Frédéric Bonell

Induction of coherent magnetization switching in a few atomic layers of FeCo using voltage pulses

The magnetization direction of a metallic magnet has generally been controlled by a magnetic field or by spin-current injection into nanosized magnetic cells. Both these methods use an electric current to control the magnetization direction; therefore, they are energy consuming. Magnetization control using an electric field is considered desirable because of its expected ultra-low power consumption and coherent behaviour. Previous experimental approaches towards achieving voltage control of magnetization switching have used single ferromagnetic layers with and without piezoelectric materials, ferromagnetic semiconductors, multiferroic materials, and their hybrid systems. However, the coherent control of magnetization using voltage signals has not thus far been realized. Also, bistable magnetization switching (which is essential in information storage) possesses intrinsic difficulties because an electric field does not break time-reversal symmetry. Here, we demonstrate a coherent precessional magnetization switching using electric field pulses in nanoscale magnetic cells with a few atomic FeCo (001) epitaxial layers adjacent to a MgO barrier. Furthermore, we demonstrate the realization of bistable toggle switching using the coherent precessions. The estimated power consumption for single switching in the ideal equivalent switching circuit can be of the order of 10(4)k(B)T, suggesting a reduction factor of 1/500 when compared with that of the spin-current-injection switching process.

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.

Quantitative Evaluation of Voltage-Induced Magnetic Anisotropy Change by Magnetoresistance Measurement

We investigated the voltage-induced perpendicular magnetic anisotropy change in an epitaxial magnetic tunnel junction (MTJ) with an ultrathin FeCo layer. Tunneling magnetoresistance (TMR) curves were measured under various bias voltage applications for different FeCo thicknesses. Clear changes in the shape of TMR curves were observed depending on the voltage-controlled perpendicular magnetic anisotropy. By evaluating the relative angle of two ferromagnetic layers, we could estimate the anisotropy energy change quantitatively. The realization of voltage-induced anisotropy change in the MTJ structure makes it possible to control the magnetization dynamics, leading to a new area of electric-field-based spintronics devices.

Reversible change in the oxidation state and magnetic circular dichroism of Fe driven by an electric field at the FeCo/MgO interface

The influence of an electric field on an ultrathin FeCo film was investigated by x-ray absorption spectroscopy and magnetic circular dichroism. Measurements were done on sub-millimeter sized pillars, with partial fluorescence yield detection. Fe L2,3 absorption spectra revealed that partial oxidation of Fe occurred during the microfabrication. The oxidation state could be reversibly controlled by an electric field, which also induced variations of the dichroic signal. These results show that electrochemical phenomena may influence the magnetism at a ferromagnet/insulator interface.

Opposite signs of voltage-induced perpendicular magnetic anisotropy change in CoFeB|MgO junctions with different underlayers

We report a voltage-induced perpendicular magnetic anisotropy (PMA) change in sputter-deposited Ta|CoFeB|MgO and Ru|CoFeB|MgO junctions. The PMA change is quantitatively evaluated by the field dependence of the tunneling magnetoresistance for various bias voltages. We find that both the sign and amplitude of the voltage effect depend on the underlayer, Ta or Ru, below the CoFeB layer. The rf voltage-induced ferromagnetic resonance spectra also support the underlayer-material-dependent direction of the voltage torque. The present study shows that the underlayer is one of the key parameters for controlling the voltage effect.

Pulse voltage-induced dynamic magnetization switching in magnetic tunneling junctions with high resistance-area product

We investigated pulse voltage-induced dynamic magnetization switchings in magnetic tunneling junctions with a high resistance-area product of 2 kΩ μm2. We found that bistable switching and the oscillatory behavior of switching probability as a function of voltage pulse duration are realized at a lower current density (−1.1 × 105 A/cm2) than in conventional spin-transfer-torque-induced magnetization switching. In addition, the switching probability at different voltage pulse strengths confirmed the existence of a voltage torque induced by a change in perpendicular magnetic anisotropy. This voltage-induced magnetization switching can be a useful technique in future spintronics devices with fast and highly reliable writing processes.

Spin-orbit torque in a bulk perpendicular magnetic anisotropy Pd/FePd/MgO system

Spin-orbit torques, including the Rashba and spin Hall effects, have been widely observed and investigated in various systems. Since interesting spin-orbit torque (SOT) arises at the interface between heavy nonmagnetic metals and ferromagnetic metals, most studies have focused on the ultra-thin ferromagnetic layer with interface perpendicular magnetic anisotropy. Here, we measured the effective longitudinal and transverse fields of bulk perpendicular magnetic anisotropy Pd/FePd (1.54 to 2.43 nm)/MgO systems using harmonic methods with careful correction procedures. We found that in our range of thicknesses, the effective longitudinal and transverse fields are five to ten times larger than those reported in interface perpendicular magnetic anisotropy systems. The observed magnitude and thickness dependence of the effective fields suggest that the SOT do not have a purely interfacial origin in our samples.

Future prospects of MRAM technologies

This paper presents a review and future prospects for the tunnel magnetoresistance (TMR) effect in magnetic tunnel junction (MTJ) and spin manipulation technologies such as spin-transfer torque (STT) for magnetoresistive random access memory (MRAM). Major challenges for ultrahigh-density STT-MRAM with perpendicular magnetization and novel functional devices related to MRAM are discussed.

Determination of the spin-lifetime anisotropy in graphene using oblique spin precession

We determine the spin-lifetime anisotropy of spin-polarized carriers in graphene. In contrast to prior approaches, our method does not require large out-of-plane magnetic fields and thus it is reliable for both low- and high-carrier densities. We first determine the in-plane spin lifetime by conventional spin precession measurements with magnetic fields perpendicular to the graphene plane. Then, to evaluate the out-of-plane spin lifetime, we implement spin precession measurements under oblique magnetic fields that generate an out-of-plane spin population. We find that the spin-lifetime anisotropy of graphene on silicon oxide is independent of carrier density and temperature down to 150 K, and much weaker than previously reported. Indeed, within the experimental uncertainty, the spin relaxation is isotropic. Altogether with the gate dependence of the spin lifetime, this indicates that the spin relaxation is driven by magnetic impurities or random spin-orbit or gauge fields.

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.