Magnetic relaxation in the nanoscale granular alloyFe20Cu20Ag6

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 Fe 20 Cu 20 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 sealed͒ and were homogeneous according to scanning electron microscopy and electron microprobe analysis. Room-temperature magnetoresistance measurements in applied fields up to Hϭ90 kOe gave a value of 5% ͑at 90 kOe͒ for the ͓R(H)ϪR(0)͔/R(0) ratio. Rietveld calculations on high-resolution image plate data using a synchrotron source (ϭ0.6920 Å) showed that the specimen comprised a dispersion of bcc Fe 60 Cu 40 ͑Im-3m, a ϭ2.951 Å͒ particles of mean size 5.5 nm in an fcc Ag 90 Cu 10 ͑Fm-3m, aϭ4.057 Å͒ matrix. This structure was stable up to 380 K as revealed by differential scanning calorimetry. dc magnetization ͑peaks in zero-fieldcooled data͒ and frequency-dependent ac susceptibility ͑in external dc magnetic fields from zero to 500 Oe͒ 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 Mössbauer 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 processes͒. 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.

Size effects in the magnetic behaviour of TbAl2milled alloys

“…As commented in the former section, a minimum size of D = 14 nm and maximum ε ≈ 1% for the 300 hours milled sample has been obtained. This values for size and strain are similar to other cubic Rare Earthalloys or Transition metal [10,32]. In addition, the reduction in size is also connected to the variation of lattice parameters (∆a/a), in coincidence with former studies in GdAl 2 [10].…”

“…Figure 5 shows that the strain increases up to ε ≈ 1 %, due to the introduction of disorder with the grinding time, similar to what is observed in GdAl 2 [10] or Fe-Cu-Ag [32]. The strain also reaches a -6-saturation for grinding times larger than 300 hours.…”

High-temperature induced ferromagnetism onγ−Feprecipitates in FeCu solid solutions

“…This is due to a decrease of particle size ͑reaching sometimes the nanometer scale͒ and microstrain effects ͑usually below 1%͒, which are typical of as-milled granular alloys. 30 On heating the peaks sharpen considerably ͓Figs. 2͑b͒-2͑d͔͒ due to structural relaxation processes in the metastable as-milled alloys.…”

Magnetic phase diagram of superantiferromagnetic TbCu2nanoparticles

“…If the magnetic interparticle interactions are absent (or negligible), the spin particle rotation under the uniaxial anisotropy is understood by a thermal activation process, following the well-known Arrhenius-Néel law and for a typical χ AC measurement (f = 10-10000 Hz) in a non-interacting SPM system (τ 0 ≈ 10 −9 s), δ = 0.13-0.24, as commonly accepted. 40 Whenever a reduction of such δ-values is found, the nanoparticles become magnetically correlated and, in the limit, a cluster glass behavior should control the spin divergence at the critical temperature. Finally, the δ-shift is practically inexistent in canonical SG involving the freezing of individual spins (δ = 0.0045 for AuMn 32 ) and negligible in phase transitions (T C , T N ).…”