ISSN:
1573-4803
Source:
Springer Online Journal Archives 1860-2000
Topics:
Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
Notes:
Abstract Separated Co-granules, of an average diameter as small as D = 2.0 nm, of FCC crystal structure have been synthesized by co-reducing Co2+ cations dispersed in a liquid. They exhibit an enhancedsaturation magnetization σs by as much as ∼34% with a more than an order of enhanced magnitude for the effective anisotropy constant K eff over the bulk values at 4.2 K. An irreversibility in the ZFC-FC (zero field cooled-field cooled) thermomagnetograms occurs at temperatures T ≤ T B, where T B is their blocking temperature. The ZFC thermomagnetogram peaks at T B according to their K eff and volume V. T B = 152 K has been found for D = 10 nm granules in an applied magnetic field of H = 1 kA/m. The sample, which is superparamagnetic(coercivity H c = 0) in nature at T ≥ T B, develops H c at T 〈 T B with a unique dependence on temperature, H c(T) = H c(0)[1 − T/T B]1/2, with H c(0) = 40.0 kA/m. The results are discussed with a two-phase modelstructure of granules. In this model, the grain-surface atoms have amodified magnetic structure of the core atoms. An inter-couplingbetween the magnetic spins in the two regions occurs in aferromagnetic or antiferromagnetic manner according to theirinterface that mediates their exchange interactions through it. Thestudies of σ, K eff, or H c as a function of temperature (4.2 to 380 K) and/or size D (2 to 20 nm) demonstrate their strong correlation with the dynamics of the surface spins (DSS). An enhanced surfaceanisotropy with large total interface-energy in small granulesgoverns the DSS. An average value of the surface anisotropy constantK s = 2.28 mJ/m2 is determined by a linear plot of K eff with D −1 at D ≤ 2.9 nm. Larger granules follow a modified K eff − D −1 plot with an order of smaller K s-value. The surface spins form a surface-spin-glass, which undergoes a magnetic transition to a spin-frozen state at a critical temperature T F = 71 K. The T F evolves following the well-known de Almeida-Thouless line, δ T F ∝ H 2/3, at H≤ 42 kA/m.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1023/A:1004861413037
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