J. Hakl

Magnetic particle hyperthermia: Néel relaxation in magnetic nanoparticles under circularly polarized field.

The mechanism of magnetization reversal in single-domain ferromagnetic particles is of interest in many applications, in most of which losses must be minimized. In cancer therapy by hyperthermia the opposite requirement prevails: the specific loss power should be maximized. Of the mechanisms of dissipation, here we study the effect of Néel relaxation on magnetic nanoparticles unable to move or rotate and compare the losses in linearly and circularly polarized fields. We present exact analytical solutions of the Landau-Lifshitz equation as derived from the Gilbert equation and use the calculated time-dependent magnetizations to find the energy loss per cycle. In frequencies lower than the Larmor frequency, linear polarization is found to be the better source of heat power, at high frequencies (beyond the Larmor frequency) circular polarization is preferable.

Coloring hot-dip galvanization of steel samples in industrial zinc-manganese baths

Colored hot dip galvanization of various steel samples was realized in an industrial bath containing 738 kg of a Zn-Mn liquid alloy at 450 oC. Zinc was alloyed in three steps to reach 0.1, 0.15 and 0.2 w% of Mn in liquid zinc, and galvanization of 9 different steel samples was performed in all three baths. The obtained colors change in the sequence blue – yellow – pink – green with increasing the Mn-content of the bath and with increasing the wall thickness of the steel samples. The results are analyzed by Glow-discharge optical emission spectroscopy (GD-OES) and Secondary Neutral Mass Spectrometry (SNMS) techniques. It is shown that depending on the Mn-content and on the wall thickness of the steel the samples are coated by MnO of various thicknesses (in the range between 30 – 230 nm). This layer forms when the samples are removed from the Zn-Mn bath into surrounding air, before the Zn-layer is solidified. Light interference on this thin MnO layer causes the colors of the galvanized coating. Different colors are obtained in different ranges of MnO thicknesses, in accordance with the laws of optics. The minimum Mn-content of liquid Zn is found as 0.025 ± 0.010 m/m% to ensure that the original outer ZnO layer on Zn is converted into the MnO layer. This minimum critical Mn-content is in agreement with chemical thermodynamics.

Larmor Precession and Debye Relaxation of Single-Domain Magnetic Nanoparticles

The numerous phenomenological equations used in the study of the behaviour of single-domain magnetic nanoparticles are described and some issues clarified by means of qualitative comparison. To enable a quantitative application of the model based on the Debye (exponential) relaxation and the torque driving the Larmor precession, we present analytical solutions for the steady states in presence of circularly and linearly polarized AC magnetic fields. Using the exact analytical solutions, we can confirm the insight that underlies Rosensweig’s introduction of the “chord” susceptibility for an approximate calculation of the losses. As an important consequence, it can also explain experiments, where power dissipation for both fields was found to be identical in “root mean square” sense. We also find that this approximation provides satisfactory numerical accuracy only up to magnetic fields for which the argument of the Langevin function reaches the value 2.8.

The Role of Iron in the Enhancement of Negative Magnetoresistance in La0.8Sr0.2FexCo1−xO3−z

The role of iron in enhancing the magnetoresistance in the compounds La0.8Sr0.2FexCo1−xO3−z was investigated by studying the electronic and magnetic structure of La0.8Sr0.2FexCo1−xO3−z as a function of temperature. For this purpose 57Fe transmission Mossbauer spectroscopy, magnetoresistance, as well as AC and DC magnetization measurements were applied. The detailed study of the temperature dependence of 57Fe Mossbauer parameters gave possibility to explore correlations between the local electronic and magnetic state of iron and the magnetic susceptibility as well as magnetoresistance in La0.8Sr0.2FexCo1−xO3−z. On the basis of the obtained results an attempt was made to explain the exotic magnetic and MR properties of these perovskites.

Disordered magnetic phase in partially oxidized bulk nanocrystalline iron

Magnetically frozen partially disordered state was observed in nanocrystalline bulk iron prepared by ball-milling of microcrystalline Fe. Role of nanostructure and oxidized fraction is pointed out. Altough we have found a small constituent which is proportional to 1/T in the ZFC curve, the magnetization scaling with T3/2 at high fields and the hysteretic behavior in FC-ZFC curves signalizes the presence of frozen ferromagnet at low temperatures.