Anomalously slow spin dynamics and short-range correlations in the quantum spin ice systemsYb2Ti2O7andYb2

Abstract

We report a positive muon spin relaxation and rotation (µSR) study of the quantum spin ice materials Yb2Ti2O7 and Yb2Sn2O7 focusing on the low field response. In agreement with earlier reports, data recorded in small longitudinal fields evidence anomalously slow spin dynamics in the microsecond range below the temperature Tc at which the specific heat displays an intense peak, namely Tc = 0.24 K and 0.15 K, respectively, for the two systems. We found that slow dynamics extends above Tc up to at least 0.7 K for both compounds. The conventional dynamical Gaussian Kubo-Toyabe model describes the µSR spectra recorded above Tc. At lower temperatures a published analytical extension of the Gaussian Kubo-Toyabe model provides a good description, consistent with the existence of short-range magnetic correlations. While the physical response of the two systems is qualitatively the same, Yb2Ti2O7 exhibits a much larger local magnetic susceptibility than Yb2Sn2O7 below Tc. Considering previously reported ac susceptibility, neutron scattering and µSR results, we suggest the existence of anomalously slow spin dynamics to be a common physical property of pyrochlore magnetic materials. The possibility of molecular spin substructures to be associated to the slow dynamics and therefore the short-range correlations is mentioned. The slow spin dynamics observed under field does not exclude the presence of much faster dynamics detected in extremely low or zero field.

A novel type of splayed ferromagnetic order observed in Yb2Ti2O7

“…The residual difference could be tentatively assigned to short-range magnetic correlations observed in several instances [31,49,32], while the diffraction experiment reported here only picks up the long-range ordered component. The signature of these short-range correlations seems to disappear below T c [31], but they are still observed in µSR data [49].…”

“…These features are suppressed at T c . The magnetic susceptibility below T c is stronger in the titanate than in the stannate [49].…”

“…Short-range correlations in the ordered state were certainly unexpected. In terms of characteristic fluctuation times, there is a large range of magnetic fluctuation modes, some of them being almost similar in the paramagnetic (still below ≈ 0.7 K) and ordered phases [49,32]. The magnetization curves along the [100] and [110] directions have been determined [48].…”

“…On the theoretical side, the ubiquitous slow spin dynamics [22,36,50,48,49], observed even below T c has to be described. In fact, the most amazing property is not the presence of magnetic order at low temperature -conventional magnetic compounds do display magnetic ordering at low temperature -but the existence of exotic spin correlations in both the paramagnetic and ordered regimes, with the possibility of competing mechanisms at their origin [53].…”

Reverse Monte Carlo Algorithm and Maximum Entropy Principle for the Analysis of Positive Muon Spin Rotation and Relaxation Spectra

“…Recently the RMC technique supplemented by the maximum entropy (ME) principle [9,10] -to simplify notations we write ME rather than the more common MaxEnthas been introduced to analyze positive muon spin rotation and relaxation (µSR) data [11][12][13][14][15][16]. It is used when no obvious fitting function can be devised for modeling an asymmetry spectrum.…”