We have performed magnetoresistance (MR) measurements on van der Waals ferromagnetic devices using quenched- (Q-) and nonquenched- (NQ-) Fe5GeTe2 crystals. A clear butterfly-shaped hysteresis has been observed for thin-film (less than 6 unit-cell layer) Q- and NQ-Fe5GeTe2 devices, but not for thicker film ones. The switching field of the butterfly-shaped MR is consistent with the coercive filed obtained from the Hall measurements. The MR ratio of the butterfly peak reaches about 10% at maximum, which is much larger than that observed with conventional magnetic materials. Such a large MR ratio would be related to magnetic fluctuations due to the complicated magnetic structure in this material.

Spintronic devices using antiferromagnets (AFMs) are promising candidates for future applications. Recently, many interesting physical properties have been reported with AFM-based devices. Here we report a butterfly-shaped magnetoresistance (MR) in a micrometer-sized triangular-lattice antiferromagnet Ag 2 CrO 2. The material consists of two-dimensional triangular-lattice CrO 2 layers with antiferromagnetically coupled S = 3/2 spins and Ag 2 layers with 2 high electrical conductivity. The butterfly-shaped MR appears only when the magnetic field is applied perpendicularly to the CrO 2 plane with the maximum MR ratio ( 15%) at the magnetic ordering temperature. These features are distinct from those observed in conventional magnetic materials. We propose a theoretical model where fluctuations of partially disordered spins with the Ising anisotropy play an essential role in the butterfly-shaped MR in Ag 2 CrO 2 .

We have performed spin transport measurements in metallic Bi/Ni nanowires using the spin absorption method in a lateral spin valve structure. Although Bi has a strong spin–orbit interaction, the spin Hall angle obtained for Bi/Ni nanowires is about 0.008. The magnitude and sign of the spin Hall angle are much smaller than and opposite to those of Cu–Bi alloys, where skew scattering at Bi impurities is dominant. The estimated spin diffusion length (3 nm) of Bi/Ni nanowires is shorter than that of thin Bi films, which indicates that Bi/Ni bilayers have different spin properties from Bi alone.

We studied resistance oscillations in two types of superconducting mesoscopic Nb rings. In a simple superconducting ring device, a resistance oscillation with a period of the quantized magnetic flux h/2e was clearly observed. On the other hand, in a micro-bridge ring device where two-narrow parts are embedded in parallel and work as superconductor-normal metal-superconductor junctions, higher harmonic resistance oscillations were obtained when the measurement current was well-tuned. We argue that such higher harmonic resistance oscillations can be detected even in the micro-bridge Nb superconducting ring device where the device size is much larger than the coherence length of Nb.

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