Abstract

The development of compact and tunable room temperature sources of coherent THz-frequency signals would open a way for numerous new applications. The existing approaches to THz-frequency generation based on superconductor Josephson junctions (JJ), free electron lasers, and quantum cascades require cryogenic temperatures or/and complex setups, preventing the miniaturization and wide use of these devices. We demonstrate theoretically that a bi-layer of a heavy metal (Pt) and a bi-axial antiferromagnetic (AFM) dielectric (NiO) can be a source of a coherent THz signal. A spin-current flowing from a DC-current-driven Pt layer and polarized along the hard AFM anisotropy axis excites a non-uniform in time precession of magnetizations sublattices in the AFM, due to the presence of a weak easy-plane AFM anisotropy. The frequency of the AFM oscillations varies in the range of 0.1–2.0 THz with the driving current in the Pt layer from 108 A/cm2 to 109 A/cm2. The THz-frequency signal from the AFM with the amplitude exceeding 1 V/cm is picked up by the inverse spin-Hall effect in Pt. The operation of a room-temperature AFM THz-frequency oscillator is similar to that of a cryogenic JJ oscillator, with the energy of the easy-plane magnetic anisotropy playing the role of the Josephson energy.

Highlights

  • An absence of compact and reliable generators and receivers of coherent signals in the frequency range 0.1– 10 THz has been identified as a fundamental physical and technological problem[1,2,3]

  • A DC spin current flowing from a current-driven Pt layer and polarized along the hard anisotropy axis of the adjacent AFM layer can excite a rotation of the AFM sublattice magnetizations[16,19,20] that is non-uniform in time due to the influence of a weak easy-plane AFM anisotropy

  • Spin current creates a non-conservative spin-transfer torque (STT) on AFM sublattice magnetizations Mj (j = 1, 2): τSTT =(τ/Ms)Mj ×(Mj ×p)[7,19,27], where p is the direction of the spin current polarization, Ms =|Mj| is the static magnetization of a sublattice, and τ is the amplitude of the spin current in the units of frequency[28]

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Summary

Introduction

An absence of compact and reliable generators and receivers of coherent signals in the frequency range 0.1– 10 THz has been identified as a fundamental physical and technological problem[1,2,3]. The presence in an AFM of two magnetic sublattices coupled by a strong exchange interaction qualitatively changes the magnetization dynamics of AFM18 It has been shown, that, in contrast with a FM, the STT acting on an AFM can lead to a dynamic instability in the magnetic sublattice orientation[3,16,19,20], which results in the rotation of the magnetizations of the AFM sublattices in the plane perpendicular to the direction of polarization of the applied spin current[16,19,20]. A DC spin current flowing from a current-driven Pt layer and polarized along the hard anisotropy axis of the adjacent AFM layer can excite a rotation of the AFM sublattice magnetizations[16,19,20] that is non-uniform in time due to the influence of a weak easy-plane AFM anisotropy This non-uniform rotation results in the THz-frequency spin-pumping back into the Pt layer, creating an AC spin current that can be detected using the inverse spin-Hall effect. The inertial nature of the AFM dynamics[18] leads to the hysteretic behavior of the AFM oscillator, which, have two different current thresholds: an “ignition” threshold, which is required to start the generation, and a lower “elimination” threshold which, in our case, is twice less the “ignition” threshold, defining the minimum current density needed to support the generation, once it has been started

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