Junyeon Kim

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.

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.

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.

Spin Hall Magnetoresistance in Metallic Bilayers.

Spin Hall magnetoresistance (SMR) is studied in metallic bilayers that consist of a heavy metal (HM) layer and a ferromagnetic metal (FM) layer. We find a nearly tenfold increase of SMR in W/CoFeB compared to previously studied HM/ferromagnetic insulator systems. The SMR increases with decreasing temperature despite the negligible change in the W layer resistivity. A model is developed to account for the absorption of the longitudinal spin current to the FM layer, one of the key characteristics of a metallic ferromagnet. We find that the model not only quantitatively describes the HM layer thickness dependence of SMR, allowing accurate estimation of the spin Hall angle and the spin diffusion length of the HM layer, but also can account for the temperature dependence of SMR by assuming a temperature dependent spin polarization of the FM layer. These results illustrate the unique role a metallic ferromagnetic layer plays in defining spin transmission across the HM/FM interface.

Three terminal magnetic tunnel junction utilizing the spin Hall effect of iridium-doped copper

We show a three terminal magnetic tunnel junction (MTJ) with a 10-nm thick channel based on an interconnection material Cu with 10% Ir doping. By applying a current density of less than 1012 A m−2 to the channel, depending on the current direction, switching of a MTJ defined on the channel takes place. We show that spin transfer torque (STT) plays a critical role in determining the threshold current. By assuming the spin Hall effect in the channel being the source of the STT, the lower bound of magnitude of the spin Hall angle is evaluated to be 0.03.

Anomalous temperature dependence of current-induced torques in CoFeB/MgO heterostructures with Ta-based underlayers

We have studied the underlayer thickness and temperature dependencies of the current-induced effective field in

Evaluation of bulk-interface contributions to Edelstein magnetoresistance at metal/oxide interfaces

\text{CoFeB}/\text{MgO}

Spin–Orbit Effects in CoFeB/MgO Heterostructures with Heavy Metal Underlayers

heterostructures with Ta-based underlayers. The underlayer thickness at which the effective field saturates is found to be different between the two orthogonal components of the effective field; i.e., the dampinglike term tends to saturate at a smaller underlayer thickness than the fieldlike term. For large underlayer thickness films in which the effective field saturates, we find that the measurement temperature significantly influences the size of the effective field. A striking difference is found in the temperature dependence of the two components: the dampinglike term decreases whereas the fieldlike term increases with increasing temperature. Using a simple spin diffusion-spin transfer model, we find that all of these results can be accounted for provided the real and imaginary parts of an effective spin mixing conductance are negative. These results imply that either spin transport in this system is different from conventional metallic interfaces or effects other than spin diffusion into the magnetic layer need to be taken into account in order to model the system accurately.

Direct measurement of interfacial Dzyaloshinskii-Moriya interaction in X vertical bar CoFeB vertical bar MgO heterostructures with a scanning NV magnetometer (X=Ta, TaN, and W)

We report a systematic study on Edelstein magnetoresistance (Edelstein MR) in Co25Fe75/Cu/Bi2O3 heterostructures with a strong spin-orbit interaction at the Cu/Bi2O3 interface. We succeed in observing a significant dependence of the Edelstein MR on both Cu layer thickness and temperature, and also develop a general analytical model considering distinct bulk and interface contributions on spin relaxation. Our analysis, based on the above model, quantitatively illustrates a unique property of the spin transport near the Rashba interface, revealing a prominent role of the spin relaxation process by determining the ratios of the spin relaxation inside and outside the interface. We further find the characteristic spin transport is unaffected by temperature. Our results provide an essential tool for exploring the transport in a system with spin-momentum-locked two-dimensional states.

Spin-orbit torque in Cr/CoFeAl/MgO and Ru/CoFeAl/MgO epitaxial magnetic heterostructures

We study effects originating from the strong spin–orbit coupling in CoFeB/MgO heterostructures with heavy metal (HM) underlayers. The perpendicular magnetic anisotropy at the CoFeB/MgO interface, the spin Hall angle of the heavy metal layer, current induced torques and the Dzyaloshinskii–Moriya interaction at the HM/CoFeB interfaces are studied for films in which the early 5d transition metals are used as the HM underlayer. We show how the choice of the HM layer influences these intricate spin–orbit effects that emerge within the bulk and at interfaces of the heterostructures.