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School of Physics

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Publication Search Results

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Data for the publication "Pressure control of magnetic order and excitations in the pyrochlore antiferromagnet MgCr2O4"

2024-01 , Mourigal, Martin

MgCr2O4 is one of the best-known realizations of the pyrochlore-lattice Heisenberg antiferromagnet. The strong antiferromagnetic exchange interactions are perturbed by small further-neighbor exchanges such that this compound may in principle realize a spiral spin liquid (SSL) phase in the zero-temperature limit. However, a spin Jahn-Teller transition below TN≈13 K yields a complicated long-range magnetic order with multiple coexisting propagation vectors. We present neutron scattering and thermo-magnetic measurements of MgCr2O4 samples under applied hydrostatic pressure up to P=1.7 GPa demonstrating the existence of multiple close-lying nearly degenerate magnetic ground states. We show that the application of hydrostatic pressure increases the ordering temperature by around 0.8 K per GPa and increases the bandwidth of the magnetic excitations by around 0.5 meV per GPa. We also evidence a strong tendency for the preferential occupation of a subset of magnetic domains under pressure. In particular, we show that the k=(0,0,1) magnetic phase, which is almost negligible at ambient pressure, dramatically increases in spectral weight under pressure. This modifies the spectrum of magnetic excitations, which we interpret unambiguously as spin waves from multiple magnetic domains. Moreover, we report that the application of pressure reveals a feature in the magnetic susceptibility above the magnetostructural transition. We interpret this as the onset of a short-range ordered phase associated with k=(0,0,1), previously not observed in magnetometry measurements.

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Quantum Magnetism in Quasi-Two-Dimensional Rare-Earth Oxides: Neutron Scattering and Instrumentation

2021-07-20 , Daum, Marcus J.

Quantum magnetism is a rich area of hard condensed matter physics where various energy scales and exchange parameters, coupled with lattice symmetries, lead to ground states of varying degrees of complexity. Through theoretical and experimental efforts, realizations of quantum many-body phenomena are found. This thesis presents recent work on several candidate materials which have been theoretically proposed to exhibit exotic states of matter as their ground state. These materials are carefully characterized using various theoretical and experimental means such as linear spin wave theory and inelastic neutron scattering to understand their ground states. In addition, work presented here displays effort to characterize, optimize, and design new neutron scattering instruments.

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Raw data and simulation code for "Quantum-to-classical crossover in generalized spin systems – the temperature-dependent spin dynamics of FeI2"

2024-01 , Mourigal, Martin

Simulating quantum spin systems at finite temperatures is an open challenge in many-body physics. This work studies the temperature-dependent spin dynamics of a pivotal compound, FeI2, to determine if universal quantum effects can be accounted for by a phenomenological renormalization of the dynamical spin structure factor S(q,ω) measured by inelastic neutron scattering. Renormalization schemes based on the quantum-to-classical correspondence principle are commonly applied at low temperatures to the harmonic oscillators describing normal modes. However, it is not clear how to extend this renormalization to arbitrarily high temperatures. Here we introduce a temperature-dependent normalization of the classical moments, whose magnitude is determined by imposing the quantum sum rule, i.e. ∫dωdqS(q,ω)=NSS(S+1) for NS dipolar magnetic moments. We show that this simple renormalization scheme significantly improves the agreement between the calculated and measured S(q,ω) for FeI2 at all temperatures. Due to the coupled dynamics of dipolar and quadrupolar moments in that material, this renormalization procedure is extended to classical theories based on SU(3) coherent states, and by extension, to any SU(N) coherent state representation of local multipolar moments.

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Versatile Spin-Wave Approaches to the Spin Dynamics of Transition-Metal Insulators

2020-03-16 , Ge, Luwei

Quantum magnetism is one of the most important branches in condensed matter physics because it serves as an excellent platform to realize model quantum many-body systems which are difficult to find elsewhere. Good understanding of the nature of magnetic excitations in such systems demands both experimental and theoretical efforts. This thesis presents comprehensive studies of the magnetic properties of several 3d transition-metal oxides for which the effective spin Hamiltonian forms quasi-1D, quasi-2D or 3D lattices. Primarily relying on advances in neutron scattering instrumentation and spin-wave theory, the work carefully examines the effectiveness of the theory of weakly interacting magnons in describing the elementary magnetic excitations of these insulators. By revealing the microscopic interactions of these systems and testing the applicability of spin-wave theory quantitatively, the work also hopes to offer useful insights or guidance to future investigations, which may extend to the entire field of quantum many-body physics.

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Data for the publication "Cryogenic platform to investigate strong microwave cavity-spin coupling in correlated magnetic materials"

2024 , Jones, Aulden , Lilly, Michael , Mounce, Andrew , Mourigal, Martin

We present a comprehensive exploration of loop-gap resonators (LGRs) for electron spin resonance (ESR) studies, enabling investigations into the hybridization of solid-state magnetic materials with microwave polariton modes. The experimental setup, implemented in a Physical Property Measurement System by Quantum Design, allows for measurements of ESR spectra at temperatures as low as 2 Kelvin. The versatility of continuous wave ESR spectroscopy is demonstrated through experiments on CuSO4·5H2O and MgCr2O4, showcasing the g-tensor and magnetic susceptibilities of these materials. The study delves into the challenges of fitting spectra under strong hybridization conditions and underscores the significance of proper calibration and stabilization. The detailed guide provided serves as a valuable resource for laboratories interested in exploring hybrid quantum systems through microwave resonators.

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