L. Lundgren

Anti-Meissner effect in the BiSrCaCuO-system

Abstract The low field susceptibility of a sintered sample of Bi 2 Sr 2 Ca 1 Cu 2 O 8+δ has been investigated in a SQUID magnetometer. In the low field FC susceptibility a new feature of high-T c superconductors is revealed, an antimeissner effect. Vortex pair fluctuations and pinning are believed to be essential for the observed behaviour.

Coexistence of aging states in spin glasses

Abstract The influence of temperature variations on the aging process of a Cu(Mn) spin glass has been investigated. It is found that the dynamics of the aging process is not very susceptible to small temperature variations around the measurement temperature. The astonishing possibility for the spin glass system to house two coexisting aging states is demonstrated.

Low field magnetic relaxation phenomena in YBaCuO

Abstract The relaxation of the magnetization of a sintered sample of YBa 2 Cu 3 O 7 has been investigated in a SQUID-magnetometer. The relaxation may adequately be described by a power law t -a at all fields (1–100 gauss) and temperatures (20–90 K) investigated. A marked field dependence of the exponent a and a corresponding field dependence in the relaxation rate (1/H ∂M/∂ln t) is observed. Three rather distinct field regimes with different values of the exponent a and different field dependences of the relaxation rate may be distinguished: a low field region where a logarithmic relaxation ( a ∼ 0) is found, an intermediate regime with a ∼ 0.04 and a high field region with a ∼ 0.07.

Non-logarithmic magnetic relaxation in PbMo6S8

The time dependence of the zero-field-cooled magnetization of the extreme type-II superconductor PbMo6S8 has been measured at temperatures 0.7≤T/Tc≤0.95 and for fields 1G≤H≤300G. The results show that the decay changes from a logarithmic to a non-logarithmic form as the field is increased, indicating that the conventional flux creep model is not sufficient to describe the phenomena.

Field and time dependence of the magnet susceptibility of single crystal YBa2Cu3O7

Abstract The susceptibility parallel and perpendicular to the crystallographic c-axis of a single crystal of YBa2Cu3O7 has been investigated in the temperature range 20–100 K in applied fields of 0.01 – 300 gauss. Relaxation and field dependent behaviours of the zero field cooled and the thermoremanent magnetizations are reported.

Non‐equilibrium dynamics in spin glasses

Some recent experiments on the non‐equilibrium dynamics in two‐ and three‐dimensional spin glasses are reviewed. The experimental results are discussed within new domain (droplet) theories of the spin glass phase. It is found that the experiments convincingly verify some fundamental concepts of these theories. Especially, it is shown that if a spin glass has been ‘‘aged’’ at constant temperature and afterwards subjected to a persistent temperature shift or a temperature cycling, the dynamics are governed by an interplay between the growth of spin glass domains and the overlap length, which is the maximum length scale where equilibrium spin correlations at two nearby temperatures are indistinguishable.

Magnetic measurements on coevaporated YBa2Cu3O7 thin films

Abstract The field an time dependence of the magnetic susceptibility of coevaporated YBa 2 Cu 3 O 7 thin films on SrTiO 3 and sapphire substrates have been investigated. Different relaxation and field dependent behaviour for the two films is reported.

Thermal magnetic fluctuations in metallic spin glasses

Abstract Thermal magnetic fluctuations of the amorphous metallic spin glasses (Fe0.10Ni0.90)75P16B6Al3 and (Fe0.06Ni0.94)75P16B6Al3 have been investigated in a SQUID magnetometer. The observed power spectra display 1 f behaviour and are found to be related to the out-of-phase component of the ac susceptibility, in accordance with the predictions of the fluctuation dissipation theorem.