Effect of lattice expansion on the Curie temperature of granular nickel films

Granular Metal Films

“…(16.3) and then relaxes with the characteristic time r G to the thermal equilibrium magnetization corresponding to Eq. In this narrow sense, the conclusion of Rayl et al, 27 ' 28 that there is no true ferromagnetism in the region χ < 0.5 is not incorrect. The blocking temperature, defined as the temperature at which the maximum in κ occurs, is given by 30 T h = AK A G/k I In O)T G ) 0 |, (16.4) where Λ is a dimensionless constant whose value is 2 for a rectangu lar distribution, 1.8 for a Poisson distribution, and 1 for a δ-function distribution, and ω is the measuring angular frequency.…”

“…A different interpretation of the magnetic behavior of the Ni-Si0 2 system has been given by Rayl et al 27,28 The authors determined only a single ordering temperature at each composition χ > 0.45, which they identified as the transition temperature from a paramagnetic state to an ordered ferromagnetic state. For χ ^ 0.45, the authors concluded that the films exhibit no magnetic ordering.…”

“…In the case where the constituent metal is a superconductor, the coherence length is very small and the penetration depth is very large, corre sponding to a dirty type-11 superconductor. Three-dimensional superconductivity 32-3 5 and ferro magnetism 27 " 30 are observed to vanish abruptly near the percolation threshold. 23 In the transition region, the dielectric inclusions become intercon nected to form a maze structure, which, with further decrease in x, progressively breaks up into isolated metal particles dispersed in a dielectric continuum.…”

Magnetism

“…The enhanced hyper®ne ®eld was due to greater exchange, a result of moving to the right on the Bethe±Slater curve due to the greater lattice spacing in the expanded interfacial phase. Rayl et al 21 cosputtered nickel and SiO 2 to create small nickel particles with a lattice constant increased by 9%. This caused a lowering of T c , which correlated well with changes caused by high-pressure studies, which decrease the lattice spacing.…”

Tunneling and percolation in metal-insulator composite materials

“…The typical size of the Ni particles increases continuously from 4 nm at x = 35 vol.% to 10 nm at x = 50 vol.%, to a continuous network at x = 90 vol.%. 4,5,23,24 For the electrical measurements Cu-Au electrodes were predeposited on the substrate, with a separation of 2.54 mm. The resistance along the substrate sections was determined by a standard fourprobe technique.…”