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.Comment: 10 pages, 7 figures (including supplementary information

Topics in two-dimensional ferromagnetism

Tesis para optar al grado de Magíster en Ciencias, Mención FísicaEsta tesis consiste de dos partes. En la parte I, se modelan los grados de libertad magnéticos en el imán bidimensional de van der Waals CrI3. Se comienza desde un cálculo ab initio obtenido, mediante la aproximación de densidad local (LDA). Empleando un método autoconsistente, se incluye la repulsión electrónica local a través de una teoría de campo medio del Hamiltoniano de Kanamori, en los iones Cr+3. Utilizando un método de funciones de Green, se deduce un Hamiltoniano de spin, a partir del modelo electrónico, y se calcula el correspondiente espectro de magnones en torno al estado basal. Se encontró que un modelo de Heisenberg-Kitaev describe de forma aproximada el ferromagnetismo en CrI3. Además, se observó una estructura topológica no trivial en las excitaciones magnéticas. La parte II incluye dos proyectos desarrollados en el contexto de simulaciones micromagnéticas. En el primero de estos, se estudió la estabilidad de un Skyrmion antiferromagnético en la capa superior de una bicapa antiferromagnética. En el segundo proyecto, se investigó el equilibrio y la dinámica de una pared de dominio magnética en un nanotubo. Al respecto, se mostró que la velocidad máxima de la pared de dominio puede aumentar en presencia deuna interaccioń de Dzyaloshinskii-Moriya.This thesis consists of two parts. In part I, we model the magnetic degrees of freedom in the van der Waals magnet CrI3. We start from an ab-initio Hamiltonian obtained with LDA calculations. Employing a self-consistent method, we include the local electronic repulsion as a mean-field theory of the Kanamori Hamiltonian on Cr+3 ions. Using a Green s functions method, we use the electronic Hamiltonian to calculate the corresponding spin Hamiltonian and the magnonic spectrum. A Heisenberg-Kitaev-Γmodel is found to describe the ferromagnetism adequately in CrI3. Also, we found a nontrivial topological structure in the spin excitations. Part II includes two projects carried out in the context of micromagnetic simulations. The first one studies the stability of an antiferromagnetic skyrmion in the top layer of a bilayer system. In the second project, we study the equilibrium and dynamics of a ferromagnetic domain wall on the surface of a nanotube. We show that the domain wall s velocity is increased in the presence of a Dzyaloshinskii-Moriya interaction.CENTRO PARA EL DESARROLLO DE LA NANOCIENCIA Y LA NANOTECNOLOGÍA (CEDENNA), Y EL PROYECTO FONDECYT REGULAR 119032

Strain control of band topology and surface states in antiferromagnetic EuCd2As2

Topological semimetal antiferromagnets provide a rich source of exotic topological states which can be controlled by manipulating the orientation of the Néel vector, or by modulating the lattice parameters through strain. We investigate via ab initio density functional theory calculations, the effects of shear strain on the bulk and surface states n two antiferromagnetic EuCd2As2 phases with out-of-plane and in-plane spin configurations. When magnetic moments are along the c-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 η4I=2. For an in-plane magnetic order, the axion insulator phase remains robust under all shear strains. We 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 η4I 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 a valuable insight in the realization of the long-sought axion states

Observation of the anomalous Nernst effect in altermagnetic candidate Mn5Si3

The anomalous Nernst effect generates transverse voltage to the applied thermal gradient in magnetically ordered systems. The effect was previously considered excluded in compensated magnetic materials with collinear ordering. However, in the recently identified class of compensated magnetic materials, dubbed altermagnets, time-reversal symmetry breaking in the electronic band structure makes the presence of the anomalous Nernst effect possible despite the collinear spin arrangement. In this work, we investigate epitaxial Mn5Si3 thin films known to be an altermagnetic candidate. We show that the material manifests a sizable anomalous Nernst coefficient despite the small net magnetization of the films. The measured magnitudes of the anomalous Nernst coefficient reach a scale of microVolts per Kelvin. We support our magneto-thermoelectric measurements by density-functional theory calculations of the material's spin-split electronic structure, which allows for the finite Berry curvature in the reciprocal space. Furthermore, we present our calculations of the intrinsic Berry-curvature Nernst conductivity, which agree with our experimental observations