Strong ferromagnetism beyond the mechanism of uncompensated surface spins in nanocrystalline GaCMn3

Abstract

Weak ferromagnetism arising from uncompensated surface spins (USS) is usually expected when an antiferromagnetic (AFM) material is diminished to nanoscale in size. Here we report strong ferromagnetism beyond the USS-mechanism in the AFM ground state of nanocrystalline GaCMn₃. The enhanced ferromagnetism can be attributed to an AFM to ferromagnetic (FM) transformation in the shell with a finite thickness of a crystallite. As the average crystallite size (<D>) decreases, the FM shell expands relative to the AFM core, leading to strengthening ferromagnetism. Through the AFM/FM interface the rotation of FM spins under a magnetic field is impeded by the AFM spins, leading to a large coercivity (H_C). The largest H_C (∼6.4 kOe at 5 K) was observed in the sample with a critical <D> of ∼15 nm. In contrast, USS-type weak ferromagnetism was observed in nanocrystalline GaNMn₃. Our results suggest a new approach to achieving strong FM order beyond the prediction of the USS mechanism, as well as to designing AFM-core/FM-shell nanostructures based on nanosized AFM materials.