Thin film quantum materials

Abstract:

We investigate the temperature-dependent electronic structure of the van der Waals ferromagnet, CrGeTe3. Using angle-resolved photoemission spectroscopy, we identify atomic- and orbital-specific band shifts upon cooling through TC. From these, together with x-ray absorption spectroscopy and x-ray magnetic circular dichroism measurements, we identify the states created by a covalent bond between the Te 5p and the Cr eg orbitals as the primary driver of the ferromagnetic ordering in this system, while it is the Cr t2g states that carry the majority of the spin moment. The t2g states furthermore exhibit a marked bandwidth increase and a remarkable lifetime enhancement upon entering the ordered phase, pointing to a delicate interplay between localized and itinerant states in this family of layered ferromagnets.

Abstract:

Insulating antiferromagnets have recently emerged as efficient and robust conductors of spin current. Element-specific and phase-resolved x-ray ferromagnetic resonance has been used to probe the injection and transmission of ac spin current through thin epitaxial NiO(001) layers. The spin current is found to be mediated by coherent evanescent spin waves of GHz frequency, rather than propagating magnons of THz frequency, paving the way towards coherent control of the phase and amplitude of spin currents within an antiferromagnetic insulator at room temperature.

Abstract:

Magnetic skyrmions are localized swirls of magnetization with a nontrivial topological winding number. This winding increases their robustness to superparamagnetism and gives rise to a myriad of novel dynamical properties, making them attractive as next-generation information carriers. Recently the equation of motion for a skyrmion was derived using the approach pioneered by Thiele, allowing for macroscopic skyrmion systems to be modeled efficiently. This powerful technique suffers from the prerequisite that one must have a priori knowledge of the functional form of the interaction between a skyrmion and all other magnetic structures in its environment. Here we attempt to alleviate this problem by providing a simple analytic expression that can generate arbitrary repulsive interaction potentials from the micromagnetic Hamiltonian, using it to provide a correction to the interaction between a skyrmion and the boundary of its material. We also discuss a toy model of the radial profile of a skyrmion, which is accurate for a wide range of material parameters.

Abstract:

The technique of x-ray detected ferromagnetic resonance (XFMR) represents an indispensable new tool in the investigation of spin current effects in complex heterostructures, as it enables the observation of magnetization and spin dynamics with element-, site-, and valence state-specificity. Here we give an overview of the development of XFMR and characterize different approaches to measure spin dynamics using synchrotron radiation. We provide a detailed description of the working principle of the technique and give an overview of recent work carried out at beamline 4.0.2 of the Advanced Light Source and beamline I10 of the Diamond Light Source using XFMR. Results from our latest publications demonstrate the capabilities and sensitivity of the technique. Element- and phase-resolution provide intriguing insights into the mechanisms of spin current propagation in multilayers, while the high sensitivity of XFMR allows for detection of even miniscule signals. Most recently, the utilization of linearly polarized x-rays for XFMR and the detection of XFMR by means of x-ray diffraction rather than x-ray absorption demonstrate two new capabilities in the investigation of spin dynamics.