Magnetic relaxation in the nanoscale granular alloyFe20Cu20Ag60

Abstract

The structural and magnetic properties of a representative member of a class of technologically relevant ternary metallic alloys have been studied in detail. The alloy, of composition ${\mathrm{Fe}}_{20}{\mathrm{Cu}}_{20}{\mathrm{Ag}}_{60},$ is a member of the family of nanoscale granular alloys that are of current interest in both giant magnetoresistive alloys and nanocrystalline soft magnets. Samples were produced by mechanical alloying (70 h, argon sealed) and were homogeneous according to scanning electron microscopy and electron microprobe analysis. Room-temperature magnetoresistance measurements in applied fields up to $H=90\mathrm{kOe}$ gave a value of 5% (at 90 kOe) for the $[R(H)\ensuremath{-}R(0)]/R(0)$ ratio. Rietveld calculations on high-resolution image plate data using a synchrotron source $(\ensuremath{\lambda}=0.6920\AA{})$ showed that the specimen comprised a dispersion of bcc ${\mathrm{Fe}}_{60}{\mathrm{Cu}}_{40}$ $(Im\ensuremath{-}3m,$ $a=2.951\AA{})$ particles of mean size 5.5 nm in an fcc ${\mathrm{Ag}}_{90}{\mathrm{Cu}}_{10}$ $(Fm\ensuremath{-}3m,$ $a=4.057\AA{})$ matrix. This structure was stable up to 380 K as revealed by differential scanning calorimetry. dc magnetization (peaks in zero-field-cooled data) and frequency-dependent ac susceptibility (in external dc magnetic fields from zero to 500 Oe) measurements showed blocking transitions between 280 and 300 K, with the onset of superparamagnetic behavior at higher temperatures. The superparamagnetic regime was confirmed at room temperature by the observation of anhysteretic $M(H)$ curves, and through zero field and applied field M\"ossbauer experiments in which a combined singlet plus doublet spectrum was transformed to a magnetically split sextet on application of an 11-kOe field. In all cases the blocking transitions were clearly affected by the existence of intergranular interactions, which shifted them to higher temperatures than would be expected from noninteracting grains. Evidence of intergranular interactions were also found in the dynamic behavior of the ac susceptibility data (small frequency-dependent shifts in the blocking temperature, Vogel-Fulcher activation processes). Muon spectroscopy was found to provide excellent corroborating information on the blocking transition, with a clear peak being found in the exponential decay rates of the depolarization spectra. The result establishes the feasibility of using muon spin relaxation to probe other superparamagnetic materials, with the advantage that measurements can be conducted in absolutely zero field.