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LOW TEMPERATURE SPECIFIC HEAT AND
MAGNETIC PROPERTIES OF Zr(Fe1-xCox)2
G. Hilscher, E. Gmelin
To cite this version:
JOURNAL DE PHYSIQUE
Cofloque
C6, suppl&ment
auno 8, Tome 39, aolif 1978, page
C6-774
LOW TEMPERATURE SPEC1 F I C H E A T AND MAGNETIC PROPERTIES OF ~r ( ~ e l
- x C ~ x )
G. Hilscher and E. ~melin*
I n s t i t u t fiir ExperirnentaZphysik, T . U . Wfen, Austria.
M m PLanck I n s t i t u t filr Festkiirperforschung, 7 Stuttgart 80, Fed. R e p , Germany
Rbsumb.- La chaleur spgcifique de cinq alliages Zr(Fe,Co)> a dtd mesurd entre 1,6 et 10 K. Les rb- sultats sont expliqubs en relation avec leurs propribtds magndtiques, La variation du terme dlectro- nique y (de C ) en fonction de la concentration (X) permet d'interprdter l'apparition du ferromagnd-
P
tisme dans le modsle du faible ferromagnbtisme itingrant.
Abstract.- Specific heat measurements between 1.6 and 10 K of five 2r(Fe,C0)~ alloys are presented. The results are discussed in connection with the magnetic properties and support the interpretation of the onset of ferromagnetism in terms of weak itinerant electron magnetism.
the framework of the Stoner Edwards Wohlfarth (SEW) X =0.72
sLD~l)!-*~---D
model / 3 / . Recently it was shown / 4 / that from ma- gneeic and resistivity measurements within the res- trictions of the' SEW model for itinerant electron
magnetism an estimation of the density of states at 0.h
--"-&S.+...---
the Fermi level (N(Ef)) can be made : the slope _/--INTRODUCTION.- Alloys of the solid solution Zr(Fe,
(1/B) of M~ versus HIM plots (Arrott plots) /3/ and the high temperature deviation of the resistivity from the linear p-T dependence p
-
p = a, T-a2T30
/ 5 / can be correlated with N(Ef) and its derivatives. N(E ) can be evaluated from the experimental data :
f
N~ ( E ~ ) = az/al 6(a2k26p4~~3)-1
B g ( 1 )
09
C O ) ~ crystallize in the cubic MgCuz structure. Star-
ting from ZrFe2 (Tc = 619 K) the magnetic moment and 0 8 - the Curie temperature drops with increasing CO con-
-
F
Y1
centration up to X = 0.75, which is the critical
2
concentration for ferromagnetism. The breakdown of
n
E 0.6
magnetic order was discussed by Kanematsu /l/ in
-
with N number of atoms per gram. The aim of this g
investigation was to compare the estimated N(Ef) values with low temperature specific heat measure-
X = 0.66 ,,A, I
....
'".-
0.7-%.- , / % W . ? . S - - -.-.--"(B#&p'w-
F-%$&.,-O*+?
"-=a7 ments.terms of local environment effects and later /2/ in
7
RESULTS AND DISCUSSION
.-
The heat capacity of the un~nealed alloys was measured in an automated adiabatic calorimeter using calibrated Ge or Carbon- Glass resistors (heat capacity of sample holder lessthan 5% of total C ) .
P
Results of the specific heat measurement are presented in a conventional C/T against T2 plot
(Fig. 1). At low temperatures these curves exhibit a deviation from linearity. The "upturns" of these
Fig. 1
.-
C /T against T2 plots of five Zr(Fel-xC~x)2 alPoys.curves are seen to become more pronounced with high- er CO concentration, or become even a maximum for
X = 0.9. In view of the paramagnon-cluster contro- versy 171, /8/ for X P 0.9 the upturn of C is seen
P to be the high temperature side of a peak, which excludes the possibility of'significant paramagnon contribution. Although for X = 0.8 and X = 0.9 it is found that the upturn of C at low temperatures
P
is proportional to 1 / ~ ~ , no decision can be made whether this upturn is due to nuclear spin-, clus- ter- or Schottky impurity contribution. Especially for binary FeCo alloys it is well known, that the
significant upturn of C is due to the nuclear spe- P
cific heat term /g/, while no upturn is found for
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19786345
Y(Fe,co),
l1
01. As already mentioned the specially for specific heat measurements prepared 25g sample were not annealed. Therefore aclusterformation can-not be excluded, but a more detailed analysis of the low temperature upturn requires further experiments.
In order to obtain an estimate of the magnetic con- tribution to C the sample with X = 0.72 (Tc = 30K)
P'
wasmeasured up to 70 K : no anomaly was found around Tc. Within the SEW model the magnetic contri- bution to y is given by Y,=-M:(ZX TZ)-' (for x=0.72
0 C
ym = 0.15 dfmole which is less than 0.3% of mea- sured y). Additionally it should be noted, that no significant change in the slope of the resistivity versus temperature curve is observed around Tc 141. From the concentration dependence of the
BD
and the lattice constant (Fig. 2) may be deduced, that the lattice becomes softer with lower CO concentration, while the pressure dependence of Tc is almost cons-tant in the critical concentration range Ill/. From
Fig. 2.- The lattice parameter (a) (o after ( I ) )
and Debye temperature (A) of Zr(Fel-Cox) 2. aTc
these almost constant
-
ap values it was concluded that the magnetic properties are mainly determined by the lattice parameter, while within the SEW model aTc
-
varies as -l/Tc. ~entatively a reason for theap aTc ~ T C
may be that the prediction
-
constant-
a
Pa?
Tc= -conSt. is balanced by the more rigid lattrce as X + xF
.
In figure 3 the y values and the corres- ponding N(Ef) values, calculated without any cor- rection due to electron-phonon enhancement or magnetic contribution (ym < 0.3% y), are compared with N(E ) values deduced from magnetic and resis-f
tivity measurements. The concentration dependence of both N(E ) values is similar, however, a signi-
f
ficant deviation is found for X
<
0.66. A reason for the significant deviation may be that thestates
N(Ef"atomev
3
I P' 1
Fig. 3.- Plot of Tc(A), N(Ef) after eq.(l) (o), N(Ef) from y (a) and
X (a)
as functionR
of X for Zr(Fe,-xC~x)2.
system tends to strong ferromagnetism with lower X values, hence the SEW model is less applicable for
X < 0.66. The susceptibility (determined at 4.2 K
and 0.5 T) as a function of X points also to de- creasing N(E ) values. Consequently the Stoner en-
f
-
1hancement S =(I-N(Ef)I) decreases monotonically from S = 155 to S = 37 (S=6) for X = 0.76, X = 0.9 (x=1.0) respectively.
Although the details of C at low temperatu- P
res are unsolvel, we conclude from the results in figure 3 that the transition from ferromagnetism to
paramagnetism at X = 0.75 can be interpreted satis- factorily in terms of the SEW model : the parama- gnetism for X > 0.75 is due to the decreasink N(E~)
References
/l/ Kanematsu, K., J. Phys.Soc.Jpn.
2
(1971) 1355/ 2 / Hilscher, G., Kirchmayr, H., Phys. Status Solidi. a
2
(1975) K169/ 3 / Wohlfarth, E.P., Physica 91B (1977) ,305,and references therein.
141
Hilscher, G., Gratz,E.,
Phys. Status Solidi to be published/S/ Jones, H., Hdb.d.Physik XIX,
2
(1956) 266 (Berlin : Springer Verlag)161 ~gdhammer, P., Gmelin, E., J. Phys.E., to be published
/7/ Hahn, A., Wohlfarth, E.P., Acta Phys.Helv.
41
(1968) 857181 de Dood, W., de Chatel, P,,
J.
Phys.F
3
(1973) 1039/ g / Kuentzler, L., Phys, Status Solidi (b)
58
(1973) 519/10/ Muraoka,
