Coherent Epitaxial Semiconductor-Ferromagnetic Insulator InAs/EuS Interfaces: Band Alignment and Magnetic Structure

Hybrid semiconductor-ferromagnetic insulator heterostructures are interesting due to their tunable electronic transport, self-sustained stray field, and local proximitized magnetic exchange. In this work, we present lattice-matched hybrid epitaxy of semiconductor-ferromagnetic insulator InAs/EuS heterostructures and analyze the atomic-scale structure and their electronic and magnetic characteristics. The Fermi level at the InAs/EuS interface is found to be close to the InAs conduction band and in the band gap of EuS, thus preserving the semiconducting properties. Both neutron and X-ray reflectivity measurements show that the overall ferromagnetic component is mainly localized in the EuS thin film with a suppression of the Eu moment in the EuS layer nearest the InAs and magnetic moments outside the detection limits on the pure InAs side. This work presents a step toward realizing defect-free semiconductor-ferromagnetic insulator epitaxial hybrids for spin-lifted quantum and spintronic applications without external magnetic fields.

Unconventional mechanisms of magnetic ordering in frustrated models of quantum magnetism

Tommaso Coletta

In this thesis we investigate in the semiclassical approximation how unconventional magnetic ordering, that is magnetic ordering which is not predicted at the classical level, emerges in a variety of frustrated models of quantum magnetism. The first part of this thesis presents our semiclassical investigation of the magnetization processes of the anisotropic triangular lattice Heisenberg antiferromanget and of the J1 − J2 square lattice Heisenberg antiferromanget. Existing analytical, numerical and experimen- tal evidences report that these systems exhibit a magnetization plateau respectively at 1/3 and 1/2 of the saturation value and whose structure can be understood in simple terms as a collinear spin structure. We develop a strategy to estimate the semiclassical energy of collinear spin structures away from their classical stability point. In this framework we are able to predict the stabilization of a collinear up-up-down structure with a magnetization equal to one third of the saturation value in a wide anisotropy range for the triangular lattice Heisen- berg antiferromanget. Similarly we find that the four sublattice up-up-up-down structure with magnetization equal to 1/2 of the saturation value is stabilized deep into the classically striped ordered region of the J1 − J2 square lattice Heisenberg antiferromanget. In the case of the anisotropic triangular lattice Heisenberg antiferromagnet a brief digression is made to the zero field case. The semiclassical approach confirms that quantum fluctuations favor collinear structures over the classical spiral states in either the weakly and strongly coupled chain regimes. The second part of the thesis is devoted to the study of fully frustrated transverse field Ising models. These models have emerged as an alternative tool to access the physics of the quan- tum dimer models (QDM) of Rokhsar and Kivelson at the purely kinetic point. We studied the classical phase diagram of this model for the square and honeycomb lattice geometries. In either case the effect of quantum fluctuations and the relevance to the physics of the QDM is discussed. In the case of the honeycomb lattice geometry we find that the classical phase diagram is rather involved with columnar and plaquette states separated by an infinite series of states of mixed nature. From the study of quantum fluctuations we conjecture that the sqrt(12) × sqrt(12) plaquette phase dominates over the columnar structure down to zero transverse field in the limit S = 1/2. In the case of the square lattice geometry we find that at the classical level the plaquette state is stabilized below the polarized state down to zero transverse field. In agreement with the most recent quantum Monte Carlo results, we show that for S = 1/2 quantum fluctuations favor columnar oder in the whole field range below saturation even though this structure is never classically stable in this parameter range.

EPFL2013

Magnetic anisotropy of Fe and Co adatoms and Fe clusters magnetically decoupled from Ni_{3}Al(111) by an alumina bilayer

Harald Brune, Markus Etzkorn, Anne Lehnert, Stefano Rusponi

The magnetic properties of individual Fe and Co atoms as well as of small Fe clusters adsorbed on two atomic layers of Al2O3 grown on Ni3Al(111) have been investigated by x-ray absorption spectroscopy and x-ray magnetic circular dichroism. We find ratios of the orbital over the effective spin magnetic moments of r=0.53 +/- 0.09 for Fe and r=0.91 +/- 0.06 for Co, which are both very close to the free-atom values of r=0.5 and 1, respectively. The magnetization curves acquired at the Fe and Ni edges demonstrate a distinctly different magnetization reversal of the transition-metal nanostructures and the substrate excluding magnetic dipolar or exchange coupling through the alumina film. Our data reveal an out-of-plane easy magnetization axis for Fe and Co originating from the spatially anisotropic hybridization of the 3d states of the adatom, with the 2p states of the oxygen terminated Al2O3 surface. We conclude that the alumina film effectively decouples the magnetic adatoms from the underlying metal substrate while providing a crystal-field environment giving rise to high magnetic anisotropy.

2010

Magnetic structure of the topological semimetal Co3Sn2S2

Henrik Moodysson Rønnow, Jian Rui Soh, Ivica Zivkovic

Co3Sn2S2 has recently been predicted to be a Weyl semimetal in which magnetic order is key to its behavior as a topological material. Here, we report unpolarized neutron diffraction and spherical neutron polarimetry measurements, supported by magnetization and transport data, which probe the magnetic order in Co3Sn2S2 below TC = 177 K. The results are fully consistent with ferromagnetic order in which the spins on the Co atoms point along the crystal c axis, although we cannot rule out some canting of the spins. We find no evidence for a type of long-ranged (k = 0) in-plane 120?? antiferromagnetic order which had previously been considered as a secondary phase present at temperatures between -90 K and TC. A discontinuous change in bulk properties and neutron polarization observed at T = 125 K when samples are cooled in a field and measured on warming is found to be due to a sudden reduction in ferromagnetic domain size. Our results lend support to the theoretical predictions that Co3Sn2S2 is a magnetic Weyl semimetal.

AMER PHYSICAL SOC2022

Unconventional mechanisms of magnetic ordering in frustrated models of quantum magnetism

Tommaso Coletta

EPFL2013