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Magnetic properties of RCo4 − xFexB compounds (R ≡ Gd, Dy)
Journal
of the Less-Common
Metals,
153 (1989) L21 - L24
L21
Letter
Magnetic properties Z. DRZAZGA Instytut
Fizyki,
of RCo4 _ xFe,B compounds
(R s Gd, Dy)
and A. WINIARSKA Uniwersytet
Slaski,
Katowice
(Poland)
F. STEIN II. Physikalisches
Institut,
Universitbl
zu Kiiln (F. R. G.)
(Received March 19,1989)
1. Introduction Rare earth transition metal compounds based on boron, unlike RT5, can be obtained with T = Fe. The introduction of iron atoms into RCo4B markedly influences the magnetic properties. In this paper we report the results of crystal and magnetic studies on GdCo4 ~ x Fe, B and DyCo, _ x Fe, B compounds. 2. Experimental details The polycrystalline RCo4 _ ,Fe,B compounds, where R - Gd, Dy and x = 0, 2, were prepared by arc furnace melting under an argon atmosphere. The rare earths used had a purity of 99.9% and the iron, cobalt and boron had a purity of 99.999%. The samples were inverted and remelted several times to ensure homogeneity. The phase purity was verified from X-ray powder patterns obtained using Fe KCYradiation. The magnetization was measured in the temperature range 4.2 < T< 800 K in an applied field up to 1.8 T. Anisotropy measurements were carried out with a torsion magnetometer in the temperature range 77 - 350 K. For this purpose fine powder particles (r#~< 10 pm) of samples were aligned in a magnetic field and fixed in that position using resin. 3. Results and discussion Table 1 and Fig. 1 present the structural and magnetic parameters for the compounds investigated. The RCo4 _ x Fe,B compounds crystallize in the hexagonal CeCo4B-type structure [l]. This structure is derived from the CaCu,-type structure by ordered substitutions of boron in alternate 2c layers. However, a disordered structure for RFe4B was also reported [2, 31. Diffraction patterns of the RCo, .~xFe,B samples depend on the x parameter. Changes in the intensities of the (h, Iz, 21+ 1) reflections suggest a partial interchange of sites between the 3d atoms and the boron atoms in the iron-substituted compounds. 0022-5088/89/$3.50
0 Elsevier Sequoia/Printed in The Netherlands
L22 TABLE 1 Lattice parameters and magnetic properties of RCo4_,Fe,B Compound
a (nm)
GdCo4B GdCozFe*B DyCo4B DyCozFezB
c (nm)
0.5058 0.5104 0.5017 0.5052
0.6889 0.6924 0.6880 0.6884
Tc
505 770 420 650
(W
@B/unit)
(,&/unit)
KI ,,,VK) (lo6 J mP3)
2.8 1.2 5.9 4.0
1.0 1.45 1.0 1.5
0.5 -0.2 -2.3 -1.3
/As (4.2
KJ
/J G’d)
GdCo4_,Fe,B
L ,. x-2
2
z\=
.
j*.
\
” I”. I XX”
‘\
$&&x;*
“r;.“,x ,.“,
0
200
LOO
e
600
800
TemperatureiKl
Fig. 1. Magnetization us. temperature recorded at B = 0.4 T.
The values of the lattice parameters increase as cobalt atoms are partially replaced by iron atoms. A more pronounced variation in lattice parameters is observed in the base plane. The small values of spontaneous magnetization and a deep minimum on the curve of magnetization against temperature (Fig. 1) indicate an antiferromagnetic coupling between the rare earth and transition metal moments. The partial replacement of the cobalt atoms by iron atoms causes the compensation point to shift towards lower temperatures. The same trend was also observed in HoCo4 _ ,Fe,B and ErCo4 _ ,Fe,B [4]. The marked increase in the Curie temperature for the compounds containing iron is noteworthy (Table 1). The RCo,Fe2B compounds studied have Tc values about 200 K higher than the RCo,B compounds. Similar results were also reported for PrCo,_,Fe,B [5] and YCo4_.Fe,B [6]. The magnetic moment per formula unit in the compounds studied at 4.2 K is given in Table 1. The magnetic moment of the 3d sublattice can be
L23
estimated from the relation ~~(4.2 K) = p(R) - 4p(3d) assuming that the magnetic moment of the rare earth atoms equals their free-ion value. The magnetic moment of the cobalt atom in GdCoJ3 and DyCo4B is approximately equal to 1.0 ,.&a, as would be expected. Nevertheless, the average magnetic moment of the 3d ions in both of the substituted iron compounds is considerably higher (approximately 1.5 /~a). This value corresponds to the iron magnetic moment observed in RFe& [7, 81. This behaviour of 3d sublattice ma~etization in the samples studied can be interpreted in terms of the rigid band model assuming a situation similar to that in ThCos _ s Fe, [9]. The decrease in the 3d electron concentration with x lowers the Fermi level in such a way that both subbands become unsaturated and the density of states increases causing an increase of the total magnetic moment. At room temperature the X-ray analysis of the aligned powders showed base plane anisotropy for DyCo4 _ ,Fe,B and GdCo,Fe2B but not for GdCo,B. GdCo&, in which Gd3+ (s state) has a negligible anisotropy, shows an easy c-axis, i.e. the easy magnetization direction of the cobalt atoms is parallel to the c-axis, as in RCo, compounds. This has been confirmed by ls5Gd M&sbauer effect studies [lo]. It is noteworthy that the introduction of iron into GdCofi causes a change in the easy direction of magnetization at room temperature (K, eff < 0 for GdCo,Fe,). This behaviour of the magnetocrystalline anisotropy, due to iron substitution, can be explained when taking the anisotropy contributions from different crystallographic 3d ion sites into account. Generally, the cobalt atoms at 2c sites have a large positive contribution favouring the c-axis, unlike those at 3i sites. The iron atoms may be anticipated to give anisotropy contributions of opposite sign in the same crystallographic sites [ll]. A significant preference of iron for the 2c sites was reported [12]. As a consequence, an easy magnetization direction perpendicular to the c-axis occurs in GdCo,Fe,B. A similar behaviour of the magnetocrystalline anisotropy as a function of the iron concentration was observed in HoCo4_ %Fe,B [4] and YCo4 _ ,Fe,B [6]. These results emphasize the importance of transition metalanisotropy in the RCo4_ xFe,B systems. In the DyCo4_,Fe,B compounds both the dysprosium and the 3d sublattices exhibit magnetocrystalline anisotropy. The existence of an easy base plane in DyCo4B at room temperature would appear to be connected with the dominance of rare earth sublattice anisotropy (the Stevens coefficient a,(Dy) < 0). The effective anisotropy constant changes sign only near the compensation point and the easy direction of magnetization shows a tendency to tilt away from the c-axis. Nevertheless, there is no evidence of spin reorientation for DyCozFezB in the temperature range studied. This suggests that the anisotropy of the Fe-Co sublattice is more negative than that of the cobalt sublattice. It may be concluded that increasing iron content in the RCo4 _ ,Fe,B (R = Gd, Dy) compounds enhances the magnetic 3d sublattice moment and decreases the overall magnetoc~stalline anisotropy, causing the occurrence of an easy base plane in the iron concentration range studied.
L24
Acknowledgments Z.D. wishes to thank Professor G. Dietz for the use of the Foner magnetometer and Dr G. Chelkowska for kindly making it possible to perform measurements in the temperature range 300 - 800 K. This investigation was partly supported by the Polish Academy of Sciences. References 1 Yu. B. Kuz’ma and N. S. Bilonizhko, Sow. Phys. Crystallogr., 18 (1974) 447. 2 H. M. van Noort, D. B. de Mooij and K. H. Buschow, J. Less-Common Met., 111 (1985) 87. 3 G. Zouganelis, A. Kostikas, A. Simopoulos and D. Niarchos, J. Magn. Magn. Mater., 75 (1988) 91. 4 Z. Drzazga, A. Winiarska and T. Mydlarz, in C. Herget, H. Kronmiiller and R. Poerschke (eds.), Proc. 5th Int. Symp. on Magnetic Anisotropy in Coercive R-T Alloys, Bad Soden, 1987, p. 297. 5 S. Y. Jiang, W. E. Wallace and E. Burzo, J. Magn. Magn. Mater., 61 (1986) 257. 6 T. T. Dung, N. P. Thuy, N. M. Hong and T. D. Hien, Private communication. 7 S. Y. Jiang, W. E. Wallace and E. Burzo, J. Less-Common Met., 119 (1986) L5. 8 P. P. Vaishnava, C. W. Kimball, A. M. Umarji, S. K. Malik and G. K. Shenoy, J. Magn. Magn. Mater., 49 (1985) 286. 9 A. M. van Diepen, K. H. J. Buschow and J. S. van Wierinngen, J. Appl. Phys., 43 (1972) 645. 10 H. H. A. Smit, R. C. Thiel and K. H. J. Buschow, J. Phys. F, 18 (1988) 295. 11 J. J. M. France, N. P. Thuy and N. M. Hong, J. Magn. Magn. Mater., 72 (1988) 361. 12 Y. Gros, F. Hartmann-Boutron, C. Meyer, M. A. Fremy and P. Tenaud, J. Phys., C,8 Suppl. 12, t. 49 (1988) 574.
of the Less-Common
Metals,
153 (1989) L21 - L24
L21
Letter
Magnetic properties Z. DRZAZGA Instytut
Fizyki,
of RCo4 _ xFe,B compounds
(R s Gd, Dy)
and A. WINIARSKA Uniwersytet
Slaski,
Katowice
(Poland)
F. STEIN II. Physikalisches
Institut,
Universitbl
zu Kiiln (F. R. G.)
(Received March 19,1989)
1. Introduction Rare earth transition metal compounds based on boron, unlike RT5, can be obtained with T = Fe. The introduction of iron atoms into RCo4B markedly influences the magnetic properties. In this paper we report the results of crystal and magnetic studies on GdCo4 ~ x Fe, B and DyCo, _ x Fe, B compounds. 2. Experimental details The polycrystalline RCo4 _ ,Fe,B compounds, where R - Gd, Dy and x = 0, 2, were prepared by arc furnace melting under an argon atmosphere. The rare earths used had a purity of 99.9% and the iron, cobalt and boron had a purity of 99.999%. The samples were inverted and remelted several times to ensure homogeneity. The phase purity was verified from X-ray powder patterns obtained using Fe KCYradiation. The magnetization was measured in the temperature range 4.2 < T< 800 K in an applied field up to 1.8 T. Anisotropy measurements were carried out with a torsion magnetometer in the temperature range 77 - 350 K. For this purpose fine powder particles (r#~< 10 pm) of samples were aligned in a magnetic field and fixed in that position using resin. 3. Results and discussion Table 1 and Fig. 1 present the structural and magnetic parameters for the compounds investigated. The RCo4 _ x Fe,B compounds crystallize in the hexagonal CeCo4B-type structure [l]. This structure is derived from the CaCu,-type structure by ordered substitutions of boron in alternate 2c layers. However, a disordered structure for RFe4B was also reported [2, 31. Diffraction patterns of the RCo, .~xFe,B samples depend on the x parameter. Changes in the intensities of the (h, Iz, 21+ 1) reflections suggest a partial interchange of sites between the 3d atoms and the boron atoms in the iron-substituted compounds. 0022-5088/89/$3.50
0 Elsevier Sequoia/Printed in The Netherlands
L22 TABLE 1 Lattice parameters and magnetic properties of RCo4_,Fe,B Compound
a (nm)
GdCo4B GdCozFe*B DyCo4B DyCozFezB
c (nm)
0.5058 0.5104 0.5017 0.5052
0.6889 0.6924 0.6880 0.6884
Tc
505 770 420 650
(W
@B/unit)
(,&/unit)
KI ,,,VK) (lo6 J mP3)
2.8 1.2 5.9 4.0
1.0 1.45 1.0 1.5
0.5 -0.2 -2.3 -1.3
/As (4.2
KJ
/J G’d)
GdCo4_,Fe,B
L ,. x-2
2
z\=
.
j*.
\
” I”. I XX”
‘\
$&&x;*
“r;.“,x ,.“,
0
200
LOO
e
600
800
TemperatureiKl
Fig. 1. Magnetization us. temperature recorded at B = 0.4 T.
The values of the lattice parameters increase as cobalt atoms are partially replaced by iron atoms. A more pronounced variation in lattice parameters is observed in the base plane. The small values of spontaneous magnetization and a deep minimum on the curve of magnetization against temperature (Fig. 1) indicate an antiferromagnetic coupling between the rare earth and transition metal moments. The partial replacement of the cobalt atoms by iron atoms causes the compensation point to shift towards lower temperatures. The same trend was also observed in HoCo4 _ ,Fe,B and ErCo4 _ ,Fe,B [4]. The marked increase in the Curie temperature for the compounds containing iron is noteworthy (Table 1). The RCo,Fe2B compounds studied have Tc values about 200 K higher than the RCo,B compounds. Similar results were also reported for PrCo,_,Fe,B [5] and YCo4_.Fe,B [6]. The magnetic moment per formula unit in the compounds studied at 4.2 K is given in Table 1. The magnetic moment of the 3d sublattice can be
L23
estimated from the relation ~~(4.2 K) = p(R) - 4p(3d) assuming that the magnetic moment of the rare earth atoms equals their free-ion value. The magnetic moment of the cobalt atom in GdCoJ3 and DyCo4B is approximately equal to 1.0 ,.&a, as would be expected. Nevertheless, the average magnetic moment of the 3d ions in both of the substituted iron compounds is considerably higher (approximately 1.5 /~a). This value corresponds to the iron magnetic moment observed in RFe& [7, 81. This behaviour of 3d sublattice ma~etization in the samples studied can be interpreted in terms of the rigid band model assuming a situation similar to that in ThCos _ s Fe, [9]. The decrease in the 3d electron concentration with x lowers the Fermi level in such a way that both subbands become unsaturated and the density of states increases causing an increase of the total magnetic moment. At room temperature the X-ray analysis of the aligned powders showed base plane anisotropy for DyCo4 _ ,Fe,B and GdCo,Fe2B but not for GdCo,B. GdCo&, in which Gd3+ (s state) has a negligible anisotropy, shows an easy c-axis, i.e. the easy magnetization direction of the cobalt atoms is parallel to the c-axis, as in RCo, compounds. This has been confirmed by ls5Gd M&sbauer effect studies [lo]. It is noteworthy that the introduction of iron into GdCofi causes a change in the easy direction of magnetization at room temperature (K, eff < 0 for GdCo,Fe,). This behaviour of the magnetocrystalline anisotropy, due to iron substitution, can be explained when taking the anisotropy contributions from different crystallographic 3d ion sites into account. Generally, the cobalt atoms at 2c sites have a large positive contribution favouring the c-axis, unlike those at 3i sites. The iron atoms may be anticipated to give anisotropy contributions of opposite sign in the same crystallographic sites [ll]. A significant preference of iron for the 2c sites was reported [12]. As a consequence, an easy magnetization direction perpendicular to the c-axis occurs in GdCo,Fe,B. A similar behaviour of the magnetocrystalline anisotropy as a function of the iron concentration was observed in HoCo4_ %Fe,B [4] and YCo4 _ ,Fe,B [6]. These results emphasize the importance of transition metalanisotropy in the RCo4_ xFe,B systems. In the DyCo4_,Fe,B compounds both the dysprosium and the 3d sublattices exhibit magnetocrystalline anisotropy. The existence of an easy base plane in DyCo4B at room temperature would appear to be connected with the dominance of rare earth sublattice anisotropy (the Stevens coefficient a,(Dy) < 0). The effective anisotropy constant changes sign only near the compensation point and the easy direction of magnetization shows a tendency to tilt away from the c-axis. Nevertheless, there is no evidence of spin reorientation for DyCozFezB in the temperature range studied. This suggests that the anisotropy of the Fe-Co sublattice is more negative than that of the cobalt sublattice. It may be concluded that increasing iron content in the RCo4 _ ,Fe,B (R = Gd, Dy) compounds enhances the magnetic 3d sublattice moment and decreases the overall magnetoc~stalline anisotropy, causing the occurrence of an easy base plane in the iron concentration range studied.
L24
Acknowledgments Z.D. wishes to thank Professor G. Dietz for the use of the Foner magnetometer and Dr G. Chelkowska for kindly making it possible to perform measurements in the temperature range 300 - 800 K. This investigation was partly supported by the Polish Academy of Sciences. References 1 Yu. B. Kuz’ma and N. S. Bilonizhko, Sow. Phys. Crystallogr., 18 (1974) 447. 2 H. M. van Noort, D. B. de Mooij and K. H. Buschow, J. Less-Common Met., 111 (1985) 87. 3 G. Zouganelis, A. Kostikas, A. Simopoulos and D. Niarchos, J. Magn. Magn. Mater., 75 (1988) 91. 4 Z. Drzazga, A. Winiarska and T. Mydlarz, in C. Herget, H. Kronmiiller and R. Poerschke (eds.), Proc. 5th Int. Symp. on Magnetic Anisotropy in Coercive R-T Alloys, Bad Soden, 1987, p. 297. 5 S. Y. Jiang, W. E. Wallace and E. Burzo, J. Magn. Magn. Mater., 61 (1986) 257. 6 T. T. Dung, N. P. Thuy, N. M. Hong and T. D. Hien, Private communication. 7 S. Y. Jiang, W. E. Wallace and E. Burzo, J. Less-Common Met., 119 (1986) L5. 8 P. P. Vaishnava, C. W. Kimball, A. M. Umarji, S. K. Malik and G. K. Shenoy, J. Magn. Magn. Mater., 49 (1985) 286. 9 A. M. van Diepen, K. H. J. Buschow and J. S. van Wierinngen, J. Appl. Phys., 43 (1972) 645. 10 H. H. A. Smit, R. C. Thiel and K. H. J. Buschow, J. Phys. F, 18 (1988) 295. 11 J. J. M. France, N. P. Thuy and N. M. Hong, J. Magn. Magn. Mater., 72 (1988) 361. 12 Y. Gros, F. Hartmann-Boutron, C. Meyer, M. A. Fremy and P. Tenaud, J. Phys., C,8 Suppl. 12, t. 49 (1988) 574.