RESEARCH ARTICLE www.advancedscience.com Strain-Driven Bidirectional Spin Orientation Control in Epitaxial High Entropy Oxide Films Zhibo Zhao, Arun Kumar Jaiswal, Di Wang,* Vanessa Wollersen, Zhengyu Xiao, Gajanan Pradhan, Federica Celegato, Paola Tiberto, Maria Szymczak, Juliusz Dabrowa, Moaz Waqar, Dirk Fuchs, Xiaoqing Pan, Horst Hahn, Robert Kruk,* and Abhishek Sarkar* 1. Introduction High entropy oxides (HEOs), based on the incorporation of multiple-principal cations into the crystal lattice, offer the possibility to explore previously Metal oxides with competing or nearly inaccessible oxide compositions and unconventional properties. Here it is degenerate physical ground states often exhibit complex and fascinating physical demonstrated that despite the chemical complexity of HEOs external stimuli, properties that are a result of the interplay such as epitaxial strain, can selectively stabilize certain magneto-electronic between different electronic, spin, and lat- states. Epitaxial (Co0.2 Cr0.2 Fe0.2 Mn0.2 Ni0.2 )3 O4 -HEO thin films are grown in tice degrees of freedom.[1] In such cases, three different strain states: tensile, compressive, and relaxed. A unique strain engineering offers a powerful tool for coexistence of rocksalt and spinel-HEO phases, which are fully coherent with selective stabilization of the desired physi- cal state by providing precise control over no detectable chemical segregation, is revealed by transmission electron the structural distortions and defects in the microscopy. This dual-phase coexistence appears as a universal phenomenon material.[2] This approach has been success- in (Co0.2 Cr0.2 Fe0.2 Mn0.2 Ni0.2 )3 O4 epitaxial films. Prominent changes in the fully utilized in magnetic oxides to enhance magnetic anisotropy and domain structure highlight the strain-induced existing functionalities or to create entirely bidirectional control of magnetic properties in HEOs. When the films are new ones.[2–4] The newly introduced class relaxed, their magnetization behavior is isotropic, similar to that of bulk of high entropy oxides (HEOs),[5–8] due to their exceptional chemical complexity, materials. However, under tensile strain, the hardness of the out-of-plane provides a natural setting for the competi- (OOP) axis increases significantly. On the other hand, compressive straining tion/coexistence of magneto-electronic in- results in an easy OOP magnetization and a maze-like magnetic domain teractions. HEOs are compositional com- structure, indicating the perpendicular magnetic anisotropy. Generally, this plex single-phase solid solutions that are study emphasizes the adaptability of the high entropy design strategy, which, based on the incorporation of multiple cations (typically ≥ 5) in near-equal con- when combined with coherent strain engineering, opens additional prospects centrations to a given cation sub-lattice.[9–11] for fine-tuning properties in oxides. This strategy, which relies on the en- hancement and control of configurational Z. Zhao, D. Wang, V. Wollersen, Z. Xiao, H. Hahn, R. Kruk, A. Sarkar A. K. Jaiswal, D. Fuchs Institute of Nanotechnology Institute for Quantum Materials and Technologies Karlsruhe Institute of Technology Karlsruhe Institute of Technology Eggenstein-Leopoldshafen, 76344 Karlsruhe, Germany Eggenstein-Leopoldshafen, 76344 Karlsruhe, Germany E-mail: di.wang@kit.edu; robert.kruk@kit.edu; abhishek.sarkar@kit.edu D. Wang, V. Wollersen Z. Zhao, H. Hahn, A. Sarkar Karlsruhe Nano Micro Facility (KNMFi) KIT-TUD-Joint Research Laboratory Nanomaterials Karlsruhe Institute of Technology Technical University Darmstadt 76131 Karlsruhe, Germany 64287 Darmstadt, Germany Z. Xiao Key Laboratory of Magnetic Molecules and Magnetic Information Materials of Ministry of Education School of Chemistry and Materials Science The ORCID identification number(s) for the author(s) of this article Shanxi Normal University can be found under https://doi.org/10.1002/advs.202304038 Taiyuan 030031, China G. Pradhan, F. Celegato, P. Tiberto © 2023 The Authors. Advanced Science published by Wiley-VCH GmbH. Advanced Materials and Life Science Divisions This is an open access article under the terms of the Creative Commons Istituto Nazionale di Ricerca Metrologica (INRiM) Attribution License, which permits use, distribution and reproduction in Turin 10135, Italy any medium, provided the original work is properly cited. DOI: 10.1002/advs.202304038 Adv. Sci. 2023, 10, 2304038 2304038 (1 of 10) © 2023 The Authors. Advanced Science published by Wiley-VCH GmbH 21983844, 2023, 27, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/advs.202304038 by Readcube (Labtiva Inc.), Wiley Online Library on [01/12/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.advancedscience.com disorder, offers the possibility to explore the unraveled composi- volves significant incoherent lattice distortions, to engineer the tion space close to the center of multinary oxide phase diagrams magnetic properties of a spinel-HEO. with anticipation of potentially unique properties.[11,13,14] Con- A well-known TM-based spinel-HEO with a bulk composi- sequently, the field of HEOs has rapidly grown to include sev- tion equivalent to (Co0.2 Cr0.2 Fe0.2 Mn0.2 Ni0.2 )3 O4 is used as the eral compositions and crystallographic structures such as tran- model system. It should be noted that unlike other HEOs, sition metal (TM)-based rocksalt-HEOs and spinel-HEOs, rare- where the configurational entropy is known to govern the ran- earth (RE)-based fluorite-HEOs, and RE-TM-based perovskite- dom cationic occupancy, (Co0.2 Cr0.2 Fe0.2 Mn0.2 Ni0.2 )3 O4 (hereafter HEOs.[6,12–14] Likewise, multiple studies showcase the wide va- termed as HEO, irrespective of their cation distribution) shows riety of unique and often improved functionalities exhibited by a strong degree enthalpy-mediated (crystal field stabilization the HEOs compared to the conventional oxide systems such energy) ionic ordering, (Co0.6 Fe0.4 )(Cr0.3 Fe0.1 Mn0.3 Ni0.3 )2 O4 .[29,30] as enhanced electrochemical cyclic stability, high catalytic activ- The HEO composition in its bulk form is known to exhibit a ity, better thermal insulation, and tailorable magneto-electronic Néel type ferrimagnetic ordering with Tc ≈ 420 K.[31] In this properties.[9,15–21] work, epitaxial HEO thin films of varying thicknesses are de- Although there are limited research works on strain en- posited on three different single crystal substrates with (001) gineering in HEOs, the available reports suggest promis- orientation, perovskite-SrTiO3 (STO), rocksalt-MgO, and spinel- ing opportunities for further exploration of structure-property MgAl2 O4 (MAO), that induce different kinds of strain (relaxed relationships.[19,20,22] Conventionally, to achieve a large magni- vs tensile vs compressive) in the HEO. Detailed structural inves- tude of coherent elastic strain in oxides, thin films are epitaxi- tigations have been performed to evaluate the strain states and ally grown on a substrate, that is, in such a way that the crys- accompanying defects at the interfaces. The results of magne- tal structure of the oxide layer is aligned with that of the sub- tometry measurements and investigations of the domain struc- strate. Strain in the oxide film, of magnitudes that far exceeds ture underline the potential of combining coherent strain engi- what can be accommodated in the bulk, stems from the substrate- neering with local chemistry-driven incoherent distortions to ma- film lattice constant differences.[23] Such strain has been ap- nipulate various magnetic characteristics, particularly magnetic plied to control spin orientation in materials, induce interfacial anisotropy, in chemically disordered HEO materials. magnetism in non-magnetic oxides, and modify ferromagnetic and superconducting transition temperatures.[4,23–25] The abil- ity to manipulate spin reorientation through magneto-crystalline 2. Results and Discussion anisotropy in spinel oxides is a technologically relevant attribute that is particularly sensitive to induced strain.[26,27] The magneto- In order to study the effects of different types of straining (tensile, crystalline anisotropy arises from a combination of spin-orbit compressive, or relaxed) and the degree of strain on the structure coupling (SOC) (between cation d-orbital electron spin (S) and or- and magnetic properties of HEO, thin films are simultaneously bital moment (Lorb )) and electrostatic interactions of d-electrons deposited on STO, MgO, and MAO to ensure identical deposition with the crystal field.[28] In the compositionally complex spinel- conditions. The surface morphology, crystallinity, and epitaxy HEOs this spin-lattice coupling is expected to exhibit significant are characterized using atomic force microscopy (AFM), high- local variations, depending on the spin-electronic state of the sur- resolution X-ray diffraction (XRD), X-ray reflectivity (XRR), and rounding ions and the local distortions of the crystal lattice. In reciprocal space mapping (RSM). To gain insight into the struc- this case study, we have combined the principles of coherent epi- ture and the interface at the local scale, high-resolution transmis- taxial straining and a HE-based approach, which inherently in- sion electron microscopy (TEM) is employed. The global and lo- cal magnetic characteristics are probed using a superconducting quantum interference device (SQUID) magnetometer and mag- netic force microscopy (MFM), respectively. M. Szymczak, J. Dabrowa AGH University of Science and Technology Faculty of Materials Science and Ceramics al. Mickiewicza 30, Kraków 30-059, Poland 2.1. Structural Features of HEO Films on MAO, MgO, and STO M. Waqar, X. Q. Pan, A. Sarkar Department of Materials Science and Engineering 2.1.1. Global Structural Features University of California Irvine, CA 92697, USA The AFM micrographs in Figure 1a–c illustrate the surface X. Q. Pan smoothness of the films with Ra (root-mean-square surface Department of Physics and Astronomy University of California roughness) of 0.145, 0.350, and 0.647 nm deposited on MAO, Irvine, CA 92697, USA MgO, and STO, respectively. The analysis of the XRR data shows X. Q. Pan that the film thicknesses are close to 20 nm as shown in Figure Irvine Materials Research Institute S1, Supporting Information. The full range 𝜃-2𝜃 XRD scans for University of California the films deposited on STO, MgO, and MAO are also presented Irvine, CA 92697, USA in Figure S1, Supporting Information. The presence of only (00l) H. Hahn School of Sustainable Chemical, Biological and Materials Engineering reflections underscores the complete epitaxy and the phase purity The University of Oklahoma of the HEO films deposited on all three substrates. The peak posi- Norman, OK 73019, USA tion of the HEO film in the vicinity of (002) reflection of STO and Adv. Sci. 2023, 10, 2304038 2304038 (2 of 10) © 2023 The Authors. Advanced Science published by Wiley-VCH GmbH 21983844, 2023, 27, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/advs.202304038 by Readcube (Labtiva Inc.), Wiley Online Library on [01/12/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.advancedscience.com Figure 1. AFM images of 20 nm HEO thin films on MAO (a), STO (b), and MgO (c) substrates. The root-mean-square surface roughness of the HEO film deposited on MAO, MgO, and STO, is 0.145, 0.350, and 0.647 nm, respectively. Schematic of lattice parameters and mismatch of HEO on various substrates (d) and corresponding observed strain states. e) HRXRD of out-of-plane diffraction of 20 nm HEO on various substrates. f) Reciprocal space map of 20 nm HEO thin film on MgO substrate, the arrow indicates the HEO reflection. g) 𝜔-scans of 20 nm HEO on various substrates. MgO, and (004) reflection of MAO, as shown in Figure 1e, is used the large lattice mismatch with the substrate, with a minor de- for determining their out-of-plane (OOP) lattice parameter. The gree of residual compressive 𝜖 of −0.4%. Further information HEO in its bulk form (as is the case in the homemade ceramic about 𝜖 can be obtained from RSM and rocking curve measure- target) has a lattice parameter of 8.348 Å. It should be noted that ments (𝜔-scans) as shown in Figure 1f,g. The 𝜔-scan of HEO on for comparison half of the lattice parameter of HEO (4.174 Å) MgO exhibits a narrow full width of half maxima (FWHM) of and spinel-MAO (4.040 Å) is used, which is equivalent to the lat- 0.078° (Figure 1g), which along with the clear thickness oscil- tice parameter of STO (3.905 Å) and MgO (4.212 Å) (Figure 1d). lations observed in the XRD pattern (Figure 1e), highlights the The corresponding lattice mismatch between the bulk HEO and superior epitaxial quality of the film. RSM for HEO on MgO (in the substrates is +0.97%, −3.25%, and −6.8% for MgO, MAO, Figure 1f) shows that the sample and the substrate have identi- and STO, respectively, as shown in Figure 1d. Consequently, dif- cal qx values, thus, further confirming the highly coherent tensile ferent types and degrees of epitaxial strain (or tetragonality) are strain in the sample. In contrast, the broad FWHM = 0.47° in the induced in the HEO films when deposited on MgO, MAO, and case of HEO on STO affirms the relaxed nature of the film. The STO. The film deposited on MgO exhibits an elongation of the in- 𝜔-scans of HEO on MAO exhibit two components, one with a plane (IP) lattice parameter (a) and corresponding compression broad FWHM of ≈0.3° and another one with a narrow FWHM of the OOP parameter (c). Thus, HEO film deposited on MgO is of ≈0.077° (Figure 1g). The narrow one indicates the superior tensile strained with the actual degree of strain (𝜖) of +0.97%, fea- epitaxial quality of the film with negligible mosaicity, while the turing perfect cube-one-cube growth as indicated by the RSM and broader one shows a significant spread of mosaicity in certain TEM results, which are discussed later in detail. 𝜖 is defined as the regions of the epitaxial film. difference between the IP lattice parameter of the HEO thin film In order to gain more insight into the evolution of mosaic- and the lattice parameter of the bulk HEO. HEO deposited on ity and imperfections in the crystal, compressively strained HEO the MAO exhibits 𝜖 = −3.19%, indicating compressive straining. films of varying thicknesses (5, 10, 20, and 55 nm) were deposited The film deposited on STO is almost completely relaxed, given on MAO. Likewise, for comparison, tensile strained films with Adv. Sci. 2023, 10, 2304038 2304038 (3 of 10) © 2023 The Authors. Advanced Science published by Wiley-VCH GmbH 21983844, 2023, 27, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/advs.202304038 by Readcube (Labtiva Inc.), Wiley Online Library on [01/12/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.advancedscience.com Figure 2. Thickness dependence of HEO thin films under tensile and compressive strain. 𝜔-scans of 55, 20, 10, and 5 nm HEO on MgO (a) and MAO (b). RSM of 10 nm HEO on MAO. Figure 3. TEM analysis of 20 nm HEO on MAO substrate. High-resolution HAADF-STEM image of the 20 nm SHEO thin film on MAO (a), the red circle marks the spinel phase and the blue circle marks the rocksalt phase. A complete coherency between the spinel and rocksalt phases is evident. The right part of the panel (a) displays the fast Fourier transforms (FFTs) obtained from the marked regions in (a). HAADF-STEM and corresponding elemental distribution maps (b) confirm the homogeneous distribution of the elements. similar thicknesses were deposited on MgO. The XRD data, pre- fully strained (see Section 2.1.2), larger mosaic components re- sented in Figure S2, Supporting Information, indicate that all the sult in a significantly weaker signal in the RSM scan (Figure S3, HEO films on both MgO and MAO are epitaxial. The 𝜔-scans, Supporting Information). Likewise, the compressively strained indicative of the crystal quality, are shown in Figure 2. In the film with the higher thickness (55 nm) shows an even increased case of MgO (Figure 2a), irrespective of the thickness, all the amount of the broad highly mosaic FWHM component, which films showed high epitaxial quality with FWHM <0.08°. How- indicates the epitaxial quality of HEO deposited on MAO lowers ever, a thickness-dependent variation in the FWHM and (more with increasing thickness. precisely) amount of the mosaic components, can be observed in the compressive strained MAO films. The films with the lower thickness (5 and 10 nm) have only a single component in 𝜔-scans 2.1.2. Local Structural Features and Dual Phase Coexistence with FWHM <0.08° (Figure 2b). This indicates that with decreas- ing thickness the compressive strained films on MAO achieve su- TEM along with energy dispersive spectroscopy (EDS) is used to perior epitaxial quality with less mosaicity compared to the afore- gain deeper insight into the local structural feature, especially at mentioned 20 nm films. The superior epitaxial quality of the thin- the substrate-film interfaces, of the 20 nm relaxed, compressive, ner film is also evident from RSM maps, shown in Figure 2c. The and tensile strained HEO that are later used for the magnetic identical in-plane position of the film and substrate indicates that studies (Figures 3 and 4). High-resolution micrographs shown in the film is completely strained. Although the 20 nm film is also Figure S4, Supporting Information, (MAO), and Figure 4a (MgO) Adv. Sci. 2023, 10, 2304038 2304038 (4 of 10) © 2023 The Authors. Advanced Science published by Wiley-VCH GmbH 21983844, 2023, 27, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/advs.202304038 by Readcube (Labtiva Inc.), Wiley Online Library on [01/12/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.advancedscience.com Figure 4. TEM analysis of 20 nm HEO on MgO substrate. High-resolution HAADF-STEM micrograph (a) indicates the fully epitaxial growth of HEO on MgO. The blue square on (a) along with (c,d) indicates a presence of an interfacial rocksalt phase (≈3 nm thick). Ni and Co enrichment at the interface (rocksalt layer) is evident from the elemental distribution maps (b). In the interior of the film (yellow square in (a)) both spinel-HEO (e,f) and rocksalt (g,h) could be observed. Elemental distribution maps (b) confirm the homogeneous distribution of the elements at the interior of the film. reveal full epitaxy at the substrate-film interfaces. This result is while the region marked by the blue circle belongs to the rocksalt consistent with the aforementioned global structural results ob- phase. Importantly, Figure 3a reveals that the rocksalt and spinel tained from XRD. TEM micrograph of HEO on STO (Figure S5, phases are completely coherent without any interfacial boundary, Supporting Information) indicates the large lattice mismatch at indicating almost identical lattice spacing. Furthermore, the EDS the film-substrate interface, however, regions of lattice coherency maps (Figure 3b) exhibit no observable local scale elemental seg- can still be overserved. In addition to the XRD findings, TEM regation. This implies that the rocksalt and spinel-HEO phases analysis of several areas reveals that two distinct, fully coherent are of very close compositions. Evidence for similar dual-phase HEO phases coexist in all the HEO films: spinel and rocksalt. coexistence, without chemical segregation, is also observed in the First, we examine the coexistence of the dual phases in HEO case of relaxed films on STO (Figures S6 and S7, Supporting In- films deposited on MAO. Their presence is quite prominent in formation). the Figure 3a micrograph, where two neighboring regions show The coexistence of completely coherent spinel and rocksalt- different atomic arrangements. Correspondingly, the fast Fourier HEO phases without any distinguishable compositional fluctu- transforms (FFTs) obtained from these regions are different. The ations can as well be observed in the bulk of the tensile thin film region indicated by the red circle corresponds to the spinel phase deposited on MgO, that is, ≈3 nm away from the film-substrate showing additional super-structure diffraction spots in the FFT, interface (Figure 4a,b,e–h). However, in addition, a 3 nm thick Adv. Sci. 2023, 10, 2304038 2304038 (5 of 10) © 2023 The Authors. Advanced Science published by Wiley-VCH GmbH 21983844, 2023, 27, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/advs.202304038 by Readcube (Labtiva Inc.), Wiley Online Library on [01/12/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.advancedscience.com rocksalt phase enriched with Ni and Co can be observed at the main an open question for future investigation. In any case, it film-substrate interface of MgO (Figure 4a,c,d). This (Ni,Co)O- seems like the spinel of (Co0.2 Cr0.2 Fe0.2 Mn0.2 Ni0.2 )3 O4 composi- rocksalt interfacial phase grows fully epitaxial on MgO, and HEO tion is an interesting system that in bulk shows a strong preferen- (spinel + rocksalt) further grows epitaxial on this interfacial rock- tial occupation[29,30] while in thin films displays a lattice-coherent salt phase. The formation of this additional interfacial rocksalt phase dissociation. Hence, exploring this system in detail, es- phase in the case of MgO, which is absent in films on MAO and pecially the origin and mechanism related to the rocksalt-HEO STO, is plausible because of the rocksalt structure of MgO. How- phase formation in (Co0.2 Cr0.2 Fe0.2 Mn0.2 Ni0.2 )3 O4 epitaxial thin ever, the reason for the formation of HEO phases, that is, rock- films, can provide valuable insights into the thermodynamics of salt + spinel, in all the thin films, irrespective of the substrate, HEOs as a whole. remains an open question. This result is rather surprising con- sidering that such dissociation of the spinel-HEO phase has not been reported in bulk.[14,29] However, a very recent study reports 2.2. Magnetic Properties and Domain Structure of 20 nm Films a similar dual-phase existence in spinel-HEO thin film grown on MAO.[32] Hence, when deposited as an epitaxial thin film on any 2.2.1. Bulk Magnetic Properties substrate (MgO, MAO, or STO), the formation or dissociation of the spinel-HEO phase into rocksalt-HEO and spinel-HEO with The temperature-dependent magnetization measurement (M–T) close lattice constants and chemical compositions appears to be curves are shown in Figure S10, Supporting Information, which a universal phenomenon that is not otherwise observed in the indicate that the magnetic transition temperature is well-above bulk. room temperature as is the case in the bulk HEO. It should be It should be noted that the exact ratio of the rocksalt-HEO noted that the magnetic properties are primarily determined by to spinel-HEO phase that is observed in all the films (on MgO, the dominant ferrimagnetic spinel-HEO phase, with insignifi- MAO, or STO) is not easy to quantify. Nevertheless, we used O-K cant contributions from the rocksalt phase, which is anticipated edge electron energy loss spectra (EELS) analysis that provides an to be antiferromagnetic.[32] The magnetic field-dependent mag- estimation of the spinel-HEO and rocksalt-HEO phases. The dif- netization measurement (M–H) curves for all the samples mea- ferentiation is done based on the hybridization of O 2p and tran- sured at 300 and 5 K are shown in Figure 5. A distinct change sition metal 3d electrons, which is distinct for spinel and rocksalt. in the magnetic anisotropy as a function of straining can be ob- The details of the O-K edge analysis are presented in Figure S8, served by comparing the M–H curves obtained when the mag- Supporting Information. The data unambiguously show that the netic field is applied parallel (IP) and perpendicular (OOP) to the spinel-HEO phase is the main phase in all cases. plane of the thin films. There are two prominent contributions The rocksalt (Fm-3m) and spinel (Fd-3m) structures are closely to the magnetic anisotropy in the case of thin films: one is the related, as they both share the same face-centered cubic (fcc) oxy- shape anisotropy that results from the OOP demagnetizing field, gen frame, where only the sites occupied by the cations differ. and the other one is the magnetocrystalline anisotropy (MA). In the rocksalt, all octahedral sites are evenly occupied by the In all the cases (Figure 5), the IP M–H curves show a rather cations. Conversely, in spinels, only a part of the octahedral sites soft magnetic behavior, that is, saturation could be reached at is occupied while the rest are vacant, and instead, one-eighth of low external magnetic fields (Hext < 7 kOe) along with low co- tetrahedral sites are occupied. Spinel to rocksalt transitions can ercivity, Hc (Table 1 and Figure S11, Supporting Information). happen due to a variety of factors, of which the primary one is In contrast, the slight disparities between the IP and OOP M–H the lowering of the effective cationic oxidation state. If ideal sto- curves (in Figure 5b,e) of the relaxed thin films on STO resem- ichiometric conditions are met, cations in spinel oxides have an ble a scenario similar to that of the bulk HEO. This indicates that effective charge of 2.66+ compared to 2+ in rock salt oxides. Con- the shape factor, although unavoidable, does not dominate the sequently, the introduction of hole doping in spinels tends to fa- anisotropy landscape in these 20 nm epitaxial HEO thin films. vor the rock salt structure. For instance, Li doping, either chemi- The ratio of the magnetic remanence to saturation magnetiza- cally or electrochemically, results in the transformation of spinel tion (MR /MS ) is shown in Table 1. The results, especially at 5 oxides to rocksalt structures.[33,34] The thin-film deposition was K, indicating that there is a coexistence of minor components carried out at an optimal O2 flow, which is similar to or higher which has dominant perpendicular magnetic anisotropy (PMA) than what has been used for depositing conventional spinel ox- alongside the major component which is close to the bulk. This ides with similar constituents.[20] Furthermore, post-annealing observation is schematically shown in Figure 6. Despite the sig- treatment of the deposited thin films in an air atmosphere (at nificant relaxation of the HEO film on STO, some degree of cube- 700 °C for 6 h) was performed to see if the rocksalt-HEO phase on-cube growth and straining are expected at the interface, which disappears. TEM results confirm that the rocksalt-HEO phase is primarily compressive in nature (𝜖 = −0.4%), as previously dis- is present even after annealing (Figure S9, Supporting Informa- cussed. Thus, the PMA component observed in the hysteresis tion). These observations, to a large extent, rule out the fact that loop is likely to correspond to the minor strained region at the the rocksalt-HEO phase stems from the synthesis conditions. interface. Hence, the presence of the rocksalt-HEO phase can plausibly be a In the case of the tensile strained films on MgO substrate, the result of the substrate-induced straining, that somehow increases OOP M–H curves at 5 and 300 K (Figure 4a,d) are significantly the preference for forming a rocksalt structure (maybe as a strain different from the IP, showing linear behavior up to Hext = 70 accommodation mechanism, which by the way is also relevant kOe. The observation differs from the aforementioned relaxed for STO) than a single spinel-HEO phase. The factors that pro- scenario (HEO on STO), where the OOP M–H curves could be mote and stabilize the formation of the dual phase certainly re- saturated at low fields, Hext < 7 kOe. Given that the thickness (20 Adv. Sci. 2023, 10, 2304038 2304038 (6 of 10) © 2023 The Authors. Advanced Science published by Wiley-VCH GmbH 21983844, 2023, 27, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/advs.202304038 by Readcube (Labtiva Inc.), Wiley Online Library on [01/12/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.advancedscience.com Figure 5. Global and local scale magnetic characterization of 20 nm HEO on various substrates. IP and OOP M–H loops at 5 K for HEO on MgO (a), STO (b), and MAO (c). 300 K M–H (d), (e), (f), and corresponding in-plane MFM micrograph (g), (h), (j) for HEO on MgO, STO, and MAO substrates, respectively. The blue circles in (i) indicate the maze domain observed in the compressive strained films. Table 1. The coercivity (HC ) values (in kOe), saturated magnetization (MS ) the MA, making the OOP direction a hard axis compared to the values (in 𝜇 B /f.u.), remnant magnetization (MR ) values (in 𝜇 B /f.u.), and in-plane one. The linear shape of the OOP curve with negligi- the ratio of MS /MR for thin film HEO and bulk (at 5 K). ble remanence confirms that initially there is no component of magnetization along the OOP direction. Hence, almost the max- Strain HC , OOP MS , IP MR , OOP MS /MR Ref. imum of the available Hext is used to rotate the spins from the Tensile (MgO) 2.5 2.54 0.18 100:7.3 This work IP to the OOP direction (Figure 6), but as soon as the field is re- Relaxed (STO) 1.4 2.53 0.71 100:27 This work moved, they rotate back to the easy axis, which in the case of the Compressive (MAO) 12.5 2.87 1.38 100:50 This work tensile straining lays in-plane. Bulk 0.23 1.73 0.17 100:10 [29] Figure 5c,f presents the IP and OOP M–H curves for the com- pressive strained HEO deposited on the MAO substrate. As men- tioned earlier, the IP M–H curves for MAO are similar to that of the relaxed and tensile strained films on STO and MgO, respec- nm) of all the samples (on STO, MgO, and MAO) are similar, the tively. However, the OOP M–H curves are remarkably different. anisotropy originating from the shape factor is practically identi- The M–H curves collected at 5 K (Figure 5e) clearly show two cal. Hence, the distinct difference in the OOP magnetic behavior magnetic components: one non-saturating linear one, which is observed in the case of MgO is expected to originate solely from the minor component, and a saturated almost square loop, which the strain effects. In-plane stretching of the HEO film (and the is the predominant component. The square-like M–H compo- accompanying compression of the OOP axis) appears to affect nent with a high Hc and MR /MS indicates that the compressive Adv. Sci. 2023, 10, 2304038 2304038 (7 of 10) © 2023 The Authors. Advanced Science published by Wiley-VCH GmbH 21983844, 2023, 27, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/advs.202304038 by Readcube (Labtiva Inc.), Wiley Online Library on [01/12/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.advancedscience.com Figure 6. Schematic of strain-driven bidirectional spin reorientation in epitaxial HEO thin films, indicating the hardness of the out-of-plane (OOP) upon tensile straining (MgO) and easiness of the OOP upon compressive strain (MAO). The relaxed film on STO behaves similar to the bulk, that is, isotropic, however, the shape anisotropy (SA) makes the in-plane (xy) axis slightly easier compared to the OOP. strained HEO has an easy axis in the OOP direction, similar to shaped IP and OOP hysteresis loops indicating the magnetiza- known systems exhibiting PMA. The room temperature M–H tion switching by coherent rotation (see Figure 5e, blue curve). curve (Figure 5c) sheds more light on the magnetic behavior of In contrast, the MFM image of the compressively strained film HEO on MAO. There are noticeable three components in the of HEO on MAO is quite different. A coexistence of two magnetic magnetization curve. The non-saturating linear one is identical to phases is visible. A typical maze domain structure on both sides that observed at low temperatures. The square-like PMA compo- is observed in Figure 5i and Figure S12, Supporting Informa- nent observed at 5 K gives rise to the remaining two components tion, which generally indicates the presence of an OOP magnetic at room temperature, one being very soft and the other relatively anisotropy. In the middle, a wide band with plain and uniform harder. The hard one, but with a lower Hc than observed at 5 K, contrast is observed similar to the sample deposited on MgO, rep- can be naturally correlated to the PMA component. Increasing resenting a softer magnetic phase. The presence of two magnetic temperature, typically, weakens the magnetization as well as the phases is also confirmed by the OOP hysteresis loop shown in MA. This explains the lower Hc at room temperature (Table 1) Figure 5f where the magnetization reversal is characterized by of the PMA component. On the other hand, faster softening of a two-step process. This further supports the conclusions made MA in some areas of the crystal may indicate magneto-electronic from the bulk magnetization data. inhomogeneity of the HEO spinel. Overall, based on magnetom- etry studies, it can be concluded that different types of strain have opposite effects on spin orientation, schematically shown in 2.3. Discussion Figure 6. The compressively strained film on MAO shows PMA with OOP direction as the easy axis. The relaxed film on STO Based on the experimental results, it appears that the type of although showing a small degree of PMA, has largely isotropic strain imposed on the HEO epitaxial films has a significant ef- character, that is, IP and OOP direction are mostly equivalent fect on the spin orientation. Applying compressive strain to the similar to the bulk HEO. In contrast, for the tensile strained MgO film on the MAO substrate results in the spin orientation be- film, the OOP direction is clearly the hard axis. In the next sec- ing perpendicular to the plane of the film, while the relaxed film tion, we discuss MFM results that offer insights into the correla- on STO is magnetically isotropic. The tensile strain in the HEO tion between local and global magnetic properties. film on the MgO substrate results in the IP direction being the easy axis. To uncover the mechanisms that drive these spin re- orientations, several factors must be considered, including the 2.2.2. Local Magnetic Domain Structure arrangement of the ions across crystallographic sites, as well as their charge and spin states. Unfortunately, because of the highly The MFM images of the tensile, relaxed, and compressively complex chemical nature of HEO, this information cannot be strained samples, acquired at room temperature in the IP mag- readily obtained. Nonetheless, by examining the bulk counterpart netization remanence, are presented in Figure 5g–i. With the of the studied HEO,[29] some meaningful conclusions may be MFM being highly sensitive to the OOP magnetization compo- drawn. In the bulk compounds, the ion distribution, the charge, nent the dependency of the domain structure on the strain is ev- and the spin states were revealed in great detail by extensive X- ident. For the tensile strained HEO (on MgO), the MFM contrast ray and X-ray magnetic circular dichroism (XMCD) spectroscopy is uniform without any OOP magnetization component, suggest- studies.[29] Particularly, the analysis of the XMCD spectra offered ing the presence of large in-plane magnetic domains (Figure 5g). precise information regarding the contribution of the cation’s or- This is consistent with the soft IP behavior observed in the M– bital moments to the overall moments.[29] If one assumes that H curve, Figure 5a,d. The relaxed HEO on STO exhibits non- the ion distribution in the bulk and epitaxial films is similar, one uniform MFM contrasts (Figure 5h) reflecting a higher OOP can conclude that only Co+2 , Co+3 ions occupying the tetrahe- component as compared to the tensile strained sample on MgO. dral and Ni+2 ions occupying octahedral sites have significant or- This is also evident from the M–H curve that represents S- bital moments. If we consider these facts, it can be inferred that Adv. Sci. 2023, 10, 2304038 2304038 (8 of 10) © 2023 The Authors. Advanced Science published by Wiley-VCH GmbH 21983844, 2023, 27, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/advs.202304038 by Readcube (Labtiva Inc.), Wiley Online Library on [01/12/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.advancedscience.com mainly Ni and Co ions would be contributing to the magneto- °C annealing in air to form a TiO2 terminated surface. The commercial crystalline anisotropy. To simplify this, one can assume that their MgO, MAO, and treated STO substrates were all cleaned in isopropanol partially quenched orbital moment, denoted as Lorb , would be and deionized water in an ultrasonic bath each for 10 min to remove the particle and contaminants and dried by nitrogen blow. The thin film was coupled to the spin S of the d-electrons through spin-orbit inter- deposited at 690 °C in 0.03 mbar oxygen pressure as optimal conditions. action, which is proportional to Lorb × S. The next step involves The KrF excimer laser with a wavelength of 248 nm was used for target the magneto-elastic coupling between Lorb and the crystal field, ablation at a repetition rate of 5 Hz. The laser fluence was controlled at which is necessary to induce spin rotation upon straining. In- 1.55 J cm−2 . The thickness of thin films was controlled by the given laser deed, in recent studies,[30] crystal field and magnetoelastic calcu- shots number: 500 shots (5 nm), 1000 shots (10 nm), 2000 shots (20 nm), lations were used to investigate strained spinels containing Co and 6000 shots (55 nm). The target-substrate distance was 25 mm. After deposition, the thin film was cooled to room temperature at 20 °C min−1 . and Ni cations. The results indicate that applying compressive Characterization: Structural properties of the thin films were charac- strain causes the easy axis to align OOP while applying tensile terized by XRR, diffraction, and RSM using a Bruker D8 Davinci diffrac- strain causes it to align IP. Thus, the strong magneto-elastic re- tometer. The surface morphology information of the S-HEO thin film was sponse observed in this study implies that there are sufficient co- scanned in tapping mode in the air by AFM of Bruker Dimension Icon. The operative spin-lattice interactions in HEO at the local level, result- scan size was 2 μm with 512 sample lines/image and a scan rate of 1 Hz. ing in a coherent rotation of the magnetization upon the strain. TEM specimen of the thin film was prepared by a focused ion beam (FIB) (Strata dual beam, FEI Company). A double aberration-corrected 3. Conclusions state-of-the art Themis Z (Thermo Fisher) TEM equipped with a Super-X energy dispersive X-Ray detector and Gatan GIF Continuum 970 HighRes This work demonstrates the use of coherent straining to control + K3 IS camera (operated at 300 kV) were to used examine the specimens. the degree of PMA in epitaxial spinel HEO thin films, with a bulk STEM imaging of annealed sample was performed on JEOL JEM-ARM300F aberration-corrected microscope (operated at 300 kV). TEM and STEM composition of (Co0.2 Cr0.2 Fe0.2 Mn0.2 Ni0.2 )3 O4 . The strain states of micrographs were taken from different regions to confirm the large-scale the epitaxial HEO films, i.e., tensile, compressive, or relaxed, are homogeneity of the samples. tailored using three different epitaxial substrates, MgO, MAO, Magnetic measurements at room temperature were performed on the and STO, respectively. TEM studies reveal a unique coexistence powder samples using a Quantum Design MPMS3 SQUID in direct cur- of two fully coherent HEO phases in the thin films, namely a rent (DC) mode. Magnetization versus temperature measurements were spinel and a rocksalt, with no discernible chemical fluctuations. carried out at 200 Oe, using a zero-field cooled (ZFC)-field cooled (FC) se- This phenomenon, which is not observed in the bulk material quence in the temperature range of 5–380 K. Magnetization versus mag- netic field measurements were carried out at different temperatures be- of the same composition, appears to be a common occurrence tween −7 and +7 T. Same measurements sequences were used for mea- when (Co0.2 Cr0.2 Fe0.2 Mn0.2 Ni0.2 )3 O4 is deposited as an epitaxial suring the bare substrates, which were used for the background subtrac- thin film. This observation implies that the free energy landscape tion. of the HEOs has multiple energy minima that can be accessed The MFM images were obtained by MultiModeTM SPM (Digital Instru- by applying strain. The strain-driven control of magnetic proper- ments Veeco Metrology Group, Version 4.31ce). MESP magnetic tips hav- ties in HEO is manifested by the marked changes in magnetic ing a resonant frequency of 75 kHz with Co/Cr reflective coating were used for acquiring the OOP magnetic signals. anisotropy, as observed through magnetometry and spatially re- solved MFM microscopy. The film grown on an STO substrate in a relaxed state displays almost isotropic magnetization behavior similar to bulk with no significant difference between in-plane Supporting Information and OOP magnetization reversal. Under tensile strain, the hard- Supporting Information is available from the Wiley Online Library or from ness of the OOP axis significantly increases, and only the coher- the author. ent spin-rotation component from in-plane to OOP is detected. Conversely, the HEO subjected to compressive strain shows an easy magnetization axis along the OOP direction, presenting a square-like magnetization-hysteresis (MH) loop and a maze do- Acknowledgements main structure that are characteristics of systems with PMA. To Z.Z., R.K., and H.H. acknowledge the support from the European summarize, the magneto-crystalline anisotropy of spinel-HEO Union’s Horizon 2020 research and innovation program under the Marie can be controlled via induced strain, despite their extreme vari- Skłodowska-Curie grant agreement No. 861145 (BeMAGIC, MSCA). X.P., ations in local bonding and magneto-electronic heterogeneity. M.W., H.H. and A.S. acknowledge the financial support from the Na- tional Science Foundation Materials Research Science and Engineering Thus, the present study demonstrates the numerous research Center program through the UC Irvine Center for Complex and Active possibilities for altering properties in oxide systems by using the Materials (DMR-2011967). A.K.J. acknowledges financial support from high entropy method in conjunction with strain tuning of epitax- the European Union’s Framework Program for Research and Innovation, ial thin films. Horizon 2020, under the Marie Skłodowska-Curie Grant Agreement No. 847471 (QUSTEC). A.S. and H.H. acknowledge financial support from the Deutsche Forschungsgemeinschaft (DFG) project HA 1344/43-2. Z.Z. 4. Experimental Section is grateful for the help of sample demagnetization treatment by Barrera Gabriele in Italian National Institute of Metrology Research for MFM mea- Materials Synthesis: HEO thin films were deposited on MgO surement. M.W., X.P. and A.S. acknowledge the use of facilities and instru- (001), STO (001), and MAO (001) substrates by laser pulsed de- mentation at the UC Irvine Materials Research Institute (IMRI) supported position technique (PLD). The material for the ceramic target of in part by the National Science Foundation Materials Research Science (Co0.2 Cr0.2 Fe0.2 Mn0.2 Ni0.2 )3 O4 was synthesized via the modified Pechini and Engineering Center program through the UC Irvine Center for Com- sol–gel method, as described in the Supporting Information. Prior to de- plex and Active Materials (DMR-2011967). position, the STO (001) substrate was treated with acid followed by 980 Open access funding enabled and organized by Projekt DEAL. Adv. Sci. 2023, 10, 2304038 2304038 (9 of 10) © 2023 The Authors. Advanced Science published by Wiley-VCH GmbH 21983844, 2023, 27, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/advs.202304038 by Readcube (Labtiva Inc.), Wiley Online Library on [01/12/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.advancedscience.com Conflict of Interest [13] A. Sarkar, R. Djenadic, D. Wang, C. Hein, R. Kautenburger, O. Clemens, H. Hahn, J. Eur. Ceram. Soc. 2018, 38, 2318. The authors declare no conflict of interest. [14] J. Dabrowa, ˛ M. Stygar, A. Mikuła, A. Knapik, K. Mroczka, W. Tejchman, M. Danielewski, M. Martin, Mater. Lett. 2018, 216, 32. [15] B. Yang, Y. Zhang, H. Pan, W. Si, Q. Zhang, Z. Shen, Y. Yu, S. Lan, F. Author Contributions Meng, Y. Liu, H. Huang, J. He, L. Gu, S. Zhang, L.-Q. Chen, J. Zhu, C.-W. Nan, Y.-H. Lin, Nat. Mater. 2022, 21, 1074. 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