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STUDY OF DOMAIN WALL OSCILLATIONS IN
FERROUS-ZINC FERRITES THROUGH
MÖSSBAUER SPECTROSCOPY
C. Srivastava, S. Shringi, A. Bommanavar
To cite this version:
JOURNAL DE PHYSIQUE Colloque CI, supplement au n° 4, Tome 38, Avril 1977, page Cl-43
STUDY OF DOMAIN WALL OSCILLATIONS
IN FERROUS-ZINC FERRITES THROUGH MOSSBAUER SPECTROSCOPY
C. M. SRIVASTAVA, S. N. SHRINGI, and A. S. BOMMANAVAR Department of Physics, Indian Institute of Technology, Powai, Bombay-400 076, India
Résumé. — On a signalé dans certains ferrites de zinc substitués des spectres Môssbauer à carac-tère de relaxation pour des températures bien plus basses que les températures de transition. Puis-que cet effet apparaît pour des températures T <g 7N, il ne peut être imputé aux fluctuations du point critique. En vue de rechercher l'origine de cet effet de relaxation on a étudié de 77 K à Ts les spectres Môssbauer de la série de ferrites de composition Zn^Fes-sC^ avec x = 0 ; 0,2 ; 0,4 ; 0,6 ; 0,8. L'étude simultanée des spectres magnétiques a montré que la relaxation observée dans la forme des raies Môssbauer paraît provenir des oscillations des parois. Une étude détaillée de la forme des raies a montré que jusqu'à une certaine température Tt les spins nucléaires à l'intérieur du domaine sont soumis à un champ hyperfin constant tandis qu'une fraction p des ions dans la gamme des oscillations de parois voit un champ moyen qui décroît avec le temps. Expérimentalement on a trouvé que p augmente avec la température. Au-dessus de Te la forme des raies indique que les spins à l'intérieur du domaine sont aussi soumis à un champ hyperfin fonction du temps. On a trouvé que le rapport de la valeur de l'aimantation h Tt à. celle à 77 K est approximativement 0,5 pour x = 0,2 ; 0,4 et 0,6.
Abstract. — In some of the zinc substituted mixed ferrites relaxed Mossbauer spectra have been reported for temperatures significantly lower than the transition temperatures. Since this effect occurs at T <§ 7V, it cannot be accounted for by the critical point fluctuations. To investigate the origin of this relaxation effect the Mossbauer spectra of the series Zn*Fe 3-SO4 (x = 0, 0.2, 0.4, 0.6, 0.8) have been studied from 77 K to TV. Simultaneous study of magnetic spectra has shown that the relaxation effects in the Mossbauer line shapes arise owing to domain wall oscillations. A detailed study of the line shape has indicated that upto a certain temperature, Tt, the nuclear spins within the domain experience a constant hyperfine field while a fraction, p, of ions in the range of domain wall oscillations see a vanishing time average field. Experimentally, p has been found to increase with temperature. Above TV, the line shape indicates that the spins within the domains also experience time-varying hyperfine field. The ratio of magnetization at Ti to that at 77 K has been found to be approximately 0.5 for x = 0.2, 0.4, and 0.6.
1. Introduction. — The Mossbauer spectra of zinc substituted ferrites like Ni-Zn [1], Co-Zn [2], Li-Zn [3], and Fe-Zn [4] show certain common features. The line shapes are functions of temperature and zinc concen-tration. As T tends to 0 K all systems show superposi-tion of two sextets from A- and B- site iron ions indicat-ing complete magnetic orderindicat-ing [5-7]. As the tempera-ture is raised at a certain value of T, depending on zinc concentration, the Mossbauer lines start showing relaxation effects. Finally as the temperature approa-ches Neel temperature there is one central line split due to quadrupole interaction.
Mossbauer spectra for ZnJ^e^^J^^ system (x = 0, 0.2, 0.4, 0.6 and 0.8) at 77 K and 300 K were recently reported by us [8]. Through simultaneous study of magnetic spectra we have shown that the relaxa-tion effects in Mossbauer spectra at T -^ TN arise
owing to domain wall oscillations. A typical relaxed spectrum contained, in addition to the two sextets from A- and B- site iron ions, a quadrupole split central line. The central doublet has been explained to
arise from the ions lying in the range of domain wall oscillations which experience rapid fluctuations of the Weiss field. The behaviour of these ions has been treated using Blume and Tjon's stochastic model [9]. Considering a 180° domain wall executing an oscilla-tion at its equilibrium posioscilla-tion, the fracoscilla-tion of ions experiencing rapid spin reversal, has already been given by us [10]. If the domain wall is considered to lie in a potential well of energy- U, the probability that it would cross the potential barrier is cot exp(— U/kT), where cor = {ml — Q2)1/2 and a)0 and Q are the reso-nance and relaxation frequencies, respectively. Hence the fraction p is given by
c [16 n2 kTx~\ 1 / 2 f. / UY
P
= A^MTa [ 1 J
fflrV ~
C X Pr W
(1) where c is a constant of order unity and depends on the nature of the domain wall, 4 nMs is the saturation magnetization, % is the susceptibility, / is the thickness
C1-44 C. M. SRIVASTAVA, S. N. SHRINGI AND A. S. BOMMANAVAR
of domain, and a is the lattice constant. It has been If a fractionp of ions lies in the range of domain wall 52 oscillations and (1 - p) is within the bulk of domain,
observed that only for 0.5 - 6 1 the domaill the equation of line shape using ~l~~~ ~ j ~ ~ . ~
W n
walls execute localized oscillations. stochastic model is given b y [8]
x { cos x or t
+
x-' sin x w, t } exp(- o, t).
(2) whereti
is the fraction of atoms on the ith (i = A, B)site, w , is the resonance frequency of the domain wall and the remaining terms are the same as in ref. [9]. 2. Results and discussion. - The analysis of the spectra of Zn,Fe, -,O, on the basis of above equation has been carried out for x = 0.2, 0.4, and 0.6 upto T = T, assuming that the Zeeman frequency is very much smaller than or. For T > T, the line shapes could not be fitted to eq. (2). Our results (Table I) indicate that T, is close to the temperature at which M, falls approximately to 50
%
of its value at 77 K. The Mossbauer spectra of Zn0.,Fe2.,O, have been shown in figure 1. The agreement between experimental and theoretical spectra is satisfactory. Similar results have been observed for x = 0.4 and 0.6 and reported else- where [Ill.The failure to fit the observed line shape to eq. (2) for T > T, indicates that in this region the spin fluc- tuations within the domain become significant. Beyond T,, as we approach
TN,
the spectra show the characteristics of critical point fluctuations and the outer lines broaden while the inner lines grow in intensity at the cost of the former. It is possible to analyze the spectra in this region with the help of Blume and Tjon's theory assuming a temperature dependent relaxation frequency for the electronic spin system. However, this has not been attempted. At TN, there is only one central line split due to quadrupole interaction.The fraction which depends on temperature in eq. (1) are M, and
X,
while I and or are unlikely to vary significantly in the temperature range studied. The temperature variation of M, has been studied byOSCILLATIONS IN FERROUS-ZINC FERRITES THROUGH M~SSBAUER SPECTROSCOPY
Y ' L
D O P P L E R V E L O C I T Y I m m l s ) 3 3 P P . E R v E ~ O C l l l I m m l r )
FIG. 1. - Mossbauer spectra of Zn0.2Fe2.~04 at temperatures 77 K-680 K (a) Experimental curves,
C1-46 C. M. SRIVASTAVA, S. N. SHRINGI AND A. S. BOMMANAVAR
us [lo] and in this temperature range it can be express- ed in the form
1 3 t
/
The plot of In p versus In T shown in figure 2 has
been used to determine the value of n. For x = 0.2 and 0.6, n is approximately 4 while for x = 0.4, it is
1 . 5 I I I I I 1 1.2. This is not unreasonable. The temperature varia-
2 . L 2 . 5 2 . 6 2 . 7 2.8 2 . 9 3.0
tion of
x
of Zn,Col-,Fe,O, ferrite system has beenI n T observed by Poltinnikov and Prikhodkina 1121. In
FIG. 2. - In p versus In T calculated on the basis of eq. (4) different regions of temperatures the (see Text) and table I. varies from 0 to 10 for this system.
0.3 0 . 5 - - I n P 0 . 7 0.9 References
-
X = O . L M s ( t ) = M s ( T ~ ) [I - a(T-
TO)] (3)A where To = 300 K . The value of a is of order
-
A SinceU
is expected to be of order 1 eV the variation0 A
x = 0 . 2 in the factor [I
-
exp(- UIkT)] is also small. Hence4
-in this region,
P
"
(xT)l12and if
x
varies as Tn,[I] RAJ, P., and KULSHRESHTHA, S. K., Phys. Status Solidi, [7] MORRISH, A. H. and CLARK, P. E., P h ~ s . Rev. B 11 (1975)
a 4 (1971) 501 ; 278.
DANIELS, J. M. and ROSENCWAIG, A., Can. J. Phys., 48 E81 SRIVASTAVA, C. M., SHRINGI, S. N. and SRIVASTAVA, R. G.,
(1970) 381. Phys. Rev. B 14 (1976) 2041.
[2] BHARGAVA, S. C. and IYENGAR, P. K., Phyx Status Solidi BLmE, M. and T~ON, J. A.y Phys. Rev. 165 (Ig6') 446.
b 46 (1971) 117 ; ibid b 53 (1972) 359. [lo] SRIVASTAVA, C. M., SHRINGI, S. N., SRIVASTAVA, R. G. and
NANADIKAR, N. G., Phys. Rev. B 14 (1976) 2032.
13'
J' W' and J'y J' App" PhyS'' 42 (1971) 2344' [ll]SRIVAS~AVA, C. M., SHRINGI, S. N. and BOMMANAVAR, A. S.
[4] DOESON, D. C., LINNET, J. W. and REHMAN M. M., J. Proc. Nuclear Phys. Solid State Phys. Symp. BARC, Phys. Chem. Solids. 31 (1970) 2727.
Calcutta (India) 1975 ; Proc. National Meeting on
[5] LEUNG, L. K., EVANS, B. J. and MORRISH, A. H., Phys. Rev. Applications of Mossbauer Effect, Kanpur (India) 1976
B 8 (1973) 29. (To be ~ublished).
[6] PETITT, G. A. and FORESTIER, D. W., Phys. Rev. B 4 (1971) 1121 POLTINNIKOV, S. A. and PRIKHODKINA, G. M., SOV. Phys.-
