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We have studied the anomalous Nernst effect (ANE) in [Fe3O4/Pt]-based heterostructures grown at high temperature, by measuring the ANE-induced electric field with a magnetic field applied normal to the sample surface, in the perpendicular magnetized configuration, where only the ANEs from the ferromagnetic layers or magnetic proximity effects can be detected. An ANE voltage is observed for [Fe3O4/Pt]n multilayers, and we further investigated its origin by performing measurements in [Fe3O4/Pt/Fe3O4] trilayers as a function of the Pt thickness. Our results suggest the presence of an interface-induced ANE in the metallic layer, possibly driven by a heat induced subnanometer interdiffusion which affects the nature of the Fe3O4/Pt interface. Despite this ANE, the spin Seebeck effect is the dominant mechanism for the transverse thermoelectric voltage in the in-plane magnetized configuration, accounting for about 70% of the voltage in the multilayers., This work was supported by the ERATO “Spin Quantum Rectification Project” (Grant No. JPMJER1402) and PRESTO “Phase Interfaces for Highly Efficient Energy Utilization” (Grant No. JPMJPR12C1) from JST, Japan; Grant-in-Aid for Scientific Research (A) (Grant No. JP15H02012), Grant-in-Aid for Scientific Research on Innovative Area, “Nano Spin Conversion Science” (Grant No. JP26103005) and Grant-in-Aid for Research Activity Start-up (No. JP18H05841) from JSPS KAKENHI, Japan; H2020-MSCA-RISE-2016 SPICOLOST (Grant No. 734187); and the Spanish Ministry of Economy and Competitiveness (Grant No. MAT2017-82970-C2, including FEDER) and the Aragon regional government (E26), Spain. The microscopy works were conducted in the LMA at INA, Universidad de Zaragoza. A.A. acknowledges MINECO [through Project Nos. FIS2016-78591-C3-1-R (SKYTRON), PCIN-2015-111 (SOgraph), and SEV-2016-0686] and Comunidad de Madrid through Grant No. S2018/NMT-4321 (NANOMAGCOST-CM).

We study the spin Hall magnetoresistance (SMR) in epitaxial γ–Fe2O3/NiO(001)/Pt stacks, as a function of temperature and thickness of the antiferromagnetic insulating NiO layer. Upon increasing the thickness of NiO from 0 nm to 10 nm, we detect a sign change of the SMR in the temperature range between 10 K and 280 K. This temperature dependence of the SMR in our stacks is different compared to that of previously studied yttrium iron garnet/NiO/Pt, as we do not find any peak or sign change as a function of temperature. We explain our data by a combination of spin current reflection from both the NiO/Pt and γ-Fe2O3/NiO interfaces and the thickness-dependent exchange coupling mode between the NiO and γ-Fe2O3 layers, comprising parallel alignment for thin NiO and perpendicular alignment for thick NiO.

Co x Fe 3−x O 4 (CFO) is a well-known spinel-type ferrite magnetic material showing large cubic magnetic anisotropy and magnetostriction[1]. Epitaxial films of CFO(001) grown on MgO(001) substrates exhibit extremely large perpendicular magnetic anisotropy (PMA) exceeding 10 Merg/cm3 owing to tensile stress originating from the lattice mismatch between the films and substrates[2]. Since the lattice constant of MgO is slightly larger than half that of the cobalt-ferrite, MgO(001) is a suitable substrate for growing CFO(001) with strong PMA. It is known that antiphase boundaries (APBs) exist at the interfaces in most of the spinel ferrite films grown on MgO(001). The high-density APBs contribute to the degradation of the magnetic properties of ferrite films such as the Fe 3 O 4 / MgO(001).[3] Therefore, it is crucial to develop a technique to reduce the number of APBs to utilize practically CFO films with large PMA. For this purpose, we attempted the insertion of spinel-type buffer layers and investigated how the insertion layers affect the magnetic properties of CFO films.

We have investigated the magnetic properties of cobalt ferrite films epitaxially grown on MgAl 2 O 4 (110) substrates. The unit cell of the cobalt ferrite film on the substrate with twofold surface symmetry is tetragonally distorted with c/a

Thin films of NiFe 2 O 4 were fabricated on MgO (001) and MgAl 2 O 4 (MAO) (001) substrates by reactive radio frequency magnetron sputtering and were evaluated with regards to their electrical and magnetic properties. A saturation magnetization of 285 emu/cm 3 was obtained with a 30 nm thick film grown on an MAO substrate at an oxygen flow rate, Q, of 9 sccm. This enhanced magnetization was found to be due to a normal spinel arrangement at the interface. From the thickness dependence of the magnetization, the intrinsic magnetization was determined as 241 emu/cm 3 with the enhancement region estimated to be around 3/4 of the lattice constant, thus providing 8 μ B /f.u. The magnetization of those films grown on an MgO substrate exhibited remarkably smaller amplitudes to those of the MAO substrates. It is suggested that this substrate-dependent magnetization can be attributed to the number of antiphase boundaries. The resistivity was found to increase with Q, with a resistivity state in the same order as that of the bulk achieved with films prepared in which Q was greater than 10 sccm.

We report the observation of the three-dimensional angular dependence of the spin Hall magnetoresistance (SMR) in a bilayer of the epitaxial antiferromagnetic insulator NiO(001) and the heavy metal Pt, without any ferromagnetic element. The detected angular-dependent longitudinal and transverse magnetoresistances are measured by rotating the sample in magnetic fields up to 11 T, along three orthogonal planes (xy-, yz- and xz-rotation planes, where the z-axis is orthogonal to the sample plane). The total magnetoresistance has contributions arising from both the SMR and ordinary magnetoresistance. The onset of the SMR signal occurs between 1 and 3 T and no saturation is visible up to 11 T. The three-dimensional angular dependence of the SMR can be explained by a model considering the reversible field-induced redistribution of magnetostrictive antiferromagnetic S- and T-domains in the NiO(001), stemming from the competition between the Zeeman energy and the elastic clamping effect of the non-magnetic MgO substrate. From the observed SMR ratio, we estimate the spin mixing conductance at the NiO/Pt interface to be greater than $2\times10^{14}$ ${\Omega}^{-1}$ $m^{-2}$. Our results demonstrate the possibility to electrically detect the N\'eel vector direction in stable NiO(001) thin films, for rotations in the xy- and xz- planes. Moreover, we show that a careful subtraction of the ordinary magnetoresistance contribution is crucial to correctly estimate the amplitude of the SMR.

Recently, we reported on the synthesis of a high-quality polyimide-Co (PI-Co) granular thin film using vapor deposition polymerization. This thin film had a tunnel magnetoresistance (TMR) ratio of 2.6% at room temperature. Because thermal annealing is considered effective for manipulating the grain size in granular systems composed of metallic ferromagnetic grains samples were annealed at temperatures between 473 K and 673 K in high vacuum. All samples were superparamagnetic at room temperature, and the average diameter of the Co particles, as deduced from Langevin analysis, increased significantly in the sample annealed at 623 K. The temperature dependence of the electrical resistance indicated tunneling conduction between Co particles in all samples. The TMR ratio increased to 4.8% at an annealing temperature of 473 K but decreased significantly after annealing above 623 K. This change was attributed to PI decomposition, as determined from the IR spectra.

A numerical simulation of magnetic materials with random magnetic anisotropy was performed. The magnetization of an assembly of magnetically-interacting grains with randomly-oriented uniaxial anisotropy was calculated using the Landau-Lifshitz-Gilbert equation. For simplicity in the simulation, the magnetizations in a grain were assumed to be aligned in the same direction; this assumption is known as the single spin model. The interaction at the interface of the grains was taken into account by including an interaction energy between the unit vectors that represent the magnetization directions of the grains. Calculations were carried out for an N × N × N system, where the grain sizes D ranged from 5 to 40 nm and N ranged from 10 to 80. The relation between the coercive forces H C and the grain size is represented by H C ∼ D k . For the case of N = 10, k = 5.7, which corresponds to the primitive theory of the random anisotropy model (RAM) where k = 6. As N increased, k decreased slightly from 5.7 to 4.2. The gradient of the log-log plot of the coercive force versus the grain size (d logH C/d logD) was deduced by smoothing the data and was found to have a peak value of approximately 6. This result suggests that the RAM is supported by the simulation within the range of grain sizes where the peak was observed.

Considering the importance of magneto-elastic (ME) effects on the magnetic anisotropy (MA) of thin films, phenomenological analysis of the MA of Co ferrites on (001) and (111) substrates is conducted. It is found that thin films of Co ferrites on (111) substrates may produce oblique magnetization, while those on (001) substrates exhibit perpendicular MA. First-principles calculations for the ME constant of L10 ordered alloys, which show large uniaxial MA, are also performed, and it is determined that the value of the ME constant is on the same order of magnitude as that for the Co ferrites.

We have been investigating the spin motive force induced by magnetization dynamics in ferromagnetic materials. Ferrimagnetic materials are expected to exhibit even larger spin motive force due to their spatially staggered magnetic structure and high frequency spin wave modes. We believe that magnetite (Fe3O4) is one of the candidates for this purpose. Here, we report magnetic resonance in Fe3O4 thin films epitaxially grown on a MgO (001) substrate. The gyromagnetic ratio and the effective saturation magnetization are estimated to be 1.71 ×1011 s-1T-1 and 364 emu/cm3, respectively. The Gilbert damping constant of 0.02 is obtained from the magnetic field dependence of the linewidth. We expect to investigate the spin motive force induced from this magnetic resonance.

The recently discovered spin Hall magnetoresistance (SMR) effect is a useful means to obtain information on the magnetization process at the interface between a nonmagnetic metal and ferromagnetic insulators. We report the SMR measurements at the interface between platinum and cobalt ferrite thin films for samples with two different preferential directions of magnetization (out-of-plane and in-plane). The directional difference of the magnetic easy axis does not seem to influence the value of SMR.

We report enhanced tunnel magnetoresistance (TMR) ratios of 188$%$ (308$%$) at room temperature and 328$%$ (479$%$) at 15 K for cation-site-disordered MgAl${}_{2}$O${}_{4}$-barrier magnetic tunnel junctions (MTJs) with Fe (Fe${}_{0.5}$Co${}_{0.5}$ alloy) electrodes, which exceed the TMR ratios theoretically calculated and experimentally observed for ordered spinel barriers. The enhancement of TMR ratios is attributed to the suppression of the so-called band-folding effect in ordered spinel MTJs [Phys. Rev. B 86, 024426 (2012)]. First-principles calculations describe a dominant role of the oxygen sublattice for spin-dependent coherent tunneling, suggesting a mechanism of coherent tunneling occurring even in the disordered systems.

Spin motive force (SMF) is induced by a time and spatial derivative of magnetizations and is dependent on spin polarization. We compare the SMF in FeNi with positive spin polarization with that in a magnetite (Fe3O4) with negative spin polarization. We observe the SMF induced by a nonuniform ferromagnetic resonance in Fe3O4 and find that the SMF in Fe3O4 is opposite to that in FeNi. This result originates from the negative spin polarization of Fe3O4. Our clear observation of the SMF depending on the sign of the spin polarization agrees well with the framework of the SMF theory.

Spin dependent quantum well resonance has been investigated in fully epitaxial magnetic tunnel junctions with Fe(001)/MgO(001)/ultrathin Fe(001)/Cr(001) structure. The dI/dV spectra clearly show the resonant peaks which shift systematically depending on the thickness of an ultrathin electrode as predicted in ab initio calculation [Zhong-Yi Lu et al, Phys. Rev. Lett. 94, 207210 (2005)]. The magnetotransport is strongly modulated at the same bias voltage as the resonant peaks. This control of the magnetotransport in magnetic tunnel junctions at a specific bias voltage will contribute to the development of active spintronic devices.

The magnetism of a typical spinel ferromagnetic oxide, Fe3O4, was controlled via ion implantation. Nitrogen ions were accelerated at 6–10 kV and irradiated to the 13-nm-thick Fe3O4 thin films with dosages of 2 × 1016 to 6 × 1016 ions/cm2. The magnetization decreased with the increase in ion dosage, and there was almost no magnetization when 6 × 1016 ions/cm2 of nitrogen was irradiated, irrespective of the acceleration voltages. The results of the temperature dependence of the magnetization and the Mössbauer study suggest that the transition from ferromagnetic to nonmagnetic phases in the Fe3O4 thin film upon N2 ion irradiation proceeds abruptly without the formation of intermediate states.

A large perpendicular magnetic anisotropy (PMA) of 1.4 MJ/m3 was observed from ultrathin Fe/MgO(001) bilayers grown on Cr-buffered MgO(001). The PMA strongly depends on the surface state of Fe prior to the MgO deposition. A large PMA energy density of 1.4 MJ/m3 was achieved for a 0.7 nm thick Fe layer having adsorbate-induced surface reconstruction, which is likely to originate from oxygen atoms floating up from the Cr buffer layer. This large magnitude of PMA satisfies the criterion that is required for thermal stability of magnetization in a few tens nanometer-sized magnetic memory elements.

Perpendicular magnetic anisotropy (PMA) of cobalt-ferrite CoxFe3-xO4 (x = 0.75 and 1.0) epitaxial thin films grown on MgO (001) by a reactive magnetron sputtering technique was investigated. The saturation magnetization was found to be 430 emu/cm3 for x = 0.75, which is comparable to that of bulk CoFe2O4 (425 emu/cm3). Torque measurements afforded PMA constants of Kueff=9.0 Merg/cm3 (Ku=10.0 Merg/cm3) and Kueff=9.7 Merg/cm3 for x = 0.75 and 1.0, respectively. The value of Kueff extrapolated using Miyajima's plot was as high as 14.7 Merg/cm3 for x = 1.0. The in-plane four-fold magnetic anisotropy was evaluated to be 1.6 Merg/cm3 for x = 0.75. X-ray diffraction measurement revealed our films to be pseudomorphically strained on MgO (001) with a Poisson ratio of 0.4, leading to a considerable in-plane tensile strain by which the extraordinarily large PMA could be accounted for.

We investigated the fundamental transport properties of epitaxial magnetite (Fe3O4) films with applied current perpendicular to the plane geometry. The devices with a junction area of 36 μm2 were fabricated with TiN/Fe3O4/ TM (TM: Ti and Fe) stacking. Both the temperature dependence and the magnetic field dependence of the junction resistance were measured. Most of the resistance properties were independent of whether the electrode was magnetic (Fe) or nonmagnetic (Ti). Below the Verwey point, the junction resistance abruptly increased with decreasing temperature, and the resistance reached maximum at approximately 30 K. The magnetoresistance showed a peak of ∼27% at T≈55 K. The observed resistance behavior was consistent with a hopping conduction mechanism with a Coulomb interaction.

A selective method for growing high-quality spinel-type epitaxial Fe3O4(0 0 1) or γ-Fe2O3(0 0 1) films was developed using conventional planar-type sputtering by controlling the flow rate of oxygen gas . Although magnetization of the oxide films is close to that of Fe3O4 or γ-Fe2O3 for all , the room-temperature resistivity and the Fe3+ and Fe2.5+ composition ratios of the films are dependent on . The films for and are identified as Fe3O4 and γ-Fe2O3 films, respectively. All the results suggest that Fe3O4 films are obtained only when sputtered in the metal mode, and γ-Fe2O3 films are obtained in the oxide mode.

Spin-dependent resonant tunneling was investigated in fully epitaxial Cr(001)/ultrathin Fe(001)/MgO(001)/Fe(001) pseudo double-barrier magnetic tunnel junctions (MTJs) by varying structural coherence of the MgO barrier through postdeposition annealing and Mg layer insertion. It was clearly demonstrated that not only the flatness of the Fe quantum-well layer, but also the structural coherence of the MgO barrier caused the appearance of sharp resonant conductance peaks which were not obtained for spin-dependent resonant tunneling in amorphous AlO-barrier MTJs. The symmetry-selective transport via MgO barrier plays a crucial role for the resonant tunneling as well as large tunneling magnetoresistance.

Fully epitaxial Fe/Ag-wedge/MgO/Fe(001) magnetic tunnel junctions (MTJs) were prepared by using a combination of magnetron sputtering and molecular beam epitaxy, and their magneto-transport properties were investigated for the Ag thickness range of 0–9 monolayer (ML). The large tunneling magnetoresistance (TMR) ratios of up to 160% at room temperature were obtained in the Fe/MgO/Fe region (Ag: 0 ML) after the optimization of the preparation conditions. The finite TMR ratio was observed until the Ag thickness reached 4 ML, although no significant oscillation due to quantum well states (QWS) was seen. This is the first demonstration of TMR in the fully epitaxial MTJs with nonmagnetic metal layer insertion which can potentially create spin-dependent QWS.

Structures and their stability of FeCo ultrathin films grown on MgO(100) were investigated. Atomic force microscopy observations revealed dramatic structural changes from a continuous ultrathin layer to coarse islands by low temperature annealing. The results suggest that the thin layered structures of Fe based alloys as grown on MgO(100) are not so stable as expected from the relatively small lattice mismatch between Fe based alloys and MgO. This possibly explains annealing temperature dependence of tunnel magnetoresistance observed in a Fe/MgO/Fe(100) junction.

We fabricated fully epitaxial Fe/MgAl2O4/Fe(001) magnetic tunnel junctions using plasma oxidation of an Mg/Al bilayer. The MgAl2O4 showed a (001)-oriented spinel-type structure, and there were few misfit dislocations at the interfaces between the MgAl2O4 and the two Fe layers due to a small lattice mismatch (∼1%). Tunnel magnetoresistance (TMR) ratios up to 117% (165%) were obtained at room temperature (15 K). The bias voltage for one-half of the zero-bias TMR ratio (Vhalf) was relatively large, ranging from 1.0 to 1.3 V at room temperature, which is attributed to the small misfit dislocation density.

Electron-beam (EB) evaporated MgO grows with (001) texture on amorphous CoFeB when the deposition rate is kept below 5 pm/s. Magnetic tunnel junctions (MTJs) fabricated using this method exhibit ∼240% magnetoresistance at room temperature for a 2.5 nm thick EB-MgO barrier, which is similar to the value for a radio frequency (rf) sputtered barrier with the same junction geometry. The average barrier height of the EB-MgO is 0.48 eV, which is higher than previously reported values for rf-MgO barriers and it increases with increasing annealing temperature. Our results show that EB-MgO could be a simpler alternative to rf-MgO in MTJs without any compromise in the tunnelling magnetoresistance.