- Page 4 of 5 -

Magnon thermal Edelstein effect in antiferromagnets

Posted on December 15, 2020 by Gyan Khatri

Efficient control of antiferromagnetic magnons in spin-dependent transport is essential for magnons to function as electrons to carry and deliver spin-based information. In an easy-plane antiferromagnet with the Dzyaloshinskii–Moriya interaction (DMI), magnons are subject to spin-momentum locking. An in-plane temperature gradient can then generate interfacial accumulation of magnons with a specified spin polarization, realizing the magnon thermal Edelstein effect. Cheng’s group theoretically investigated the injection and detection of this thermally driven spin polarization through an adjacent heavy metal. It is found that the detectable inverse spin Hall voltage has a sign determined not only by the thermal gradient but also by the DMI, which is in principle controllable by a gate voltage. This allows for the electrical and thermal control of the magnon spin polarization in an antiferromagnet, enabling new device functionalities.

This work has been published in Applied Physics Letters and can be accessed here.

Electrically tunable THz-detector based on an antiferromagnet

Posted on December 15, 2020 by Gyan Khatri

A concept of an electrically tunable resonance detector of THz-frequency signals based on an antiferromagnetic/heavy metal (AFM/HM) heterostructureis proposed. The conversion of a THz-frequency input signal into DC voltage is done using the inverse spin Hall effect in an (AFM/HM) bilayer. An additional bias DC in the HM layer can be used to vary the effective anisotropy of the AFM and, therefore, to tune the antiferromagnetic resonance (AFMR) frequency. The proposed AFM/HM heterostructure works as a resonance-type quadratic detector, which can be tuned by the bias current in the range of at least 10% of the AFMR frequency, and our estimations show that the sensitivity of this detector could be comparable to that of modern detectors based on the Schottky, Gunn, or graphene-based diodes. This work has been published in Applied Physics Letters and can be accessed here.

Using NV two-magnon relaxometry to detect high frequency, high wave-vector magnons generated by antiferromagnetic Néel Order switching

Posted on November 15, 2020 by Gyan Khatri

Identification of magnon populations in antiferromagnets and understanding of the associated magnon-magnon interactions is essential for the development of AF spintronic applications. A single NV spin is a powerful, nanoscale probe for local detection of magnons. As we and others have shown, the relaxation rate of the NV spin is sensitive to fluctuations of the magnetic dipolar fields generated by magnons. However, this detection technique was long thought to be limited to fluctuations at the NV resonance frequency (~2.9 GHz). We recently showed the effectiveness of two-magnon processes in relaxing NV moments. These two-magnon processes involve very high frequency/wavevector magnons whose difference frequency is constrained to match the NV frequency. Magnon systems in antiferromagnets remain poorly understood in large part due to the technical challenge of detecting high frequency magnetics. Our recent advance opens a route to NV detection and study of very high frequency magnon dynamics. This work has been published in nature communications and can be accessed here.

Nonlocal Uniform-Mode Ferromagnetic Resonance Spin Pumping

Posted on October 1, 2020 by Gyan Khatri

Magnonic transport is an important component in realizing THz antiferromagnetic spintronic devices. Due to the fundamental similarities between ferrimagnetic and antiferromagnetic insulators, magnonic transport studies in ferrimagnetic insulator heterostructures can provide valuable insights for their antiferromagnetic counterparts. Nonlocal spin transport using lateral structures is ideal for investigating lateral magnonic transport. Typically, a spin current is generated by a ferro-/ferri-magnetic (FM) or a heavy metal (HM) electrode in a nonlocal structure, which can be detected by another FM or HM electrode. The Yang and Hammel groups reported a new nonlocal spin injection scheme using uniform-mode ferromagnetic resonance spin pumping in Pt/Y₃Fe₅O₁₂ (YIG) lateral structures. This scheme is enabled by well-separated resonant fields of Pt/YIG and bare YIG due to substantial change of anisotropy in YIG films induced by a Pt overlayer, allowing for clearly distinguishable local and nonlocal spin pumping. These results show that the spin decay length of nonlocal uniform-mode spin pumping in 20 nm YIG films is 2.1 μm at room temperature, opening up a new path for spin current generation, propagation, and detection for future spintronic application.

This work has been published in Nano Letters and can be accessed here.

Ultrafast Sweep-Tuned Spectrum Analyzer with Temporal Resolution Based on a Spin-Torque Nano-Oscillator

Posted on September 2, 2020 by Gyan Khatri

We demonstrate that a spin-torque nano-oscillator (STNO) rapidly sweep-tuned by a bias voltage can be used to perform an ultrafast time-resolved spectral analysis of frequency-manipulated microwave signals. The critical reduction in the time of the spectral analysis comes from the naturally small-time constants of a nanosized STNO (1−100 ns). The demonstration is performed on a vortex-state STNO generating in a frequency range around 300 MHz, when frequency down-conversion and matched filtering is used for signal processing. It is shown that this STNO-based spectrum analyzer can perform analysis of frequency-agile signals, having multiple rapidly changing frequency components with temporal resolution in a μs time scale and frequency resolution limited only by the “bandwidth” theorem. Our calculations show that using uniform magnetization state STNOs it would be possible to increase the operating frequency of a spectrum analyzer to tens of GHz. This work has been published in Nano Letters and can be accessed here.

Unconventional spin currents in magnetic films

Posted on September 2, 2020 by Gyan Khatri

A spin current—a flow of spin angular momentum—can be carried either by spin-polarized free electrons or by magnons, the quanta of a moving collective oscillation of localized electron spins, i.e., a spin wave. Traditionally, it was assumed that a spin wave in a magnetic film with spin-sink-free surfaces can transfer energy and angular momentum only along its propagation direction. In this work, using Brillouin light-scattering spectroscopy in combination with a theory of dipole-exchange spin-wave spectra, we show that in obliquely magnetized free magnetic films, the in-plane propagation of spin waves is accompanied by a transverse spin current along the film normal without any corresponding transverse transport of energy. This work has been published in Physical Review Research and can be accessed here.

Theoretical study on antiferromagnetic-piezoelectric heterostructure

Posted on September 2, 2020 by Gyan Khatri

Voltage controlled anisotropy and current-induced magnetization dynamics in antiferromagnetic-piezoelectric heterostructures

It is shown theoretically that in a layered heterostructure comprising piezoelectric, dielectric antiferromagnetic crystal, and heavy metal (PZ/AFM/HM), it is possible to control the anisotropy of the AFM layer by applying a dc voltage across the PZ layer. In particular, we show that by varying the dc voltage across the heterostructure and/or the dc current in the HM, it is possible to vary the frequency of the antiferromagnetic resonance of the AFM in a passive (subcritical) regime and, also, to reduce the threshold of the current-induced terahertz-frequency generation. Our analysis also shows that, unfortunately, the voltage –induced reduction of the generation threshold leads to the proportional reduction of the amplitude of the terahertz-frequency signal generated in the active (supercritical) regime. The general results are illustrated by a calculation of the characteristics of experimentally realizable PZT-5H/NiO/Pt. This work has been published in Physical Review Applied and can be accessed here.

Spin-wave transmission through an internal boundary

Posted on September 2, 2020 by Gyan Khatri

Spin-wave transmission through an internal boundary: Beyond the scalar approximation

The transmission and reflection of a spin wave at an internal boundary created by the local variation of anisotropy (or a bias magnetic field) are studied taking into account not only the changes in the wave amplitude, but also the changes in the wave polarization. It is shown that the account of the changes in the spin-wave polarization before and after the boundary leads to (i) increase of the spin-wave amplitude reflection coefficient, (ii) appearance of an additional phase shift 0,π in both transmitted and reflected waves, and (iii) creation of additional evanescent waves in the vicinity of the boundary. It is also shown that even when significant changes in the transmitted wave polarization take place at the boundary, a spin wave could pass a finite-width boundary without reflection, if a certain resonance condition is satisfied. The effect of the polarization change at an internal boundary is especially pronounced for the exchange-dominated spin waves, while in the case of the dipole-dominated spin waves, this effect can vanish completely for certain configurations of the static magnetization. This work has been published in PRB and can be accessed here.

Bose-Einstein condensation of quasiparticles by rapid cooling

Posted on September 2, 2020 by Gyan Khatri

BEC of quasiparticles by rapid cooling

The fundamental phenomenon of Bose–Einstein condensation has been observed in different systems of real particles and quasiparticles. The condensation of real particles is achieved through a major reduction in temperature, while for quasiparticles, a mechanism of external injection of bosons by irradiation is required. Here, we present a new and universal approach to enable Bose–Einstein condensation of quasiparticles and to corroborate it experimentally by using magnons as the Bose-particle model system. The critical point to this approach is the introduction of a disequilibrium of magnons with the phonon bath. After heating to an elevated temperature, a sudden decrease in the temperature of the phonons, which is approximately instant on the time scales of the magnon system, results in a large excess of incoherent magnons. The consequent spectral redistribution of these magnons triggers the Bose–Einstein condensation. This work has been published in Nature Nanotechnology and can be accessed here.

THz frequency Spectrum analysis

Posted on September 2, 2020 by Gyan Khatri

THz frequency spectrum analysis with a nanoscale AFM tunnel junction

A method to perform spectrum analysis on low power signals having frequency between 0.1 and 10 THz is proposed. It utilizes a nanoscale antiferromagnetic tunnel junction (ATJ) that produces an oscillating tunneling anisotropic magnetoresistance, whose frequency is dependent on the magnitude of the driving DC current. It is shown that the ATJ output voltage is strongly dependent on the thickness of the dielectric tunneling barrier. Spectrum analysis can be performed using an ATJ, whose frequency is linearly swept with time, and a low-pass filter, and a matched filter are used for signal processing. It is also found by simulation and analytical calculations that for an ATJ with a 0.1 μm² footprint, the spectrum analysis can be performed over a 0.25 THz bandwidth in just 25 ns on signals that are just above the Johnson–Nyquist thermal noise floor. This work has been published in Journal of Applied Physics and can be accessed here.