arXiv subjects

Martin Jourdan

Publications and source records attributed to Martin Jourdan.

At least 19 recordsLinked to original sources

arXiv2026

Engineering altermagnetic symmetry to enable anomalous Hall response in Cr$_{1-x}$Mn$_x$Sb

Altermagnets are a promising class of materials for spintronic applications. However, compounds that simultaneously combine the symmetry required to support an anomalous Hall effect with good metallic conductivity and magnetic ordering temperatures well above room temperature have remained elusive. Here, we demonstrate that partial substitution of Cr by Mn in epitaxial CrSb(100) thin films provides a viable route to engineer the combined structural and magnetic symmetry necessary to enable an otherwise symmetry-forbidden anomalous Hall effect. We systematically explore the magnetic phase diagram of Cr(Mn)Sb thin films including neutron diffraction. This allows us to identify a magnetic symmetry that supports the anomalous Hall effect, which we demonstrate in Cr(0.75)Mn(0.25)Sb. Guided by Landau theory, we model the field-driven reorientation of the Neel vector and the resulting anomalous Hall response, achieving good qualitative agreement with the experimental observations.

cond-mat.mtrl-sci↗

arXiv2025

Terahertz switching of antiferromagnetic order by N\'eel spin-orbit torques

Ultrafast electric manipulation of magnetic order in solids is critical for the development of future terahertz data processing. A fascinating concept for such high-speed operation is offered in metallic antiferromagnets by N\'eel spin-orbit torque. It should allow one to coherently rotate the ordered spins by simply applying an electric current of suitable amplitude and polarity. However, such switching has been severely hampered by competing heat-induced effects, and it has not yet been achieved on the intrinsically ultrafast time scales of antiferromagnets. Here, we report robust, direction-controlled and non-thermal rotation of the N\'eel vector $\mathbf{L}$ by $\pm$90{\deg} at room temperature in the antiferromagnet Mn$_2$Au driven by phase-locked terahertz current pulses. All observed features are consistent with ultrafast N\'eel spin-orbit torque: First, nonlinear optical imaging reveals that the terahertz current direction sets the final orientation of $\mathbf{L}$ in the absence of any bias field for at least two months. Second, transient optical birefringence shows that the switching proceeds ultrafast in less than 15 picoseconds. Finally, atomistic spin-dynamics simulations reproduce the observed dynamics and confirm the minor role of thermal effects. While the switching is already one order of magnitude faster than in ferromagnets at comparable dissipated energy, our simulations predict routes toward switching times and energies which are another order of magnitude lower. Our approach can be transferred to electric-field-driven switching in many more antiferromagnets, including magnetoelectric insulators. The engineering of spin torques, resonance frequencies and read-out mechanisms provides an exciting pathway toward on-chip applications of terahertz antiferromagnetic spin-orbitronics.

cond-mat.mes-hall↗

arXiv2024

Identifying Switching of Antiferromagnets by Spin-Orbit Torques

Antiferromagnets are promising candidates for ultrafast spintronic applications, leveraging current-induced spin-orbit torques. However, experimentally distinguishing between different switching mechanisms of the staggered magnetization (N\'eel vector) driven by current pulses remains a challenge. In an exemplary study of the collinear antiferromagnetic compound Mn$_2$Au, we demonstrate that slower thermomagnetoelastic effects predominantly govern switching over a wide parameter range. In the regime of short current pulses in the nanosecond range, however, we observe fully N\'eel spin-orbit torque driven switching. We show that this ultrafast mechanism enables the complete directional alignment of the N\'eel vector by current pulses in device structures.

physics.app-ph↗

arXiv2024

All-optical magnetometric characterization of the antiferromagnetic exchange-spring system Mn$_2$Au|Py by terahertz spin-torques

Antiferromagnetic materials have great potential for spintronic applications at terahertz (THz) frequencies. However, in contrast to ferromagnets, experimental studies of antiferromagnets are often challenging due to a lack of straightforward external control of the N\'eel vector $\mathbf{L}$. Here, we study an AFM|FM stack consisting of an antiferromagnetic metal layer (AFM) of the novel material Mn2Au and a ferromagnetic metal layer (FM) of NiFe. In this exchange-spring system, $\mathbf{L}$ of AFM Mn2Au can be controlled by the application of an external magnetic field B_ext. To characterize the AFM|FM stack as a function of the quasi-static $\mathbf{B}_{\mathrm{ext}}$, we perform THz-pump magneto-optic probe experiments. We identify signal components that can consistently be explained by the in-plane antiferromagnetic magnon mode excited by field-like N\'eel spin-orbit torques (NSOTs). Remarkably, we find that the $\mathbf{B}_{\mathrm{ext}}$- and THz-pump-induced changes in the optical response of the sample are dominated exclusively by the spin degrees of freedom of AFM. We fully calibrate the magnetic circular and magnetic linear optical birefringence of AFM and extract the efficiency of the NSOTs. Finally, by selective excitation of domains with different orientation of $\mathbf{L}$, we are able to determine the relative volume fraction of 0{\deg}, 90{\deg}, 180{\deg} and 270{\deg} domains distribution during the quasi-static reversal of $\mathbf{L}$ by $\mathbf{B}_{\mathrm{ext}}$. Our insights are an important prerequisite for future studies of ultrafast coherent switching of spins by THz NSOTs and show that THz-pump magneto-optic-probe experiments are a powerful tool to characterize magnetic properties of antiferromagnets.

cond-mat.mes-hall↗

arXiv2023

Direct observation of altermagnetic band splitting in CrSb thin films

Altermagnetism represents an emergent collinear magnetic phase with compensated order and an unconventional alternating even-parity wave spin order in the non-relativistic band structure. We investigate directly this unconventional band splitting near the Fermi energy through spinintegrated soft X-ray angular resolved photoemission spectroscopy. The experimentally obtained angle-dependent photoemission intensity, acquired from epitaxial thin films of the predicted altermagnet CrSb, demonstrates robust agreement with the corresponding band structure calculations. In particular, we observe the distinctive splitting of an electronic band on a low-symmetry path in the Brilliouin zone that connects two points featuring symmetry-induced degeneracy. The measured large magnitude of the spin splitting of approximately 0.6 eV and the position of the band just below the Fermi energy underscores the signifcance of altermagnets for spintronics based on robust broken time reversal symmetry responses arising from exchange energy scales, akin to ferromagnets, while remaining insensitive to external magnetic fields and possessing THz dynamics, akin to antiferromagnets.

cond-mat.mtrl-sci↗

arXiv2023

Nonlinear terahertz N\'eel spin-orbit torques in antiferromagnetic Mn$_2$Au

Antiferromagnets have large potential for ultrafast coherent switching of magnetic order with minimum heat dissipation. In novel materials such as Mn$_2$Au and CuMnAs, electric rather than magnetic fields may control antiferromagnetic order by N\'eel spin-orbit torques (NSOTs), which have, however, not been observed on ultrafast time scales yet. Here, we excite Mn$_2$Au thin films with phase-locked single-cycle terahertz electromagnetic pulses and monitor the spin response with femtosecond magneto-optic probes. We observe signals whose symmetry, dynamics, terahertz-field scaling and dependence on sample structure are fully consistent with a uniform in-plane antiferromagnetic magnon driven by field-like terahertz NSOTs with a torkance of (150$\pm$50) cm$^2$/A s. At incident terahertz electric fields above 500 kV/cm, we find pronounced nonlinear dynamics with massive N\'eel-vector deflections by as much as 30{\deg}. Our data are in excellent agreement with a micromagnetic model which indicates that fully coherent N\'eel-vector switching by 90{\deg} within 1 ps is within close reach.

cond-mat.mes-hall↗

arXiv2023

Fiber-tip spintronic terahertz emitters

Spintronic terahertz emitters promise terahertz sources with an unmatched broad frequency bandwidth that are easy to fabricate and operate, and therefore easy to scale at low cost. However, current experiments and proofs of concept rely on free-space ultrafast pump lasers and rather complex benchtop setups. This contrasts with the requirements of widespread industrial applications, where robust, compact, and safe designs are needed. To meet these requirements, we present a novel fiber-tip spintronic terahertz emitter solution that allows spintronic terahertz systems to be fully fiber-coupled. Using single-mode fiber waveguiding, the newly developed solution naturally leads to a simple and straightforward terahertz near-field imaging system with a 90%-10% knife-edge-response spatial resolution of 30 ${\mu}m$.

physics.optics↗

arXiv2023

Magnetic domain engineering in antiferromagnetic CuMnAs and Mn$_2$Au devices

Antiferromagnetic materials hold potential for use in spintronic devices with fast operation frequencies and field robustness. Despite the rapid progress in proof-of-principle functionality in recent years, there has been a notable lack of understanding of antiferromagnetic domain formation and manipulation, which translates to either incomplete or non-scalable control of the magnetic order. Here, we demonstrate simple and functional ways of influencing the domain structure in CuMnAs and Mn2Au, two key materials of antiferromagnetic spintronics research, using device patterning and strain engineering. Comparing x-ray microscopy data from two different materials, we reveal the key parameters dictating domain formation in antiferromagnetic devices and show how the non-trivial interaction of magnetostriction, substrate clamping and edge anisotropy leads to specific equilibrium domain configurations. More specifically, we observe that patterned edges have a significant impact on the magnetic anisotropy and domain structure over long distances, and we propose a theoretical model that relates short-range edge anisotropy and long-range magnetoelastic interactions. The principles invoked are of general applicability to the domain formation and engineering in antiferromagnetic thin films at large, which will pave the way towards realizing truly functional antiferromagnetic devices.

cond-mat.mtrl-sci↗

arXiv2023

Coupling of ferromagnetic and antiferromagnetic spin dynamics in Mn$_{2}$Au/NiFe thin-film bilayers

We investigate magnetization dynamics of Mn$_{2}$Au/Py (Ni$_{80}$Fe$_{20}$) thin film bilayers using broadband ferromagnetic resonance (FMR) and Brillouin light scattering spectroscopy. Our bilayers exhibit two resonant modes with zero-field frequencies up to almost 40 GHz, far above the single-layer Py FMR. Our model calculations attribute these modes to the coupling of the Py FMR and the two antiferromagnetic resonance (AFMR) modes of Mn2Au. The coupling-strength is in the order of 1.6 T$\cdot$nm at room temperature for nm-thick Py. Our model reveals the dependence of the hybrid modes on the AFMR frequencies and interfacial coupling as well as the evanescent character of the spin waves that extend across the Mn$_{2}$Au/Py interface

cond-mat.mtrl-sci↗

arXiv2022

Current-driven writing process in antiferromagnetic Mn2Au for memory applications

Current pulse driven Neel vector rotation in metallic antiferromagnets is one of the most promising concepts in antiferromagnetic spintronics. We show microscopically that the Neel vector of epitaxial thin films of the prototypical compound Mn2Au can be reoriented reversibly in the complete area of cross shaped device structures using single current pulses. The resulting domain pattern with aligned staggered magnetization is long term stable enabling memory applications. We achieve this switching with low heating of 20 K, which is promising regarding fast and efficient devices without the need for thermal activation. Current polarity dependent reversible domain wall motion demonstrates a Neel spin-orbit torque acting on the domain walls.

physics.app-ph↗

arXiv2022

Optically-triggered strain-driven N\'{e}el vector manipulation in a metallic antiferromagnet

The absence of stray fields, their insensitivity to external magnetic fields, and ultrafast dynamics make antiferromagnets promising candidates for active elements in spintronic devices. Here, we demonstrate manipulation of the N\'{e}el vector in the metallic collinear antiferromagnet Mn$_2$Au by combining strain and femtosecond laser excitation. Applying tensile strain along either of the two in-plane easy axes and locally exciting the sample by a train of femtosecond pulses, we align the N\'{e}el vector along the direction controlled by the applied strain. The dependence on the laser fluence and strain suggests the alignment is a result of optically-triggered depinning of 90$^{\mathrm{o}}$ domain walls and their sliding in the direction of the free energy gradient, governed by the magneto-elastic coupling. The resulting, switchable, state is stable at room temperature and insensitive to magnetic fields. Such an approach may provide ways to realize robust high-density memory device with switching timescales in the picosecond range.

cond-mat.mtrl-sci↗

arXiv2021

Direct Observation of Antiferromagnetic Parity Violation in the Electronic Structure of Mn$_2$Au

Parity symmetric photoemission spectra are ubiquitous in solid state research, being prevalent in many highly active areas, such as unconventional superconductors, nonmagnetic and antiferromagnetic topological insulators, and weakly relativistic collinear magnets, among others. The direct observation of parity-violating metallic Kramers degenerate bands has remained hitherto experimentally elusive. Here we observe the antiferromagnetic parity violation (APV) in the bandstructure of Mn$_2$Au thin films by using momentum microscopy with sub-$mu$m spatial resolution, allowing momentum resolved photoemission on single antiferromagnetic domains. The APV arises from breaking the P symmetry of the underlying crystal structure by the collinear antiferromagnetism, while preserving the joint space-time inverison PT -symmetry and in combination with large spin-orbit coupling. In addition, our work also demonstrates a novel tool to directly image the Neel vector direction by combining spatially resolved momentum microscopy with ab-initio calculations.

cond-mat.mtrl-sci↗

arXiv2021

Optical read-out of the N\'eel vector in metallic antiferromagnet Mn$_{2}$Au

Metallic antiferromagnets with broken inversion symmetry on the two sublattices, strong spin-orbit coupling and high N\'{e}el temperatures offer new opportunities for applications in spintronics. Especially Mn$_{2}$Au, with high N\'{e}el temperature and conductivity, is particularly interesting for real-world applications. Here, manipulation of the orientation of the staggered magnetization,\textit{\ i.e.} the N\'{e}el vector, by current pulses has been recently demonstrated, with the read-out limited to studies of anisotropic magnetoresistance or X-ray magnetic linear dichroism. Here, we report on the in-plane reflectivity anisotropy of Mn$_{2}$Au (001) films, which were N\'{e}el vector aligned in pulsed magnetic fields. In the near-infrared, the anisotropy is $\approx$ 0.6\%, with higher reflectivity for the light polarized along the N\'{e}el vector. The observed magnetic linear dichroism is about four times larger than the anisotropic magnetoresistance. This suggests the dichroism in Mn$_{2}$Au is a result of the strong spin-orbit interactions giving rise to anisotropy of interband optical transitions, in-line with recent studies of electronic band-structure. The considerable magnetic linear dichroism in the near-infrared could be used for ultrafast optical read-out of the N\'{e}el vector in Mn$_{2}$Au.

cond-mat.mtrl-sci↗

arXiv2019

Surface resonance of the Heusler half metal Co2MnSi probed by SX-ARPES

Heusler compounds are promising materials for spintronics with adjustable electronic properties including 100% spin polarization at the Fermi energy. We investigate the electronic states of AlOx capped epitaxial thin films of the ferromagnetic half metal Co2MnSi ex-situ by soft X-ray angular resolved photoemission spectroscopy (SX-ARPES). Good agreement between the experimental SX- ARPES results and photoemission calculations including surface effects was obtained. In particular, we observed in line with our calculations a large photoemission intensity at the center of the Brillouin zone, which does not originate from bulk states, but from a surface resonance. This provides strong evidence for the validity of the previously proposed model based on this resonance, which was applied to explain the huge spin polarization of Co2MnSi observed by angular-integrating UV-photoemission spectroscopy.

cond-mat.mtrl-sci↗

arXiv2018

Signature of a highly spin polarized resonance state at Co2MnSi(001)/Ag(001) interfaces

We investigated interfaces of halfmetallic Co2MnSi(100) Heusler thin films with Ag(100), Cr(100), Cu and Al layers relevant for spin valves by high energy x-ray photoemission spectroscopy (HAXPES). Experiments on Co2MnSi samples with an Ag(100) interface showed a characteristic spectral shoulder feature close to the Fermi edge, which is strongly diminished or suppressed at Co2MnSi (100) interfaces with the other metallic layers. This feature is found to be directly related to the Co2MnSi(100) layer, as reflected by control experiments with reference non-magnetic films, i.e. without Heusler layer. By comparison with HAXPES calculations, the shoulder feature is identified as originating from an interface state related to a highly spin polarized surface resonance of Co2MnSi (100).

cond-mat.mtrl-sci↗

arXiv2017

Complex THz and DC inverse spin Hall effect in YIG/Cu$_{1-x}$Ir$_{x}$ bilayers across a wide concentration range

We measure the inverse spin Hall effect of Cu$_{1-x}$Ir$_{x}$ thin films on yttrium iron garnet over a wide range of Ir concentrations ($0.05 \leqslant x \leqslant 0.7$). Spin currents are triggered through the spin Seebeck effect, either by a DC temperature gradient or by ultrafast optical heating of the metal layer. The spin Hall current is detected by, respectively, electrical contacts or measurement of the emitted THz radiation. With both approaches, we reveal the same Ir concentration dependence that follows a novel complex, non-monotonous behavior as compared to previous studies. For small Ir concentrations a signal minimum is observed, while a pronounced maximum appears near the equiatomic composition. We identify this behavior as originating from the interplay of different spin Hall mechanisms as well as a concentration-dependent variation of the integrated spin current density in Cu$_{1-x}$Ir$_{x}$. The coinciding results obtained for DC and ultrafast stimuli show that the studied material allows for efficient spin-to-charge conversion even on ultrafast timescales, thus enabling a transfer of established spintronic measurement schemes into the terahertz regime.

cond-mat.mtrl-sci↗

arXiv2016

Evidence for eight node mixed-symmetry superconductivity in a correlated organic metal

We report a combined theoretical and experimental investigation of the superconducting state in the quasi-two-dimensional organic superconductor $κ$-(ET)$_2$Cu[N(CN)$_2$]Br. Applying spin-fluctuation theory to a low-energy material-specific Hamiltonian derived from ab initio density functional theory we calculate the quasiparticle density of states in the superconducting state. We find a distinct three-peak structure that results from a strongly anisotropic mixed-symmetry superconducting gap with eight nodes and twofold rotational symmetry. This theoretical prediction is supported by low-temperature scanning tunneling spectroscopy on in situ cleaved single crystals of $κ$-(ET)$_2$Cu[N(CN)$_2$]Br with the tunneling direction parallel to the layered structure.

cond-mat.supr-con↗

arXiv2015

Spin Currents injected electrically and thermally from highly spin polarized Co$_2$MnSi

We demonstrate the injection and detection of electrically and thermally generated spin currents probed in Co$_2$MnSi/Cu lateral spin valves. Devices with different electrode separations are patterned to measure the non-local signal as a function of the electrode spacing and we determine a relatively high effective spin polarization $α$ of Co$_2$MnSi to be 0.63 and the spin diffusion length of Cu to be 500 nm at room temperature. The electrically generated non-local signal is measured as a function of temperature and a maximum signal is observed for a temperature of 80 K. The thermally generated non-local signal is measured as a function of current density and temperature in a second harmonic measurement detection scheme. We find different temperature dependences for the electrically and thermally generated non-local signals, which allows us to conclude that the temperature dependence of the signals is not just dominated by the transport in the Cu wire, but that there is a crucial contribution from the different generation mechanisms, which has been largely disregarded to date.

cond-mat.mes-hall↗