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.
We present a detailed study of vortex motion in a type-II superconductor using the muon spin rotation ͑SR͒ technique. The vortices were set in motion by an alternating transport current. By adjusting the frequency and amplitude of the driving force so that vortices "visible" to the implanted muons did not cross the sample boundaries, a SR line shape was obtained corresponding to almost perfectly ordered vortex motion. We also observed sidebands to the SR line shape which correspond to the frequency at which vortices pass the implanted muon, and allow a direct measurement of the vortex velocity. Both these features of the line shape confirm numerical predictions reported in a previous study ͓see Charalambous et al., Phys. Rev. B 66, 054506 ͑2002͔͒.
Superconducting and magnetic order are usually mutually exclusive, and are found to coexist in relatively few materials. We have obtained direct evidence for a spin-density wave (SDW) coexisting with bulk superconductivity in a ferromagnetic-superconducting trilayer. In the superconducting state the amplitude of the SDW is enhanced and modeling the data also suggests a pi/2 phase shift of one component of the SDW, implying a profound coupling of these two forms of order.
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