Cu2GaBO5和Cu2AlBO5类陆氏体中长程磁序的外磁场破坏及其自旋动力学
Destruction of long-range magnetic order in an external magnetic field and the associated spin dynamics in Cu2GaBO5 and Cu2AlBO5 ludwigites
摘要：
具有路德维希晶体结构的量子自旋系统Cu $_2$ M 'BO $_5$（M' = Al，Ga）由结构有序的Cu $^{2+}$亚晶格组成，该亚晶格呈三腿梯状，并被结构无序的亚晶格贯穿，磁性Cu $^{2+}$和Ga $^{3+}$或Al $^{3+}$离子在亚晶格中具有统计上的随机占位。基于密度泛函理论计算的Cu $_2$ GaBO $_5$的微观分析揭示了具有五个不等价反铁磁交换的受抑准二维自旋模型。一个广泛的低温$^{11}$ B核磁共振指向一个相当大的自旋无序的系统。在零磁场下，Ga和Al化合物的反铁磁有序分别在低于$T_\text{N}$ $\approx$ 4.1 K和~2.4 K的温度下发生.从中子衍射中，我们发现Cu $_2$ GaBO $_5$中的磁传播矢量是公度的，位于（H_0 L）面的布里渊区边界上，$\mathbf{q}_\text{m}$ =（0.45 0 - 0.7），对应于一个具有大磁晶胞的复杂的非共线长程有序结构.μ子自旋弛豫是单调的，由复杂的非共线自旋系统典型的快速静态分量和源自低能自旋波动弛豫的缓慢动态分量组成。中子非弹性散射也证明了扩散准弹性峰形式的无隙自旋动力学。最值得注意的是，施加高于1 T的磁场会破坏静态长程有序，这表现为磁布拉格峰的逐渐加宽。我们认为，这种从磁长程有序状态的自旋玻璃制度的交叉可能会导致孤儿自旋的结构无序的磁子晶格，这是极化的磁场，从而作为一个调谐旋钮场控制的磁无序。
英文摘要：
The quantum spin systems Cu$_2$M'BO$_5$ (M' = Al, Ga) with the ludwigite crystal structure consist of a structurally ordered Cu$^{2+}$ sublattice in the form of three-leg ladders, interpenetrated by a structurally disordered sublattice with a statistically random site occupation by magnetic Cu$^{2+}$ and nonmagnetic Ga$^{3+}$ or Al$^{3+}$ ions. A microscopic analysis based on density-functional-theory calculations for Cu$_2$GaBO$_5$ reveals a frustrated quasi-two-dimensional spin model featuring five inequivalent antiferromagnetic exchanges. A broad low-temperature $^{11}$B nuclear magnetic resonance points to a considerable spin disorder in the system. In zero magnetic field, antiferromagnetic order sets in below $T_\text{N}$ $\approx$ 4.1 K and ~2.4 K for the Ga and Al compounds, respectively. From neutron diffraction, we find that the magnetic propagation vector in Cu$_2$GaBO$_5$ is commensurate and lies on the Brillouin-zone boundary in the (H0L) plane, $\mathbf{q}_\text{m}$ = (0.45 0 -0.7), corresponding to a complex noncollinear long-range ordered structure with a large magnetic unit cell. Muon spin relaxation is monotonic, consisting of a fast static component typical for complex noncollinear spin systems and a slow dynamic component originating from the relaxation on low-energy spin fluctuations. Gapless spin dynamics in the form of a diffuse quasielastic peak is also evidenced by inelastic neutron scattering. Most remarkably, application of a magnetic field above 1 T destroys the static long-range order, which is manifested in the gradual broadening of the magnetic Bragg peaks. We argue that such a crossover from a magnetically long-range ordered state to a spin-glass regime may result from orphan spins on the structurally disordered magnetic sublattice, which are polarized in magnetic field and thus act as a tuning knob for field-controlled magnetic disorder.
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