Dao Nguyen Hoai Nam

Field dependence of zero-field-cooled magnetization of La1-xSrxCoO3 (x=0.05-0.5)

Zero-field-cooled magenetizations with field up to 50000e and temperature up to 250 K were measured for La 1-x Sr x CoO 3 compounds with Sr concentrations of x = 0.05, 0.15, 0.2, 0.3, 0.4 and 0.5. It is shown that all M ZC curves exhibit a maximum at a temperature T a (<T c ), which scales well with the power law T a /T c 1 - (C AT /T c )H.The variation of parameter n with the concentration x shows very clearly the existence of two regions bording at a value x c of around 0.18. For the low-Sr region the parameter n does not depend on x and its values are similar to those suggested by the Almaida. Thouless model for canonical spin glass (n = 2/3), whereas in the high x-region n increases monotonically from 0.23 to 0.58, correspondingly, for x varying from 0.2 to 0.5.

Magnetoresistance and magnetocaloric properties of La0.7Sr0.3Co0.95Mn0.05O3 compound

The magnetoresistance and magnetocaloric effects were studied via magnetic and electric properties measurements. The ferromagnetic transition temperature and critical exponents, which are determined by analyzing the Arott plots, to be TC = 190.665K, ? = 0.456, ? = 1.0623 and ? = 3.2845, respectively. Although ferro-paramagnetic transition was observed clearly with TC of 190 K but the insulating behaviour is remained in a whole in a temperature region of 5 K up to 300 K. Moreover the negative magnetoresistance shows a shoulder at ~TC, broadens toward lower temperature and reach maximum of 11.5% at 174 K in applied field of 5 T. This result is explained as due to the competing between ferromagnetic and non-ferromagnetic phases in the sample. The magnetization measurements were performed in a narrow temperature range, close to TC and external magnetic fields up to 4.5T. The adiabatic magnetic entropy changes, ?S, determined from magnetization data, shows a maximum at TC.

Magnetic memory effect in a strong phase competition system La0.7Ca0.3Mn0.925Ti0.075O3

A magnetic memory effect was observed by means of zero-field-cooled magnetization measurements in a temperature range below TC. The magnitude of magnetic memory effect, determined by the difference between the magnetization curve with a pause of temperature during the cooling process and its reference curve without the pause, showed that it is independent of the number of pauses. The memory effect is supposed to be related to the isolated states with an infinite degeneration, which is associated with the presence of magnetic frustration in a system with a strong magnetic phase competition. Besides, a resistance memory effect was concomitantly observed at the same pause temperatures.

High-energy ball milling preparation of La0.7Sr0.3MnO3 and (Co,Ni)Fe2O4 nanoparticles for microwave absorption applications

Microwave and radar absorbing materials (MAM and RAM) are widely used for reducing electromagnetic interference (EMI) for electronic equipment and devices, in electromagnetic anechoic technique, and especially in radar stealth technology. Ferromagnetic and ferrimagnetic nanoparticles have been known to have a strong microwave absorbing capability. To study magnetic MAMs and RAMs, we have prepared La 0.7 Sr 0.3 MnO 3 ferromagnetic and several (Co,Ni)Fe 2 O 4 ferrimagnetic nanoparticle powders using high-energy ball milling technique, which is capable of producing nanoparticles in reasonably larger scale comparing to conventional chemical methods. The magnetic properties of the nanoparticle powders are strongly dependent on the preparation conditions. The milling process produces damages and defects not only on the surface, but also the crystal structure inside the particles, that cause an undesired strong reduction of saturation magnetization ( M s ) and an increase of coercivity ( H c ). A suitable post-milling heat treatment is able to heal the particles and recover most of their saturation magnetization and magnetic softness. The nanoparticles are then mixed with paraffin for microwave and radar absorption measurements.

Magnetic Properties of Annealed Powders Prepared By Mechanical Alloying

Fe-Co alloy powders were prepared by mechanical alloying of the elemental Fe and Co powders in air and subsequently annealed at various temperatures. Structural and magnetic characteristics of the annealed powders were studied in detail as a function of annealing temperatures by using an X-ray diffractometer (XRD), a field emission scanning electron microscope, a vibrating sample magnetometer, and a physical property measurement system. The XRD results showed an existence of the nanocrystals with sizes of 15-50 nm. The magnetic studies indicated a strong increase of magnetization and a sharp decrease of coercivity as annealing temperature increased. Both the effect of the oxidation on the magnetic properties as well as magnetization stability of the annealed samples will be discussed.