铁掺杂对LiNiPO $_4$中自旋波色散的调制和无公度相的抑制
Tweaking the spin-wave dispersion and suppressing the incommensurate phase in LiNiPO$_4$ by iron substitution
摘要：
Li（Ni $_{1-x}$ Fe $_{x}$）PO $_4$单晶的弹性和非弹性中子散射研究揭示了沿平行于磁非公度相特征波矢的结晶方向的反常自旋波色散。反常的自旋波色散（{ \it磁软模}）表明类伊辛基态的不稳定性，随着温度的升高，最终演变成无公度相。纯LiNiPO $_4$体系（$x=0$）在{ \it T} $_N$ = 20.8 K时经历了从长程无公度相到反铁磁基态的一级磁相变。在20%Fe浓度下，虽然AFM基态在很大程度上保持为纯系统的基态，但相变是二阶的，并且无公度相被完全抑制。使用海森堡自旋哈密顿量，包括面内和面内最近和次近邻耦合的色散曲线的分析揭示了挫折，由于最近和次近邻站点之间的强竞争相互作用，与所观察到的无公度结构一致。Fe取代仅略微降低了挫折的程度，足以抑制IC相。色散曲线中的能隙随着Fe含量的增加而逐渐减小，从纯体系（$x=0$）的$\sim$ 2 meV到$x=0.2$的$\sim$ 0.9 meV。
英文摘要：
Elastic and inelastic neutron scattering studies of Li(Ni$_{1-x}$Fe$_{x}$)PO$_4$ single crystals reveal anomalous spin-wave dispersions along the crystallographic direction parallel to the characteristic wave vector of the magnetic incommensurate phase. The anomalous spin-wave dispersion ({\it magnetic soft mode}) indicates the instability of the Ising-like ground state that eventually evolves into the incommensurate phase as the temperature is raised. The pure LiNiPO$_4$ system ($x=0$), undergoes a first-order magnetic phase transition from a long-range incommensurate phase to an antiferromagnetic ground state at {\it T}$_N$ = 20.8 K. At 20% Fe concentrations, although the AFM ground state is to a large extent preserved as that of the pure system, the phase transition is second-order, and the incommensurate phase is completely suppressed. Analysis of the dispersion curves using a Heisenberg spin Hamiltonian that includes inter- and in-plane nearest and next-nearest neighbor couplings reveals frustration due to strong competing interactions between nearest- and a next-nearest neighbor site, consistent with the observed incommensurate structure. The Fe substitution only slightly lowers the extent of the frustration, sufficient to suppress the IC phase. An energy gap in the dispersion curves gradually decreases with the increase of Fe content from $\sim$2 meV for the pure system ($x=0$) to $\sim$0.9 meV for $x=0.2$.
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