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SPECIFIC HEAT OF EuS/SrS
MULTILAYERS-DEPENDENCE ON MAGNETIC
FIELD AND LAYER THICKNESS RATIO
J. Wosnitsa, H. V. Löhneysen, W. Zinn
To cite this version:
JOURNAL DE PHYSIQUE
Colloque C8, Supplement au no 12, Tome 49, dkcembre 1988
SPECIFIC HEAT OF EuS/SrS MULTILAYERS-DEPENDENCE ON MAGNETIC
FIELD AND LAYER THICKNESS RATIO
J. Wosnitaa
('1,
H. v. Lahneysen ( I ) and W. Zinn (2)(I) Physikafisches Institut, UnkersitGt Karfsr~lhe, Engesserstrape 7, 0-7500 Karisruhe, F.R.G. (') Institut fiir Festk6rperforschung der KFA Jiilich, 0-51 70 Jiilich, F.R. G.
Abstract.
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The low-temperature specific heat C of a EuS/SrS multilayer with layer thickness dM = d~ = 1.25 nm(M = EuS, N = SrS) is identical to that of bulk Eu,Srl-,S with x = 0.54 in magnetic fields up to 3 T, due to the
formation of Eu,Srl-,S by diffusion. For multilayers with increasing dM/dN, C evolves towards pure EuS, thus exhibiting a transition from spin-glass to ferromagnet.
Multilayers consisting of alternating layers of magnetic and nonmagnetic layers have become of in- terest recently. In a previous work [I] we reported on EuS/SrS multilayers with fixed layer thickness ratio r = d ~ / d ~ = 1. The EuS/SrS system is particularly well suited because EuS is a Heisenberg ferromagnet and the nonmagnetic SrS has the same NaCl struc- ture and almost equal lattice constant, thus allowing epitaxial growth of a multilayer [2]. With decreasing modulation length A = d~
+
d ~ , a transition from fer- romagnetic to spin-glass behavior was observed which was attributed to interfacial Eu,Srl-,S layers formed by diffusion. In continuation of this work, we present specific-heat results for a sample with d~ = d~ =1.25 nm in magnetic fields up t o 3 T. We also report on
Fig. 1. - Specific heat C vs. temperature T of a EuS/SrS multilayer consisting of alternating EuS and SrS layers (number nM = nN = 64), each nominally 1.25 nm thick,
in magnetic fields up to 3 T. For comparison, C for bulk Euo.54Sro.46S is also shown. Solid lines indicate theoretical results for the specific heat for this concentration [3].
measurements of a series of multilayers where A is kept roughly constant (between 13 and 8 nm). This allows us to consider changes brought about by varying the layer thickness ratio r.
The multilayers were prepared by sequential depo- sition of EuS and SrS onto Si (111) substrates as de- scribed previously [I, 21. Substrate temperature was always 1 000 O C . The specific heat was measured with
the standard heat-pulse technique.
Figure 1 shows the specific heat C as a function of temperature T for a multilayer with T = 1.25/1.25 in
several applied magnetic fields B. The zero-field re- sults have already been reported [I]. For comparison, the specific heat of bulk Eu,Srl-,S with x = 0.54, i.e. of roughly the same overall Eu concentration, is also shown [3]. This concentration lies in the range
of "reentrant" ferromagnetism [4] where upon cooling, the system first goes from paramagnetic t o ferromag- netic state and a t lower temperatures exhibits features of a spin-glass (e.g. no long-range magnetic correla- tions). Between 0.2 K and the upper limit of our T range (2.5 K), C is dominated by the magnetic con- tribution (the phonon contribution being negligible), while the rise towards lower T indicates the hyperhe contribution. The similarity of C for the bulk x = 0.54
sample and the multilayer can be understood [I] in terms of interdiffusion of EuS and SrS layers during film growth at elevated temperatures. A quantitative analysis with a simple linear diffusion profile, based on measurements for several A, shows that an interfacial Eu,Srl-,S layer of thickness
-
3.5 nm is formed [I]. Of course, for the sample with A = 2.5 nm this diffu- sion process results in a sample of quite homogeneous concentration x 0.5.As can be seen from figure 1, we find the same
C (B, T) dependence for multilayer and bulk sample.
This confirms the above analysis. On the other hand, this result can be regarded as an indication of con- centration homogeneity for the bulk sample down t o 1.25 nm, since already for the multilayer with d~ =
d~ = 5 nm, C is reduced by 30 % [I]. It appears unlikely that concentration inhomogeneities would de-
C8 - 1774 JOURNAL DE PHYSIQUE velop on the same length scale ( w 1 nm) in the two
samplei'of figure 1, which were prepared on totally different routes.
We now turn t o the results with various thickness ratios r = d ~ / d ~ . As can be seen from figure 2, C
is largest for small r and decreases smoothly towards
C of pure EuS with increasing T , in much the same
way as multilayers with r = 1 fixed and increasing
A [I]. As discussed above, we attribute the enhance-
ment of C over pure EuS t o the presence of interfa- cial Eu,Srl-,S layers produced by diffusion. The data can be quite well explained assuming a linear diffusion profile (a: = 0 t o z = 1) with a thickness of 3-4 nm.
Fig. 2.
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C vs. T for EuS/SrS multilayers with different thickness ratio d M / d N . For comparison, the results for a80 nm EuS film are also shown. They agree well with liter- ature data for bulk EuS 151. Dashed line indicates nuclear specific heat expected for EuS.
Then, for the r = 3/10 sample almost no pure EuS re- mains. Since C for EuS diluted with Sr is much larger
than for EuS a t low T, this results in the large spe- cific heat observed. As more and more EuS is present in samples with increasing r and roughly constant dif- fusion layer thickness, C decreases towards the data
for pure EuS. In agreement with this interpretation, the specific heat for a multilayer system with a given
r becomes smaller when the substrate temperature is reduced from 1 000 O C t o 600 OC. A more detailed analysis will be published elsewhere.
As a final point we note that magnon quantization effects which should be observable in the present range of temperature and thickness ratio [I], are probably masked by the large heat capacity of the diffusion lay- ers.
Acknowledgments
We thank J. Kijhne for the sample preparation. This work was supported by the Deutsche Forschungsge- meinschaft through SFB 125 Aachen-Jiilich-Koln.
[I] Wosnitza, J., v. Lohneysen, H. and Zinn, W.,
Solid State Commun. 65 (1988) 509.
121 Zinn, W., S a i c , B., Rasula, N., Mirabal, M. and Kohne, J., J. Magn. Magn. Mater. 35 (1983) 329.
[3] Wosnitza, J., v. Lijhneysen, H., Zinn, W. and Krey, U., Phys. Rev. B 33 (1986) 3436.
[4] Maletta, H., 3. Appl. Phys. 53 (1982) 2185. [5] Dietrich, 0. W., Henderson, A. J., Jr. and Meyer,
