Person:
Mourigal, Martin

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https://hdl.handle.net/1853/71536
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https://hdl.handle.net/1853/72958
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    Data for the publication "Pressure control of magnetic order and excitations in the pyrochlore antiferromagnet MgCr2O4"
    (Georgia Institute of Technology, 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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    Raw data and simulation code for "Quantum-to-classical crossover in generalized spin systems – the temperature-dependent spin dynamics of FeI2"
    (Georgia Institute of Technology, 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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    The New Wave of Quantum Magnetism
    (Georgia Institute of Technology, 2022-02-08) Mourigal, Martin
    Magnetism is a fascinating phenomenon with roots in the ancient world. Although its precise understanding calls for relativistic quantum mechanics and field theory, it is integral to everyday technologies. In magnetic insulators, electrons are closely bound to a crystal lattice and carry strongly interacting magnetic dipoles; as a result, phases of matter with no classical analogs are possible. Such quantum magnetic phases are of great fundamental interest as a testbed of our understanding of many-particle quantum mechanics. In the first part of this lecture, I will discuss some of the central ideas in quantum magnetism, from the Heisenberg model to the more recent concepts introduced by Kitaev and others. Then, I will explain our research program to search for these simple models in bulk materials and understand their properties using neutron spectroscopy. Finally, I will discuss the challenges of utilizing these quantum magnets in electronic devices and beyond.