R.V. Upadhyay

Magnetic properties of Fe-Zn ferrite substituted ferrofluids

Abstract In the present paper Zn x Fe 1− x Fe 2 O 4 nanoparticles were synthesized to achieve a low Curie temperature magnetic fluid with moderately high value of magnetization for use in energy conversion devices. Certain structural and magnetic properties of these ferrites are studied and discussed in detail.

Viscosity measurements of a ferrofluid: comparison with various hydrodynamic equations

Effective viscosity of a magnetic fluid as a function of applied magnetic field oriented in the perpendicular direction of the capillary flow is determined. Close agreement with the Shliomis expression derived on the basis of effective field method is observed.

Electron spin resonance study of a temperature sensitive magnetic fluid

Electron spin resonance spectra of a temperature sensitive magnetic fluid involving polydispersed Mn0.5Zn0.5Fe2O4 ferrite particles are scanned from 200 to 400 K. For such a polydispersed system, the deconvolutions of the spectra suggest coexistence of two different resonance modes: (i) a broad line due to ferrimagnetic resonance and (ii) a sharp line at g=2 due to intrinsic superparamagnetic phase. From the intensity variation of the line at g=2, it is found that the contribution due to the superparamagnetic phase increases with temperature. A study of peak-to-peak linewidth variation with temperature indicates that the weakening of the magnetic moment is responsible for the observed reduction in the linewidth.

Electron magnetic resonance of ferrofluids: Evidence for anisotropic resonance at 77 K in samples cooled in a magnetic field

Abstract Evidence is presented from studies of electron magnetic resonance for the formation of linear chains in kerosene based Mn 0.1 Fe 0.9 Fe 2 O 4 (MF1) ferrite ferrofluid cooled in a magnetic field. The resonance field at 77 K was found to depend on the field at which the sample was cooled. More interestingly, the samples cooled in a magnetic field exhibited anisotropy in a resonance field with 180° periodicity, giving evidence for frozen chains.

Magnetic properties of laboratory synthesized magnetic fluid and their temperature dependence

Abstract Ferrite particles with 10% Mn substitution in Fe 3 O 4 are synthesized. These particles are used to prepare kerosene based magnetic fluid. Detailed characteristics of these particles are studied by X-ray, SAXS, EM, magnetization and Mossbauer techniques. Magnetization measurements at 4.2 K suggest the presence of canting on the surface layer. This was further confirmed by Mossbauer measurements. The anisotropy constant of the particle was calculated using the temperature decay of remanence. Particle size distribution determined by SAXS, SANS, EM and magnetization measurements are well in agreement.

Field induced rotational viscosity of ferrofluid : Effect of capillary size and magnetic field direction

In the present investigation we report the effect of capillary diameter and the direction of applied magnetic field on the rotational viscosity of water and kerosene based ferrofluids. We found that changes in the field induced rotational viscosity are larger in the case of water based magnetic fluid than that of kerosene based fluid. The field induced rotational viscosity is found to be inversely proportional to the capillary diameter and it falls exponentially as a function of the angle between the direction of field and vorticity of flow. Magnetophoretic mobility and hydrodynamic volume fraction of nanomagnetic particles are determined for above cases.

Magnetocaloric effect in temperature-sensitive magnetic fluids

The magnetocaloric properties of three different temperature-sensitive magnetic fluids were studied. The pyromagnetic coefficient for all the materials were obtained and it was found that this property depends on physical and magnetic properties like size, magnetization and Curie temperature. A theoretical model was developed to explain the behaviour of change in entropy with temperature.

When does a living polymer live? - case of CTAB/NaSal

SANS and zero-shear viscosity from a 0.1 M solution of CTAB at various concentrations (c) of NaSal are reported. The viscosity shows two pronounced maxima as a function of c. The SANS distributions for salt concentrations beyond the first viscosity maximum are significantly different from those for lower salt concentrations. In particular, analysis of the data shows that the micelle lengths for higher salt concentrations are exponentially distributed which indicates the living polymer regime.

Magnetic properties of Zn-substituted Co-Ge-Fe-O ferrites near the dilution limit

Abstract The magnetic properties of Co-doped spinel system Co 1.4− x Zn x Ge 0.4 Fe 1.2 O 4 ( x = 0.4, 0.5, 0.6) have been investigated through Mossbauer spectroscopy, ac susceptibility and dc-magnetization measurements. In these alloys the A-sublattice has been diluted to below the percolation threshold and hence ferrimagnetic long range order cannot be retained. The observation of the (i) 57 Fe Mossbauer data, (ii) broad peaks in the real component of the ac-susceptibility χ( T ), (iii) similar temperature dependencies for χ′ and χ″ and (iv) irreversibility of magnetization during field cooling leads to a picture of local spin canting structure in this system. At high magnetic fields these systems behave as highly anisotropic ferrimagnets.

Magnetic properties of ultra-fine particles of Mn0.5Fe0.5Fe2O4 spinel system

A systematic study of the magnetic properties of ultra-fine particles of Mn0.5Fe0.5Fe2O4 spinel system has been undertaken. The effect of temperature on the magnetic properties of particles and the ferrofluid has been studied. Analysis of the data yields information on the anisotropy constant, particle size distribution and superparamagnetic behaviour. The results are explained on the basis of existing theories.