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] CONTENTS I. Introduction 911 II. Basic Concepts 914 A. Flux exclusion and flux lines in superconductors 914 B. The vortex lattice 915 C. Defected superconductors: irreversible magnetic properties 916 D. The critical-state model 917 E. Magnetic relaxation: basic mechanism 918 III. Overview of Magnetic-Relaxation Experiments 920 A. Experimental issues in magnetic-relaxation measurements 920 1. Sample inhomogeneities 920 2. Field inhomogeneities in SQUID magnetometers 921 3. Establishment of a fully penetrated flux distribution 922 4. Complex demagnetizing fields and anisotropy of high-temperature-superconductor crystal platelets 922 5. Complexities of the remanent state 923 6. Determination of the irreversible component of the magnetization 923 7. Step-by-step procedure for measurements of magnetic relaxation 924 B. Summary of experimental results 925 1. Temperature dependence 925 2. Field dependence 927 3. Deviations from time-logarithmic relaxation 928 4. Magnetic relaxation over surface barriers 929 5. Memory effect 930 6. V-I curves 930 7. Experimental determination of U(J) 931 C. Suppression of magnetic relaxation 932 1. Modulation of flux profiles and flux annealing 932 2. Effect of introducing defects 933 IV. Theoretical Approaches to Magnetic Relaxation in High-Temperature Superconductors 934 A. The electrodynamic equation of flux creep 935 B. Linear U(J)-the Anderson and Kim model 936 C. Nonlinear U(J) 937 1. Collective-creep theory 938 2. Logarithmic barrier 939 3. Barrier distribution 939 V. Concluding Remarks 940 Acknowledgments 941 References 941

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Local magnetic measurements in a highly anisotropic Nd-Ce-Cu-O crystal reveal a sharp onset of an anomalous magnetization peak at a temperature-dependent field B on . The same field marks a change in the field profiles across the sample, from profiles dominated by geometrical barriers below B on to Bean-like profiles above it. The temperature dependence of B on and the flux distribution above and below B on imply a disorder-induced transition at B on from a relatively ordered vortex lattice to a highly disordered, entangled vortex solid. Local magnetic relaxation measurements above B on show evidence for plastic vortex creep associated with the motion of dislocations in the entangled vortex structure.[S0031-9007 (97)04113-6]

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We present magnetic measurements in a single crystal of the newly discovered superconducting ironpnictide Ba͑Fe 0.925 Co 0.075 ͒ 2 As 2. The magnetization loops exhibit a second magnetization peak ͑SMP͒ similar to that observed in most high-temperature superconductors ͑HTSs͒. Magnetic relaxation measurements reveal a minimum in the normalized relaxation rate, S = d ln M / d ln t, located in between the SMP onset and the peak fields. The SMP in HTSs is commonly associated with the vortex order-disorder phase transition. However, in Ba͑Fe 0.925 Co 0.075 ͒ 2 As 2 the onset and peak fields, as well as the minimum point in S, exhibit strong temperature dependence down to low temperatures, excluding the possibility for such a transition. We suggest that the SMP in Ba͑Fe 0.925 Co 0.075 ͒ 2 As 2 is associated with a vortex structural phase transition from rhombic to square lattice taking place at field and temperatures corresponding to the minimum point of S. A theoretical fit to the transition line, based on a recent theoretical model for vortex structural phase transition, shows good agreement with the experimental results.