ELSEVIER

Journal of Magnetism and Magnetic Materials 140-144 (1995) 1919-1920

~ i Journal of magnetism and magnetic materials

Disaccommodation spectra of single crystal nickel ferrites

L. Torres a,* M. Zazo a j. Ifiiguez a C. de Francisco b, J.M. Mufioz b a Departamento de Ffsica Aplicada, Universidad de Salamanca, E-37071 Salamanca, Spain

b Departamento de Electricidad, Universidad de Valladolid, E-47071 Valladolid, Spain

Abstract

Disaccommodation (DA) spectra for single crystal nickel ferrites of composition NixFe3_xO 4 with x = 0.9, 1.0 and 1.2 have been measured. Our results are compared with measurements of ferromagnetic resonance (FMR) on the same samples. Qualitative comparation between the two kinds of measurements (DA and FMR) by means of induced anisotropy formalism is provided.

Disaccommodation (DA) in nickel ferrites has been deeply studied in the case of polycrystaUine samples [1,2]. In this paper we present DA spectra for single crystal nickel ferrites of composition NixFe3_xO 4 with x = 0.9, 1.0 and 1.2 grown by a flux melt technique using a B a O / B i 2 0 3 / B 2 0 3 flux and characterized by X-ray diffraction analysis.

DA measurements were carried out with the help of a computer-aided system based on the use of an automatic LCR bridge [3]. The DA spectrum is obtained by recording at each temperature the temporal variation of the perme- ability after sample demagnetization. In this way isother- mal curves are obtained for each temperature but, as it is usual in the literature, we have chosen a representation in terms of the isochronal relaxation curves which show the relative variation of permeability at different rate windows:

A/Z(tl , t 2, T ) /Z(tl, T) - p~(t2, T)

Ju.(tl, T) -- /-t(t 1, T) ' (1)

where in our experiences t 1 = 2 s and t 2 varies between 4 and 128 s. The experimental results are processed by using the analysis techniques described in Ref. [4].

The isochronal spectra for single crystal samples can be observed in Fig. 1. The samples of composition NixFe 3_xO4 with x = 0.9 exhibit a relaxation process for temperatures lower than 200 K, another process at higher temperatures from 250 to 400 K is detected for x = 1.2. The composition with x = 1.0 presents a nearly flat spec- trum.

As it can be seen in Fig. 1 the amplitude of the DA processes is around 1.5% for samples with x = 0.9 and

* Corresponding author. Email: luis@rs6000.usal.es; fax: 34- 23-294584.

0304-8853/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0304-8853(94)00509-5

around 0.2% for x = 1.2. These amplitudes are clearly weaker than those of polycrystalline samples of nickel ferrite which were around 20% [1,2]. The lower amplitude in the single crystal samples was expected due to the lower content of defects.

2 . 0 , , J I , ,

1 . 5

1 . 0

0 . 5

0 . 0

~-o.1 ::1.

%

0 . 0

&

Nio.aF%.: 04

NilFe204

0 . 2

0 . 1

0 . 0

- 0 . 1 I I L I I I I

1 0 0 1 5 0 2 0 0 2 5 0 3 0 0 3 5 0 4 0 0 4 5 0

T E M P E R A T U R E K

Fig. 1. Isochronal spectra for single crystal nickel ferrites.

1920 L. Torres et al. /Journal of Magnetism and Magnetic Materials 140-144 (1995) 1919-1920

The process observed for x = 0.9 is very wide and the maximum in the isochronal curves lies out of our range of temperatures. Although a rigorous numerical fitting [4] is not possible due to the range of temperatures of the measurement system, the characteristics of the spectrum, where the isochronal curves do not superpose in the high temperature region, indicate a diffusion relaxation process

[4]. The samples with x = 1.0 present a nearly fiat spec-

trum indicating that this single crystals have the lowest content of defects of all our samples. The composition x = 1.2 presents two relaxation processes from 250 to 400 K. These relaxation processes could be related to those found in polycrystalline nickel ferrite samples [1,2] al- though the very weak amplitude ( < 0.2%) makes not possible a conclusive comparation.

Single crystal nickel ferrites have been previously stud- ied by our group by means of FMR [5,6]. We have found that valence exchange contribution to the FMR linewidth is present for compositions with x < 1 while slowly relax- ing mechanism contributes to the FMR linewidth when x > 1 [5,6].

According to Krupicka [7] and the first studies of Gait [8] the effect of the induced anisotropy in FMR can be

2 . 0 i i , [ 2 0 0

1.5 x

o - l o

"~ 0.5 I

o o

Q 150

~oo~

o 5O

0.9 1.0 1.1 1.2

COMPOSITION (x )

Fig. 2. Comparation of the effect of induced anisotropy in DA amplitude ( v ) (left axis) and in C constant of FMR peak pro- cesses (O) (right axis).

observed in the amplitude of the contribution of the peak processes (valence exchange and slowly relaxing mecha- nisms) to the FMR linewidth. The contribution of a peak process to the FMR linewidth is written as [9]:

tot r A H = C (2)

I -{- ( t o T r ) 2 '

where C is a constant, to = 2"trf, being f the measuring frequency and 7" r is the relaxation time of the mechanism.

On the other hand the effect of induced anisotropy can be observed directly in the amplitude of the DA spectrum [7]. In this way a qualitative comparation of the effect of induced anisotropy in DA and FMR measurements can be done analyzing the values of the constant C of Eq. (2) and the amplitude of the DA peak in the isochronal spectrum. In Fig. 2 the values of C [6] and the amplitude of the DA peak are shown.

Looking at Fig. 2 we can see how the qualitative effect of the induced anisotropy in both DA and FMR measure- ments is similar although some discrepancies are observed for x = 1.2. We can observe that the induced anisotropy is patent for the samples with x 4= 1 while for the stoichio- metrical nickel ferrite ( x = 1) the effect of induced anisotropy is not perceptible in neither DA nor FMR measurements.

References

[1] J. Rivas, J. Ifiiguez, J. Ayala, Appl. Phys. A 19 (1979) 71. [2] J. Ifiiguez, J. Rivas, J. Abell~in, Appl. Phys. A 25 (1981) 35. [3] C. de Francisco, J. Ifiiguez, J.M. Mufioz, J. Ayala, IEEE

Trans. Magn. 23 (1987) 1866. [4] C. de Francisco, J. Ifiiguez, J.M. Mufioz, IEEE Trans. Magn.

25 (1989) 4413. [5] L. Torres, M. Zazo, J. Ifiiguez, M. Quintilian, Appl. Phys. A

55 (1992) 154. [6] L. Torres, M. Zazo, J. Ifiiguez, C. de Francisco, J.M. Mufioz,

IEEE Trans. Magn. 29 (1993) 3434. [7] S. Krupicka, K. Zaveta, 'Anisotropy, induced anisotropy and

related phenomena' in Magnetic Oxides, ed. D.J. Craik (Wi- ley, New York, 1975).

[8] J.K. Gait, E.G. Spencer, Phys. Rev. 127 (1962) 1572. [9] M. Sparks, Ferromagnetic Relaxation Theory (McGraw-Hill,

New York, 1964).