J. Hammann

Memory and Chaos Effects in Spin Glasses

New low frequency ac susceptibility measurements on two different spin glasses show that cooling/heating the sample at a constant rate yields an essentially reversible (but rate dependent)

EXTRACTION OF THE SPIN GLASS CORRELATION LENGTH

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Spin anisotropy and slow dynamics in spin glasses

curve; a downward relaxation of

Contrasting effects of field and temperature variations on ageing in spin glasses

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Glassy dynamics of nanometric magnetic particles

occurs during a temporary stop at constant temperature (aging). Two main features of our results are (i) when cooling is resumed after such a stop,

Neutron diffraction study of antiferromagnetism in YbVO4

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A new experimental procedure for characterizing quantum effects in small magnetic particle systems

goes back to the reversible curve (chaos), and (ii) upon reheating,

Barrier heights versus temperature in spin glasses

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Antiferromagnetic ordering in the ytterbium aluminum perovskite YbAlO3

perfectly traces the previous aging history (memory). We discuss implications of our results for a real space (as opposed to phase space) picture of spin glasses.

Spin-glass dynamics and the barrier model: Extraction of the Parisi physical order parameter

A change in magnetic field H quenches the barriers in a spin glass for heights less than an associated change in Zeeman energy, E_z. The shift of the peak of S(t)=-dM_{TRM}(t,t_w)/H/{d lnt} with H generates E_z={N_s}{X_fc}{H^2}, with X_fc the field cooled magnetic susceptibility per spin, and N_s the number of spins participating in barrier quenching (and barrier hopping). Experiments on Cu:Mn 6at.% and CdCr_{1.7}In_{0.3}S_4 for ranges of H, T, and waiting times t_w generate the correlation length, Ksi(t_w,T) ~ {N_s}^{1/3}, fitted by both the hierarchical model, Ksi(t_w,T)=0.635(t_w}/tau_0)^{0.169T/T_g}, in numerical accord with simulations, or the droplet model, Ksi(t_w,T)={10^{-5}}{{[(T/T_g)Ln(t_w/tau_0)]}^{1/0.21}}, with too small a prefactor for simulation time scales, and exponent 1/psi at the lower limit for psi.