Ferromagnetic Transition of Pyrochlore Compound Yb₂Ti₂O₇

Abstract

R 2 Ti 2 O 7 (R = Y and various rare earth elements) has pyrochlore type structure, which consists of two kinds of three-dimensional networks individually formed by the corner-sharing R 4 -and Ti 4 -tetrahedra, respectively. Due to this structural characteristic, magnetic moments at the R sites are expected to be frustrated, if their nearest neighbor interaction is antiferromagnetic.For R = Tb, the value of Weiss temperature w is À19 K, indicating the Tb 3þ moments have antiferromagnetic nearest neighbor interaction and the system does not exhibit clear evidence for magnetic ordering. Previously, we investigated both the dynamical and static magnetic properties of Tb 2 Ti 2 O 7 by means of neutron scattering on a single crystal. 1-3) Based on results of measurements of the specific heat C and ac magnetic susceptibility , low temperature state of Tb 2 Ti 2 O 7 was discussed in ref. 4, too.The frustration is also expected even for the system with the ferromagnetic nearest neighbor interaction, if the moments have strong uniaxial anisotropy, where each moment lies along the local principal axis corresponding to the line which connects the site with the center of gravity of the tetrahedron. (There are four principal axes along [111] and other crystallographically equivalent directions.) Such the situation can be found in R 2 Ti 2 O 7 with R = Dy 5) and Ho, 6) which are called ''spin ice''.For Yb 2 Ti 2 O 7 , w is equal to be $0:53 K as shown in the inset of Fig. 1, indicating the nearest neighbor interaction between the Yb 3þ moments is ferromagnetic. The electronic ground state of Yb 3þ ion was reported to be a Kramers doublet with relatively small planar anisotropy, g ? ¼ 4:27 and g k ¼ 1:70, where g ? and g k are the g-values perpendicular to and along the local principal axis, respectively. 7) Sengupta et al. reported that the system has uniaxial anisotropy, g ? ¼ 0 and g k ¼ 3:4. 8) A sharp peak of C-T curve was reported at 0.24 K, indicating the existence of the phase transition. 9) However, Hodges et al. did not observe magnetic reflection except the small angle diffuse scattring. 7) In order to identify the specific heat anomaly at 0.24 K, we have carried out neutron diffraction and other magnetic measurements on a single crystal of Yb 2 Ti 2 O 7 down to 0.03 K by using dilution refrigerator. Here, we report that the system exhibits ferromagnetic transition at T C ¼ 0:24 K. We also discuss the low temperature behavior of the moments.A single crystal of Yb 2 Ti 2 O 7 was grown by a floating zone (FZ) method. The magnetization M was measured by using a SQUID magnetometer. The method of the ac magnetic susceptibility is described in ref. 4. Neutron measurements were carried out by using the triple axis spectrometer HQR (T1-1) installed at the thermal guide of JRR-3 of JAERI in Tokai. The crystal was oriented with [hh0] and [00l] axes in the scattering plane. Figure 1 shows the M-H curves of Yb 2 Ti 2 O 7 obtained at 5 K with the magnetic fields along [001], [110] and [111], where the anisotropy of the curves is ...

A novel type of splayed ferromagnetic order observed in Yb2Ti2O7

“…The site numbering refers to Table 1. Discussion -The collinear ferromagnetic structure [27] previously deduced from measurements on a single crystal with a specific heat anomaly at T c ≈ 195 mK [31] is inconsistent with our result. In Table 2 we compare the magnetic intensities measured at the position of the first three Bragg peaks authorised in a face-centred cubic structure with the intensities obtained for the best fit to the collinear ferromagnetic structure.…”

“…However, within our experimental uncertainties we found them equal, consistent with the 170 Yb Mössbauer spectroscopy results [22]. Their amplitude is m sp = 0.95 (2) µ B , a value reasonably close to the estimate 1.1 (1) µ B from neutron diffraction on a single crystal [27], although interpreted with a different magnetic structure -see below -and 1.15 µ B from Mössbauer spectroscopy [22]. In Fig.…”

“…Magnetic Bragg reflections are reported only in Refs. [27,31] which describe data measured on the same crystal. The diversity of the specific heat [33,34,31,35] and muon spin relaxation (µSR) results [22,36,25,23] adds to the difficulty in understanding the ground state.…”

Neutron scattering studies of spin ices and spin liquids

“…Yb 2 Ti 2 O 7 was originally thought to be a type of XY -ferromagnet on the pyrochlore lattice [125,126], but definitive evidence of static order below a phase transition at 0.24 K remained conflicting [24,[126][127][128]. This was compounded by irregular existence of the phase transition in different samples [129,130] which is now attributed to very small variations in stoichiometry [130,131].…”

Phase Competition in the Palmer-Chalker XY Pyrochlore Er2Pt2O<

“…Of these states, all but Γ 7 have been experimentally observed in the XY pyrochlores. This XY family is made up of Yb 2 B 2 O 7 and Er 2 B 2 O 7 with B = Ge, Ti, and Sn, where: (i) The order-by-disorder candidate Er 2 Ti 2 O 7 orders into ψ 2 [17] (ii) Er 2 Ge 2 O 7 and Yb 2 Ge 2 O 7 have as-of-yet unidentified ordered states within Γ 5 (ψ 2 or ψ 3 ) [12,18] and (iii) Both Yb 2 Sn 2 O 7 [19,20] and some samples of Yb 2 Ti 2 O 7 [21][22][23][24][25] order into the Γ 9 splayed ferromagnetic state. This ensemble of magnetic ground states supports the picture of a rich phase space.…”