Akio Fukushima

We study the perpendicular magnetic anisotropy (PMA) in the MgO/CoFeB (CFB)/MgO junctions with an angstrom-thick Mo spacer layer separating the CFB layer. Perpendicularly magnetized CFB/Mo/CFB films are achieved for a wide range of CFB thicknesses, and a large PMA energy density of > 0.3 mJ m⁻² is demonstrated by tuning the thickness ratio of the two CFB layers as well as the thickness of the Mo spacer layer. The PMA in the MgO/CFB/Mo/CFB/MgO is controlled by a voltage applied across the junction, and a sign inversion in the voltage-controlled magnetic anisotropy effect is clearly observed between the ‘top free’ and ‘bottom free’ magnetic tunnel junctions, in which the CFB/Mo/CFB layers are fabricated on top and bottom of an MgO barrier layer, respectively. Nanostructural analyses reveal the difference in the morphology of the top free and bottom free magnetic tunnel junctions and also suggest that the flatness of CFB/MgO interface is rather important for improving the efficiency of the voltage-controlled magnetic anisotropy.

Brand new mechanism of strain-induced magnetic anisotropy is proposed to secure retention energy while keeping writing current small for SOT-MRAM. The new mechanism succeeded in making switching current threshold independent on retention energy.

The performance of magnetoresistive sensors is today mainly limited by their 1/f low-frequency noise. Here, we study this noise component in vortex-based TMR sensors. We compare the noise level in different magnetization configurations of the device, i.e., vortex state or uniform parallel or antiparallel states. We find that the vortex state is at least an order of magnitude noisier than the uniform states. Nevertheless, by activating the spin-transfer-induced dynamics of the vortex configuration, we observe a reduction of the 1/f noise, close to the values measured in the AP state, as the vortex core has a lower probability of pinning into defect sites. Additionally, by driving the dynamics of the vortex core by a non-resonant rf field or current, we demonstrate that the 1/f noise can be further decreased. The ability to reduce the 1/f low-frequency noise in vortex-based devices by leveraging their spin-transfer dynamics thus enhances their applicability in the magnetic sensors' landscape.

Voltage pulse-driven switching of nano-magnets has gained distinct attention because of its high-speed writing with ultralow power consumption. One of the key advantages is that the external voltage applied to a nano-magnet reduces the magnetic anisotropy energy and excites a precessional motion of magnetization. By adjusting the duration and amplitude of the voltage pulse, a switching probability close to 50% can be attained, suggesting that the magnetic state of nano-magnets can be used as a source for generating binary random numbers (RNs) in principle. Because the bi-directional switching of nano-magnets is induced by unipolar voltage pulses, which is essentially different from the case of spin transfer torque (STT) switching, the results are a mixture of two switching polarities: from parallel (“0” state) to antiparallel (“1” state) and vice versa. Here, we focus our attention on the appearance probabilities of four cases, “00,” “01,” “10,” and “11,” all of which change linearly as functions of voltage. By tuning the probabilities of “00” or “11” to 25%, well-balanced RNs can be generated. A clear advantage of the voltage-pulse driven random number generator (RNG) over the conventional STT-driven one is lower consumption, which enables integration and heavily parallel operations of a large number of RNGs.

Magnetic tunnel junctions (MTJs) have been identified as promising candidates for the development of high-performance, ultra-low field magnetometers due to their high sensitivity, low cost, low power consumption, and small size. However, 1/f noise is often quite large at low frequencies and inevitably becomes one of the most difficult issues in developing a magnetic field sensor with ultrahigh sensitivity. Low-frequency 1/f noise can have both electric and magnetic origins, and it is a result of complex non-linear interactions between many degrees of freedom inside a sensor. Therefore, a reduction of the 1/f noise can be expected for the magnetic sensor with very small dimensions. Here, the dependence of the 1/f noise on voltage and strong hard-axis bias field in deep submicrometer-sized MgO-based MTJs is investigated with various junction sizes. The noise spectra were measured by using a home-built low-frequency noise measurement setup with maximum frequency up to 30 kHz. We find that the noise spectral power density is 1/f-like at low frequencies. The experimental results suggest a relative reduction of 1/f noise with respect to the intrinsic thermal noise in small-sized MTJs. The results may open a new approach for reducing the 1/f noise level in MTJ nanosensors.

We study the perpendicular magnetic anisotropy in (Fe_100−xNi_x)₈₀B₂₀ (FeNiB) films with various Ni contents. Perpendicularly magnetized films are achieved when the Ni content is in the range of 30 at. %–70 at. %. An effective perpendicular magnetic anisotropy (PMA) constant of 1.1× 10⁵ J/m³ is achieved for the (Fe₅₀Ni₅₀)₈₀B₂₀ film. We also fabricate magnetic tunnel junction devices containing FeNiB films, and electrical measurements show that a tunneling magnetoresistance ratio of more than 20% can be achieved for devices having an orthogonal magnetization configuration. The PMA of the FeNiB film clearly changes by varying the bias voltage applied along the FeNiB/MgO junction, and a voltage-controlled magnetic anisotropy (VCMA) efficiency of over 30 fJ/Vm is demonstrated. From systematic investigations, there is no clear correlation between PMA and VCMA efficiency in the FeNiB/MgO junction. These experimental results should facilitate the development of energy-efficient magnetic random-access memory.

We show that magnetic energy can be converted into electrical energy via inverse of voltage-controlled magnetic anisotropy effect.

We report the existence of a spin-orbit torque generated from charge to spin conversion in perpendicularly magnetized ferromagnetic material through spin-torque ferromagnetic resonance (ST-FMR) measurement. We fabricated a trilayer structure consisting of a Cu spacer layer sandwiched between an in-plane magnetized FeB layer and perpendicularly magnetized Co/Ni multilayers. From the ferromagnetic resonance under DC bias current application, we observed a significant linewidth modulation in the FeB layer attributed to the charge to spin conversion generated from Co/Ni multilayers. In-plane angular dependence and the magnetization polarity dependence of the linewidth modulation reveal that two types of charge to spin conversion with different symmetry coexist in the single Co/Ni multilayers.