arXiv subjects

P. Wadley

Publications and source records attributed to P. Wadley.

At least 19 recordsLinked to original sources

arXiv2026

Discerning ground state and photoemission-induced spin textures in altermagnetic $\alpha$-MnTe

The recently discovered class of altermagnets provide a physical realization of an unconventional compensated magnetic phase with a higher partial-wave type of ordering, reminiscent of unconventional superfluid phases. Their stability under normal conditions has sparked significant research interest, spanning fields from spintronics to topological and correlated quantum materials. Spin- and angle-resolved photoemission spectroscopy (SARPES) has great promise to resolve the momentum-dependent spin textures, which are intricately interweaved with the altermagnetic direct space spin order. Using the relativistic $d$-wave-like collinear spin polarization on one of the non-relativistic nodal surfaces of the altermagnetic band structure of $\alpha$-MnTe as an example, we here identify and resolve the challenges associated with (S)ARPES studies on altermagnets. We focus particularly on the role of photoemission-induced electron polarization and the coupling between light and the N\'eel vector of a magnetic domain. Our findings reveal an atypical behaviour of photoemission selection rules while using linearly-polarized light. Our methods allow to distinguish polarization of photoelectrons originating from the sample's ground state spin texture, on one hand, and from the photoemission process, on the other hand. Our experimental results are supported by a combination of ab initio band-structure and 1-step photoemission calculations.

cond-mat.str-el↗

arXiv2025

Unconventional relativistic spin polarization of electronic bands in an altermagnet

Altermagnetism is a recently identified phase with a d, g or i-wave spin symmetry of magnetic ordering. Its discovery opens new research fronts at intersections of magnetism and spintronics with fields ranging from superconductivity to topological and relativistic quantum physics. Here we demonstrate an unconventional relativistic spin polarization in an altermagnet by spin and angle resolved photoemission spectroscopy of electronic bands in single-domain MnTe. The relativistic spin-orbit coupling origin is revealed by observing that the alternating momentum-dependent spin polarization is orthogonal to the magnetic-ordering vector. The collinearity, even-parity and time-reversal-odd nature of the demonstrated relativistic spin polarization in the altermagnet is unparalleled in conventional forms of the relativistic spin polarization. Our experimental results and methodology are supported by non-relativistic spin-symmetry and relativistic magnetic-symmetry analyses, and microscopic ab initio ground-state and photoemission theory.

cond-mat.mes-hall↗

arXiv2025

Altermagnetic spintronics

The research landscape of magnetism has been recently enriched by the discovery of altermagnetism. It is an unconventional phase of matter characterized by a d-wave (or higher even-parity-wave) collinear compensated spin ordering, which enables strongly spin-polarized currents in the absence of magnetization, and features fast spin dynamics. Simultaneously, on the applied magnetism front, spintronic memories based on conventional ferromagnets are currently turning from a niche to a mass produced integrated-circuit technology as they start to complement semiconductors on advanced-node microprocessor chips. Our review connects these two rapidly developing science and technology fields by discussing how the unique signatures of altermagnetism can impact the functionality and scalability of future spintronic devices. As a reference, we first briefly recall the merits and physical limitations of the present ferromagnetic spintronic technology, and of proof-of-concept spintronic devices based on conventional collinear antiferromagnets and non-collinear compensated magnets. The main part of the review then focuses on physical concepts of the altermagnetic spintronics, and its potential interplay with ferroelectricity or superconductivity. We conclude with an outlook on the nascent experimental research of altermagnetic spintronics, and on the role of relativistic phenomena.

cond-mat.mtrl-sci↗

arXiv2024

Altermagnetism imaged and controlled down to the nanoscale

Nanoscale detection and control of the magnetic order underpins a broad spectrum of fundamental research and practical device applications. The key principle involved is the breaking of time-reversal ($\cal{T}$) symmetry, which in ferromagnets is generated by an internal magnetization. However, the presence of a net-magnetization also imposes severe limitations on compatibility with other prominent phases ranging from superconductors to topological insulators, as well as on spintronic device scalability. Recently, altermagnetism has been proposed as a solution to this restriction, since it shares the enabling $\cal{T}$-symmetry breaking characteristic of ferromagnetism, combined with the antiferromagnetic-like vanishing net-magnetization. To date, altermagnetic ordering has been inferred from spatially averaged probes. Here, we demonstrate nanoscale imaging and control of altermagnetic ordering ranging from nanoscale vortices to domain walls to microscale single-domain states in MnTe. We combine the $\cal{T}$-symmetry breaking sensitivity of X-ray magnetic circular dichroism with magnetic linear dichroism and photoemission electron microscopy, to achieve detailed imaging of the local altermagnetic ordering vector. A rich variety of spin configurations can be imposed using microstructure patterning or thermal cycling in magnetic fields. The demonstrated detection and control of altermagnetism paves the way for future research ranging from ultra-scalable digital and neuromorphic spintronic devices, to the interplay of altermagnetism with non-dissipative superconducting or topological phases.

cond-mat.mtrl-sci↗

arXiv2024

Thermally Stable Peltier Controlled Vacuum Chamber for Electrical Transport Measurements

The design, manufacture and characterisation of an inexpensive, temperature controlled vacuum chamber with millikelvin stability for electrical transport measurements at and near room temperature is reported. A commercially available Peltier device and high-precision temperature controller are used to actively heat and cool the sample space. The system was designed to minimise thermal fluctuations in spintronic and semiconductor transport measurements but the general principle is relevant to a wide range of electrical measurement applications. The main issues overcome are the mounting of a sample with a path of high thermal conductivity through to the Peltier device and the heat-sinking of said Peltier device inside of a vacuum. A copper slug is used as the mount for a sample and a large copper block is used as a thermal feedthrough before a passive heatsink is used to cool this block. The Peltier device provides 20 W of heating and cooling power achieving a maximum range of 30 K below and 40 K above the ambient temperature. The temperature stability is within 5 mK at all set points with even better performance above ambient temperature. A vacuum pressure of 1e-8 hPa is achievable. As a demonstration, we present experimental results from current-induced electrical switching of a CuMnAs thin film. Transport measurements with and without the Peltier control emphasise the importance of a constant temperature in these applications. The thermal lag between the sample space measurement and the sample itself is observed through magnetoresistance values measured during a temperature sweep.

physics.ins-det↗

arXiv2023

Altermagnetic lifting of Kramers spin degeneracy

Lifted Kramers spin-degeneracy has been among the central topics of condensed-matter physics since the dawn of the band theory of solids. It underpins established practical applications as well as current frontier research, ranging from magnetic-memory technology to topological quantum matter. Traditionally, lifted Kramers spin-degeneracy has been considered to originate from two possible internal symmetry-breaking mechanisms. The first one refers to time-reversal symmetry breaking by magnetization of ferromagnets, and tends to be strong due to the non-relativistic exchange-coupling origin. The second mechanism applies to crystals with broken inversion symmetry, and tends to be comparatively weaker as it originates from the relativistic spin-orbit coupling. A recent theory work based on spin-symmetry classification has identified an unconventional magnetic phase, dubbed altermagnetic, that allows for lifting the Kramers spin degeneracy without net magnetization and inversion-symmetry breaking. Here we provide the confirmation using photoemission spectroscopy and ab initio calculations. We identify two distinct unconventional mechanisms of lifted Kramers spin degeneracy generated by the altermagnetic phase of centrosymmetric MnTe with vanishing net magnetization. Our observation of the altermagnetic lifting of the Kramers spin degeneracy can have broad consequences in magnetism. It motivates exploration and exploitation of the unconventional nature of this magnetic phase in an extended family of materials, ranging from insulators and semiconductors to metals and superconductors, that have been either identified recently or perceived for many decades as conventional antiferromagnets.

physics.app-ph↗

arXiv2023

X-ray Magnetic Circular Dichroism in Altermagnetic $\alpha$-MnTe

Altermagnetism is a recently identified magnetic symmetry class combining characteristics of conventional collinear ferromagnets and antiferromagnets, that were regarded as mutually exclusive, and enabling phenomena and functionalities unparalleled in either of the two traditional elementary magnetic classes. In this work we use symmetry and ab initio theory to explore X-ray magnetic circular dichroism (XMCD) in the altermagnetic class. Our results highlight the distinct phenomenology in altermagnets of this time-reversal symmetry breaking response, and its potential utility for element-specific spectroscopy and microscopy in altermagnets. As a representative material for our XMCD study we choose $\alpha$-MnTe with the compensated antiparallel magnetic order in which an anomalous Hall effect has been already demonstrated both in theory and experiment. The predicted magnitude of XMCD lies well within the resolution of existing experimental techniques.

cond-mat.mtrl-sci↗

arXiv2022

Antiferromagnetic half-skyrmions electrically generated and controlled at room temperature

Topologically protected magnetic textures, such as skyrmions, half-skyrmions (merons) and their antiparticles, constitute tiny whirls in the magnetic order. They are promising candidates for information carriers in next-generation memory devices, as they can be efficiently propelled at very high velocities using current-induced spin torques. Antiferromagnets have been shown to host versions of these textures, which have gained significant attention because of their potential for terahertz dynamics, deflection free motion, and improved size scaling due to the absence of stray field. Here we show that topological spin textures, merons and antimerons, can be generated at room temperature and reversibly moved using electrical pulses in thin film CuMnAs, a semimetallic antiferromagnet that is a testbed system for spintronic applications. The electrical generation and manipulation of antiferromagnetic merons is a crucial step towards realizing the full potential of antiferromagnetic thin films as active components in high density, high speed magnetic memory devices.

cond-mat.other↗

arXiv2021

Spin flop and crystalline anisotropic magnetoresistance in CuMnAs

Recent research works have shown that the magnetic order in some antiferromagnetic materials can be manipulated and detected electrically, due to two physical mechanisms: Neel-order spin-orbit torques and anisotropic magnetoresistance. While these observations open up opportunities to use antiferromagnets for magnetic memory devices, different physical characterization methods are required for a better understanding of those mechanisms. Here we report a magnetic field induced rotation of the antiferromagnetic Neel vector in epitaxial tetragonal CuMnAs thin films. Using soft x-ray magnetic linear dichroism spectroscopy, x-ray photoemission electron microscopy, integral magnetometry and magneto-transport methods, we demonstrate spin-flop switching and continuous spin reorientation in antiferromagnetic films with uniaxial and biaxial magnetic anisotropies, respectively. From field-dependent measurements of the magnetization and magnetoresistance, we obtain key material parameters including the anisotropic magnetoresistance coefficients, magnetocrystalline anisotropy, spin-flop and exchange fields.

cond-mat.mtrl-sci↗

arXiv2021

Low-Energy Switching of Antiferromagnetic CuMnAs/ GaP Using sub-10 Nanosecond Current Pulses

The recently discovered electrical-induced switching of antiferromagnetic (AF) materials that have spatial inversion asymmetry has enriched the field of spintronics immensely and opened the door for the concept of antiferromagnetic MRAM. CuMnAs is one promising AF material that exhibits such electrical switching ability, and has been studied to switch using electrical pulses of length millisecond down to picosecond, but with little focus on nanosecond regime. We demonstrate here switching of CuMnAs/GaP using nanosecond pulses. Our results showed that in the nanosecond regime low-energy switching, high readout signal with highly reproducible behaviour down to a single pulse can be achieved. Moreover, a comparison of the two switching methods of orthogonal switching and polarity switching was done on same device showing two different behaviours that can be exploited selectively for different future memory/processing applications.

cond-mat.mtrl-sci↗

arXiv2020

Investigation of Magnetic Anisotropy and Heat Dissipation in Thin Films of Compensated Antiferromagnet CuMnAs by Pump-probe Experiment

We recently reported on a method to determine the easy axis position in a 10 nm thick film of the fully compensated antiferromagnet CuMnAs. The film had a uniaxial magnetic anisotropy and the technique utilized a magneto-optical pump and probe experiment [Nature Photonics 11, 91 (2017)]. In this contribution we discuss the applicability of this method for the investigation of a broader set of epitaxial CuMnAs films having different thicknesses. This work reveals that the equilibrium magnetic anisotropy can be studied only in samples where this anisotropy is rather strong. However, in the majority of CuMnAs films, the impact of a strong pump pulse induces nano-fragmentation of the magnetic domains and, therefore, the magnetic anisotropy measured by the pump-probe technique differs substantially from that in the equilibrium conditions. We also demonstrate that optical pump-probe experiment can be used very efficiently to study the local heating and heat dissipation in CuMnAs epitaxial layers. In particular, we determined the electron-phonon relaxation time in CuMnAs. We also observed that for a local film heating by a focused laser the thinner films are heated more, but the heat is dissipated considerably faster than in the case of thicker films. This illustrates that the optical pump-probe experiment is a valuable characterization tool for the heat management optimization in the CuMnAs memory devices and can be applied in a similar way to those used during heat-assisted magnetic recording (HAMR) technology development for the latest generation of hard drive disks.

cond-mat.mtrl-sci↗

arXiv2019

Current-induced fragmentation of antiferromagnetic domains

Electrical and optical pulsing allow for manipulating the order parameter and magnetoresistance of antiferromagnets, opening novel prospects for digital and analog data storage in spintronic devices. Recent experiments in CuMnAs have demonstrated giant resistive switching signals in single-layer antiferromagnetic films together with analog switching and relaxation characteristics relevant for neuromorphic computing. Here we report simultaneous electrical pulsing and scanning NV magnetometry of antiferromagnetic domains in CuMnAs performed using a pump-probe scheme. We observe a nano-scale fragmentation of the antiferromagnetic domains, which is controlled by the current amplitude and independent on the current direction. The fragmented antiferromagnetic state conserves a memory of the pristine domain pattern, towards which it relaxes. Domain fragmentation coexists with permanent switching due to the reorientation of the antiferromagnetic moments. Our simultaneous imaging and resistance measurements show a correlation between the antiferromagnetic domain fragmentation and the largest resistive switching signals in CuMnAs.

cond-mat.mtrl-sci↗

arXiv2019

Gating effects in antiferromagnetic CuMnAs

Antiferromagnets (AFs) attract much attention due to potential applications in spintronics. Both the electric current and the electric field are considered as tools suitable to control properties and the Néel vector direction of AFs. Among AFs, CuMnAs has been shown to exhibit specific properties that result in the existence of the current-induced spin-orbit torques commensurate with spin directions and topological Dirac quasiparticles. Here, we report on the observation of a reversible effect of an electric field on the resistivity of CuMnAs thin films, employing ionic liquid as a gate insulator. The data allow to determine the carrier type, concentration, and mobility independently of the Hall effect that may be affected by an anomalous component.

cond-mat.mtrl-sci↗

arXiv2019

Stability and lifetime of antiferromagnetic skyrmions

The two-dimensional Heisenberg exchange model with out-of-plane anisotropy and a Dzyaloshinskii-Moriya interaction is employed to investigate the lifetime and stability of antiferromagnetic (AFM) skyrmions as a function of temperature and external magnetic field. An isolated AFM skyrmion is metastable at zero temperature in a certain parameter range set by two boundaries separating the skyrmion state from the uniform AFM phase and a stripe domain phase. The distribution of the energy barriers for the AFM skyrmion decay into the uniform AFM state complements the zero-temperature stability diagram and demonstrates that the skyrmion stability region is significantly narrowed at finite temperatures.We show that the AFM skyrmion stability can be enhanced by an application of magnetic field, whose strength is comparable to the spin-flop field. This stabilization of AFM skyrmions in external magnetic fields is in sharp contrast to the behavior of their ferromagnetic counterparts. Furthermore, we demonstrate that the AFM skyrmions are stable on timescales of milliseconds below 50 K for realistic material parameters, making it feasible to observe them in modern experiments.

cond-mat.mes-hall↗

arXiv2018

Negative spin-Hall angle and anisotropic spin-orbit torques in epitaxial IrMn

A spin-torque ferromagnetic resonance study is performed in epitaxial $\mathrm{Fe / Ir_{15}Mn_{85}}$ bilayers with different Fe thicknesses. We measure a negative spin-Hall angle of a few percent in the antiferromagnetic IrMn in contrast to previously reported positive values. A large spin-orbit field with Rashba symmetry opposing the Oersted field is also present. Magnitudes of measured spin-orbit torques depend on the crystallographic direction of current and are correlated with the exchange bias direction set during growth. We suggest that the uncompensated moments at the Fe / IrMn interface are responsible for the observed anisotropy. Our findings highlight the importance of crystalline and magnetic structures for the spin-Hall effect in antiferromagnets.

cond-mat.mes-hall↗

arXiv2017

Band structure of CuMnAs probed by optical and photoemission spectroscopy

Tetragonal phase of CuMnAs progressively appears as one of the key materials for antiferromagnetic spintronics due to efficient current-induced spin-orbit torques whose existence can be directly inferred from crystal symmetry. Theoretical understanding of spintronic phenomena in this material, however, relies on the detailed knowledge of electronic structure (band structure and corresponding wave functions) which has so far been tested only to a limited extent. We show that AC permittivity (obtained from ellipsometry) and UV photoelectron spectra agree with density functional calculations. Together with the x-ray diffraction and precession electron diffraction tomography, our analysis confirms recent theoretical claim [Phys.Rev.B 96, 094406 (2017)] that copper atoms occupy lattice positions in the basal plane of the tetragonal unit cell.

cond-mat.mtrl-sci↗

arXiv2017

Current-polarity dependent manipulation of antiferromagnetic domains

Antiferromagnets have a number of favourable properties as active elements in spintronic devices, including ultra-fast dynamics, zero stray fields and insensitivity to external magnetic fields . Tetragonal CuMnAs is a testbed system in which the antiferromagnetic order parameter can be switched reversibly at ambient conditions using electrical currents . In previous experiments, orthogonal in-plane current pulses were used to induce 90 degree rotations of antiferromagnetic domains and demonstrate the operation of all-electrical memory bits in a multi-terminal geometry . Here, we demonstrate that antiferromagnetic domain walls can be manipulated to realize stable and reproducible domain changes using only two electrical contacts. This is achieved by using the polarity of the current to switch the sign of the current-induced effective field acting on the antiferromagnetic sublattices. The resulting reversible domain and domain wall reconfigurations are imaged using x-ray magnetic linear dichroism microscopy, and can also be detected electrically. The switching by domain wall motion can occur at much lower current densities than those needed for coherent domain switching.

cond-mat.mtrl-sci↗

arXiv2017

THz electrical writing speed in an antiferromagnetic memory

The speed of writing of state-of-the-art ferromagnetic memories is physically limited by an intrinsic GHz threshold. Recently, an alternative research direction has been initiated by realizing memory devices based on antiferromagnets in which spin directions periodically alternate from one atomic lattice site to the next. In our work we experimentally demonstrate at room temperature that the speed of reversible electrical writing in a memory device can be scaled up to THz using an antiferromagnet. Efficient current-induced spin-torque mechanism is responsible for the switching in our memory devices throughout the twelve orders of magnitude range of writing speeds from Hz to THz. Our work opens the path towards the development of memory-logic technology reaching the elusive THz band.

physics.app-ph↗