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 $1\times 1$ (primitive CrN unit cell) to $\sqrt{2}\times \sqrt{2}-R{45}^{\circ }$ with a transition temperature of ($278\pm 3$) 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 $\pm$ 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 ($270\pm 2$) K or ($280\pm 2$) 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.

We studied exchange bias and exchange spring effects in magnetic bilayer thin films of Fe and CrN grown by molecular beam epitaxy. First, the relationships between exchange bias, coercivity, and blocking temperature and the Fe and CrN layers thicknesses are studied. Second, the exchange spring effect is observed and studied in all samples. The exchange spring breaks free at critical applied field strengths while creating planar domain walls at lower applied fields. First-principles calculations are performed to understand the magnetic interactions between the Fe and CrN layers at their interfaces. The calculations are key to understanding the experimental observations.