Giant coercive-field (Hc @ 2 K > 17 T) by freezing of magnetic domains in BaFe2(PO4)2

Abstract

In the 2D-Ising BaFe2(PO4)2 ferromagnet (FM), the competition between giant magneto-crystalline anisotropy and strong FM couplings creates remarkably narrow magnetic domain walls. These domains freeze below TF ≈ 15 K which is accompanied by progressive “soft-magnet” → “super-hard magnet” transition at both sides of TF. In the hard-regime, the coercive force is above 17 T at 2 K. At 5 K, we calculate a B.Hmax figure of merit (i.e., energy product) of 6.5 MG Oe superior to what observed in most permanent magnet oxides. This allows for the printing above TF and robust locking below TF of any magnetization between the saturation limits. The outstanding Hc value surpasses the standards by far, and brings questions about origin of such unusually strong pinning effects. They have to be surely attributed to narrow domain walls, estimated to be only ∼16 Å thick. Besides the observation of the domain wall structure by magnetic-force microscopy in various conditions, their thermodynamic signature appears in the specific heat data.