Magnetic Ordering and Negative Thermal Expansion in PrFeAsO

TL;DR

This study investigates the magnetic structure and thermal expansion behavior of PrFeAsO, revealing a phase transition, striped magnetism, and a unique negative thermal expansion linked to Fe and Pr magnetic interactions.

Contribution

It provides detailed neutron diffraction analysis of PrFeAsO's magnetic ordering and uncovers the negative thermal expansion phenomenon associated with magnetic interactions.

Findings

Tetragonal-orthorhombic phase transition below 136 K

Striped Fe magnetism detected below 85 K

Negative thermal expansion of 0.015% at 2 K

Abstract

We report the structure and magnetism of PrOFeAs, one of the parent phases of the newly discovered Fe-As superconductors, as measured by neutron powder diffraction. In common with other REOFeAs materials, a tetragonal-orthorhombic phase transition is found on cooling below 136 K and striped Fe magnetism with $k =$(1,0,1) is detected below $\sim$ 85 K. Our magnetic order parameter measurements show that the ordered Fe moment along the a axis reaches a maximum at $\sim$ 40 K, below which an anomalous expansion of the c axis sets in, which results in a negative thermal volume expansion of 0.015 % at 2 K. We propose that this effect, which is suppressed in superconducting samples, is driven by a delicate interplay between Fe and Pr ordered moments.