The Role of Disorder in the Magnetic Properties of Mechanically Milled Nanostructured Alloys

Abstract

Mechanical milling provides a unique means of studying the influence of grain size and disorder on the magnetic properties of nanostructured alloys. This paper compares the role of milling in the nanostructure evolution of two ferromagnets -SmCo 5 and GdAI 2 -and the subsequent impact of nanostructure on magnetic properties and phase transitions. The ferromagnetic properties of SmCo 5 are enhanced by short (< 2 hours) milling times, producing up to an eight-fold increase in coercivity and high remanence ratios. The coercivity increase is attributed to defect formation and strain. Additional milling increases the disorder and produces a mix of ferromagnetic and antiferromagnetic interactions that form a magnetically glassy phase. GdA1 2 , which changes from ferromagnetic in its crystalline form to spin-glass-like in its amorphous form, is a model system for studying the dependence of magnetically glassy behavior on grain size and disorder. Nanostructured GdAI 2 with a mean grain size of 8 nm shows a combination of ferromagnetic and magnetically glassy behavior, in contrast to previous studies of nanostructured GdA1 2 with a grain size of 20 nm that show only spin-glass-like behavior.

Disorder-induced depression of the Curie temperature in mechanically milledGdAl2

“…7 Mechanically milling initially crystalline GdAl 2 produces a spin-glass-like peak, irreversibility between FC and ZFC magnetizations, and a decrease in the magnitude of the ferromagnetic transition. [8][9][10][11] The spinglass-like peak in M͑T͒, however, occurs 20-40 K higher than that in amorphous GdAl 2 . Zhou and Bakker interpret the low-temperature peak as evidence of a "new class" of spin glass in which milling-induced disorder creates the necessary randomness and frustration, 8,9 although there remains disagreement on whether the peak is due to a true spin-glass transition.…”