Gd-substituted ferrite ferrofluid: a possible candidate to enhance pyromagnetic coefficient

 

R.V. Upadhyay^a, R.V. Mehta^a'*, Kinnari Parekh^a, D. Srinivas^b, R.P. Pantc

 

a Department of Physics, Bhavnagar University, Bhavnagar-364 002, Gujrat, India

b Central Salt & Marine Chemicals Research Institute, Bhavnagar-364 002, Gujrat, India

c National Physical Laboratory, Dr. K.S. Krishnan Road, New Delhi-110012, India

 

Abstract

Physical and magnetic properties of newly synthesized Gd-substituted ferrite ferrofluid is studied. The particle size is derived from X-ray, magnetisation and viscosity measurements. This fluid has low-curie temperature, 348 K. The study indicates that it is possible to enhance the pyromagnetic coefficient of the magnetic fluid using Gd. © 1999 Elsevier Science B.V. All rights reserved.

Keywords: Viscosity; Magnetisation; Pyromagnetic coefficient

1. Introduction

Ferrites are a group of technologically important magnetic materials of current interest, in particular Gd-ferrite, which has a low-curie temperature (298 K) and high-pyromagnetic coefficient (i.e. high (дМ/дТ)_н). To synthesise such materials and dis­perse them in a suitable carrier is a difficult task. The interest for this dispersion in a suitable carrier is to use it in magnetocaloric energy conversion (MEC) devices. MEC using magnetic fluid has been studied by a few researchers [1,2]. For an efficient heat transfer one has to develop highly temperature sensitive as well as highly thermal conducting mag­netic fluids. Recently, we have synthesised fine particles of Mn-Zn ferrites and used them for a temperature sensitive magnetic fluid [3]. The Curie temperature (T_c) of this fluid was 340 K and the particle saturation magnetisation is 205 emu/cm³ at 300 K. This gives us a pyromag­netic coefficient of the fluid as high as 5 emu/cm³ K. In order to enhance the saturation magnetisation of the fluid, keeping the volume fraction of the particle same we have for the first time substituted Gd in this system and synthesised magnetic fluid. The Curie temperature of this Gd-doped fluid is 348 K. In this paper we report certain physical and mag­netic properties of this fluid.

2. Sample preparation

Particles of Gd substituted Mn}Zn ferrite were synthesised by a chemical co-precipitation technique.

GPR grade FeCl 6HO, MnCl 4HO, GdCl and sulphate of Zn were used to obtain Fe³⁺, Mn²⁺, Zn²⁺ and Gd³⁺ ions in the aqueous solu­tion. Aqueous solution containing these ions in the appropriate molar proportion was added to 8 M NaOH solution at room temperature. The detailed preparation conditions are described elsewhere [4]. These particles were coated with oleic acid and dispersed first in kerosene and later transferred to a diester. The sample was solid below 200 K and liquid above this temperature.

3. Results and discussion

3.1.   Structural characterisation

The structure of the particles was characterised using a Philips X-ray diffractometer. The Cu K radiation was used. The X-ray diffraction pat­tern was analysed using the Rietveld refinement programme. The analysis confirms the formation of single phase FCC spinel structure. The structural parameters derived from the fit are (i) the particle size D  = 60 As, (ii) lattice parameter, a = 8.453 As.

3.2.   Viscosity measurements

Viscosity measurements were carried out using a Wells-Brookfield cone/plate viscometer. The temperature of the sample cup was maintained upto an accuracy of +- 0.013C with the help of EX-100 constant temperature bath. The instrument was calibrated using a Brookfield viscosity stan­dard fluid. Using the Rosensweig [1] method the particle size for the Gd-doped ferrofluid was deter­mined. The value thus obtained was 40 A (Fig. 1). This size corresponds to the median diameter of the number distribution.

3.3.  Magnetic measurements

 

The room temperature magnetisation measure­ments were carried out using the search coil method. The magnetisation curve was fitted using the earlier [3] method (Fig. 2). In Fig. 2 for com­parison, Mn}Zn and Mn}Zn with Gd substituted fluid magnetisation is shown. The values of the median diameter of the lognormal volume distribu­tion, D, the standard deviation of the logarithmic of the diameter, a, and the domain magnetisation, M, were obtained. The values are 60 As, 0.33 and 300 emu/cm³, respectively, for Gd-doped ferrofluid and 67 A 0.36 and 205 emu/cm³ for Mn}Zn ferro­fluid.

The following conclusion may be drawn from Fig. 2: (i) The substitution of Gd has increased the domain magnetisation of the sample. (ii) The par­ticle size has decreased from 67 to 60 A for Gd-doped fluid.

The observed increase in magnetisation is due to the substitution of octahedral site (B-site) Fe ion by Gd ion, which has a high spin only magnetic mo­ment compare of the Fe ion. The substitution of Gd on B-site can a!ect the inter-sublattice exchange energy between Mn-O-Gd, Mn-O-Fe as well as the intrasublattice exchange energy. It may be noted that the Mn-O-Fe interaction energy is two times smaller than the Fe-O-Fe interaction energy. Therefore, the observed increase in magnetisation clearly indicates the change in intersublattice ex­change energy values for Gd-doped ferrite particles. If this is true then one can observe two di!erent e!ects in the magnetic properties they are (i) change in Curie temperature and (ii) change in anisotropy energy. To find the e!ect on Curie temperature a Quincke's method was used. The basic set-up and calculation are described in Ref. [5]. Fig. 3 shows the change in the field average magnetisation (AM) with the temperature for both the fluids.

The result of this study indicates that the Curie temperature of Gd substituted ferrite ferrofluid has increased slightly (348 K) compared to that of pure Mn-Zn ferrite ferrofluid (340 K). Thus, the pyro-magnetic coefficient for Gd substituted ferrofluid is higher compared to pure Mn-Znfluid. To under­stand the e!ect of Gd on anisotropy energy ESR measurements were carried out.

3.4.  ESR measurements

The ESR patterns were recorded using Bruker ESP-200 spectrometer for different temperatures.

The data were fitted using Gaussian line shape. Fig. 4 shows the ESR spectra recorded for parallel and perpendicular geometry for Mn-Zn and Mn-Zn-Gd ferrofluids under field cooled (field applied at 300 K was 1 T and was then cooled to 100 K) condition at 100 K. The spectra shows that the anisotropy field is present in the case of Gd-substituted ferrite ferrofluid which is not the case for pure Mn}Zn ferrofl#uid. The values of an-isotropy constant calculated from the spectra for Gd substituted sample is two orders of magnitude higher than pure Mn}Zn ferrofluid.

The data were also recorded for higher temper­ature (from 290 to 473 K). Fig. 5 shows the typical spectra for the Mn-Zn-Gd fluid. The intensity, linewidth and resonance field analy­sis show that there is a transition at 341 K which is close to the Curie temperature of the Gd-sub­stituted fluid.

4. Conclusion

The present investigations indicate that a partial substitution of Gd in Mn}Zn ferrite increases the

pyromagnetic coefficient of the fluid. A detailed investigation of this fluid is in progress.

Acknowledgements

This work was carried out under IUC-DAEF project PB.36 and IFCPAR (CEFIPRA) project No. 1508-2.

References

[1] R.E. Rosensweig, Ferrohydrodynamics, Cambridge Univer­sity Press, Cambridge, 1985.

[2] M. Matsuki, K. Yamasawa, K. Murakami, IEEE Trans. Magn. MAG-13 (1977) 1143.

[3] T. Upadhyay, R.V. Upadhyay, R.V. Mehta, P.S. Goyal, V.K. Aswal, Phys. Rev. B 55 (1997) 5585.

[4] R.V. Upadhyay, Unpublished.

[5] K. Parekh, R.V. Upadhyay, Indian J. Pure Appl. Phys. 35 (1997) 523.