Setting of the magnetic structure of chiral kagome antiferromagnets by a seeded spin-orbit torque.

The current-induced spin-orbit torque switching of ferromagnets has had huge impact in spintronics. However, short spin-diffusion lengths limit the thickness of switchable ferromagnetic layers, thereby limiting their thermal stability. Here, we report a previously unobserved seeded spin-orbit torque (SSOT) by which current can set the magnetic states of even thick layers of the chiral kagome antiferromagnet Mn3Sn. The mechanism involves setting the orientation of the antiferromagnetic domains in a thin region at the interface with spin currents arising from an adjacent heavy metal while also heating the layer above its magnetic ordering temperature. This interface region seeds the resulting spin texture of the entire layer as it cools down and, thereby, overcomes the thickness limitation of conventional spin-orbit torques. SSOT switching in Mn3Sn can be extended beyond chiral antiferromagnets to diverse magnetic systems and provides a path toward the development of highly efficient, high-speed, and thermally stable spintronic devices.

Josephson diode effect from Cooper pair momentum in a topological semimetal.

Cooper pairs in non-centrosymmetric superconductors can acquire finite centre-of-mass momentum in the presence of an external magnetic field. Recent theory predicts that such finite-momentum pairing can lead to an asymmetric critical current, where a dissipationless supercurrent can flow along one direction but not in the opposite one. Here we report the discovery of a giant Josephson diode effect in Josephson junctions formed from a type-II Dirac semimetal, NiTe2. A distinguishing feature is that the asymmetry in the critical current depends sensitively on the magnitude and direction of an applied magnetic field and achieves its maximum value when the magnetic field is perpendicular to the current and is of the order of just 10 mT. Moreover, the asymmetry changes sign several times with an increasing field. These characteristic features are accounted for by a model based on finite-momentum Cooper pairing that largely originates from the Zeeman shift of spin-helical topological surface states. The finite pairing momentum is further established, and its value determined, from the evolution of the interference pattern under an in-plane magnetic field. The observed giant magnitude of the asymmetry in critical current and the clear exposition of its underlying mechanism paves the way to build novel superconducting computing devices using the Josephson diode effect.

Intrinsic 2D-XY ferromagnetism in a van der Waals monolayer.

The physics and universality scaling of phase transitions in low-dimensional systems has historically been a topic of great interest. Recently, two-dimensional (2D) materials exhibiting intriguing long-range magnetic order have been in the spotlight. Although an out-of-plane anisotropy has been shown to stabilize 2D magnetic order, the demonstration of a 2D magnet with in-plane rotational symmetry has remained elusive. We constructed a nearly ideal easy-plane system, a single CrCl3 monolayer on graphene/6H-SiC(0001), and observed robust ferromagnetic ordering with critical scaling characteristic of a 2D-XY system. These observations indicate the realization of a finite-size Berezinskii-Kosterlitz-Thouless phase transition in a large-area, quasi­free-standing van der Waals monolayer magnet with an XY universality class. This offers a material platform to host 2D superfluid spin transport and topological magnetic textures.

Enhanced Spontaneous Polarization in Ultrathin SnTe Films with Layered Antipolar Structure.

2D SnTe films with a thickness of as little as 2 atomic layers (ALs) have recently been shown to be ferroelectric with in-plane polarization. Remarkably, they exhibit transition temperatures (Tc ) much higher than that of bulk SnTe. Here, combining molecular beam epitaxy, variable temperature scanning tunneling microscopy, and ab initio calculations, the underlying mechanism of the Tc enhancement is unveiled, which relies on the formation of γ-SnTe, a van der Waals orthorhombic phase with antipolar inter-layer coupling in few-AL thick SnTe films. In this phase, 4n - 2 AL (n = 1, 2, 3…) thick films are found to possess finite in-plane polarization (space group Pmn21 ), while 4n AL thick films have zero total polarization (space group Pnma). Above 8 AL, the γ-SnTe phase becomes metastable, and can convert irreversibly to the bulk rock salt phase as the temperature is increased. This finding unambiguously bridges experiments on ultrathin SnTe films with predictions of robust ferroelectricity in GeS-type monochalcogenide monolayers. The observed high transition temperature, together with the strong spin-orbit coupling and van der Waals structure, underlines the potential of atomically thin γ-SnTe films for the development of novel spontaneous polarization-based devices.