High T-Dependence of Local Order in Magnetic FeM Alloys from Neutron Diffuse Scattering

Abstract

We present in this paper the first “in situ” at high temperature investigation of the atomic short-range order (SRO) T-dependence in magnetic alloys by neutron diffuse scattering (NDS). Up to now analogous experimental studies were done on quenched samples by either NMR in dilute alloys [1–3] or NDS in concentrated ones [2, 4, 5]. NDS was generally done at room or at 4 K temperature in alloys annealed and quenched. In the large samples necessary for neutron experiments, the equilibrium state at the annealing temperature T can be frozen in by a quench only for low values of T . At such temperatures, the annealing time necessary to attain equilibrium makes it impossible to vary T on a large range. This limitation can be overcome by doing “in situ” at temperature measurements. NDS is then the most suitable technique because it allows one to separate the elastic contribution in contrast to X-rays. In this work, we focus on the high-temperature aspects of these NDS measurements: metallurgy and Debye-Waller factors; the paramagnetic contribution evaluation will be detailed elsewhere. We show that in spite of all these difficulties and in spite of the increased noise due to the oven and the smoothing of SRO at high temperature, it is possible to perform such measurements and so to obtain SRO parameters at temperatures as high as 1500 K.