Physical Properties of Al-Co-Ce (Dy) Amorphous Alloys
S.A. Uporov1 , V.A. Bykov2 , V.E. Sidorov1 , N.S. Uporova1 ,
K.Yu. Shunyaev2 , P. Svec3, D. Janickovic3.
1
Ural State Pedagogical University, Ekaterinburg, Russia;
Institute of Metallurgy UD RAS, Ekaterinburg, Russia;
3
Institute of Physics SAV, Bratislava, Slovakia
2
INTRODUCTION
The progress in electronics and communication technique requires the
production of new materials with improved properties. Aluminum alloys with
additions of rare-earth and 3d-transition metals are considered to be very promising
materials due to their good mechanical and electric properties, especially in
amorphous state. However, the mechanism of glass formation in these alloys differs
from those for alloys with deep eutectic – the concentration region of high glassforming ability is shifted from eutectic composition to the first intermetallic
compounds with high melting points [1]. In this paper we investigated structure (by
X-rays), DSC, electroresistivity and magnetic susceptibility of Al91Co2Ce7 and
Al89Co5Dy6 alloys in amorphous, crystal and liquid states.
Results and discussion
Master alloys were prepared by remelting of initial components in the
resistance furnace during half an hour at 17000 C in helium atmosphere. In order to
remove oxide film, the surface of the samples was mechanically cleaned. The ribbons
were produced using standard planar flow method. The chamber was preliminary
outgased and than filled in with argon up to 103 Pa. The melt was overheated to 130015000 C in the induction furnace and then injected onto copper water-cooling rotating
drum.
According to X-ray analysis, all the ribbons were amorphous. In order to
investigate crystallization kinetics in these ribbons, we studied DSC and
electroresistivity during heating and subsequent cooling with the rate of 10 K/min.
The DSC curve for Al91Co2Ce7 alloy is shown at fig. 1. It has two exothermal
and one endothermal peak at heating. The first peak (t1=181,4 ˚С; ΔH=18.6 J/g) can
be connected with the beginning of crystallization in the ribbon. By X-ray diffraction
it was found that the first stage corresponds to crystallization of aluminum matrix.
The second peak (t2=309.7 ˚С; ΔH=78.6 J/g) matches the crystallization of Al11Ce3
compound. The third (endothermal) peak corresponds to the reaction
Al+Al11Ce3+Al9Co2 ↔ L+Al11Ce3+Al9Co2, and thus its temperature t3=635,9 ˚С can
be considered as solidus in this system. The latest fact is in good agreement with
phase diagram [2]. As for Al89Co5Dy6 ribbon, the situation is vice versa – the first
peak on DSC curve (t1=307,4 ˚С; ΔH=69.5 J/g) characterizes the crystallization of
Al3Dy compound, whereas the second one (t2=340.3 ˚С; ΔH=44.3 J/g) is the evidence
of crystallization of aluminum matrix (fig.2). The solidus temperature was found to be
ts = 643,6 0С here.
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Fig.1. Temperature dependent heat flow rate (DSC) for Al91Co2Ce7 alloy
Fig.2. Temperature dependent heat flow rate (DSC) for Al89Co5Dy6 alloy
Two stages of crystallization can be distinctly seen on electroresistivity
temperature curves as well (fig. 3 and 4). In amorphous state resistivity practically
does not depend on temperature, and remains for 20 % higher than in liquid state and
for 80 % higher than in crystal.
Magnetic susceptibility temperature and field dependences were studied for
both ribbons. Susceptibility temperature curves χ(T) for amorphous Al91Co2Ce7 and
Al89Co5Dy6 ribbons in comparison with crystalline analogues are presented at fig. 5
and 6. It was obtained that in amorphous state magnetic susceptibility decreases
follow Curie-Weiss law; no anomalies at the beginning of crystallization were fixed.
However, the pronounced field dependence of susceptibility was stated for both
ribbons. In crystal and liquid state the results are practically the same as for nonquenched samples, no hysteresis of property was found out.
In order to compare parameters of electronic structure in amorphous and
crystalline state, the experimental susceptibility curves were fitted by generalized
Curie-Weiss law using less-square method
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Fig. 3. Relative electroresistivity vs temperature for Al-Co-Ce ribbon
Fig. 4. Relative electroresistivity vs temperature for Al-Co-Dy ribbon
 p (T )   0 
C
, (1)
T 
here C is Curie constant, Θ - paramagnetic Curie temperature and χ0 - the
temperature independent susceptibility depending mainly on density of electron states
at the Fermi level.
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Fig.5. Magnetic susceptibility temperature curves for Al91Co2Ce7 alloy:
○,● – amorphous; ∆,▲ – crystalline; ∆,○ – heating; ●,▲ – cooling
5
.10 , эме/г
7
5
3
1
0
300
600
900
1200
0
t, С
1500
Fig.6. Magnetic susceptibility temperature curves for Al89Co5Dy6 alloy:
○,● – amorphous; ∆,▲ – crystalline; ∆,○ – heating; ●,▲ – cooling,
Density of states at Fermi level N(EF) can be derived from temperature
independent term χ0 using eq. (2)
χ 0  2N A M 1 μ Б N(E F )ξ ,
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(2)
here NA – Avogadro number, M – molar mass, ξ – exchange amplification
factor (for rare earth metals ξ = 1.5 [3-5]).
Thus aluminum is a very weak paramagnetic material and cobalt atoms exist
here in nonmagnetic state [6], the effective magnetic moment per rare earth atom was
calculated using
μ eff 
3kC(M 1  M 2 ) ,
αN A (μ Б ) 2
(3)
here k – Boltsman constant, µB – Bohr magneton, М1, М2 ,α ,β – molar masses
and atomic fractions of REM and aluminum respectively.
The results of calculations are given in table 1.
Table 1.
Parameters of electronic structure
Al91Co2Ce7
Al89Co5Dy6
amorphous.
crystalline
amorphous.
crystalline
0·106,
emu/g
N(EF), eV-1
, К
C·105,
emu*K/g
eff, B
1.484
1.490
6.929
6.915
0.545
5
0.545
98
1.928
108
1.925
70
58.75
74.19
1419.06
1455.11
0.97
1.09
5.545
5.615
Amorphous state can be considered, in first approximation, as frozen liquid.
Thus the values of electronic parameters, obtained for amorphous state, can be
transferred to liquid state as well. The effective magnetic moments and densities of
states at Fermi level are practically the same in crystal and amorphous states. The
essential difference was fixed for paramagnetic temperature only. It can be connected
either with the changes in magnetic environment of rare earth atoms, or with the
difference in interatomic distances in crystalline and amorphous phases.
Amorphous ribbons are characterized by the field dependence of magnetic
susceptibility. This fact demonstrates superparamagnetic behavior of susceptibility.
Thus, the structure of the samples can be represented as paramagnetic aluminum
matrix with the inclusions of microparticles consisting of AlxREMy and AlxTMy
quasymolecules. These microparticles are characterized by specific magnetic
ordering, and the total susceptibility of the alloys can be approximated by [3]:
 (T , B) 
NM 02  M 0 B 
f
   p (T ) , (4)
4kT  kT 
In eq. (4) N is the number of particles per mass unit and Mo is the average
magnetic moment of one particle. The first term corresponds to superparamagnetic
particles and the second describes the susceptibility of the remaining paramagnetic
matrix. The “generalized” Curie-Weiss law can be used for the second term as it was
done above.
Fitting experimental χ(B) curves by eq. (4), one can get parameters for
superparamagnetic particles. For Al91Co2Ce7 alloy the following results were
obtained: the average magnetic moment per superparamagnetic particle Mo = 2165 µB,
the mass density of these particles N ≅ 1.9·1015 g-1. For Al89Co5Dy6: Mo = 4040 µB, N
≅ 1.45·1015 g-1 respectively. The obtained values N = (1.45÷1.9)·1015 g-1 seem to be
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very small and that is why such superparamagnetic particles can not be registered
using standard X-ray diffraction method.
Taking into account that effective magnetic moments per rare earth atom in
Al2Ce and Al11Ce3 compounds are µef ≅2.5 µB and µef ≅1.1 µB, and in Al2Dy and
Al3Dy compounds - µef ≅9.5 µB and µef ≅5.6 µB respectively, using equation Mo = n *
μeff, one can estimate n - the effective number of “noncompensated” magnetic
moments in the particle. For the ribbon Al91Co2Ce7 the results are: n1(µef ≅2.5 µB) =
870 and n2(µef ≅1.1 µB) = 1970, as for Al89Co5Dy6 ribbon - n1(µef ≅5.6 µB) = 425 and
n2(µef ≅9.5 µB) = 720 respectively.
The obtained n values demonstrate the fact that superparamagnetic particles
consist of a large number of atoms (molecules). It becomes possible to suppose that
these superparamagnetic particles do not appear during quenching the melt, but exist
in liquid state even at high overheating above liquidus. This fact is necessary to take
into account while creating a model of liquid alloys with tendency to amorphization.
Conclusions
The obtained values of electronic parameters for Al-Сo-Ce and Al-Co-Dy alloys in
crystalline and amorphous states allow us to make the following conclusions:
the effective magnetic moments per cerium (1.1 µB) or dysprosium (5.6 µB)
atoms are the evidence of the fact that in investigated alloys rare earth atoms
exist not in R3+ state, as it was considered before [4], but create directed bonds
with aluminum atoms (this idea was generated in our previous papers [7,8]).
This situation takes place in amorphous, crystalline and liquid states. 4felectrons located earlier on REM ions are involved now into directed bonds
formation, i.e. some part of them become delocalized. Because of that
effective magnetic moment on REM atom decreases and becomes lower than
for R3+ ion. We think that in Al-Co-Ce(Dy) alloys the probability of Al2Ce
(Al2Dy) quasymolecules existence is rather high;
density of electron states at Fermi level has small values in Al-Co-Ce(Dy)
alloys independently on state (amorphous, crystal or liquid). It means that 4fand 3d-zones do not overlap and have maximums rather far from Fermi level.
The work is supported by RFBR (grants N 07-02-01049 and N 09-03-90450) and
Federal Target Program (Contract П895)
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