Interplay between superparamagnetic and blocked behavior in an ensemble of lanthanum-strontium manganite nanoparticles.

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.