Abstract
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.
Highlights
In contrast to the Fe superconductors no structural distortion is apparent in NdMnAsO0.95F0.05 down to 4 K17; yet, neutron diffraction studies show that several magnetic transitions exist
Recent high pressure neutron diffraction studies revealed that the AFM ordering of Mn spins in NdMnAsO0.95F0.05 are robust up to pressures of 8.59 GPa and TMn is enhanced from 360–383 K upon applying an external pressure of 4.97 GPa18
The results demonstrate that there is no change in crystal structure within the temperature range studied in contrast to PrMnAsO0.95F0.05 and the superconducting Fe analogues
Summary
Study of Colossal Magnetoresistant received: 29 September 2015 accepted: 11 January 2016. 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. In order to further investigate the relationship between the crystal structure and electronic and magnetic properties of the CMR material NdMnAsO0.95F0.05, we have performed a variable temperature synchrotron X-ray diffraction study between 4 K and 290 K. Subtle discontinuities in the a lattice parameter and c/a ratio are observed at TSR
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