JOURNAL OF MATERIALS SCIENCE 35 (2 0 0 0 ) 3561 – 3571 Surface structure and surface-spin induced magnetic properties and spin-glass transition in nanometer Co-granules of FCC crystal structure S. RAM Materials Science Centre, Indian Institute of Technology, Kharagpur-721302, India E-mail: sram@matsc.iitkgp.ernet.in Separated Co-granules, of an average diameter as small as D = 2.0 nm, of FCC crystal structure have been synthesized by co-reducing Co 2+ cations dispersed in a liquid. They exhibit an enhanced saturation magnetization σ s 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 model structure of granules. In this model, the grain-surface atoms have a modified magnetic structure of the core atoms. An inter-coupling between the magnetic spins in the two regions occurs in a ferromagnetic or antiferromagnetic manner according to their interface that mediates their exchange interactions through it. The studies 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 surface anisotropy with large total interface-energy in small granules governs the DSS. An average value of the surface anisotropy constant K s = 2.28 mJ/m 2 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, δT F ∝ H 2/3 , at H ≤ 42 kA/m. C  2000 Kluwer Academic Publishers 1. Introduction Small magnetic particles of size of a nanometer scale are a subject of intense research in these days owing to their unique magnetic properties which make them very appealing from both the theoretical and the tech- nological points of view [1–10]. They are widely used as permanent magnets [1, 6, 11], information storage systems [3, 12, 13], magnetic toner in xerography [14], ferrofluids [15], contrast agents in magnetic resonance imaging [12, 14], magnetooptic or magnetoresistance devices [8, 16], and chemical catalysts [17, 18]. Below a critical size D c , i.e. ∼20 nm for cobalt or iron [1], they become single domain in nature in contrast with the usual multidomain structure of the bulk material and exhibit unique phenomena of quantum size effects [19], superparamagnetism [14, 20], quantum tunneling of magnetization [21], and unusually large surface ( H s ) and magnetocrystalline ( H a ) anisotropies [3, 5]. A large fraction of 20 to 60% atoms in such small par- ticles are the surface atoms. These surface atoms have a modified electronic or magnetic structure of the core atoms [17, 18] that leads to a significant change in their H a and H s anisotropies and magnetic moment μ n . It is well established from first principles self-consistent local spin-density calculations that the magnetic mo- ment in the topmost layer(s) in magnetic 3d-transuition metals is generally enhanced as much as 50% over the bulk value [8, 22]. This is ascribed to the reduction in coordination number and coordination symmetry of the surface atoms. It causes the bands narrow and hence, in general, enhances the paramagnetic density at the Fermi level E F [22]. Considerably modified magnetic hyper- fine fields of the core atoms have been shown in the surface atoms [23]. Chen et al. observed that as small Co-granules as 1.8 nm have ∼30% increased μ n over the bulk value [5]. Recently, there has been a consid- erable work on atomic clusters [24–26]. The cluster of iron, cobalt, or nickel assumes a similar increase in μ n as the number of atoms in it falls from several hundred to a few tens of atoms. Rhodium, which is paramagnetic 0022–2461 C  2000 Kluwer Academic Publishers 3561