Spin-orbit torques in ferrimagnetic GdFeCo alloys

Abstract

The spin-orbit torque switching of ferrimagnetic Gdx(Fe90Co10)100−x films was studied for both transition metal (TM)-rich and rare earth (RE)-rich configurations. The spin-orbit torque driven magnetization switching follows the same handedness in TM-rich and RE-rich samples with respect to the total magnetization, but the handedness of the switching is reversed with respect to the TM magnetization. This indicates that the sign of the spin-orbit-torque-driven magnetic switching follows the total magnetization, although transport based techniques such as anomalous Hall effect are only sensitive to the transition metal magnetization. These results provide important insight into the physics of spin angular momentum transfer in materials with antiferromagnetically coupled sublattices.Magnetization dynamics at interfaces has been investigated extensively over the last three decades 1-3 . In that context 'spin-orbit torque' (SOT) has received a lot of interest recently. Here, a charge current in a heavy metal is converted into a spin current via spin-orbit coupling and injected into an adjacent ferromagnet 4-7 . The transfer of angular momentum from the spins to the ferromagnet causes a torque on the magnetization which can switch the magnet 6,7 . To date, most studies of SOT have concentrated on 3d ferromagnets such as Co 4,6,8,9 FeCo 10 , FeCoB 11-13 , FePd 14 or transition metal rich ferrimagnetic alloys such as TbFeCo 15 . Here we report SOT switching of ferrimagnetic GdFeCo alloys with both rare earth (RE) rich or transition metal (TM) rich configurations with bulk perpendicular magnetic anisotropy (PMA) at room temperature.Our study is based on Gd x (Fe 90 Co 10 ) 100−x thin films, where the antiferromagnetic ordering between Gd 4f and FeCo 3d magnetic moments is mediated by Gd 5d electrons via 4f -5d exchange interaction and 3d-5d hybridization 16,17 and indirect RKKY conduction band exchange. The magnetic properties of Gd x (Fe 90 Co 10 ) 100−x can be varied by changing the composition x. A comparison between TM-rich and RE-rich samples allows us to conclude that the anomalous Hall effect (AHE) changes sign for these two different samples, in accordance with literature 18-21 , while the SOT-driven magnetic switching has the same sign in both samples. Further, the effective magnetic fields, induced by damping-like and field-like SOT, do not show any change in the sign.Ta (10)/Gd x (Fe 90 Co 10 ) 100−x (5)/SiN(5) films (thickness in nm) were grown by RF magnetron sputtering on thermally oxidized silicon substrates with compositions x = 21 (TM-rich) and x = 28 (RE-rich). The base pressure during deposition was lower than 1 × 10 −8 Torr. The SiN overlayer is used to prevent oxidation. After growth, PMA was confirmed with magnetometry. Hall bar mesa structures with a width of 20 µm were patterned using optical lithography and ion milling. The layout of the sample and the measurement setup are shown in Fig. 1. DC or AC currents are applied along the x-direction while the transverse voltage is measured. The orientatio...

Temperature dependence of spin-orbit torques across the magnetic compensation point in a ferrimagnetic TbCo alloy film

“…At this temperature, the coercivity diverges since the torque from the applied field scales inversely with saturation magnetization M s . The reversal of the sign of V AHE is in good agreement with previous studies reporting that the AHE originates predominantly from spindependent interactions with the TM sublattice [32,33,44,45]. From these data we estimate T M ∼ 200 K, below (above) which the Tb (Co) sublattice dominates M s .…”

“…We repeated the measurements for several polar angles to verify that the results below are independent of θ except for small (or zero) θ , at which we observe a decreased V AHE signal indicative of a multidomain state when the in-plane field component is large. We find that the signs of the V ω1 and V ω2 loops invert across T M as expected since the sign of the AHE voltage reverses [44].…”

“…Since these materials consist of two magnetic sublattices whose magnetic moments possess different orbital character, their dynamics and interactions with spin currents are especially rich. Recently, SOTs in ferrimagnet/heavy-metal bilayer films have been investigated, and the transport mechanisms were discussed [31][32][33]43,44]. Finley and Liu reported a maximum in the dampinglike SOT efficiency at the smallest net magnetization in Ta/Tb x Co 1−x bilayer films grown with * kubond@mit.edu † gbeach@mit.edu a series of different compositions [32] measured at room temperature.…”

“…However, a systematic temperature-dependent study of spin-transport and SOTs near the magnetization compensation temperature is lacking. For example, it is known that the anomalous Hall effect (AHE) in RE-TM alloys tracks the TM sublattice magnetization [32,33,44,45], implying predominant spin-dependent interactions with the 3d orbitals as opposed to the magnetic 4f orbitals on the RE atoms. If the dampinglike SOT likewise interacts predominantly with one sublattice, then the current-induced effective field should vary smoothly through the compensation temperature, whereas if it is exerted on the net magnetization, then the effective field should diverge.…”

“…Unlike ferromagnetic systems with interfacial PMA, ferrimagnetic films based on rare-earth (RE)-transition-metal (TM) alloys possess strong bulk PMA [31][32][33][34][35][36][37][38][39][40][41][42][43][44] as well as a low net magnetization. Moreover, the net magnetization can be tuned by the composition and temperature [32,[34][35][36]38], making these materials of great interest for examining phenomena such as ultrafast optical switching [35][36][37][38], currentinduced switching [39], and domain wall motion [31,[40][41][42], which rely on maximizing the torque exerted on the net magnetization.…”