Wildman E J, Mclaughlin A C
The Chemistry Department, University of Aberdeen, Meston Walk, Aberdeen, AB24 3UE, Scotland.
Sci Rep. 2016 Feb 15;6:20705. doi: 10.1038/srep20705.
The recent discovery of high temperature superconductivity in Fe arsenides has invigorated research into transition metal pnictides. Colossal magnetoresistance (CMR) has recently been reported for NdMnAsO1-xFx for x = 0.05-0.08, with a maximum magnetoresistance achieved at low temperature (MR9T(3 K)) = -95%). This appears to be a novel mechanism of CMR, which is as a result of a second order phase transition in field from an insulating antiferromagnet to a semiconducting paramagnet. Here we report a variable temperature synchrotron X-ray powder diffraction study of the CMR oxypnictide NdMnAsO0.95F0.05 between 4 K-290 K. An excellent fit to the tetragonal unit cell with space group P4/nmm is obtained over the entire temperature range, with no change in crystal structure detected down to 4 K. A coupling of the lattice and magnetic order is observed, where subtle discontinuities in the temperature variation of a and the c/a ratio are apparent as the Nd spins order antiferromagnetically and the Mn moments reorient into the basal plane at TSR. The results suggest that very small changes in lattice parameters effect the coupling between lattice, electronic and magnetic degrees of freedom.
近期在铁砷化物中发现高温超导现象，这激发了对过渡金属磷化物的研究。最近有报道称，对于x = 0.05 - 0.08的NdMnAsO1 - xFx，其具有巨磁电阻（CMR），在低温下实现了最大磁电阻（MR9T(3 K)）= -95%。这似乎是一种新型的CMR机制，它是由磁场中从绝缘反铁磁体到半导体顺磁体的二级相变导致的。在此，我们报道了对CMR氧磷化物NdMnAsO0.95F0.05在4 K至290 K之间进行的变温同步辐射X射线粉末衍射研究。在整个温度范围内，均获得了与空间群为P4/nmm的四方晶胞的极佳拟合，直至4 K均未检测到晶体结构变化。观察到晶格与磁有序之间存在耦合，当Nd自旋反铁磁有序且Mn磁矩在TSR时重新取向到基面时，a和c/a比值的温度变化中会出现细微的不连续性。结果表明，晶格参数的非常小的变化会影响晶格、电子和磁自由度之间的耦合。
