Magnetic Properties of Gadolinium Silicide Thin Films for Different Heat Treatments
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283 Mat. Res. Soc. Symp. Proc. Vol. 384 01995 Materials Research Society
Figure 2: A-film TEM plan view and its associated electron diffraction
Figure 3: C-film TEM plan view and its associated electron diffraction
284
hexagonal superstructure. The C diffraction pattern is of the 1*1 type. It is characteristic [1] of the A1B 2 diagram, the silicide having no Si vacancy. A TEM (Transmission Electron Microscopy) study on A and C layers was performed (fig 2 and 3). They clearly are single crystals. They contain a rather high density of planar defects. The annealing process dramatically decreases this defect density. Diffraction superlattice reflections similar to those found for other rareearth silicides (eg T.L. Lee [2] and F.H. Kaatz [3]) strongly suggest the ordering of vacancies in the Si sublattice of GdSi 2 . A more detailed TEM study is reported elsewhere [3]. II b Resistivity measurements From resistivity measurements it is possible to deduce the residual resistivity Po due to the collision of conduction electrons with impurity atoms or mechanical stress in the lattice, and also the magnetic resistivity Pm due to crystal field and to spin disorder. The values obtained are summarised in Table I. The residual resistivity is lower for TABLE I: Thin film residual and films elaborated at higher temperature. The magnetic resistivity PO and Pm high temperature decreases the crystalline resistivity Po Pm defect
density,
as
seen
on
previous
micrographs. Magnetic-ordering temperatures are better determined from the first derivative of the resistivity curves (fig. 4, 5, 6). Two well separated anomalies are observed for samples B and C, whereas one anomaly or two closely
(AU..cm)
film A film B film C
15.25 59 108.5
8.75 14 15.5
located anomalies occur for sample A. Characteristic temperatures are given in
Table II, the highest temperature is always close to 50 K and is obviously a Ndel temperature. Note that the knee on the curves around 15 K is not related to any magnetic transition, but to the rate of the thermal population of excited magnetic levels. .4 .3.-,
• .'"
.3I
.2.2
.21
"•,.••••".1~
/"2 (K)
Temperature (K) 0
10
20
30
40
0 20 40 60 80 Figure 5: First derivative of the B-film resistivity versus the temperature
50
Figure 4: First derivative of the A-film resistivity versus the temperature
285
.4
TABLE II: Thin film critical
temperatures .2
critical temperature
TN
Tx
film A film B film C
50 49 55
48 40 41
(K)
"0. 0
,Temperature 20
40
60
(K) 80
Figure 6: First derivative of the
C-film resistivity versus the temperature III. Discussion Magnetic and transport measurements have been previously performed in bulk silicides. Orthorhombic GdSi (FeB type) is reported to order at 50K. The hexagonal phase GdSil. 65 has a N6el point near 33 K, whereas the orthorhombic phase GdSi 1.8 orders at 25 K [5, 6] and presents a modification of its magnetic structure near 23 K. The high transition temperature is close to that of bulk GdSi, whereas the low one is closer to that of hex
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