INFLUENCE OF MAGNETIC FIELD ON RELAXATION EFFECTS IN Ni0.25Zn0.75Fe2O4

(1)

HAL Id: jpa-00216731

https://hal.archives-ouvertes.fr/jpa-00216731

Submitted on 1 Jan 1976

HAL is a multi-disciplinary open access

archive for the deposit and dissemination of sci-entific research documents, whether they are pub-lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

INFLUENCE OF MAGNETIC FIELD ON

RELAXATION EFFECTS IN Ni0.25Zn0.75Fe2O4

S. Bhargava, S. Mørup, J. Knudsen

To cite this version:

(2)

JOURNAL DE PHYSIQUE Colloque C6, supplément au n° 12, Tome 37, Décembre 1976, page C6-93

INFLUENCE OF MAGNETIC FIELD ON RELAXATION EFFECTS

IN Nio.

25

Zno.

75

Fe

2

0

4

S. C. BHARGAVA (*), S. M0RUP and J. E. KNUDSEN Laboratory of Applied Physics II

Technical University of Denmark, DK-2800 Lyngby, Denmark

Résumé. — La variation en fonction de la température des spectres Mossbauer anormale de Nio,2sZno,7 5Fe204 dans un champs magnétique externe a été étudiée. On a trouvé que la variation anormale de < Sz > des ions Fe3+ avec la température antérieurement rapportée disparaît

partiel-lement quand le champ est présent. La variation avec la température du temps relaxation des spins des ions Fe3+ ressemble a celle trouvée antérieurement à l'absence d'un champ magnétique.

Abstract. — The temperature dependence of the anomalous shapes of the Mossbauer spectra of

Nio.25Zno.7sFe2C>4 in presence of an external transverse field of 12.3 kG has been studied. It is found that the anomalous dependence of < Sz > of Fe3+ ions on temperature, noted in an earlier

investigation, is partly removed in the presence of this field. The temperature dependence of the ionic spin relaxation time, in the presence of the external field is similar to the behaviour found earlier without the external field.

1. Introduction. — Earlier investigations [1-3] of the

anomalous shapes of the Mossbauer spectra of (Co, Z n ) F e204 and (Ni, Zn) F e204 revealed the

presence of fluctuation effects. The study of the tempe-rature and the concentration dependences of the shape of the spectra of these mixed ferrites showed that ionic spin relaxation is the principal source of the fluctuations. These investigations, however, revealed an anomalous temperature dependence of < Sz > of Fe3 + ions in mixed ferrites with large concentrations of

the diamagnetic ion, which deviates considerably from the sublattice magnetization determined using the neutron diffraction method [4].

In the present study, Mossbauer spectra of Ni0.2 5Zn0.7 5Fe2O4

at various temperatures and external magnetic fields have been obtained to study the influence of the external field on the anomalous behaviour of < Sz > , and to determine the temperature dependence of the ionic spin relaxation time in presence of the external field.

2. Experimental. — The sample used in the present measurements was studied earlier [4] using the neutron diffraction method. The method of preparation has been described elsewhere [2, 4]. A conventional Mossbauer spectrometer operated in the constant

(*) On leave from Nuclear Physics Division, Bhabha Atomic Research Centre, Bombay, India.

acceleration mode has been used to obtain the spectra. The sources used are Co5 7 in Pd for all measurements

above the liquid nitrogen temperature, and Co5 7 in

Rh for spectra at 4.3 K. The temperature of the absorber was kept stable and uniform within + 0 . 5 K during the measurements. Transverse magnetic fields up to 12.3 kG with stability better than + 0.1 kG were obtained using a water cooled electromagnet.

3. Results. — Mossbauer spectra obtained at 4.3 K in external transverse fields of 38 G and 6.2 kG are shown in figure 1. The spectra were fitted with Lorent-zian lines. The results thus obtained have been given in

-14 -10 - 6 - 2 2 6 10 14 1 1 1 1 1 1 2 38 G '• / • .* *•; %" • • * • 4 . . . - " S 6 * ' z 8 : .. o S 2 6.2 KG * : '• ." "•; • ; •• •' \ • a * • , < 6 8 \ . '•* 1 1 1 1 1 1 -14 -10 - 6 - 2 2 6 10 14 VELOCITY (MM/S)

FIG. 1. — Mossbauer spectra of Nio.2sZno.75Fe204 in pre-sence of transverse magnetic fields at 4.3 K.

(3)

C6-94 S. C. BHARGAVA, S. M0RUP AND J. E. KNUDSEN

table I. The quadrupole shifts in the two spectra are negligible (smaller than 0.02 mm/s).

Absorber : Nio .25Zn, .,,Fe,O,. Temperature : 4.3 K.

External He~n A1.6 : A2.5 : A3.4 I-I : r 2 , 3 : r 4 . 5 : r 6

field (kG) (arb. units) (mm/s)

- - -

-

38 G 513 =t 1 2.7 f 0.2 : 0.48 f 0.05 :

1.8 4 0.2 : 0.47 f 0.06 :

1.0 f 0.2 0.4 f 0.1 :

0.67 f 0.06

A and

r

refer to area and width of the lines, respectively, in Tables I and 11.

Mossbauer spectra at several temperatures, above the liquid nitrogen temperature, in presence of a small field of 38 G (the remanent field of the electro- magnet) are shown in figure 2a. The separation of the

-8 -4 0 4 8 -8 -4 0 4 8 VELOCITY (MM/S)

FIG. 2. - (a) Mossbauer spectra of Ni0.25Zno.7~Fe204 in pre-

sence of a remanent field (transverse) of 38 G. (b) Theoretical spectra which fit best the experimental spectra.

inner lines (A,,) decreases rapidly at low temperatures but very slowly above 200 K. The intensities of the central lines continue to increase far above this tempe- rature. This is also evident from the spectra in figure 3, obtained with increased resolution of the velocity scale. Mossbauer spectra, at several temperatures above 77 K, in an external transverse field produces significant changes in the shapes of the spectra at these

-4 -3 -2 -1 0 I 2 3 4

VELOCITY (MM/S)

FIG. 3. - The dependence of the shape of the spectra of

Ni0.2~Zn0.75Fe204 on temperature recorded with increased resolution of the velocity scale in the presence of the remanent

field (38 G).

I I I

- 8 -4 0 4 8 - 8 -4 0 4 8 VELOCITY (MM/S)

FIG. 4. -(a) Mossbauer spectra of Ni0.25Zn0.75Fe204 in

presence of an external transverse field of 12.3 kG. (6) Theore- tical spectra which fit best the experimental spectra.

temperatures. In particular, the line separations increase, instead of decreasing, when the field is applied. The intensities of the innermost lines continue to increase even above 300 K, where the zero-field spectra showed no significant increase in the intensity (Fig. 5).

The dependence of the shape of the spectrum at 199 K on external fields, in the range 140 G to 12.3 kG, showed that the change in the shape occurs gra- dually, and there is no distinct saturation up to

(4)

INFLUENCE OF MAGNETIC FIELD ON RELAXATION EFFECTS IN Nio. 25Zn0. 75Fe204 C6-95

VELOCITY (MM/S)

FIG. 5.

-

Massbauer spectra of Ni0.25Zn0.75Fe204 in presence of an external transverse field of 12.3 kG. 4. Computation of theoretical spectra.

-

The sto- chastic model of ionic spin relaxation [5, 61 which provides a good description of the fluctuations in magnetically ordered materials has been used for the calculation of theoretical line shapes. The quadrupole interaction has not been included in the calculations, in accordance with the absence of quadrupole shifts at

4.3 K. The procedure of the numerical computation of the theoretical spectra has been described elsewhere [I-31. For simplicity, it has been assumed that the spectra of ions at A and B sites are characterized by identical sets of parameters (6, s, and the relaxation time). The values of 6 were obtained from the spectra at 4.3 K (10 6 = separation of outermost lines at very low temperatures). The values of other parameters used for the computation and the results thus obtained from the comparison of the theoretical

spectra with the experimental spectra are given in table 11. The theoretical spectra which give the best fit to the experimental data are shown in figures 2b and 4b.

The assumption 6, = 6 , is justified by the spectra at 4.3 K (Table I). On the other hand, the assumptions sA = s, (s = exp(2 PH,/kT), where Ha is the Weiss field) and zA = z, appears to be responsible for small misfits in the central part below 160 K. Nevertheless, the values given in Table I1 provide good description of the ions at B-sites, as only 12

%

of the Fe3+ ions occupy A-sites. It is understandable that the assumptions are not good when the concentration of Zn2+ ions is large and the external magnetic field is strong (due to the ferrimagnetic alignment of the spins). To improve agreement in the central parts, the values of the linewidths used are a little larger than the values provided by 4.3 K data. This, however, necessitated the use of lower values of the relative intensities of the outer lines for the computation of the spectra in figure 4b. At higher temperatures (T

>

160 K) the large broadening due to the relaxation effects reduces the dependence of the line shapes on the values of the relative linewidths and intensities considerably. The shapes are, however, very sensitive to the values of s and z in this temperature range. The values of

<

SZ

>

obtained from the fitting procedures are shown in figure 6, together with the A-sublattice magnetization, determined using the neutron diffraction method [4]. The comparison has not been made with B-sublattice magnetization because the moment of this sublattice has contribution from ~ i , ' ions also. The magnetization of Fe3+ ions alone on the B-sublattice is close to the temperature dependence of A-sublattice magnetization [4].

Absorber : Ni,,25Zn,,,,Fe204

H = 3 8 G H = 12.3 kG

TI,, : ~ 2:

r,,,

, ~= 0.66 : 0.59 : 0.52 _:r2,5 : r3,4 = 0.66 : 0.59 : 0.52

: : A3,4 = 2.6 : 2.2 : 1.0 A, : A, : A,,, : A, : A, = 1.83 : 2.47 : 1.0 : 2.4 : 1.75 Temperature (K) A , , (mmls)

<

Sz

>

z(ns) Temperature (K) A,, (mm/s) <Sz> z(ns)

(5)

(26-96 S. C. BHARGAVA. S. M0RUP AND J. E. KNUDSEN

t Neutron D ~ f f r a c t ~ o n r e s u l t s

x Mossbauer r e s u l t s . H = 12 3 KG

Mossbouer results. H=IO G

I

- ,

0 100 200 300 LOO

TEMPERATURE (K)

FIG. 6.

-

The temperature dependence of

<

SZ

>

of Fe3+ ions determined from the comparison of theoretically computed spectra with the experimental spectra in presence of the transverse fields (H). The temperature dependence of the A-sublattice magnetization, containing Fe3f ions alone, determined using the neutron diffraction method by Satya Murthy et al. [4] has also been reproduced for comparison. The saturation magnetization

has been normalized to 2.5.

5. Discussion.

-

The small changes observed in the

4.3 K spectra (see Table I) upon the application of an external magnetic field show that the spins at the B-sites are non-collinar, in agreement with the results of the neutron diffraction studies.

The presence of the external field has little effect on the ionic spin flip frequency and its temperature inde- pendence below

TN.

This is in agreement with the interpretation given earlier [I] for the origin of spin relaxation effects in the spectra of the mixed ferrites which is briefly mentioned below :

In these mixed ferrites, the neighbouring ions, connected by the exchange interactions, are not magnetically similar, and possess different magnetic envi- ronments. There is no translational symmetry in the distribution of magnetically similar ions. This ran- domness in the distribution and the presence of Zn2+ ions inhibit propagation of spin waves and the ionic Zeeman levels remain discrete. The narrowness and the presence of different Zeeman splitting of neigh-

bouring ions make the relaxation times large. The situation is similar to an assembly of paramagnetic ions having ionic Zeeman splitting much greater than the crystal field splitting. In such a system, the relaxation time is independent of temperature and magnetic field.

It is remarkable that the temperature dependences of

<

Sz

>

obtained from the neutron diffraction method and Mossbauer spectroscopy are different. This difference is diminished when an external field is applied. In the ferrites with lower z n 2 + concentration, the behaviour of

<

SZ

>,

determined using the two methods, agree very well. Therefore, the phenomenon cannot be due to shortcoming of the fitting procedure. The neutron diffraction method yields a space average value of sublattice magnetizations. The time scale of this technique is

-

lo-'' s. Mossbauer spec- troscopy yields time average of Sz of single ion (Fe3+ ion). The measurement time is

-

5 x lo-' s.

It might be suggested that the discrepancy could be explained by fluctuations of the magnetization in magnetic clusters with a relaxation time which is short compared to the characteristic measurement time of Mossbauer spectroscopy, but long compared to the interaction time of the neutron. Upon application of a magnetic field, such fluctuations are suppressed and this would result in larger values of

<

S,

>.

However, the spectra do not exhibit the characteristics of isolated superparamagnetic particles as discussed earlier [I-31. For example, the characteristic effects due to particle size distributions (the coexistence of paramagnetic and magnetically split components) are not observable even though the effect is spread over a large temperature range. Therefore, the present effect cannot be explained by the influence of normal superparamagnetic relaxation.

Acknowledgements. - We are indebted to Prof. Trumpy for his keen interest in this work. One of us (S. C. B.) wishes to acknowledge the valueable support of DANIDA.

References

[I] BHARWA, S. C., Ph. D. Thesis (1974), Bombay University, [4] SATYA MURTHY, N. S., NATERA, M. G., YOUSSEF, S. I., India. BEGUM, R. J. and SRIVASTAVA, C. M., Phys. Rev. 181

[2] BHARGAVA, S. C. and IYENGAR, P. K., J. Physique Colloq. 35 (1969) 969.

(1974) C 6-669. [5] BLUME, M. and TJON, J. A., Phys. Rev. 165 (1968) 446.

[3] BHARGAVA, S. C. and IYENGAR, P. K., Phys. Stat. Sol. 46@) [6] SRNASTAVA, J. K. and SHARMA, R. P., Phys. Stat. Sol. 35