Research

A publication with Helen Gomonay and Tobias Wagner revealing ultrafast domain wall motion in Mn2⁢Au through permalloy capping has been published in Physical Review B.

Antiferromagnets offer much faster dynamics compared to their ferromagnetic counterparts but their order parameter is extremely difficult to detect and control. So far, controlling the Néel order parameter electrically is limited to only very few materials where Néel spin-orbit torques are allowed by symmetry. In this work, they show that coupling a thin ferromagnet (permalloy) layer on top of an antiferromagnet (Mn2⁢Au) solves a major roadblock—the controlled reading, writing, and manipulation of antiferromagnetic domains. They confirm by atomistic spin dynamics simulations that the domain wall patterns in the Mn2⁢Au are imprinted on the permalloy, therefore allowing for indirect imaging of the Néel order parameter. Their simulations show that the coupled domain wall structures in Mn2⁢Au-Py bilayers can be manipulated by either acting on the Néel order parameter via Néel spin-orbit torques or by acting on the magnetization (the ferromagnetic order parameter) via magnetic fields. In both cases, they predict ultrahigh domain wall speeds on the order of 8.5 km/s. Thus, employing a thin ferromagnetic layer has the potential to easily control the Néel order parameter in antiferromagnets even where Néel spin-orbit torques are forbidden by symmetry. The controlled manipulation of the antiferromagnetic order parameter provides a promising basis for the development of high-density storage and efficient computing technologies working in the THz regime.

A publication with Libor Šmejkal about a tool to check whether a symmetry-compensated collinear magnetic material is antiferro- or altermagnetic has been published in SciPost Phys. Codebases.

Altermagnets (AM) is a recently discovered class of collinear magnets that share some properties (anomalous transport, etc) with ferromagnets, some (zero net magnetization) with antiferromagnets, while also exhibiting unique properties (spin-splitting of electronic bands and resulting spin-splitter current). Since the moment compensation in AM is driven by symmetry, it must be possible to identify them by analyzing the crystal structure directly, without computing the electronic structure. Given the significant potential of AM for spintronics, it is very useful to have a tool for such an analysis. This work presents an open-access code implementing such a direct check.

A publication with Jairo Sinova  and Helen Gomonay about magnetic domain engineering in antiferromagnetic Cu⁢Mn⁢As and Mn2Au has been published in Physical Review Applied.

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 nonscalable control of the magnetic order. Here, they demonstrate simple and functional ways of influencing the domain structure in Cu⁢Mn⁢As and Mn2Au, two key materials of antiferromagnetic spintronics research, using device patterning and strain engineering. Comparing x-ray microscopy data from two different materials, they reveal the key parameters dictating domain formation in antiferromagnetic devices and show how the nontrivial interaction of magnetostriction, substrate clamping, and edge anisotropy leads to specific equilibrium domain configurations. More specifically, they 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 hopefully pave the way toward realizing truly functional antiferromagnetic devices.

A publication with Jairo Sinova, Libor Šmejkal, Anna Hellenes and Rafael González Hernández about the observation of a spontaneous anomalous Hall response in the Mn5Si3 d-wave altermagnet candidate has been published in Nature Communications.

Phases with spontaneous time-reversal symmetry (T) breaking are sought after for their anomalous physical properties, low-dissipation electronic and spin responses, and information-technology applications. Recently predicted altermagnetic phase features an unconventional and attractive combination of a strong T-symmetry breaking in the electronic structure and a zero or only weak-relativistic magnetization. In this work, they experimentally observe the anomalous Hall effect, a prominent representative of the T-symmetry breaking responses, in the absence of an external magnetic field in epitaxial thin-film Mn₅Si₃ with a vanishingly small net magnetic moment. By symmetry analysis and first-principles calculations they demonstrate that the unconventional d-wave altermagnetic phase is consistent with the experimental structural and magnetic characterization of the Mn₅Si₃epilayers, and that the theoretical anomalous Hall conductivity generated by the phase is sizable, in agreement with experiment. An analogy with unconventional d-wave superconductivity suggests that our identification of a candidate of unconventional d-wave altermagnetism points towards a new chapter of research and applications of magnetic phases.

A publication with Libor Šmejkal about the spontaneous formation of altermagnetism from orbital ordering has been published in Physical Review Letters.

Altermagnetism has emerged as a third type of collinear magnetism. In contrast to standard ferromagnets and antiferromagnets, altermagnets exhibit extra even-parity wave spin order parameters resulting in a spin splitting of electronic bands in momentum space. In real space, sublattices of opposite spin polarization are anisotropic and related by rotational symmetry. In the hitherto identified altermagnetic candidate materials, the anisotropies arise from the local crystallographic symmetry. Here, they show that altermagnetism can also form as an interaction-induced electronic instability in a lattice without the crystallographic sublattice anisotropy. They provide a microscopic example of a two-orbital model showing that the coexistence of staggered antiferromagnetic and orbital order can realize robust altermagnetism. They quantify the spin-splitter conductivity as a key experimental observable and discuss material candidates for the interaction-induced realization of altermagnetism.

A publication with Jairo Sinova, Libor Šmejkal, Atasi Chakraborty and Rafael González Hernández about strain-induced phase transitions from antiferromagnets to altermagnets has been published in Physical Review B.

A key challenge for future applications and functionalization of altermagnets is to demonstrate controlled transitioning to the altermagnetic phase from other conventional phases in a single material. Here they prove a viable path toward overcoming this challenge through a strain-induced transition from an antiferromagnetic to an altermagnetic phase in ReO2. Combining spin group symmetry analysis and ab initio calculations, they demonstrate that under compressive strain ReO2 undergoes such transition, lifting the Kramers degeneracy of the band structure of the antiferromagnetic phase in the nonrelativistic regime. In addition, they show that this magnetic transition is accompanied by a metal-insulator transition, and calculate the distinct spin-polarized spectral functions of the two phases, which can be detected in angle-resolved photoemission spectroscopy experiments.

A publication with Libor Šmejkal about x-ray magnetic circular dichroism in altermagnetic -MnTe has been published in Physical Review Letters.

In this work they use symmetry, ab initio theory, and experiments to explore x-ray magnetic circular dichroism (XMCD) in the altermagnetic class. As a representative material for our XMCD study they choose -MnTe with compensated antiparallel magnetic order in which an anomalous Hall effect has been already demonstrated. They predict and experimentally confirm a characteristic XMCD line shape for compensated moments lying in a plane perpendicular to the light propagation vector. Their results highlight the distinct phenomenology in altermagnets of this time-reversal symmetry breaking response, and its potential utility for element-specific spectroscopy and microscopy.

A publication with Olena Gomonay about current-controlled chirality dynamics in a mesoscopic magnetic domain wall has been published in Physical Review B.

Chirality as internal degree of freedom of a mesoscopic domain wall inside a quasi-one-dimensional fixture can be controlled by spin-polarized current for ferro- as well as antiferromagnetic domain walls. They show that the current density required for the chirality manipulation can be significantly reduced in the low-temperature regime where the chirality dynamics exhibits quantum effects. In this quantum regime, weak currents can excite Bloch oscillations of the domain wall angular rotation velocity, with the oscillation frequency proportional to the current, modulated by a much higher magnon-range frequency. In addition to that, the Wannier-Stark localization effects enable controlled switching between different chiral states, suppressing inertial effects characteristic for the classical regime. They also show that for recently discovered novel class of magnetic materials — altermagnets — chirality switching can be driven by the usual charge current (not spin polarized).

A publication with Jairo Sinova, Libor Šmejkal, Nayra Alvarez and Venkata K. Bharadwaj about Strain control of band topology and surface states in antiferromagnetic EuCd2As2 has been published in Physical Review B.

They investigate via ab initio density functional theory calculations, the effects of shear strain on the bulk and surface states in two antiferromagnetic EuCd2⁢As2 phases with out-of-plane and in-plane spin configurations. When magnetic moments are along the axis, a 3% longitudinal or diagonal shear strain can tune the Dirac semimetal phase to an axion insulator phase, characterized by the parity-based invariant 4⁢=2. For an in-plane magnetic order, the axion insulator phase remains robust under all shear strains. They further find that for both magnetic orders, the bulk gap increases, and a surface gap opens on the (001) surface up to 16 meV. Because of a nonzero 4⁢ index and gapped states on the (001) surface, hinge modes are expected to happen on the side surface states between those gapped surface states. This result can provide valuable insight into the realization of the long-sought axion states.

A publication with Jairo Sinova, Libor Šmejkal, Anna B. Hellenes, Rodrigo Jaeschke Ubiergo and Venkata K. Bharadwaj about supercell altermagnets has been published in Physical Review B.

They substantially broaden the family of altermagnetic candidates by predicting supercell altermagnets. Their magnetic unit cell is constructed by enlarging the nonmagnetic primitive unit cell, resulting in a nonzero propagation vector for the magnetic structure. This connection of the magnetic configuration to the ordering of sublattices gives an extra degree of freedom to supercell altermagnets, which can allow for the control over the order parameter spatial orientation. They identify realistic candidates ${\mathrm{MnSe}}_2$ with a $d$-wave order, and ${\mathrm{RbCoBr}}_3,{\mathrm{CsCoCr}}_3$, and ${\mathrm{BaMnO}}_3$ with $g$-wave order. They demonstrate the reorientation of the order parameter in ${\mathrm{MnSe}}_2$, which has two different magnetic configurations, whose energy difference is only 5 meV, opening the possibility of controlling the orientation of the altermagnetic order parameter by external perturbations.