London Centre for Nanotechnology and Department of Physics and Astronomy, University College London, 17-19 Gordon Street, London WC1H 0AJ, UK.
J Phys Condens Matter. 2013 Sep 25;25(38):386002. doi: 10.1088/0953-8984/25/38/386002. Epub 2013 Aug 29.
We present an experimental determination of the isothermal magnetic susceptibility of the spin ice materials Dy2Ti2O7 and Ho2Ti2O7 in the temperature range 1.8-300 K. The use of spherical crystals has allowed accurate correction for demagnetizing fields and allowed the true bulk isothermal susceptibility χT(T) to be estimated. This has been compared against a theoretical expression based on a Husimi tree approximation to the spin ice model. Agreement between experiment and theory is excellent at T > 10 K, but systematic deviations occur below that temperature. Our results largely resolve an apparent disagreement between neutron scattering and bulk measurements that has been previously noted. They also show that the use of non-spherical crystals in magnetization studies of spin ice may introduce very significant systematic errors, although we note some interesting--and possibly new--systematics concerning the demagnetizing factor in cuboidal samples. Finally, our results show how experimental susceptibility measurements on spin ices may be used to extract the characteristic energy scale of the system and the corresponding chemical potential for emergent magnetic monopoles.
我们在 1.8-300 K 的温度范围内对自旋冰材料 Dy2Ti2O7 和 Ho2Ti2O7 的等温磁化率进行了实验测定。球形晶体的使用允许对退磁场进行精确修正，并允许估计真实的体等温磁化率 χT(T)。这与基于自旋冰模型的 Husimi 树近似的理论表达式进行了比较。在 T > 10 K 时，实验与理论之间的一致性非常好，但在该温度以下会出现系统偏差。我们的结果在很大程度上解决了先前指出的中子散射和体测量之间明显的不一致。它们还表明，在自旋冰的磁化研究中使用非球形晶体可能会引入非常显著的系统误差，尽管我们注意到关于立方样品的退磁因子的一些有趣的——可能是新的——系统。最后，我们的结果表明，自旋冰的实验磁化率测量如何用于提取系统的特征能量尺度和出现磁单极子的相应化学势。
