Magnetization dynamics of the ferrimagnet CoGd near the compensation of magnetization and angular momentum

Abstract

Magnetization dynamics in alloys of ferrimagnetic CoGd have been studied in the vicinity of the magnetization and angular momentum compensation point as a function of alloy composition and temperature. In agreement with standard mean-field treatments of the dynamics of the total magnetization we observe an increase of the precessional frequency and the effective damping parameter near the angular momentum compensation point. We demonstrate the consistency of the magnetization dynamics extracted from frequency domain methods such as ferromagnetic resonance and time resolved laser pump-probe measurements. DOI: 10.1103/PhysRevB.74.134404 PACS number͑s͒: 75.40.Gb, 75.50.Gg, 76.50.ϩg Transition metal ͑TM͒ rare earth ͑RE͒ ferrimagnets are ideal canonical systems to probe magnetization dynamics. Typically, TM-RE alloys are nearly amorphous materials. The TM sublattice is antiferromagnetically ͑AF͒ coupled to the RE sublattice. When the coupling is strong, as, e.g., in CoGd, there are two transition temperatures, the magnetization compensation temperature T M where M Gd = M Co , and the angular momentum compensation temperature T L , where M Gd / ␥ Gd = M Co / ␥ Gd , and ␥ is the gyromagnetic ratio. These temperatures are sensitive functions of the relative concentration. At the magnetic compensation temperature, applied magnetic fields cannot couple to the magnetization to alter its energy since M Gd − M Co = M eff = 0. Angular momentum is quenched at the angular momentum compensation point, where the AF coupled sublattices gyrate 180°out of phase about the magnetic field. Studying the dynamics in ferrimagnetic systems are complicated by these tightly coupled AF sublattices. As T L is approached from low temperatures, the phenomenological mean-field damping parameter ␣ eff which governs how fast the system as a whole dissipates energy increases quickly, and the gyromagnetic frequency changes sign as the angular momentum of the dominant sublattice changes from Gd to Co. An ideal ferrimagnet should dissipate angular momentum instantaneously at T L .1,2 CoGd was chosen for this study because T M and T L are very close to each other, and the intrinsic orbital moment of Gd is essentially zero, thereby eliminating additional loss channels due to spin-orbit coupling. 3 We compare experimental results obtained by a frequency domain method used to study the dynamics of the total magnetization of M eff -namely, ferromagnetic resonance ͑FMR͒-to time domain ultrafast laser pump/probe experiments.The most straight forward method to excite magnetization dynamics uses strong magnetic field pulses that couple directly to the magnetization ͑spin͒.4,5 These field pulses are typically produced by external sources. However, these methods cannot excite the magnetization at the magnetization compensation point in a ferrimagnet since there is no net magnetic moment one can couple to. Another method to excite spin-systems employs ultrashort laser pulses that alter the magnetic system by heating across a critical temperature ͑Curie, Néel, ...

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

“…Based on the expression above, our TbCo composition is expected to have T M < T A considering the g factors of Co (∼2.2) and Tb (∼1.5) [47,48] and the weak temperature dependence of the Co sublattice magnetization in the measurement range. The difference between T M and T A is ∼50 K for GdFeCo [35] and ∼80 K for GdCo [36], and our TbCo is expected to show a greater temperature difference, with T A > 300 K. Jiang et al reported current-induced switching (CIS) of a spin-valve system using a CoGd free layer and found that magnetoresistance and CIS showed different temperature dependence [39], with the former showing an anomaly near T M and the latter near T A . Our results do not indicate any obvious anomalies in either spin transport or SOTs other than near T M .…”

“…Hence, our results for the SOT effective field are particularly surprising and unexpected. Previously, Stanciu et al [35] and Binder et al [36] have demonstrated ultrafast switching as well as a divergence of the damping constant near T A (not T M ) using optical pump probe and vector network analyzer-ferromagnetic resonance techniques. While at…”

“…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.…”

“…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. 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.…”

Novel behaviors of anomalous Hall effect in TbFeCo ferrimagnetic thin films

“…It was reported that AHE sign dramatically changes as the Tb composition varies. 24 Thus, even slight decrease of Tb composition results in AHE sign change. The hysteresis as shown in Fig.…”