Spin Correlations of Geometrically Frustrated Magnetic System

几何阻挫自旋系统由于其迷人的磁性已经被研究了几十年。在立方烧绿石晶格上的伊辛模型，由平行于局部[111]易轴的自旋组成，在基态表现出宏观简并性，与水冰中的质子无序相同。局部易轴，最近邻（NN）交换相互作用，和角共享四面体的几何形状在这个系统中引起挫折。由于基态的局域自旋构型模拟了水冰中的“冰规则”，伊辛模型被称为“自旋冰”。通过对NN自旋冰模型施加磁场，基态的宏观简并性可以部分或完全提升，这取决于磁场的方向和大小。对于平行于[111]方向的场，烧绿石晶格是kagome和三角形层交替堆叠的，中等强度的场部分地提高了简并度，同时引起层间自旋的退耦在烧绿石氧化物如R_2Ti_2O_7中发现了NN铁磁自旋冰模型的阻挫行为，R_2Sn_2O_7（R=Dy，Ho）。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NN铁磁耦合。在这项工作中，我们进行了中子散射实验的Dy_2Ti_2O_7与场沿着[111]在turned kagome冰制度，旨在微观演示kagome冰状态。我们比较了中子数据与MC模拟的基础上偶极相互作用模型。模拟再现了可果美冰的主要特征：剩余熵，自旋相关接近波矢（2/3，-2/3，0），和场诱导的降维，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.