Gyan Khatri, Author at

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.

Paper published in Science

Posted on September 2, 2020 by Gyan Khatri

Sub-THz coherent spin pumping from an insulating antiferromagnet

Sub-terahertz coherent spin pumping from an insulating antiferromagnet (del Barco, Lederman and Cheng with van Tol, and Brataas)

Spin-transfer torque and spin Hall effects combined with their reciprocal phenomena, spin pumping and inverse spin Hall effects (ISHEs), enable the reading and control of magnetic moments in spintronics. The MURI team has reported for the first time sub-terahertz spin pumping at the interface of the uniaxial insulating antiferromagnet manganese difluoride and platinum. The measured ISHE voltage arising from spin-charge conversion in the platinum layer depends on the chirality of the dynamical modes of the antiferromagnet, which is selectively excited and modulated by the handedness of the circularly polarized sub-terahertz irradiation. Our results open the door to the controlled generation of coherent, pure spin currents at terahertz frequencies. You can access the paper here.