Bulletin of the American Physical Society
APS March Meeting 2024
Monday–Friday, March 4–8, 2024; Minneapolis & Virtual
Session K22: Pyrochlores, Spinels, and Spin IcesFocus
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Sponsoring Units: GMAG Chair: Michel Gingras, University of Waterloo Room: 101B |
Tuesday, March 5, 2024 3:00PM - 3:36PM |
K22.00001: Spin–orbital liquid state and liquid–gas metamagnetic transition on a pyrochlore lattice Invited Speaker: Nan Tang Crystal structures with degenerate electronic orbitals are generally unstable, as the system lowers the overall energy by lifting the degeneracy through Jahn-Teller (JT) distortions. Typically, in e.g. 3d transition metal compounds, energy scales of orbital-lattice couplings and exchange interactions are orders of magnitude different, which ensures that the JT distortions occur independently from magnetic orderings. Therefore, it is hard to suppress orbital order down to low temperatures, not to mention to obtain a “spin-orbital liquid” state. Nevertheless, rare earth systems provide a promising platform for realizing such novel states. In even number electron (non-Kramers) systems, e.g. Pr3+, an interlocking between the spin and the orbital moment is realized due to the strong spin-orbit coupling. Our focus is in the pyrochlore oxide Pr2Zr2O7, a multipolar spin ice whose ground doublet comprises of longitudinal magnetic dipole and transverse electric quadrupole (orbital) moments. Such non-Kramers doublet is extremely sensitive to disorders/ distortions, setting a high bar for the sample quality. |
Tuesday, March 5, 2024 3:36PM - 3:48PM |
K22.00002: Revisiting spin ice physics in the ferromagnetic Ising pyrochlore Pr$_2$Sn$_2$O$_7$ Adam A Aczel, Paul M Sarte, Brenden R Ortiz, Ganesh Pokharel, Steven J Gomez Alvarado, Andrew F May, Stuart Calder, Lucile Mangin-Thro, Andrew R Wildes, Haidong Zhou, Gabriele Sala, Christopher R Wiebe, Stephen D Wilson, Joseph Paddison Pyrochlore materials are characterized by their hallmark network of corner-sharing rare-earth tetrahedra, which can produce a wide array of complex magnetic ground states. Ferromagnetic Ising pyrochlores often obey the ``two-in-two-out" spin ice rules, which can lead to a highly-degenerate spin structure. Large moment systems, such as Ho$_2$Ti$_2$O$_7$ and Dy$_2$Ti$_2$O$_7$, tend to host a classical spin ice state with low-temperature spin freezing and emergent magnetic monopoles. Systems with smaller effective moments, such as Pr$^{3+}$-based pyrochlores, have been proposed as excellent candidates for hosting a ``quantum spin ice" characterized by entanglement and a slew of exotic quasiparticle excitations. However, experimental evidence for a quantum spin ice state has remained elusive. Here, we show that the low-temperature magnetic properties of Pr$_2$Sn$_2$O$_7$ satisfy several important criteria for continued consideration as a quantum spin ice. We find that Pr$_2$Sn$_2$O$_7$ exhibits a partially spin-frozen ground state with a large volume fraction of dynamic magnetism. Our comprehensive bulk characterization and neutron scattering measurements enable us to map out the magnetic field-temperature phase diagram, producing results consistent with expectations for a ferromagnetic Ising pyrochlore. We identify key hallmarks of spin ice physics, and show that application of small magnetic fields ($mu_0 H_{ ext{c}} sim$~0.75~T) suppresses the spin ice state and induces a long-range ordered magnetic structure. Together, our work clarifies the current state of Pr$_2$Sn$_2$O$_7$ and encourages future studies aimed at exploring the potential for a quantum spin ice ground state in this system. |
Tuesday, March 5, 2024 3:48PM - 4:00PM |
K22.00003: Atlas of pyrochlore CSLs in the large-N limit Daniel Lozano-Gomez, Owen Benton, Han Yan The pyrochlore lattice spin system has been one of the most fruitful platforms in the search for spin liquids, both experimentally and theoretically. In this work, we develop the field theory for the emergence of generalized Gauss’s law constraints on the pyrochlore lattice and use it to systematically construct various (if not all) spin liquids in the large-N limit with nearest neighbor interactions. We also perform Monte Carlo simulations to examine what happens to these models with classical vector spins. |
Tuesday, March 5, 2024 4:00PM - 4:12PM |
K22.00004: Sublattice pairing in pyrochlore Heisenberg antiferromagnets Cecilie Glittum, Olav F Syljuasen We argue that classical pyrochlore Heisenberg antiferromagnets with small further-neighbor couplings can order in a state where pairs of sublattices form antiparallel spirals. The spiral ordering wave vectors of the two pairs are in general different from each other and are constrained by which sublattices are being paired. This sublattice pairing state generally breaks inversion and most rotation symmetries. Its existence depends on the antiferromagnetic nearest-neighbor coupling which favors the spins on each tetrahedron to sum to zero. To substantiate our argument, we extend the nematic bond theory, a diagrammatic large-Ns method, to non-Bravais lattices, and we demonstrate that the predicted state is indeed realized at low temperatures in a large region of exchange coupling space. We also carry out a spin wave calculation which suggests that the sublattice pairing state is coplanar. |
Tuesday, March 5, 2024 4:12PM - 4:48PM |
K22.00005: The Dipole-Octupole Quantum Spin Ice Candidate Ce2Zr2O7 Invited Speaker: Evan M Smith High-energy neutron scattering measurements on the insulating pyrochlore magnet Ce2Zr2O7 reveal that its Ce3+ pseudospin-1/2 degrees of freedom possess dipole-octupole character, making Ce2Zr2O7 a candidate for novel quantum spin ice ground states at low temperature. Muon spin relaxation measurements on single crystal Ce2Zr2O7 show a lack of both magnetic order and spin freezing down to T = 0.02 K. Furthermore, low-energy inelastic neutron scattering measurements at low temperature show a lack of magnetic order and a snowflake-like pattern of diffuse scattering that is characteristic of quantum spin ice. No obvious spin waves are observed in the low-energy excitation spectrum of Ce2Zr2O7, in both zero and non-zero magnetic field, and this eliminates the most common method of estimating the microscopic spin Hamiltonian by fitting spin wave dispersions using linear spin wave theory. Instead, we use heat capacity, magnetic susceptibility, and diffuse neutron scattering measurements on Ce2Zr2O7, combined with theoretical results from numerical linked cluster calculations, in order to estimate the terms in the nearest-neighbor exchange Hamiltonian expected for Ce2Zr2O7. Polarized neutron diffraction measurements reveal zone-boundary diffuse scattering in the non-spin-flip channel which is not predicted by the nearest-neighbor Hamiltonian, and which we tentatively attributed to weak interactions beyond nearest neighbors. Finally, our in-field neutron scattering measurements show that magnetic fields along the [1,-1,0] and [0, 0, 1] directions induce partially-polarized and polarized spin ice phases at low temperatures, respectively, with the partially-polarized spin ice phase being particularly interesting due to the presence of field-decoupled, quasi-one-dimensional quantum spin chains within this phase. Altogether, our low-temperature characterization of Ce2Zr2O7 makes a strong case for a pure quantum spin ice ground state in zero magnetic field, and sheds light on the exotic field-induced magnetic behaviour in Ce2Zr2O7 for two magnetic field directions. |
Tuesday, March 5, 2024 4:48PM - 5:00PM |
K22.00006: Emergence of dominant liquid state in artificial honeycomb spin ice Pousali Ghosh, Deepak K Singh, George Yumnam, Jiasen Guo, Lisa DeBeer-Schmitt, Valeria Lauter, Laura R Stingaciu, Piotr Zolnierczuk, Artur Glavic Emergence of dominant liquid state in artificial honeycomb spin ice Geometrically frustrated 2D artificial magnetic honeycomb lattices provide a unique platform for the exploration of emergent phenomena of fundamental importance with possible implications for quantum computing devices and algorithms. At the honeycomb vertices, quantum mechanical entities represented by spin-1/2 Pauli matrices, emerge due to converging or diverging fluxes. Among the many novel magnetic properties, our homegrown nanoscopic magnetic honeycomb lattice renders a thermally tunable system providing a disorder-free environment for exploring liquid-like short-range magnetic charge correlation resembling the spin liquid state of atomistic origin. The magnetic charge correlation exhibits massively degenerate ground state at low temperature, which remains unperturbed even in large magnetic field applications. Another significant aspect of these systems is that the magnetic charges are in perpetual dynamic state with ultra-fast picosecond relaxation kinetics between the honeycomb vertices. |
Tuesday, March 5, 2024 5:00PM - 5:12PM |
K22.00007: Deconstructing Magnetization Noise: Degeneracies, Phases, and Mobile Fractionalized Excitations in Tetris Artificial Spin Ice Mateusz M Goryca, Xiaoyu Zhang, Justin Ramberger, Justin D Watts, Cristiano Nisoli, Chris Leighton, Peter Schiffer, Scott A Crooker Direct detection of spontaneous spin fluctuations, or magnetization noise, is emerging as a powerful means of studying magnetic excitations in both natural and artificial frustrated magnets. These excitations can often be described as fractionalized quasiparticles possessing an effective magnetic charge. They are topologically protected, can diffuse through the crystal lattice in thermal equilibrium, and can move in response to applied magnetic fields, motivating studies of magnetricity. In archetypal square ASI lattices, magnetization noise was detected via optical magnetometry and the appearance of excess noise at certain applied magnetic fields revealed the presence of phases rich in mobile magnetic charges. |
Tuesday, March 5, 2024 5:12PM - 5:24PM |
K22.00008: Dynamics of a macroscopic artificial spin ice: experiment and modeling Ezio Iacocca, Lawrence Scafuri, Renju R. Peroor, Dmytro Bozhko Artificial spin ices (ASIs) are ensembles of geometrically structured, interacting magnetic nano-elements that exhibit frustration [1]. Here, we present a macroscopic ASI where 1-inch magnets mounted on low-friction rotors are arranged in a square lattice. The system can be driven into a nonlinear regime by an external coil with a field oscillating in the 1 to 20 Hz range. Mixing and coupling between high-symmetry modes is observed, evidenced by an increase in the spectral content at approximately 10 Hz. The dynamics are experimentally captured by a high-frame-rate camera and then digitized to recover the spectrum for each magnet. Modeling based on the torque equation for each elongated magnet and their coupling using a monopole-charge approximation [2] reproduces the dynamics with remarkable accuracy. While at completely different spatial and temporal scales, these dynamics are similar to those observed at the nanoscale when ASIs are driven into a nonlinear regime [3]. Our macroscopic ASI can be considered as a testbed for nonlinear phenomena at a scale appropriate for dissemination to the general public. |
Tuesday, March 5, 2024 5:24PM - 5:36PM |
K22.00009: Exploring dynamical modes in tilted square artificial spin ice Ghanem Alatteili, Alison Roxburgh, Ezio Iacocca Artificial Spin ices (ASIs) are periodic arrays of interacting nanomagnets exhibiting geometric frustration [1]. Due to their periodic nature, ASIs can be viewed as magnonic crystals [2], exhibiting reconfigurable magnon modes [3] and band structures [4] depending on the magnetization state. A new frontier is the exploration of ASIs in three-dimensional geometries [5]. Here, we use Gænice [6], a novel semi-analytical code for arbitrary artificial spin ices, to investigate the frequency response of a square ASI geometry with nanomagnets tilted out of the x-y plane and keeping the center-to-center distance between nanomagnets constant. In our study, we investigated both vortex and remanent states. For both states, we found a redshift for the high-frequency modes. Low-frequency modes emerged at critical tilt angles of 30 deg and 78 deg in the vortex state and 25 deg and 41 deg for the remanent state. Micromagnetic simulations recover these two features and quantitatively agree with the predictions done by Gænice. Our results pave the way to explore dynamical modes in the realm of 3D ASI geometries. |
Tuesday, March 5, 2024 5:36PM - 5:48PM |
K22.00010: Investigation of Magnetic Phase Transitions in Spinel Cobalt Vanadate Thin Films using XAS and XMCD Sangsoo Kim, Elizabeth M Skoropata, Christianne Beekman Epitaxial thin films of cobalt vanadate spinels have been successfully grown on SrTiO3 (001) substrates. Due to an in-plane compression of the unit cell, previous magnetization studies via neutron scattering1,2 and torque magnetometry3 show a 90 K magnetization reorientation from out-of-plane to in-plane that is not present in bulk crystals of CoV2O4. To elucidate the nature of this phase transition, the L3,2 edge is investigated using soft X-ray Absorption Spectroscopy (XAS) and X-ray Magnetic Circular Dichroism (XMCD). In this talk, we review the XAS and XMCD data of Cobalt Vanadate thin films and share how the magnetic reorientation affects the XAS and XMCD data on both the Cobalt and Vanadium sites. |
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