Dipolar Spin Ice States with a Fast Monopole Hopping Rate in CdEr2X4 ( X=Se

Abstract

Excitations in a spin ice behave as magnetic monopoles, and their population and mobility control the dynamics of a spin ice at low temperature. CdEr_{2}Se_{4} is reported to have the Pauling entropy characteristic of a spin ice, but its dynamics are three orders of magnitude faster than the canonical spin ice Dy_{2}Ti_{2}O_{7}. In this Letter we use diffuse neutron scattering to show that both CdEr_{2}Se_{4} and CdEr_{2}S_{4} support a dipolar spin ice state-the host phase for a Coulomb gas of emergent magnetic monopoles. These Coulomb gases have similar parameters to those in Dy_{2}Ti_{2}O_{7}, i.e., dilute and uncorrelated, and so cannot provide three orders faster dynamics through a larger monopole population alone. We investigate the monopole dynamics using ac susceptometry and neutron spin echo spectroscopy, and verify the crystal electric field Hamiltonian of the Er^{3+} ions using inelastic neutron scattering. A quantitative calculation of the monopole hopping rate using our Coulomb gas and crystal electric field parameters shows that the fast dynamics in CdEr_{2}X_{4} (X=Se, S) are primarily due to much faster monopole hopping. Our work suggests that CdEr_{2}X_{4} offer the possibility to study alternative spin ice ground states and dynamics, with equilibration possible at much lower temperatures than the rare earth pyrochlore examples.

Deviation from the dipole-ice model in the spinel spin-ice candidate MgEr2Se4

“…The H = 0 T data reveal a sizable residual entropy which is partially relieved with small applied fields, broadly consistent with spin-ice behavior 6 , but significantly less than the value of 1/2 ln (3/2) predicted for the DSI model 6,27 . This is true for both the current data, but also the data taken from Ref 66 on related material CdEr 2 Se 4 after correcting for contributions from subsequently measured CEF excitations measured recently 67 . Intriguingly, one sees that the remnant entropy of both systems approach the same value, but fall far short of the full Pauling value.…”

“…Ferromagnetically coupled 111 easy axis spins reminiscent of spin ices have been reported in several spinels leading to frustration observed through diffuse scattering in single crystals 62 , or leading to two-in-two-out ordered states of the Bsite sublattice 60,[63][64][65] . In the singular system, CdEr 2 Se 4 , remnant Pauling entropy has been reported 66 , and a very recent study on the same material has claimed DSI-like spin-correlations and an anomalously fast monopole hopping rate 67 .…”

Design and implementation of a low temperature, inductance based high frequency alternating current susceptometer

“…Spin ices are materials in which geometric frustration on the crystal lattice causes novel magnetic behavior, including emergent quasi-particles and a residual entropy in the ground state 13 . High frequency susceptibility measurements on CdEr 2 S 4 with the detailed in-phase and out-of-phase components shown in figure 7a and b were performed as part of a recent study into this material 14 .…”

Crystal electric field excitations in the quantum spin liquid candidate NaErS2

“…In order to facilitate the QSL search in the delafossites, it is crucial to have an overview of their CEF environment. As exemplified by the spin ice compounds [40][41][42][43], the CEF parameters in systems with similar structures normally obey the scaling rule. Therefore, compared to the Yb 3+ ions with J = 7/2 [35,39], rare earth ions with a larger J allow more CEF transitions, which will enable a more accurate fit of the CEF parameters and thus provide a reference in the study of the similar delafossite compounds.…”