Ferromagnetic Transition of Pyrochlore Compound Yb2Ti2O7

摘要: R2Ti2O7 (R = Y and various rare earth elements) has pyrochlore type structure, which consists of two kinds three-dimensional networks individually formed by the corner-sharing R4and Ti4-tetrahedra, respectively. Due to this structural characteristic, magnetic moments at R sites are expected be frustrated, if their nearest neighbor interaction is antiferromagnetic. For Tb, value Weiss temperature w 19K, indicating Tb3þ have antiferromagnetic system does not exhibit clear evidence for ordering. Previously, we investigated both dynamical static properties Tb2Ti2O7 means neutron scattering on a single crystal. Based results measurements specific heat C ac susceptibility , low state was discussed in ref. 4, too. The frustration also even with ferromagnetic interaction, strong uniaxial anisotropy, where each moment lies along local principal axis corresponding line connects site center gravity tetrahedron. (There four axes [111] other crystallographically equivalent directions.) Such situation can found Dy 5) Ho, called ‘‘spin ice’’. Yb2Ti2O7, equal 0:53K as shown inset Fig. 1, between Yb3þ ferromagnetic. electronic ground ion reported Kramers doublet relatively small planar g? 1⁄4 4:27 gk 1:70, g-values perpendicular axis, Sengupta et al. that 0 3:4. A sharp peak C–T curve 0.24K, existence phase transition. However, Hodges did observe reflection except angle diffuse scattring. In order identify anomaly carried out diffraction crystal Yb2Ti2O7 down 0.03K using dilution refrigerator. Here, report exhibits transition TC 0:24K. We discuss behavior moments. grown floating zone (FZ) method. magnetization M measured SQUID magnetometer. method described 4. Neutron were triple spectrometer HQR (T1-1) installed thermal guide JRR-3 JAERI Tokai. oriented [hh0] [00l] plane. Figure 1 shows M–H curves obtained 5K fields [001], [110] [111], anisotropy small. calculated considering an anisotropic molecular field treatment coupling constant . By fitting data, parameters estimated 0:64 0:10T/ B, 3:9 0:2 2:6 0:4. Judging from rather differences among curves, deduced surprisingly large, but least much smaller than refs. 7 8. This understood follows. Because Yb divided into sets different three directions, averaged over these becomes very small, though large within set saturation 1:8 B/Yb. T-dependence real part increasing T 2, been observed 0.24K. 0.24K agree 1=4 N experimental error bar, demagnetization coefficient used sample. M=H 0=ð1þ 4 0NÞ, approaches ! result indicates At 0:03K (