Low-temperature giant coercivity in Co$_{6.2}$Ga$_{3.8-x}$Ge$_{x}$ ($x$=2.4 to 3.2)

TL;DR

This study reports low-temperature giant coercivity exceeding 20 kOe in Co-Ga-Ge alloys with a hexagonal structure, highlighting their potential as rare-earth-free permanent magnets with tunable magnetic states.

Contribution

It demonstrates the emergence of giant coercivity in Co$_{6.2}$Ga$_{3.8-x}$Ge$_{x}$ alloys and links magnetic ground states to Ge content, providing new insights into rare-earth-free magnet development.

Findings

Giant coercivity (>20 kOe) observed at low temperatures.

Magnetic ground state transitions from ferrimagnetism to ferromagnetism with Ge content.

Coercive fields reach 26 kOe and 44 kOe at 2 K for different compositions.

Abstract

The observation of giant coercivity exceeding 20 kOe at low temperatures in several transition-metal-based compounds has attracted significant attention from a fundamental perspective. This research is also relevant to developing rare-earth-free permanent magnets, wherein cobalt is one of the primary elements used. To facilitate easy fabrication, rare-earth-free and Co-based inorganic bulk magnets that exhibit giant coercivity are highly demanded but rarely reported. Herein, we report the observation of low-temperature giant coercivity in polycrystalline metallic Co$_{6.2}$Ga$_{3.8-x}$Ge$_{x}$ ($x$=2.4 to 3.2) with the hexagonal Fe$_{13}$Ge$_{8}$-type structure composed of Kagome and triangular lattices. As the Ge content $x$ decreases from 3.2, the magnetic ground state changes from ferrimagnetism to ferromagnetism at $x$=2.6. In the ferrimagnetic state, we observed a signature of spin frustration arising from the Kagome and/or triangular lattices of Co atoms. The ferromagnetic ordering temperatures for the $x$=2.6 and 2.4 samples are 46 K and 60 K, respectively. The coercive fields rapidly increase upon cooling and reach values of 26 kOe and 44 kOe in the $x$=2.6 and 2.4 samples, respectively, at 2 K.