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MAGNETIC AFTEREFFECTS IN A COBALT ALUMINOSILICATE SPIN GLASS
H. Rechenberg, A. de Graaf
To cite this version:
H. Rechenberg, A. de Graaf. MAGNETIC AFTEREFFECTS IN A COBALT ALUMI- NOSILICATE SPIN GLASS. Journal de Physique Colloques, 1978, 39 (C6), pp.C6-934-C6-935.
�10.1051/jphyscol:19786414�. �jpa-00217885�
JOURNAL DE PHYSIQUE Colloque C6, supplément au n" 8, Tome 39, août 1978, page C6-934
MAGNETIC AFTEREFFECTS IN A COBALT ALUMINOSILICATE SPIN GLASS H.R. Rechenberg, and A.M. De Graaf t+
Institute: de Fisiaa da Universidade de Sao Paulo, C.P. 20516, Sao Paulo, Brazil + Department of Physics, Wayne State University, Detroit, MI 48202, U.S.A.
Résumé.- On présente des mesures de susceptibilité statique d'un verre alumino-silicate contenant 35,5 % at. de Co, à des températures proches de celle (<v 7,8 K) où la susceptibilité en champ al- ternatif présente un pic. Aucune anomalie n'a été observée à cette température. Des effets de ré- manence apparaissent en-dessous de 7 K. On discute ces résultats en termes du modèle à domaines superparamagnétiques.
Abstract.- Low-field DC susceptibility measurements on a 35.5 at.% Co aluminosilicate glass, at tem- peratures around the AC susceptibility peak temperature (1> 7.8 K ) , are reported. No anomaly is found at that temperature. Remanence effects are apparent below 7 K. The results will be discussed in terms of the superparamagnetic domain model.
Cobalt and manganese aluminosilicate glasses have striking magnetic properties at low temperatu- res h i . The low-field, AC susceptibility of these glasses obeys a Curie-Weiss law down to approxima- tely 50 K, and exhibits a sharp peak characteristic of spin-glass behavior at low temperatures. It has been shown that this peak can be explained by assu- ming the formation of (essentially non-interacting) superparamagnetic domains of correlated spins below a- bout 50 K. Measurements of the heat capacity /2,3/, the sound velocity A / , Mossbauer hyperfine fields /5/, muon depolarization rates /6/ and the frequency-de- pendent low-field AC susceptibility 111 have yielded results consistent with the superparamagnetic inter- pretation. The frequency dependence of the low-field susceptibility clearly indicated the importance of relaxation effects near the AC susceptibility peak temperature. In view of this we performed measure- ments of the DC susceptibility of a 35.5 at.% Co glass in the same temperature range. In the course of these measurements, remanence effects became ap- parent at lower temperatures. The long-time (t£lOs) decay of the magnetization in zero field has already been discussed in reference /8/ in terms of the do- main model. It is the purpose of the present paper
to discuss the DC susceptibility and the initial (t < 10 s) decay of the remanent magnetization.
•The composition of the glass was 83. 1 mole % CoO, 15.5 mole % A1,,03, and 1.4 mole % SiO,,. The magnetization was measured with a PAR vibrating-
sample magnetometer between 4.2 and 15 K. The ap- plied field (98 G) was small enough to ensure a linear response.
Magnetization versus temperature data are shown in figure 1 as open circles. The time needed for the magnetization to reach its equilibrium value after turning on the field was strongly temperature
Fig. 1 : Open circles : field-cooled magnetization at H = 98 gauss. AC susceptibility peak temperatures are indicated. Closed circles : reversible magneti- zation AM at the same field
r
+ Supported in part by CNPq, Brazil.
Supported in part by the National Science Foun- dation under grant numbers DMR 76-17036 A01, OIP 75-15713
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19786414
dependent, being unmeasurably small above % 7 K,and extremely long (% hours) at the lowest measuring temperatures. In an attempt to avoid this effect, all measurements were done by slowly decreasing the temperature in the presence of the field. The resul- ting field-cooled values are indistinguishable from the equilibrium ones down to about 6 K, but it is still doubtful if the equilibrium magnetization has actually been reached at the lower temperatures.
It should be noted that no anomaly occurs in the temperature region (7.5 to 8 K) where the AC susceptibility of this sample has a sharp cusp 171.
This behavior seems to be in contrast with that observed in metallic spin glasses 191, but it is consistent with the superparamagnetic domain inter- pretation.
We also studied the time decay of the magne- tization after the field was suddenly turned off.
Typical data are shown in figure 2. Changes during
TIME (SECONDS)
Fig. 2 : Decay of magnetization at T = 4.05 K, after turning off the field at t = 0. Reversible magneti- zation AMr is indicated
the first 10 seconds are too rapid to be followedin detail. The change, AMr, of the magnetization in this time interval is reversible in the followi'ng sense : (i) it is independent of the past hi'story of the sample, and (ii) it has the same absolute value when the field is either decreased or increa- sed by the same amountAH. ValuesofAM ,corresponding
r
to AH = 98 G, are shown as full circles in figure 1
.
Within experimental error, these values coincide with the equilibrium magnetizati'on above % 7 K.
This is consistent with the absence of relaxatl'on effects at this and higher temperatures, as stated above.
The aMr versus T curve of ffgure 1 can be
explained, at least qualitatively, in terms of the domain model. In previous work, it has been shown that the specific heat /2/, the ~sssbauer spectra
/5/, the frequency-dependent AC suscepti'bility 17 /
,
and the long-time decay of the remanent magneti'za- tion /8/ of Co and Mn aluminosilicate glasses can only be explained by assuming a rather broad dis- tribution, P(E), of domain anisotropy energl'es. Thi's implies a wide range of relaxation times,because
T = T exp (E/kT)
.
0 (1)
Domains with ani'sotropy energies 100 K have rela- xation times in excess of 10 s at low temperatures.
Only domains for which T < 10 s will then contribu- te to the reversible magnetization. We may write approximately
E' (TI AMr(T) = AH
1.
X(E,T) P(E) dE,
(2) where X(E,T) is the susceptibility of domains with anisotropy energy E, and E1(T) is the energy that makes T = 10 s in equation (1). As T increases, the upper limit of the integral will also increase, un- til all domains are able to reach equilibrium within 10 s. Unfortunatelywe werenot able to test equation (2) quantitatively, since the true equilibrium va- lues of M(T), from which the function X(E,T) must be extracted, could not be determined experimentally.At present measurements are in progress on a glass sample with a lower AC susceptibility peak temperature. Relaxation effects are less severe in this case, enabling us to test equation (2) more easily. The results of this investigation will be published elsewhere.
References
/I/ Hooper, H.O. et al., Amorphous Magnetism, (Plenum, N.Y.), 1973, p. 47
/2/ Kline, R.W. et al., AIP Conf. Proc.
2
(1976)169
/3/ Wenger, L.E. and Keesom, P.H., Amorphous Magne- tism II (Plenum, N.Y .)
,
1977, p. 577/4/ Moran, T.J. et al., Phys. Rev. (1975) 4436 /5/ Bieman, L.H. et al., Amorphous Magnetism 11,
(Plenum, N.Y.) 1977, p. 587
161 Bieman, L.H. et al., to appear in Hyperfine Interactions
/7/ Huang, F.S. et al., to appear in J. Phys. C.
/8/ Rechenberg, H.R. et al., to appear in J. Appl.
Phys
.
/9/ Guy, C.N., J. Phys. F
