A.K.M. Akther Hossain

Colossal magnetoresistance in spinel type Zn1−xNixFe2O4

Spinel-type ferrites are widely used in practical applications. A fascinating property of Zn-Ni ferrites which reveals a direction for application is reported. A large negative magnetoresistance effect has been observed in ZnFe2O4 and Ni substituted Zn1−xNixFe2O4 ferrites of spinel structure. These materials are either ferrimagnetic or paramagnetic at room temperature and a spin (cluster) glass transition was found for some compositions at low temperatures. The magnetoresistance is either parabolic or linear with respect to applied field up to 9 T depending on the compositions and temperatures. It was found that the magnetoresistance effect increases as the Ni content increases in Zn1−xNixFe2O4 up to x=0.2 and then again decreases and finally become negligible for x=1.0, i.e., NiFe2O4. This magnetoresistance effect can be explained with the help of spin-dependent scattering and the Yafet-Kittel angle of the Ni-substituted Zn-Ni ferrites.

Influence of strain and microstructure on magnetotransport in La0.7Ca0.3MnO3 thin films

A La0.7Ca0.3MnO3 thin film made by pulsed laser deposition (PLD) and another film of the same composition made by metal organic chemical vapor deposition (MOCVD), both on single crystal LaAlO3, were subject to a series of six, short, controlled anneals. The oxygen content was purposely not changed in the films from the first anneal to subsequent anneals. After each anneal, the film microstructures were characterized to determine average grain size, lattice constants, nonuniform strain, and crystalline mosaic spread, and these parameters were correlated with the magnetotransport properties. For both sets of films, the influence of annealing was to both increase the temperature at which the maximum in the magnetoresistance occurs (Tm) and the maximum magnetoresistance (MR) value. The improvement in film properties occurred in conjunction with stress relaxation and improved crystallinity, as a result of grain growth. The MOCVD films showed poorer grain coupling and poorer epitaxy compared to the PLD films. T...

Influence of oxygen vacancies on magnetoresistance properties of bulk La0.67Ca0.33MnO3-δ

Abstract Bulk polycrystalline samples of La 0.67 Ca 0.33 MnO 3− δ were annealed in controlled oxygen atmospheres. Two types of conditions were chosen, firstly with the aim of taking oxygen from the grain boundaries and secondly with the aim of removing oxygen uniformly from the sample. Reducing conditions increase the overall resistivity and suppress the Curie–Weiss temperature ( T c ). A second peak at temperatures below T c appears in the resistivity and magnetoresistivity when the samples are deoxygenated. However, the presence of this peak does not strongly influence the low-temperature magnetoresistance.

Influence of oxygen stoichiometry on Raman phonon spectroscopy, lattice parameters and physical properties of La0.7Ca0.3MnO3 thin films

Abstract In this study the influence of annealing at reduced oxygen pressure on electrical transport, Raman active phonon modes and structural properties of several La 0.7 Ca 0.3 MnO 3 thin films has been determined. The out-of-plane lattice parameters and some of the Raman phonon modes were found to be insensitive to oxygenation condition. The temperature at which the peak magnetoresistance occurs and other of the Raman phonon modes, are found to change continuously as a function of decreasing oxygen partial pressure. The Raman studies indicate that small variations of oxygen content induce a structural change.

High-temperature cluster glass state and photomagnetism in Zn- and Ti-substituted NiFe2O4 films

We report on the cluster glass nature and photoinduced magnetization (PIM) of (Ni,Zn,Fe,Ti)3O4 thin films prepared using a pulsed-laser deposition (PLD) technique. The films exhibit cluster glass behaviors with a spin-freezing temperature Tf of approximately 230 K. It was found that the magnetization increases following light irradiation below Tf. The experimental results suggest that the electronic transitions induced by photons are intrinsic to the observed PIM rather than the thermal heating effects resulting from light irradiation. The change in the magnetization from the original dark value at 10 K was approximately 40% for the Ni0.7Zn0.3Fe1.7Ti0.3O4 film, and it was approximately 5% for the Ni0.4Zn0.6Fe2O4 film when the excitation intensity is 1.48mW∕mm2. The analysis of the excitation energy dependence of PIM suggests that an intervalence charge transfer Ti4++Fe2+→Ti3++Fe3+ contributes to the observed enhancement in the PIM of the Ti-substituted films.

Electrical transport properties of Mn?Ni?Zn ferrite using complex impedance spectroscopy

Polycrystalline Mn0.45Ni0.05Zn0.50Fe2O4 was prepared by a standard solid state reaction technique. We report the electrical properties of this ferrite using ac impedance spectroscopy as a function of frequency (20 Hz–10 MHz) at different temperatures (50–350 °C). X-ray diffraction patterns reveal the formation of cubic spinel structure. Complex impedance analysis has been used to separate the grain and grain boundary resistance of this ferrite. The variation of grain and grain boundary conductivities with temperature confirms semiconducting behavior. The dielectric permittivity shows dielectric dispersion at lower frequency and reveals that it has almost the same value on the high-frequency side. The non-coincidence of peaks corresponding to modulus and impedance indicates deviation from Debye-type relaxation. A similar value of activation energy is obtained from impedance and modulus spectra, indicating that charge carriers overcome the same energy barrier during relaxation. Electron hopping is responsible for ac conduction in this ferrite. The electron hopping shifts toward higher frequency with increasing temperature, below which the conductivity is frequency independent. The frequency-independent ac conductivity has been observed at and above 300 °C in the frequency range 20 Hz–1 MHz. This frequency-independent ac conductivity is due to the long-range movement of the mobile charge carriers.

Complex impedance and electric modulus studies of magnetic ceramic Ni 0.27 Cu 0.10 Zn 0.63 Fe 2 O 4

The electrical properties of Ni0.27Cu0.10Zn0.63Fe2O4 (NCZF) prepared from auto combustion synthesis of ferrite powders have been studied by impedance and modulus spectroscopy. We studied frequency and temperature dependencies of impedance and electric modulus of NCZF in a wide frequency range (20 Hz–5 MHz) at different measuring temperatures TSM (30–225°C). The complex impedance spectra clearly showed both grain and grain boundary effects on the electrical properties. The observed impedance spectra indicated that the magnitude of grain boundary resistance Rgb becomes more prominent compared to grain resistance Rb at room temperature, and with the increase in TSM, Rgb decreases faster than the intrinsic Rgb. The frequency response of the imaginary part of impedance showed relaxation behavior at every TSM, and the relaxation frequency variation with TSM appeared to be of Arrhenius nature and the activation energy has been estimated to be 0.37 eV. A complex modulus spectrum was used to understand the mechanism of the electrical transport process, which indicated that a non-Debye type of conductivity relaxation characterizes this material.

Relaxation mechanism of (x)Mn0.45Ni0.05Zn0.50Fe2O4 + (1 − x)BaZr0.52Ti0.48O3 multiferroic materials

Polycrystalline (x)Mn0.45Ni0.05Zn0.50Fe2O4 + (1 − x)BaZr0.52Ti0.48O3 (with 0.2 ≤ x ≤ 0.8) multiferroic materials were prepared by the standard solid state reaction technique and samples prepared from these composites were sintered at 1200 °C. The impedance, electrical modulus, ac conductivity and dielectric permittivity were investigated over a wide range of frequencies (20 Hz–1 MHz) and at various temperatures (room temperature to 600 °C) to understand the relaxation phenomenon in these materials. X-ray diffraction patterns confirm the presence of a simple cubic spinel structure for the ferromagnetic phase and tetragonal peroveskite for the ferroelectric phase. Frequency-independent conductivity was observed in the low frequency region, which shifts to a higher frequency and dominates over a wide range of frequency (up to 1 MHZ) at higher temperature (600 °C). The transition temperature (~675 °C) of these composites is higher than that of ferrite and ferroelectric phases. The frequency response electric modulus graphs for some composites show two maxima in the relaxation process. The first relaxation process appears at lower temperature and higher frequency with a lower value of activation energy for the composites containing more than 20% ferrimagnetic phase. This relaxation process is due to the first ionization energy of oxygen vacancies. The second relaxation process appears at all temperatures and at lower frequency, which shifts to higher frequency with increasing temperature, possessing a comparatively higher value of activation energy. This relaxation process is attributed to the Maxwell–Wagner–Sillars relaxation phenomenon. The frequency-dependent impedance and modulus plots exhibit a non-coincidence of relaxation peaks, indicating the deviation from the Debye-type relaxation process.

Colossal magnetoresistance in screen printed La0.67Ca0.33MnO3 thick films

Abstract Thick films of La 0.67 Ca 0.33 MnO 3 were fabricated on (100) oriented single crystal LaAlO 3 (LAO), yttria stabilized zirconia (YSZ) and on polycrystalline Al 2 O 3 by a screen printing technique. A range of sintering conditions were explored with temperature of 1200, 1300 and 1400°C, and in air, oxygen and nitrogen atmospheres. Magnetic and DC resistance properties were measured on all films. The so-called ‘colossal magnetoresistance’ (CMR) behaviour was found to occur for films on all substrates but for specific preparation conditions. The salient feature of the CMR observed in these thick films is that MR is not limited to a small temperature window near the metal–insulator transition (M–I), T p1 . The MR peak is very broad and for some films a temperature independent CMR is observed at temperatures below T p1 . Several repeat films were made and the reproducibility of the results obtained in the first batch was confirmed.

Highly sensitive low-temperature low-field colossal magnetoresistance in screen printed La0.63Y0.07 Ca0.30MnO3 thick films

Abstract Thick films of La0.63Y0.07Cao.30MnO3 were fabricated on (100) oriented single crystal LaAlO3 (LAO) and on polycrystalline Al2O3 substrates by a screen printing technique. The films were sintered at 1200°C in air and oxygen atmosphere. All these thick films show a metal-insulator (M-I) transition at Tp1∼220 K. For both substrates, the films sintered in oxygen flow have slightly higher Tp1. The film on polycrystalline Al2O3 sintered in oxygen flow shows 71% magnetoresistance (MR) at 220 K in the presence of an 8 T applied field. The salient feature of the colossal magnetoresistance observed in these thick films is that low-temperature low-field MR is highly sensitive than only Ca doped manganites.