Microstructural dependence of magnetic properties of Nd-based bulk metallic glasses

Abstract

With unique amorphous structures, bulk metallic glasses exhibit a series of novel physical and chemical properties, and attract considerable research interest. Among the family of bulk metallic glasses, rare earth-transition metal (RE-TM) based bulk metallic glasses (BMGs) are known to exhibit large coercivity at room temperature, and the coercivity is very sensitive to the variation in microstructure. Although a considerable amount of research effort has been spent in studying their magnetic properties, the origin of their large coercivity, and the relationship between their microstructure and magnetic properties are still not fully understood. In this project, the dependence of magnetic properties on the microstructures of Nd-based BMGs has been investigated. Different microstructures of the BMGs were realized by minor alloying, adjusting the cooling rate and varying the temperatures. It is found that the glass-forming ability of (Nd60Fe30Al10)100-xNix alloys can be significantly enhanced with the addition of Ni. The content of the non-magnetic phases is increased by adding Ni, which strengthens the effect as a pinning center to the magnetic phase and increases the coercivity in the BMGs. The cooling rate induced evolution in microstructure, from amorphous to partial crystalline precipitation, was also observed in one as-cast Nd60Fe30Al10 sample. A featureless amorphous phase was observed at the periphery, and a network-like structure, consisting of the Fe-rich, Nd-rich and amorphous regions, was formed in the center of the sample. As a consequence of different microstructures, two magnetic structures were observed at the periphery and the center of the cross section, respectively. At room temperature, the as-cast Nd55Fe28Al9Ni8 BMG, which consists of nanoclusters no larger than 5nm embedded in amorphous matrix, exhibits one magnetic phase behavior. However, the appearance of an apparent step in the hysteresis loops at low temperature implies the presence of at least two ferromagnetic phases in the alloy. Based on Monte Carlo simulation, the coercivity of the clusters of the BMG is found to be closely related to the change of temperature and the anisotropy of the clusters. By overlapping the hysteresis loops of the two cluster systems at different temperatures, a step is also observed in the hysteresis loop when the system is at low temperature. This result is consistent with the experimental findings.The findings of the present investigation do not only result in a better understanding of the microstructure-magnetic properties of the BMG, but also lay down a good foundation for further improving their magnetic properties by tailoring their microstructures.