MOSSBAUER AND X-RAY STUDY OF THE STRUCTURE AND MAGNETIC-PROPERTIES OFAMORPHOUS AND NANOCRYSTALLINE FE81ZR7B12 AND FE79ZR7B12CU2 ALLAYS

The specialized technique of radio-frequency-induced collapse of Mossb auer spectra combined with conventional Mossbauer spectroscopy, x-ray diffraction (XRD), small-angle x-ray scattering (SAXS), and differenti al scanning calorimetry (DSC) are used to investigate in detail the ma gnetic and structural properties of the two magnetic materials Fe81Zr7 B12 and Fe79Zr7B12Cu2. Thermal treatments to convert the as-quenched, fully amorphous state into mixtures of nanocrystalline and amorphous s tates and the effect of the small Cu addition were of primary interest due to the improved magnetic behavior in the mixed state. DSC shows t hat the Cu leads to a lowering of the onset temperature for formation of the nanocrystalline phase and also to an increase in the range of t emperatures over which this phase forms. XRD and Mossbauer data show t he nanoscale phase to be bcc Fe and the Mossbauer spectral parameters demonstrate it to be essentially pure Fe (i.e., with little or no Zr, B, or Cu substitutional impurities). The electron density contrast bet ween the amorphous matrix and the bcc Fe permits the detection of the Fe grains by SAXS and significant volume fractions with sizes of only 2.8-8 nm are shown to exist. Larger sizes are also present as demonstr ated by the XRD and Mossbauer data and a bimodal size distribution is suggested. The Mossbauer experiments in which the radio-frequency-indu ced effects (rf collapse and rf sidebands) are used, allows the nanocr ystalline bcc phase to be distinguished from magnetically harder micro crystalline alpha-Fe. The complete rf collapse of the magnetic hyperfi ne structure occurs only in the amorphous and nanocrystalline phases a nd is suppressed by the formation of larger grains. The rf sidebands d isappear when the nanocrystalline phase is formed, revealing that magn etostriction vanishes. The rf-Mossbauer studies are shown to be partic ularly sensitive to magnetic softness of the material in that large ch anges in the spectra are observed for applied field changes as small a s 2 Oe. (C) 1996 American Institute of Physics.