T. Nozaki

Large voltage-induced magnetic anisotropy change in a few atomic layers of iron

In the field of spintronics, researchers have manipulated magnetization using spin-polarized currents. Another option is to use a voltage-induced symmetry change in a ferromagnetic material to cause changes in magnetization or in magnetic anisotropy. However, a significant improvement in efficiency is needed before this approach can be used in memory devices with ultralow power consumption. Here, we show that a relatively small electric field (less than 100 mV nm(-1)) can cause a large change (approximately 40%) in the magnetic anisotropy of a bcc Fe(001)/MgO(001) junction. The effect is tentatively attributed to the change in the relative occupation of 3d orbitals of Fe atoms adjacent to the MgO barrier. Simulations confirm that voltage-controlled magnetization switching in magnetic tunnel junctions is possible using the anisotropy change demonstrated here, which could be of use in the development of low-power logic devices and non-volatile memory cells.

Voltage-induced perpendicular magnetic anisotropy change in magnetic tunnel junctions

A voltage-induced perpendicular magnetic anisotropy change in an ultrathin FeCo layer was observed in an epitaxial magnetic tunnel junction (MTJ) structure. A spin-transfer induced ferromagnetic resonance measurement technique was used under various bias voltage applications to evaluate the anisotropy change. From the peak frequency shifts, we could estimate that a surface magnetic anisotropy change of 15 μJ/m2 was induced by an electric field application of 400 mV/nm in the MTJ with a 0.5 nm thick FeCo layer. The realization of voltage-induced anisotropy changes in an MTJ structure should have a large impact on the development of electric-field driven spintronic devices.

Highly sensitive nanoscale spin-torque diode

Highly sensitive microwave devices that are operational at room temperature are important for high-speed multiplex telecommunications. Quantum devices such as superconducting bolometers possess high performance but work only at low temperature. On the other hand, semiconductor devices, although enabling high-speed operation at room temperature, have poor signal-to-noise ratios. In this regard, the demonstration of a diode based on spin-torque-induced ferromagnetic resonance between nanomagnets represented a promising development, even though the rectification output was too small for applications (1.4 mV mW(-1)). Here we show that by applying d.c. bias currents to nanomagnets while precisely controlling their magnetization-potential profiles, a much greater radiofrequency detection sensitivity of 12,000 mV mW(-1) is achievable at room temperature, exceeding that of semiconductor diode detectors (3,800 mV mW(-1)). Theoretical analysis reveals essential roles for nonlinear ferromagnetic resonance, which enhances the signal-to-noise ratio even at room temperature as the size of the magnets decreases.

Large change in perpendicular magnetic anisotropy induced by an electric field in FePd ultrathin films

The magnetic properties of FePd ultrathin films and their variation under the influence of an electric field are investigated by magneto-optical Kerr effect (MOKE) measurements. L10-ordered FePd shows a spin reorientation transition when varying the thickness. The easy axis of magnetization is found to be normal to the plane at thicknesses above 9 monolayers (MLs) and in-plane below 9 ML. The coercive field, the perpendicular magnetic anisotropy and the MOKE signal at saturation vary with the applied electric field. The sensitivity of the interface magnetic anisotropy is estimated to be 602 fJ/V m.

Unified understanding of both thermally assisted and precessional spin-transfer switching in perpendicularly magnetized giant magnetoresistive nanopillars

We report on the spin-transfer magnetization switching properties of CoFe/Pd-based perpendicularly magnetized giant magnetoresistive cells over a wide current pulse duration time range. Analytic expressions without empirical parameters like attempt frequency are tested experimentally for the thermally assisted and precessional regimes. Good agreement with the experiment data is obtained using a common parameter set in both regimes, which leads to a comprehensive understanding of the switching properties including the origin of the attempt frequency.

Voltage control of in-plane magnetic anisotropy in ultrathin Fe/n-GaAs"001… Schottky junctions

We report on the realization of voltage control of in-plane magnetic anisotropy at room temperature in ultrathin Fe∕n-GaAs(001) Schottky junctions. Clear voltage-induced changes in magnetic anisotropy were observed in a Kerr ellipticity hysteresis loop using a lock-in modulation technique. The maximum change reached 4.5% of the saturation ellipticity under the application of a sinusoidal voltage signal of 1V peak-to-peak in an Fe layer with a thickness of 0.64nm. These results reveal the feasibility of controlling the in-plane magnetization process by the use of a perpendicular electric field. This can be a useful technique in ultralow power magnetization switching.

rf amplification in a three-terminal magnetic tunnel junction with a magnetic vortex structure

We fabricated a three-terminal MgO-based magnetic tunnel junction including a free layer for use as a vortex-type spin structure. The resonant motion of the magnetic vortex core, excited by the application of rf current to the free layer, was successfully detected through the tunneling magnetoresistance effect using a homodyne detection technique. By analyzing the detected dc signal, we were able to qualitatively estimate the efficiency of the spin-transfer induced excitation. From the parameters obtained in the experiment, we found that substantial voltage gain can be realized with this device.

Brillouin Light Scattering Study of the Magnetic Anisotropy in bcc-Fe(100) Ultrathin Films Grown on GaAs(100) Surfaces With Different Reconstructions

We investigated the magnetic anisotropy of ultrathin bcc-Fe (001) films with a fcc-Au(001) cover layer, grown onto GaAs (001) (4 times 1) and (2 times 6)/(3 times 6) reconstructed surfaces. The magneto-optical Kerr effect hysteresis loops revealed existence of a strong in-plane uniaxial magnetic anisotropy in the Fe films grown on both the surfaces. Details of the in-plane uniaxial anisotropy (KU), cubic anisotropy (KC), and effective demagnetization field (4piMeff) were studied by Brillouin light scattering method. We observed a stronger reduction of KU, KC, and 4piMeff in thinner Fe layers (below 1.3 nm) grown on both the surfaces. In the case of Fe films grown on GaAs (4 times 1) surface, anisotropies observed were smaller than that for the (2 times 6)/(3 times 6) surface. This reflects the role of surface structure in determining the magnetic anisotropies.

Radio-frequency amplification property of the MgO-based magnetic tunnel junction using field-induced ferromagnetic resonance

The radio-frequency (RF) voltage amplification property of a tunnel magnetoresistance device driven by an RF external-magnetic-field-induced ferromagnetic resonance was studied. The proposed device consists of a magnetic tunnel junction and an electrically isolated coplanar waveguide. The input RF voltage applied to the waveguide can excite the resonant dynamics in the free layer magnetization, leading to the generation of an output RF voltage under a DC bias current. The dependences of the RF voltage gain on the static external magnetic field strength and angle were systematically investigated. The design principles for the enhancement of the gain factor are also discussed.

Variation of the perpendicular magnetic anisotropy with bias voltage in ultra thin ferromagnetic layers

We investigated the voltage controlled perpendicular magnetic anisotropy in ultra thin ferromagnetic layers.