Quantum Versus Classical Spin Fragmentation in Dipolar Kagome Ice Ho3Mg2Sb3O<

Abstract

A promising route to realize entangled magnetic states combines geometrical frustration with quantumtunneling effects. Spin-ice materials are canonical examples of frustration, and Ising spins in a transverse magnetic field are the simplest many-body model of quantum tunneling. Here, we show that the tripodkagome lattice material Ho 3 Mg 2 Sb 3 O 14 unites an icelike magnetic degeneracy with quantum-tunneling terms generated by an intrinsic splitting of the Ho 3þ ground-state doublet, which is further coupled to a nuclear spin bath. Using neutron scattering and thermodynamic experiments, we observe a symmetrybreaking transition at T Ã ≈ 0.32 K to a remarkable state with three peculiarities: a concurrent recovery of magnetic entropy associated with the strongly coupled electronic and nuclear degrees of freedom; a fragmentation of the spin into periodic and icelike components; and persistent inelastic magnetic excitations down to T ≈ 0.12 K. These observations deviate from expectations of classical spin fragmentation on a kagome lattice, but can be understood within a model of dipolar kagome ice under a homogeneous transverse magnetic field, which we survey with exact diagonalization on small clusters and mean-field calculations. In Ho 3 Mg 2 Sb 3 O 14 , hyperfine interactions dramatically alter the single-ion and collective properties, and suppress possible quantum correlations, rendering the fragmentation with predominantly single-ion quantum fluctuations. Our results highlight the crucial role played by hyperfine interactions in frustrated quantum magnets and motivate further investigations of the role of quantum fluctuations on partially ordered magnetic states.

Fragmentation in Frustrated Magnets: A Review

“…The physics of Dy and Ho systems being very similar in pyrochlore oxides R 2 M 2 O 7 (R = rare-earth, M = metal), it is natural to think of the Ho member of this family as a possible candidate for the stabilization of a fragmented kagome ice state. In Ho 3 Mg 2 Sb 3 O 14 powder samples, the coexistence of partial all in / all out ordering with diffuse scattering akin to that expected for a Coulomb phase has indeed been observed [101]. Nevertheless, the ordered magnetic moment is smaller than expected, (1.7 instead of 3.3 µ B ) and than in the Dy compound.…”

“…In R 3 Mg 2 Sb 3 O 14 systems, the symmetry of the rare-earth site is lower than in the pyrochlore lattice, thus lifting the degeneracy of the ground doublet for the non-Kramers ions such as Ho. Due to the small size of the crystal field splitting, Ho 3 Mg 2 Sb 3 O 14 was proposed to be an example of a kagome ice in a transverse field Ising model [102] coupled to a nuclear spin bath, resulting in a quantum spin fragmented state modified by quantum dynamics [101,103].…”

Absence of moment fragmentation in the mixed B -site pyrochlore Nd2GaSbO7

“…Alternatively, this phase can be understood through the superposition of two components: an ordered, divergence-full "all-in-all-out" (AIAO) component and a divergence-free "two-in-two-out" component, which remains fluctuating and provides a gain in entropy [17]. Experimental realizations of such "ground-state moment fragmentation" have been demonstrated in the tripod Kagome 2469-9950/2021/103(21)/214419 (11) 214419-1 ©2021 American Physical Society materials RE 3 Mg 2 Sb 3 O 14 (RE = Dy [18], Ho [19]) and via a staggered magnetic field in the iridate pyrochlores RE 2 Ir 2 O 7 (RE = Dy [20], Ho [21]).…”

PyCrystalField: software for calculation, analysis and fitting of crystal electric field Hamiltonians

“…The user must provide a global 2 function and PyCrystalField will minimize 2 by varying any user-specified variables. The user may fit any set of variables to the data: B m n , point charge location [demonstrated by Scheie et al (2020); a similar method is discussed by Dun et al (2020)], effective charge of the point charges, width of neutron scattering peaks etc. Any constraints may be imposed.…”