Anisotropic Band-Split Magnetism in Magnetostrictive CoFe$_2$O$_4$

TL;DR

This paper investigates the anisotropic magnetic properties of CoFe$_2$O$_4$, revealing how site mixing and competing magnetic interactions lead to large magnetostriction and domain locking, with implications for industrial and medical uses.

Contribution

It uncovers the microscopic origin of anisotropic magnetism and magnetostriction in CoFe$_2$O$_4$, highlighting the role of site mixing and competing magnetic fields.

Findings

Large magnon band splitting (~60 meV) due to site mixing.

Weak magnetocrystalline anisotropy (~3 meV).

Domain locking enhances magnetostriction.

Abstract

Single crystal spinel CoFe$_2$O$_4$ exhibits the largest room-temperature saturation magnetostriction among non-rare-earth compounds and a high Curie temperature ($T_c \sim 780$ K), properties that are critical to a wide range of industrial and medical applications. Neutron spectroscopy reveals a large band splitting ($\sim$ 60 meV) between two ferrimagnetic magnon branches, which is driven by site mixing between Co$^{2+}$ and Fe$^{3+}$ cations, and a significantly weaker magnetocrystalline anisotropy ($\sim$ 3 meV). Central to this behavior is the competition between extremely large mismatched molecular fields on the tetrahedral $A$-site and octahedral $B$-site sublattices and the single-ion anisotropy on the $B$-site. This creates a strong energetic anisotropy that locks the magnetic moment within each structural domain in place. As a result of these differing energy scales, switching structural domains is energetically favored over a global spin reorientation under applied magnetic fields, and this is what amplifies the magnetostrictive nature of CoFe$_2$O$_4$.