Ferromagnetic transition of pyrochlore compound Yb2Ti2O7
Ferromagnetic transition of pyrochlore compound Yb2Ti2O7
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DOI：
10.1143/jpsj.72.3014
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发表时间：
2003-11
影响因子：
1.7
通讯作者：
Y. Yasui;M. Soda;S. Iikubo;Masafumi Ito;M. Sato;Nobuko Hamaguchi;T. Matsushita;N. Wada;T. Takeuchi;N. Aso;K. Kakurai
中科院分区：
文献类型：
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作者：
Y. Yasui;M. Soda;S. Iikubo;Masafumi Ito;M. Sato;Nobuko Hamaguchi;T. Matsushita;N. Wada;T. Takeuchi;N. Aso;K. Kakurai
R2Ti2O7 （R = Y和各种稀土元素）具有焦绿石型结构，由共角的r4和ti4四面体分别形成的两种三维网络组成。由于这种结构特征，如果它们最近的邻居相互作用是反铁磁性的，则R位的磁矩预计会受到挫折。当R = Tb时，Weiss温度w值为19K，表明Tb3þ矩存在反铁磁最近邻相互作用，系统没有明显的磁有序证据。在此之前，我们利用单晶上的中子散射研究了Tb2Ti2O7的动态和静态磁性能。参考文献4也根据比热C和交流磁化率的测量结果，讨论了Tb2Ti2O7的低温态。即使对于具有铁磁最近邻相互作用的系统，如果力矩具有很强的单轴各向异性，其中每个力矩都位于与四面体重心连接的位置对应的局部主轴上，也会出现挫折。（沿[111]和其他晶体学等效方向有四个主轴。）这种情况可以在R2Ti2O7 （R = dy5）和Ho中发现，它们被称为“自旋冰”。对于Yb2Ti2O7， w = 0:53K，如图1所示，表明Yb3þ矩之间的最近邻相互作用是铁磁性的。据报道，yb3 +离子的电子基态是具有相对较小的平面各向异性的克莱默斯双重态，g？1⁄4 4:27和gk 1⁄4 1:70，其中g？gk分别是垂直于和沿着局部主轴的g值。Sengupta等人报道该体系具有单轴各向异性，g？1 / 4 0和gk 1 / 4 3:4。在0.24K处，C-T曲线出现尖峰，表明相变的存在。而Hodges等人除了小角度漫反射外，没有观察到磁反射。为了确定在0.24K处的比热异常，我们利用稀释制冷机对低至0.03K的Yb2Ti2O7单晶进行了中子衍射和其他磁测量。在这里，我们报告了系统在TC 1 / 4 0:24K时表现出铁磁跃迁。我们还讨论了这些矩的低温行为。采用浮区法生长Yb2Ti2O7单晶。用SQUID磁强计测量磁化强度M。交流磁化率的测定方法见文献4。利用安装在东海JAERI JRR-3热导板上的三轴光谱仪HQR （T1-1）进行中子测量。晶体在散射面上以[h0]和[00l]轴取向。图1为5K下沿[001]、[110]、[111]方向磁场下得到的Yb2Ti2O7的M-H曲线，其中各向异性较小。我们计算了考虑各向异性Kramers双重态的M-H曲线，并使用带耦合常数的铁磁相互作用的分子场处理。通过将计算曲线拟合到数据中，估计参数为1 / 4 0:64 0:10T/ B， g？1⁄4 3:9 0:2和1⁄4 2:6 0:4。从M-H曲线之间相当小的差异判断，推断出的各向异性惊人地大，但至少比参考文献中报道的要小得多。7和8。这可以理解如下。由于Yb点沿[111]被分成四个具有不同局部主轴和三个等效方向的集合，这些集合上的矩的平均各向异性变得非常小，尽管每个Yb矩集合内的各向异性相当大。由M-H曲线估计的饱和磁化强度为1:8 B/Yb。随着T的增加，Yb2Ti2O7的交流磁化率0实部的T依赖性如图2的插入部分所示，其中在0.24K处的T依赖性中观察到明显的异常。在0.24K处发现0的值与实验误差条内1=4 N的值一致，其中N为所用样品的退磁系数。因为M=H被描述为0=ð1þ 4 0NÞ，接近1=4 N为0 ！结果表明，该体系在TC为1⁄4 0:24K时发生铁磁跃迁。在1:4 0:03K (
R2Ti2O7 (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 R4and Ti4-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 19K, indicating the Tb3þ 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 Tb2Ti2O7 by means of neutron scattering on a single crystal. Based on results of measurements of the specific heat C and ac magnetic susceptibility , low temperature state of Tb2Ti2O7 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 R2Ti2O7 with R = Dy 5) and Ho, which are called ‘‘spin ice’’. For Yb2Ti2O7, w is equal to be 0:53K as shown in the inset of Fig. 1, indicating the nearest neighbor interaction between the Yb3þ moments is ferromagnetic. The electronic ground state of Yb3þ ion was reported to be a Kramers doublet with relatively small planar anisotropy, g? 1⁄4 4:27 and gk 1⁄4 1:70, where g? and gk are the g-values perpendicular to and along the local principal axis, respectively. Sengupta et al. reported that the system has uniaxial anisotropy, g? 1⁄4 0 and gk 1⁄4 3:4. A sharp peak of C–T curve was reported at 0.24K, indicating the existence of the phase transition. However, Hodges et al. did not observe magnetic reflection except the small angle diffuse scattring. In order to identify the specific heat anomaly at 0.24K, we have carried out neutron diffraction and other magnetic measurements on a single crystal of Yb2Ti2O7 down to 0.03K by using dilution refrigerator. Here, we report that the system exhibits ferromagnetic transition at TC 1⁄4 0:24K. We also discuss the low temperature behavior of the moments. A single crystal of Yb2Ti2O7 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 Yb2Ti2O7 obtained at 5K with the magnetic fields along [001], [110] and [111], where the anisotropy of the curves is found to be relatively small. We calculated the M–H curves considering an anisotropic Kramers doublet and using a molecular field treatment of the ferromagnetic interaction with a coupling constant . By fitting the calculated curve to the data, the parameters are estimated to be 1⁄4 0:64 0:10T/ B, g? 1⁄4 3:9 0:2 and gk 1⁄4 2:6 0:4. Judging from the rather small differences among the M–H curves, the deduced anisotropy is surprisingly large, but at least much smaller than that reported in refs. 7 and 8. This can be understood as follows. Because Yb sites are divided into four sets with different local principal axes along [111] and three equivalent directions, the averaged anisotropy of the moments over these sets becomes very small, even though the anisotropy is rather large within each set of the Yb moments. The value of saturation magnetization estimated from the M–H curves is 1:8 B/Yb. The T-dependence of the real part of the ac magnetic susceptibility 0 of Yb2Ti2O7 measured with increasing T is shown in the inset of Fig. 2, where a clear anomaly has been observed in the T-dependence at 0.24K. The value of 0 at 0.24K is found to agree with the value of 1=4 N within the experimental error bar, where N is the demagnetization coefficient of the used sample. Because M=H is described as 0=ð1þ 4 0NÞ, which approaches 1=4 N as 0 ! 1, the result indicates that the system exhibits a ferromagnetic transition at TC 1⁄4 0:24K. At T 1⁄4 0:03K (