Rie Matsumoto

Voltage-Induced Magnetic Anisotropy Changes in an Ultrathin FeB Layer Sandwiched between Two MgO Layers

We investigated the effect of voltage on the perpendicular magnetic anisotropy in an ultrathin Fe80B20 layer sandwiched between two MgO barrier layers with different thicknesses, in which the bias voltage is predominantly applied to one of the MgO layers. The application of both positive and negative bias voltages enhanced the perpendicular anisotropy, in contrast with the odd function dependence previously observed in a MgO/ferromagnetic metal/non-magnetic metal structure. Moreover, for positive bias voltages, the large anisotropy change slope of 108 fJ/Vm was demonstrated. These results indicate that the MgO-sandwich structure has high potential to extend the availability of the voltage effect.

High domain wall velocities via spin transfer torque using vertical current injection

Domain walls, nanoscale transition regions separating oppositely oriented ferromagnetic domains, have significant promise for use in spintronic devices for data storage and memristive applications. The state of these devices is related to the wall position and thus rapid operation will require a controllable onset of domain wall motion and high speed wall displacement. These processes are traditionally driven by spin transfer torque due to lateral injection of spin polarized current through a ferromagnetic nanostrip. However, this geometry is often hampered by low maximum wall velocities and/or a need for prohibitively high current densities. Here, using time-resolved magnetotransport measurements, we show that vertical injection of spin currents through a magnetic tunnel junction can drive domain walls over hundreds of nanometers at ~500 m/s using current densities on the order of 6 MA/cm2. Moreover, these measurements provide information about the stochastic and deterministic aspects of current driven domain wall mediated switching.

Oscillation of giant tunneling magnetoresistance with respect to tunneling barrier thickness in fully epitaxial Fe∕MgO∕Fe magnetic tunnel junctions

The authors fabricated fully epitaxial Fe(001)∕MgO(001)∕Fe(001) magnetic tunnel junctions (MTJs) with various MgO thicknesses (tMgO) and investigated spin-dependent transport properties. Both the tunneling resistance in the parallel magnetic state (RP) and that in the antiparallel magnetic state (RAP) exhibited short-period oscillations as functions of tMgO with the same period of 3.2A and different phases. RAP also showed a long-period oscillation with a period of 9.9A. As a result, tMgO dependence of magnetoresistance is expressed as a superposition of the short- and long-period oscillations. These results provide important clues for understanding the oscillatory tMgO dependence of the tunneling magnetoresistance effect.

A magnetic synapse: multilevel spin-torque memristor with perpendicular anisotropy

Memristors are non-volatile nano-resistors which resistance can be tuned by applied currents or voltages and set to a large number of levels. Thanks to these properties, memristors are ideal building blocks for a number of applications such as multilevel non-volatile memories and artificial nano-synapses, which are the focus of this work. A key point towards the development of large scale memristive neuromorphic hardware is to build these neural networks with a memristor technology compatible with the best candidates for the future mainstream non-volatile memories. Here we show the first experimental achievement of a multilevel memristor compatible with spin-torque magnetic random access memories. The resistive switching in our spin-torque memristor is linked to the displacement of a magnetic domain wall by spin-torques in a perpendicularly magnetized magnetic tunnel junction. We demonstrate that our magnetic synapse has a large number of intermediate resistance states, sufficient for neural computation. Moreover, we show that engineering the device geometry allows leveraging the most efficient spin torque to displace the magnetic domain wall at low current densities and thus to minimize the energy cost of our memristor. Our results pave the way for spin-torque based analog magnetic neural computation.

Spin-Torque Diode Measurements of MgO-Based Magnetic Tunnel Junctions with Asymmetric Electrodes

We present a detailed study of the spin-torque diode effect in CoFeB/MgO/CoFe/NiFe magnetic tunnel junctions. From the evolution of the resonance frequency with magnetic field at different angles, we clearly identify the free-layer mode and find an excellent agreement with simulations by taking into account several terms for magnetic anisotropy. Moreover, we demonstrate the large contribution of the out-of-plane torque in our junctions with asymmetric electrodes compared to the in-plane torque. Consequently, we provide a way to enhance the sensitivity of these devices for the detection of microwave frequency.

Spin-torque-induced oscillation at zero bias field in a magnetoresistive nanopillar with a free layer with first- and second-order uniaxial anisotropy

Spin-torque-induced magnetization dynamics in a nanopillar having a perpendicularly magnetized free layer with first- and second-order uniaxial anisotropy and an in-plane magnetized reference layer is studied theoretically on the basis of the macrospin model. It is shown that in the presence of second-order uniaxial anisotropy, self-oscillation is induced even at zero bias magnetic field. Analytical expressions for the threshold current, condition of the second-order anisotropy constant required for oscillation, and current dependence of the oscillation frequency are obtained.

Spin-transfer-torque switching in a spin-valve nanopillar with a conically magnetized free layer

Spin-transfer-torque switching in a spin-valve nanopillar with a conically magnetized free layer (c-FL) was theoretically studied using the macrospin model. The c-FL is shown to have advantages such as a low switching current density and short switching time compared with a perpendicularly magnetized free layer (p-FL). A c-FL with a thermal stability of 60kBT exhibited a switching current density 22% smaller and a switching time 56% shorter than those of a p-FL with the same thermal stability.

Spin dependent tunneling spectroscopy in single crystalline bcc-Co/MgO/ bcc-Co"001… junctions

We have measured the derivative conductance spectra of magnetic tunnel junctions with fully epitaxial bcc-Co electrodes and MgO barrier layers of various thicknesses. For all samples, clear peaks, caused by a minority Δ1 band in the bcc Co(001) electrodes, were observed at around 0.5 V. Below 0.7 V, the dI/dV spectra are essentially independent of the barrier layer thickness indicating the absence of scattering of any significance in the barrier layer and the validity of the coherent tunneling description in the single crystalline structure. Above 0.7 V, the spectra depend on the MgO barrier layer thickness. The contribution made to the conductivity by the intrinsic electronic structure in the MgO layer is discussed.

Magnetic field angle dependence of out-of-plane precession in spin torque oscillators having an in-plane magnetized free layer and a perpendicularly magnetized reference layer

Out-of-plane (OP) precession in spin torque oscillators having an in-plane (IP) magnetized free layer and a perpendicularly magnetized reference layer was studied. The bias voltage (V B) and magnetic field angle (θ) dependence of the OP precession were investigated. The absolute values of the critical magnetic fields ( and ) between which OP precession is excited increased as V B increased and as θ changed from the IP to the OP direction. The IP components of converged to a constant value regardless of θ. This result indicates that excitation of OP precession is suppressed entirely by the IP component of the magnetic field, and the contribution of the OP component can be ignored. The experimentally observed precession behavior was successfully modeled by macrospin simulations.

Time-resolved observation of fast domain-walls driven by vertical spin currents in short tracks

We present time-resolved measurements of the displacement of magnetic domain-walls (DWs) driven by vertical spin-polarized currents in track-shaped magnetic tunnel junctions. In these structures, we observe very high DW velocities (600 m/s) at current densities below 107 A/cm2. We show that the efficient spin-transfer torque combined with a short propagation distance allows avoiding the Walker breakdown process and achieving deterministic, reversible, and fast (≈1 ns) DW-mediated switching of magnetic tunnel junction elements, which is of great interest for the implementation of fast DW-based spintronic devices.