Critical Behaviour of the Fe 3 Nd Cluster Glass Phase in Bulk Glassy Nd 60 Fe 30 Al 10 Hard Magnets
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Critical Behaviour of the Fe3Nd Cluster Glass Phase in Bulk Glassy Nd60Fe30Al10 Hard Magnets R. J. Ortega-Hertogs,a, b A. Inouec and K.V. Raob a
Departamento de Física, Universidad Pública de Navarra, E-31006 Pamplona, Spain Department of Materials Science, Royal Inst. of Technology, S-10044 Stockholm, Sweden c Institute for Materials Research, Tohoku University, Sendai 980-77, Japan b
ABSTRACT The magnetic properties of novel bulk glassy Nd60Fe30Al10 magnets around the transition to the paramagnetic state have been studied in order to understand the magnetic microstructure in these materials. Analyses of the critical behavior of the magnetic isotherms suggest a transition from a low field magnetic state to a ferromagnetic-like state upon the application of high enough magnetic fields. The ferromagnetic-like state is evidenced by a scaling of the magnetization curves with critical exponents γ=1.5(1), β=0.65(5), and TC=465 K. These results are consistent with the predictions for weak random magnetic anisotropy systems. INTRODUCTION Bulk metallic glasses are attracting much interest because of 1) the diversity of multicomponent alloys that can be produced, with a possibility to tailor their specific properties, and 2) the slow quench rates required to amorphize from the liquidus state, enables one to fabricate them in different shapes with dimensions of the order of several millimeters [1,2]. One important issue in understanding their remarkable glass forming ability is whether they are indeed metallic glasses or if they posses a short range order reminiscent of crystalline structure at a nanoscale. How does such a structure manifest itself in a bulk glassy magnetic material? Recently, ternary NdxFe1-xAl10 bulk glassy alloys have been found to possess hard magnetic properties[3]. Furthermore, the alloy x=60 at.% among the series was found to exhibit the highest coercivity and (BH)max energy product at room temperature. A structural analyses indicates that, despite the absence of any crystalline trace in X-ray diffraction (XRD) and transmission electron microscopy (TEM) studies (lateral resolution 5 nm), the Nd-rich alloy contains regions richer in Fe than expected for a random alloy[4]. Our previous magnetic studies at temperatures above 1 K, in magnetic fields up to 30 tesla [5], reveal the existence of two main magnetic phases: a Fe-rich phase, of approximate composition Fe3Nd, and a Nd-rich phase. Low field AC-susceptibility data indicated that the Nd-rich phase is mainly antiferromagnetic. At temperatures above 70 K, the magnetization loop was found to evolve towards a Stoner-Wohlfarth type, with pronounced thermal activation effects, responsible mainly for the deterioration of the intrinsic coercivity. These observations suggest a microstructure of non-interacting magnetic entities or clusters (Fe3Nd) dispersed in a Nd-rich matrix, despite the observed glassy nature of the alloy from XRD and TEM (lateral resolution 5 nm), for which reason we like to name it magnetically granular. In this work, we report on a sc
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