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Growth Des. 2019, 19, 2765−2771 pubs.acs.org/crystal Nonstoichiometry Driven Ferromagnetism in Double Perovskite La2Ni1−xMn1+xO6 Insulating Thin Films Monica Bernal-Salamanca,*,† Zorica Konstantinovic,́ †,‡ Lluis Balcells,† Elisa Pannunzio-Miner,†,§ Felip Sandiumenge,† Laura Lopez-Mir,† Bernat Bozzo,† Javier Herrero-Martín,⊥ Alberto Pomar,† Carlos Frontera,† and Benjamín Martínez† † Institut de Ciència de Materials de Barcelona, ICMAB-CSIC, Campus UAB, 08193 Bellaterra, Spain ‡ Center for Solid State Physics and New Materials, Institute of Physics Belgrade, University of Belgrade, Pregrevica 118, 11080 Belgrade, Serbia See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles. § Facultad de Ciencias Exactas, Físicas y Naturales, Centro de Investigaciones en Ciencias de la Tierra (CICTERRA-CONICET-UNC), Av. Velez Sarfield 1611, X5016GCA Ciudad Universitaria, Cordoba, Argentina ⊥ ALBA Synchrotron Light Source, C. de la Llum 2-26, E 08920 Cerdanyola del Vallès, Spain * S Supporting Information Downloaded via 83.56.229.240 on May 11, 2021 at 07:46:37 (UTC). ABSTRACT: In this work we report on the epitaxial growth of La2NiMnO6 double perovskite thin films on top of (001) oriented SrTiO3 substrates by RF magnetron sputtering. The influence of oxygen pressure (PO2) and growth temperature on the microstructure, stoichiometry of the films, and magnetic and transport properties is thoroughly investigated. It is found that high oxygen pressure promotes the growth of stoichiometric films, with a Ni/Mn ratio almost equal to 1. However, these films exhibit poor ferromagnetic properties with respect to the expected optimum values corresponding to ferromagnetic ordering mediated by superexchange interaction between Mn4+ and Ni2+ according to the Goodenough-Kanamori rules. Most interestingly, films grown at low PO2 exhibit Ni/Mn ratios below 1, but ferromagnetic properties close to the optimal ones. The valence balance between Ni and Mn ions in nonstoichiometric sample has been elucidated by X-ray absorption spectros- copy. The results indicate that Ni deficiency plays a crucial role in the puzzling insulating ferromagnetic behavior observed in nonstoichiometric samples. ■ INTRODUCTION The urgent need to solve the problems of energetic efficiency according to the Goodenough-Kanamori rules,10 and it is very sensitive to the ordered distribution of cations in the B- and data storage and processing speed of current micro- sublattice of the DP structure.11 In the case of LNMO, cationic electronic devices has boosted the search for new materials and order gives rise to 180° ferromagnetic interactions between paradigms capable of circumventing those problems. Spin- Ni2+(d8, t62ge2g, S = 1) and Mn4+ (d3, t32ge0g, S = 3/2) ions12 and tronics has been signaled as a potential alternative technology results in a TC very close to RT (TC ≈ 280 K). Disordering of to develop multifunctional electronics that combines logic Ni and Mn ions, and oxygen vacancies may strongly suppress operations, data storage and transmission with an improved FM superexchange interaction.13 In fact, cationic disorder energetic efficiency. The development of spintronic devices reduces the saturation magnetization from 5 μB/f.u. (the requires the generation and control of highly spin-polarized maximum expected for the FM contribution of Ni2+ and Mn4+) currents1 or pure spin currents.2 A way of obtaining a highly since, according to the Goodenough-Kanamori rules, disorder spin polarized current is by using a spin filtering effect through introduces antiferromagnetic (AF) Mn4+−O−Mn4+ and Ni2+− ferromagnetic-insulator-ferromagnetic tunnel barriers.3,4 The O−Ni2+ interactions.8,15 same effect can be obtained simply by using a ferromagnetic In this work we report on the growth and characterization of insulating barrier.5,6 However, ferromagnetic insulators (FMI) La2Ni1−xMn1+xO6 thin films prepared by RF magnetron are very scarce; among the few FMI known double perovskite sputtering on top (001)-oriented SrTiO3 (STO) substrates. (DP) oxides, like La2NiMnO6 (LNMO) and La2CoMnO6, Substrates have been cleaned in an ultrasonic bath with Milli-Q have attracted considerable attention due to their high Curie temperature, TC, and magnetodielectric properties.7−9 In both Received: December 21, 2018 cases, the ferromagnetic (FM) ordering is explained by Revised: March 4, 2019 superexchange interaction between Mn4+ and Ni2+ or Co2+ Published: March 15, 2019 © 2019 American Chemical Society 2765 DOI: 10.1021/acs.cgd.8b01897 Cryst. Growth Des. 2019, 19, 2765−2771 Crystal Growth & Design Article Table 1. Chemical Composition and Stoichiometry Ratio of LNMO-δD (140 mTorr) and LNMO-δG (300 mTorr) Samples PO2 (mTorr) La (atom %) Ni (atom %) Mn (atom %) O (atom %) La/Ni + Mn Ni/Mn x 140 19.2 ± 0.2 6.0 ± 0.1 14.0 ± 0.1 60.0 ± 0.5 0.96 0.4 0.43 300 20.0 ± 0.1 9.0 ± 0.1 9.0 ± 0.1 60.0 ± 0.2 1.11 1.0 0.0 water and then annealed at 1000 °C in air for 2 h to obtain a temperature from 900 to 850 °C, at a partial oxygen pressure clean and smooth surface typical for single TiO2 termination.16 of 140 mTorr, TC increases from TC ∼160 K to ∼240 K Growth conditions, such as temperature, annealing, and (estimated from inflection point). As a result, the optimal oxygen partial pressure, have been optimized to obtain Curie temperature, TC ≈ 240 K, has been obtained for samples LNMO thin films with MS and TC close to the optimum grown/annealed at 850 °C and at an oxygen partial pressure of values. However, it is found that Ni/Mn stoichiometric 140 mTorr. This sample was labeled as LNMO-δD. samples always exhibit values of TC and MS far from the According to previous results reported for LNMO samples optimum ones. In contrast, samples with a remarkable Ni prepared by PLD, good FM properties (i.e., TC ≈ 260 K and deficiency show excellent magnetic properties. We report a MS ≈ 5 μB/f.u. at low T) are obtained in samples prepared at detailed study of structural, magnetic, and transport properties higher oxygen pressures, i.e., 300 mTorr,17 or even 800 mTorr, of these two characteristic types of samples. and temperatures around 750−800 °C,13,14 with in situ ■ EXPERIMENTAL DETAILS A series of LNMO samples were grown with oxygen partial pressures annealing (in 760 Torr of O2) to promote B site cationic ordering and minimize the oxygen deficiency.21 In general, for lower oxygen pressures, samples present a low saturation ranging from 70 to 300 mTorr and temperatures between 500 and magnetization,13 with the exception of ref 17 where films 900 °C. As-grown samples were annealed in-situ at the same prepared under pO2 > 180 mTorr are reported to have MS ≈ 5 temperature for 1 h under high oxygen pressure (420 Torr, see μB/f.u.. Supporting Information SI1c). The surface quality of samples was characterized by atomic force microscopy (AFM), using a MFP-3D These results are in contrast with our own observations, so AFM (Asylum Research) in taping mode. Structural characterization to clarify the origin of those discrepancies we have carefully was made by means of X-ray diffraction and reflectivity techniques analyzed the stoichiometry (using EPMA) of our samples. using a X’Pert MRD-Panalytical and a Siemens D5000 diffractom- Results are shown in Supporting Information SI2a. It is found eters. The chemical composition and stoichiometry ratio of LNMO that all the samples present a La:(Ni + Mn) atomic ratio near thin films were determined by wavelength dispersive spectrometer 1, but the Ni/Mn ratio strongly depends on pO2 used in the (WDS) electron probe microanalysis (EPMA) using a CAMECA SX- 50 electron microprobe equipped with four wavelength-dispersive X- growth process. Thus, the actual composition of the samples ray spectrometers. High-resolution transmission electron microscopy should be expressed as La2Ni1−xMn1+xO6. Samples grown at (HRTEM) was used to study the microstructure of cross section high pressure present a Ni/Mn ratio close to 1 (and thus good specimens of LNMO thin films on a FEITecnai G2F20 S-TWIN stoichiometry), but at lower pressures samples become Ni- HR(S)TEM operated at 200 kV. Cross section specimens were deficient, reaching values of a Ni/Mn ratio of about 0.3 (x ≈ prepared by focus ion beam (FIB). The transport properties were 0.54, La2Ni0.46Mn1.54O6). In spite of this remarkable Ni measured in a Physical Properties Measurement System (PPMS, deficiency, samples exhibit excellent FM properties (see Quantum Design) by a four-probe technique. Magnetization measurements were done using a superconducting quantum Supporting Information SI1a). It must be remarked that interferometer device (SQUID, Quantum Design). X-ray absorption Blasco et al. report, for bulk LaNi0.25Mn0.75O3, a FM behavior spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) with TC = 200 K and MS = 2.75 μB/f.u. (equivalent to 5.5 μB/ were investigated at the Ni and Mn L2,3 edges in BL29-BOREAS f.u. in double perovskite notation). It is also worth to remark beamline at ALBA Synchrotron Light Source (Barcelona, Spain). The that the same work reports stoichiometric oxygen content spectra were measured in both the total electron yield (TEY) and when samples were sintered in O-poor atmosphere (Ar). On total fluorescence yield (TFY) modes, under ultrahigh vacuum another hand, a very detailed local study of LNMO films conditions (2 × 10−10 mbar). The applied magnetic field (parallel to grown by molecular beam epitaxy (at very low oxygen the X-ray beam) was 4 T. ■ pressure) has shown a spontaneous segregation of the stoichiometric La2NiMnO6 into a La2Ni1−xMn1+xO6 (x ≈ RESULTS AND DISCUSSION 0.2) matrix and NiO inclusions.22 Previous results corresponding to LNMO/STO thin films Lowering the growth/annealing temperature to 850 °C has prepared by pulsed laser deposition (PLD)13,17−20 make little effect on the stoichiometry of the films (see Supporting evident that magnetic properties depend on film thickness Information SI2b) which are still Ni deficient. Actually, sample and growth conditions. Similarly, samples prepared by RF LNMO-δD has a Ni/Mn ratio of 0.40 (x = 0.43) as shown in sputtering, in a broad range of temperatures (500 °C ≤ T ≤ Table 1. Next, we will focus on two samples prepared by using 900 °C) and oxygen pressure (70 mTorr ≤ PO2 ≤ 300 mTorr), the same conditions: growth temperature 850 °C and exhibit strong dependence on the growth conditions. Samples annealing at 850 °C for 60 min, one under 140 mTorr of O2 grown/annealed at high temperature and oxygen pressures pressure (LNMO-δD), and the second under 300 mTorr above ∼200 mTorr show very deficient magnetic properties (LNMO-δG). This second sample has nearly the nominal Ni/ with TC below 150 K. On lowering oxygen pressure Mn = 1 stoichiometry but poor ferromagnetic properties. (maintaining the same annealing temperature), TC rises up XRR curves of nonstoichiometric (LNMO-δD) and to ∼225 K and then decreases again (see Supporting stoichiometric (LNMO-δG) samples are depicted in Figure Information SI1a). We have also found a non-monotonous 1a,b, which correspond to the highest and lowest TC obtained, dependence of TC on the deposition temperature (see the fitting to these data (black lines) has been done using XRay Supporting Information SI1b). On lowering the growth utilities library.23 Thickness values found for both samples are 2766 DOI: 10.1021/acs.cgd.8b01897 Cryst. Growth Des. 2019, 19, 2765−2771 Crystal Growth & Design Article Figure 1. XRR data of (a) LNMO-δD (140 mTorr O2) and (b) LNMO-δG (300 mTorr O2) thin films grown on STO(001) Figure 2. High-resolution θ/2θ XRD scans of the (002) reflections substrate, with thickness values of 42 and 44 nm; insets show AFM for (a) LNMO-δD and (c) LNMO-δG films. Panels (b) and (d) show topography images of a small area of the samples. RSM around (103) reflections for the same samples. very similar (42 and 44 nm). Inset in Figure 1 shows AFM FT of a large area shows different spots corresponding to the topography images in tapping mode of both films, topography enlargement of the LNMO lattice with respect to STO shows a high quality flat surface with a terrace-steps type (LNMO cell parameters are of the type a ≈ b ≈ √2ap, c ≈ 2ap morphology. The root-mean-square (rms) value of surface where ap is the primitive perovskite cell parameter). These roughness is found to be very small about of 0.2 nm and steps spots can be indexed as (0 k/2 l) (k odd), (0 k l/2) (l odd), height of about 0.4 nm, corresponding to the perovskite unit and (0 k/2 l/2) (k and l odd) and indicate different cell. Annealing at high oxygen pressures (420 Torr) favors the orientations of the LNMO cell in the film. This is better formation of well-defined terraces mimicking the STO illustrated by FT of small regions. For some regions only spots substrate. corresponding to (0 k/2 l) (k odd) are found thus in those In addition, structural properties have been investigated by regions cLNMO is parallel to (010)-STO substrate direction; in high resolution θ/2θ X-ray diffraction (XRD), Figure 2a,c other zones only spots of the type (0 k l/2) (l odd) can be shows scans around (002)STO reflection using Cu−Kα1 seen, which correspond to cLNMO parallel to (001)-STO; and monochromatic radiation of LNMO-δD and LNMO-δG finally a third type of regions where spots of the type (0 k/2 l/ samples, respectively. From these θ/2θ XRD scans the 2) (k and l odd) are present corresponds to cLNMO parallel to corresponding out-of-plane lattice parameters are determined: (100)-STO. On the other hand, in Figure 3b sample LNMO- c = 3.895 Å for LNMO-δD and c = 3.908 Å LNMO-δG, which δG was cut along the (110)-STO direction. In this case, the are slightly higher than the pseudocubic perovskite lattice sample is more homogeneous, only one type of spot parameter of bulk LNMO, namely, 3.879 Å,24 and this corresponding to the larger cell of LNMO is observed, and difference may be caused by the presence of Mn3+. It is also superstructure spots are indexed as (h/2 −h/2 l/2) (h and l worth mentioning here that Laue fringes can only be seen in odd). This indicates that the orientation of cLNMO axis is LNMO-δG sample but not in LNMO-δD one. Reciprocal parallel to (100) or (010) (in the plane of the film). Therefore, space maps, around (103)STO reflection of the same samples, HRTEM observations confirm that domains with in-plane and are shown in Figure 2b,d. Measures evidence that films grow out-of-plane orientations of cLNMO coexist in LNMO-δD fully strained. In fact, for LNMO-δD sample, a small peak sample, while only in-plane orientation of cLNMO is stabilized coming from the LNMO film can be seen just above the for LNMO-δG sample. This multiple orientations of c-axis in substrate peak. For LNMO-δG sample, both peaks are fully LNMO-δD must be related to the absence of Laue fringes in overlapped due to the high similarity between the out-of-plane the θ/2θ scans. lattice parameters. Zero field cooling-field cooling (ZFC-FC) magnetization A deeper insight into the microstructural features can be curves of both, LNMO-δD and LNMO-δG, samples measured obtained by using high resolution transmission electron using a field of H ≈ 0.1 T, are depicted in Figure 4a. This microscopy (HRTEM) (see Figure 3a,b. For HRTEM figure evidences the strong dependence of the magnetic observations in Figure 3a sample LNMO-δD was cut along behavior on the oxygen pressure during the growth process. the (100) direction of STO. HRTEM images show the M(T) curves of LNMO-δG sample do not show evidence of coexistence of domains with different orientations as can be ferromagnetic ordering. On the other hand, M(T) curves of appreciated in the Fourier transform (FT) of the image. The LNMO-δD display the expected temperature dependence of 2767 DOI: 10.1021/acs.cgd.8b01897 Cryst. Growth Des. 2019, 19, 2765−2771 Crystal Growth & Design Article Figure 4. (a) Magnetization measured after ZFC and FC as a function of temperature under an applied field of 1 kOe (in the plane of the film) of LNMO-δD (140 mTorr O2, red curve) and LNMO-δG (300 mTorr O2, blue curve). (b) M(H) hysteresis loops of LNMO-δD at 10 K for H applied in-plane and out-of-plane, the inset shows in detail the low field region. (c) Temperature dependent resistivity of LNMO- δD under zero and 9 T magnetic field. Inset, ln ρ vs (1/T)1/4. Figure 3. (a) Cross-section HRTEM image of the LNMO-δD (140 mTorr) film viewed along the STO [100] zone axis and the field applied in-plane and out-of-plane configurations, corresponding Fourier filtered image emphasizing the strain state of respectively. Accordingly, magnetization remanence for the in- the film and the coexistence of nanodomains with different plane configuration (Mr ≈ 3.1 μB/f.u) is larger than that found orientations. Bottom panels show, from left to right, an FFT of the for the out-of-plane (Mr ≈ 1.7 μB/f.u). A saturation whole image exhibiting superlattice spots at (0 0 l/2), (0 k/2 l/2), and magnetization of Ms ≈ 5.3 μB/f.u, is found, i.e., somewhat (0 k/2 0) (indexed according to STO orientation), and FFT patterns larger than the expected theoretical value (∼5 μB/f.u.) for obtained from windows 1, 2 and 3, respectively, revealing the perfectly ordered stoichiometric La2Ni2+Mn4+O6,26 and also orientations of different nanodomains: In 1 the c-axis of the film is in- larger than the experimental bulk value (∼4.96 μB/f.u.). At this plane and parallel to the image, in 2 the c-axis is in-plane and point it is worth mentioning that nonstoichiometry enhances perpendicular to the image, and in 3 it is oriented out-of-plane. In 1, the maximum value achievable for the magnetization for full 2, and 3, peaks and zone axes are referred to the pseudocubic unite cell of LNMO. (b) Schematic models for the observed epitaxial FM ordering. For the stoichiometry (Ni/Mn ratio) found in relationships. (c) Cross-section HRTEM image and corresponding our case Ms should be around 5.9 μB/f.u., as 2 μB/Ni (× 0.57 FFT pattern of the LNMO-δG (300 mTorr) film viewed along the Ni/f.u.) are attributed to Ni2+ and 3.34 μB/Mn (× 1.43 Mn/ [110] zone axis, exhibiting high crystalline quality and an in-plane f.u.) to Mn3.66+ (as a fraction of Mn4+ must reduce to Mn3+ to orientation of the monoclinic cLNMO axis. The encircled spot compensate Ni deficiency). corresponds to the (h/2 h/2 −l/2) superlattice. Indices are referred On the other hand, according to Figure 4b the easy to the pseudocubic unit cell of LNMO. magnetization direction lies in the film plane. Although this is the usual behavior in thin films since shape anisotropy tends to an FM with no signal of double transition, and a Curie place magnetization in plane, the large remanence coercive temperature of about 240 K. The field dependence of the field of the out-of-plane magnetization curve cannot be magnetization, M(H), after correcting the diamagnetic understood by considering only shape anisotropy and indicates contribution from substrate and other instrumental contribu- that magnetocrystalline anisotropy also plays an important tions,25 is shown in Figure 4b. M(H) loops corresponding to role. Furthermore, HRTEM characterization has shown that sample LNMO-δD at T = 10 K exhibit the expected hysteretic domains with different crystallographic orientations coexist. behavior with a coercive field of about 1.4 kOe and 1.2 kOe for Consequently, magnetocrystalline anisotropy would contribute 2768 DOI: 10.1021/acs.cgd.8b01897 Cryst. Growth Des. 2019, 19, 2765−2771 Crystal Growth & Design Article to both in-plane and out-of-plane directions, in accordance with the observed behavior. Figure 4c shows the temperature and field-dependence of the resistivity of the LNMO-δD film. The measurements show that the resistivity values increase when cooling, thus exhibiting insulating behavior in the whole temperature range. Almost no magnetic field dependence of the resistivity is detected even when crossing TC (∼240 K). The temperature dependence can be well described by using a three-dimensional (3D) variable- range hopping (VRH) model: ρ(T) = ρ0 exp(T0/T)1/4, where ρ0 is the prefactor and T0 is the characteristic hopping temperature, related to the electron hopping probability, P, of the material.27 The slope of the curve corresponds to T1/40 , and in this case, we have obtained a value of T0 ≈ 0.4 × 108 K, which is similar to the values reported for other transition metal oxides.28,29 As mentioned in the introduction, the FM character of La2NiMnO6 is understood in terms of the superexchange interactions between Ni2+ and Mn4+ according to the Goodenough-Kanamori rules. However, the large Ni deficiency found in LNMO-δD sample puts a question mark over this interpretation. To determine the valence states of Ni and Mn ions in Ni-deficient samples, X-ray absorption spectroscopy (XAS) experiments have been carried out in LNMO-δD (x ≈ 0.43) sample (at two different temperatures: 20 and 300 K). The L2,3 (2p → 3d transition) XAS spectra, measured by using the TEY technique, corresponding to Mn and Ni are shown in Figure 5a,b. The Ni L3 edge (see Figure 5b) is overlapped with the La M (3d3/2 → 4f) absorption peak and is hard to analyze. In contrast, the Ni L2 edge (ℏυ ≈ 865−875 eV) absorption peak (shown in the inset) can be easily compared with that of Figure 5. XAS spectra for an epitaxial LNMO-δD thin film. (a) Mn NiO (Ni2+) also shown in the inset. All three edges present a L2,3 and (b) Ni L2,3, the inset shows the Ni 2p1/2 absorption peak, double peak feature that is quite similar to all Ni-divalent absorption spectrum of NiO is also plotted for comparison (black compounds and is well understood in terms of a covalent curve). XMCD of (c) Mn L2,3 and (d) Ni L2,3, measured at 20 K (red ground state of mainly Ni2+ (3d8) character plus an anion- curves) and room temperature (RT) (blue curves); the dashed line dependent fraction of the 3d9 L and 3d10 L configurations, shows the corresponding integral at 20 K (right axis, note that dashed where L stands for an anion (ligand) hole.30,32 This double lines start at zero value). (e) Absorption-corrected FY (left) and TEY (right) Mn L2,3 XAS edge of LNMO-δD film compared with reference peak is also displayed by L2 edge of a stoichiometric (and with spectra such as LaMnO3, LSMO, and CaMnO3 at 300 K respectively. almost full cationic ordering) sample of La2NiMnO6, and very different from the L 2 peak of PrNiO 3 and NdNiO 3 corresponding to (Ni3+).31,32 Therefore, we can conclude saturation magnetization might be observed in Ni-deficient that the oxidation states of Ni ions in our Ni-deficient sample samples. is 2+. It is worth mentioning that a XANES study in bulk X-ray magnetic circular dichroism (XMCD) has been used samples of LaNi1−xMnxO3 has concluded that for x ≥ 0.5 the to investigate the specific magnetic properties of Mn and Ni valence of Ni stabilizes to 2+.33 ions in the LNMO-δD sample. All the XMCD spectra (see In order to determine the valence of Mn, we have also Figure 5c,d) have been normalized to the integrated area of the measured the L2,3 edges of reference samples LaMnO3 (Mn3+), corresponding XAS spectra.33 These spectra were measured by La0.7Sr0.3MnO3 (Mn3.3+), and CaMnO3 (Mn4+). Spectra the TEY method at 20 K and 300 K. It is found that L3 measured in both TEY and TFY detection modes at room absorption peak shows a strong negative XMCD signal at both temperature are compared in Figure 5e. A shift of the Mn L3 Ni and the Mn edges, which indicates that the Ni2+ and Mn4+ peak toward higher energies for all XAS spectra in the series of ions are aligned ferromagnetically. The XMCD signal nearly samples is detected. By comparing L3 absorption peaks of disappears at room temperature (above TC). LaMnO3 at 642.5 eV (Mn3+) and CaMnO3 located at 644.1 eV In order to extract quantitative information about orbital (Mn4+), with that of LNMO-δD centered between both magnetic moment μorb and spin magnetic moment μspin compounds at 643.5 eV, the valence of the latter can be contributions to the magnetic moment of Mn 3d and Ni 3d estimated to be around +3.6. Therefore, assuming a 2+ valence states, we made use of the so-called sum-rules in the XMCD for Ni, and recovering sample stoichiometry values determined spectra.34−36 by EPMA, a mixed valence of 3.64+ is obtained for Mn. Thus, XAS measurements lead us to conclude that Ni-deficient samples (LNMO-δD) contain a certain quantity of Mn3+, and μL = − ( 4 ∫ ΔI(E) dE + ∫ ΔI(E) d E L 3 L 2 ) (10 − N ) 3d therefore, Mn must be in a mixed-valence Mn3+/Mn4+ oxidation state. For that reason, since Mn3+ ions have a larger ( 3 ∫ I (E ) d E + ∫ I (E ) d E L 3 L 2 ) magnetic moment that Mn4+ ones, larger values of the (1) 2769 DOI: 10.1021/acs.cgd.8b01897 Cryst. Growth Des. 2019, 19, 2765−2771 Crystal Growth & Design Article 2 ∫ ΔI(E) dE − 4 ∫ ΔI(E) dE Additional M(T) curves and stoichiometry of the μS + 7μT = − L3 L2 LNMO films (PDF) ■ ∫L I(E) dE + ∫L I(E) dE 3 2 AUTHOR INFORMATION (10 − N3d) (2) Corresponding Author where ΔI = I+ − I−; I = I+ + I−; N3d is the 3d electron *E-mail: mbernal@icmab.es. Telephone: +34 935 801 853. occupation number and μT is the magnetic-dipole moment ORCID (usually negligible for transition metals in octahedral environ- Carlos Frontera: 0000-0002-0091-4756 ment). The corresponding integral of the XMCD signal is also Benjamín Martínez: 0000-0001-9879-7748 depicted in (Figure 5c,d). In the case of Mn ions from the Notes integral of XMCD signal it is found that μMn L ≈ 0. In contrast The authors declare no competing financial interest. ■ we obtained μNi Ni L /μS = 0.32 at 20 K for Ni. Thus, XMCD study reveals that orbital moment of Ni is not quenched. It is worth ACKNOWLEDGMENTS mentioning that this orbital contribution is necessary for spin− orbit interactions giving rise to magnetocrystalline anisotropy M.B.-S., Ll.B., F.S., B.B., C.F., B.M., and A.P. acknowledge and reinforces the interpretation that this must be the origin of financial support from the Spanish Ministry of Economy and the magnetic anisotropy evidenced in Figure 4b. Competitiveness through the “Severo Ochoa” Programme for ■ Centres of Excellence in R&D (SEV-2015-0496), and Project CONCLUSIONS MAT2015-71664-R. This work has received funding from the European Union’s Horizon 2020 Research and Innovation In summary, we have shown that high-quality epitaxial thin Programme under the Marie Sklodowska-Curie Grant Agree- films of the double perovskite LNMO, with Curie temper- ment No. 645658 (DAFNEOX Project). Z.K. acknowledge the atures of Tc = 240 K and saturation magnetization of 5.3 μB/ support of the Serbian Ministry of Education, Science and f.u., can be prepared by using the RF sputtering technique. Technological Development (III45018). Authors thank ALBA However, magnetic properties turn out to be strongly synchrotron (Spain) for the provision of beamtime. ■ dependent on the preparation conditions, particularly on the oxygen partial pressure, pO2, used during the growth process. REFERENCES Samples prepared at low pO2 (≤140 mTorr) are Ni-deficient (1) Bibes, M.; Villegas, J. E.; Barthélémy, A. 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