Spin glass behavior in an interacting gamma-Fe2O3 nanoparticle system

In this paper we investigate the superspin glass behavior of a concentrated assembly of interacting maghemite nanoparticles and compare it to that of canonical atomic spin glass systems. ac versus temperature and frequency measurements show evidence of a superspin glass transition taking place at low temperature. In order to fully characterize the superspin glass phase, the aging behavior of both the thermo-remanent magnetization (TRM) and ac susceptibility has been investigated. It is shown that the scaling laws obeyed by superspin glasses and atomic spin glasses are essentially the same, after subtraction of a superparamagnetic contribution from the superspin glass response functions. Finally, we discuss a possible origin of this superparamagnetic contribution in terms of dilute spin glass models

Memory interference effects in spin glasses

When a spin glass is cooled down, a memory of the cooling process is imprinted in the spin structure. This memory can be disclosed in a continuous heating measurement of the ac-susceptibility. E.g., if a continuous cooling process is intermittently halted during a certain aging time at one or two intermediate temperatures, the trace of the previous stop(s) is recovered when the sample is continuously re-heated [1]. However, heating the sample above the aging temperature, but keeping it below Tg, erases the memory of the thermal history at lower temperatures. We also show that a memory imprinted at a higher temperature can be erased by waiting a long enough time at a lower temperature. Predictions from two complementary spin glass descriptions, a hierarchical phase space model and a real space droplet picture are contested with these memory phenomena and interference effects. [1] K. Jonason, E. Vincent, J. Hammann, J. P. Bouchaud and P. Nordblad, Phys. Rev. Lett. 31, 3243 (1998).Comment: 7 pages, 1 LaTex file + 5 figures in EPS Revised version of June 17, 1999 (minor changes), to appear in EPJ B around November 9

Spin Anisotropy and Slow Dynamics in Spin Glasses

We report on an extensive study of the influence of spin anisotropy on spin glass aging dynamics. New temperature cycle experiments allow us to compare quantitatively the memory effect in four Heisenberg spin glasses with various degrees of random anisotropy and one Ising spin glass. The sharpness of the memory effect appears to decrease continuously with the spin anisotropy. Besides, the spin glass coherence length is determined by magnetic field change experiments for the first time in the Ising sample. For three representative samples, from Heisenberg to Ising spin glasses, we can consistently account for both sets of experiments (temperature cycle and magnetic field change) using a single expression for the growth of the coherence length with time.Comment: 4 pages and 4 figures - Service de Physique de l'Etat Condense CNRS URA 2464), DSM/DRECAM, CEA Saclay, Franc

Aging phenomena in spin glass and ferromagnetic phases: domain growth and wall dynamics

We compare aging in a disordered ferromagnet and in a spin glass, by studying the different phases of a reentrant system. We have measured the relaxation of the low-frequency ac susceptibility, in both the ferromagnetic and spin-glass phases of a CdCr_{1.9}In_{0.1}S_4 sample. A restart of aging processes when the temperature is lowered (`chaos-like' effect) is observed in both phases. The memory of previous aging at a higher temperature can be retrieved upon re-heating, but in the ferromagnetic phase it can rapidly be erased by the growth of ferromagnetic domains. We interpret the behaviour observed in the ferromagnetic phase in terms of a combination of domain growth and pinned wall reconformations, and suggest that aging in spin glasses is dominated by such wall reconformation processes.Comment: SPEC, CEA Saclay, 91191 Gif sur Yvette Cedex, France, to appear in Europhys. Lett. (2000

The Equivalence of the Dekel-Fudenberg Iterative Procedure and Weakly Perfect Rationalizability

Two approaches have been proposed in the literature to refine the rationalizability solution concept: either assuming that players make small errors when playing their strategies, or assuming that there is a small amount of payoff uncertainty. We show that both approaches lead to the same refinement if errors are made according to the concept of weakly perfect rationalizability, and there is payoff uncertainty as in Dekel and Fudenberg [Journal of Economic Theory 52 (1990), 243-267]. For both cases, the strategies that survive are obtained by starting with one round of elimination of weakly dominated strategies followed by many rounds of elimination of strictly dominated strategies.Rationalizability, refinements

Public contracts as accountability mechanisms: assuring quality in public healthcare in England and Wales

Contracting in the public sector is designed to enhance the accountability of service providers to their funders. The idea is that quality is improved by the use of service specifications, monitoring of performance and imposition of contractual sanctions. Socio-legal and economic theories of contract indicate that it will be difficult to make and enforce contracts to achieve this. The results of a study of National Health Services contracting in England and Wales are reported. We conclude that contracts alone are not sufficient to improve accountability – collibration of various regulatory measures (including more hierarchical mechanisms such as performance targets) is required

Functional Literacy, Educational Attainment and Earnings - A Multi-Country Comparison

In this paper a rich and innovative dataset, the International Adult Literacy Survey, is used to examine the impact of functional literacy on earnings. We show that the estimated return to formal education is sensitive to the inclusion of literacy - excluding it biases the return to education in many countries by significant amounts. Literacy itself has a well-determined effect on earnings in all countries though with considerable variation in the size of the effect. The benefits of literacy do not only arise from increasing low levels of literacy: increases at already high levels generate substantial increases in earnings in some countries. In general we find little interaction between schooling and literacy though for a few countries they appear to complement each other.

The social geography of childcare: 'making up' the middle class child

Childcare is a condensate of disparate social forces and social processes. It is gendered and classed. It is subject to an excess of policy and political discourse. It is increasingly a focus for commercial exploitation. This is a paper reporting on work in progress in an ESRC funded research project (R000239232) on the choice and provision of pre-school childcare by middle class (service class) families in two contrasting London locations. Drawing on recent work in class analysis the paper examines the relationships between childcare choice, middle class fractions and locality. It suggests that on the evidence of the findings to date, there is some evidence of systematic differences between fractions in terms of values, perspectives and preferences for childcare, but a more powerful case for intra-class similarities, particularly when it comes to putting preferences into practice in the 'making up of a middle class child' through care and education