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Magnetic properties of Ho2Co17−xGax compounds
Journal of Alloys and Compounds 267 (1998) 37–40
L
Magnetic properties of Ho 2 Co 172x Ga x compounds C. Zhang
a ,b ,
*, J.C.P. Klaasse b , E. Bruck ¨ b , F.R. de Boer b , K.H.J. Buschow b
a Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110015, P.R. China Van der Waals-Zeeman Institute, University of Amsterdam, Valckenierstraat 65, 1018 XE Amsterdam, Netherlands
b
Received 1 October 1997
Abstract We have studied the magnetic properties of Ho 2 Co 172x Gax compounds (x#7) by means of magnetic measurements and X-ray diffraction on magnetically aligned powders. The Curie temperature and the spontaneous moment decrease strongly with increasing Ga concentration. The compounds that are magnetically ordered at room temperature have an easy magnetisation direction at room temperature which is perpendicular to the c-direction. 1998 Elsevier Science S.A. Keywords: Rare earth compounds; Co rich compounds; Magnetic properties
1. Introduction In previous investigations, we studied the magnetic properties of R 2 Co 172x Ga x compounds with R5Pr, Sm, and Gd by means of magnetic measurements and X-ray diffraction on magnetically aligned powders [1,2]. The Tb compounds were also studied by neutron diffraction [3]. These studies have shown that increasing Ga concentration leads to a sign reversal of the magnetocrystalline Cosublattice anisotropy from easy-plane anisotropy for low Ga concentration to easy-axis anisotropy for higher Ga concentration. A similar sign change of the Co-sublattice anisotropy has also been observed in R 2 Co 172x Al x compounds. In the latter compounds, the competition between the R-sublattice anisotropy and the Co-sublattice anisotropy frequently leads to spin-reorientation transitions. For instance, in the series of Pr 2 Co 172x Al x compounds, we have observed spin-reorientation transition temperatures that decrease strongly with increasing x in the range 0#x#4 [4,5]. Surprisingly, no such transitions were observed in the Pr 2 Co 172x Ga x compounds. In order to be able to assess the difference in Co-sublattice anisotropy in R 2 Co 172x Ga x and R 2 Co 172x Al x compounds more clearly, we have extended our investigation to Ho 2 Co 172x Ga x compounds.
purity. After arc melting, the ingots were wrapped into Ta foil, sealed into an evacuated quartz tube and annealed for two weeks at 9008C. The X-ray-diffraction diagrams showed that the annealed materials were approximately single phase and that their crystal structure corresponded to the hexagonal Th 2 Ni 17 type for 0#x#3 and to the rhombohedral Th 2 Zn 17 -structure type for x55 and 7. The Ho 2 Co 172x Ga x compounds with 0#x#5 have Curie temperatures above room temperature. For these compounds, X-ray diffraction data were also taken on finely ground powder samples of which the particles were magnetically aligned at room temperature and fixed in the alignment direction with glue. The magnetic measurements were made on a SQUID magnetometer in the temperature range 5–300 K in magnetic fields up to 5 T. For the measurements above 300 K, we used a home-built magnetometer based on the Faraday principle. In the latter case, the measurements were made on pieces of polycrystalline bulk material sealed into an evacuated silica tube in order to avoid oxidation at elevated temperatures as far as possible. The temperature dependence of the AC-susceptibility was measured for all compounds in the temperature range 5–300 K.
2. Experimental
3. Results
Ho 2 Co 172x Ga x compounds with x51, 3, 5 and 7 were prepared by arc melting starting materials of at least 99.9%
The lattice constants derived from the X-ray diagrams are listed in Table 1, together with the corresponding unit-cell volumes. From the X-ray diffraction data on the magnetically aligned samples, it was derived that the
*Corresponding author. 0925-8388 / 98 / $19.00 1998 Elsevier Science S.A. All rights reserved. PII S0925-8388( 97 )00567-7
C. Zhang et al. / Journal of Alloys and Compounds 267 (1998) 37 – 40
38
Table 1 Lattice parameters a and c, unit-cell volume V, Curie temperature T c (K) spontaneous moment M0 at 5 K, structure type and easy magnetisation direction at room temperature of Ho 2 Co 172x Ga x compounds x
a ˚ (A)
c ˚ (A)
V ˚ 3) (A
Tc (K)
M0 at 5 K ( mB / f.u.)
Structure type
EMD (RT)
1 3 5 7
8.350 8.397 8.483 8.595
8.143 8.182 12.351 12.301
491.69 499.68 769.78 787.01
1070 750 390 70
5.0 1.9 6.5 9.9
Hexagonal Hexagonal Rhombohedral Rhombohedral
Plane Plane Plane –
compounds with 0#x#5 have their easy magnetisation direction at room temperature perpendicular to the c-axis. Examples of X-ray diagrams are shown in Fig. 1. The easy magnetisation directions at room temperature are also listed in Table 1. Results of magnetic measurements showing the temperature dependence of the magnetisation are displayed in Fig. 2. The Curie temperatures derived from these measurements have been included in Table 1. It is seen that the magnetic-ordering temperatures of the Ho 2 Co 172x Ga x compounds investigated decrease strongly with increasing Ga concentration. The concentration dependence of the Curie temperature is presented in Fig. 3. For the Curie temperature of Ho 2 Co 17 , we used the value 1177 K given in Ref. [6]. The field dependence of the magnetisation at 5 K of the Ho 2 Co 172x Ga x compounds investigated is shown in Fig. 4. The values of the spontaneous moment were derived by extrapolating the linear parts in the high-field region of the magnetic isotherms shown in Fig. 4 to zero field. The corresponding values have been included in Table 1. Results of AC-susceptibility measurements are displayed in Fig. 5. For the compounds with x53 and 5, the temperature dependence of the AC-susceptibility does not show any pronounced structure that would indicate a spin reorientation below room temperature. Somewhat more
Fig. 1. X-ray diagrams obtained on magnetically aligned powder samples (a and b), and on non-aligned powder samples (c and d) of Ho 2 Co 172x Ga x . The data shown for the compounds x53 and 5 refer to the hexagonal and rhombohedral structure, respectively.
Fig. 2. Temperature dependence of the magnetisation of Ho 2 Co 172x Ga x compounds measured in a field of 0.05 T.
Fig. 3. Concentration dependence of the Curie temperature of Ho 2 Co 172x Ga x compounds.
C. Zhang et al. / Journal of Alloys and Compounds 267 (1998) 37 – 40
39
Fig. 4. Field dependence of the magnetic moment of free powder material of Ho 2 Co 172x Ga x compounds measured with decreasing field at 5 K.
Fig. 6. Spontaneous moment per formula unit of Ho 2 Co 172x Ga x versus Ga concentration.
structure is shown in the compound with x51 in the form of a shallow maximum around 160 K. Two maxima are found in the curve of the compound with x57. The maximum at about 75 K can be identified as representing the magnetic-ordering temperature of this compound. The maximum at about 15 K is of different origin and may reflect the occurrence of a spin-reorientation transition.
4. Discussion
Fig. 5. Temperature dependence of the ac-susceptibility of polycrystalline bulk material of Ho 2 Co 172x Ga x compounds.
In Fig. 6, we have plotted the concentration dependence of the spontaneous moment. For Ho 2 Co 17 , we used the value M0 58.0 m B / f.u. [7]. As can be seen in Fig. 5, the compounds with x53 and 5 show compensation temperatures in their temperature dependence of the magnetisation, meaning that for x$3 the Ho-sublattice magnetisation dominates the Co-sublattice magnetisation at 5 K. This fact is indicated in the plot of Fig. 6 by negative values for the compounds with x$3. It is seen in Fig. 6 that Ga substitution gives rise to a fairly strong decrease of the spontaneous moments. This decrease is much stronger than expected on the basis of magnetic dilution, represented by the broken line in Fig. 6. It means that Ga not only acts as magnetic diluent but simultaneously reduces the average Co moment of the remaining Co atoms. Recent results of X-ray diffraction, made on magnetically aligned Gd 2 Co 172x Ga x samples, have shown that the Co-sublattice anisotropy changes sign from negative to positive with increasing Ga concentration [1]. This sign change arises as a consequence of the fact that Ga atoms substitute almost exclusively for the 18 h-site Co atoms [3]. Apparently, the 18 h-site Co atoms contribute negatively to the total Co-sublattice anisotropy, but if they are absent, the total Co-sublattice anisotropy can become positive, i.e. easy-axis type. In the previous section, we have shown that the Ho 2 Co 172x Ga x compounds with x#5 have Curie temperatures above room temperature and the results listed in Table 1 show that the easy magnetisation direction at room temperature is perpendicular to the c-axis. This means that, even at room temperature, the easy-plane Ho-sublattice
40
C. Zhang et al. / Journal of Alloys and Compounds 267 (1998) 37 – 40
anisotropy dominates the easy-axis Co-sublattice anisotropy. Similar results were also found [1,2,8] in other series of R 2 Co 172x Ga x compounds in which the R component has the same sign of the second-order Stevens factor (aJ ,0). These results are furthermore consistent with an easy c-axis anisotropy observed [1] in the series with R5Sm (aJ .0). As discussed in more detail elsewhere [1,5], one may expect that the R-sublattice anisotropy strongly increases with decreasing temperature, due to the strong temperature dependence of the thermal average of the Stevens operator kO 02 l. This thermal average determines the temperature dependence of the first-order anisotropy constant in the expression K R1 523 / 2 aJ kr 2 lkO 02 lA 02 . Because the Ho-sublattice anisotropy is already dominant at room temperature, for the compounds with x#5, no change in the easy magnetisation direction is expected upon cooling. This agrees with the fact that we did not observe any spinreorientation phenomena in the temperature dependence of the AC-susceptibility of the compounds with x53 and 5. As shown in Fig. 5, there is a shallow maximum in the curve of the compound with x51 at about 160 K, which may point to a spin reorientation at this temperature. From the discussion given above, it seems unlikely, however, that this is a spin reorientation caused by the competition between the Ho- and Co-sublattice anisotropies. It cannot be excluded that it may be caused by changes of the Ho-sublattice anisotropy due to higher-order contributions. We also cannot offer an explanation for the low-tempera-
ture maximum observed in the AC-susceptibility curve of the compound with x57 because, for this compound, information on the easy direction at any temperature could not be obtained from X-ray diffraction. Neutron-diffraction studies of these materials are planned in the near future.
Acknowledgements The present investigation has been carried out within the scientific exchange program between China and The Netherlands.
References ¨ [1] D. Zhang, D.P. Middleton, E. Bruck, F.R. de Boer, Z.D. Zhang, K.H.J. Buschow, J. Alloys Comp. 259 (1997) 65. ¨ [2] C. Zhang, J.C.P. Klaasse, E. Bruck, F.R. de Boer, K.H.J. Buschow, to be published. [3] O. Moze, L. Giovanelli, W. Kockelmann, C.H. de Groot, F.R. de Boer, K.H.J. Buschow, J. Alloys Comp. (1997). ¨ [4] D. Zhang, C.H. de Groot, E. Bruck, F.R. de Boer, K.H.J. Buschow, J. Alloys Comp. 259 (1997) 42. ¨ [5] D. Zhang, E. Bruck, F.R. de Boer, Z.D. Zhang, K.H.J. Buschow, Physica B 240 (1997) 110. [6] K.H.J. Buschow, Rep. Prog. Phys. 40 (1977) 1179. [7] J.J.M. Franse, R.J. Radwanski, in: K.H.J. Buschow (Ed.), Handbook of Magnetic Materials, Vol. 7, North Holland, Amsterdam, 1993. [8] S.Y. Zhang, B.G. Shen, B. Liang, Z.H. Cheng, J.X. Zhang, H.W. Zhang, J.G. Zhao, W.S. Zhan, J. Alloys Comp. (1997).
L
Magnetic properties of Ho 2 Co 172x Ga x compounds C. Zhang
a ,b ,
*, J.C.P. Klaasse b , E. Bruck ¨ b , F.R. de Boer b , K.H.J. Buschow b
a Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110015, P.R. China Van der Waals-Zeeman Institute, University of Amsterdam, Valckenierstraat 65, 1018 XE Amsterdam, Netherlands
b
Received 1 October 1997
Abstract We have studied the magnetic properties of Ho 2 Co 172x Gax compounds (x#7) by means of magnetic measurements and X-ray diffraction on magnetically aligned powders. The Curie temperature and the spontaneous moment decrease strongly with increasing Ga concentration. The compounds that are magnetically ordered at room temperature have an easy magnetisation direction at room temperature which is perpendicular to the c-direction. 1998 Elsevier Science S.A. Keywords: Rare earth compounds; Co rich compounds; Magnetic properties
1. Introduction In previous investigations, we studied the magnetic properties of R 2 Co 172x Ga x compounds with R5Pr, Sm, and Gd by means of magnetic measurements and X-ray diffraction on magnetically aligned powders [1,2]. The Tb compounds were also studied by neutron diffraction [3]. These studies have shown that increasing Ga concentration leads to a sign reversal of the magnetocrystalline Cosublattice anisotropy from easy-plane anisotropy for low Ga concentration to easy-axis anisotropy for higher Ga concentration. A similar sign change of the Co-sublattice anisotropy has also been observed in R 2 Co 172x Al x compounds. In the latter compounds, the competition between the R-sublattice anisotropy and the Co-sublattice anisotropy frequently leads to spin-reorientation transitions. For instance, in the series of Pr 2 Co 172x Al x compounds, we have observed spin-reorientation transition temperatures that decrease strongly with increasing x in the range 0#x#4 [4,5]. Surprisingly, no such transitions were observed in the Pr 2 Co 172x Ga x compounds. In order to be able to assess the difference in Co-sublattice anisotropy in R 2 Co 172x Ga x and R 2 Co 172x Al x compounds more clearly, we have extended our investigation to Ho 2 Co 172x Ga x compounds.
purity. After arc melting, the ingots were wrapped into Ta foil, sealed into an evacuated quartz tube and annealed for two weeks at 9008C. The X-ray-diffraction diagrams showed that the annealed materials were approximately single phase and that their crystal structure corresponded to the hexagonal Th 2 Ni 17 type for 0#x#3 and to the rhombohedral Th 2 Zn 17 -structure type for x55 and 7. The Ho 2 Co 172x Ga x compounds with 0#x#5 have Curie temperatures above room temperature. For these compounds, X-ray diffraction data were also taken on finely ground powder samples of which the particles were magnetically aligned at room temperature and fixed in the alignment direction with glue. The magnetic measurements were made on a SQUID magnetometer in the temperature range 5–300 K in magnetic fields up to 5 T. For the measurements above 300 K, we used a home-built magnetometer based on the Faraday principle. In the latter case, the measurements were made on pieces of polycrystalline bulk material sealed into an evacuated silica tube in order to avoid oxidation at elevated temperatures as far as possible. The temperature dependence of the AC-susceptibility was measured for all compounds in the temperature range 5–300 K.
2. Experimental
3. Results
Ho 2 Co 172x Ga x compounds with x51, 3, 5 and 7 were prepared by arc melting starting materials of at least 99.9%
The lattice constants derived from the X-ray diagrams are listed in Table 1, together with the corresponding unit-cell volumes. From the X-ray diffraction data on the magnetically aligned samples, it was derived that the
*Corresponding author. 0925-8388 / 98 / $19.00 1998 Elsevier Science S.A. All rights reserved. PII S0925-8388( 97 )00567-7
C. Zhang et al. / Journal of Alloys and Compounds 267 (1998) 37 – 40
38
Table 1 Lattice parameters a and c, unit-cell volume V, Curie temperature T c (K) spontaneous moment M0 at 5 K, structure type and easy magnetisation direction at room temperature of Ho 2 Co 172x Ga x compounds x
a ˚ (A)
c ˚ (A)
V ˚ 3) (A
Tc (K)
M0 at 5 K ( mB / f.u.)
Structure type
EMD (RT)
1 3 5 7
8.350 8.397 8.483 8.595
8.143 8.182 12.351 12.301
491.69 499.68 769.78 787.01
1070 750 390 70
5.0 1.9 6.5 9.9
Hexagonal Hexagonal Rhombohedral Rhombohedral
Plane Plane Plane –
compounds with 0#x#5 have their easy magnetisation direction at room temperature perpendicular to the c-axis. Examples of X-ray diagrams are shown in Fig. 1. The easy magnetisation directions at room temperature are also listed in Table 1. Results of magnetic measurements showing the temperature dependence of the magnetisation are displayed in Fig. 2. The Curie temperatures derived from these measurements have been included in Table 1. It is seen that the magnetic-ordering temperatures of the Ho 2 Co 172x Ga x compounds investigated decrease strongly with increasing Ga concentration. The concentration dependence of the Curie temperature is presented in Fig. 3. For the Curie temperature of Ho 2 Co 17 , we used the value 1177 K given in Ref. [6]. The field dependence of the magnetisation at 5 K of the Ho 2 Co 172x Ga x compounds investigated is shown in Fig. 4. The values of the spontaneous moment were derived by extrapolating the linear parts in the high-field region of the magnetic isotherms shown in Fig. 4 to zero field. The corresponding values have been included in Table 1. Results of AC-susceptibility measurements are displayed in Fig. 5. For the compounds with x53 and 5, the temperature dependence of the AC-susceptibility does not show any pronounced structure that would indicate a spin reorientation below room temperature. Somewhat more
Fig. 1. X-ray diagrams obtained on magnetically aligned powder samples (a and b), and on non-aligned powder samples (c and d) of Ho 2 Co 172x Ga x . The data shown for the compounds x53 and 5 refer to the hexagonal and rhombohedral structure, respectively.
Fig. 2. Temperature dependence of the magnetisation of Ho 2 Co 172x Ga x compounds measured in a field of 0.05 T.
Fig. 3. Concentration dependence of the Curie temperature of Ho 2 Co 172x Ga x compounds.
C. Zhang et al. / Journal of Alloys and Compounds 267 (1998) 37 – 40
39
Fig. 4. Field dependence of the magnetic moment of free powder material of Ho 2 Co 172x Ga x compounds measured with decreasing field at 5 K.
Fig. 6. Spontaneous moment per formula unit of Ho 2 Co 172x Ga x versus Ga concentration.
structure is shown in the compound with x51 in the form of a shallow maximum around 160 K. Two maxima are found in the curve of the compound with x57. The maximum at about 75 K can be identified as representing the magnetic-ordering temperature of this compound. The maximum at about 15 K is of different origin and may reflect the occurrence of a spin-reorientation transition.
4. Discussion
Fig. 5. Temperature dependence of the ac-susceptibility of polycrystalline bulk material of Ho 2 Co 172x Ga x compounds.
In Fig. 6, we have plotted the concentration dependence of the spontaneous moment. For Ho 2 Co 17 , we used the value M0 58.0 m B / f.u. [7]. As can be seen in Fig. 5, the compounds with x53 and 5 show compensation temperatures in their temperature dependence of the magnetisation, meaning that for x$3 the Ho-sublattice magnetisation dominates the Co-sublattice magnetisation at 5 K. This fact is indicated in the plot of Fig. 6 by negative values for the compounds with x$3. It is seen in Fig. 6 that Ga substitution gives rise to a fairly strong decrease of the spontaneous moments. This decrease is much stronger than expected on the basis of magnetic dilution, represented by the broken line in Fig. 6. It means that Ga not only acts as magnetic diluent but simultaneously reduces the average Co moment of the remaining Co atoms. Recent results of X-ray diffraction, made on magnetically aligned Gd 2 Co 172x Ga x samples, have shown that the Co-sublattice anisotropy changes sign from negative to positive with increasing Ga concentration [1]. This sign change arises as a consequence of the fact that Ga atoms substitute almost exclusively for the 18 h-site Co atoms [3]. Apparently, the 18 h-site Co atoms contribute negatively to the total Co-sublattice anisotropy, but if they are absent, the total Co-sublattice anisotropy can become positive, i.e. easy-axis type. In the previous section, we have shown that the Ho 2 Co 172x Ga x compounds with x#5 have Curie temperatures above room temperature and the results listed in Table 1 show that the easy magnetisation direction at room temperature is perpendicular to the c-axis. This means that, even at room temperature, the easy-plane Ho-sublattice
40
C. Zhang et al. / Journal of Alloys and Compounds 267 (1998) 37 – 40
anisotropy dominates the easy-axis Co-sublattice anisotropy. Similar results were also found [1,2,8] in other series of R 2 Co 172x Ga x compounds in which the R component has the same sign of the second-order Stevens factor (aJ ,0). These results are furthermore consistent with an easy c-axis anisotropy observed [1] in the series with R5Sm (aJ .0). As discussed in more detail elsewhere [1,5], one may expect that the R-sublattice anisotropy strongly increases with decreasing temperature, due to the strong temperature dependence of the thermal average of the Stevens operator kO 02 l. This thermal average determines the temperature dependence of the first-order anisotropy constant in the expression K R1 523 / 2 aJ kr 2 lkO 02 lA 02 . Because the Ho-sublattice anisotropy is already dominant at room temperature, for the compounds with x#5, no change in the easy magnetisation direction is expected upon cooling. This agrees with the fact that we did not observe any spinreorientation phenomena in the temperature dependence of the AC-susceptibility of the compounds with x53 and 5. As shown in Fig. 5, there is a shallow maximum in the curve of the compound with x51 at about 160 K, which may point to a spin reorientation at this temperature. From the discussion given above, it seems unlikely, however, that this is a spin reorientation caused by the competition between the Ho- and Co-sublattice anisotropies. It cannot be excluded that it may be caused by changes of the Ho-sublattice anisotropy due to higher-order contributions. We also cannot offer an explanation for the low-tempera-
ture maximum observed in the AC-susceptibility curve of the compound with x57 because, for this compound, information on the easy direction at any temperature could not be obtained from X-ray diffraction. Neutron-diffraction studies of these materials are planned in the near future.
Acknowledgements The present investigation has been carried out within the scientific exchange program between China and The Netherlands.
References ¨ [1] D. Zhang, D.P. Middleton, E. Bruck, F.R. de Boer, Z.D. Zhang, K.H.J. Buschow, J. Alloys Comp. 259 (1997) 65. ¨ [2] C. Zhang, J.C.P. Klaasse, E. Bruck, F.R. de Boer, K.H.J. Buschow, to be published. [3] O. Moze, L. Giovanelli, W. Kockelmann, C.H. de Groot, F.R. de Boer, K.H.J. Buschow, J. Alloys Comp. (1997). ¨ [4] D. Zhang, C.H. de Groot, E. Bruck, F.R. de Boer, K.H.J. Buschow, J. Alloys Comp. 259 (1997) 42. ¨ [5] D. Zhang, E. Bruck, F.R. de Boer, Z.D. Zhang, K.H.J. Buschow, Physica B 240 (1997) 110. [6] K.H.J. Buschow, Rep. Prog. Phys. 40 (1977) 1179. [7] J.J.M. Franse, R.J. Radwanski, in: K.H.J. Buschow (Ed.), Handbook of Magnetic Materials, Vol. 7, North Holland, Amsterdam, 1993. [8] S.Y. Zhang, B.G. Shen, B. Liang, Z.H. Cheng, J.X. Zhang, H.W. Zhang, J.G. Zhao, W.S. Zhan, J. Alloys Comp. (1997).