Department of Physics, Royal Holloway, University of London, Egham TW20 0EX, UK.
ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot OX11 0QX, UK.
Nat Mater. 2014 May;13(5):488-93. doi: 10.1038/nmat3924. Epub 2014 Apr 13.
The idea of magnetic monopoles in spin ice has enjoyed much success at intermediate temperatures, but at low temperatures a description in terms of monopole dynamics alone is insufficient. Recently, numerical simulations were used to argue that magnetic impurities account for this discrepancy by introducing a magnetic equivalent of residual resistance in the system. Here we propose that oxygen deficiency is the leading cause of magnetic impurities in as-grown samples, and we determine the defect structure and magnetism in Y2Ti2O7-δ using diffuse neutron scattering and magnetization measurements. These defects are eliminated by oxygen annealing. The introduction of oxygen vacancies causes Ti(4+) to transform to magnetic Ti(3+) with quenched orbital magnetism, but the concentration is anomalously low. In the spin-ice material Dy2Ti2O7 we find that the same oxygen-vacancy defects suppress moments on neighbouring rare-earth sites, and that these magnetic distortions markedly slow down the long-time monopole dynamics at sub-Kelvin temperatures.
在中间温度下，自旋冰中磁单极子的概念取得了很大的成功，但在低温下，仅用单极子动力学来描述是不够的。最近，数值模拟被用来证明，磁性杂质通过在系统中引入磁等效残余电阻来解释这种差异。在这里，我们提出在生长的样品中，氧空位是磁性杂质的主要原因，并使用漫散射中子散射和磁化率测量来确定 Y2Ti2O7-δ 的缺陷结构和磁性。通过氧退火可以消除这些缺陷。氧空位的引入导致 Ti(4+)转变为具有淬灭轨道磁矩的磁性 Ti(3+)，但浓度异常低。在自旋冰材料 Dy2Ti2O7 中，我们发现相同的氧空位缺陷抑制了相邻稀土位的磁矩，并且这些磁畸变显著降低了亚开尔文温度下的长时间磁单极子动力学。
