Inhomogeneous structure and magnetic properties of granularCo10Cu90alloys

Abstract

Granular Co 10 Cu 90 alloys displaying giant magnetoresistance have been obtained by melt spinning followed by an appropriate heat treatment in the range 0-700°C. Their structural and magnetic properties have been studied on a microscopic scale using 59 Co NMR technique and thermoremanent magnetization measurements. The study reveals that in the as-quenched samples Co is involved in two main structural components: small, irregular, strained Co particles ͑60% of the entire Co population͒ and a composition modulated CoCu alloy. A high modulation amplitude of the concentration profile in the alloy subdivides the latter in two parts with distinctly different properties. One part consists of ferromagnetic alloy ͑average Cu concentration of about 20%͒ with a blocking temperature of about 35 K ͑involving 6% of the entire Co population in a sample͒. The other part, containing the remaining 34% of the entire Co population, is a paramagnetic alloy with a blocking temperature below 4.2 K. The ferromagnetic alloy is magnetically soft-its transverse susceptibility is lower by a factor of 7 than the transverse susceptibility of the quenched-in Co particles. The latter population has a blocking temperature of about 150-200 K. During the heat treatment, each of the two main structural components undergoes respective decomposition processes: both of them display two temperature regimes. One process consists in dissolving the quenched-in Co particles after annealing at around 400°C, followed at higher temperatures by a nucleation and growth of the more regular in shape Co particles. The other process resembles a spinodal decomposition of the quenched-in CoCu alloy, resulting in sharpening of the concentration profile and eventually leading to Co cluster formation in samples annealed above 450°C. Both processes end at about T an ϭ700°C, in formation of large, pure Co clusters that are ferromagnetic at least up to 400 K.

Temperature dependence of the coercive field in single-domain particle systems

“…The obtained f (T B ) confirm the inhomogeneous magnetic nanostructure observed by other authors. 24,25,26,27 …”

Thermoremanence and zero-field-cooled/field-cooled magnetization study ofCox(SiO2)Denardin1, Brandl2, Knobel3 et al. 2002Phys. Rev. BSelf Cite244111462View full textAdd to dashboardCite

“…5 Several magnetic techniques are currently employed to indirectly infer the structural parameters and intrinsic magnetic properties. In addition to hysteresis loops that can be fitted using properly weighted Langevin functions, 6 the most used techniques are the so-called zero-field-cooled͑ZFC͒/ field-cooled͑FC͒ magnetization 7 and thermoremanent magnetization ͑TRM͒, 8 from which it is possible to extract information about the average blocking temperature ͑and therefore the mean grain diameter, if the particles considered are of spherical shape with a known anisotropy constant͒.The dispersion of grain sizes can be estimated from the difference between the maximum in the ZFC curve and the bifurcation between the ZFC and FC curves or from the fit of the TRM curve. Although these data are widely used to characterize nanocrystalline systems, it is worth noting that they should be employed with extreme care, because the dipolar interactions may cause strong deviations in the extracted structural parameters, although the shapes of the curves remain similar to the ones expected for noninteracting systems.…”

Specific heat and magnetization studies of spin-glass like transition in nanogranular Cu90Co10 ribbon

“…absorption fine structure spectroscopy (XAFS) [14], 59 Co nuclear magnetic resonance (Co-NMR) [15], probe-field ion microscopy [12] and other standard methods; but up to date no results on specific heat measurements were reported. In this work, we show that by using specific heat data in addition to magnetic measurements allows us to determine experimentally the volume fraction of Co solute atoms in melt-spun Co-Cu solid solution and its role in the establishment of the SSG state at low temperature.…”