θ XRD scan disappears completely and only the substrate peak can be clearly ob- Nanomaterials 2021, 11, 804 6 of 13 plane lattice parameter is found to decrease to a⊥ = 3.929 ± 0.002 Å, remaining elongated respect to the corresponding pseudocubic perovskite lattice parameter of bulk LNMO (abulk = 3.876 Å) [23]. However, under annealing process at 900 ◦ C in Figure 3c, the shoul- der in θ/2θ XRD scan disappears completely and only the substrate peak can be clearly observed. Nevertheless, the RSM around (103) reflection indicates that the LNMO film peak is just above the substrate peak corresponding to a slight asymmetry detected in θ/2θ XRD scan (see arrow in left-hand Figure 3c). While the in-plane lattice parameter still remains constant (a⊥ = 3.905 ± 0.005 Å), the out-of-plane lattice parameter further decreases to a⊥ = 3.883 ± 0.001 Å. The variation of the in-plane and the out-of-plane cell parameters in pseudo-cubic notation after different annealing processes are depicted in Figure 4a. The in-plane lattice parameters (red dashed line in Figure 4a) match the SrTiO3 Nanomaterials 2021, 11, x FOR PEER REVIEW 7 of 13 lattice parameter indicating that the film remains strained even after various annealing processes. On the other hand, the out-of-plane lattice parameter (black dashed line in Figure 4a) decreases progressively towards the LNMO bulk value (blue dashed line). Figure 4. (a) Variation of in-plane (red dashed line) and out-of-plane (black dashed line) lattice Figure 4. (a) Variation of in-plane (red dashed line) and out-of-plane (black dashed line) lattice parameters ofofLNMO parameters LNMOfilm filmafter afterdifferent differentex-situ ex-situ annealing annealing processes. processes. The The blue blue dashed dashed lineline repre- repre- sentsthe sents thebulk bulkcounterpart counterpartvaluevalue [23] [23] and and (b)(b) corresponding corresponding temperature temperature dependence dependence of in-plane of in-plane magnetizationatatµµ0H magnetization 0H==0.50.5 T.T. ItItisisworth worthto tonote notethat thatnon-annealed non-annealedas-grown as-grownfilmfilmhas hasaalarger largerout-of-plane out-of-planelattice lattice parameterthan thanexpected expectedandand consequentially consequentially larger larger unit unit cell cell volume volume (~60.06 (~60.06 Å 3 Å3).). This This parameter larger unit cell usually denotes some oxygen deficiency and the presence of Mn 3+ [25]. In larger unit cell usually denotes some oxygen deficiency and the presence of Mn [25]. In 3+ fact,asascan fact, canbebededuced deduced from from Figure Figure 4a,4a, thethe shrinkage shrinkage of the of the out-of-plane out-of-plane lattice lattice param- parameter eter while in-plane parameters remain unchanged results in a reduction while in-plane parameters remain unchanged results in a reduction of the unit cell volume of the unit cell volume after successive annealing, which is after successive annealing, which is consistent with 4+ consistent with oxygen incorporation oxygen incorporation during the an- during the annealing nealing processes processes and the formation and the formation of Mn4+, asofpreviously Mn , as previously observed inobserved in similar similar manganite manganite systems [25–27]. Indeed, as-grown non-annealed film exhibit systems [25–27]. Indeed, as-grown non-annealed film exhibit clearly depressed values of clearly depressed values of Curie temperature (black curve in Figure 4b while Tc is enhanced in annealed Curie temperature (black curve in Figure 4b while Tc is enhanced in annealed films (blue films (blue and red curves), in agreement with previous observations in the LNMO sys- and red curves), in agreement with previous observations in the LNMO system [15]. It tem [15]. It should be noted that we cannot exclude the presence of NiOx segregations should be noted that we cannot exclude the presence of NiOx segregations inside the film inside the film matrix and that the presence of NiO secondary phase may also contribute to matrix and that the presence of NiO secondary phase may also contribute to the increment the increment of the out-of-plane lattice parameter in as-grown samples, besides oxygen of the out-of-plane lattice parameter in as-grown samples, besides oxygen deficiency. deficiency. These results agree with previous studies showing that annealing processes in These results agree with previous studies showing that annealing processes in oxygen rich oxygen rich atmosphere promote an increase of the Curie temperature in La2 NiMnO6 films atmosphere promote an increase of the Curie temperature in La2NiMnO6 films and other and other double perovskites [15,16]. Furthermore, it has been shown that post-growth double perovskites [15,16]. Furthermore, it has been shown that post-growth annealing annealing treatments are also effective to reduce the number of antisite defects [17] and to treatments are also effective to reduce the number of antisite defects [17] and to dissolve dissolve NiO segregations, thus contributing to increase the saturation magnetization [18]. NiO segregations, thus contributing to increase the saturation magnetization [18]. How- However, in our case, in spite of a clear improvement of the ferromagnetic properties after ever, in our case, post-growth in spite annealing of a clear (ex-situ improvement or in-situ, of the A1 see also Figure ferromagnetic in Appendixproperties after A), nanometric post-growth annealing (ex-situ or in-situ, see also Figure nickel-oxide segregations are still present at the LNMO surface. A1 in Appendix A), nanometric nickel-oxide segregations It is worth mentioningare that still Curie present at the LNMOfound temperatures surface. in our samples although sig- It is worth mentioning that Curie temperatures nificantly improved, are still lower than that found in the bulk found in our counterpart samples although sig- even after nificantly improved, are still lower than that found in the bulk counterpart even after ox- ygen annealing at the highest temperature (900 °C). This could be attributed to the non- homogeneity of the sample detected by HRTEM (see Figure A2 in Appendix A), that causes the formation of NiO secondary phases (as detected from EDX measurements) and cationic vacancies, giving rise to octahedral distortions in the film, and thus affecting their Nanomaterials 2021, 11, 804 7 of 13 oxygen annealing at the highest temperature (900 ◦ C). This could be attributed to the non-homogeneity of the sample detected by HRTEM (see Figure A2 in Appendix A), that causes the formation of NiO secondary phases (as detected from EDX measurements) and cationic vacancies, giving rise to octahedral distortions in the film, and thus affecting their ferromagnetic properties [28,29]. Moreover, as Ni agglomeration at the surface is larger after annealing process it is sound to assume that the Ni deficiency inside the film increases that, as a consequence, could further affect the ferromagnetic transition, as Tc is known to Nanomaterials 2021, 11, x FOR PEER REVIEW 8 of 13 decrease when reducing the Ni content [3,12]. In conclusion, the strong structural strain exerted by the substrate on thin layers clamped to it allows only the contraction of the out-of-plane cell parameter. Thus, high-temperature annealing in oxygen atmosphere Additionally, promotes a vertical the presence cationic of nickel-oxide migration (the entry ofnanoparticles oxygen ions asatwellthe as surface nickel of LNMO migration films may towards strongly surface), influence allowing thelocal electrical formation properties. To of nickel-oxide analyze the at nanoparticles electronic proper- film surface. ties of the different Additionally, thephases presencepresent at the LNMO of nickel-oxide film surface, nanoparticles at theconductive atomic films surface of LNMO force microscopy measurements were performed in films prepared after an in-situ may strongly influence local electrical properties. To analyze the electronic properties of annealing at 900 different the °C. Figure 5a shows phases topographic present at the LNMOimage insurface, film a 2 × 2 μm 2 area of aatomic conductive LNMO filmmicroscopy force with NiOx nanoparticles formed measurements on its surface, were performed while prepared in films Figure 5b after depictanitsin-situ simultaneous annealing acquired ◦ C. at 900cur- rent map Figure 5a(applying 1.5 V). shows topographic 2 image in a 2 × 2 µm area of a LNMO film with NiOx nanoparticles formed on its surface, while Figure 5b depict its simultaneous acquired current map (applying 1.5 V). Figure Localelectric 5. Local Figure 5. electric properties properties of the of the LNMOLNMO films films withwith NiOxNiO x nanoparticles nanoparticles at the at the surface surface meas- measured by C-AFM. (a) Topography image × 22µm 2 ured by C-AFM. (a) Topography image of aof 2 ×a 2 μm areaarea of film. of the the film. (b) Simultaneous (b) Simultaneous current current map of the same same area area taken taken(applying (applying1.51.5V), V),where wherethetheless lessconductive conductivepartsparts cancanbebe related related with with the particles particles observed observedin inthe thetopography. topography.(c)(c)Current Current (red) (red)andandheight height(black) profiles (black) of aofNiO profiles a NiO x x nanoparticle and nanoparticle and the the surrounding surroundingLNMO LNMOfilm filmmarked markedinin(a,b). (a,b).(d) (d)I-V I-Vcurves curvesononthe LNMO the LNMO film film (red) and (red) and on on aa particle particle (black), (black), respectively. respectively. In the topography image, NiOxx nanoparticles nanoparticles can can be identified as the parts of the the surface with higher height (bright circles) while in the current current mapmap aa conductive conductive matrix matrix (white background) (white background)isisdisrupted disruptedbybyless less conductive conductive circles circles (black). (black). Comparing Comparing bothboth im- images ages allows identifying NiO nanoparticles as the less conductive allows identifying NiOx nanoparticles as the less conductive parts of the current x parts of the current map, i.e., NiOxx particles particles are more resistive than the LNMO film. This correlation can be be better better appreciated in appreciated inFigure Figure5c, 5c,where wherethethe profiles profiles of the of the topography topography (black)(black) and theandcurrent the current (red) (red) of theof the(see lines linesFigure (see Figure 5a,b) 5a,b) are are depicted. depicted. In FigureIn Figure 5c,beit clearly 5c, it can can be observed clearly observed that, in the that,higher in theheight higher(z) region, height (z)which region,corresponds to a NiOxtoparticle, which corresponds a NiOx the current particle, thedrops to current ≈ 1 nA, while in the flat surrounding regions, where the LNMO film drops to ≈1 nA, while in the flat surrounding regions, where the LNMO film is directlyis directly measured, the current the measured, is almost current two orders two is almost of magnitude higher (≈100 orders of magnitude nA). (≈100 nA). higher Furthermore, the response of the LNMO film and the NiOx particles to voltage cy- cling is clearly different, as evidenced in Figure 5d. Voltage cycles taken directly on the LNMO surface (red curve) show a hysteretic behavior pinned at 0 V characteristic of a resistive switching behavior. At the same time, it is also observed that the application of a Nanomaterials 2021, 11, 804 8 of 13 Furthermore, the response of the LNMO film and the NiOx particles to voltage cycling is clearly different, as evidenced in Figure 5d. Voltage cycles taken directly on the LNMO surface (red curve) show a hysteretic behavior pinned at 0 V characteristic of a resistive switching behavior. At the same time, it is also observed that the application of a negative voltage (−5 V) locally increases the resistance of the film, switching the affected area to the so-called high resistance state (HRS). This HRS can be reverted back by the application of a positive voltage (+5 V) which returns the affected area of the film back into a more conductive state, i.e., the low resistance state (LRS). It is important to mention that both HRS and LRS are of a non-volatile nature and can be switched between them multiple times, stating the base for a bipolar resistive switching behavior suitable for the implementation of memory devices. A totally different behavior is observed sweeping the voltage on the top of a particle (black line). In this case, a non- hysteretic symmetrical curve is obtained, where the application of positive or negative voltage does not have any influence in the resistivity properties of the particles. The absence of resistive switching in the particles can be also visualized in Figure A3 (Appendix A). After switching an area to the HRS, the LNMO current is heavily reduced while the particles included in the area maintain their initial current, thus showing more conductivity than the high resistive state. Thus, our results show that nanostructured LNMO films may be switched from a system of poor conductive particles onto a conductive matrix to the opposite case, with conductive particles onto an insulating matrix, thus behaving as a model to study interfacial local transport. 4. Conclusions In conclusion, we report on the controlled formation of nickel-oxide nanocuboids at the surface of La2 Ni1−x Mn1+x O6 (x~0.47) thin films deposited by the RF magnetron sputtering technique. It is shown that the LNMO functional properties are highly influ- enced by the post-growth annealing process. Even LNMO films remains strained after various consecutive annealing processes, annealing treatments in oxygen atmosphere at high temperature allow a partial structural relaxation reflected in a reduction of the unit cell volume towards the LNMO bulk value. This structural relaxation, detected through a reduction of the out-of-plane cell parameter, allows some kind of vertical cationic migration and promotes an improvement of the ferromagnetic properties that affects both the Curie temperature and the saturation magnetization of the films. On the other hand, oxygen annealing processes promote Ni segregation towards the surface of the films where the for- mation of NiOx nanoparticles is detected. The presence of NiOx nanocuboids significantly changes the local surface transport properties, allowing one to obtain a composite surface in which areas exhibiting a resistive switching behavior alternate with other insulating behaviors. Author Contributions: Thin film preparation and characterization, M.B.-S.; supervision, Z.K. and L.B.; structural properties, C.F.; local transport properties, V.F.; writing—original draft preparation, M.B.-S., Z.K., L.B., and C.F.; writing—review and editing, C.F., A.P., and B.M. All authors have read and agreed to the published version of the manuscript. Funding: We acknowledge financial support from the Spanish Ministry of Science, Innovation and Universities through Severo Ochoa Program (CEX2019-000917-S) and RTI2018-099960-B-I00 (SPINCURIOX) and funding from the European Union’s Horizon 2020 research and innovation program under the Marie-Sklodowska-Curie grant agreement No. 645658 (DAFNEOX Project) and FEDER Program. Z.K. acknowledge the support of the Serbian Ministry of Education, Science and Technological Development (III45018). This work has been performed in the framework of the PhD program of the Universitat Autonoma de Barcelona (UAB). Data Availability Statement: The data that support the findings of this study are available on request from the corresponding authors. Acknowledgments: We acknowledge Jordi Arbiol and Jeremy David for electronic microscopy assistance and ICMAB technical services. Nanomaterials 2021, 11, 804 9 of 13 Conflicts of Interest: The authors declare no conflict of interest. Appendix A Figure A1 shows structural and magnetic properties of La2 Ni0.53 Mn1.47 O6 thin film (thickness, t = 42 nm) with in-situ annealing temperature at 900 ◦ C for 1 h under 5 104 Pa O2 (growth at 900 ◦ C under PO2 ~19 Pa). The reciprocal space map, around (103)STO reflection, erials 2021, 11, x FOR PEER REVIEW indicates strained film as discussed above (Figure A1a). The Tc value10found of 13 in our samples is slightly lower compared to the bulk value, as expected for nickel deficient samples [3,12] (Figure A1b). In-plane and out-of-plane hysteresis loops M(H) at 10 K (Figure A1c) exhibit A1c) exhibit thethe expected expected hysteretic hysteretic behaviorwith behavior withsimilar similarcoercive coercivefield, field,H Hcc~400 ~400Oe Oe(see (see corresponding inset). On corresponding inset). Onthetheother otherside, side,a saturation a saturation magnetization magnetization of (M of (M s ≈μB/f.u) s ≈5.8 5.8 µB/f.u) is is found to be somewhat found to be somewhat larger larger than thethan the expected expected theoretical theoretical value (∼5value (∼5 for μB/f.u.) µB/f.u.) fully or-for fully ordered stoichiometric dered stoichiometric LNMO, as LNMO, as discussed discussed previously previously [11]. [11]. More More about details detailsthe about the ferromagnetic ferro- nature in LNMO films with similar magnetic behavior, magnetic nature in LNMO films with similar magnetic behavior, including X-ray mag- including X-ray magnetic circular dichroism, can be found netic circular dichroism, can be found in [11]. in [11]. 5 6 7.8 6 4 2 4 4 0 Hip M (μ Β/f.u.) M ( μ B /f.u.) -2 2 Hop qz (nm-1) -4 3 -6 -0.2 0.0 0.2 7.7 0 2 -2 1 -4 7.6 (a) (b) -6 (c) 0 -2.6 -2.5 -2.4 0 50 100 150 200 250 300 -6 -4 -2 0 2 4 6 qx (nm-1) T (K) μΒH (T) (a) RSM around ◦ C thin film (thickness, t = 42 nm) with in-situ annealing Figure A1.Figure A1. (a) RSM (103) reflection around of LNMO900 (103) reflection of LNMO900 °C thin film (thickness, t = 42 nm) with in- temperature ◦ situatannealing 900 C (b)temperature at 900 °C (b) In-plane In-plane magnetization magnetization as a function as a function of temperature underofantemperature applied fieldun- of µ0 H = 0.5 T. (c) In-planeder andanout-of-plane of μ0H = 0.5 applied fieldhysteresis T. (c) loops M(H)In-plane at 10. and Insetout-of-plane hysteresis shows the low loopsinM(H) field region at 10. Inset detail. shows the low field region in detail. Figure A2 shows a plan-view in high resolution TEM (HRTEM) images, acquired in Figure A2 shows order toa plan-view in high resolution obtain information about theTEM (HRTEM) images, microstructure acquired in features of a and crystallographic order to obtain LNMO information about film with the microstructure in-situ annealed-900 ◦ Cand crystallographic process. The sample was features of a to the sample cut parallel surface (perpendicular to the (00l) planes). HRTEM LNMO film with in-situ annealed-900 °C process. The sample was cut parallel to the sam-images show a non-homogeneous surface withtothe ple surface (perpendicular thecoexistence (00l) planes). of HRTEM domains images with different show aorientations. non-homogeneous The Fourier transform surface with the(FT) of selected coexistence areas of the of domains withimage wereorientations. different computed inThe order to assess Fourier the periodicity of the transform (FT) of selected crystal areas oflattice. The were the image FT of computed a large areainwas marked order by squares, to assess which shows the periodicity of the different spots corresponding crystal lattice. The FT of a large to thewas area enlargement marked by of the LNMO lattice with respect to STO (LNMO cell √squares, which shows different spots parameters are of the type corresponding to the enlargement of the LNMO latticea ≈ b ≈ c ≈ 2a 2ap ,with p where respect to aSTO p is the (LNMOprimitive cell perovskite cell parameters are of the type a ≈ b ≈ √2ap, c ≈ 2ap where ap is the primitive perovskite cell l/) (k odd), and parameter). These spots can be indexed as (0 k/2 l/2) (k and l odd), (0 k/2 parameter). These (0 kspots l/2) (lcan odd) be and indexedindicate as (0different orientations k/2 l/2) (k and l odd),of(0thek/2LNMO cell inand l/) (k odd), the film. The Resistive (0 k l/2) (l odd) and indicate differentswitching characteristics orientations of the LNMO (or their cell inabsence), the film. depicted in Figure 5d of the main text, has also been probed on large surface areas to assess its reproducibility. Figure A3a,b depicts the topography and current images of a 5 × 5 µm2 area. In the center of this area a 3 × 3 µm2 square has been previously scanned at −4 V. As a consequence of this initial scan, the area has been switched into the HRS without harming the surface, as can be noticed in the topography image. Figure A3c,d show a detail picture (2 × 2 µm2 ) of the previous image. Adjusting the scale of the current map it can be observed that, inside the switched area, the particles are more conductive than the LNMO film in the HRS, while in the pristine state the particles are more resistive. The lack of resistance change after the biased scan supports the I-V curves presented in the main text and confirms the absence of RS behavior in the NiOx particles. surface with the coexistence of domains with different orientations. The Fourier transform (FT) of selected areas of the image were computed in order to assess the periodicity of the crystal lattice. The FT of a large area was marked by squares, which shows different spots corresponding to the enlargement of the LNMO lattice with respect to STO (LNMO cell Nanomaterials 2021, 11, 804 parameters are of the type a ≈ b ≈ √2ap, c ≈ 2ap where ap is the primitive perovskite cell 10 of 13 parameter). These spots can be indexed as (0 k/2 l/2) (k and l odd), (0 k/2 l/) (k odd), and (0 k l/2) (l odd) and indicate different orientations of the LNMO cell in the film. Nanomaterials 2021, 11, x FOR PEER REVIEW 11 of 13 The Resistive switching characteristics (or their absence), depicted in Figure 5d of the main text, has also been probed on large surface areas to assess its reproducibility. Figure A3a,b depicts the topography and current images of a 5 × 5 μm2 area. In the center of this area a 3 × 3 μm2 square has been previously scanned at −4 V. As a consequence of this initial scan, the area has been switched into the HRS without harming the surface, as can be noticed in the topography image. Figure A3c,d show a detail picture (2 × 2 μm2) of the previous image. Adjusting the scale of the current map it can be observed that, inside the switched area, the particles are more conductive than the LNMO film in the HRS, while in the pristine state the particles are more resistive. The lack of resistance change after the Figurescan biased Figure A2.A A2. Asupports plan-view plan-view HRTEM the imageof I-V curves HRTEM image ofLNMO900 LNMO900 presented °Cthin in◦ C thethin main film. film. Fourier text transform(FT) andtransform Fourier confirms (FT)shows the showsthe absence the coexistence ofcoexistence RS behavior of nanodomains in the NiOx with of nanodomains with different particles. orientations. different orientations. Figure A3. Switching of a 3 × 3 µm2 area of a LNMO film into the HRS. (a) Topography and Figure A3. Switching (b) current map images of of a 3a ×5 3μm 2 area of a LNMO film into the HRS. (a) Topography and (b) × 5 µm 2 area, where the central 3 × 3 µm2 has been previously scanned current map images of a 5 × 5μm2 area, where the central 3 × 3 μm2 has been previously scanned at at −4 V of sample Voltage. The current map shows a switching to the HRS of the LNMO film in the −4 V of sample Voltage. The current map shows a switching to the HRS of the LNMO film in the scanned area, which does not induce any changes in the topography. (c) Topography and (d) Current scanned area, which does not induce any 2changes in the topography. (c) Topography and (d) Cur- mapmap rent images of aof images detailed a detailed (2 ×(22×µm areaarea ) showing 2 μm thethe 2) showing lack of switching lack of of of switching NiOx particles NiOx that particles are that more resistive than the film in the pristine state but more conductive than the film in the are more resistive than the film in the pristine state but more conductive than the film in the HRS. HRS. AppendixBB Appendix The size of nanocuboids was determined from SEM micrographs using image analysis The size of nanocuboids was determined from SEM micrographs using image anal- (ImageJ software [30]) averaging measurements of a large number of particles at different ysis (ImageJ software [30]) averaging measurements of a large number of particles at dif- regions of the film surface. The distribution of nanoparticle size is well described by the ferent regions of the film surface. The distribution "   #of nanoparticle size is well described 2 S ln S log-normal function f (S) = √ 1 exp − 2σ20 , where the fitting parameters S0 and by the log-normal function 𝑓 𝑆2πσS = exp − , where the fitting parameters S0 √ σ are and σ the are most probable the most nanoparticle probable sizesize nanoparticle and and the width of the the width of distribution, respectively. the distribution, respec- The average nanoparticle size SM is determined from the most probable nanoparticle tively. The average nanoparticle size SM is determined from the most probable nanoparti- size cle size (SM = S0 exp(σ /2)). Examples of the analysis performed for different thin films are 2 shown in Figures A4 (in-situ annealed film) and B2 (ex-situ annealed film). A summary of the obtained values for the surface density and average nanoparticle volume is presented in Tables A1 and A2. Nanomaterials 2021, 11, 804 11 of 13 (SM = S0 exp(σ2 /2)). Examples of the analysis performed for different thin films are shown in Figure A4 (in-situ annealed film) and Figure A5 (ex-situ annealed film). A summary Nanomaterials 2021, 11, x FOR PEER REVIEW 12 ofof 13 Nanomaterials 2021, 11, x FOR PEER REVIEW 12 of 13 the obtained values for the surface density and average nanoparticle volume is presented in Tables A1 and A2. × of ◦ C thin film (thickness, Figure A4. SEM FigureA4. SEM micrograph micrograph with withaamagnification magnificationof of50,000 50,000× of LNMO-900 LNMO-900 °C thin film (thickness, Figure tt== 42 A4. SEM micrograph with a magnification ◦ of 50,000× of LNMO-900 °C thin image film (thickness, 42 nm, nm, in-situ in-situ annealing at 900 annealing at 900 °C). C).Filtered Filteredimage imageofofits itscorresponding correspondingSEM SEM (medium) image (medium) t = 42 nm, in-situ annealing at 900 °C). Filtered image of its corresponding SEM image (medium) and andautomated automatedgrains grainscounting countingwith withImage ImageJJsoftware software(right). (right). and automated grains counting with Image J software (right). Table A1.Surface TableA1. Surfacedensity, density,average averagenanoparticle nanoparticle volume, volume, total total volume volume of the nanoparticles (NiO (NiOXX)) Table A1. Surface density, average nanoparticle volume, total volume of the 22) and ◦ Cnanoparticles (NiOX) perunit per unitsurface surface (1 (1 µm μm nanoparticle fraction volume of LNMO-900 LNMO-900 °C thin thinfilm film (determined (determined per unit surface (1 μm2) and nanoparticle fraction volume of LNMO-900 °C thin film (determined fromFigure from FigureA4). A4). from Figure A4). AnnealingTT(In-Situ)Density Density NPVolume NP Volume NPsNPs Volume VolumeNP Fraction NP Fraction Annealing Density 2) NP Volume 3 )3) NPs Volume 3 )/µm2 2NP Fraction Volume Annealing T (In-Situ)(NP/µm (In-Situ) (NP/µm 2) (nm (nm (nm 3)/µm (nm Volume Volume (NP/µm ) 2 (nm ) 3 (nm )/µm 3 2 900 ◦ °C ~1.04 × 10− 1× 10−1 ~4.2××101044 ~4.4 × 10 5 5 ~0.011 900 900C°C ~1.04~1.04 × 10−1 ~4.2 ~4.2 × 104 × 10× ~4.4~4.4 5 10 ~0.011 ~0.011 FigureA5. A5. SEM SEM micrographs micrographs withwith aa magnification magnificationofof200,000 200,000× of the same LNMO thin film (thick- Figure Figure A5. SEM micrographs with a magnification of 200,000××ofofthe thesame sameLNMO LNMOthin thinfilm film(thick- (thick- ness, t = 48 nm) ness,t t==4848nm) after nm)after different afterdifferent ex-situ differentex-situ ex-situ annealing annealing temperatures: temperatures:(a) Anneal-800 °C ◦ and (b) Anneal- ness, annealing temperatures: (a)(a) Anneal-800 Anneal-800 C and °C and (b)(b) Anneal- Anneal- 900 ◦°C. Filtered image of its corresponding SEM image (medium) and automated grains counting 900°C. 900 C.Filtered Filteredimage imageofofitsitscorresponding correspondingSEM SEMimage image(medium) (medium)and andautomated automated grains grains counting counting with Image J software (right). with withImage ImageJJsoftware software(right). (right). Table A2. Surface density, average nanoparticle volume, total volume of the nanoparticles (NiOX) Table Table A2. A2. Surface Surfacedensity, density, average averagenanoparticle nanoparticlevolume, volume,total totalvolume volumeofofthe thenanoparticles nanoparticles(NiO (NiO X)X ) per unit surface (1 μm 2) and nanoparticle fraction volume of the same LNMO thin film after differ- 22) and nanoparticle fraction volume of the same LNMO thin film after differ- per perunit unit surface surface (1 (1 μm µm ) and nanoparticle fraction volume of the same LNMO thin film after different ent ex-situ annealing temperatures, determined from Figure A5. ent ex-situ ex-situ annealing annealing temperatures, temperatures, determined determined from from Figure Figure A5.A5. Annealing T (Ex- Density NP Volume NPs Volume NP Fraction Vol- Annealing Annealing T (Ex- Density 2 NP NPVolume NPs Volume NP Fraction Vol- Situ)T Density (NP/µm ) Volume (nm 3) NPs (nm 3Volume )/µm 2 NP ume Fraction Situ) (Ex-Situ) (NP/µm (NP/µm 2 ) 2) (nm (nm )) 2 3 3 (nm (nm)/µm 3 3 2 )/µm5 2 ume Volume 800 °C ~5.34 × 10 2 ~3.7 × 10 ~2.0 × 10 ~0.004 800◦ C°C ~5.34 × × 2102 2 ~3.7 × 10 22 ~2.0 × 10 5 5 ~0.004 800900 °C ~5.34 10 ~3.70 ×2 10 ~1.8××1010 ~3.7 3 ~2.0 ××10 ~6.5 105 ~0.004 ~0.014 900◦ C°C 900 ~3.70 ~3.70 × 102 × 10 ~1.8 × 10 ~1.8 × 1033 ~6.5 ~6.5× ×1010 5 5 ~0.014 ~0.014 References References 1. Chumak, A.V.; Vasyuchka, V.I.; Serga, A.A.; Hillebrands, B. Magnon spintronics. Nat. Phys. 2015, 11, 453–461. 1. Chumak, A.V.; Vasyuchka, V.I.; Serga, A.A.; Hillebrands, B. Magnon spintronics. Nat. Phys. 2015, 11, 453–461. 2. Hoffmann, A.; Bader, S.B. Opportunities at the Frontiers of Spintronics. Phys. Rev. Appl. 2015, 4, 047001. 2. Hoffmann, A.; Bader, S.B. 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