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Tweaking the spin-wave dispersion and suppressing the incommensurate phase in LiNiPO_4 by iron substitution

机译:调节自旋波色散并通过铁取代抑制LiNiPO_4中不相称的相

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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 (magnetic soft mode) indicates the instability of the Ising-type 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 (AFM) ground state at 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 interplane and in-plane nearest- and next-nearest-neighbor couplings reveals frustration due to strong competing interactions between nearest- and next-nearest-neighbor sites, consistent with the observed incommensurate structure. The Fe substitution only slightly lowers the extent of the frustration, sufficient to suppress the incommensurate phase. An energy gap in the dispersion curves gradually decreases with the increase in Fe content from ~2 meV for the pure system (x=0) to ~0.9 meV for x=0.2.
机译:Li(Ni_(1-x)Fe_x)PO_4单晶的弹性和非弹性中子散射研究表明,沿着平行于磁性不相称特征波矢量的晶体学方向异常的自旋波色散。异常的自旋波色散(软磁模式)表示伊辛型基态的不稳定性,随着温度的升高最终发展成不相称的相。纯LiNiPO_4系统(x = 0)在T_N = 20.8 K时经历了从远距离不相称相到反铁磁(AFM)基态的一阶磁性相变。在20%Fe浓度下,尽管AFM基态为在很大程度上保留了纯系统的相变是二阶的,并且不相称的相被完全抑制了。使用海森堡自旋哈密顿量分析色散曲线,其中包括平面内和平面内最近和下一个最近的耦合,揭示了由于最近和下一个最近的站点之间强烈的竞争性相互作用而产生的挫败感,与观察到的不对称结构一致。 Fe的替代只会稍微降低无奈程度,足以抑制不相称的相。随着Fe含量的增加,色散曲线中的能隙逐渐减小,从纯系统的〜2 meV(x = 0)到x = 0.2的〜0.9 meV。

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