K. Yakushiji

Ultrathin Co/Pt and Co/Pd superlattice films for MgO-based perpendicular magnetic tunnel junctions

Ultrathin [Co/Pt]n and [Co/Pd]n superlattice films consisting of 0.14–0.20-nm-thick Co and Pt(Pd) layers were deposited by sputtering. A large perpendicular magnetic anisotropy [(3–9)×106 ergs/cm3] and an ideal square out-of-plane hysteresis loop were attained even for ultrathin superlattice films with a total thickness of 1.2–2.4 nm. The films were stable against annealing up to 370 °C. MgO-based perpendicular magnetic tunnel junctions with this superlattice layer as the free layer showed a relatively high magnetoresistance ratio (62%) and an ultralow resistance-area product (3.9 Ω μm2) at room temperature. The use of these films will enable the development of gigabit-scale nonvolatile memory.

Influence of perpendicular magnetic anisotropy on spin-transfer switching current in CoFeB∕MgO∕CoFeB magnetic tunnel junctions

We investigated the spin-torque diode effect in submicron-scale Co60Fe20B20∕MgO∕(CoxFe1−x)80B20 (0⩽x⩽0.9) magnetic tunnel junctions (MTJs) under perpendicular magnetic fields Hext up to 10kOe. A single peak was clearly observed in every spin-torque diode spectrum and the dependence of resonant frequency fres on Hext was well explained by using Kittel’s formula. It was found that effective demagnetizing fields in the perpendicular-to-plane direction of the Fe-rich CoFeB free layers obtained from the spectra were considerably smaller than those expected from the magnetizations of the free layers. This suggested that the Fe-rich CoFeB free layers exhibited a perpendicular magnetic anisotropy, which agreed well with the reduced switching current density in the MTJs.

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.

Observations of thermally excited ferromagnetic resonance on spin torque oscillators having a perpendicularly magnetized free layer

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.

Coherent microwave generation by spintronic feedback oscillator.

The transfer of spin angular momentum to a nanomagnet from a spin polarized current provides an efficient means of controlling the magnetization direction in nanomagnets. A unique consequence of this spin torque is that the spontaneous oscillations of the magnetization can be induced by applying a combination of a dc bias current and a magnetic field. Here we experimentally demonstrate a different effect, which can drive a nanomagnet into spontaneous oscillations without any need of spin torque. For the demonstration of this effect, we use a nano-pillar of magnetic tunnel junction (MTJ) powered by a dc current and connected to a coplanar waveguide (CPW) lying above the free layer of the MTJ. Any fluctuation of the free layer magnetization is converted into oscillating voltage via the tunneling magneto-resistance effect and is fed back into the MTJ by the CPW through inductive coupling. As a result of this feedback, the magnetization of the free layer can be driven into a continual precession. The combination of MTJ and CPW behaves similar to a laser system and outputs a stable rf power with quality factor exceeding 10,000.

Integer, Fractional, and Sideband Injection Locking of a Spintronic Feedback Nano-Oscillator to a Microwave Signal

In this article we demonstrate the injection locking of recently demonstrated spintronic feedback nano oscillator to microwave magnetic fields at integers as well fractional multiples of its auto oscillation frequency. Feedback oscillators have delay as a new degree of freedom which is absent for spin-transfer torque based oscillators, which gives rise to side peaks along with a main peak. We show that it is also possible to lock the oscillator on its side band peaks, which opens a new avenue to phase locked oscillators with large frequency differences. We observe that for low driving fields, side band locking improves the quality factor of the main peak, whereas for higher driving fields the main peak is suppressed. Further, measurements at two field angles provide some insight into the role of symmetry of oscillation orbit in determining the fractional locking.

Spin-RAM for Normally-Off Computer

Spin-RAM technologies for operation speed faster than 30 ns, memory capacity larger than 1 Gbits and practically infinite read/write endurance have been developed by using magnetic tunnel junctions with perpendicular magnetization layers. Combination of Spin-RAM and power-gating technology will enable ultra low power computer called Normally-Off Computer.

Spin dice (physical random number generator using spin torque switching) and its thermal response

Stochastic switching of spin-torque switching in a small-sized magnetic tunnel junction (MTJ) has a unique nature in any kind of solid state devices working at room temperature. The switching probability (Psw) can be controlled by the amplitude of the injected current with good reproducibility. This means that the results of the spin-torque switching events can be used as an ideal physical random number generator which produces completely unpredictable random numbers. Meanwhile, the unpredictable random numbers are required as the key factor for data encryption. For this reason, we have been developing a physical random number generator, called spin dice [1], using the stochastic spin-torque switching.

Nonlinear thermal effect on sub-gigahertz ferromagnetic resonance in magnetic tunnel junction

Sub-gigahertz ferromagnetic resonance (FMR) is investigated in a magnetic tunnel junction (MTJ) with small anisotropy fields. Distinct FMR spectra down to 0.36 GHz are obtained by applying an external field that cancels out the anisotropy. A macrospin model simulation reveals that the difficulty in obtaining a smaller frequency excitation is not due to a magnetization inhomogeneity mainly attributed to domain creation but to a nonlinear thermal effect. The results indicate that anisotropy engineering through interfacial magnetic anisotropy together with voltage control will assist in realizing MTJs for low-frequency spin-torque devices.

Generation of highly stable 5 GHz microwave from a spin torque oscillator by phase locked loop referenced to a 80 MHz clock

In this work, we have developed a PLL circuit implementing a high performance spin torque oscillator (STO) into it . The STO consists of a magnetic tunnel junction (MTJ) having an in-plane magnetized reference layer and perpendicularly magnetized free layer [6][7] . This STO shows a very high power and Q factor simultaneously, thanks to a large magnetoresistance (MR) ratio of the MTJ stack and absence of edge effects in the perpendicular free layer magnetization, respectively . Fig . 1 shows a block diagram of the PLL built in this work . A STO showing a Q factor of about 800 under free running oscillation and red shift for a small change of bias voltage is nominally biased through a pick-off tee to generate a 5.12 GHz microwave signal . This signal is amplified by a broadband low noise amplifier (LNA), and its frequency is down converted to 80 MHz by a 1/64 down counter . This signal and 80 MHz reference clock are fed to a phase frequency detector (PFD), which outputs a voltage signal proportional to the phase difference between the two signals (phase error signal, PES) . The PES is low pass filtered and fed back to the bias control circuit to fine tune the STO frequency . That way, the phase of the STO output should be locked to that of the 80 MHz reference clock .