We review experimental studies of the time decay of the nonequilibrium magnetization in high-temperature superconductors, a phenomenon known as magnetic relaxation. This effect has its origin in motion of flux lines out of their pinning sites due to thermal activation or quantum tunneling. The combination of relatively weak flux pinning and high temperatures leads to rich properties that are unconventional in the context of low temperature superconductivity and that have been the subject to intense studies. The results are assessed from a purely experimental perspective and discussed in the context of present phenomenological theories. [S0034-6861(96)00403-5]

Magnetic relaxation in high-temperature superconductors

Magnetic properties of electron-doped La0.23Ca0.77MnO3 manganite nanoparticles, with average size of 12 and 60 nm, prepared by the glycine–nitrate method, have been investigated in the temperature range 5–300 K and magnetic fields up to 90 kOe. It is suggested that weak ferromagnetic moment results from ferromagnetic shells of the basically antiferromagnetic nanoparticles and from domains of frustrated disordered phase in the core. Assumption of two distinct sources of ferromagnetism is supported by the appearance of two independent ferromagnetic contributions in the fit of the T
 3/2 Bloch law to spontaneous magnetization. The ferromagnetic components, which are more pronounced in smaller particles, occupy only a small fraction of the nanoparticle volume and the antiferromagnetic ground state remains stable. It is found that the magnetic hysteresis loops following field cooled processes, display size-dependent horizontal and vertical shifts, namely, exhibiting exchange bias effect. Time-dependent magnetization dynamics demonstrating two relaxation rates were observed at constant magnetic fields upon cooling to T < 100 K.

Magnetic properties of electron-doped La0.23Ca0.77MnO3 nanoparticles

Local magnetic relaxation in high-temperature superconductors.

Influence of magnetic field on critical behavior near a first order transition in optimally doped manganites: The case of La1−xCaxMnO3 (0.2≤x≤0.4)

Magnetic properties of compacted La0.8Ca0.2MnO3 manganite nanoparticles with average particle size of 18 and 70 nm and Curie temperatures TC 231 K and TC 261 K, respectively, have been investigated. The relative volume of the ferromagnetic phase has been estimated to be 52% for ensembles of 18 nm particles and 92% for 70 nm particles. It was found that applied hydrostatic pressure enhances TC of La0.8Ca0.2MnO3 nanoparticles at a rate dTC /dP 1.8– 1.9 K / kbar, independently on the average particle size. Pronounced irreversibility of magnetization below Tirr 208 K and strong frequency dependent ac susceptibility below TC for smaller 18 nm particles have been observed. 18 nm particles have also shown aging and memory effects in zero-field-cooled ZFC and field-cooled magnetization. These features indicate the appearance of spin-glasslike state, partially reminiscent the behavior of La1�xCaxMnO3 crystals, doped below the percolation threshold xxC = 0.225. In contrast, ensembles of larger 70 nm particles have shown insignificant irreversibility of magnetization only and no frequency dependence of ac susceptibility, similarly to the behavior of La1�xCaxMnO3 crystals with xxC. The temperature of the ZFC magnetization maximum for 18 nm particles decreases with increasing magnetic field and forms a critical line with an exponent 1.89 0.56. The results suggest that superspin-glass features in ensembles of interacting 18 nm particles appear along with superferromagnetic-like features.

Spin-glass-like properties ofLa0.8Ca0.2MnO3nanoparticles ensembles

Disorder-induced phase coexistence in bulk doped manganites and its suppression in nanometer-sized crystals: The case ofLa0.9Ca0.1MnO3

Random telegraph voltage noise signals strongly dependent on externally applied magnetic field and transport current have been detected in c-axis-oriented BiSrCaCuO thin films at temperatures just below the onset of superconductivity. Signals appeared only in a narrow range of bias currents and associated magnetic fields. Such signals, already observed in granular YBaCuO films at helium temperatures, seem to be a universal feature of high-Tc superconducting films. Observed signals are consistently interpreted as an evidence of vortex hopping between two distinct pinning sites. Resistivity fluctuations and intrinsic SQUID detection are proposed as alternative mechanisms converting flux noise into random telegraph voltage signals.

Flux Origin of Random Telegraph Voltage Signals in High-Tc Superconducting Thin Films

Magnetic properties of compacted Sm0.1Ca0.9MnO3 particles with 25 and 60 nm average sizes have been investigated. Particular attention has been paid to Griffiths-like features at temperatures above magnetic transition temperature TC and to the system glassiness at low temperatures. Griffiths-like features in inverse magnetic susceptibility of Sm0.1Ca0.9MnO3 nanoparticles have been linked to the presence of short range ferromagnetically correlated spin clusters above TC. Glassy behavior has been revealed in temperature and frequency dependence of ac-susceptibility, temperature and field dependence of thermoremanent and isothermoremanent magnetization, and time decay of the remanent magnetization. Experiments revealed the major impact of the glassy component on magnetic properties of investigated nanoparticles. The magnetic relaxation associated with glassy features was found to be much more pronounced in smaller particles, where a formation of collective state in an ensemble of phase separated nanoparticle...

Irreversibility, remanence, and Griffiths phase in Sm0.1Ca0.9MnO3 nanoparticles

Magnetic properties of compacted Sm0.1Ca0.9MnO3 nanoparticles with average particle size of 25 and 60 nm have been investigated. It was found that the relative volume of the ferromagnetic phase decreases with decreasing particle size. Magnetization curves measured in field cooled and zero field cooled mode separate near the transition temperature TC and remain different even in magnetic field of 15 kOe. AC-susceptibility is strongly frequency dependent below TC, although the temperature of the maximum depends on frequency only slightly. Magnetization hysteresis loops exhibit horizontal and vertical shifts, relatively small in 60 nm and much larger in 25 nm particles, due to size-dependent exchange bias effect. The exchange bias field and the coercive field depend in a non-monotonic way on cooling magnetic field, while the asymmetry of remanence magnetization and magnetic coercivity increase monotonously with the increase of cooling field. Applied pressure enhances Curie temperature TC of nanoparticles wit...

Magnetic properties of Sm0.1Ca0.9MnO3 nanoparticles

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