P. S. Anil Kumar

The relationship between field-cooled and zero-field-cooled susceptibilities of some ordered magnetic systems

Analysis of the irreversible field-cooled (FC) and the zero-field-cooled (ZFC) magnetic susceptibilities of one ferrimagnetic and three ferromagnetic systems, measured at different applied magnetic fields, shows that the irreversibility indicated by the difference between the FC and the ZFC susceptibilities arises from magnetic anisotropy. The two susceptibilities are related to each other through the coercivity which is a measure of the anisotropy. The ZFC susceptibility can be calculated from the FC susceptibility (or vice versa) and the coercivity.

Enhancement of (BH)max in a hard-soft-ferrite nanocomposite using exchange spring mechanism

We have observed the exchange spring behavior in the soft (Fe3O4)-hard (BaCa2Fe16O27)-ferrite composite by tailoring the particle size of the individual phases and by suitable thermal treatment of the composite. The magnetization curve for the nanocomposite heated at 800 °C shows a single loop hysteresis showing the existence of the exchange spring phenomena in the composite and an enhancement of 13% in (BH)max compared to the parent hard ferrite (BaCa2Fe16O27). The Henkel plot provides the proof of the presence of the exchange interaction between the soft and hard grains as well as its dominance over the dipolar interaction in the nanocomposite.

Systematic study of magnetic parameters of Ni–Zn ferrite synthesized by soft chemical approaches

Active powders of Ni–Zn ferrite were synthesized by soft chemical approaches and were extensively characterized using x-ray diffraction, scanning electron micrographs, vibrating sample magnetometer, resistivity, and permeability/loss factor techniques. The variation in the sintering conditions established a useful microstructure–magnetic permeability correlation.

Origin of the cluster-glass-like magnetic properties of the ferromagnetic system

The magnetic behaviour of La 0.5 Sr 0.5 CoO 3 at low magnetic fields has been studied by ac susceptibility, and field cooled (FC) and zero field cooled (ZFC) magnetization measurements. The cluster-glass-like magnetic behaviour of the compound is found to originate from its magnetocrystalline anisotropy as similar properties are observed for ferromagnetic systems also. The cluster glass freezing temperature and its magnetic field dependence, the irreversibility between the FC and ZFC magnetization curves. the shape of the low-field susceptibility curves, etc are related to the magnitude and temperature variation of the coercivity (H t ) which is a measure of the anisotropy, and the ratio H A /H C where H A is the applied magnetic field.

Transfer ratio of the spin-valve transistor

We describe the factors that control the transfer ratio of the spin-valve transistor. An increase in transfer ratio is obtained by a systematic variation of the height of emitter and collector Schottky barrier, and of the nonmagnetic metals. Next, we found that in some cases, a thicker base leads to a higher transfer ratio. Finally, the thickness of the magnetic layers in the Ni 80 Fe 20 /Au/Co spin-valve base can be optimized for a maximum absolute change of collector current. An overall increase by a factor of 24 was achieved, without loss of the magnetocurrent.

On the origin of itinerant electron behaviour and long-range ferromagnetic order in La1-xSrxCoO3

The itinerant electron ferromagnetism in La1-xSrxCoO3 (0.1 or=0.3). Electrical resistivity studies show that itinerant electron behaviour with a positive temperature coefficient of resistance at low temperatures is obtained for x >or=0.2. Metallization and long-range ferromagnetic order thus seems to be associated with different origins. Chemical systematics obtained from the changes in lattice parameters show that the expansion of LaCoO3 on the substitution of La by Sr stabilizes the paramagnetic states of Co ions which are in predominantly diamagnetic low-spin state in LaCoO3. The core S=1 spins of trivalent Co seems to be coupled ferromagnetically by the itinerant electron in a manner similar to the Zener double-exchange mechanism.

A novel spectrometer for spin-polarized electron energy-loss spectroscopy

By making use of advanced methods for the calculation of electron trajectories in the presence of space charge fields we have designed and built a novel electron energy-loss spectrometer for the purpose of measuring spin flip excitations at surfaces. The measurement of the spin asymmetry requires a deflection angle between the photocathode and the sample position which amounts to 90° in total. That deflection angle is not provided by standard electron deflectors, or by a combination of those. We show that a 90° deflection angle can be realized by a combination of two novel deflectors with 90° and 180° deflection angles each. The performance of the new spectrometer matches the performance of the best electron spectrometers developed for surface vibration spectroscopy.

Low-field magnetocurrent above 200% in a spin-valve transistor at room temperature

A spin-valve transistor (SVT) that employs hot electrons is shown to exhibit a huge magnetotransport effect at room temperature in small magnetic fields. The SVT is a ferromagnet-semiconductor hybrid structure in which hot electrons are injected into a NiFe/Au/Co spin valve, and collected on the other side with energy and momentum selection. This makes the collector current extremely sensitive to spin-dependent scattering. The hot-electron current output of the device changes by more than a factor of three in magnetic fields of only a few Oe, corresponding to a magnetocurrent above 200% at room temperature.

Comparison of the irreversible thermomagnetic behaviour of some ferro- and ferrimagnetic systems

The magnetic behaviour of two ferromagnetic oxides and two ferrimagnetic oxides (ferrites) are compared to study the effect of magnetocrystalline anisotropy on thermal-history-dependence of magnetization of these ordered magnetic systems. All four compounds show thermomagnetic irreversibility (MFC >MZFC) below a certain temperature,Tirr. The highly anisotropic ferromagnetic oxide, SrRuO3 and the hard ferrite, SrFe12O19 show sharp peaks belowTc in theirMZFC(T) curves, whereas for the soft ferrite Ni0.8Zn0.2Fe2O4 and the low anisotropic ferromagnetic oxide La0.7Ca0.3MnO3 only a broad maximum is observed inMZFC, when measured in small magnetic fields. The shapes of theMZFC(T) curves are inversely-related to the magnitude of the coercivities (Hc) of the compounds in relation to the applied field, and the temperature dependence of Hc.MFC andMZFC are related through the coercivity for all four magnetic systems.

Influence of lattice distortion on the Curie temperature and spin-phonon coupling in LaMn0.5Co0.5O3

Two distinct ferromagnetic phases of LaMn(0.5)Co(0.5)O(3) having monoclinic structure with distinct physical properties have been studied. The ferromagnetic ordering temperature T(c) is found to be different for both the phases. The origin of such contrasting characteristics is assigned to the changes in the distance(s) and angle(s) between Mn-O-Co resulting from distortions observed from neutron diffraction studies. Investigations on the temperature dependent Raman spectroscopy provide evidence for such structural characteristics, which affects the exchange interaction. The difference in B-site ordering which is evident from the neutron diffraction is also responsible for the difference in T(c). Raman scattering suggests the presence of spin-phonon coupling for both the phases around the T(c). Electrical transport properties of both the phases have been investigated based on the lattice distortion.