Researchers at DGIST, Shinshu University and Vietnam National University Ho Chi Minh City (VNU-HCM) have shown that the compensation temperature of a ferrimagnetic cobalt-gadolinium (CoGd) film can be lowered electrically. Instead of changing the alloy's composition or thickness, the team used spin-orbit torque (SOT) to rearrange the spins inside the film. The size of the shift could be set by the amplitude and duration of the current pulses. The researchers suggest this could enable ferrimagnetic memory devices whose magnetic properties are tailored locally, after fabrication.
In a ferrimagnet such as CoGd, the cobalt and gadolinium sublattices are magnetized in opposite directions. At the compensation temperature, the two cancel out and the net magnetization drops to zero. Near that point, ferrimagnets switch efficiently under SOT and show fast spin dynamics, which makes the compensation temperature a key design parameter for ferrimagnetic spintronics. It is normally fixed by composition or film thickness, or adjusted by processes such as annealing, hydrogen exposure or ion migration. These approaches are either slow or cannot target specific locations within a finished device.
The team deposited Pt/IrMn₃/CoGd multilayers, in which IrMn₃ is an antiferromagnet. Current pulses through the platinum layer generate SOT through the spin Hall effect, which realigns the IrMn₃ spins in the film plane. The researchers found that IrMn₃ couples strongly to cobalt, producing an exchange bias of about 447 Oe at a Co interface, but has almost no coupling to gadolinium. As a result, cobalt spins near the interface are pulled in-plane, while those farther away keep their original perpendicular orientation, and the gadolinium spins follow only weakly. This twists the normally antiparallel Co and Gd spins into a noncollinear spiral through the thickness of the film, reducing how fully the two sublattices compensate each other and lowering the compensation temperature, most strongly near the interface.
In a Co₀.₅Gd₀.₅ film, the average compensation temperature fell by about 70 K after a current pulse, from roughly 350–360 K to 280–290 K. In a cobalt-richer Co₀.₆₈Gd₀.₃₂ film, a 60 mA, 50 ms pulse lowered it from about 140–150 K to 80–90 K, and longer or stronger pulses pushed it lower still. The change appeared only above a current density of about 10¹¹ A/m², the level needed for SOT to reorient the antiferromagnet. A control device with gold in place of platinum, which produces little spin Hall effect, showed no change at all, ruling out Joule heating as the cause. Mean-field calculations of a spiral spin configuration reproduced the measured trend. Reversing the current polarity reproducibly flipped the sign of the exchange bias, but left the compensation temperature essentially unchanged.
"This study demonstrates that the magnetic properties of a material can vary significantly depending not only on the types and spatial arrangement of its atoms but also on the configuration of its internal spins," said Professor Jung-Il Hong, who led the research. He added that the approach has significant potential for spin memory technologies in which different magnetic properties are created at specific locations on a chip.