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

The grain-size effects on the charge ordering and exchange bias in CE-type antiferromagnetic charge ordered manganite compound ${\text{Pr}}_{0.5}{\text{Ca}}_{0.5}{\text{MnO}}_{3}$ are explored by magnetometry. Reducing the size suppresses antiferromagnetism and charge ordering that does not occur any more when the size is smaller than 40 nm. At the same time ferromagnetic clusters appear and gradually become larger; their fraction increases to about 19.7% when the particle size is reduced to 20 nm. The magnetic hysteresis loops of the nanosized samples exhibit both horizontal and vertical shifts in the field-cooled process. The exchange bias field ${H}_{\text{eb}}$ and the coercivity ${H}_{\text{C}}$ do not depend monotonously on size but show a maximum at about 85 nm, which indicates that in antiferromagnetic charge-ordered manganites ${H}_{\text{eb}}$ can be effectively tuned by changing the grain size. These notable phenomena can be understood when the evolution of the spin configuration is considered that results from the surface-phase separation and surface effect.

Grain-size effects on the charge ordering and exchange bias in Pr 0.5 Ca 0.5 MnO 3 : The role of spin configuration

Exchange coupled bilayers of soft and hard ferromagnetic thin films show remarkable analogies to conventional antiferromagnetic/ferromagnetic exchange bias heterostructures. Not only do all these ferromagnetic bilayers exhibit a tunable exchange bias effect, they also show a distinct training behavior upon cycling the soft layer through consecutive hysteresis loops. In contrast with conventional exchange bias systems, such all ferromagnetic bilayer structures allow the observation of training induced changes in the bias-setting hardmagnetic layer by means of simple magnetometry. Our experiments show unambiguously that the exchange bias training effect is driven by deviations from equilibrium in the pinning layer. A comparison of our experimental data with predictions from a theory based upon triggered relaxation phenomena shows excellent agreement.

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Exchange bias training effect in coupled all ferromagnetic bilayer structures

Perovskite manganites exhibit a wide range of functional properties, such as colossal magneto-resistance, magnetocaloric effect, multiferroic property, and some interesting physical phenomena including spin, charge, and orbital ordering. Recent advances in science and technology associated with perovskite oxides have resulted in the feature sizes of microelectronic devices down-scaling into nanoscale dimensions. The nanoscale perovskite manganites display novel magnetic and electronic properties that are different from their bulk and film counterparts. Understanding the size effects of perovskite manganites at the nanoscale is of importance not only for the fundamental scientific research but also for developing next generation of electronic and magnetic nanodevices. In this paper, the current understanding and the fundamental issues related to the size effects on the magnetic properties and charge ordering in manganites are reviewed, which covers lattice structure, magnetic and electronic properties in both ferromagnetic and antiferromagnetic based manganites. In addition to review the literatures, this article identifies the promising avenues for the future research in this area.

Magnetic and charge ordering in nanosized manganites

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

Transport, magnetic, and thermal properties of phase-separated ${\mathrm{La}}_{0.8}{\mathrm{Ca}}_{0.2}\mathrm{Mn}{\mathrm{O}}_{3}$ crystal were studied in a wide temperature range down to $10\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. At low temperatures below the Curie point ${T}_{C}=184\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, the sample resistance is characterized by spontaneous transitions to higher resistivity metastable states. Metastability becomes more pronounced when enforced by the application of current pulses at low temperatures. Metastable states are characterized by long-term memory surviving even thermal cycling to room temperatures. Only heating to $Tg{T}_{e}\ensuremath{\approx}350\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ erases the previously imprinted state of the system. At temperatures close to the low temperature resistivity maximum, a slow relaxation of the resistance has been observed following changes in the bias current. Ac susceptibility, low-field magnetization, and specific heat data indicate that there is a spin-cluster glass-like transition at temperatures corresponding to the maximum of the relaxation time. Phase separation and coexistence of metallic and insulating ferromagnetic phases with different orbital order at a wide temperature range are claimed to be responsible for the observed electric-field and current effects. The disappearance of the resistance memory effects at temperatures above ${T}_{e}$ may be considered an indirect proof for the existence of one more temperature scale in disordered manganites.

Electric-field and current-induced metastability and resistivity relaxation in La 0.8 Ca 0.2 Mn O 3 at low temperatures

Excess low‐frequency noise extending to MHz frequencies was observed in dc current biased granular high‐Tc thin films. At particular bias conditions random telegraph signal produced by a single, fast two‐level fluctuator dominated the noise properties of the sample. Lifetimes of the low‐ and high‐voltage states of the fluctuating system were found to be exponentially distributed. Power spectra of the excess noise signal could be well fitted with a single Lorentzian contribution. Duty cycle dependence of the random telegraph signal on bias conditions was used to get an insight into physical mechanism causing the fluctuations. Charge trapping events in the intergranular intrinsic Josephson junctions and trapped flux hopping were identified as possible alternative sources of the observed noise.

Random telegraph signals and low-frequency voltage noise in Y-Ba-Cu-O thin films

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.

G. Jung

Papers

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

Electric-field and current-induced metastability and resistivity relaxation in La 0.8 Ca 0.2 Mn O 3 at low temperatures

Random telegraph signals and low-frequency voltage noise in Y-Ba-Cu-O thin films

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