Michihiko Yamanouchi

Current-induced domain-wall switching in a ferromagnetic semiconductor structure

Magnetic information storage relies on external magnetic fields to encode logical bits through magnetization reversal. But because the magnetic fields needed to operate ultradense storage devices are too high to generate, magnetization reversal by electrical currents is attracting much interest as a promising alternative encoding method. Indeed, spin-polarized currents can reverse the magnetization direction of nanometre-sized metallic structures through torque; however, the high current densities of 107–108 A cm-2 that are at present required exceed the threshold values tolerated by the metal interconnects of integrated circuits. Encoding magnetic information in metallic systems has also been achieved by manipulating the domain walls at the boundary between regions with different magnetization directions, but the approach again requires high current densities of about 107 A cm-2. Here we demonstrate that, in a ferromagnetic semiconductor structure, magnetization reversal through domain-wall switching can be induced in the absence of a magnetic field using current pulses with densities below 105 A cm-2. The slow switching speed and low ferromagnetic transition temperature of our current system are impractical. But provided these problems can be addressed, magnetic reversal through electric pulses with reduced current densities could provide a route to magnetic information storage applications.

Layer thickness dependence of the current-induced effective field vector in Ta|CoFeB|MgO

Current-induced effective magnetic fields can provide efficient ways of electrically manipulating the magnetization of ultrathin magnetic heterostructures. Two effects, known as the Rashba spin orbit field and the spin Hall spin torque, have been reported to be responsible for the generation of the effective field. However, a quantitative understanding of the effective field, including its direction with respect to the current flow, is lacking. Here we describe vector measurements of the current-induced effective field in Ta|CoFeB|MgO heterostructrures. The effective field exhibits a significant dependence on the Ta and CoFeB layer thicknesses. In particular, a 1 nm thickness variation of the Ta layer can change the magnitude of the effective field by nearly two orders of magnitude. Moreover, its sign changes when the Ta layer thickness is reduced, indicating that there are two competing effects contributing to it. Our results illustrate that the presence of atomically thin metals can profoundly change the landscape for controlling magnetic moments in magnetic heterostructures electrically.

Electric field-induced magnetization reversal in a perpendicular-anisotropy CoFeB-MgO magnetic tunnel junction

The electric field-induced ∼180° magnetization reversal is realized for a sputtered CoFeB/MgO-based magnetic tunnel junction with perpendicular magnetic easy axis in a static external magnetic field. Application of bias voltage with nanoseconds duration results in a temporal change of magnetic easy axis in the free layer CoFeB to in-plane, which induces precessional motion of magnetization in the free layer. The magnetization reversal takes place when the bias voltage pulse duration is adjusted to a half period of the precession. We show that the back and forth magnetization reversal can be observed by using successive application of half-period voltage pulses.

Perpendicular-anisotropy CoFeB-MgO magnetic tunnel junctions with a MgO/CoFeB/Ta/CoFeB/MgO recording structure

We investigated perpendicular CoFeB-MgO magnetic tunnel junctions (MTJs) with a recording structure consisting of two CoFeB-MgO interfaces, MgO/CoFeB (1.6 nm)/Ta (0.4 nm)/CoFeB (1.0 nm)/MgO. Thermal stability factor of MTJ with the structure having junction size of 70 nmφ was increased by a factor of 1.9 from the highest value of perpendicular MTJs with single CoFeB-MgO interface having the same device structure. On the other hand, intrinsic critical current for spin transfer torque switching of the double- and single-interface MTJs was comparable.

Velocity of domain-wall motion induced by electrical current in the ferromagnetic semiconductor (Ga,Mn)As.

Current-induced domain-wall motion with velocity spanning over 5 orders of magnitude up to 22 m/s has been observed by the magneto-optical Kerr effect in (Ga,Mn)As with perpendicular magnetic anisotropy. The data are employed to verify theories of spin transfer by the Slonczewski-like mechanism as well as by the torque resulting from spin-flip transitions in the domain-wall region. Evidence for domain-wall creep at low currents is found.

Interface control of the magnetic chirality in CoFeB/MgO heterostructures with heavy-metal underlayers

Recent advances in the understanding of spin orbital effects in ultrathin magnetic heterostructures have opened new paradigms to control magnetic moments electrically. The Dzyaloshinskii-Moriya interaction (DMI) is said to play a key role in forming a Néel-type domain wall that can be driven by the spin Hall torque. Here we show that the strength and sign of the DMI can be changed by modifying the adjacent heavy-metal underlayer (X) in perpendicularly magnetized X/CoFeB/MgO heterostructures. The sense of rotation of a domain wall spiral is reversed when the underlayer is changed from Hf, Ta to W and the strength of DMI varies as the filling of 5d orbitals, or the electronegativity, of the heavy-metal layer changes. The DMI can even be tuned by adding nitrogen to the underlayer, thus allowing interface engineering of the magnetic texture in ultrathin magnetic heterostructures.

Junction size effect on switching current and thermal stability in CoFeB/MgO perpendicular magnetic tunnel junctions

Junction size dependence of critical current (IC0) for spin transfer torque switching and thermal stability factor (E/kBT) was examined in CoFeB/MgO perpendicular magnetic tunnel junctions (p-MTJs). The IC0 increased with increasing recording layer area (Srec). On the other hand, the E/kBT showed almost constant values even though the Srec was increased from ∼1500 nm2 (44 nmφ) to ∼5000 nm2 (76 nmφ). Both IC0 and E/kBT behavior can be explained with assuming that the nucleation type magnetization reversal takes place in CoFeB/MgO p-MTJs.

Properties of magnetic tunnel junctions with a MgO/CoFeB/Ta/CoFeB/MgO recording structure down to junction diameter of 11 nm

We investigate properties of perpendicular anisotropy magnetic tunnel junctions (MTJs) with a recording structure of MgO/CoFeB/Ta/CoFeB/MgO down to junction diameter (D) of 11 nm from 56 nm. Thermal stability factor (Δ) of MTJ with the structure starts to decrease at D = 30 nm. D dependence of Δ agrees well with that expected from magnetic properties of blanket film taking into account the change in demagnetizing factors of MTJs. Intrinsic critical current (IC0) reduces with decrease of D in the entire investigated D range. A ratio of Δ to IC0 shows continuous increase with decrease of D down to 11 nm.

Current-induced domain wall motion in perpendicularly magnetized CoFeB nanowire

Current-induced domain wall motion in perpendicularly magnetized CoFeB nanowires with a stack structure of Ta(1.0 nm)/CoFeB(1.2 nm)/MgO(2.0 nm)/Ta(1.0 nm) was investigated. Domain wall motion driven by adiabatic spin-transfer torque was observed at a current of about 74 μA, corresponding to a current density of 6.2×107 A/cm2. The obtained results were compared with those of a micromagnetic simulation and the spin polarization of the CoFeB was estimated to be 0.72.

Quantitative characterization of the spin-orbit torque using harmonic Hall voltage measurements

Solid understanding of current induced torques is key to the development of current and voltage controlled magnetization dynamics in ultrathin magnetic heterostructures. To evaluate the size and direction of such torques, or effective fields, a number of methods have been employed. Here we examine the adiabatic (low frequency) harmonic Hall voltage measurement that has been used to study the effective field. We derive an analytical formula for the harmonic Hall voltages to evaluate the effective field for both out of plane and in-plane magnetized systems. The formula agrees with numerical calculations based on a macrospin model. Two different in-plane magnetized films, Pt|CoFeB|MgO and CuIr|CoFeB|MgO are studied using the formula developed. The effective field obtained for the latter system shows relatively good agreement with that estimated using a spin torque switching phase diagram measurements reported previously. Our results illustrate the versatile applicability of harmonic Hall voltage measurement for studying current induced torques in magnetic heterostructures.