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A 155Gd Mössbauer spectral study and magnetic properties of Gd2Fe17 − xGax compounds
ELSEVIER
Journal
of Alloys and Compounds
232 (1996) 90-94
A 15’Gd Miissbauer spectral study and magnetic properties of GdPe,,-x Ga, compounds D.P. MiddletonaTb, D.A. van de Straatb, R.C. Thielb, K.H.J. BuschowC “Philips Research Laboratories 5656 AA Eindhoven, Netherlands bKamerlingh Onnes Laboratory, Leiden University, 2300 RA Leiden, Netherlands ‘Van der Waals-Zeeman Laboratory, University of Amsterdam, 1018 XE Amsterdam, Netherlands
Received 13 March 1995; in final form 29 May 1995
Abstract
We have investigated the effect of Ga substitution on the magnetic properties and the lssGd MGssbauer spectra of Gd,Fe I,_xGax solid solutions. An X-ray diffraction study indicates that the compounds adopt the Th2Zn17 type of structure. The substitution of Ga for Fe in Gd,Fe,, leads to an increase of the unit cell volume. The c-axis lattice parameter increases linearly from x = 0 to 6 and then decreases between n = 7 and 8. Magnetization studies indicate that the Curie temperature increases from 516 K for Gd,Fe,, to 601 K for Gd,Fe,,Ga,, and then decreases with further increase in x. The magnitude of the electric field gradient was derived from the quadrupole splitting of the “‘Gd Miissbauer spectra. The variation in the hypefine field across the series has been attributed to a gradual reduction in the transfered hyperfine field due to the Fe moments. The variation in the isomer shift was shown to be in agreement with model predictions. Keywords:
Rare-earth compounds; Magnetic properties; “‘Gd MGssbauer spectroscopy
1.Introduction Interstitially and substitutionally modified Fe-based rare-earth (R) transition metal intermetallic compounds have been shown to lead to good permanent magnet properties [1,2], which distinguishes them from their analogous binary compounds, all of which are unsuitable for magnetic applications due to their low Curie temperatures. The interstitially modified Sm,Fe,,(N, C), compounds, in particular, have good magnetic properties, such as high-energy product and Curie temperature, but they are difficult to process as permanent magnets. Therefore, we have been searching for alternative routes to increase the Curie temperature by replacing some of the iron by a nonmagnetic element, e.g. Al, Ga, or Si. Previous investigations have shown that Al substitutional solid solutions of R2Fe17 compounds lead to an increase of the Curie temperature and to an expansion of the lattice [3-81. There is a Curie temperature enhancement in the range of 0 s x c 5 with a maximum at x = 3.5. A similar optimum in the Curie temperature has been observed in the Ga substituted 0925~8388/%/$15.00
0 19%
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Elsevier Science S.A. All rights reserved
R,Fe,, compounds [9,10]. A mean-field analysis of the Curie temperature and high-field magnetization [lo] has shown that the maximum in Curie temperature is accompanied by a maximum in the Fe-Fe exchange interaction constants. This is a combined result of preferential substitution of Ga into distinct Fe sites as well as of an increase in lattice volume, overcompensating the effect of magnetic dilution by the nonmagnetic Ga atoms. It is generally believed that the small 6c-6c or ‘dumbbell’-site bond lengths are responsible for the low Curie temperature in R2Fe1, compounds, because they may involve antiferromagnetic exchange interactions. We have previously reported in a magnetic, neutron and Fe-M(issbauer study [9], that the initial enhancement in the Curie temperature in the Tb,Fe,,_,Ga, compounds is not a result of the removal of Fe 6c atoms by substitution of Ga into these sites. From our point of view, the increase in 9d-18h and 18h-18h Fe bond lengths may be more important for the enhancement of the Curie temperature. Another interesting feature of the Ga substitution in Tb2Fe17-x Ga, is the change of the easy axis of magnetization from basal to axial with increasing
D.P. Middleton et al. I Journal of Alloys and Compounds
gallium content. The total magnetocrystalline anisotropy constant is the sum of the first-order anisotropy constants K1, of the R and Fe sublattices, K, = K, [Fe] + K, [RI. For the R,Fe,, compounds, F, [Fe] is negative [2]. The R sublattice anisotropy can be expressed in lowest order by the crystal field terms: K,[R] = -(3/2)aj(r~,)(O~)A~, where cu, is the second-order Stevens’ factor, 0: the corresponding operator equivalent and Ai the second-order crystalfield parameter. The latter is predominately determined by the R valence electron asphericity [ll] and can be viewed as an electric field gradient at the 4f site. Depending on the R element considered (and hence the sign of aJ) and the sign and magnitude of Ai, K, can be positive, leading to uniaxial anisotropy. A similar, but not identical, electric field gradient is experienced by the Gd nuclei in isostructural compounds, and can be observed as the quadrupole interaction in “‘Gd Mossbauer spectroscopy. The 4f moment of Gd does not contribute to the magnetocrystalline anisotropy, due to the absence of an orbital moment (L = 0). Therefore, we have extended our previous investigations of the Tb,Fe,,_,Ga, system, to Ga, solid solutions, and focused our attenGd,Fe,,-, tion on the magnetic properties, as well as on the changes in hyperfine parameters observed by means of 155Gd Mossbauer spectroscopy.
2. Experimental methods The samples were prepared from 99.9% pure elements by arc melting in an argon atmosphere. After arc melting, the samples were wrapped in Ta foil and were vacuum annealed in quartz tubes at 900°C for 2 weeks. The samples were investigated by powder Xray diffraction with Cu Ka radiation on a Philips PW 1800/10 X-ray diffractometer equipped with a single crystal monochromator. The magnetic measurements were performed on 5 T field-cooled free particle samples, on a SQUID magnetometer between 5 and 350K, and on a Faraday magnetometer between 350 and 1000 K. The Curie temperatures were determined in small magnetic fields of 0.1 T by plotting the squared magnetization vs. temperature and extrapolating the steepest part of the curves to zero magnetization. Values of the saturation magnetization at 5.0 K were obtained from the magnetic isotherms by extrapolating the M( l/B) curves to l/B=O. “‘Gd Mossbauer spectra of Gd,Fe,,_,Ga, compounds were measured on a spectrometer described in detail elsewhere [12], using the 86.5 keV resonance of ‘55Gd. The source was neutron irradiated SmPd,, for which we employed Sm enriched to 98% in ‘54Sm. The spectra have been analysed by means of a least
232 (1996) N-94
91
squares fitting procedure that involved diagonalization of the full nuclear Hamiltonian and use of a transmission integral. The independently refined variables considered in the fitting procedure are the isomer shift (IS), the effective hyperfine field Heff, and the quadrupole splitting (QS). From the latter quantity we obtained the electric field gradient tensor element V,, via the relation: QS = +eQV,,(3 cos20 - 1) using the value Q = 1.30 X 10e2’ m* given by Tanaka et al. [13]. The angle 0 between H,,, and the c-axis was kept at 90” or 0”. The line width of the absorber and source were constrained for the transmission integral to 0.25 mm s-‘.
3. Results and discussion 3.1. Crystallographic and magnetic properties X-ray diffraction of Gd,Fe,,_,Ga, showed these samples to be approximately single phase, with the rhombohedral Th,Zn,, type of structure. The samples contained a few percent of elemental Fe as an impurity phase for low Ga concentrations (x c 3). The lattice constants were derived from the diffraction diagrams of Gd,Fe,,_, Ga, (Table l), and change considerably with increasing Ga content. The c-axis parameter increases almost linearly with a slope of 0.043 A per Ga from x = 1 to x = 5, remains constant at x = 5 and 6, but decreases slightly at x = 8. This behaviour is very similar to that found for Tb,Fe,,_,Ga,. The a-axis increase linearly with a slope of 0.023 A per Ga between x = 1 and 4, and linearly with a slope of 0.043 A per Ga for x between 5 and 8. As a ;esult, the unit cell volume increases linearly by, 6.7 A per Ga for x between 1 and 4, and by 8.5 A per Ga (Fig. 1) for larger Ga concentrations. The c/a lattice parameter ratio is very similar to the c/a ratio lattice parameter ratio of the Tb,Fe,,_,Ga, system (Fig. 2). Table 1 Lattice parameters of Gd,Fe,,_,Ga, tion at room temperature, saturation netic moment Nominal x
Ga content
0” 1 2 3 4 5 6 7 8 a According
to Ref. [14].
as measured by X-ray diffracmagnetization, and Fe mag-
a-axis
c-axis
(fQ
(A)
$Zm’kg--‘)
Z,
8.538 8.564 8.585 8.608 8.631 8.658 8.680 8.738 8.786
12.431 12.458 12.495 12.539 12.586 12.627 12.653 12.652 12.650
93.3 80.3 69.4 62.2 42.3 22.6 12.6
2.06 2.03 2.04 2.06 2.03 1.89 1.89
D.P. Middleton et al. I Journal of Alloys and Compounds 232 (1996) 90-94
92
840
-
830
-
820
-
_J
0 >
0
2
4 Go
6
10
8
content
(x)
0
Fig. 1. The room temperature unit cell volume of the rhombohedral Gd,Fe,,_,Ga, solid solutions as a function of Ga content. The unit cell volume at x = 0 is obtained from Ref. [ll].
i
1.455
1.450
1
2
3
4
5
6
I@ t-0 Fig. 3. Magnetic isotherms at 5.0 K of Gd,Fe,,_,Ga,, for x = 1 (top curve), 2, 3, 5, 7 and 8 (lowest curve). The solid line is a guide to the eye.
5o: 40
m 3 1.445
1.440 1 1.435 e 0
2
4
6
8
10
Ga content (x) Fig. 2. The room temperature c/a lattice parameter ratio of the rhombohedral Gd,Fe,,-, Ga, solid solutions as a function of Ga content. The solid line is a guide to the eye.
10
0 0
The magnetic isotherms at 5.0 K have been measured, and are shown in Fig. 3. From the isotherms, the saturation magnetization M, was derived (Table 1). Values of the saturation magnetization were obtained as is described in Section 2. The values of the Fe moment he (see Table 1) were derived from the corresponding values of M,, assuming a moment of 7,+ per Gd atom and an antiparallel coupling between the Gd sublattice magnetization and the transition metal sublattice magnetization. From the temperature dependence of the magnetization o(T) of Gd,Fe,,_,Ga,, as shown in Fig. 4, the Curie temperature was derived (Table 1). As in the case of Tb2Fe,7_xGax, gallium substitution leads to a maximum in the concentration dependence of the Curie temperature for Gd,Fe,,_,Ga, at about x = 4, and further decreases for increasing Ga concentrations (Fig. 5).
100
200
300
400
500
600
700
Temperature (K) Fig. 4. Temperature dependence of the magnetization u(T) of GD,Fe,,_,Ga, for x = 1 (upper curve), 5, 6 and 8 (lower curve), measured at an applied field p,,H = 0.1 T. The samples were fieldcooled.
At low Ga concentrations, x s 3, the increase in the Curie temperature can be explained by magnetovolume effects, which are counteracted by magnetic dilution. For large x values, the reduction with x of the Fe moments also has to be considered (see Table 1). 3.2. 15*Gd Miissbauer
effect study
The “‘Gd Mossbauer spectra measured at 4.2 K for the Gd,Fe,,_,Ga, compounds are shown in Fig. 6. The (full) curve through the data points represents a fit to the spectrum. The hyperfke parameters corre-
D.P. Middleton et al. I Journal of Alloys and Compounds
zz 2 1 F? g
232 (1996) 90-94
93
650
Table 2 Hyperfine parameters derived from fitting the “‘Gd spectra at 4.2 K for various Gd,Fe,,_xGa, compounds
600
Nominal Ga content 0”
550
1 2 3 4 5 6 7 8
500
Mossbauer
Vzz (lo*] Vm-‘)
F&G
1s
(T)
(mm so’)
(Bdeg)
4.4 5.4 5.7 5.5 5.5 5.2 4.9 4.8 4.5
21 14.2 11.7 8.5 6.9 6.2 7.2 8.1 9.1
0.27 0.25 0.29 0.32 0.35 0.37 0.39 0.42 0.43
90
90 90 90 90 90 90 90 90
a According to Ref. [7]. 2
4
8
6
10
Ga content (x) Fig. 5. The Curie temperature Gd,Fe,,_,Ga,.
as a function of the Ga content x for
sponding to these fits have been listed in Table 2. The concentration dependence of the isomer shift may be compared with a simple model based on chemical effects (charge transfer and intra-atomic s-d electron redistribution) and volume effects [1.5]. On the basis of this model, one expects that the ultimate value of the isomer shift that would be reached for x = 17 is 0.64 mm s-l. There is satisfactory agreement between this value and that obtained by extrapolation of the IS values (see fig. 7). The total hyperfine field can be decomposed into various contributions (see Ref. [7] and references cited therein):
peft= B,, + B,, + B”,’+ B:
(1)
where B,,is the contribution of core polarization. The contributions of the polarization of the conduction electrons originating from the polarizing effect of the on-site Gd-spin is given by B,,,that of the neigh-
98
F
0.6
-
i z .-
0.5
-
,,/’ ,/’ ,A ,,/’ ,.,’ _,/’ _,I. _,.’ ,_I’ ,,1’
.
0.2 0
-5-4-3-2-l
Velocity
1
2
3
4
5
0
(mm/s)
Fig. 6. “‘Gd M&batter spectra of Gd,Fe,,_,Ga, compounds at 4.2 K. The solid line through the data points represents a fit.
1 5 Ga
Fig. 7. Concentration Gd,Fe,,_,Ga,.
dependence
content
/
I
10
15 (x)
of the isomer
shift (IS) in
94
D.P. Middleton
et al. I Journal of Alloys
bouring Gd sites by B:, and that due to neighbouring iron moments, by B fp. B,, = -32.2 T, Bg = +50.2 T, and For Gd,Fe,,, the sum of the Gd contributions B,, + Bz = +4.0 T. The hyperhne field is mainly determined by the 6s conduction electron polarization, and it is clear that the effect resulting from the iron moments will gradually disappear. The values of the hyperhne field IH,,,I are listed in Table 2. The values and sign of the electric field gradient are an indication of the magnitude and sign of the secondorder crystal field parameter A:. In the Tb,Fe,,_,Ga, compounds it has been shown that, for high Ga concentrations, the easy axis of magnetization direction changes from planar to axial. For the Gd,Fe,,_,Ga, solid solutions studied, no reliable indication has been obtained about a sign reversal of the electric field gradient across the series.
4. Conclusions In conclusion, we have studied the magnetic properties of the Gd,Fe,,_,Ga, solid solutions. From the quadrupole splitting of the “‘Gd Mossbauer spectra, we derived the magnitude of the electric field gradient V’, at the nuclear site. The variation of the Gd hyperfine field was explained in terms of a reduced transferred hyperfine field contribution resulting from the Fe moments. The isomer shift was found to be in agreement with previous model predictions.
Acknowledgements D.P.M. and D.A.S. would like to acknowledge the stimulating support of L.J. de Jongh and to thank the
and Compounds
232 (1996) 90-94
‘Stichting voor Fundamenteel Onderzoek der Materie’ (FOM), which is financially supported by the ‘Nederlandse Organisatie voor Wetenschappelijk Onderzoek’ (NWO).
References [l] J.F. Herb& Rev. Mod. Phys., 63 (1991) 819. [2] K.H.J. Buschow, Rep. Prog. Phys., 54 (1991) 1123. [3] D. Plusa, R. Pfranger and B. Wyslocki, J. Less-Common
Met.,
99 (1984) 87. [4] D. Plusa, R. Pfranger, B. Wyslocki and T. Mydlara, J. LessCommon Met., 99 (1986) 1. [5] T.H. Jacobs, K.H.J. Buschow, G.F. Zhou and F.R. de Boer, Physica B, 179 (1992) 177. [6] T.H. Jacobs, Ph.D. Thesis, Leiden University, 1992. [7] F.M. Mulder, R.C. Thiel and K.H.J. Buschow, J. Alloys Comp., 197 (1993) 25. [8] G.J. Long, G.K. Marasinghe, S. Mishra, O.A. Pringle, Z. Hu,
W.B. Yelon, D.P. Middleton, K.H.J. Buschow and F. Grandjean, J. Appl. Phys. 76 (1994) 5383. [9] Z. Hu, W.B. Yelon, S. Mishra, G.J. Long, O.A. Pringle, D.P. Middleton, K.H.J. Buschow and F. Grandjean, J. Appl. Phys., 76 (1994) 443.
[lo] Y.H. Gao, N. Tang, X.P. Zhong, J.L. Wang, W.Z. Li, W.D. Gin, F.M. Yang, D.M. Zhang and F.R. de Boer, J. Magn. Magn. Mater.,
137 (1994) 275.
[ll] R. Coehoorn
and K.H.J. Buschow, J. Appl.
Phys.,
69 (1991)
5590. [12] H. de Graaf, Ph.D.
Thesis, Leiden University, 1982. [13] Y. Tanaka, O.B. Laubacher, R.M. Stoffen, E.B. Shera, H.D. Wohlfarth and M.V. Hoehn, Phys. Lett. B, 108 (1982) 8. [14] K.H.J. Buschow, Rep. Prog. Phys., 40 (1977) 1179. [15] J.W.C. de Vries, R.C. Thiel and K.H.J. Buschow, J. Phys. F:, (1985) 1413.
Journal
of Alloys and Compounds
232 (1996) 90-94
A 15’Gd Miissbauer spectral study and magnetic properties of GdPe,,-x Ga, compounds D.P. MiddletonaTb, D.A. van de Straatb, R.C. Thielb, K.H.J. BuschowC “Philips Research Laboratories 5656 AA Eindhoven, Netherlands bKamerlingh Onnes Laboratory, Leiden University, 2300 RA Leiden, Netherlands ‘Van der Waals-Zeeman Laboratory, University of Amsterdam, 1018 XE Amsterdam, Netherlands
Received 13 March 1995; in final form 29 May 1995
Abstract
We have investigated the effect of Ga substitution on the magnetic properties and the lssGd MGssbauer spectra of Gd,Fe I,_xGax solid solutions. An X-ray diffraction study indicates that the compounds adopt the Th2Zn17 type of structure. The substitution of Ga for Fe in Gd,Fe,, leads to an increase of the unit cell volume. The c-axis lattice parameter increases linearly from x = 0 to 6 and then decreases between n = 7 and 8. Magnetization studies indicate that the Curie temperature increases from 516 K for Gd,Fe,, to 601 K for Gd,Fe,,Ga,, and then decreases with further increase in x. The magnitude of the electric field gradient was derived from the quadrupole splitting of the “‘Gd Miissbauer spectra. The variation in the hypefine field across the series has been attributed to a gradual reduction in the transfered hyperfine field due to the Fe moments. The variation in the isomer shift was shown to be in agreement with model predictions. Keywords:
Rare-earth compounds; Magnetic properties; “‘Gd MGssbauer spectroscopy
1.Introduction Interstitially and substitutionally modified Fe-based rare-earth (R) transition metal intermetallic compounds have been shown to lead to good permanent magnet properties [1,2], which distinguishes them from their analogous binary compounds, all of which are unsuitable for magnetic applications due to their low Curie temperatures. The interstitially modified Sm,Fe,,(N, C), compounds, in particular, have good magnetic properties, such as high-energy product and Curie temperature, but they are difficult to process as permanent magnets. Therefore, we have been searching for alternative routes to increase the Curie temperature by replacing some of the iron by a nonmagnetic element, e.g. Al, Ga, or Si. Previous investigations have shown that Al substitutional solid solutions of R2Fe17 compounds lead to an increase of the Curie temperature and to an expansion of the lattice [3-81. There is a Curie temperature enhancement in the range of 0 s x c 5 with a maximum at x = 3.5. A similar optimum in the Curie temperature has been observed in the Ga substituted 0925~8388/%/$15.00
0 19%
SSDZ 0925-8388(95)01932-4
Elsevier Science S.A. All rights reserved
R,Fe,, compounds [9,10]. A mean-field analysis of the Curie temperature and high-field magnetization [lo] has shown that the maximum in Curie temperature is accompanied by a maximum in the Fe-Fe exchange interaction constants. This is a combined result of preferential substitution of Ga into distinct Fe sites as well as of an increase in lattice volume, overcompensating the effect of magnetic dilution by the nonmagnetic Ga atoms. It is generally believed that the small 6c-6c or ‘dumbbell’-site bond lengths are responsible for the low Curie temperature in R2Fe1, compounds, because they may involve antiferromagnetic exchange interactions. We have previously reported in a magnetic, neutron and Fe-M(issbauer study [9], that the initial enhancement in the Curie temperature in the Tb,Fe,,_,Ga, compounds is not a result of the removal of Fe 6c atoms by substitution of Ga into these sites. From our point of view, the increase in 9d-18h and 18h-18h Fe bond lengths may be more important for the enhancement of the Curie temperature. Another interesting feature of the Ga substitution in Tb2Fe17-x Ga, is the change of the easy axis of magnetization from basal to axial with increasing
D.P. Middleton et al. I Journal of Alloys and Compounds
gallium content. The total magnetocrystalline anisotropy constant is the sum of the first-order anisotropy constants K1, of the R and Fe sublattices, K, = K, [Fe] + K, [RI. For the R,Fe,, compounds, F, [Fe] is negative [2]. The R sublattice anisotropy can be expressed in lowest order by the crystal field terms: K,[R] = -(3/2)aj(r~,)(O~)A~, where cu, is the second-order Stevens’ factor, 0: the corresponding operator equivalent and Ai the second-order crystalfield parameter. The latter is predominately determined by the R valence electron asphericity [ll] and can be viewed as an electric field gradient at the 4f site. Depending on the R element considered (and hence the sign of aJ) and the sign and magnitude of Ai, K, can be positive, leading to uniaxial anisotropy. A similar, but not identical, electric field gradient is experienced by the Gd nuclei in isostructural compounds, and can be observed as the quadrupole interaction in “‘Gd Mossbauer spectroscopy. The 4f moment of Gd does not contribute to the magnetocrystalline anisotropy, due to the absence of an orbital moment (L = 0). Therefore, we have extended our previous investigations of the Tb,Fe,,_,Ga, system, to Ga, solid solutions, and focused our attenGd,Fe,,-, tion on the magnetic properties, as well as on the changes in hyperfine parameters observed by means of 155Gd Mossbauer spectroscopy.
2. Experimental methods The samples were prepared from 99.9% pure elements by arc melting in an argon atmosphere. After arc melting, the samples were wrapped in Ta foil and were vacuum annealed in quartz tubes at 900°C for 2 weeks. The samples were investigated by powder Xray diffraction with Cu Ka radiation on a Philips PW 1800/10 X-ray diffractometer equipped with a single crystal monochromator. The magnetic measurements were performed on 5 T field-cooled free particle samples, on a SQUID magnetometer between 5 and 350K, and on a Faraday magnetometer between 350 and 1000 K. The Curie temperatures were determined in small magnetic fields of 0.1 T by plotting the squared magnetization vs. temperature and extrapolating the steepest part of the curves to zero magnetization. Values of the saturation magnetization at 5.0 K were obtained from the magnetic isotherms by extrapolating the M( l/B) curves to l/B=O. “‘Gd Mossbauer spectra of Gd,Fe,,_,Ga, compounds were measured on a spectrometer described in detail elsewhere [12], using the 86.5 keV resonance of ‘55Gd. The source was neutron irradiated SmPd,, for which we employed Sm enriched to 98% in ‘54Sm. The spectra have been analysed by means of a least
232 (1996) N-94
91
squares fitting procedure that involved diagonalization of the full nuclear Hamiltonian and use of a transmission integral. The independently refined variables considered in the fitting procedure are the isomer shift (IS), the effective hyperfine field Heff, and the quadrupole splitting (QS). From the latter quantity we obtained the electric field gradient tensor element V,, via the relation: QS = +eQV,,(3 cos20 - 1) using the value Q = 1.30 X 10e2’ m* given by Tanaka et al. [13]. The angle 0 between H,,, and the c-axis was kept at 90” or 0”. The line width of the absorber and source were constrained for the transmission integral to 0.25 mm s-‘.
3. Results and discussion 3.1. Crystallographic and magnetic properties X-ray diffraction of Gd,Fe,,_,Ga, showed these samples to be approximately single phase, with the rhombohedral Th,Zn,, type of structure. The samples contained a few percent of elemental Fe as an impurity phase for low Ga concentrations (x c 3). The lattice constants were derived from the diffraction diagrams of Gd,Fe,,_, Ga, (Table l), and change considerably with increasing Ga content. The c-axis parameter increases almost linearly with a slope of 0.043 A per Ga from x = 1 to x = 5, remains constant at x = 5 and 6, but decreases slightly at x = 8. This behaviour is very similar to that found for Tb,Fe,,_,Ga,. The a-axis increase linearly with a slope of 0.023 A per Ga between x = 1 and 4, and linearly with a slope of 0.043 A per Ga for x between 5 and 8. As a ;esult, the unit cell volume increases linearly by, 6.7 A per Ga for x between 1 and 4, and by 8.5 A per Ga (Fig. 1) for larger Ga concentrations. The c/a lattice parameter ratio is very similar to the c/a ratio lattice parameter ratio of the Tb,Fe,,_,Ga, system (Fig. 2). Table 1 Lattice parameters of Gd,Fe,,_,Ga, tion at room temperature, saturation netic moment Nominal x
Ga content
0” 1 2 3 4 5 6 7 8 a According
to Ref. [14].
as measured by X-ray diffracmagnetization, and Fe mag-
a-axis
c-axis
(fQ
(A)
$Zm’kg--‘)
Z,
8.538 8.564 8.585 8.608 8.631 8.658 8.680 8.738 8.786
12.431 12.458 12.495 12.539 12.586 12.627 12.653 12.652 12.650
93.3 80.3 69.4 62.2 42.3 22.6 12.6
2.06 2.03 2.04 2.06 2.03 1.89 1.89
D.P. Middleton et al. I Journal of Alloys and Compounds 232 (1996) 90-94
92
840
-
830
-
820
-
_J
0 >
0
2
4 Go
6
10
8
content
(x)
0
Fig. 1. The room temperature unit cell volume of the rhombohedral Gd,Fe,,_,Ga, solid solutions as a function of Ga content. The unit cell volume at x = 0 is obtained from Ref. [ll].
i
1.455
1.450
1
2
3
4
5
6
I@ t-0 Fig. 3. Magnetic isotherms at 5.0 K of Gd,Fe,,_,Ga,, for x = 1 (top curve), 2, 3, 5, 7 and 8 (lowest curve). The solid line is a guide to the eye.
5o: 40
m 3 1.445
1.440 1 1.435 e 0
2
4
6
8
10
Ga content (x) Fig. 2. The room temperature c/a lattice parameter ratio of the rhombohedral Gd,Fe,,-, Ga, solid solutions as a function of Ga content. The solid line is a guide to the eye.
10
0 0
The magnetic isotherms at 5.0 K have been measured, and are shown in Fig. 3. From the isotherms, the saturation magnetization M, was derived (Table 1). Values of the saturation magnetization were obtained as is described in Section 2. The values of the Fe moment he (see Table 1) were derived from the corresponding values of M,, assuming a moment of 7,+ per Gd atom and an antiparallel coupling between the Gd sublattice magnetization and the transition metal sublattice magnetization. From the temperature dependence of the magnetization o(T) of Gd,Fe,,_,Ga,, as shown in Fig. 4, the Curie temperature was derived (Table 1). As in the case of Tb2Fe,7_xGax, gallium substitution leads to a maximum in the concentration dependence of the Curie temperature for Gd,Fe,,_,Ga, at about x = 4, and further decreases for increasing Ga concentrations (Fig. 5).
100
200
300
400
500
600
700
Temperature (K) Fig. 4. Temperature dependence of the magnetization u(T) of GD,Fe,,_,Ga, for x = 1 (upper curve), 5, 6 and 8 (lower curve), measured at an applied field p,,H = 0.1 T. The samples were fieldcooled.
At low Ga concentrations, x s 3, the increase in the Curie temperature can be explained by magnetovolume effects, which are counteracted by magnetic dilution. For large x values, the reduction with x of the Fe moments also has to be considered (see Table 1). 3.2. 15*Gd Miissbauer
effect study
The “‘Gd Mossbauer spectra measured at 4.2 K for the Gd,Fe,,_,Ga, compounds are shown in Fig. 6. The (full) curve through the data points represents a fit to the spectrum. The hyperfke parameters corre-
D.P. Middleton et al. I Journal of Alloys and Compounds
zz 2 1 F? g
232 (1996) 90-94
93
650
Table 2 Hyperfine parameters derived from fitting the “‘Gd spectra at 4.2 K for various Gd,Fe,,_xGa, compounds
600
Nominal Ga content 0”
550
1 2 3 4 5 6 7 8
500
Mossbauer
Vzz (lo*] Vm-‘)
F&G
1s
(T)
(mm so’)
(Bdeg)
4.4 5.4 5.7 5.5 5.5 5.2 4.9 4.8 4.5
21 14.2 11.7 8.5 6.9 6.2 7.2 8.1 9.1
0.27 0.25 0.29 0.32 0.35 0.37 0.39 0.42 0.43
90
90 90 90 90 90 90 90 90
a According to Ref. [7]. 2
4
8
6
10
Ga content (x) Fig. 5. The Curie temperature Gd,Fe,,_,Ga,.
as a function of the Ga content x for
sponding to these fits have been listed in Table 2. The concentration dependence of the isomer shift may be compared with a simple model based on chemical effects (charge transfer and intra-atomic s-d electron redistribution) and volume effects [1.5]. On the basis of this model, one expects that the ultimate value of the isomer shift that would be reached for x = 17 is 0.64 mm s-l. There is satisfactory agreement between this value and that obtained by extrapolation of the IS values (see fig. 7). The total hyperfine field can be decomposed into various contributions (see Ref. [7] and references cited therein):
peft= B,, + B,, + B”,’+ B:
(1)
where B,,is the contribution of core polarization. The contributions of the polarization of the conduction electrons originating from the polarizing effect of the on-site Gd-spin is given by B,,,that of the neigh-
98
F
0.6
-
i z .-
0.5
-
,,/’ ,/’ ,A ,,/’ ,.,’ _,/’ _,I. _,.’ ,_I’ ,,1’
.
0.2 0
-5-4-3-2-l
Velocity
1
2
3
4
5
0
(mm/s)
Fig. 6. “‘Gd M&batter spectra of Gd,Fe,,_,Ga, compounds at 4.2 K. The solid line through the data points represents a fit.
1 5 Ga
Fig. 7. Concentration Gd,Fe,,_,Ga,.
dependence
content
/
I
10
15 (x)
of the isomer
shift (IS) in
94
D.P. Middleton
et al. I Journal of Alloys
bouring Gd sites by B:, and that due to neighbouring iron moments, by B fp. B,, = -32.2 T, Bg = +50.2 T, and For Gd,Fe,,, the sum of the Gd contributions B,, + Bz = +4.0 T. The hyperhne field is mainly determined by the 6s conduction electron polarization, and it is clear that the effect resulting from the iron moments will gradually disappear. The values of the hyperhne field IH,,,I are listed in Table 2. The values and sign of the electric field gradient are an indication of the magnitude and sign of the secondorder crystal field parameter A:. In the Tb,Fe,,_,Ga, compounds it has been shown that, for high Ga concentrations, the easy axis of magnetization direction changes from planar to axial. For the Gd,Fe,,_,Ga, solid solutions studied, no reliable indication has been obtained about a sign reversal of the electric field gradient across the series.
4. Conclusions In conclusion, we have studied the magnetic properties of the Gd,Fe,,_,Ga, solid solutions. From the quadrupole splitting of the “‘Gd Mossbauer spectra, we derived the magnitude of the electric field gradient V’, at the nuclear site. The variation of the Gd hyperfine field was explained in terms of a reduced transferred hyperfine field contribution resulting from the Fe moments. The isomer shift was found to be in agreement with previous model predictions.
Acknowledgements D.P.M. and D.A.S. would like to acknowledge the stimulating support of L.J. de Jongh and to thank the
and Compounds
232 (1996) 90-94
‘Stichting voor Fundamenteel Onderzoek der Materie’ (FOM), which is financially supported by the ‘Nederlandse Organisatie voor Wetenschappelijk Onderzoek’ (NWO).
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