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${\mathrm{Cr}}_{2}{\mathrm{O}}_{3}$ is the archetypal magnetoelectric (ME) material, which has a linear coupling between electric and magnetic polarizations. Quadratic ME effects are forbidden for the magnetic point group of ${\mathrm{Cr}}_{2}{\mathrm{O}}_{3}$, due to space-time inversion symmetry. In ${\mathrm{Cr}}_{2}{\mathrm{O}}_{3}$ films grown by sputtering, we find a signature of a quadratic ME effect that is not found in bulk single crystals. We use Raman spectroscopy and magnetization measurements to deduce the removal of space-time symmetry and corroborate the emergence of the quadratic ME effect. We propose that metastable site-selective trace dopants remove the space, time, and space-time inversion symmetries from the original magnetic point group of bulk ${\mathrm{Cr}}_{2}{\mathrm{O}}_{3}$. We include the quadratic ME effect in a model describing the switching process during ME field cooling and estimate the effective quadratic susceptibility value. The quadratic magnetoelectric effect in a uniaxial antiferromagnet is promising for multifunctional antiferromagnetic and magnetoelectric devices that can incorporate optical, strain-induced, and multiferroic effects.

Parasitic magnetism plays an important role in magnetoelectric spin switching of antiferromagnetic oxides, but its mechanism has not been clearly investigated. Unlike the widely obtained surface boundary magnetization in magnetoelectric Cr

An inserted metal (IM) layer (such as Pt, Ru, or Cr) is typically fabricated between Cr2O3 and the ferromagnet in magnetoelectric switched antiferromagnet/ferromagnet exchange-biased thin film systems. The IM layer is used as a “spacer layer” to prevent oxidization of Co and enhance the blocking temperature. In this work, we found that the magnetic properties of these thin film systems vary depending on the type of IM, and they could be reproduced by bilayers or multilayers of Co and IM thin films. We also found that the IM layer was spin-polarized by the Co ferromagnetic layer based on X-ray magnetic circular dichroism data. These results suggest that the IM does not work separately as a “spacer layer”, but rather it is an integral component of Co-based stacked ferromagnetic films in magnetoelectric switched Cr2O3/ferromagnet exchange-biased systems. Furthermore, we found that some stacked ferromagnetic films easily exhibit a low unidirectional anisotropy energy (Jk), which may be a good way to decrease the magnetoelectric switching energy of antiferromagnets for practical device applications.

The present investigation deals with the effect of thickness and a platinum (Pt) buffer layer on the dynamic magnetic properties of polycrystalline yttrium iron garnet (YIG) films fabricated after post-annealing to recrystallize from the amorphous state. The dynamic magnetic properties of films without a buffer layer deteriorate as the thickness decreases as indicated by the increased line-widths in the ferromagnetic resonance spectra. On the other hand, a strong rejuvenation in the static and dynamic magnetic properties of the films is observed when a Pt buffer layer is used. The Pt buffer layer decreases the Gilbert damping parameter from $11.4 \times 10^{-4}$ and $3.5 \times 10^{-4}$ for a 100 nm-thick YIG film. These observations mainly originate from controlling the microstructure or the surface morphology of the films.

In this work, we investigated the temperature dependence of interface exchange coupling in an Ir-doped Fe2O3/Co system with variations in the Ir–Fe2O3 thickness. Low-temperature perpendicular exchange bias was observed in the samples with an Ir–Fe2O3 thickness of 5 nm or less, when it cooled from 360 K in an external magnetic field of 1 T perpendicular to the sample surface. For 5 nm-thick Ir–Fe2O3, a maximum exchange bias value of 1200 Oe was obtained at 200 K. This is the first report on perpendicular exchange bias of an Fe2O3 thin film coupled with a ferromagnet. Enhancements to the Morin temperature and the magnetic anisotropy of Fe2O3 caused by lattice strain, Ir doping, and the finite-size scaling effect, as well as the high quality (good crystal orientation, small roughness, etc.) of the Fe2O3 film fabricated by our process, are considered to be mostly responsible for the perpendicular exchange bias obtained.

The electric control of magnetic anisotropy has important applications for nonvolatile memory and information processing. By first-principles calculations, we show a large nonvolatile control of magnetic anisotropy in the ferromagnetic/ferroelectric CoPt/ZnO interface. Using the switched electric polarization of ZnO, the density-of-states and magnetic anisotropy at the CoPt surface show a large change. Due to a strong Co/Pt orbitals hybridization and a large spin-orbit coupling, a large control of magnetic anisotropy was found. We experimentally measured the change of effective anisotropy by tunneling resistance measurements in CoPt/Mg-doped ZnO/Co junctions. Additionally, we corroborate the origin of the control of magnetic anisotropy by observations on tunneling anisotropic magnetoresistance.

We show, by solving Maxwell’s equations, that an electric charge on the surface of a slab of a linear magnetoelectric material generates an image magnetic monopole below the surface provided that the magnetoelectric has a diagonal component in its magnetoelectric response. The image monopole, in turn, generates an ideal monopolar magnetic field outside of the slab. Using realistic values of the electric and magnetic field susceptibilities, we calculate the magnitude of the effect for the prototypical magnetoelectric material Cr2O3. We use low-energy muon spin rotation to measure the strength of the magnetic field generated by charged muons as a function of their distance from the surface of a Cr2O3 film and show that the results are consistent with the existence of the monopole. We discuss other possible routes to detecting the monopolar field, and show that, while the predicted monopolar field generated by Cr2O3 is above the detection limit for standard magnetic force microscopy, the detection of the field using this technique is prevented by surface charging effects., Physical Review X, 9 (1), ISSN:2160-3308

In Cr2O3 thin films doped with Al or Ir, we have discovered a parasitic magnetization, accompanied by the antiferromagnetic order, with tunable direction and magnitude. In this study, by utilizing the parasitic magnetization, the antiferromagnetic anisotropy KAF of the doped Cr2O3 thin films was evaluated. A much greater improvement of KAF was obtained for Al-doped Cr2O3 films than that of bulk. The maximum KAF in this study was ∼9 × 104 J/m3, obtained for the Al 3.7%-doped Cr2O3 film sample. The enhancement of the magnetic dipole anisotropy KMD due to the site-selective substitution is speculated for the dominant origin of the enhancement. Furthermore, based on the obtained KAF, the influence of the parasitic magnetization on the exchange bias blocking temperature TB of the doped-Cr2O3/Co exchange coupled system was discussed. TB greatly increases when the parasitic magnetization is coupled antiparallel to ferromagnetic moment, such as Al-doped Cr2O3/Co systems.

We studied the relationship between the magnetic anisotropy properties and crystal structure of an artificial B2 state of Fe 0.5 Co 0.5 epitaxial films on a Rh(001) seed layer with a tetragonal distortion structure, which exhibited large perpendicular uniaxial magnetic anisotropy over a wide range of thicknesses and c/a ratios at room temperature. We obtained the bulk anisotropy energy of the Fe 0.5 Co 0.5 films, which reached 2.5× 10 7 erg/cm 3 at c/a=1.20. The magnetization easy axis of the Fe 0.5 Co 0.5 films transferred from out-of-plane to in-plane when the thickness was increased from 24 to 30 monolayers. This is due to the lattice relaxation caused by the increase of the total thickness. We observed that the cubic anisotropy component around the in-plane of the films was much smaller than the perpendicular effective magnetic anisotropy, which demonstrates that our films exhibited the uniaxial anisotropy property. Furthermore, we found the bulk anisotropy values of our artificial B2-state Fe 0.5 Co 0.5 to be larger than those of the A2 state under the same lattice distortion. Thus, we suggest that tuning the degree of crystal order of FeCo alloys is an effective way to experimentally enhance their perpendicular uniaxial magnetic anisotropy.

Recently, yttrium iron garnet (YIG, Y3Fe5O12) has been proved to be an efficient material for magnonics and spintronics application after discovery of spin pumping [1], spin seebeck effect [2] and spin hall magnetoresistance [3] phenomena.

The scaling of antiferromagnetic ordering temperature of corundum-type chromia films has been investigated. N\'eel temperature ${T}_{N}$ was determined from the effect of perpendicular exchange bias on the magnetization of a weakly-coupled adjacent ferromagnet. For a thick-film case, the validity of detection is confirmed by a susceptibility measurement. Detection of ${T}_{N}$ was possible down to 1-nm-thin chromia films. The scaling of ordering temperature with thickness was studied using different buffering materials and compared with Monte-Carlo simulations. The spin-correlation length and the corresponding critical exponent were estimated, and they were consistent between experimental and simulation results. The spin-correlation length is an order of magnitude less than cubic antiferromagnets. We propose that the difference is from the change of number of exchange-coupling links in the two crystal systems.

A combined analysis of Ir-doped α-Fe2O3 (001) films, which show a drastic enhancement in the Morin transition temperature as a result of Ir doping, was performed by employing X-ray absorption near edge structure (XANES), extended X-ray absorption fine structure (EXAFS), and Mossbauer spectroscopy analyses at room temperature. The Mossbauer spectra reveal that the valency of Fe remains 3+ after Ir doping. The XANES spectra indicate that the valency of the Ir ion in Ir-doped α-Fe2O3 varies from 3+ to 4+. The EXAFS analysis results clarified that the Ir ions occupy the Fe-sites of α-Fe2O3. These findings help elucidate the mechanism for controlling the Morin transition temperature in α-Fe2O3 films toward antiferromagnetic spintronic applications.

We have observed high exchange bias for a Cr2O3 film, which was crystallized from an amorphous Cr-oxide by annealing in O2 flow. To clarify the origin of the high exchange bias, we characterized the morphology of the Cr2O3 film. From X-ray diffraction, atomic force microscopy, and transmission electron microscopy measurements, we discovered that our polycrystalline Cr2O3 film was found to grow not to be entirely random orientation, but to form large R-planes on the surface. That is, R-planes were self-organized during crystallization. Since uncompensated Cr spins exist on R-planes of Cr2O3, the origin of the high exchange bias would be the self-organized R-planes. We successfully explained the peculiar temperature-dependent exchange bias and coercivity of the Cr2O3 film. For the Cr2O3 film, perpendicular exchange bias was obtained whereas out of plane direction is hard magnetization axis. This is because of the oblique Cr spin directions from out of plane direction of the film. The results demonstrated that exchange bias higher than coercivity (μ0Hex ≫ μ0Hc) was realized near room temperature.

The linear magnetoelectric effect was measured in 500-nm Cr2O3 films grown by radio frequency sputtering on Al2O3 substrates between top and bottom thin film Pt electrodes. Magnetoelectric susceptibility was measured directly by applying an alternating current (ac) electric field and measuring the induced ac magnetic moment using superconducting quantum interference device magnetometry. A linear dependence of the induced ac magnetic moment on the ac electric field amplitude was found. The temperature dependence of the magnetoelectric susceptibility agreed qualitatively and quantitatively with prior measurements of bulk single crystals, but the characteristic temperatures of the film were lower than those of single crystals. It was also possible to reverse the sign of the magnetoelectric susceptibility by reversing the sign of the magnetic field applied during cooling through the Neel temperature. A competition between total magnetoelectric and Zeeman energies is proposed to explain the difference between film and bulk Cr2O3 regarding the cooling field dependence of the magnetoelectric effect.

The electric manipulation of antiferromagnets has become an area of great interest recently for zero-stray-field spintronic devices, and for their rich spin dynamics. Generally, the application of antiferromagnetic media for information memories and storage requires a heterostructure with a ferromagnetic layer for readout through the exchange-bias field. In magnetoelectric and multiferroic antiferromagnets, the exchange coupling exerts an additional impediment (energy barrier) to magnetization reversal by the applied magnetoelectric energy. We proposed and verified a method to overcome this barrier. We controlled the energy required for switching the magnetic domains in magnetoelectric ${\mathrm{Cr}}_{2}{\mathrm{O}}_{3}$ films by compensating the exchange-coupling energy from the ferromagnetic layer with the Zeeman energy of a small volumetric spontaneous magnetization found for the sputtered ${\mathrm{Cr}}_{2}{\mathrm{O}}_{3}$ films. Based on a simplified phenomenological model of the field-cooling process, the magnetic and electric fields required for switching could be tuned. As an example, the switching of antiferromagnetic domains around a zero-threshold electric field was demonstrated at a magnetic field of 2.6 kOe.

Ferromagnetic resonance (FMR) modes for Fe70Co30 magnetic nano-dots of 100 nm in diameter in a mono-domain state are studied under different in-plane and out-of-plane magnetic fields using a reflection measurement from a vector network analyzer with a coplanar wave guide (CPW VNA FMR) and a micromagnetics simulation. The observed real and imaginary parts of the reflection signal become a combination of symmetric and antisymmetric Lorentzian forms in FMR. The combinations are given different susceptibility components of χxx and χzx by changing the sample position on the CPW. The magnetic dots exhibit triple modes in FMR spectra from an influence of the demagnetization field distribution in the mono-domain state. The magnetic dots also show a different variation of FMR amplitude in χxx and χzx for an out-of-plane field increase.

We studied the formation mechanism of ferromagnetic nanocontacts (NCs) around 1-2 nm in diameter in thin AlOx nano-oxide layer (NOL) through in situ conductive atomic force microscopy (c-AFM), which is the only method suitable for measuring morphological changes in features this small. Spin valves (SVs) with Fe0.5Co0.5NCs in AlOx NOL fabricated by ion-assisted-oxidation (IAO) irradiated onto 1.3-nm-thick Al had nanocontact magnetoresistance (NCMR) characteristics with the MR ratio of 5% for 1 Ω μm2. According to measurements with c-AFM, these NCs were mostly located at the valleys between the AlOx grains or the edge of the grain. In other words, NCs were formed in the oxidation and cohesion process of Al by the IAO. Moreover, we found that the high-temperature annealing process improved the MR ratio to around 10% for 0.5 Ω μm2. The high-temperature annealing process may involve the AlOx morphological change and have led the Al atoms in NCs to migrate to AlOx by combining with oxygen as impurities. Then, the morphological changes and migration of Al caused NCs to be the low resistivity and the short length.

The ferromagnetic resonance response (FMR) in the variously sized magnetic dot arrays was measured by using network-analyzer ferromagnetic resonance technique. We first demonstrated that the magnetization excitation related to the susceptibility χzx by the coplanar wave-guide (CPW). In this χzx, the edge mode excitation modified the FMR response appreciably and found that the damping constant is strongly affected by the size of the nano-magnet. In larger nano-magnets, two edge modes were observed. We showed that this edge mode can be exploited as a means to separately characterize the magnetization state of the nano-magnet as well as the size variation. In addition, the simulation results are also presented here to account for the experimental observations.

We measured microwave spectra of nano-contacts magneto-resistive (NCMR) devices with a current-perpendicular-to-plane (CPP) spin-valve (SV) structure under dc current. This CPP-SV structure has a FeCo/FeCo-AlOx nano-oxide layer (NOL)/FeCo structure as reference/spacer/free layer, respectively. NCMR is a new MR effect whose origin is thought to be spin-dependent scattering due to confined domain walls at nano-contacts (NCs) inside NOL with a potentially high MR ratio. We have observed high-level oscillation with narrow line-width which has been reported in NCMR-SV devices with the MR ratio of 3%-5%. In this study, we investigated current-induced microwave oscillation in NCMR devices with high MR ratio of 7%-11% after high-temperature annealing in order to obtain larger integrated oscillation power. High-temperature annealing not only increases the MR ratio, but also causes instability in the reference layer magnetization. It also leads to an increase in the oscillation power but broadening of the line-width, compared with low MR ratio samples. It is suggested that large integrated oscillation power in NCMR device is caused by the distorted alignment between free and reference layer magnetization at ferromagnetic NCs.

We confirmed the structural properties of trilayered epitaxial films with AML [Fe/Co]n and Au spacer on Au electrode by RHEED and TEM. A considerably large DeltaRA (2.61 mOmegamum2) resistance area product was observed in all metallic current perpendicular-to-plane giant magneto-resistance (CPP-GMR) spin-valve elements by using alternate monatomic epitaxial [Fe/Co]n ferromagnetic layer with IrMn layer on the top. The estimated spin scattering asymmetry coefficient beta and interface asymmetry coefficient gamma was 0.81 and 0.43 plusmn 0.05, respectively. This value of beta is higher than that of Fe50Co50/Cu laminated with Cu spacer or Heusler alloy which is largest value ever reported at room temperature.

We investigated the current induced magnetization dynamics, so called spin torque oscillation, in magnetic nano-contact MR element with a synthetic antiferromagnetic type spin-valve structure under high in-plane applied magnetic field of 0.9-1 kOe. Very high level oscillation of 18 nVHz-1/2 with narrow FWHM of 12 MHz was observed in the condition of negative applied current where electron-spin is injected from free layer to reference one. Applied current dependency on an oscillation frequency is a blueshift with good linearlity, and same dependency on level and FWHM is almost constant in the range of -10 to -14 mA, while applied field dependency on frequency shows redshift with two different slopes, -2.5 MHz/Oe in less than 1 kOe and -8.7 MHz/Oe in more than 1 kOe which field is smaller than the spin-flop field for the synthetic antiferromagnet, besides very clear and reasonable peak and bottom for oscillation level and FWHM are observed, respectively. It is thought that these magnetization dynamics are originated from synthetic antiferromagnet.

The spin valve films with the insertion of a nano-oxide-layer are expected as nano-constriction SV MR devices such as nano-contact MR. However, the nature of those magnetic coupling, such as the exchange bias and interlayer coupling in SV films with the insertion of a NOL have been not well discussed. In this paper, the interlayer coupling field through Cu spacer and its temperature dependency in SVs with Co1-xFex-NOL and Co1-xFex/Cr-NOL are studied to confirm the influence of the insertion of NOL into SV films. In the case of the SVs with Co0.9Fe0.1-NOL and Co0.9Fe0.1/Cr-NOL, negative interlayer coupling field increases with decreasing temperature in spite of the strength of the Neel coupling of those are almost same with the full metal SV at room temperature. It suggests that the antiferromagnetic interlayer coupling appears by the insertion of NOL into the SVs even though the rough interface. From the result of the temperature dependencies of Hin for each sample, it is considered that the phase of the antiferromagnetic interlayer coupling which related the intensity is varied by the insertion of NOL into the SVs.

Body-centered-cubic (bcc) Fe50Co50 material is reported to show a high bulk spin scattering coefficient on current perpendicular to plane–giant magneto-resistance (CPP–GMR) system. But the origin of that phenomenon does not make sure yet. We prepared artificially alternate monatomic layered (AML) [Fe/Co] 41 MLs epitaxial films (Ts: 75, 250 °C) by monatomic deposition method and investigated the topology of AML [Fe/Co]n epitaxial films on MgO substrate with different orientation (001), (011) by the scanning tunnel microscopy (STM) and reflection high energy electron diffraction (RHEED), which we could confirm Frank–van der Merwe (FM) growth mode for AML [Fe/Co]n on MgO(001) and Volmer–Weber (VW) growth mode for that on Mg(011). The roughness of surface, Ra (0.20 nm) of AML [Fe/Co] 41 MLs epitaxial film grown at 75 °C on MgO(001) is smaller than that (0.46 nm) of AML [Fe/Co] grown at 250 °C on MgO(001), which has the large terraces of over 50 nm (Ra: 0.17 nm), even though there are some valleys between large terraces. Moreover we confirmed the structural properties of trilayered epitaxial films with AML [Fe/Co]n (Ra: 0.18 nm) and Fe50Co50 alloy epitaxial film on Au electrode by RHEED before confirming the characteristics of CPP–GMR devices.

The deposition and the di usion behaviors of Al atoms on Cu surfaces of various orientations were investigated by using classical molecular dynamics simulations and molecular static calculations. Al atoms with a kinetic energy of 0.1 eV were deposited at room temperature. On the Cu(001) surface, the deposited Al atoms tend to agglomerate only with adjacent atoms. In the case of the Cu(111) surface, surface di usion of Al atoms is signi cant even as a time scale between two consecutive depositions (5 ps). Most deposited atoms are, thus, agglomerated near the surface step. In contrast, Al atoms deposited on Cu(011) hardly di use on the surfaces but intermix with the Cu atoms, resulting in an atomistically rough interface. These behaviors are consistent with changes in the activation barrier for a possible kinetic process that depends on the orientation of the substrate.

We investigated the influence of a direct current on domain wall magnetoresistance (DWMR) elements with Fe0.5Co0.5-AlOx nano-oxide-layer including ferromagnetic metal nanocontacts. The current induced microwave oscillation and changes in the shape of R-H curves with increase of sense current were observed. The origin of these results are considered to be spin transfer effect on the confined domain wall in ferromagnetic metal nanocontacts and current induced magnetic field adjacent to ferromagnetic metal nanocontacts because of high current density. Furthermore, our study indicates a high likelihood of correlation between microwave oscillation and R-H curves in high sense current. We infer current induce microwave oscillation in the higher resistance state which was observed after the jump of resistance by high sense current. Moreover, it is suggested that there is some correlation between oscillation frequency and degree of twist of magnetization in a domain wall.

The exchange bias field and its training effect, the MR ratio as a function of the measured angles, and the interlayer coupling of Fe0.7Co0.3/Cr-nano-oxide-layer (NOL) spin-valves (SVs) were studied to investigate the magnetism and the structure of Fe0.7Co0.3/Cr-NOL. Strong negative interlayer coupling was observed, which suggests Fe0.7Co0.3/Cr-NOLs have very flat surfaces. However, the unusual tendency of the dependence on temperature of the exchange bias field and its training effect were observed due to the twisted coupling through on Fe0.7Co0.3/Cr-NOL. As a result, this suggested that Fe0.7Co0.3/Cr-NOLs include several different magnetic components, such as ferrimagnetic components of Co2+Fe3+2-tCr3+tO4 and Fe2+Fe3+2-tCr3+tO4 and antiferromagnetic components of Cr2O3-Fe2O3 and CoO.

Spin-torque oscillators are strong candidates as nano-scale microwave generators and detectors. However, because of large amplitude-phase coupling (non-linearity), phase noise is enhanced over other linear auto-oscillators. One way to reduce nonlinearity is to use ferromagnetic layers as a resonator and excite them at localized spots, making a resonator-excitor pair. We investigated the excitation of oscillations in dipole-coupled ferromagnetic layers, driven by localized current at ferromagnetic nano-contacts. Oscillations possessed properties of optical-mode spin-waves and at low field ($\approx$200 Oe) had high frequency (15 GHz), a moderate precession amplitude (2--3$^\circ$), and a narrow spectral linewidth ($

Nano-contact magnetoresistance (NCMR) spin-valves (SVs) using an AlOx nano-oxide-layer (NOL) have numerous nanocontacts in the thin AlOx oxide layer. The NCMR theoretically depends on the bulk scattering spin asymmetry (β) of the ferromagnetic material in the nanocontacts. To determine the relationship between NCMR and β, we investigated the dependence of NCMR on the composition of the ferromagnetic material Co1−xFex. The samples were annealed at 270 °C and 380 °C to enhance the MR ratio. For both annealing temperatures, the magnetorsistance ratio in the low-resistance area product region at less than 1 Ω μm2 was maximized for Co0.5Fe0.5. To evaluate β exactly, we fabricated current-perpendicular-to-plane giant magnetoresistance SVs with Co1−xFex/Cu/Co1−xFex layers and used Valet and Fert's theory to solve the diffusion equation of the spin accumulation for a ferromagnetic layer/non-ferromagnetic layer of five layers with a finite diffusion length. The evaluated β for Co1−xFex was also maximized for Co0.5Fe0.5. Additionally, to determine the difference between the experimental MR ratio of NCMR SVs and the theoretical MR ratio, we fabricated Co0.5Fe0.5 with oxygen impurities and estimated the decrease in β with increasing oxygen impurity concentration. Our Co0.5Fe0.5 nano-contacts fabricated using ion-assisted oxidation may contain oxygen impurities, and the oxygen impurities might cause a decrease in β and the MR ratio.

We have investigated the soft magnetic characteristics of an ultrathin Co-rich CoFeNi free layer in spin-valve films. By addition of Ni to a Co-rich CoFe free layer, magnetostriction ( λ ) of the films increased positively with Ni concentration, in contrast to which a Co90Fe10 free layer showed a negatively large λ . However, Ni addition also caused an increase in coercivity of the easy axis direction (Hce.a.). To avoid this problem, a slight decrease in the Co contents of a CoFeNi free layer was found to be effective for decreasing Hce.a.. In order to satisfy both the small λ and Hce.a., a free layer of (Co86Fe14)88−94Ni12−6 proved to be an optimum composition in spin-valve films. Moreover, the zero λ composition of the CoFeNi free layer was changed by a high-conductance Cu layer deposited on the free layer, which was considered to come from the lattice strain of a free layer.

To confirm the origin of the exchange coupling through the nano-oxide layer (NOL), the detailed analyses of the magnetization curve and the temperature dependence on magnetization for the Co 1− x Fe x -NOL samples with various oxidation processes are investigated. The field cooling effect is observed for NOL-inserted samples, which suggests the existence of the antiferromagnetic (AFM) component in the NOL. The characteristic temperature (blocking temperature) increases with increase in the Fe content for Co 1− x Fe x ( x =0.08,0.17,0.26)-NOL. These results support that NOL pinning appears higher than room temperature. Further, the twisted coupling through the NOL observed for the Co 0.50 Fe 0.50 -NOL is discussed from the viewpoint of the magnetic roughness due to the ferrimagnetic nature of the very thin NOL in specular spin valve (SPSV).

A novel method is proposed for increasing the giant magnetoresistance (GMR) of dual spin valves of the CoFe/Cu/CoFe/Cu/CoFe type by inserting nano-oxide layers (NOLs) into the pinned layers. Using this method, MR ratio of 23.5% was obtained, a value equal to those of specular spin valves with antiferromagnetic oxide, e.g., NiO. This method allows the selection of metallic materials for the antiferromagnetic layer. In addition, we obtained the specularity factor of upper NOLs, 0.8, and that of lower NOLs, 0.7, by calculating Boltzmann equations taking into account the roughness of each interlayer. This model shows that the MR ratio would be 27.5% for dual spin valves with ideal NOLs.

In this paper, we mainly discuss the extendability of current perpendicular to plane giant magnetoresistive (CPP-GMR) heads beyond 100 Gbpsi by using micromagnetic simulation and simulation techniques. An area map of CPP-GMR's resistance-area product (RA) and magnetoresistive ratio for several areal densities is made, which suggests that downsizing of a CPP-GMR head does not cause a decrease in output voltage differently from a current in plane GMR (CIP-GMR) head. At this point, a CPP-GMR has the advantage of extendability to a higher areal density over CIP-GMR, and is the major candidate for the next generation. A CIP-GMR seems to have a scalability limit around 100-200 Gbpsi. On the other hand, the area map also suggests that the RA of the CPP-GMR is a more important keyfactor for higher areal density than for CIP-GMR. The CPP-GMR, therefore, requires not only higher MR, but also less RA as areal density gets higher. Tunneling MR (TMR) has exactly the same problem, so CPP-GMR also has many advantages over TMR, with too high an RA. Additionally, we present CPP-GMR films with the potential for around 500 Gbpsi and discuss a suitable read-head structure for CPP-GMR. Micromagnetic simulation results indicate that the read track width will be controlled by a magnetic field due to the sense current of CPP. We will discuss overall studies on scalability of CPP-GMR.

We investigated the saturation magnetostriction (λs) of an ultrathin CoFe free-layer on double-layered underlayers of Ru/Cu. By increasing the Ru underlayer thickness, λs becomes positively large; on the other hand, λs becomes negatively large by increasing the Cu underlayer thickness. This tendency can be explained by the model where λs is changed by the film strain. The λs sensitivity on the Cu underlayer thickness becomes large with increasing the Co concentration of the CoFe free-layer. This indicates that CoFe λs becomes sensitive to strain with increasing Co concentration. In order to realize small magnetostriction spin-valve films with an ultrathin CoFe free-layer, we have to take care of the lattice strain of the film.

We investigated the effect of Al doping on the perpendicular exchange bias in Cr2O3/Co thin film systems. By Al doping, a large unidirectional anisotropy energy (J K ~ 0.74 erg/cm2) and a high blocking temperature (T B) were obtained simultaneously. The variations in J K and T B induced by Al doping were discussed on the basis of Mauri's model and explained from an increase in magnetic anisotropy and a decrease in the exchange stiffness constant of Cr2O3. Our first-principles calculations suggest a factor to improve the magnetic anisotropy of Cr2O3 markedly, which is different from the previously established lattice strain effect.

We investigated perpendicular exchange bias switching by a magnetoelectric field cooling process in a Pt-spacer-inserted Cr2O3/Co exchange-coupled system exhibiting small Cr2O3 magnetization. Although higher magnetoelectric switching energies with decreasing Cr2O3 thickness due to the exchange bias were reported in Cr2O3/Co all-thin-film systems, in this study, we demonstrated low-energy switching in a magnetoelectric field cool process regardless of the exchange-bias magnitude; we balanced the exchange-bias energy with the Zeeman energy associated with finite magnetization in Cr2O3. We proposed a guideline for realizing low-energy switching in thin Cr2O3 samples.

A pure-phase, unstrained and epitaxial α-Fe2O3 film of thickness 250 nm has been fabricated by reactive rf sputtering over c-Al2O3 substrate with nominal average surface roughness of 0.71 nm. Field induced spin-reorientation temperature (TSR) and temperature dependent spin-reorientation field (HSR) have been investigated. A linear relationship in the result has been found with ∂TSR/∂HSR equal to −0.92 ± 0.05 K kOe−1 and −0.89 ± 0.175 K kOe−1 by both methods, respectively. The field induced entropy change is argued to be the possible cause for the shift in the spin-reorientation temperature with applied field. The entropy change associated with the first-order phase transition is calculated on the basis of Clausius–Clapeyron equation and found to be 0.029 J kg−1 K−1.