Published papers

TEM studies have revealed the temperature dependence of the magnetic domain structure in the crystal MnV2O4, which has attracted considerable attention because of the intriguing coupling of structural orders (magnetism, orbital, and lattice) and potential applications of these phenomena. Although this crystal forms well-defined magnetic domains because of long-range ferrimagnetic order, in both the cubic and the tetragonal phases, the magnetic information is dramatically reduced at temperatures below similar to 42 K. The results provide useful information for understanding magnetic anomalies, such as glassy behavior in the thermomagnetization curves, which have been reported in this temperature range.

We have investigated the temperature dependence of magnetic dynamics in a spinel-type vanadium oxide MnV2O4 by inelastic neutron scattering. The scattering intensity of excitation around 20 meV disappears in the collinear intermediate-temperature cubic-ferrimagnetic phase, which reveals that this excitation should be peculiar to the orbital ordered phase. We have found a weakly dispersive mode emergent from a non-integer wavevector (1.4,1.4,0) at 56 K, which lies in the cubic-ferrimagnetic phase between non-coplanar ferrimagnetic and paramagnetic phases. This indicates that the probable presence of an incommensurate instability in the simple collinear structure.

Magnetic-field(H)-induced shape memory effect without temperature variation is demonstrated by a combination of H-sweep and H-rotation in an orbital-spin-coupled spinel-type insulating system MnV2O4. A rotation of the direction of a magnetic field of 5 T in the low-temperature tetragonal phase gives rise to a large macroscopic strain up to 1%, which stems from a 90 degrees rotation of tetragonal domains. The results show several possible ways of macroscopic shape control in this class of matter, and may open a possibility of high-speed actuators free from the eddy current. (C) 2012 American Institute of Physics. [doi: 10.1063/1.3681581]

We fabricate a surface acoustic wave (SAW) device on a multiferroic BiFeO3 crystal while SAW devices are usually fabricated on nonmagnetic piezoelectrics and commercially available as bandpass filters. By using the time-domain technique, we demonstrate the SAW excitation on BiFeO3. The amplitude and phase of the SAW signal are modulated by the external magnetic field reflecting the multiferroicity of BiFeO3. The magnetic controllability of the multiferroic SAW device seems useful for the further functionalization of the SAW device.

AbstractInterconversion between electron spin and other forms of angular momentum is useful for spin-based information processing. Well-studied examples of this are the conversion of photon angular momentum and rotation into ferromagnetic moment. Recently, several theoretical studies have suggested that the circular vibration of atoms work as phonon angular momentum; however, conversion between phonon angular momentum and spin-moment has yet to be demonstrated. Here, we demonstrate that the phonon angular momentum of surface acoustic wave can control the magnetization of a ferromagnetic Ni film by means of the phononic-to-electronic conversion of angular momentum in a Ni/LiNbO3 hybrid device. The result clearly shows that the phonon angular momentum is useful for increasing the functionality of spintronic devices.

The control of physical properties by external fields is essential in many contemporary technologies. For example, conductance can be controlled by a gate electric field in a field effect transistor, which is a main component of integrated circuits. Optical phenomena induced by an electric field such as electroluminescence and electrochromism are useful for display and other technologies. Control of microwave propagation is also important for future wireless communication technology. Microwave properties in solids are dominated mostly by magnetic excitations, which cannot be easily controlled by an electric field. One solution to this problem is to use magnetically induced ferroelectrics (multiferroics). Here we show that microwave nonreciprocity, that is, different refractive indices for microwaves propagating in opposite directions, could be reversed by an external electric field in a multiferroic helimagnet Ba2Mg2Fe12O22. This approach offers an avenue for the electrical control of microwave properties.

We have investigated the anomalous Hall effect and magnetoresistance of the noncentrosymmetric itinerant ferromagnet Cr11Ge19. We have found a characteristic temperature (similar to 30 K) below the ferromagnetic transition temperature T-C = 85 K. While a conventional anomalous Hall conductivity that is proportional to the magnetization smoothly grows below T-C, the anomalous Hall conductivity shows an additional enhancement below the characteristic temperature. Concomitantly, the AMR emerges and the magnitude of negative magnetoresistance begins to increase toward low temperature. Because there is no anomaly in the temperature dependence of magnetization around the characteristic temperature, the origin of these observations in transport properties is ascribed to some electronic structure with the energy scale of 30 K. We speculate this is caused by the spin splitting due to breaking of spatial inversion symmetry.

Abstract Time-resolved X-ray magnetic circular dichroism under the effects of ferromagnetic resonance (FMR), known as X-ray ferromagnetic resonance (XFMR) measurements, enables direct detection of precession dynamics of magnetic moment. Here we demonstrated XFMR measurements and Bayesian analyses as a quantitative probe for the precession of spin and orbital magnetic moments under the FMR effect. Magnetization precessions in two different Pt/Ni-Fe thin film samples were directly detected. Furthermore, the ratio of dynamical spin and orbital magnetic moments was evaluated quantitatively by Bayesian analyses for XFMR energy spectra around the Ni $$L_{2,3}$$ absorption edges. Our study paves the way for a microscopic investigation of the contribution of the orbital magnetic moment to magnetization dynamics.

We have investigated microwave nonreciprocity in the non-centrosymmetric magnet CuB2O4. We simultaneously observed nonreciprocities with different origins: the classical magnetic dipolar nonreciprocity and the magnetochiral (MCh) nonreciprocity. By rotating a magnetic field in a tetragonal plane, we clearly unveiled a qualitative difference between them. The MCh signal reveals chiral transitions from one enantiomer to the other via an intermediate achiral state. We showed that the magnetoelectric effect plays an essential role in the emergence of the microwave MCh effect. Our demonstration may give a new insight into the classical dipolar-type and emergent spin-orbit-type microwave nonreciprocities in media with broken time-reversal symmetry and spatial inversion symmetry.

We investigated the microwave response in room-temperature multiferroic BaSrCo2Fe11AlO22. Microwave absorption ascribed to magnetic resonances was observed in the frequency range from 6 to 20 GHz. When the ferroelectric polarization is aligned by the electric and magnetic fields, the magnitude depends on the sign of the microwave propagation vector that indicates the nonreciprocal directional dichroism (NDD). The phenomenon can be observed even at room temperature, the sign can be controlled by the external electric and magnetic fields, and the magnitude of NDD attained is approximately 11% at 300 K and 28% at 200 K. Such microwave properties will open the avenue of practical applications for future wireless communications.

We investigated surface acoustic wave propagation in a Ni/LiNbO3 hybrid device. We found that the absorption and phase velocity are dependent on the sign of the wave vector, which indicates that the surface acoustic wave propagation has nonreciprocal characteristics induced by simultaneous breaking of time-reversal and spatial inversion symmetries. The nonreciprocity was reversed by 180 degrees rotation of the magnetic field. The origin of the nonreciprocity is ascribed to interference of shear-type and longitudinal-type magnetoelastic couplings.

Breaking of spatial inversion symmetry induces unique phenomena in condensed matter. In particular, by combining this symmetry with magnetic fields or another type of time-reversal symmetry breaking, noncentrosymmetric materials can be made to exhibit nonreciprocal responses, which are responses that differ for rightward and leftward stimuli. However, the effect of spatial inversion symmetry breaking on thermal transport in uniform media remains to be elucidated. Here, we show nonreciprocal thermal transport in the multiferroic helimagnet TbMnO3. The longitudinal thermal conductivity depends on whether the thermal current is parallel or antiparallel to the vector product of the electric polarization and magnetization. This phenomenon is thermal rectification that is controllable with external fields in a uniform crystal. This discovery may pave the way to thermal diodes with controllability and scalability.

Spinel FeV2O4 exhibits successive structural phase transitions, reflecting the interplay between the Fe2+ (3d(6)) and V3+ (3d(2)) ions, both of which have orbital and spin degrees of freedom. The temperature-dependent orbital shapes of Fe2+ and V3+ were investigated by means of single-crystal structure analysis, and were compared with those in MnV2O4, where only the V3+ ions are Jahn-Teller active. The highest-temperature transition from the cubic to the high-temperature tetragonal phase was driven by a ferroic Fe2+ 3z(2)-r(2) orbital order (OO). At 110 K, where the ferrimagnetic transition takes place, the magnetic order modified the orbital shape through intratomic spin-orbit coupling, causing an orthorhombic distortion. The V3+ orbital order (V-OO) contributed to the lowest temperature transition from the orthorhombic to the low-temperature tetragonal phase. The V-OO in FeV2O4 was qualitatively different from that in MnV2O4. We propose that ferro-OO contains a complex orbital in FeV2O4 in contrast to the V-OO of real orbitals observed in MnV2O4.

The elastic properties of a spinel vanadate MnV2O4 that has an orbital degree of freedom in the triply degenerate t2g orbital at V3+(d2) sites are investigated by ultrasonic pulse-echo measurement. Considerable softening of the shear elastic constant (C 11-C12)/2 is observed as the temperature approaches the first-order transition from the high-temperature orbital-disordered cubic phase to the low-temperature orbital-ordered tetragonal phase. The softening is attributed to fluctuations between dyz and dzx. Moreover, the elastic anomaly is found to be sensitive to the external magnetic field, revealing a coupling between spin and orbital fluctuations. The elastic anomaly is well reproduced theoretically based on Landau theory and elucidates a characteristic precursor phenomenon in MnV2O4. © 2013 American Physical Society.

The elastic properties of a spinel vanadate MnV2O4 that has an orbital degree of freedom in the triply degenerate t(2g) orbital at V3+(d(2)) sites are investigated by ultrasonic pulse-echo measurement. Considerable softening of the shear elastic constant (C-11 - C-12)/2 is observed as the temperature approaches the first-order transition from the high-temperature orbital-disordered cubic phase to the low-temperature orbital-ordered tetragonal phase. The softening is attributed to fluctuations between d(yz) and d(zx). Moreover, the elastic anomaly is found to be sensitive to the external magnetic field, revealing a coupling between spin and orbital fluctuations. The elastic anomaly is well reproduced theoretically based on Landau theory and elucidates a characteristic precursor phenomenon in MnV2O4. DOI: 10.1103/PhysRevB.87.085111

Abstract Helimagnetic structures, in which the magnetic moments are spirally ordered, host an internal degree of freedom called chirality corresponding to the handedness of the helix. The chirality seems quite robust against disturbances and is therefore promising for next-generation magnetic memory. While the chirality control was recently achieved by the magnetic field sweep with the application of an electric current at low temperature in a conducting helimagnet, problems such as low working temperature and cumbersome control and detection methods have to be solved in practical applications. Here we show chirality switching by electric current pulses at room temperature in a thin-film MnAu2 helimagnetic conductor. Moreover, we have succeeded in detecting the chirality at zero magnetic fields by means of simple transverse resistance measurement utilizing the spin Berry phase in a bilayer device composed of MnAu2 and a spin Hall material Pt. These results may pave the way to helimagnet-based spintronics.

Helimagnetic structures, in which the magnetic moments are spirally ordered, host an internal degree of freedom called chirality (or helicity) corresponding to the handedness of the helix. The chirality seems quite robust against disturbances and is therefore promising for next-generation magnetic memory. While the chirality control was recently achieved by the magnetic field sweep with the application of an electric current at low temperature in a conducting helimagnet, problems such as low working temperature and cumbersome control sequence have to be solved in practical applications. Another issue is the thin film fabrication that enables the development of spintronic devices based on helimagnets. Here we show chirality switching by electric current pulses at room temperature in a thin-film MnAu$_2$ helimagnetic conductor. The result demonstrates the feasibility of helimagnet-based spintronics that can overcome all the above problems.