MAGNETIC PROPERTIES OF MAGNESIOFERRITE IN MAGNESIOWÜSTITE MATRIX

Transcription

1 MAGNEIC PROPERIES OF MAGNESIOFERRIE IN MAGNESIOWÜSIE MARIX W. S. D. Folly, W. C. Nunes, M. A. Novak and R. S. de iasi * Federal University of Rio de Janeiro - Institute of Physics * Military Institute of Engineering - Department of Materials Science

2 Main properties of the system easy control of the size distributions magnesioferrite particles are coherent with the matrix cubic magnetocrystalline anisotropy very weak magnetic interparticle interactions

3 Sample preparation MgO single crystal pressed in MgO + Fe 2 O 3 powder annealing at 1600 K for two weeks single crystal of magnesiowüstite homogenization annealing at 1600 K for 1 hour quenching to ambient temperature homogeneous single crystal of magnesiowüstite annealing at 973 K to precipitate the magnesioferrite phase

4 Structure he magnesioferrite exhibits structure of partially inverse spinel, with chemical formula: [ Mg,Fe ] [ Mg,Fe ] O x 1-x A 1-x 1+ x 4 where A are the tetrahedral sites and the octahedral ones. he inversion parameter 1-x depends on the precipitation temperature. magnesiowüstite magnesioferrite

5 Experimental results Employed techniques: magnetic resonance X band magnetization M and M H ac magnetic susceptibility 3 Hz to 125 khz

6 Magnetic resonance spectra of impurities observed in the original MgO single crystal a and in the quenched magnesiowüstite b.

7 Magnetic resonance spectra at various measurement temperatures. he central narrowest line is due to unblocked particles whereas the largest line, is due to blocked ones precipitation annealing at 700 o C for 8 hours.

8 Angular variation of resonant field due to blocked particle sample annealed at 700 o C for 10 hours

9 ,00 0,01 0,02 0,03 0,04 Anisotropy field as function of 1 /. samples annealed at 973 K for 2, 3, 6 and 8 hours

10 Particle size distributions obtained from magnetic resonance spectra measured at various temperatures [3,4]. samples annealed at 973 K for 2, 3, 6 and 8 hours

11 Magnetization as function of H / at 50 and 300 K. sample annealed at 973K for 1400 hours

12 Magnetization versus temperature. sample annealed at 973 K for 1400 hours

13 FCM and ZFCM curves and the obtained reduced size distribution of a sample annealed for 3 h a and for 8 h b.

14 AC Magnetization curves. sample annealed at 973 K for 1400 hours

15 ln τ ,00 0,02 0,04 0,06 0,08 0,10 0,12 0,14 0,16 1 / K -1 2 h 3 h 8 h 1400 h Arrhenius plot obtained from imaginary part of AC susceptibility measured at frequencies between 3 Hz and 125 khz. samples annealed at 973 K for 2, 3, 8 and 1400 hours

16 Hysteresis loops measured in [100]directions and corresponding coercive fields at various temperatures.

17 Coercive field calculation H M M H C r S P r C + + = χ χ where C P = χ S r d f M M = α S S d f K M = χ = d f d f = S C M K H α = 0,48 1 α unique orientation random orientation

18 PARIAL RESULS he particle size distributions determined by magnetic resonance [3,4] for samples with 2,2 mol% Fe annealed at 973 K for 2, 3, 6 and 8 hours are in agreement with the predictions of the theory of diffusion controlled coarsening developed by Lifshitz and Slyozov [5]. hese distributions also confirm the results obtained for Wirtz and Fine, which studied the same system using the technique of ransmission Electron Microscopy. For all analyzed samples, AC susceptibility data provide τ s for the Arrhenius law τ = τ 0 exp KV k, which remains valid in the frequency range from 3Hz to 125 khz. he DC magnetization curves measured at different temperatures do not show an overlap, as expected for superparamagnetic particles, when are plotted as functions of H/. At low temperatures, a 1/ behavior of the magnetization indicates the presence of free ions in the magnesiowüstite matrix.

19 REFERENCES [1] J. L. Dormann, D. Fiorani and E. ronc, Adv. Chem. Phys., XCVIII, [2] [3] [4] [5] C. J. Kriessman and S. E. Harrison, Phys. Rev., 103, R. S. de iasi and W. S. D. Folly, Physica, 321, W. S. D. Folly and R. S. de iasi, raz. J. Phys., 313, I. M. Lifshitz and V. V. Slyozov, J. Phys. Chem. Solids, 19, Acknowledgment: o CNPq and CAPES for financial support.