Electron Spin Resonance on Magnetic Van der Waals Compounds

M.Sc. Joyal John Abraham

AutorIn

M.Sc. Joyal John Abraham

Titel

Electron Spin Resonance on Magnetic Van der Waals Compounds

Zitierfähige Url:

https://nbn-resolving.org/urn:nbn:de:bsz:14-qucosa2-993574

Erstveröffentlichung

2025

Datum der Einreichung

04.03.2025

Datum der Verteidigung

19.09.2025

Abstract (EN)

Suppressing interactions along one direction in three-dimensional (3D) systems offers a unique opportunity to explore exotic low-dimensional physics. Magnetic van der Waals (vdW) materials, by virtue of weak interlayer interaction, serve as ideal platforms to realize fundamental aspects of magnetism in reduced spatial dimensions. The potential to host novel quantum ground states, exotic spin textures, unique spin dynamics, and the relevance in next-generation spintronics, quantum computing, and magnetic memory devices has positioned vdW systems at the forefront of condensed matter research. In this dissertation, broad-band electron spin resonance (ESR) spectroscopy is employed to investigate several quasi-two-dimensional (2D) magnetic vdW compounds, focusing on their ground state, magnetic excitations, and spin dynamics. The first part of this research is dedicated to the investigation of the singlecrystalline material (Mn1−xNix)2P2S6. At the X-band frequency of 9.56 GHz, the temperature and angular dependence of the resonance field and the linewidth for x = 0, 0.5, and 1 are studied above the transition temperature TN. Only for Mn2P2S6, a significant change in the spin dynamics from the dominance of the 3D-like fluctuations close to the magnetic order to a relative increase of the 2D-like spin fluctuations at higher temperatures is observed. Moreover, high-frequency/field ESR (HF-ESR) experiments are performed on Mn2P2S6 and MnNiP2S6, and the results are compared with prior studies of Ni2P2S6. Analysis of magnetic excitations reveals that in comparison to Mn2P2S6, increasing the Ni content yields a larger magnon gap in the ordered state and an increased g-factor value and its anisotropy in the paramagnetic state. These two studied compounds are strongly anisotropic, each having a unique ground state and type of magnetic order. A stronger deviation of the g-factor from the free electron value in the samples containing Ni suggests that the anisotropy of the exchange is an important contributor to stabilizing a certain type of magnetic order with particular anisotropy. At temperatures above the magnetic order, the spin-spin correlations, resulting in a development of slowly fluctuating short-range order, have been analyzed. They are much more pronounced in MnNiP2S6 compared to Mn2P2S6. The enhanced spin fluctuations in MnNiP2S6 are attributed to the competition of different types of magnetic order. Finally, the analysis of the temperature-dependent critical behavior of the magnon gaps below the ordering temperature in Mn2P2S6 suggests that the character of the spin wave excitations in this compound undergoes a field-induced crossover from a 3D-like toward a 2D XY regime. In the second part of this dissertation, investigations on a related magnetic vdW material CuCrP2S6, featuring interpenetrating antipolar Cu+ and antiferromagnetic Cr3+ sublattices, are presented. A detailed ESR study in a broad range of excitation frequencies, magnetic fields, and temperatures reported here provides detailed insights into the low-energy spin dynamics of this material both above and below the antiferromagnetic (AFM) ordering temperature TN ≈ 30 K. Strong ferromagnetic (FM) correlations persist far above TN, indicating an inherently two-dimensional and anisotropic character of the spin dynamics. AtT < TN, a complex field dependence of collective excitations of the AFM-ordered spin-lattice was observed, featuring two non-degenerate magnon gaps at H = 0. The results can be accurately modeled with the linear spin wave theory analysis, yielding the values of interlayer exchange energy, magnetocrystalline anisotropies, and gap energies. A remarkable tuning of the excitations from the AFM type to the FM type with increasing the field strength was demonstrated, which, together with the non-degeneracy of the gaps favorable for the excitation of the spin-transfer torque, put forward CuCrP2S6 as an interesting functional material for use in magnonic devices. The ESR results do not provide clear evidence of the magneto-electric coupling, suggesting the weakness of the cross-coupling between the electrical and magnetic subsystems of CuCrP2S6, similar to other type-I multiferroics. In the final part, a technical project is outlined, detailing the design and construction of a new HF-ESR transmission probehead aimed at improving and enhancing the capabilities of the HF-ESR setup. The theoretical motivation behind the project is discussed, followed by a detailed description of its construction. By modifying the mirror geometry, transmission losses of broadband microwaves across different frequency regimes are measured and compared. Additionally, a modular design approach is adopted, ensuring convenient and cost-effective future upgrades and a more accessible sample space.

Freie Schlagwörter (EN)

Magnetic anisotropy, spin dynamics, Electron Spin Resonance, Van der Waals Compounds, magnetic excitations

Klassifikation (DDC)

530

Klassifikation (RVK)

UP 9340

GutachterIn