Orientation dependence of the magnetic phase diagram of Yb2Ti2O7

Abstract

In the quest to realize a quantum spin liquid (QSL), magnetic long-range order is hardly welcome. Yet it can offer deep insights into a complex world of strong correlations and fluctuations. Much hope was placed in the cubic pyrochlore Yb2Ti2O7 as a putative U(1) QSL but a new class of ultrapure single crystals make it abundantly clear the stoichiometric compound is a ferromagnet. Here we present a detailed experimental and theoretical study of the corresponding field-temperature phase diagram. We find it to be richly anisotropic with a critical endpoint for B 100 , while field parallel to 110 and 111 enhances the critical temperature by up to a factor of two and shifts the onset of the field-polarized state to finite fields. Landau theory shows that Yb2Ti2O7 in some ways is remarkably similar to pure iron. However, it also pinpoints anomalies that cannot be accounted for at the classical mean-field level including a dramatic enhancement of TC and reentrant phase boundary by fields with a component transverse to the easy axes, as well as the anisotropy of the upper critical field in the quantum limit.

Multiphase magnetism in Yb 2 Ti 2 O 7

“…We also collected scattering data at 20 K for use as a background. [The elastic line for the in-field data appears oversubtracted-this is due to the field-dependent extinction in Yb2Ti2O7 (31).] We subtracted this background from all datasets, divided by the squared Yb 3+ magnetic form factor, symmetrized the data using Mantid (32), and made cuts along high-symmetry directions Γ (000) → K (220) → UL (211) → Γ (000).…”

Understanding Reentrance in Frustrated Magnets: the Case of the Er$_2$Sn$_2$O$_7$ Pyrochlore

“…In semi-classical and quantum systems, our mechanism may work together with the field-induced suppression of quantum fluctuations [29] to produce even larger reentrant lobes. We hope our work will motivate others to pursue a microscopic interpretation of future observations of reentrance (and possibly to revisit old ones [8][9][10][11][12][13][14][15][16]) in light of zero-temperature transitions. Since magnetic systems often afford us with minimal models to understand other areas of physics, our results raise a more general question: if reentrance is observed by varying a given parameter, when is it actually due to a nearby transition in a broader parameter space?…”

“…1(b,c)]. Because of this anisotropy, the response of Er 2 Sn 2 O 7 to an applied field is expected to differ with field direction, as has been poignantly illustrated with the experimental exploration of rare earth pyrochlore titanates [5,6,11,12,35]. As such, the present study critically relies on the recently gained ability to synthesize pyrochlore stannate single crystals [40], including Er 2 Sn 2 O 7 .…”

“…This is particularly important when T c is just above the experimental baseline temperature, so that temperatures which are low relative to T c cannot be reached. Here we precisely consider such a situation, as arises in the Er 2 Sn 2 O 7 pyrochlore antiferromagnet, and which provides an opportunity to study a recurrent aspect of frustrated magnetic systems observed at nonzero temperature: reentrance [8][9][10][11][12][13][14][15][16].…”

“…Despite its ubiquity, reentrance is typically unexpected and its explanation in terms of entropic contributions to the free-energy from the underlying microscopic degrees of freedom is usually subtle. In this context, while frequently observed in frustrated magnets, the microscopic mechanism leading to reentrance often remains obscure [8][9][10][11][12][13][14][15]. Two mechanisms have commonly been invoked: a field-dependent suppression of quantum fluctuations [29][30][31] and the partial disorder of an intervening phase [18,[32][33][34].…”