High-Coercivity Spin Glasses in Anisotropic Mo₂FeB₂-Type M_1.5Mn_1.5B₂ (M = Mo, W)

Magnetic materials combining spin-glass behavior with hard-magnet coercivity are important for fundamental and applied research. Herein, we report spin-glass behaviors coupled to remarkably large intrinsic coercivity values of 302.4 kA/m (H_c = 3.80 kOe) in Mo_1.5Mn_1.5B₂ and 175.1 kA/m (H_c = 2.20 kOe) in W_1.5Mn_1.5B₂. Both phases belong to the Mo₂FeB₂-type structure known for enabling excellent structural properties and predicted altermagnetic candidates. Single-phase products were obtained via a compositionally tuned solid-state route that resolves prior phase-purity challenges. Temperature-dependent susceptibility measurements revealed spin-glass behavior below the freezing temperatures T_g = 29 K for Mo_1.5Mn_1.5B₂ and T_g = 43 K (reentrant cluster spin glass) as well as a ferromagnetic (FM) transition at 70 K for W_1.5Mn_1.5B₂. AC susceptibility (χ') measurements exhibited frequency-dependent peaks that shifted to higher temperatures with increasing frequency, confirming the presence of dynamic spin freezing in both compounds. DFT calculations confirmed the presence of strongly competing FM and antiferromagnetic (AFM) interactions leading to competing FM, AFM and ferrimagnetic (FiM) orderings with a greater spin-exchange energy (ΔE_ex = 28-98 meV/f.u.) for FiM/AFM compared to FM in Mo_1.5Mn_1.5B₂ and even greater ΔE_ex = 53-131 meV/f.u. in W_1.5Mn_1.5B₂. The magnetocrystalline anisotropy energy (MAE) is uniaxial, favoring the c-axis, with MAE = -0.17 meV/f.u. (Mo) and -0.10 meV/f.u. (W), which align well with the large hysteresis found experimentally, while the unexpectedly larger coercivity of Mo_1.5Mn_1.5B₂ correlates with its greater AFM interactions. These findings enable the development of Mo₂FeB₂-type multifunctional materials, expanding their applications beyond traditional structural material properties.