Minori Goto

Electric spectroscopy of vortex states and dynamics in magnetic disks

Spin-polarized radio frequency (RF) currents and RF-Oersted fields resonantly excite a magnetic vortex core confined in a micron-scale soft magnetic disk. In this study, we measured the rectifying voltage spectra caused by the anisotropic magnetoresistance oscillation due to the gyration of the vortex with different polarity and chirality. The measured spectra are presented such that we can determine the vortex properties and strength of the spin torques and Oersted field accurately and directly through analytical calculation.

Voltage-controlled magnetic anisotropy in Fe|MgO tunnel junctions studied by x-ray absorption spectroscopy

In this study, voltage-controlled magnetic anisotropy (VCMA) in Fe|MgO tunnel junctions was investigated via the magneto-optical Kerr effect, soft x-ray absorption spectroscopy, and magnetic circular dichroism spectroscopy. The Fe|MgO tunnel junctions showed enhanced perpendicular magnetic anisotropy under external negative voltage, which induced charge depletion at the Fe|MgO interface. Despite the application of voltages of opposite polarity, no trace of chemical reaction such as a redox reaction attributed to O2− migration was detected in the x-ray absorption spectra of the Fe. The VCMA reported in the Fe|MgO-based magnetic tunnel junctions must therefore originate from phenomena associated with the purely electric effect, that is, surface electron doping and/or redistribution induced by an external electric field.

Voltage controlled interfacial magnetism through platinum orbits

Electric fields at interfaces exhibit useful phenomena, such as switching functions in transistors, through electron accumulations and/or electric dipole inductions. We find one potentially unique situation in a metal–dielectric interface in which the electric field is atomically inhomogeneous because of the strong electrostatic screening effect in metals. Such electric fields enable us to access electric quadrupoles of the electron shell. Here we show, by synchrotron X-ray absorption spectroscopy, electric field induction of magnetic dipole moments in a platinum monatomic layer placed on ferromagnetic iron. Our theoretical analysis indicates that electric quadrupole induction produces magnetic dipole moments and provides a large magnetic anisotropy change. In contrast with the inability of current designs to offer ultrahigh-density memory devices using electric-field-induced spin control, our findings enable a material design showing more than ten times larger anisotropy energy change for such a use and highlight a path in electric-field control of condensed matter.

Tunnel anisotropic magnetoresistance in CoFeB|MgO|Ta junctions

We found that CoFeB|MgO|Ta tunnel junctions exhibit tunnel anisotropic magnetoresistance (TAMR) at room temperature. The tunnel junctions exhibit positive magnetoresistance with the application of a magnetic field normal to the film plane. The dependencies on the applied magnetic field angle and MgO thickness reveal that the magnetoresistance originates from the TAMR, caused by the spin polarization and the spin-orbit interaction at the CoFeB|MgO interface. We also found that the TAMR can be used to detect ferromagnetic resonance in the CoFeB. This detection method could be useful for the characterization of nanomagnets that are free from the spin-transfer effect and the stray field of a reference layer, unlike conventional magnetic tunnel junctions.

Electric-field-induced changes of magnetic moments and magnetocrystalline anisotropy in ultrathin cobalt films

In this study, the microscopic origins of the voltage-controlled magnetic anisotropy (VCMA) in 3d-ferromagnetic metals are revealed. Using in-situ X-ray fluorescence spectroscopy that provides a high quantum efficiency, electric-field-induced changes in orbital magnetic moment and magnetic dipole Tz terms in ultrathin Co films are demonstrated. An orbital magnetic moment difference of 0.013{\mu}B. was generated in the presence of electric fields of +(-)0.2 V/nm. The VCMA of Co was properly estimated by the induced change in orbital magnetic moment, according to the perturbation theory model. The induced change in magnetic dipole Tz term only slightly contributed to the VCMA in 3d-ferromagnetic metals.

Electrical detection of vortex states in a ferromagnetic disk using the rectifying effect

A magnetic vortex core confined in a micron-scale magnetic disk is resonantly excited by both spin-polarized rf current and rf field. We found that rectifying voltage spectra caused by the resonance of vortex core are dependent not only on the core polarity, but also the chirality. These experimental results can be explained by analytically calculating the anisotropic magnetoresistance effect induced by the motion of the vortex core.

Criteria for electric determination of antivortex creation in ferromagnetic thin film

Antivortex creation in a cross-shaped wire was demonstrated by using magnetoresistance and magnetic force microscopy measurements. The magnetization process was stochastic and exhibited a wide variety of magnetoresistance curves. By developing a conventional method, we have achieved electrical determination criteria to distinguish antivortex creation from many other magnetization states. The electrical criteria provide a simple method without large-scale apparatus and promote further experiments such as spin-current driven antivortex dynamics.

Progress in time-resolved photoemission electron microscopy at BL25SU, SPring-8: Radiofrequency field excitation of magnetic vortex core gyration

We developed a system to carry out time-resolved photoemission electron microscopy under RF excitation at the BL25SU beamline in SPring-8. RF fields of up to ~42 MHz could be applied to microsized magnetic disks with a power loss of less than 0.9 dB. We performed time-resolved real-space observations of the gyrotropic motion of the magnetic vortex core in Ni81Fe19 disks under an RF magnetic field. We confirmed that significant core gyrations can be seen only when the resonant frequency of the disks matches with the excitation frequency of the system.

Characterization of the magnetic moments of ultrathin Fe film in an external electric field via high-precision X-ray magnetic circular dichroism spectroscopy

The magnetic moments of a Pt|Fe(0.5 nm)|MgO film at an external electric field of in a 0.35 V/nm were characterized by analyzing the X-ray magnetic circular dichroism (XMCD) at the L-edges of Fe using a partial fluorescence yield method with a precision 40 times greater than that in our previous study. The XMCD induced by the electric field was negligible (<0.2%). Furthermore, although an induced orbital magnetic moment seemingly existed, it was within the precision error (0.3%). This paper demonstrates that slight electron doping and/or redistribution without any electrochemical reaction causes voltage-controlled magnetic anisotropy at Fe|MgO interfaces.

Real-space observation of magnetic vortex core gyration in a magnetic disc both with and without a pair tag

We demonstrate the time-resolved real-space observation of vortex gyration in a magnetic circular disc by X-ray magnetic circular dichroism photoemission microscopy at the SPring-8 beamline BL25SU, both with and without an additional structure called a pair tag, which serves to control the magnetic curling direction. By comparing the gyration orbits of the vortices, we found that the gyration properties are dependent on the presence of the additional structure in spite of the discs having the same diameter. Furthermore, we reproduced the vortex gyration by micromagnetic simulation. The confinement potential of the vortex can be controlled by the additional structure.