The high‐pressure nitridation of nickel was investigated using a laser‐heated diamond‐anvil cell. Marcasite‐type nickel pernitride (NiN2) was synthesized at approximately 40 GPa, and it transformed into the tetragonal phase at approximately 3 GPa along with the decompression. The structural refinement of marcasite‐type NiN2 at 36 GPa gives an N‐N distance of 1.24 Å. The first‐principles calculation reveals that the marcasite‐type NiN2 is a narrow‐gap semiconductor, and high‐pressure in‐situ X‐ray diffraction measurements revealed a zero‐pressure bulk modulus of 172(6) GPa. The axial ratios (c/a and b/a) of marcasite‐type NiN2 are close to the upper‐limit values of marcasite‐type structures, which suggests that the stability of marcasite‐type transition metal pernitrides strongly depends on the d‐electrons.

We have studied the transmission of surface acoustic waves (SAWs) in ferromagnetic/non-magnetic/ferromagnetic tryilayers. The SAW scattering matrix is studied for devices with various non-magnetic spacer thickness, which defines the strength of the interlayer exchange coupling. We find the SAW transmission amplitude depends on its propagation direction when the two ferromagnetic layers are coupled antiferromagnetically. The degree of such SAW nonreciprocity increases with increasing exchange coupling strength and reaches 37 dB/mm for a device with the thinnest spacer layer. These results show the potential of interlayer exchange coupled synthetic antiferromagnets for viable acoustic nonreciprocal transmission devices, such as circulators and isolators.

We report on the observation of the acoustic spin Hall effect that facilitates lattice motion–induced spin current via spin-orbit interaction (SOI). Under excitation of surface acoustic wave (SAW), we find that a spin current flows orthogonal to the SAW propagation in nonmagnetic metals (NMs). The acoustic spin Hall effect manifests itself in a field-dependent acoustic voltage in NM/ferromagnetic metal bilayers. The acoustic voltage takes a maximum when the NM layer thickness is close to its spin diffusion length, vanishes for NM layers with weak SOI, and increases linearly with the SAW frequency. To account for these results, we find that the spin current must scale with the SOI and the time derivative of the lattice displacement. These results, which imply the strong coupling of electron spins with rotating lattices via the SOI, show the potential of lattice dynamics to supply spin current in strong spin-orbit metals.

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The electromotive forces induced by surface acoustic waves (SAWs) are investigated in ferromagnetic thin films. CoFeB thin films deposited on LiNbO3 substrates are patterned into Hall-bars to study the acoustoelectric transport properties of the device. The longitudinal and transverse dc voltages that develop in the Hall bars, which are parallel and orthogonal to the flow of the SAW, respectively, are measured under application of an in-plane magnetic field. The longitudinal voltage scales linearly with the SAW power and reverses its polarity upon changing the direction to which the SAW propagates, suggesting generation of a dc acoustic current via the SAW excitation. The magnetic field has little influence on the acoustic current. In contrast, the SAW induced transverse voltage shows significant dependence on the relative angle between the magnetic field and the SAW propagation direction. Such field angle dependent voltage resembles that of the planar Hall voltage induced by electric current. Interestingly, the angle dependent acoustic transverse voltage does not depend on the SAW propagation direction. Moreover, the magnitude of the equivalent angle dependent acoustic transverse resistance is more than one order of magnitude larger than that of the planar Hall resistance. These results show the unique acoustoelectric transport properties of ferromagnetic thin films.