A Variable Temperature Synchrotron X-ray Diffraction Study of Colossal Magnetoresistant NdMnAsO0.95F0.05.

Area of Science:

• Condensed Matter Physics
• Materials Science
• Solid State Chemistry

Background:

• High-temperature superconductivity in iron arsenides has spurred interest in transition metal pnictides.
• Colossal magnetoresistance (CMR) was recently observed in NdMnAsO1-xFx (x = 0.05-0.08), reaching -95% at 3 K.
• This CMR is linked to a second-order phase transition from an insulating antiferromagnet to a semiconducting paramagnet.

Purpose of the Study:

• To investigate the crystal structure of the CMR oxypnictide NdMnAsO0.95F0.05 using variable temperature synchrotron X-ray powder diffraction.
• To understand the relationship between lattice parameters, magnetic ordering, and the observed colossal magnetoresistance.

Main Methods:

• Synchrotron X-ray powder diffraction was performed on NdMnAsO0.95F0.05 across a temperature range of 4 K to 290 K.
• The diffraction data were analyzed to determine the crystal structure and unit cell parameters.
• Subtle changes in lattice parameters were correlated with magnetic ordering transitions.

Main Results:

• The tetragonal crystal structure (space group P4/nmm) remained consistent from 4 K to 290 K, with no structural phase transitions observed.
• Coupling between the lattice and magnetic order was identified.
• Discontinuities in the temperature dependence of the lattice parameters (a and c/a ratio) were observed at the magnetic ordering temperature (TSR), corresponding to Nd spin ordering and Mn moment reorientation.

Conclusions:

• The crystal structure of NdMnAsO0.95F0.05 is stable across the studied temperature range, including below the magnetic ordering temperature.
• Subtle lattice changes associated with magnetic ordering play a significant role in the coupling of electronic and magnetic degrees of freedom.
• These findings suggest that minute variations in lattice parameters are critical for the observed colossal magnetoresistance mechanism in this material.