Microstructure and Magnetic Entropy Change in Amorphous Alloy

Microstructure and some thermomagnetic properties of the amorphous Fe₇₆Mo₁₀Cu₁B₁₃ alloy in the as-quenched state and after annealing at 623 and 673 K are studied. High-resolution electron microscopy, apart from the amorphous phase, reveals the existence of medium range ordered (MRO) regions 1-2 nm in size. Transmission Mössbauer spectroscopy confirms that these regions are α-Fe MRO ones and grow during annealing. The Curie point of the alloy is 277, 272, and 284 K in the as-quenched state and after mentioned above heat treatments, respectively. The maximum of the magnetic entropy change is rather modest and equal to 0.88 J · kg^-1· K^-1 in the as-quenched state and reaches the value of 1.12 J · kg^-1 · K^-1 after annealing at 673 K. Magnetic entropy change depends not only on the temperature, but on the maximum of the magnetizing flux density, Bm = μ₀H_m (according to ASM = C(T) · Bnm) as well. The exponent n is close to 1 below the Curie point and does not reach 2 in high and equal to 2 in low magnetizing flux density range above the Curie point like in Curie-Weiss paramagnets. Such behavior is ascribed to the presence of MRO regions beside the amorphous phase and the material can be treated as biphasic.