H. Gencer

Electrical Transport and Magnetoresistance of La 0.67 Ca 0.33 MnO 3 : Ag x (x = 0, 0.1, 0.2, 0.3, 0.4) Composites

The structural, magnetic and magnetotransport properties of La 0.67 Ca 0.33 MnO 3 : Agx (x = 0, 0.1, 0.2, 0.3 and 0.4) composites were investigated systematically. X-ray and EDX analysis indicated that Ag is not substituted into the main La 0.67 Ca 0.33 MnO 3 phase and remains an additive to the second phase at the grain boundary. The Curie temperature first decreased from 269 K for x = 0 to 257 K for x = 0.1 and then remained nearly unchanged with increasing Ag content. For the x > 0.1 samples, a second transition temperature (T MI2 ) was observed in the resistance curves. At temperatures below 150 K, a significant enhancement in MR was observed while high temperature MR decreased with increasing Ag content. The maximum MR was observed to be 55% in the x = 0.4 sample at 10 K and a 6T magnetic field, this value is larger than that of pure La 0.67 Ca 0.33 MnO 3 (53% at 265 K and 6 T). In addition, at low fields (H < 1T), a sharp increase in the MR was observed.

ELECTRICAL TRANSPORT AND MAGNETORESISTANCE PROPERTIES OF La0.67Ca0.33MnO3 FILM COATED ON PYREX GLASS SUBSTRATE

The electrical transport and magnetoresistance properties of the polycrystalline La0.67Ca0.33MnO3 film produced on a Pyrex substrate were investigated for the first time. X-ray powder diffraction indicated that the film sample has a perovskite structure. Scanning electron microscope indicated that La0.67Ca0.33MnO3 film thickness is approximately 500 nm, and the average grain size of this sample varies between 40 and 50 nm. La0.67Ca0.33MnO3 film showed a phase transition from paramagnetic to ferromagnetic at (TC) 80 K and a metal–insulator transition at (TMI) 77.5 K and at 2 mT magnetic field. The upturn of the resistance observed at low temperatures (<36 K) was attributed to the Coulomb blockade, and the strong structural disorder is due to the large lattice mismatch and strain relaxation. A large magnetoresistance ratio [MR (%)] of 780% was observed at 100 K and 6 T magnetic field.

INFRARED, STRUCTURAL AND MAGNETIC PROPERTIES OF La0.67Ca0.33MnO3 COMPOUND

La0.67Ca0.33MnO3 samples prepared by polymeric precursor route method are investigated using the thermogravimetric, differential thermal analysis, X-ray powder diffraction, scanning electron microscope, infrared spectroscopy and magnetic measurements. The unit cell parameters and grain size for all the samples are refined using X-ray diffraction and scanning electron microscope. It is observed that the IR bands at about 400 cm-1 is are most likely due to Mn–O–Mn bending mode and the band at 600 cm-1 is the result of the in plane Mn–O stretching mode. Magnetization measurements indicated that the Curie temperature increases from 241.3 K for the sample sintered at 600°C to 268.86 K for the sample sintered at 1000°C as a function of sintering temperature. The magnetic entropy change reaches the maximum value of about 4.8J/kg·K for the sample sintered at 1150°C at an applied field of 1 T.

THE VARIATION OF EXCHANGE CONSTANT AND MAGNETOCALORIC EFFECT IN LaFe13-xSix (x = 1.6, 1.9 AND 2.2) COMPOUNDS

In this work, we present the variation of exchange constant (Jexc) and magnetocaloric effect (MCE) in LaFe13-xSix (x = 1.6, 1.9 and 2.2) compounds. NaZn13-type LaFe13-xSix intermetallic compounds were prepared by arc melting in an atmosphere of highly pure argon gas. X-ray results indicated a slight decrease of lattice parameter with increasing Si content. The maximum magnetic entropy change of the LaFe13-xSix (x = 1.6, 1.9 and 2.2) samples decreases with increasing Si contents. Also, the magnetic entropy change was calculated theoretically at various magnetic fields and compared with experimental data. The spin-wave dispersion coefficients (D) and exchange constants (Jexc) were calculated for the fist time for these samples. It was found that D and Jexc increase with increasing Si content.

Magnetic and Magnetocaloric Properties of (Gd 1-x Ce x )Al 2 (x = 0, 0.25, 0.5, 0.75) Compounds

The magnetic and magnetocaloric properties of Gd 1-x Ce x Al 2 (x = 0, 0.25, 0.5, 0.75) intermetallic compounds alloys have been investigated in detail for the first time. XRD patterns indicated that all the samples were crystallized in a single phase with MgCu 2 -type structure (Laves phase). Ce substitution for Gd increased the lattice parameters and decreased the Curie temperature from 163 K for x = 0 to 37 K for x = 0.75. A maximum entropy change of 3.82 J/kg K was observed when a 2 T magnetic field was applied to the x = 0 sample. This entropy change decreased with increasing Ce content to 2.04 J/kg K for the x = 0.75 sample.

SIZE-INDUCED EFFECTS IN NANOSTRUCTURED NiFe2O4

NiFe2O4 nanoparticles were successfully prepared by a hydrothermal process, and the effect of temperature on them was studied. The particles were annealed at various temperatures ranging from 413 to 1473 K. Studies were carried out using powder X-ray diffraction, scanning electron microscopy, infrared spectroscopy, differential thermal analysis, thermogravimetric analysis and a vibrating sample magnetometer. The annealing temperature had a significant effect on the magnetic and structural parameters, such as the crystallite size, lattice parameter, magnetization and coercivity.