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Magnetic properties of NiFeAlO4 nanoparticles

When the size of the ferromagnetic material is reduced bellow some critical value (~100 nm), it is energetically favorable that the magnetic structure of the particle is singe-domain. Single-domain particles can be characterized by their magnetic moment. These materials also have magnetic anisotropy which tends to direct magnetic moment of the particle along the certain axis (so-called easy axis). Two energetically favorable orientations are separated by the magnetic anisotropy barrier. Because of the thermal energy magnetic moment of the particle can fluctuate over the anisotropy barrier and change its orientation. At high temperatures thermal energy is comparable to the anisotropy barrier so fluctuations of the magnetic moment are quick in respect to the time of the measurement and the system behaves as paramagnet. These particles are called superparamagnets because the values of their magnetic moment are much larger than magnetic moment of paramagnetic ions. When the temperature is lowered, thermal energy is small compared to the anisotropy barrier and the magnetic moment of the particle is blocked on the one side of the barrier during the time of the experiment. Relaxation of the magnetic moment of the system of many magnetic nanoparticles is thermally activated process and relaxation time is given by the Arrhenius law. Nanosized magnetic particles have variety of the new interesting properties so they offer many possibilities for technological applications, such as magnetic data storage, biomedical applications (magnetic drug delivery, magnetic separations of cells, contrast agent for magnetic resonance imaging) etc. Magnetic properties of the NiFeAlO4 nanoparticles have been investigated. The size of the particles is 6 nm and the structure is cubic spinel structure. The magnetic moment of the sample of the nanoparticles has been measured in the temperature dependence for different values of the applied magnetic field. The zero field cooled (ZFC) and field cooled (FC) curves have been obtained. The splitting of the ZFC and FC curves is a consequence of the slow relaxation of the magnetic moment of the sample with respect to the time of the measurement of the one point. The temperature bellow which the splitting occurs is called blocking temperature (240 K for measurements in magnetic field of 0.1 T). It was observed that the blocking temperature is lower for the higher values of the applied magnetic field. Magnetic moment of the sample in magnetic field dependence has been measured at different temperatures. Hysteresis loops have been obtained at all temperatures up to the room temperature and the value of the coercive field is lower for the measurements at higher temperatures. At the room temperature the value of the coercive field is 0.003 T and therefore it is expected that at some higher temperature m(H) curve should be reversible. Relaxation of the magnetic moment of the sample has also been measured during ~2.5 h at different temperatures and it has been observed that the time dependence is logarithmic. Measured magnetic properties of the NiFeAlO4 nanoparticles fit very well within the frame of superparamagnetic particles with thermally activated relaxation of the magnetic moment. Detailed qualitative and quantitative analysis of the NiFeAlO4 nanoparticles will be subject of the further investigation.

nanoparticle; superparamagnets; magnetic properties; thermal relaxation

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