Magnetic Oxide Thin Films Research Articles

Robust Ferrimagnetism and Switchable Magnetic Anisotropy in High‐Entropy Ferrite Film

AbstractFerrimagnetic insulator materials are the enabling technology for the development of next‐generation magnetic devices with low power consumption, high operation speed, and high miniaturization capability. To achieve a high‐density memory device, a combined realization of robust saturation magnetization (Ms), controllable magnetic anisotropy, and high resistivity (ρ) are highly demanded. Despite significant efforts that have been made recently, simultaneously achieving significant enhancements in these properties in a soft magnetic insulator material still remains a great challenge, severely limiting their practical application. Herein, a high‐entropy strategy in an ultra‐thin spinel ferrite (CrMnFeCoNi)3O4 film is reported that exhibits concurrently a superior saturation magnetization (MS = 1198 emu cm−3), low coercivity (HC = 90 Oe), and excellent resistivity (ρ = 1233 Ω cm), as well as switchable magnetic anisotropy. The comprehensive lattice probing and microstructure analysis studies reveal that such desirable ferromagnetic properties originate from the high‐quality structurally ordered but compositionally disordered single‐crystal epitaxial structure. The switchable magnetic anisotropy demonstrated in the high‐entropy ferrite film can be attributed to the new antiferromagnetic rock‐salt phase. This work unveils the critical benefits of the high‐entropy strategy for magnetic oxide thin films, which opens up new opportunities for the development of high‐performance magnetic materials.

Epitaxial lift-off of flexible single-crystal magnetite thin films with tunable magnetic performances by mechanical deformation

Integrating magnetic oxide thin films without restriction of substrates is crucial for their applications in electronic and spintronic devices. Here the water-soluble Sr3Al2O6 (SAO) layer is employed as the sacrificial layer to transfer the millimeter-size single-crystal Fe3O4 films from SrTiO3 (STO) to silicon and flexible PET substrates. Structural characterizations indicate that the transferred films have the same epitaxial quality with the as-grown films. The hysteresis loops measurements of flexible Fe3O4/PET samples show a deceasing magnetic anisotropy under both of convex and concave deformations. Such tunable magnetic anisotropy is confirmed by fitting the angular dependence of ferromagnetic resonance (FMR) fields. These results provide a damage-free way to fabricate single-crystal spinel ferrites films on the selected substrates for flexible spintronic devices.

Switching Magnetic Anisotropy of SrRuO3 by Capping-Layer-Induced Octahedral Distortion

Materials with large perpendicular magnetic anisotropy (PMA) are candidates for spintronic devices, such as magnetic random-access memory, etc., due to their stable magnetic reference states. Because of shape anisotropy, the magnetic easy axis of oxide thin films favors in-plane orientation. In this Paper, we demonstrate a convenient means to control the magnetic anisotropy of ${\mathrm{Sr}\mathrm{Ru}\mathrm{O}}_{3}$ (SRO) ultrathin layers from in-plane to out-of-plane by capping with nonmagnetic materials. Tuning the anisotropy is achieved by imposition of symmetry mismatch at the interface-induced structural transition of SRO with suppressed octahedral tilt. These results suggest a potential direction for engineering magnetic oxide thin films with flexible tunable PMA using capping-layer-induced dissimilar symmetry.

Exchange bias in La0.7Sr0.3CrO3/La0.7Sr0.3MnO3/La0.7Sr0.3CrO3 heterostructures

In the recent past, heterostructures of magnetic oxide thin films have attracted a great deal of research excitement due to very interesting physical properties such as antiferromagnetic interlayer coupling, tunable exchange-bias, interfacial driven magnetic properties and high mobility electron gas across the interfaces. In this work, we report on the comprehensive magnetic properties observed from the heterostructures of (2 unit cells) La0.7Sr0.3CrO3/(8 unit cells) La0.7Sr0.3MnO3/(2 unit cells) La0.7Sr0.3CrO3, which are epitaxially deposited on SrTiO3 substrate by plasma-assisted oxide molecular beam epitaxy. Using SQUID magnetometer, the magnetic properties are studied when the magnetic field was applied both in plane and out of plane. The Curie temperature of this structure is found to be at 290 K. Most significantly, at 2 K, we observed a complete up/down shift (along magnetization axis) of hysteresis loop when the sample was cooled under a magnetic field of ± 5000 Oe in the in-plane configuration. We believe that the strong antiferromagnetic (super) exchange coupling of Mn-Cr across the two interfaces is responsible for the observed exchange bias. We will present and discuss our in-detailed experimental findings collected on this heterostructure as a function of temperature and magnetic field.

Ultrathin interfacial layer with suppressed room temperature magnetization in magnesium aluminum ferrite thin films

Low-damping magnetic oxide thin films with small thicknesses are essential for efficient insulator spintronic devices, particularly those driven by spin torque effects. Here, we investigate the depth-resolved compositional and magnetic properties of epitaxial spinel MgAl0.5Fe1.5O4 (MAFO), which has recently been reported as a promising low-damping insulator. We find that ≈11 nm films exhibit optimal Gilbert damping, with a typical damping parameter of 0.001. While defects due to strain relaxation in the bulk of the film contribute to increased damping for large film thickness, the damping increase in thinner films is attributed to the presence of a chemically disordered magnetic dead layer at the film/substrate interface. This interfacial dead layer arises from an Fe-deficient MAFO layer. Notably, this layer is only about one-sixth the thickness of that found at the interface between yttrium iron garnet films and gadolinium gallium garnet substrates, making MAFO an ideal thin-film insulator for spin-torque applications.

Integration of Both Invariable and Tunable Microwave Magnetisms in a Single Flexible La0.67Sr0.33MnO3 Thin Film.

High-quality flexible magnetic oxide thin films have promoted a wide range of potential applications in spintronic devices due to their unique physical properties. To obtain the optimized microwave magnetism for future all-oxide-based spintronic applications, high-quality oxide materials with excellent epitaxial quality as well as specific bending properties related to ferromagnetic resonance are high in demand. Here, (001)-oriented La0.67Sr0.33MnO3 epitaxial thin films with different thicknesses have been grown and subsequently transferred onto flexible poly(dimethylsiloxane) substrates. The microwave magnetisms of these film samples have been investigated under various bending states. Under bending, the ferromagnetic resonance lineshape of the film gradually transits from a single mode to a superposition of multimodes, possibly because of the uneven distribution of magnetization in the bending film at X-band. This phenomenon is more apparent when the direction of the applied magnetic field goes close to the out-of-plane of the film. Hence, an integration of invariable and continuous tuning of ferromagnetic resonance field under various mechanical bending can be achieved in one same sample by just tuning the direction of the applied magnetic field, which reveals that the flexible La0.67Sr0.33MnO3 thin films have huge potential in the applications in future flexible multifunctional devices.

Temperature dependence of Raman spectra and first principles study of NiMn2O4 magnetic spinel oxide thin films. Application in efficient photocatalytic removal of RhB and MB dyes.

Magnetic spinel oxide NiMn2O4 thin films were grown on glass substrates using the spray pyrolysis deposition technique at 350 °C. First, structural optimization, electronic structure and vibrational properties were obtained for NiMn2O4 by density functional theory (DFT) with an all electron basis set and the hybrid functional B3LYP implemented in the CRYSTAL14 code. Our DFT results show good agreement with previous experimental and theoretical data. Second, Raman spectra of as-prepared thin films have been measured over the temperature range from 100 K to 620 K. We report on the temperature dependence of their dominant first-order scattering F2g and A1g phonons modes. The phonon dynamic behavior is well described by a model taking into account explicit lattice anharmonicity associated to both cubic and quartic phonon decay processes and thermal expansion related quasiharmonicity. Finally, the photocatalytic activities of the obtained thin films were evaluated through the photocatalytic degradation of Rhodamine B (RhB) and methylene blue (MB) solutions under solar light irradiation as a function of time. All the results indicate that NiMn2O4 thin film can be used as a potential solar-light-driven photocatalyst.

Effect of in-situ oxidation on structure and ferromagnetic properties of Fe3Al and FeAl2O4 thin films prepared by electrodeposition

Magnetic oxide thin films are potentially important in the coming era in data storage and spintronic devices. But the effects of oxygen positional parameter on different properties of oxide thin films are not completely understood and scarcely reported. Using DC electro-deposition, FeAl2O4 thin films were deposited on copper substrates by in-situ oxidation of intermetallic Fe3Al at 2 V. Deposition time for each intermetallic film was a 30 min while oxidation time was varied from 0 min to 30 min with the interval of 5 min. All the thin films were annealed for 1 h at 300 °C in the 500 Oe magnetic field. XRD patterns confirmed the formation of spinel structure for the FeAl2O4 thin films with no preferred orientation. Variations in lattice parameters and crystallite size might be due to surface activation energy and in cooperation of oxygen atoms at the tetrahedral sites for charge compensation. Cation distribution and lowest stacking fault probability confirmed the formation of defect free pure phase of FeAl2O4 for the thin film with the 30 min oxidation. Tolerance factor found to very close to unity representing the formation of defect free thin films. Magnetic results showed that all the thin films carried the ferromagnetic behavior and FeAl2O4 thin film with the 30 min oxidation carried maximum saturation magnetization.

Magnetic oxide thin solid films deposited at room temperature under ambient pressure using an electro-spray method

The deposition and characterization of magnetic metal-oxide, Eu1−xSrxCoO3 (x=0.2, 0.5) thin films using an electrostatic spray method are presented. The nano-sized particles were electro-sprayed from a colloidal suspension onto Pt/TiO2/SiO2/Si substrates at room temperature under ambient pressure. X-ray diffraction patterns showed a perovskite tetragonal structure with an I4/mmm space group. Optical and scanning electron microscopy of the thin films revealed their crack-free and uniform nature. The temperature-dependent magnetization showed a ferromagnetic-paramagnetic transition above 350K. Magnetic moments hysteresis loops demonstrate that the thin films exhibited a hard ferromagnetic state at 10K and a soft ferromagnetic one at room temperature. The Hall measurement revealed metallic behavior at room temperature, confirming the transition from a metallic state to an insulating one at T>300K. The correlation is explained based on the double exchange interaction and spin state transitions at both low and high temperatures.

Preparation and structural-optical characterization of dip-coated nanostructured Co-doped ZnO dilute magnetic oxide thin films

We demonstrate the feasibility to prepare Co-doped ZnO thin films (Zn1−xCoxO, x = 0; 0.01; 0.03 and 0.05) via the dip-coating technique, a very simple and a low-cost process. We focus the structural and the optical analyses in the context of the DMOs.

High Quality Magnetic Oxide Thin Films Prepared via Aqueous Solution Processing

Recent progress in multiferroic materials and spintronic devices has renewed interest in metal oxide ferromagnetic and ferrimagnetic materials. Here, we report the preparation of thin films of nanocrystalline ferrimagnetic CoFe2O4 (CFO) using an environmentally benign aqueous solution processing route. The evolution of the structural, optical, and magnetic properties as a function of postdeposition annealing temperature is reported. For the highest annealing temperature (800 °C), the remanence, coercivity, and resistivity values are comparable with those of films fabricated by epitaxial growth methods and exceed the quality of CFO nanocrystals prepared by other wet-chemical syntheses. In addition to the ability to make high quality ferrite films, the aqueous solution processing strategy offers great flexibility for tuning film properties by incorporating or substituting additional transition metal ions.

Effect of Bloch wave electron propagation and momentum-resolved signal detection on the quantitative and site-specific electron magnetic chiral dichroism of magnetic spinel oxide thin films

Electron magnetic chiral dichroism (EMCD) in a transmission electron microscope is an element-specific magnetic characterization technique and is extremely powerful for understanding magnetism of materials at the nanoscale. However, quantitative EMCD remains a challenge. In the present paper, we have highlighted and overcome major difficulties associated with the technique. For example, the experimentally observed low dichroic signal and imbalance between the ${L}_{3}$ and ${L}_{2}$ edge have been explained based on the oscillatory nature of electron propagation through the crystal thickness and specific momentum resolved signal detection, respectively. With this advancement in understanding, site-specific quantitative EMCD has been accomplished in epitaxial thin films of two important ferrimagnetic spinel oxides, ${\mathrm{NiFe}}_{2}{\mathrm{O}}_{4}$ (NFO) and ${\mathrm{CoFe}}_{2}{\mathrm{O}}_{4}$ (CFO), with varying degree of cation mixing and $A$ site cation defects. A simple model based on phenomenological absorption has been developed for different site-specific signal contributions for the inverse spinel structure. The experimental moment values for NFO and CFO obtained using EMCD are in good agreement with first principle based theoretical calculations and the results strengthen the promise of utilizing EMCD as a routine nanoscale magnetic characterization technique.

On the Nature of the Magnetism-Promoting States in Dilute Magnetic Semiconductor and Oxide Thin Films

Ferromagnetic (FM) ordering is observed in intermediate and wide gap dilute magnetic semiconductors as well as in oxides. While the interpretation of the experimental results is often clouded by the existence of non-homogeneous and non ideal nanostructures related to the fact that impurity concentration tends to far exceed the thermodynamic solubility limit, a general physical picture as to the physical origin of the FM interactions has emerged. We discuss the physical mechanism of ferromagnetism mediated by the carriers. We show that what stabilizes the FM spin arrangement is the energy-lowering due to interaction between partially occupied states in the band gap, localized on different transition atoms. These partially occupied states are hybrids between the d impurity band states and host vacancy orbitals, never host-like states as imagined in model Hamiltonian approaches. The theory uses both the model and first principle approach and can be applied to various types of systems such as dilute magnetic semiconductors [(Ga, Mn)As, (Ga, Mn)N, etc.] and oxides [(Ti, Co)O2, (Zn, Mn)O, etc.] as well as nanodevices prepared of these materials.

The Kondo effect and carrier transport in amorphous Cr-doped In2O3 thin films

Understanding the interaction between spin of the charge carriers and local magnetic moments in diluted magnetic oxide is an important issue for applications in spintronic devices. This study examines amorphous Cr-doped In2O3 diluted magnetic oxide thin films for the existence of the Kondo effect and a general s-d scattering effect on the magneto transport, as well as for the well known 3D weak localization effect that explains the low temperature transport behavior of transparent conducting oxides. The carrier transport behavior at low temperature can be accurately described and well fit by a combination of these effects. At temperatures lower than the minimum resistivity temperature, the Kondo effect dominates the magnetoresistance effect and is responsible for the enhancement of resistivity.

Interplay between magnetism and chemical structure at spinel-spinel interfaces

By utilizing a graded wedge sample geometry in combination with surface sensitive soft x-ray techniques, we explain the enhanced magnetic properties observed at the interface between two dissimilar magnetic spinel oxide thin films in terms of the chemical and magnetic character of the constituent cations. Through x-ray absorption spectroscopy and magnetic circular dichroism studies, we find that the interfacial cations exhibit chemical valences and site-occupancies which differ remarkably from the bulk of either film. This results in enhanced magnetic properties localized to the interface region. While this phenomena likely arises from cation interdiffusion of 1–2 nm near the interface due to the open spinel crystal structure, this dramatic change in the magnetic properties localized to a thin interface region may provide a route to obtaining isolated interfacial properties in other spinel-structured heterostructures.

Transparent magnetic oxide thin films of Fe 3O 4 on glass

Transparent magnetic oxide (TMO) thin films of magnetite (Fe 3O 4) were grown on top of a (Zn,Al)O transparent conducting oxide on a glass substrate. The polycrystalline magnetite thin films show the typical Raman spectrum of Fe 3O 4, a sharp Verwey transition at 120 K and hysteretic behavior. The highest achieved average transmittance in the visible range (400–800 nm) for the entire multilayer was slightly below 80%. TMOs permit the inclusion of magnetic functionalities into transparent electronics. Our results show that TMOs can be successfully used to add magnetic functionalities to transparent conducting oxides.

Synthesis and characterization of recombinant factor VIIa‐conjugated magnetic iron oxide nanoparticles for hemophilia treatment

Maghemite (gamma-Fe(2)O(3)) nanoparticles of 15.0 +/- 2.1 nm in diameter were prepared by nucleation, followed by controlled growth of magnetic iron oxide thin films onto gelatin nuclei. Functionalization of these magnetic nanoparticles with activated double bonds was accomplished by interacting divinyl sulfone with the gelatin coating of the gamma-Fe(2)O(3) nanoparticles. The activated double bonds were then used for covalent binding, via Michael addition reaction, of recombinant factor VIIa and human serum albumin to the surface of these nanoparticles. Recombinant factor VIIa was also physically bound to the magnetic nanoparticles by interacting this factor with the human serum albumin conjugated gamma-Fe(2)O(3) nanoparticles. The influence of factor VIIa concentration on the immobilization yield has been elucidated. Leakage of the bound factor VIIa into PBS containing 4% albumin was insignificant. The coagulant activity of the physically adsorbed recombinant factor VIIa was similar to that of the free one and was significantly better than that of the covalently bound. The blood half-life of free factor VIIa is short, about 2-3 h, because of digestion by proteolytic enzymes and inhibitory effects. Stabilization of factor VIIa against trypsin (a model proteolytic enzyme) and chloromethyl ketone-type inhibitor was accomplished by conjugation of the factor to the gamma-Fe(2)O(3) nanoparticles. This stabilization may extend the blood half-life of factor VIIa. Therefore, IV injection of factor VIIa conjugated gamma-Fe(2)O(3) nanoparticles instead of free factor may avoid the frequent dosing and reduce the cost of hemophilia treatment.

Neutron studies of magnetic oxide thin films

We describe the use of neutron scattering techniques such as reflectivity and diffractionfor the study of oxide thin films. We first describe how neutron reflectivity cancomplement x-ray reflectivity for the study of some oxide materials. We then emphasizemagnetic thin films which have become an important field of study in the 1990s,following the discovery of new phenomena in heterostructures: magnetic exchangecoupling, exchange bias coupling at antiferro/ferromagnetic interfaces, enhancedmagnetism in ultrathin films or tunnel magnetoresistance for example. We showhow neutron scattering can provide detailed quantitative information about themagnetization depth profiles of thin films and about the magnetic order in epitaxialfilms.

Structural, magnetic and spectroscopic study of a diluted magnetic oxide: Co dopedCeO2−δ

A structural, spectroscopic and magnetic study of Co dopedCeO2−δ diluted magnetic oxide (DMO) thin films grown by pulsed laser deposition ispresented. No secondary phase or metallic clusters could be detected. Thesamples are ferromagnetic at room temperature and epitaxial films display alarge magnetic anisotropy with an out-of-plane easy axis. The evolution of themagnetization with temperature did not reveal any superparamagnetic signalrelated to nanosized clusters, reinforcing the conclusion that the ferromagnetism isintrinsic in this system. The magnetization at saturation has the same magnitude(∼1.4–1.5 μB/Co) in epilayers and textured films and shows no clear dependence on the structural quality,contrary to other DMOs. It is also shown that ferromagnetism is not restricted to aparticular region of the films, such as the interface. The ferromagnetic signal dependssensitively on the oxygen pressure during growth and post-growth annealing. The valenceof Ce and Co was deduced from x-ray spectroscopies, revealing a predominantCo2+ state. The origin of ferromagnetism in Co dopedCeO2−δ is discussed in connection with possible charge-compensating defects and existing modelsdescribing indirect exchange in DMOs.

Material damage induced by nanofabrication processes in manganite thin films

We investigated the use of the focused ion-beam (FIB) technique as a nanofabrication toolfor the implementation of oxide-based magnetic and magnetoelectronic functional devices.In particular, we studied the effect of using FIB lithography for the patterning ofLa2/3Ca1/3MnO3 magnetic oxide thin films. Results obtained show that the transport propertiesof patterned areas were strongly degraded after the patterning process. Incontrast, no degradation was detected when the patterning was performed usinga less aggressive technique. The origin of this degradation correlates withGa+ ion implantation, as indicated by Auger spectroscopy analysis of the patterned films.