On the limits of coercivity in permanent magnets

TL;DR

This paper estimates the maximum coercivity in permanent magnets by modeling energy barriers for domain reversal, revealing fundamental limits influenced by microstructure, temperature, and material properties.

Contribution

It introduces a theoretical approach to determine the upper bounds of coercivity in magnetic alloys considering idealized microstructures and thermal effects.

Findings

Maximum coercivity is about 60% of the anisotropy field at room temperature.

Coercivity drops to about 50% of the anisotropy field at 473K.

Misorientation, demagnetizing fields, and thermal fluctuations reduce coercivity.

Abstract

The maximum coercivity that can be achieved for a given hard magnetic alloy is estimated by computing the energy barrier for the nucleation of a reversed domain in an idealized microstructure without any structural defects and without any soft magnetic secondary phases. For Sm$_{1-z}$Zr$_z$(Fe$_{1-y}$Co$_y$)$_{12-x}$Ti$_x$ based alloys, which are considered an alternative to Nd$_2$Fe$_{14}$B magnets with lower rare-earth content, the coercive field of a small magnetic cube is reduced to 60 percent of the anisotropy field at room temperature and to 50 percent of the anisotropy field at elevated temperature (473K). This decrease of the coercive field is caused by misorientation, demagnetizing fields and thermal fluctuations.