Role of dimensional crossover on spin-orbit torque efficiency in magnetic insulator thin films

Abstract

Magnetic insulators (MIs) attract tremendous interest for spintronic applications due to low Gilbert damping and the absence of Ohmic loss. Spin-orbit torques (SOTs) on MIs are more intriguing than magnetic metals since SOTs cannot be transferred to MIs through direct injection of electron spins. Understanding of SOTs on MIs remains elusive, especially how SOTs scale with the MI film thickness. Here, we observe the critical role of dimensionality on the SOT efficiency by studying the MI layer thickness-dependent SOT efficiency in tungsten/thulium iron garnet (W/TmIG) bilayers. We show that the TmIG thin film evolves from two-dimensional to three-dimensional magnetic phase transitions as the thickness increases. We report the significant enhancement of the measured SOT efficiency as the TmIG thickness increases, which is attributed to the increase of the magnetic moment density. We demonstrate the current-induced SOT switching in the W/TmIG bilayers with a TmIG thickness up to 15 nm.

Efficient Spin‐Orbit Torque Switching of the Semiconducting Van Der Waals Ferromagnet Cr2Ge2Te6

“…Note that the counterclockwise and clockwise switching under positive and negative charge currents, respectively, is opposite compared to Ta/CoFeB and W/CoFeB systems due to an inversion of the stacking sequence of HM and FM layers. Our observations are also consistent with the observed SOT switching in a Tm 3 Fe 5 O 12 /Tungsten (TMIG/W) heterostructure as reported before, since W in their experiments and Ta in our experiments exhibit a spin Hall angle with the same polarity. On the other hand, no such R AHE loops are observed while sweeping H y (Figure d).…”

“…Due to the semiconducting nature and low Curie temperature of CGT, direct electrical characterization of its magnetization has proven to be difficult, and most of the studies on CGT have utilized magneto‐optic Kerr effect (MOKE) or magnetic force microscopy (MFM) . A stack consisting of HMs such as Pt or W deposited on an insulating FM such as Tm 3 Fe 5 O 12 (TMIG) has shown to result in an induced anomalous Hall effect in the metal, thus providing an electrical path toward reading the magnetization of insulating FMs . In fact, a recent study has used a similar approach to characterize a CGT/Pt heterostructure .…”

“…It is apparent that metallic HM/FM heterostructures such as Ta/CoFeB or Pt/FGT require in general high critical current densities of around 10 7 A cm –2 for field‐assisted SOT switching as a result of current shunting through the magnet and large M s values of the magnets used. On the other hand, integration of insulating FMs such as TMIG or usage of TI as GSHE material that exhibits large spin Hall angles can reduce the required critical current densities to ≈10 6 A cm –2 . Here, using semiconducting 2D‐FM CGT, we have shown that even lower critical current densities of ≈5 × 10 5 A cm –2 can be sufficient for field‐assisted SOT switching.…”

“…We believe that the reason for the further reduced current densities in our structures lies in the quality of the films and interfaces. TMIG films (which are usually grown on Nd 3 Ga 5 O 12 ) suffer from finite size and strong surface modification effects, which collectively reduce the saturation magnetization ( M s ) in such films . In contrast, in the case of CGT, which is a vdW material, the 2D bond structure results in a much lower defect density and better crystallinity even for ultra‐thin flakes.…”

Van der Waals magnets: Wonder building blocks for two‐dimensional spintronics?

“…It has been demonstrated that the spin current generated in the Pt layer could switch the magnetization of the FGT layer by exerting a damping‐like spin‐orbital torque. Comparing with prior switching devices, a better switching efficiency of 2.2 has been found in this case . According to Figure E, the critical current density required to switch the FGT magnetization strongly depends on the magnitude of an in‐plane applied magnetic field …”