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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.

The spin-wave coupled spin-torque-oscillator is studied for a processing unit of an artificial neural network by numerically solving the Landau-Lifshitz-Gilbert equation incorporating the Slonczewski spin-torque term. The spin-wave coupling is controlled by tuning the Heisenberg and Dzyaloshinskii-Moriya interactions and also by externally applying the oscillating magnetic field. The activation function of the processing unit is varied by the spin-wave coupling. The artificial neural network computation is also demonstrated by performing the recognition of the handwritten digits in the MNIST database. By using the sigmoid-like activation function, the learning is conducted by the algorithm used for the conventional feedforward neural network. The obtained recognition accuracy is 0.853.The spin-wave coupled spin-torque-oscillator is studied for a processing unit of an artificial neural network by numerically solving the Landau-Lifshitz-Gilbert equation incorporating the Slonczewski spin-torque term. The spin-wave coupling is controlled by tuning the Heisenberg and Dzyaloshinskii-Moriya interactions and also by externally applying the oscillating magnetic field. The activation function of the processing unit is varied by the spin-wave coupling. The artificial neural network computation is also demonstrated by performing the recognition of the handwritten digits in the MNIST database. By using the sigmoid-like activation function, the learning is conducted by the algorithm used for the conventional feedforward neural network. The obtained recognition accuracy is 0.853.

We fabricated a magnetic tunnel junction (MTJ) using an MgO barrier and MnGa electrode, which shows large magnetic anisotropy, and we investigated the spin torque diode effect in the MTJ. The magnetoresistance ratio increased up to approximately 40% by inserting thin FeB/Fe layers at the MnGa/MgO interface. The obtained diode effect was as high as 70 GHz, which is thought to occur due to the coupled precession acoustic mode in the MnGa/Fe-B multi-layer. Numerical simulation suggests that a very high frequency diode signal of more than 150 GHz can be expected in the optical mode precession.

The magnetization switching in a perpendicularly magnetized nanomagnet by applying a circularly polarized rf field is analyzed as a function of the thickness (d) of the specimen using an effective one-dimensional model. It is found that there exist two kinds of critical thicknesses, dc1 and dc2 (≥dc1). When d

Magnetic anisotropy energy (MAE) of a Ru monolayer on MgO(001) substrate under the application of an electric field is investigated by using first-principles calculations. Owing to the large spin orbit coupling in Ru atoms, both MAE and its rate of change against electric field are about 3 times as large as those of an Fe monolayer on MgO(001) substrate. The change rate of MAE of Ru/MgO has an opposite sign to that of Fe/MgO, which originates from the modification of the band structure near the Fermi energy around Γ ¯ point caused by the application of an electric field.

We systematically investigated the spin wave-assisted magnetization switching (SWAS) of a L10-FePt / Ni81Fe19 (Permalloy; Py) exchange-coupled bilayer using a pulse-like rf field (hrf) and mapped the switching events in the magnetic field (H) - hrf frequency (f) plane to reveal the switching conditions. The switching occurred only in a limited region following the dispersion relationship of the perpendicular standing spin wave (PSSW) modes in Py. The results indicate that SWAS is a resonant magnetization switching process, which is different from the conventional microwave assisted switching, and has the potential to be applied to selective switching for multilevel recording media., Comment: 11 pages, 4 figures

The effect of hydrostatic pressure on structure and magnetism was investigated by a combined theoretical and experimental study for Cr2O3 showing the linear magnetoelectric effect below the Neel temperature TN ≈ 307 K. Based on first-principles calculations and Monte-Carlo simulations, we predicted that TN linearly increases with increasing applied pressure P with the slope of (1/TN)(dTN/dP) = +1.9 × 10−2 GPa−1. This theoretical prediction was qualitatively verified by our experimental observation about the pressure dependence of TN, which was detected by measurements of dielectric constant through the magnetoelectric coupling [(1/TN)(dTN/dP) = +1.6 × 10−2 GPa−1]. These results provide an effective way to enhance magnetoelectric operating temperature in the rare room-temperature magnetoelectric, Cr2O3.

Microwave-assisted magnetization switching (MAS) in an exchange-coupled-composite (ECC) nano-magnet was analyzed based on the macrospin-model. The ECC nano-magnet consists of the perpendicularly magnetized soft and hard magnetic layers which are ferromagnetically coupled with each other through the interlayer exchange coupling, χ. With increasing χ a reentrant transition from the MAS-non-effective state to the MAS-effective state appears. There exists a region of χ where the switching field is reduced by application of the microwave field. The width of this MAS-effective region of χ decreases with an increase of the frequency of the microwave field and vanishes at a certain frequency.

The dependence on diameter of the emission power in MgO-based nano-pillar spin torque oscillators (STOs) was systematically investigated. A maximum emission power of over 2.5 μW was obtained around 300 nm in diameter, which is the largest reported to date among the out-of-plane precession STOs. By analyzing physical quantities, precession cone angle of the free-layer magnetization was evaluated. In the diameter range below 300 nm, the increase in power was mainly due to the increase of the injected current. The power decrease above 300 nm is possibly attributed to the decrease in the averaged precession cone angle, suggesting spatial phase difference of magnetization precession. This study provides the method for estimating the optimum STO diameter, which is of great importance in practical use.

We theoretically study the spin-polarized current flowing through a Josephson junction (JJ) in a spin injection device. When the spin-polarized current is injected from a ferromagnet (FM) in a superconductor (SC), the charge current is carried by the superconducting condensate (Cooper pairs), while the spin-up and spin-down currents flow in the equal magnitude but in the opposite direction in SC, because of no quasiparticle charge current in SC. This indicates that not only the Josephson current but also the spin current flow across JJ at zero bias voltage, thereby generating Joule heating by the spin current. The result provides a new method for detecting the spin current by measuring Joule heating at JJ., Comment: 3 pages, 2 figures

We have fabricated granular nanobridge structures consisting of electrodes separated by a nanometer-sized gap in which a thin insulating CoAlO granular film is filled, and measured the current–bias voltage characteristics in a magnetic field to investigate the spin-dependent transport. The Coulomb blockade with a clear threshold voltage (Vth) is observed at 4.2 K. Tunnel magnetoresistance (TMR) is enhanced by fabricating nanobridges. TMR shows a maximum exceeding about 30% at the voltage slightly above Vth. This enhancement is explained by the orthodox theory of single electron tunneling in ferromagnetic multiple tunnel junctions.

Oscillation frequency of spin torque oscillator with a perpendicularly magnetized free layer and an in-plane magnetized pinned layer is theoretically investigated by taking into account the field-like torque. It is shown that the field-like torque plays an important role in finding the balance between the energy supplied by the spin torque and the dissipation due to the damping, which results in a steady precession. The validity of the developed theory is confirmed by performing numerical simulations based on the Landau-Lifshitz-Gilbert equation., Comment: conference proceeding of MMM 2014

We showed the high rf power (Pout) emission from an all-metal spin torque oscillator (STO) with a Co2Fe0.4Mn0.6Si (CFMS)/Ag/CFMS giant magnetoresistance (GMR) stack, which was attributable to the large GMR effect thanks to the highly spin-polarized CFMS. The oscillation spectra were measured by varying the magnetic field direction, and the perpendicular magnetic field was effective to increase Pout and the Q factor. We simultaneously achieved a high output efficiency of 0.013%, a high Q of 1124, and large frequency tunability. CFMS-based all-metal STO is promising for overcoming the difficulties that conventional STOs are confronted with.

We studied lattice deformation effect on exchange interaction in the corundum-type Cr2O3 theoretically. First-principles electronic structure calculations were performed to evaluate the total energy and exchange coupling constants of Cr2O3 under lattice deformation. We found that a few percent elastic deformation is expected via misfit strain and that the first- and second-nearest neighbor exchange coupling constants of Cr2O3 strongly depend on the lattice deformation. These results imply a possibility for improving the thermal stability of Cr2O3 based magnetoelectric devices by lattice deformation.

Measurements of thermally excited ferromagnetic resonance were performed on spin torque oscillators having a perpendicularly magnetized free layer and in-plane magnetized reference layer (abbreviated as PMF-STO in the following) for the purpose of obtaining magnetic properties in the PMF-STO structure. The measured spectra clearly showed a large main peak and multiple smaller peaks on the high frequency side. A Lorentzian fit on the main peak yielded Gilbert damping factor of 0.0041. The observed peaks moved in proportion to the out-of-plane bias field. From the slope of the main peak frequency as a function of the bias field, Lande g factor was estimated to be about 2.13. The mode intervals showed a clear dependence on the diameter of the PMF-STOs, i.e., intervals are larger for a smaller diameter. These results suggest that the observed peaks should correspond to eigenmodes of lateral spin wave resonance in the perpendicularly magnetized free layer.

We found the characteristic dependence of magnetization switching assisted by spin wave excitation on the magnetic field angle (θ) for exchange-coupled L10-FePt/Permalloy bilayers, where θ was the angle from the easy magnetization axis of L10-FePt. Without the spin wave being excited, an incoherent switching occurred through the domain wall motion. The frequency for the switching field (Hsw) reduction under the spin wave excitation did not depend on θ. This characteristic dependence is attributable to less variation in the resonance frequencies of the spin waves against θ. We also increased the efficiency of the Hsw reduction further with θ.

Using micromagnetic simulations, we studied spin-wave excitations induced by spin current through a nanometer-sized magnetic point contact with a confined domain wall. We found that the spin-wave excitations could be classified into three different modes by their spatial distribution profiles and oscillation frequencies. One is a localized mode while the other two are spreading modes. The oscillation frequency of the localized mode can be controlled in a wide range from 5 GHz to 147 GHz by varying the applied current density.

We studied thermal fluctuations in magnetoresistance (MR) signals originating from geometrically confined magnetic walls (GCMWs) of nanometer size. To this end, we developed a novel numerical simulation method which quantitatively evaluates the magnitude of thermal fluctuations in MR signals of magnetic nanostructures. Using the method, we first investigated the case when the twist angle Θ between the magnetization in a fixed layer and that in a free layer is 180°. We found that the thermal fluctuations of the magnetic structure of the 180° GCMW do not induce any crucial fluctuations in the MR signal because there is no significant difference among the MR values of the magnetic structures caused by the thermal fluctuations. We next investigated the dependence on the twist angle Θ of thermal fluctuations in MR signals. Since the GCMW is stabilized by decreasing Θ from 180°, the standard deviation (SD) of the MR signal is reduced with decreasing Θ. On the contrary, the SD/M ratio (M is the mean of the MR si...

The thermally assisted spin torque switching of a synthetic free layer was studied theoretically. Based on the rate equation, theoretical formulas of the switching probability, the switching current, and the standard deviation of the current were obtained. We showed that the switching time is proportional to the coupling field. We also showed that the standard deviation of the current is independent of the coupling field.

We investigated systematically the spin torque diode spectrum of a ferromagnetically coupled (FeB/CoFe)/Ru/(CoFe/FeB) synthetic free layer in an MgO-based magnetic tunnel junction. In the spectra, we observed single peaks shifted to higher frequency with increasing the in-plane magnetic fields, as expected from the ferromagnetic resonance of the FeB/CoFe adjacent to the MgO tunnel barrier. On the other hand, under the perpendicular fields, we observed several peaks below 6 GHz and around 10 GHz, which were rather insensitive to the field. These behaviors are different from our simple calculation taking account the interlayer coupling, suggesting that the excitation of magnetizations occurs in a complicated manner for the synthetic free layers.

This paper theoretically analyzes the Andreev reflection in normal-conductor/multi-band super-conductor junctions. We applied Bogoliubov-de Gennes equations to the case of superconductors with extended s-waves and calculated the respective point-contact Andreev reflection (PCAR) spectra. Our results showed that the PCAR measurement with high-Tc superconductivity can be used as a tool for measuring the spin-polarization of ferromagnetic materials.

We investigated the microwave oscillation of resistance due to spin-transfer torque in ferromagnetic nano-contact spin-valves under an external in-plane magnetic field. We varied the direction of the external magnetic field and therefore the relative angle, θ, between magnetizations of the free and reference layers. We showed that the level of microwave oscillation takes a maximum value at around θ = 140°. We also showed that the microwave oscillation frequency increases with increases in the applied direct current.

Based on the Keldysh Green’s function method within the tight-binding model, the spin transfer torque is analyzed in a magnetic tunnel junction with a synthetic ferrimagnetic (SyF) free layer in the ballistic regime. The spin transfer torque exerted on the magnetizations of ferromagnetic bilayers in the SyF free layer tends to rotate the magnetizations in the same direction as a combined motion of the SyF free layer regardless of the bias direction. This rotation causes a reduction of the critical current for the magnetization switching of the SyF free layer which is consistent with experimental observations.

We theoretically analyzed the noncontact Andreev reflection in the normal-conductor/superconductor junction through the vacuum. We solved the Bogoliubov–de Gennes equations with the vacuum barrier by using the recursion-transfer-matrix method and showed that the noncontact Andreev reflection could be observed in the effective range of STM. The results showed that the noncontact Andreev reflection can be used as a tool for measuring the spin polarization of ferromagnetic materials.

Current-perpendicular-to-plane giant-magnetoresistance effect in a ferromagnetic/nonmagnetic multilayer is studied theoretically by solving the diffusion equation of the spin accumulation without assuming the periodic boundary condition. We show that the Valet and Fert theory underestimates spin asymmetries with decreasing the number of layers.

We studied magnetization dynamics of a geometrically confined domain wall under dc current by solving simultaneously the Landau-Lifshitz-Gilbert equation and diffusion equation for spin accumulation. We showed that the oscillation motion of the domain wall is driven by the spin-transfer torque and the dc current is converted to the ac voltage signal. The results means that the geometrically confined domain wall is applicable as a source of microwave oscillator., Comment: 3 pages, 3 figures

The dependence of the critical current of spin transfer torque-driven magnetization dynamics on the free-layer thickness was studied by taking into account both the finite penetration depth of the transverse spin current and spin pumping. We showed that the critical current remains finite in the zero-thickness limit of the free layer for both parallel and anti-parallel alignments. We also showed that the remaining value of the critical current of parallel to anti-parallel switching is larger than that of anti-parallel to parallel switching., Comment: accepted by J. Appl. Phys. (Conference Proceedings of 53rd Annual Conference on Magnetism and Magnetic Materials)