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Crystallographic and magnetic investigations of the intermetallic system ErCo2-ErAl2
JoumaloftheLess-CommonMetals,81 (198X)5
5
- 13
~R~STALLOGR~HrC AND MAGNETIC INVESTIGATIONS OF THE INTERMETALLIC SYSTEM ErCo,-ErAla
D. KONOPKA and W. ZAREK fnstitute of
Physics, Silesian U~iue~~t~, ~O-~U7K~to~i~~~Poland~
(Received February 4, 1981)
Summary The crystalline structure and magnetic properties of polycrystalline ErCo2-ErAI, Laves phase compounds with the Cl4 and Cl5 crystal structures were investigated. It was found that for the Cl5 cubic structure it is possible to obtain compounds with a disordered structure. X-ray examination showed that for the E&o, compound the exchange of cobalt atoms between 8a and 16d sites is almost 1 atom per unit cell. After thermal treatment the crystal structure of the ErCoz compound shows a tendency towards ordering. For compounds with the hexagonal Cl4 structure the possibility of a layer distribution of atoms, alternately magnetic and nonmagnetic, was tested. This assumption is well satisfied by the ErCoAl compound. For this compound a certain anomaly was also observed, ie. a small lattice constant c. Magnetic investigations indicated that the magnetic prop erties of the compounds investigated, particularly the Curie temperature, are markedly dependent on the degree of crystal structure ordering. For the remaining compounds with the Cl5 structure it was not possible to obtain fully ordered compounds but after thermal treatment they exhibit a tendency towards ordering.
1. Introduction The study of the magnetic and structural properties of pseudobinary RTa-RAla systems (where R is a rare earth element and T is a transition metal) of Laves phase types [l - 41 has received significant attention in recent years. Researchers have reported the large differences in magnetic properties between the ErCoa-ErAls system and the other RTa-RAls systems. For the ErCoa-ErAls system it was found that the gyration magnetic moment as a function of the composition has maxima at the contents 10 and 50 mol.% ErAla, while the Curie temperature remains constant except for the cobalt-rich compounds. This anomaly was not observed for other intermetallic systems of the RT2-RA12 type. @ Elsevier ~quoia/~i~ted
in The Netherl~ds
6
Oesterreicher [l] has postulated that these anomalies are associated with the crystal structure and he has studied the distribution of the aluminium, cobalt and erbium atoms in the unit cell of the compounds with the Cl5 and Cl4 structures. He has suggested that compounds of the Cl5 type have an ideal “ordered” structure with erbium atoms in 8a sites and the cobalt and aluminium atoms statistically distributed among the 16d sites. For compounds with the Cl4 crystal structure Oesterreicher has postulated that the erbium atoms occupy 4f sites while the cobalt and aluminium atoms are statistically distributed between the 2a and 6h sites. He has applied plate techniques and visual estimation of the diffracted X-ray intensity [l] . We believe that Oesterreicher incorrectly chose to use Cu Ka radiation. Although this wavelength is strongly absorbed by cobalt, no correction for anomalous absorption was applied. For this reason detailed investigations of ErCos-ErAl, compounds were undertaken and are reported here. Crystallographic and magnetic tests were performed on specimens before and after heat treatment. Earlier investigations were performed on quenched samples [ 5,6] .
2. Preparation The ErCo,-ErAl, compounds were prepared from the elements (erbium, 99.5% pure; aluminium, 99.999% pure; cobalt, 99.999% pure) by arc melting stoichiometric quantities in an argon atmosphere. Phase analysis of diffraction patterns for the samples obtained did not show any foreign phases. After melting, some of the compounds were annealed at 923 K for about 100 h in an argon atmosphere. The X-ray investigations (lattice constant and diffraction line intensities) and magnetic investigations were performed both before and after thermal treatment.
3. X-ray investigations X-ray measurements were made with a DRON-2 diffractometer on powdered samples with the Debye-Scherrer method using Fe Ke radiation. The primary beam stability was 0.1%. Lattice constants of the samples tested were measured using the method of location of the diffraction line maxima, which were determined by measuring intensity every 0.01” at the extreme points on the line contour, with a scaling line of 40”. The radiation was detected using a scintillation counter with a scanning velocity of 2’ mm-‘. The lattice constants of the compounds with the cubic structure were determined from an extrapolation of the data measured to 90” with for absorption and for the extrapolation function cos28. Corrections horizontal divergence of the X-ray beam were made. For compounds with the hexagonal Cl4 structure the lattice constants were calculated with the method of successive approximations. The lattice constants of the samples tested are given in Table 1.
7 TABLE
1
Lattice constants Compound
of the ErCorErAln
ErAI,
(mol.%) ErCoz 0 E*o,.aAlo.~ 10 E~ol.6Ab.4 20 '+‘%.4A'o.6 30 ErCoAl 50 E~oo.~All.z 60 Efloo.sA11.r70 15 J3*00.54.5 60 E~"o.&l.5 so E~oo.z&, ErA12 100
intermetallic
Lattice
Loves phase
After thermal treatment
con&tint
a t.4
c(A)
7.145+ 0.001 7.164i 0.05 1.266+ 0.05 5.216+ 0.001 5.300+ b.007 5.364+ 0.006 5.3962 0.007 5.401f 0.009 1.670+_0.001 7.766* 0.004 7.794i 0.01
Cl5 Cl5 Cl5 Cl4 Cl4 Cl4 Cl4 Cl4 Cl5 Cl5 Cl5
compounds
6.445f 0.001 6.400f 0.007 8.469+ 0.007 6.603f 0.001 6.646+ 0.009
a(&
c(A)
7.151* 0.001 7.205t 0.003 7.380f 0.004 5.209+ 0.006 5.339* 0.006 5.370* 0.006 5.395* 0.06
6.471+0.006 6.419+ 0.006 6.619f 0.006 8.611+ 0.06
7.671+ 0.002 7.7102 0.002 7.808? 0.002
The distribution of atoms in the unit cells of the compounds Er(CoI_,Al,), was determined from diffraction line intensities, and the structures of the compounds were calculated by the trial and error method. The structure factor Fcalc was calculated taking into account the Lorentz and polarization factor, the multiplicity factor and the temperature factor. For compounds with the Cl5 cubic structure Fcalc was calculated from the formula
+ 2{0.5yf,,
+ (1 -x)(1
+ (1- 0.5y)fA1)
- 0.5Y)fc,
+
g expt 27ri(hUk + kuk + 1wk))
(1)
where y expresses the exchange of erbium and cobalt (aluminium) atoms between 8a and 16d sites, fEr, fc, and fAl are atomic scattering factors, u,, u, and w, denote 8a Sitis[71 and &, uk and wk denote 16d sites [ 71. The observed structure factor values Fobs obtained experimentally were fitted to the calculated structure factor values Fcalc using the equation
z IFca~c-
R(y)
=
hkr
Fobs1
100%
2 lFobsl
hkl
where R is the divergence factor. The function R(y) was minimized for y
(2)
0
Q04
008
412
Q’S
020
0
y
0,OL
0,08
0.12
O,l6
a20
y
Fig. 1. The function R(y) for ErCorErAlz compounds with the Cl5 crystal structure (before thermal treatment): 0, ErCoz ; X, ErCol_8Alo.2 ; + , ErCoo.zA11.8 ; 0, ErA12. Fig. 2. The function R(y) for ErCog-ErAl2 compounds with the Cl5 crystal structure (after thermal treatment): 0, ErCoo ; X, ErCol_8Alo.2 ; + , ErCoo.~All_8 ; 0, ErA12.
the range 0.0 - 0.2 for intervals of 0.01. The structures of the compounds, i.e. the distribution of erbium, cobalt and aluminium atoms between the 8a and 16d sites, were determined from the minima of R(y). The results obtamed are presented in Figs. 1 and 2. For compounds with the Cl4 crystal structure Fcalc was calculated from the formula in
F talc= fEr5 exp {2ni(hu + kv + Iw)} + + YU - x)fco $
exp{2ni(hu’
f x(1-
i
0.5y)f,,
+ (1 -x)(1 f xyf,,
+ ku’ + Zw’)} +
exp{2ni(hu’
- 0.5y)fc0
+ ku’ + Zw’)} +
2 exp{ BA(hu” + ku” + Iw”)} +
$ exp{ 2S(hu” + ku” + IW”)}
(3)
TABLE 2 The distribution of atoms in the EuCoz-EuAl2 compounds
intermetallic
Compound
ErAlz (mol.%) in ErCoz
R (%I
Y
E~%&o.G
30 50 60 70
26.7 23.2 27.2 24.5
0.5 1.6 0.6 0.8
E*o,.oAh.o BrCoo.8Al1.2 BrCoo.6Al1.4
where y expresses the occupancy of the cobalt atoms in 2a sites, fEr, fc, and fAl are atomic scattering factors, u, u and w denote 4f sites [7] , u’, u’ and w’ denote 2a sites [7] and u”, u” and w” denote 6th sites [7]. The experimentally obtained values Fobs were fitted to the values of Fcalc using eqn. (3). The function R(y) was minimized for parameters y in the range y = 0 to y = 2 .O with intervals of 0 .Ol.The crystal structures of the compounds tested were determined from the minimum of the function R(y), i.e. the distribution of cobalt and aluminium atom8 between the 2a and 6h sites (when y = 0, aluminium atoms occupy the 2a sites; when y = 1, there is a statistical distribution of aluminium and cobalt atoms between the 2a and 6h sites; when y = 2, cobalt atoms occupy the 2a sites). The value8 of the parameters 3cand z from which the 4f and 6h sites were determined were taken as z = 0.062 and x = 0.830 from the MgZn,type structure. The results obtained are presented in Table 2.
4. Magnetic investigations The magnetization u and magnetic susceptibility x as functions of temperature Tin a magnetic field H were determined using the Faraday method. The Curie temperatures Tc of ErCo2 and ErC01.8A10.2were determined from magnetic investigations in low magnetic field8 up to 500 Oe and from the H/o = f(a2) plots. Figure 3 shows the thermal variation8 in the reciprocal susceptibility for the compounds tested (before thermal treatment). The thermal variations in the reciprocal susceptibility for ErCo2 and ErCo1.8A10.z before and after thermal treatment are shown in Figs. 4 and 5 and the results of the magnetic investigations are presented in Table 3.
5. Results and diBCU88ion X-ray investigations of the degree of ordering in the crystal structures of ErCo2-ErAl, compounds indicated that in compounds with the Cl5 crystal structure an exchange of atoms between the 8a and 16d site8 takes
Fig. 3. The thermal variation in the reciprocal susceptibility for ErCoa-ErAlz compounds (before thermal treatment): curve 1, ErCoz; curve 2, ErCo1_8Al02 ; curve 3, ErCo1.4Al0.~ ; curve 4, ErCoo.aAll.2; curve 5, ErCo0.5A11.5; curve 6, ErCo,-,sAl1.8; curve 7, ErA12.
place (Fig. 1). For ErCoz this exchange is approximately 1 atom per unit cell, whereas for ErC0iV8A10.zonly 0.5 cobalt atoms move to 8a sites. Compounds rich in aluminium have an ordered crystal structure. After thermal treatment, only ErCoz compounds exhibit “ideal” ordering of the crystal structure with erbium atoms in 8a sites and cobalt atoms in 16d sites. X-ray investigations of the compound with the hexagonal Cl4 structure showed that only the ErCoAI compound contains alternate magnetic layers. In this compound, cobalt atoms occupy 2a sites and together with erbium atoms occupy the 4f sites, forming the magnetic layers. For the remaining ErCoz-ErAlz compounds with the hexagonal structure, the cobalt and aluminium atoms are statistically distributed between 2a and 6h sites. The alternating magnetic and non-magnetic layer arrangement may be the cause of the anomalously large saturation magnetic moment found for ErCoAl [31* The magnetic tests on ErCoz-ErAlz compounds showed that those with x < 1 do not obey the Curie-Weiss law. The shape of the dependence x-l(T) for compounds with 3t< 1 indicates that the cobalt atoms have a non-zero magnetic moment and exhibit strong exchange interactions between erbium and cobalt. Compound8 with x > 1 obey the Curie-Weiss law. This fact plus the values of the effective magnetic moments of these compounds (Table 3)
11
01
SO
250
200
150
100
XXI
TPKI
4. The thermal variation in reciprocal susceptibility for ErCo2 : 0, before thermal treatment; 0, after thermal treatment.
Fig.
20-
15-
IO-
50
100
150
200
250
3L-OTl"K1
Fig. 5. The thermal variation in the reciprocal susceptibility for ErC01,8A&-,.~: 0, before thermal treatment; 0, after thermal treatment.
indicate that the cobalt atoms are non-ma~etic. This result is in agreement with earlier results reported by Oesterreicher [ 31. Magnetic investigations of the compounds ErCo2 and ErCo1.sAlo.2 with the Cl5 structure showed that their magnetic properties depend on the
12 TABLE 3 Magnetic data for ErCoz-ErA12 Compound
ErAtz (mol.%) in ErCo,
intermetallic compounds Effective parum~e~c moment peff
0
Cl5
Does not obey the CurieWeiss law
101.5 ? 0.5 40 [31
Ec"l.8A10.2
10
Cl5
Does not obey the CurieWeiss law
115.5 * 0.5 66 [31
B~o1.4Alo.6
30
Cl4
Does not obey the CurieWeiss law
E~"0.6A11.4
70
Cl4
9.0
ES02
Weiss constant f3 (Q
Curie temperature
Type of Laues phase
14
75
Cl4
9.2
14
E~oo.,All.~
90
Cl5
9.4
24
100
Cl5
9.0 9.56 f3]
40 16
ErAI
60 24 [33 26 131
9.3 [3] Efloo.5A11.5
Tc
(K)
22 r31
E31
degree of ordering in the crystal structure. It was found that these compounds both before and after thermal treatment do not obey the CurieWeiss law. The lack of agreement between our results and those given in ref. 3 could be due to the fact that our investigations were performed in a considerably wider temperature range. The ordered and disordered phases of ErCo2 and ErCo1.sAlo.2 compounds were found to have different Curie temperatures and qualitative variations in the x-l (2’) dependence (Fig. 4). The Curie temperatures of ErCoa and ErCo1sA10.2 before thermal treatment were Tc = 101.5 K and Tc = 115.5 K respectively and after thermal treatment Tc = 40 K and Tc = 123 K respectively. In ErCo2 we distinguish the Er-Er and Co-Co magnetic ~tera~tions, which in the 8a and 16d sub~tti~e are ferromagnetic, and the Er-Co magnetic interaction between sublattice 8a and 16d sites which leads to the antiferromagnetic alignment of the magnetic moment of the erbium and cobalt atoms. The passage of cobalt atoms to $a sites in the disordered structure of ErCo, has the effect of enhancing the Co-Co interaction, thus leading to an increase in the Curie temperature. For ErCo1.8Alo.2 no differences in ordering in the crystal structure before and after annealing were found. However, the observed difference in Curie temperature (10 K) suggests that such structure changes may take place but owing to the small resolving power of the X-ray method (about 1%) they may remain undetected.
13
Acknowledgments The authors would like to express their sincere thanks to Professor A. Chelkowski for helpful discussions. The work was partly supported by the Institute of Physics, Polish Academy of Sciences.
References 1 2 3 4 5
H. Oesterreicher, J. Less-Common Met., 33 (1973) 25. H. Oesterreicher and W. E. Wallace, J. Less-Common Met., 13 (1967) 91. H. Oesterreicher, L. M. Corliss and J. M. Hastings, J. Appl. Phys., 41 (1970) 2326. H. Oesterreicher, J. Appl. Phys., 42 (5) (1971) 137. A. Slebarski, W. Zarek, D. Konopka, A. Winiarska, E. Derejczyk and A. Chelkowski, Znt. Conf. on Physics of Transition Metals, Toronto, 1977, in Inst. Phys. Conf. Ser., 39 (1978) Chap. 6. 6 D. Konopka and W. Zarek, 11 th Znt. Congr. of Crystallography, Warsaw, August 3 - 12, 1978, Collected Abstracts 13.4 - 13.5. 7 N. F. M. Henry and K. Lonsdale (eds.), International Tables for X-ray Crystallography, Vol. I, Kynoch, Birmingham, 1963.
5
- 13
~R~STALLOGR~HrC AND MAGNETIC INVESTIGATIONS OF THE INTERMETALLIC SYSTEM ErCo,-ErAla
D. KONOPKA and W. ZAREK fnstitute of
Physics, Silesian U~iue~~t~, ~O-~U7K~to~i~~~Poland~
(Received February 4, 1981)
Summary The crystalline structure and magnetic properties of polycrystalline ErCo2-ErAI, Laves phase compounds with the Cl4 and Cl5 crystal structures were investigated. It was found that for the Cl5 cubic structure it is possible to obtain compounds with a disordered structure. X-ray examination showed that for the E&o, compound the exchange of cobalt atoms between 8a and 16d sites is almost 1 atom per unit cell. After thermal treatment the crystal structure of the ErCoz compound shows a tendency towards ordering. For compounds with the hexagonal Cl4 structure the possibility of a layer distribution of atoms, alternately magnetic and nonmagnetic, was tested. This assumption is well satisfied by the ErCoAl compound. For this compound a certain anomaly was also observed, ie. a small lattice constant c. Magnetic investigations indicated that the magnetic prop erties of the compounds investigated, particularly the Curie temperature, are markedly dependent on the degree of crystal structure ordering. For the remaining compounds with the Cl5 structure it was not possible to obtain fully ordered compounds but after thermal treatment they exhibit a tendency towards ordering.
1. Introduction The study of the magnetic and structural properties of pseudobinary RTa-RAla systems (where R is a rare earth element and T is a transition metal) of Laves phase types [l - 41 has received significant attention in recent years. Researchers have reported the large differences in magnetic properties between the ErCoa-ErAls system and the other RTa-RAls systems. For the ErCoa-ErAls system it was found that the gyration magnetic moment as a function of the composition has maxima at the contents 10 and 50 mol.% ErAla, while the Curie temperature remains constant except for the cobalt-rich compounds. This anomaly was not observed for other intermetallic systems of the RT2-RA12 type. @ Elsevier ~quoia/~i~ted
in The Netherl~ds
6
Oesterreicher [l] has postulated that these anomalies are associated with the crystal structure and he has studied the distribution of the aluminium, cobalt and erbium atoms in the unit cell of the compounds with the Cl5 and Cl4 structures. He has suggested that compounds of the Cl5 type have an ideal “ordered” structure with erbium atoms in 8a sites and the cobalt and aluminium atoms statistically distributed among the 16d sites. For compounds with the Cl4 crystal structure Oesterreicher has postulated that the erbium atoms occupy 4f sites while the cobalt and aluminium atoms are statistically distributed between the 2a and 6h sites. He has applied plate techniques and visual estimation of the diffracted X-ray intensity [l] . We believe that Oesterreicher incorrectly chose to use Cu Ka radiation. Although this wavelength is strongly absorbed by cobalt, no correction for anomalous absorption was applied. For this reason detailed investigations of ErCos-ErAl, compounds were undertaken and are reported here. Crystallographic and magnetic tests were performed on specimens before and after heat treatment. Earlier investigations were performed on quenched samples [ 5,6] .
2. Preparation The ErCo,-ErAl, compounds were prepared from the elements (erbium, 99.5% pure; aluminium, 99.999% pure; cobalt, 99.999% pure) by arc melting stoichiometric quantities in an argon atmosphere. Phase analysis of diffraction patterns for the samples obtained did not show any foreign phases. After melting, some of the compounds were annealed at 923 K for about 100 h in an argon atmosphere. The X-ray investigations (lattice constant and diffraction line intensities) and magnetic investigations were performed both before and after thermal treatment.
3. X-ray investigations X-ray measurements were made with a DRON-2 diffractometer on powdered samples with the Debye-Scherrer method using Fe Ke radiation. The primary beam stability was 0.1%. Lattice constants of the samples tested were measured using the method of location of the diffraction line maxima, which were determined by measuring intensity every 0.01” at the extreme points on the line contour, with a scaling line of 40”. The radiation was detected using a scintillation counter with a scanning velocity of 2’ mm-‘. The lattice constants of the compounds with the cubic structure were determined from an extrapolation of the data measured to 90” with for absorption and for the extrapolation function cos28. Corrections horizontal divergence of the X-ray beam were made. For compounds with the hexagonal Cl4 structure the lattice constants were calculated with the method of successive approximations. The lattice constants of the samples tested are given in Table 1.
7 TABLE
1
Lattice constants Compound
of the ErCorErAln
ErAI,
(mol.%) ErCoz 0 E*o,.aAlo.~ 10 E~ol.6Ab.4 20 '+‘%.4A'o.6 30 ErCoAl 50 E~oo.~All.z 60 Efloo.sA11.r70 15 J3*00.54.5 60 E~"o.&l.5 so E~oo.z&, ErA12 100
intermetallic
Lattice
Loves phase
After thermal treatment
con&tint
a t.4
c(A)
7.145+ 0.001 7.164i 0.05 1.266+ 0.05 5.216+ 0.001 5.300+ b.007 5.364+ 0.006 5.3962 0.007 5.401f 0.009 1.670+_0.001 7.766* 0.004 7.794i 0.01
Cl5 Cl5 Cl5 Cl4 Cl4 Cl4 Cl4 Cl4 Cl5 Cl5 Cl5
compounds
6.445f 0.001 6.400f 0.007 8.469+ 0.007 6.603f 0.001 6.646+ 0.009
a(&
c(A)
7.151* 0.001 7.205t 0.003 7.380f 0.004 5.209+ 0.006 5.339* 0.006 5.370* 0.006 5.395* 0.06
6.471+0.006 6.419+ 0.006 6.619f 0.006 8.611+ 0.06
7.671+ 0.002 7.7102 0.002 7.808? 0.002
The distribution of atoms in the unit cells of the compounds Er(CoI_,Al,), was determined from diffraction line intensities, and the structures of the compounds were calculated by the trial and error method. The structure factor Fcalc was calculated taking into account the Lorentz and polarization factor, the multiplicity factor and the temperature factor. For compounds with the Cl5 cubic structure Fcalc was calculated from the formula
+ 2{0.5yf,,
+ (1 -x)(1
+ (1- 0.5y)fA1)
- 0.5Y)fc,
+
g expt 27ri(hUk + kuk + 1wk))
(1)
where y expresses the exchange of erbium and cobalt (aluminium) atoms between 8a and 16d sites, fEr, fc, and fAl are atomic scattering factors, u,, u, and w, denote 8a Sitis[71 and &, uk and wk denote 16d sites [ 71. The observed structure factor values Fobs obtained experimentally were fitted to the calculated structure factor values Fcalc using the equation
z IFca~c-
R(y)
=
hkr
Fobs1
100%
2 lFobsl
hkl
where R is the divergence factor. The function R(y) was minimized for y
(2)
0
Q04
008
412
Q’S
020
0
y
0,OL
0,08
0.12
O,l6
a20
y
Fig. 1. The function R(y) for ErCorErAlz compounds with the Cl5 crystal structure (before thermal treatment): 0, ErCoz ; X, ErCol_8Alo.2 ; + , ErCoo.zA11.8 ; 0, ErA12. Fig. 2. The function R(y) for ErCog-ErAl2 compounds with the Cl5 crystal structure (after thermal treatment): 0, ErCoo ; X, ErCol_8Alo.2 ; + , ErCoo.~All_8 ; 0, ErA12.
the range 0.0 - 0.2 for intervals of 0.01. The structures of the compounds, i.e. the distribution of erbium, cobalt and aluminium atoms between the 8a and 16d sites, were determined from the minima of R(y). The results obtamed are presented in Figs. 1 and 2. For compounds with the Cl4 crystal structure Fcalc was calculated from the formula in
F talc= fEr5 exp {2ni(hu + kv + Iw)} + + YU - x)fco $
exp{2ni(hu’
f x(1-
i
0.5y)f,,
+ (1 -x)(1 f xyf,,
+ ku’ + Zw’)} +
exp{2ni(hu’
- 0.5y)fc0
+ ku’ + Zw’)} +
2 exp{ BA(hu” + ku” + Iw”)} +
$ exp{ 2S(hu” + ku” + IW”)}
(3)
TABLE 2 The distribution of atoms in the EuCoz-EuAl2 compounds
intermetallic
Compound
ErAlz (mol.%) in ErCoz
R (%I
Y
E~%&o.G
30 50 60 70
26.7 23.2 27.2 24.5
0.5 1.6 0.6 0.8
E*o,.oAh.o BrCoo.8Al1.2 BrCoo.6Al1.4
where y expresses the occupancy of the cobalt atoms in 2a sites, fEr, fc, and fAl are atomic scattering factors, u, u and w denote 4f sites [7] , u’, u’ and w’ denote 2a sites [7] and u”, u” and w” denote 6th sites [7]. The experimentally obtained values Fobs were fitted to the values of Fcalc using eqn. (3). The function R(y) was minimized for parameters y in the range y = 0 to y = 2 .O with intervals of 0 .Ol.The crystal structures of the compounds tested were determined from the minimum of the function R(y), i.e. the distribution of cobalt and aluminium atom8 between the 2a and 6h sites (when y = 0, aluminium atoms occupy the 2a sites; when y = 1, there is a statistical distribution of aluminium and cobalt atoms between the 2a and 6h sites; when y = 2, cobalt atoms occupy the 2a sites). The value8 of the parameters 3cand z from which the 4f and 6h sites were determined were taken as z = 0.062 and x = 0.830 from the MgZn,type structure. The results obtained are presented in Table 2.
4. Magnetic investigations The magnetization u and magnetic susceptibility x as functions of temperature Tin a magnetic field H were determined using the Faraday method. The Curie temperatures Tc of ErCo2 and ErC01.8A10.2were determined from magnetic investigations in low magnetic field8 up to 500 Oe and from the H/o = f(a2) plots. Figure 3 shows the thermal variation8 in the reciprocal susceptibility for the compounds tested (before thermal treatment). The thermal variations in the reciprocal susceptibility for ErCo2 and ErCo1.8A10.z before and after thermal treatment are shown in Figs. 4 and 5 and the results of the magnetic investigations are presented in Table 3.
5. Results and diBCU88ion X-ray investigations of the degree of ordering in the crystal structures of ErCo2-ErAl, compounds indicated that in compounds with the Cl5 crystal structure an exchange of atoms between the 8a and 16d site8 takes
Fig. 3. The thermal variation in the reciprocal susceptibility for ErCoa-ErAlz compounds (before thermal treatment): curve 1, ErCoz; curve 2, ErCo1_8Al02 ; curve 3, ErCo1.4Al0.~ ; curve 4, ErCoo.aAll.2; curve 5, ErCo0.5A11.5; curve 6, ErCo,-,sAl1.8; curve 7, ErA12.
place (Fig. 1). For ErCoz this exchange is approximately 1 atom per unit cell, whereas for ErC0iV8A10.zonly 0.5 cobalt atoms move to 8a sites. Compounds rich in aluminium have an ordered crystal structure. After thermal treatment, only ErCoz compounds exhibit “ideal” ordering of the crystal structure with erbium atoms in 8a sites and cobalt atoms in 16d sites. X-ray investigations of the compound with the hexagonal Cl4 structure showed that only the ErCoAI compound contains alternate magnetic layers. In this compound, cobalt atoms occupy 2a sites and together with erbium atoms occupy the 4f sites, forming the magnetic layers. For the remaining ErCoz-ErAlz compounds with the hexagonal structure, the cobalt and aluminium atoms are statistically distributed between 2a and 6h sites. The alternating magnetic and non-magnetic layer arrangement may be the cause of the anomalously large saturation magnetic moment found for ErCoAl [31* The magnetic tests on ErCoz-ErAlz compounds showed that those with x < 1 do not obey the Curie-Weiss law. The shape of the dependence x-l(T) for compounds with 3t< 1 indicates that the cobalt atoms have a non-zero magnetic moment and exhibit strong exchange interactions between erbium and cobalt. Compound8 with x > 1 obey the Curie-Weiss law. This fact plus the values of the effective magnetic moments of these compounds (Table 3)
11
01
SO
250
200
150
100
XXI
TPKI
4. The thermal variation in reciprocal susceptibility for ErCo2 : 0, before thermal treatment; 0, after thermal treatment.
Fig.
20-
15-
IO-
50
100
150
200
250
3L-OTl"K1
Fig. 5. The thermal variation in the reciprocal susceptibility for ErC01,8A&-,.~: 0, before thermal treatment; 0, after thermal treatment.
indicate that the cobalt atoms are non-ma~etic. This result is in agreement with earlier results reported by Oesterreicher [ 31. Magnetic investigations of the compounds ErCo2 and ErCo1.sAlo.2 with the Cl5 structure showed that their magnetic properties depend on the
12 TABLE 3 Magnetic data for ErCoz-ErA12 Compound
ErAtz (mol.%) in ErCo,
intermetallic compounds Effective parum~e~c moment peff
0
Cl5
Does not obey the CurieWeiss law
101.5 ? 0.5 40 [31
Ec"l.8A10.2
10
Cl5
Does not obey the CurieWeiss law
115.5 * 0.5 66 [31
B~o1.4Alo.6
30
Cl4
Does not obey the CurieWeiss law
E~"0.6A11.4
70
Cl4
9.0
ES02
Weiss constant f3 (Q
Curie temperature
Type of Laues phase
14
75
Cl4
9.2
14
E~oo.,All.~
90
Cl5
9.4
24
100
Cl5
9.0 9.56 f3]
40 16
ErAI
60 24 [33 26 131
9.3 [3] Efloo.5A11.5
Tc
(K)
22 r31
E31
degree of ordering in the crystal structure. It was found that these compounds both before and after thermal treatment do not obey the CurieWeiss law. The lack of agreement between our results and those given in ref. 3 could be due to the fact that our investigations were performed in a considerably wider temperature range. The ordered and disordered phases of ErCo2 and ErCo1.sAlo.2 compounds were found to have different Curie temperatures and qualitative variations in the x-l (2’) dependence (Fig. 4). The Curie temperatures of ErCoa and ErCo1sA10.2 before thermal treatment were Tc = 101.5 K and Tc = 115.5 K respectively and after thermal treatment Tc = 40 K and Tc = 123 K respectively. In ErCo2 we distinguish the Er-Er and Co-Co magnetic ~tera~tions, which in the 8a and 16d sub~tti~e are ferromagnetic, and the Er-Co magnetic interaction between sublattice 8a and 16d sites which leads to the antiferromagnetic alignment of the magnetic moment of the erbium and cobalt atoms. The passage of cobalt atoms to $a sites in the disordered structure of ErCo, has the effect of enhancing the Co-Co interaction, thus leading to an increase in the Curie temperature. For ErCo1.8Alo.2 no differences in ordering in the crystal structure before and after annealing were found. However, the observed difference in Curie temperature (10 K) suggests that such structure changes may take place but owing to the small resolving power of the X-ray method (about 1%) they may remain undetected.
13
Acknowledgments The authors would like to express their sincere thanks to Professor A. Chelkowski for helpful discussions. The work was partly supported by the Institute of Physics, Polish Academy of Sciences.
References 1 2 3 4 5
H. Oesterreicher, J. Less-Common Met., 33 (1973) 25. H. Oesterreicher and W. E. Wallace, J. Less-Common Met., 13 (1967) 91. H. Oesterreicher, L. M. Corliss and J. M. Hastings, J. Appl. Phys., 41 (1970) 2326. H. Oesterreicher, J. Appl. Phys., 42 (5) (1971) 137. A. Slebarski, W. Zarek, D. Konopka, A. Winiarska, E. Derejczyk and A. Chelkowski, Znt. Conf. on Physics of Transition Metals, Toronto, 1977, in Inst. Phys. Conf. Ser., 39 (1978) Chap. 6. 6 D. Konopka and W. Zarek, 11 th Znt. Congr. of Crystallography, Warsaw, August 3 - 12, 1978, Collected Abstracts 13.4 - 13.5. 7 N. F. M. Henry and K. Lonsdale (eds.), International Tables for X-ray Crystallography, Vol. I, Kynoch, Birmingham, 1963.