Origin of large coercivity in charge-ordered lanthanide-free magnets

Abstract

Lanthanide-based permanent magnets are essential for a wide range of applications, from nanotechnology to industrial engineering. However, the limited availability and escalating costs of rare-earth elements have spurred efforts to develop alternative lanthanide-free magnets. Low-dimensional magnetic oxides, such as Co₃BO₅ and Co₂FeBO₅ single crystals (space group Pbam), offer a promising solution due to their structural properties and potential for stabilizing charge-ordered states. This study investigates the influence of nanodomains on macroscopic coercivity in these materials, revealing that domain wall pinning and high-energy barriers significantly impede domain wall motion, resulting in exceptional coercive fields. Notably, Co₂FeBO₅ exhibits a giant coercive field exceeding 9 Tesla at low temperatures. X-ray absorption and single crystal X-ray diffraction confirmed the mixed-valent character of Co and Fe ions, showing a 3+ oxidation state at the M4 sites and 2+ at other sites (M1, M2, M3). X-ray magnetic circular dichroism (XMCD) further revealed element-selective magnetizations in opposing directions below the Néel temperature, indicative of strong antiferromagnetic interactions persisting even in the paramagnetic state. These unprecedented coercivities are attributed to the interaction of alternating magnetic sublattices formed by adjacent ions, influenced by the crystallographic symmetry. By precisely substituting ions at specific crystallographic sites (M1–M4), it is possible to modulate local magnetic anisotropy and establish regions with high energy barriers, effectively enhancing the material's resistance to demagnetization. This targeted optimization of magnetic properties positions these materials as strong candidates for applications demanding stable and robust magnetic performance under challenging conditions.