Spin phase detection by spin current in a chiral helimagnet

TL;DR

This work demonstrates electrical spin-phase detection in a nanoscale van der Waals chiral helimagnet CrNb$_3$S$_6$ using nonlocal spin valves, leveraging the material’s short spin diffusion length to probe surface moments that encode the spin phase. Complementary micromagnetic simulations reproduce the field-driven evolution of the surface magnetization, including 180$^{\circ}$ spin-phase rotations for thicknesses near $L_0$ and at $1.5L_0$, while inverse spin Hall effect measurements reveal spin fluctuations that cause a sign change in the spin Hall angle near the Curie temperature. Collectively, the results establish spin currents as a powerful probe of the spin phase and its fluctuations in helimagnets, highlighting the spin phase as a tunable internal degree of freedom for nanoscale spintronic devices.

Abstract

Helimagnets, characterized by a helical arrangement of magnetic moments, possess unique internal degrees of freedom, including the spin phase, defined by the phase of the helical magnetic structure. Electrical detection of the spin phase is essential for both practical applications and fundamental research in helimagnets. Here, we demonstrate the electrical detection of the spin phase in a van der Waals nanoscale chiral helimagnet CrNb$_3$S$_6$ using nonlocal spin valve measurements. Due to the short spin diffusion length in CrNb$_3$S$_6$ ($\sim5$~nm), the surface magnetic moment direction, which corresponds to the spin phase, can be detected via spin currents. The experimentally observed magnetic field dependence of the nonlocal spin valve signal is consistent with that of the surface magnetic moment in the helical magnetic structure, as supported by micromagnetic simulations. Our results establish spin currents as a powerful tool for detecting the spin phase in helimagnets, opening avenues for utilizing the spin phase as a novel internal degree of freedom in nanoscale spintronic devices.

Figures (9)

• Figure 1: Basic properties of CrNb$_3$S$_6$.a, Definition of the spin phase in a helical magnetic structure. The 0$\tcdegree$ and 180$\tcdegree$ phases are defined when the surface magnetic moment is parallel and antiparallel to an in-plane reference axis (taken here as the $y$-axis), respectively. b, Crystal structure of CrNb$_3$S$_6$. The black solid line represents a unit cell. c, A scanning electron microscope image of a typical nonlocal spin valve (NLSV) device. The scale bar is 1 $\mu$m. d, e, Magnetoresistance of CrNb$_3$S$_6$ with thicknesses of 43 nm (d) and 72 nm (e) measured at 50 K. The red and blue data correspond to the magnetic field increasing and decreasing processes, respectively.
• Figure 1: Temperature dependence of the NLSV signal with and without a 30 nm thick CrNb$_3$S$_6$ flake. The temperature dependence of $\Delta R_{\rm S1}$ for devices with and without the CrNb$_3$S$_6$ flake.
• Figure 2: Determination of spin diffusion length of CrNb$_3$S$_6$.a, b, Schematic illustrations of NLSV measurements without (a) and with (b) an insertion of a CrNb$_3$S$_6$ flake. c, NLSV signal $R_{\rm S1}$ with (blue) and without (red) a 30 nm thick CrNb$_3$S$_6$ flake measured at $T = 10$ K. d, Spin diffusion length of the CrNb$_3$S$_6$ flake as a function of temperature. The inset shows the temperature dependence of the resistance $R$ of the CrNb$_3$S$_6$ flake.
• Figure 2: Reproducibility of the NLSV signal using CrNb$_3$S$_6$ flakes. NLSV signals $R_{\rm S2}$ at $T = 50$ K using $t\approx L_{0}$ (a, b) and $t\approx 1.5L_{0}$ (c) CrNb$_3$S$_6$ flakes.
• Figure 3: Spin phase detection in CrNb$_3$S$_6$ flakes.a, A schematic illustration of the spin phase detection measurement. b, c, NLSV signal $R_{\rm S2}$ at $T = 50$ K using a 43 nm ($\approx L_{0}$) thick CrNb$_3$S$_6$ flake for a low magnetic field region (b) and a 52 nm ($\approx L_{0}$) thick CrNb$_3$S$_6$ flake for a high magnetic field region (c). d, The expected magnetic structures at characteristic magnetic fields during the magnetic field increasing process for the $t \approx L_{0}$ thick CrNb$_3$S$_6$ flake. e, f, NLSV signal $R_{\rm S2}$ at $T = 50$ K using a 72 nm ($\approx 1.5L_{0}$) thick CrNb$_3$S$_6$ flake for a low magnetic field region (e) and a high magnetic field region (f). g, The expected magnetic structures at characteristic magnetic fields during the magnetic field increasing process for the $t \approx 1.5L_{0}$ thick CrNb$_3$S$_6$ flake.