Magnetic nanoparticles constitute promising tools for addressing medical and health-related issues based on the possibility to obtain various kinds of responses triggered by safe remote stimuli. However, such richness can be detrimental if different performances are not adequately differentiated and controlled. The aim of this work is to understand and systemize different kinds of magnetic-field-induced response for an ensemble of lanthanum-strontium manganite nanoparticles, which are considered as promising materials for self-controlled magnetic hyperthermia. A complex set of static and dynamic magnetic measurements accompanied by a numerical simulation of DC and AC magnetic behavior has been carried out. It is shown that to achieve adequate results, the dispersion of particle sizes and/or magnetic parameters should necessarily be taken into account. A quantitative description of the magnetic behavior of the ensemble should comprise two groups of nanoparticles differentiated according to the regime of their magnetization reversal: one group, which demonstrates non-hysteretic behavior similar to a superparamagnet and another one, which shows magnetic hysteresis characteristic of blocked particles. The fraction of nanoparticles in each group depends not only on the nanoparticles' parameters (in particular, their size), but also on the parameters of the external AC magnetic field (amplitude and frequency) used for remagnetization. The main outcome of this work is the development of a procedure which allows one to separately analyze contributions from different groups of nanoparticles and find the regularities of the redistribution of nanoparticles between these groups on changing the parameters of the external AC magnetic field. The results show the directions to enhance the heating efficiency of ensembles of magnetic nanoparticles and pave the way for further optimization of their characteristics and the parameters of the external field.

The magnetic field and temperature dependences of the critical current density jc(H,T) were studied in thin epitaxial YBCO films (laser ablated on single-crystal SrTiO3 substrates). Two complementary magnetic methods, namely, measurements of virgin magnetization m(H = const,T) and magnetic susceptibility loss-maximum on alternating magnetic field h, were used. The slope of the obtained magnetic field dependences of jc(H,T)/jc(0,T) in the inflection point is analysed in semi-logarithmic (jc–ln(H)) coordinates. It is shown that this parameter is rather informative for the governing pinning mechanism in the specific film, and together with the low-field ‘plateau’ temperature dependence it can be made the basis of the respective identification criterion. Of the five critical current limitation mechanisms considered, only one meets this criterion—the model of collective pinning of the distorted vortex lattice on a system of low-angle grain boundaries formed by edge dislocation walls threading through the whole film thickness. Scaling of the magnetic field and temperature curves in this model allows the emergence of all experimental data to a single general experimental curve specific for an individual film. Fitting of such curves with the theoretical formulae provides important information about the film parameters such as average grain size, dispersion of grain size distribution, and the average misorientation angle of crystallites. The results obtained lead to the conclusion that the laser ablated films studied are characterized by rather large sizes and low misorientation angles of single-crystal blocks as compared with films made using other procedures.

The temperature dependence of the dynamic relaxation rate Q for epitaxial thin films of YBa2Cu3O7−δ is obtained from measurements of the ac magnetic susceptibility at different frequencies in the temperature range from 77K to the critical temperature Tc. The critical current density is determined from the measurements according to two known methods: using the “loss maximum” and a “high constant amplitude” of the ac exciting field. The results obtained by these two methods are compared to each other and with the theoretical predictions of the critical state model as modified by Clem and Sanchez. It is found that for the temperature and field conditions used in the loss maximum measurements, the deviations from that model are small and independent of temperature right up to Tc. Under these conditions the value of Q is determined correctly and is found to be independent of temperature. The results obtained from the measurements by the method of high constant amplitude of the ac field cannot be interpreted correctly because of practical limitations encountered in the whole range of temperatures studied. With increasing distance from Tc the approximate formulas used in that method diverge rapidly from the exact formulas of the critical state model. On approach to Tc the amplitude of the ac field becomes much greater than the corresponding loss maximum, and it is shown that under those conditions the relations among the parameters obtained diverge from the predictions of the Clem–Sanchez theory. It is concluded that this last circumstance is due to the deviations from the critical state model at such high amplitudes of the ac field. As a result, the use of this theory to obtain data on Q becomes incorrect. At high amplitudes of the ac field there is a region about 1K wide below Tc in which one observes a regime where the real part of the complex magnetic susceptibility falls off practically to zero and only the imaginary part remains. This corresponds to a flux-flow-induced resistive state of the superconductor.