Surface structure and surface-spin induced magnetic properties and spin-glass transition in nanometer Co-granules of FCC crystal structure

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 Keff over the bulk values at 4.2 K. An irreversibility in the ZFC-FC (zero field cooled-field cooled) thermomagnetograms occurs at temperatures T ≤ TB, where TB is their blocking temperature. The ZFC thermomagnetogram peaks at TB according to their Keff and volume V. TB = 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 Hc = 0) in nature at T ≥ TB, develops Hc at T < TB with a unique dependence on temperature, Hc(T) = Hc(0)[1 − T/TB]1/2, with Hc(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, Keff, or Hc 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 constantKs = 2.28 mJ/m2 is determined by a linear plot of Keff with D−1 at D ≤ 2.9 nm. Larger granules follow a modified Keff − D−1 plot with an order of smaller Ks-value. The surface spins form a surface-spin-glass, which undergoes a magnetic transition to a spin-frozen state at a critical temperature TF = 71 K. The TF evolves following the well-known de Almeida-Thouless line, δ TF ∝ H2/3, at H≤ 42 kA/m.