Department of Chemistry , University of Manitoba , Winnipeg , MB R3T 2N2 , Canada.
ISIS Pulsed Neutron and Muon Source , STFC Rutherford Appleton Laboratory , Harwell Campus , Didcot OX11 0QX , United Kingdom.
Inorg Chem. 2019 Apr 1;58(7):4300-4309. doi: 10.1021/acs.inorgchem.8b03419. Epub 2019 Mar 13.
SrLaMnO is a layered antiferromagnetic (AF) oxide with the tetragonal ( I4/ mmm) n = 1 Ruddlesden-Popper phase structure (also known as the KNiF structure) with T = 128 K. Remarkably, substitution of Sr by Ba, forming the solid solution BaSr LaMnO, results in the destruction of long-range magnetic order and of the ordered moment on Mn for x > 0.35, although the effective paramagnetic moment remains unchanged, an unprecedented behavior for this class of magnetic materials. Four members, x = 0.0, 0.25, 0.35, and 1.0, have been studied using XYZ neutron polarization analysis which permits isolation of the magnetic, nuclear and nuclear spin components of the scattering and the measurement of the absolute value of the magnetic cross section. Data analysis is done using model independent reverse Monte Carlo methods (SPINVERT). The results for x = 0.0 (SrLaMnO), T > T(128 K) show an asymmetric diffuse peak which evolves into resolution limited Bragg peaks below T and a fully ordered AF ground state with a Mn moment of 3.06 μ. For x = 0.25 the magnetic scattering below T displays a remarkable phase separation-Bragg reflections coexisting with diffuse scattering. The ordered Mn moment is 1.1 μ, much reduced from that obtained via unpolarized neutrons. There are no Bragg peaks for x = 0.35 at any measured temperature ( T > 3 K) but there is highly structured diffuse scattering indicating strong short-range order reminiscent of x = 0 and 0.25 above their respective transition temperatures. For x = 1.00 (BaLaMnO) the diffuse scattering roughly follows a paramagnetic form-factor indicating no short or long-range magnetic correlations. It is argued that the observed phenomena are due to a competition between AF and ferromagnetic (F) superexchange interactions for the 180° Mn-O-Mn geometry within the ab plane and that the changes in the local geometry of the Mn-O octahedron leads to reduction of the AF interaction with a likely enhancement of the F interaction with increasing Ba content, ultimately giving rise to a glassy ground state. Analysis of the diffuse magnetic components show clear 2D AF spin correlations above T for x = 0.00, 0.25, and 0.35 with correlation lengths, ξ ∼ 14-7 Å and no spin correlations for x = 1.00.
SrLaMnO 是一种层状反铁磁（AF）氧化物，具有四方（ I4/ mmm）n = 1 的钙钛矿相结构（也称为 KNiF 结构），T = 128 K。值得注意的是，用 Ba 取代 Sr，形成固溶体 BaSrLaMnO，导致长程磁有序和 Mn 的有序矩破坏，对于 x > 0.35，尽管有效顺磁矩保持不变，但这是此类磁材料前所未有的行为。已经使用 XYZ 中子极化分析研究了四个成员，x = 0.0、0.25、0.35 和 1.0，该分析允许隔离散射的磁、核和核自旋分量，并测量磁横截面积的绝对值。数据分析使用无模型的反蒙特卡罗方法（SPINVERT）进行。对于 x = 0.0（SrLaMnO），T > T（128 K）的结果显示出不对称的漫散射峰，该峰在 T 以下演变成分辨率受限的布拉格峰，并具有完全有序的 AF 基态，Mn 矩为 3.06 μ。对于 x = 0.25，T 以下的磁散射显示出显著的相分离-布拉格反射与漫散射共存。有序 Mn 矩为 1.1 μ，比非极化中子得到的矩小得多。在任何测量温度（T > 3 K）下，x = 0.35 都没有布拉格峰，但存在高度结构化的漫散射，表明在各自的转变温度以上，存在强烈的短程有序，类似于 x = 0 和 0.25。对于 x = 1.00（BaLaMnO），漫散射大致遵循顺磁形式因子，表明没有短程或长程磁关联。有人认为，观察到的现象是由于 ab 平面内 180°Mn-O-Mn 几何结构的反铁磁（AF）和铁磁（F）超交换相互作用之间的竞争引起的，并且 Mn-O 八面体的局部几何结构的变化导致 AF 相互作用的减小，可能增强了 F 相互作用，随着 Ba 含量的增加，最终导致玻璃态基态。对漫散射磁分量的分析表明，x = 0.00、0.25 和 0.35 在上具有明显的二维 AF 自旋关联，关联长度 ξ~14-7 Å，而 x = 1.00 则没有自旋关联。
