Structural and magnetic phase transitions in chromium nitride thin films grown by rf nitrogen plasma molecular beam epitaxy

A magnetostructural phase transition is investigated in single-crystal chromium nitride (CrN) thin films grown by rf plasma molecular beam epitaxy on MgO(001) substrates. While still within the vacuum environment following molecular beam epitaxy growth, in situ low-temperature scanning tunneling microscopy, and in situ variable low-temperature reflection high-energy electron diffraction are applied, revealing an atomically smooth and metallic CrN(001) surface, and an in-plane structural transition from 1×1 (primitive CrN unit cell) to 2×2-R45 with a transition temperature of (278±3) K, respectively. Ex situ temperature-dependent measurements using neutron diffraction are also performed, looking at the structural peaks and likewise revealing a first-order structural transition along the [111] out-of-plane direction, with transition temperatures of (268 ± 3) K. Turning to the magnetic peaks, neutron diffraction confirms a clear magnetic transition from paramagnetic at room temperature to antiferromagnetic at low temperatures with a sharp, first-order phase transition and a Néel temperature of (270±2) K or (280±2) K for two different films. In addition to the experimental measurements of structural and magnetic ordering, we also discuss results from first-principles theoretical calculations which explore various possible magnetostructural models.