Spin Correlations of Geometrically Frustrated Magnetic System

几何受挫自旋系统由于其迷人的磁性，已经被研究了几十年。一个由平行于局部易轴[111]的自旋组成的立方焦绿石晶格上的Ising模型显示出基态的宏观简并，其方式与水冰中的质子无序相同。局部易轴、最近邻交换相互作用以及角共享四面体的几何形状导致了该系统的受挫。由于基态的局部自旋构型模拟了水冰中的“冰规则”，因此伊辛模型被称为“自旋冰”。通过对神经网络自旋冰模型施加磁场，可以根据磁场的方向和大小部分或完全解除基态的宏观简并。对于平行于[111]方向的场，焦绿石晶格是kagome层和三角形层的交替堆叠，中等强度的场部分提升了简并并同时诱导了层之间的自旋解耦（kagome冰）。在r_2ti_2o_7， R_2Sn_2O_7 （R=Dy, Ho）等焦绿盐氧化物中发现了神经网络铁磁自旋冰模型导致的受挫行为。虽然这些氧化物中的自旋主要通过偶极相互作用相互作用，但蒙特卡罗（MC）模拟研究表明，偶极相互作用带来了有效的神经网络铁磁耦合。本文在推测的kagome冰态下，对Dy_2Ti_2O_7进行了沿场[111]的中子散射实验，目的是对kagome冰态进行微观表征。我们将中子数据与基于偶极相互作用模型的MC模拟结果进行了比较。模拟重现了kagome冰的主要特征：残差熵、接近波矢量的自旋相关（2/3,-2/3,0）和场致降维，与观测结果一致。

Geometrically frustrated spin systems have been investigated for decades because of their fascinating magnetic properties. An Ising model on a cubic pyrochlore lattice consisting of spins parallel to local [111] easy axes was shown to exhibit macroscopic degeneracy in the ground states in the same way as the proton disorder in water ice does. The local easy axis, nearest-neighbor (NN) exchange interaction, and the geometry of corner sharing tetrahedra give rise to frustration in this system. Since the local spin configuration of the ground states simulates the "ice rule" in water ice, the Ising model is referred to as "spin ice". By applying magnetic fields to the NN spin-ice model the macroscopic degeneracy of the ground states can be partly or fully lifted depending on the field direction and magnitude. For a field parallel to a [111] direction, along which the pyrochlore lattice is alternating stacking of kagome and triangular layers, the field with intermediate strengths partly lifts the degeneracy and simultaneously induces decoupling of spins between the layers (kagome ice).Frustrated behavior attributable to the NN ferromagnetic spin-ice model was discovered in pyrochlore oxides, e.g.R_2Ti_2O_7, R_2Sn_2O_7 (R=Dy, Ho). Although spins in these oxides interact dominantly by the dipolar interaction, Monte Carlo (MC) simulation studies showed that the dipolar interaction brings about an effective NN ferromagnetic coupling. In this work, we performed neutron scattering experiments on Dy_2Ti_2O_7 with field along [111] in the conjectured kagome ice regime, aiming at microscopic demonstration of the kagome ice state. We compared the neutron data with MC simulations based on the dipolar interacting model. The simulations reproduce the main characteristics of the kagome ice : the residual entropy, the spin correlations close to the wave vector (2/3,-2/3, 0), and the field-induced dimensional reduction, being consistent with observations.