arXiv subjects

T. Janda

Publications and source records attributed to T. Janda.

9 recordsLinked to original sources

arXiv2026

Sign-resolved nanoscale readout and control of hidden antiferromagnetic spin order

Antiferromagnetic memories promise ultrafast, stray-field-free information storage. Yet perfect magnetic compensation conceals the information carrier itself: the sign of the N\'eel vector distinguishing two time-reversed states. Moreover, in future dense memories, the local polarity of N\'eel domains would need to be read out on the nanoscale. We make this hidden polarity visible in a fully-compensated, high-N\'eel-temperature, PT-symmetric antiferromagnet by driving interband electric-dipole transitions by mid-infrared near-fields confined at a scanning probe. The excitation generates a N\'eel-order-dependent quantum-metric photocurrent, a Hall-like signal reversing with N\'eel order, which we term the optical nonlinear anomalous Hall effect. This optically induced electrical readout maps opposite N\'eel polarities with sub-100-nm resolution at room temperature and, combined with spin-orbit-torque writing, reveals N\'eel-texture polarization and reversible domain-wall motion, establishing electrical-write/optoelectronic-read antiferromagnetic functionality.

cond-mat.mtrl-sci↗

arXiv2022

Ultrashort spin-orbit torque generated by femtosecond laser pulses

To realize the very objective of spintronics, namely the development of ultra-high frequency and energy-efficient electronic devices, an ultrafast and scalable approach to switch magnetic bits is required. Magnetization switching with spin currents generated by the spin-orbit interaction at ferromagnetic/non-magnetic interfaces is one of such scalable approaches, where the ultimate switching speed is limited by the Larmor precession frequency. Understanding the magnetization precession dynamics induced by spin-orbit torques (SOTs) is therefore of great importance. Here we demonstrate generation of ultrashort SOT pulses that excite Larmor precession at an epitaxial Fe/GaAs interface by converting femtosecond laser pulses into high-amplitude current pulses in an electrically biased p-i-n photodiode. We control the polarity, amplitude, and duration of the current pulses and, most importantly, also their propagation direction with respect to the crystal orientation. The SOT origin of the excited Larmor precession was revealed by a detailed analysis of the precession phase and amplitude at different experimental conditions.

cond-mat.mtrl-sci↗

arXiv2018

A jigsaw puzzle metamaterial concept

A concept of a planar modular mechanical metamaterial inspired by the nature's principle of local adaptivity is proposed. The metamaterial consists of identical pieces similar to jigsaw puzzle tiles. Their rotation within assembly provides a substantial flexibility in terms of structural behavior and mechanical interlocks enable reassembly. The tile design with a diagonal elliptical opening allows us to vary elastic properties--from stiff to compliant, with positive, zero, or negative Poisson's ratio. The outcomes of experimental testing on additively manufactured specimens confirm that the assembly properties can be accurately designed using optimization approaches with finite element analysis at heart.

cond-mat.mtrl-sci↗

arXiv2018

Voigt effect-based wide-field magneto-optical microscope integrated in a pump-probe experimental setup

In this work we describe an experimental setup for spatially-resolved pump-probe experiment with integrated wide-field magneto-optical (MO) microscope. The MO microscope can be used to study ferromagnetic materials with both perpendicular-to-plane and in-plane magnetic anisotropy via polar Kerr and Voigt effects, respectively. The functionality of the Voigt effect-based microscope was tested using an in-plane magnetized ferromagnetic semiconductor (Ga,Mn)As. It was revealed that the presence of mechanical defects in the (Ga,Mn)As epilayer alters significantly the magnetic anisotropy in their proximity. The importance of MO experiments with simultaneous temporal and spatial resolutions was demonstrated using (Ga,Mn)As sample attached to a piezoelectric actuator, which produces a voltage-controlled strain. We observed a considerably different behavior in different parts of the sample that enabled us to identify sample parts where the epilayer magnetic anisotropy was significantly modified by a presence of the piezostressor and where it was not. Finally, we discuss the possible applicability of our experimental setup for the research of compensated antiferromagnets, where only MO effects even in magnetic moments are present.

cond-mat.mtrl-sci↗

arXiv2016

Inertial displacement of a domain wall excited by ultra-short circularly polarized laser pulses

Domain wall motion driven by ultra-short laser pulses is a prerequisite for envisaged low-power spintronics combining storage of information in magneto electronic devices with high speed and long distance transmission of information encoded in circularly polarized light. Here we demonstrate the conversion of the circular polarization of incident femtosecond laser pulses into inertial displacement of a domain wall in a ferromagnetic semiconductor. In our study we combine electrical measurements and magneto-optical imaging of the domain wall displacement with micromagnetic simulations. The optical spin transfer torque acts over a picosecond recombination time of the spin polarized photo-carriers which only leads to a deformation of the internal domain wall structure. We show that subsequent depinning and micro-meter distance displacement without an applied magnetic field or any other external stimuli can only occur due to the inertia of the domain wall.

cond-mat.mes-hall↗

arXiv2016

Enhancement of the spin Hall voltage in a reverse-biased planar pn-junction

We report an experimental demonstration of a local amplification of the spin Hall voltage using an expanding depletion zone at a pn-junction in GaAs/AlGaAs Hall-bar microdevices. It is demonstrated that the depletion zone can be spatially expanded by applying reverse bias by at least 10~$\mu$m at low temperature. In the depleted regime, the spin Hall signals reached more than one order of magnitude higher values than in the normal regime at the same electrical current flowing through the micro-device. It is shown that the pn-bias has two distinct effects on the detected spin Hall signal. It controls the local drift field at the Hall cross which is highly non-linear in the pn-bias due to the shift of the depletion front. Simultaneously, it produces a change in the spin-transport parameters due to the non-linear change in the carrier density at the Hall cross with the pn-bias.

cond-mat.mes-hall↗

arXiv2015

Nanosecond spin-transfer over tens of microns in a bare GaAs/AlGaAs layer

The spin-conserving length-scale is a key parameter determining functionalities of a broad range of spintronic devices including magnetic multilayer spin-valves in the commercialized magnetic memories or lateral spin transistors in experimental spin-logic elements. Spatially resolved optical pump-and-probe experiments in the lateral devices allow for the direct measurement of the lengthscale and the time-scale at which spin-information is transferred from the injector to the detector. Using this technique, we demonstrate that in an undoped GaAs/AlGaAs layer spins are detected at distances reaching more than ten microns from the injection point at times as short as nanoseconds after the pump-pulse. The observed unique combination of the long-range and highrate electronic spin-transport requires simultaneous suppression of mechanisms limiting the spin life-time and mobility of carriers. Unlike earlier attempts focusing on elaborate doping, gating, or heterostructures we demonstrate that the bare GaAs/AlGaAs layer intrinsically provides superior spin-transport characteristics whether deposited directly on the substrate or embedded in complex heterostructures.

cond-mat.mes-hall↗

arXiv2015

Optical investigation of magneto-structural phase transition in FeRh

Magneto-structural phase transition in FeRh epitaxial layers was studied optically. It is shown that the transition between the low-temperature antiferromagnetic phase and the high-temperature ferromagnetic phase is accompanied by a rather large change of the optical response in the visible and near infrared spectral ranges. This phenomenon was used to measure the phase transition temperature in FeRh films with thicknesses from 6 to 100 nm and it was observed that the hysteretic transition region broadens significantly in the thinner films.

cond-mat.mtrl-sci↗

arXiv2014

Optical spin transfer torque driven domain wall motion in ferromagnetic semiconductor

We demonstrate optical manipulation of the position of a domain wall in a dilute magnetic semiconductor, GaMnAsP. Two main contributions are identified. Firstly, photocarrier spin exerts a spin transfer torque on the magnetization via the exchange interaction. The direction of the domain wall motion can be controlled using the helicity of the laser. Secondly, the domain wall is attracted to the hot-spot generated by the focused laser. Unlike magnetic field driven domain wall depinning, these mechanisms directly drive domain wall motion, providing an optical tweezer like ability to position and locally probe domain walls.

cond-mat.mes-hall↗