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Magnetic domain structures, domain wall energies and magnetization processes in UFe10Si2 and UCo10Si2 intermetallic compounds
Journal of the Less-Common
115
Metals, 163 ( 1990) 115-I 2 1
MAGNETIC DOMAIN STRUCTURES, DOMAIN WALL ENERGIES AND MAGNETIZATION PROCESSES IN UFe,,Si, AND UCo,,Si, INTERMETALLIC COMPOUNDS .I. J. WYSIrDCKI Institute of Physics, Technical University, Al. Zawadzkiego 19, 42-200 Cz
January
8,199O)
Summary Applying the powder pattern method, magnetic domain structures of UFe,,Si, and UCo,,Si, compounds were observed on the sample surfaces parallel and perpendicular to the magnetic easy direction. The domain structure observed is typical for uniaxial materials. By measuring the spacings D, of domain boundaries on surfaces perpendicular to the easy axis, values of domain wall energy y have been obtained using the technique of Bodenberger and Hubert as equal to 18 erg cm -* and 5 erg cm -* for UFe,,Si, and UCo,,Si, respectively. The influence of the applied magnetic field on the domain structure has been observed throughout a complete hysteresis cycle.
1. Introduction The rare earth-transition metal compounds with uniaxial symmetry provide the basis for permanent magnet materials with large magnetocrystalline anisotropy [l]. Extensive investigations of uranium compounds proved that uranium atoms often also cause a strong anisotropy in numerous compounds [2]. The ternary uranium compounds (UT,Al,,_,, where T is a 3d element) with the ThMn,, type of structure exhibit interesting physical properties. Recently, some compounds with iron and cobalt as the 3d element with x > 6 and with other elements instead of Al (Si, MO) have been obtained [3,4]. The UFe,,Si, compound exhibits a considerably large saturation magnetization i&f,= 850 G at room temperature and a high Curie temperature T,=650 K. However, the UCo,,Si, compound has M, = 390 G and T,= 530 K. The common feature of both ferromagnets is their very small hysteresis, negligible in the case of UFe,,Si, [3]. Up to now, according to our best knowledge, the domain structures of the ThMn,,-type compounds, both with rare earths and with uranium, have not been 0022-5088/90/$3.50
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in The Netherlands
116
investigated. Therefore the aim of this work is to observe the domain structure, to determine the domain wall energy y and to study the magnetization processes in UFe,,Si, and UCo,,,Si, compounds.
2. Material and experimental methods The polycrystalline samples were prepared by melting the ultrapure elements in stoichiometric amounts in an arc furnace in a purified argon atmosphere gettered with titanium. This process for UFe,,,Si, and UCo,,Si, was followed by annealing at 1170 K for 1 week in an evacuated silica ampoule. The X-ray diffraction patterns confirm that both compounds are single phase of the ThMn,, type of tetragonal crystal structure. The surfaces for the observation of domain structures were prepared by polishing using emery papers and aluminium oxide. The domain structure was observed by means of metallographic oxide. The domain structure was observed by means of metallographic microscopy using the powder pattern method. The domain wall energy y was obtained from the surface domain width using the stereologic method described by Bodenberger and Hubert [5].
3. Results and discussion 3.1. Magnetic domain observations The domain structures observed in the UFe,,Si, and UCo,,Si, compounds are of the same type as those for crystals with high uniaxial magnetocrystalline anisotropy. Typical magnetic domains in the demagnetized state are shown in Fig. 1 and Fig. 2 for the UFe,,S& and UCo,,Si, compounds respectively. The upper parts of Figs. l(a) and 2(a) show the grain surfaces parallel to the easy axis (axial plane) whereas the lower parts of Figs. l(a) and 2(a) show the grain surface per-
Fig. 1. Typical domain structure in the UFe,,,S& compound: (a) two grains with magnetic easy direction parallel (upper part) and perpendicular (lower part) to the plane of the sample surface; (b) magnitied lower part of (a); (c) the grains with different crystallographic orientation of the sample surface.
117
Fig. 2. Typical domain structure in the UCo,,,Siz compound: (a) two grains with magnetic easy direction parallel (upper part) and perpendicular (lower part) to the plane of the sample surface; (b) magnified lower part of (a); (c) the grains with different crystallographic orientation of the sample surface.
pendicular to the easy axis (basal plane). On grain surfaces that are parallel to the easy direction, domains with 180” Bloch walls are apparent (upper parts of Figs. l(a) and 2(a)). At the ends of such domains, i.e. at the grain boundaries, closure domains occur in the form of spikes with magnetization antiparallel to that of the main domains in which they lie. However, the domain structures observed on the grain surfaces that are perpendicular to the easy direction are slightly different (compare magnified lower parts of Figs. l(a) and 2(a) presented in Figs. l(b) and 2(b)). A characteristic star-like structure (Fig. l(b)) is observed in UFe,,Si, (as in Nd2Fe,,B [6]) and a labyrinth-like structure (Fig. 2(b)) in UCo,,Si, (as in Dy,Fe,,B [6]). Circular domains on the perpendicular surface, such as shown in Figs. 1(b) and 2(b), are also spike-like closure domains. A further complication of the domain structure on the basal plane of uniaxial magnets is due to undulation of Bloch walls and the formation of secondary spikes with increasing thickness of the grains. If the grain surface makes an angle with the crystallographic axial planes, more complex domains arise (Figs. 1(c) and 2(c)). 3.2. Domain wall energy determination The domain wall energy y is determined mostly from the dependence of the domain width D on the crystal thickness L in the magnetically preferred direction. The relationship D(L) can be found from measurements of the widths of 180” main domains on the axial plane of the crystals or for thin crystals from the basal plane (when the domain structure on this plane is not complicated by waviness of domain walls near the surface or by the occurrence of spike domains). However, for more complicated surface structures on the basal plane, when the range of the crystal thicknesses for which the width of the surface domains does not depend on the thickness of the sample or when it is impossible to observe domain structures on the axial plane, the method of Bodenberger and Hubert [5] can be applied to calculate the domain wall energy y. Bodenberger and Hubert showed that the surface domain width D, for large sample thicknesses (or large grains in the
118
case of polycrystalline materials) is independent of thickness and is given by D, = 4n/3y/Mi, where MS is the magnetization and /I is a geometrical factor depending on the type of surface structure. In this paper we assumed /3 = 0.3 1, as was done, for example, for the SmCo, system [5] because the structures described in our work appeared similar to those on SmCo,. To measure D,, a statistical method was employed. A large number of lines in random directions were drawn on the basal plane patterns like those in Figs. l(b) and 2(b). The total length was divided by the number of intersections with domain walls to give a mean spacing of intersections. This mean value was then multiplied by 2/~r to give the average surface domain width D,. Using these mean values of D, and the values of MS we calculated the domain wall energies y for the UFe,,Si, and UCo,,Si, compounds which are listed in Table 1. The value of the domain wall energy for UFe,,Si,, is close to the values obtained for Nd,Fe,,B single crystals ( y = 20 y=18ergcme2, erg cm-2 [7]). 3.3. Magnetic domains under appliedfield The influence of the magnetic field (up to 5 kOe, i.e. to a fully magnetized state of the samples) on the domain wall motion was studied in the UFe,,Si, and UCo,,Si, compounds. It was found that the behaviour of these compounds was similar, the only difference being the magnitude of the field needed to create a single domain. Therefore, in this paper, the behaviour of domains under an applied field was shown only for the UCo,,Si, compound. In particular, special attention was paid to the domain wall pinning and the nucleation of the reverse domains in order to explain the origin of the low values of the coercive force and remanence. In Fig. 3 magnetizing and demagnetizing processes are illustrated by showing changes in domain structure of the large grains in the thermally demagnetized UCo,,Si, compound. As the field H, applied parallel to the easy direction in this grain, is increased from 0 to 390 Oe (Figs. 3(a) and 3(b)) domains with 180” domain walls which were magnetized antiparallel to the applied magnetic field direction shrink by smooth movement of domain walls until they are pinned by inclusions. On further increasing the field (Fig. 3(c), H= 730 Oe) these domains begin to narrow at the spherical inclusions where domain walls are pinned and then the increasing field causes the domain walls to break up (Fig. 3(d), H= 1080 Oe) and in this way two separate spike domains are created, which become smaller with any further’ increase in the field (Fig. 3(e), H = 2180 Oe). However, existing spike TABLE 1 The saturation magnetization M, (at room temperature), the average surface domain width D, and the domain wall energy yin the UFe,,,Si, and UCo,,,Si, compounds Compound
M, (G)
D, (m4
UFe,,,Si, UCo,,,Si,
850 390
0.98 1.15
18 5
Fig. 3. The influence of the magnetic field H on the domain structure of the UCo,,Si,: (a) H= 0 Oe, (b) 390 Oe, (c) 730 Oe, (d) 1080 Oe, (e) 2180 Oe, (f) 730 Oe, (g) 390 Oe and (h) 0 Oe. The arrow(b) indicates the direction of the applied field H.
domains with a magnetization opposite to the applied field direction begin to disappear during this process. As the field is decreased newly created spike domains (in the nucleation field H,, = 1080 Oe for this grain) begin to grow (Fig. 3(f), H= 730 Oe) and on further decreasing the field these domains are recombined into one domain with 180” domain walls (Fig. 3(g), H= 390 Oe). At zero field (the sample in remanent state, Fig. 3(h)) there are no distinct differences in domain structure in comparison with the initial state, which might be one of the reasons for the very low values of coercivity and remanence in the UCo,,Si, and UFe,,Si, compounds. In Fig. 4 the magnetization process on the grain surface of the thermally demagnetized UCo,,Si, compound is shown (H= 0 + 2.5 kOe -, 0 -, - 0.73 kOe-0). It has been observed that the magnetization process in the left and middle part of the grain takes place by shrinking of the spike domains which are magnetized antiparallel to the applied field direction. However, domains with 180” domain walls in which spike domains are apparent (see Fig. 4(b), upper grain boundary in the right part of the grain) are more difficult to magnetize. At a field of H= 2500 Oe the grain is single domain up to the maximum field (Fig. 4(c)). On decreasing the field spike domains reappeared spontaneously (in the nucleation field H, = 1250 Oe for this grain) almost at the same positions as those at which these spike domains disappeared upon saturation (Fig. 4(d), H= 390 Oe). Domain structures of the sample in the remanent (Fig. 4(e), H= 0 Oe) and initial states are similar and differ only in details. An increase in the applied field in the opposite
120
directiun causes new spike domains with reverse magnet~ation to be nucleated as a result of a disruption of the existing spike domains with magnetization parallel to the applied field direction and further convergence of adjacent domain walls which surround the area with reverse magnetization (compare middle part of Figs. 4(e)
Fig. 4. The influence of the magnetic field Han the domain structure of the UCo,,Si,: (a) H-0 Oe, (b) 730 Oe, (c) 2500 Oe, (d) 390 Qe, (e) 0 Oe, (f) - 390 Oe, (g) - 730 Oe and (h) 0 Oe. The arrows(b) and (ff indicate the direction of the appiied field H.
121
and 4(f)). On further increasing the field these newly created spike domains shrink (Fig. 4(g), H= 730 Oe) and finally disappear (when saturation is achieved). Decreasing the field to zero causes rising of the remanent domain structure, which is shown in Fig. 4(h). From the comparison of the remanent (Figs. 4(e) and 4(h)), and initial (Fig. 4(a)) domain structures it follows that these structures differ only in details. It is not possible to suggest any preferable sites for domain nucleation from the domain structure observations undertaken in the present paper.
4. Concluding
remarks
(1) The domain structures observed in UFe,,Si, and UCo,,Si, compounds are of the same type as those for crystals with high uniaxial magnetocrystalline anisotropy. (2) Using the values of average surface domain width D, domain wall energies y were calculated for UFe,,Si, and UCo,,,Si, and found to be equal to 18 erg cm-? and 5 erg cm- 2 respectively. (3) The magnetization process of the UFe,,,Si, and UCo,,,Si, compounds showed that there are no distinct differences between domain structures in the remanent and the initial state, which might be one of the reasons for the very low values of the coercivity and the remanence in these compounds.
References I K. H. J. Buschow, Mater. Sci. Rep., 1 (1986) 1. 2 J. M. Fournier and R. Trod, in A. J. Freeman and G. H. Lander (eds.), Handbook on the Physics and Chemisrry ofthe Actinides, Vol. 2, North-Holland, Amsterdam, 1985, p. 29. 3 W. Suski, A. Baran and T. Mydlarz, Whys. Lett. A, I36 (1989) 89. 4 A. Baran, M. kwkasiak, W. Suski, J. Suwalski, H. Figiel, J. Opti, K. Turek and T. Mydlarz, J. Map. Map. Mater., 83 (1990) 262. 5 R. Bodenberger and A. Hubert, Phys. Status Solidi, A, 44 (1977) K7. 6 D. Phrsa, J. J. Wyskxki, B. Wystocki and R. Pfranger, J. Less-Common Met., 133 (1987) 231. 7 R. Szymczak, D. Givord and H. S. Li, Acra Phys. Pal. A, 72 ( 1987) 113.
115
Metals, 163 ( 1990) 115-I 2 1
MAGNETIC DOMAIN STRUCTURES, DOMAIN WALL ENERGIES AND MAGNETIZATION PROCESSES IN UFe,,Si, AND UCo,,Si, INTERMETALLIC COMPOUNDS .I. J. WYSIrDCKI Institute of Physics, Technical University, Al. Zawadzkiego 19, 42-200 Cz
January
8,199O)
Summary Applying the powder pattern method, magnetic domain structures of UFe,,Si, and UCo,,Si, compounds were observed on the sample surfaces parallel and perpendicular to the magnetic easy direction. The domain structure observed is typical for uniaxial materials. By measuring the spacings D, of domain boundaries on surfaces perpendicular to the easy axis, values of domain wall energy y have been obtained using the technique of Bodenberger and Hubert as equal to 18 erg cm -* and 5 erg cm -* for UFe,,Si, and UCo,,Si, respectively. The influence of the applied magnetic field on the domain structure has been observed throughout a complete hysteresis cycle.
1. Introduction The rare earth-transition metal compounds with uniaxial symmetry provide the basis for permanent magnet materials with large magnetocrystalline anisotropy [l]. Extensive investigations of uranium compounds proved that uranium atoms often also cause a strong anisotropy in numerous compounds [2]. The ternary uranium compounds (UT,Al,,_,, where T is a 3d element) with the ThMn,, type of structure exhibit interesting physical properties. Recently, some compounds with iron and cobalt as the 3d element with x > 6 and with other elements instead of Al (Si, MO) have been obtained [3,4]. The UFe,,Si, compound exhibits a considerably large saturation magnetization i&f,= 850 G at room temperature and a high Curie temperature T,=650 K. However, the UCo,,Si, compound has M, = 390 G and T,= 530 K. The common feature of both ferromagnets is their very small hysteresis, negligible in the case of UFe,,Si, [3]. Up to now, according to our best knowledge, the domain structures of the ThMn,,-type compounds, both with rare earths and with uranium, have not been 0022-5088/90/$3.50
0 Elsevier
Sequoia/Printed
in The Netherlands
116
investigated. Therefore the aim of this work is to observe the domain structure, to determine the domain wall energy y and to study the magnetization processes in UFe,,Si, and UCo,,,Si, compounds.
2. Material and experimental methods The polycrystalline samples were prepared by melting the ultrapure elements in stoichiometric amounts in an arc furnace in a purified argon atmosphere gettered with titanium. This process for UFe,,,Si, and UCo,,Si, was followed by annealing at 1170 K for 1 week in an evacuated silica ampoule. The X-ray diffraction patterns confirm that both compounds are single phase of the ThMn,, type of tetragonal crystal structure. The surfaces for the observation of domain structures were prepared by polishing using emery papers and aluminium oxide. The domain structure was observed by means of metallographic oxide. The domain structure was observed by means of metallographic microscopy using the powder pattern method. The domain wall energy y was obtained from the surface domain width using the stereologic method described by Bodenberger and Hubert [5].
3. Results and discussion 3.1. Magnetic domain observations The domain structures observed in the UFe,,Si, and UCo,,Si, compounds are of the same type as those for crystals with high uniaxial magnetocrystalline anisotropy. Typical magnetic domains in the demagnetized state are shown in Fig. 1 and Fig. 2 for the UFe,,S& and UCo,,Si, compounds respectively. The upper parts of Figs. l(a) and 2(a) show the grain surfaces parallel to the easy axis (axial plane) whereas the lower parts of Figs. l(a) and 2(a) show the grain surface per-
Fig. 1. Typical domain structure in the UFe,,,S& compound: (a) two grains with magnetic easy direction parallel (upper part) and perpendicular (lower part) to the plane of the sample surface; (b) magnitied lower part of (a); (c) the grains with different crystallographic orientation of the sample surface.
117
Fig. 2. Typical domain structure in the UCo,,,Siz compound: (a) two grains with magnetic easy direction parallel (upper part) and perpendicular (lower part) to the plane of the sample surface; (b) magnified lower part of (a); (c) the grains with different crystallographic orientation of the sample surface.
pendicular to the easy axis (basal plane). On grain surfaces that are parallel to the easy direction, domains with 180” Bloch walls are apparent (upper parts of Figs. l(a) and 2(a)). At the ends of such domains, i.e. at the grain boundaries, closure domains occur in the form of spikes with magnetization antiparallel to that of the main domains in which they lie. However, the domain structures observed on the grain surfaces that are perpendicular to the easy direction are slightly different (compare magnified lower parts of Figs. l(a) and 2(a) presented in Figs. l(b) and 2(b)). A characteristic star-like structure (Fig. l(b)) is observed in UFe,,Si, (as in Nd2Fe,,B [6]) and a labyrinth-like structure (Fig. 2(b)) in UCo,,Si, (as in Dy,Fe,,B [6]). Circular domains on the perpendicular surface, such as shown in Figs. 1(b) and 2(b), are also spike-like closure domains. A further complication of the domain structure on the basal plane of uniaxial magnets is due to undulation of Bloch walls and the formation of secondary spikes with increasing thickness of the grains. If the grain surface makes an angle with the crystallographic axial planes, more complex domains arise (Figs. 1(c) and 2(c)). 3.2. Domain wall energy determination The domain wall energy y is determined mostly from the dependence of the domain width D on the crystal thickness L in the magnetically preferred direction. The relationship D(L) can be found from measurements of the widths of 180” main domains on the axial plane of the crystals or for thin crystals from the basal plane (when the domain structure on this plane is not complicated by waviness of domain walls near the surface or by the occurrence of spike domains). However, for more complicated surface structures on the basal plane, when the range of the crystal thicknesses for which the width of the surface domains does not depend on the thickness of the sample or when it is impossible to observe domain structures on the axial plane, the method of Bodenberger and Hubert [5] can be applied to calculate the domain wall energy y. Bodenberger and Hubert showed that the surface domain width D, for large sample thicknesses (or large grains in the
118
case of polycrystalline materials) is independent of thickness and is given by D, = 4n/3y/Mi, where MS is the magnetization and /I is a geometrical factor depending on the type of surface structure. In this paper we assumed /3 = 0.3 1, as was done, for example, for the SmCo, system [5] because the structures described in our work appeared similar to those on SmCo,. To measure D,, a statistical method was employed. A large number of lines in random directions were drawn on the basal plane patterns like those in Figs. l(b) and 2(b). The total length was divided by the number of intersections with domain walls to give a mean spacing of intersections. This mean value was then multiplied by 2/~r to give the average surface domain width D,. Using these mean values of D, and the values of MS we calculated the domain wall energies y for the UFe,,Si, and UCo,,Si, compounds which are listed in Table 1. The value of the domain wall energy for UFe,,Si,, is close to the values obtained for Nd,Fe,,B single crystals ( y = 20 y=18ergcme2, erg cm-2 [7]). 3.3. Magnetic domains under appliedfield The influence of the magnetic field (up to 5 kOe, i.e. to a fully magnetized state of the samples) on the domain wall motion was studied in the UFe,,Si, and UCo,,Si, compounds. It was found that the behaviour of these compounds was similar, the only difference being the magnitude of the field needed to create a single domain. Therefore, in this paper, the behaviour of domains under an applied field was shown only for the UCo,,Si, compound. In particular, special attention was paid to the domain wall pinning and the nucleation of the reverse domains in order to explain the origin of the low values of the coercive force and remanence. In Fig. 3 magnetizing and demagnetizing processes are illustrated by showing changes in domain structure of the large grains in the thermally demagnetized UCo,,Si, compound. As the field H, applied parallel to the easy direction in this grain, is increased from 0 to 390 Oe (Figs. 3(a) and 3(b)) domains with 180” domain walls which were magnetized antiparallel to the applied magnetic field direction shrink by smooth movement of domain walls until they are pinned by inclusions. On further increasing the field (Fig. 3(c), H= 730 Oe) these domains begin to narrow at the spherical inclusions where domain walls are pinned and then the increasing field causes the domain walls to break up (Fig. 3(d), H= 1080 Oe) and in this way two separate spike domains are created, which become smaller with any further’ increase in the field (Fig. 3(e), H = 2180 Oe). However, existing spike TABLE 1 The saturation magnetization M, (at room temperature), the average surface domain width D, and the domain wall energy yin the UFe,,,Si, and UCo,,,Si, compounds Compound
M, (G)
D, (m4
UFe,,,Si, UCo,,,Si,
850 390
0.98 1.15
18 5
Fig. 3. The influence of the magnetic field H on the domain structure of the UCo,,Si,: (a) H= 0 Oe, (b) 390 Oe, (c) 730 Oe, (d) 1080 Oe, (e) 2180 Oe, (f) 730 Oe, (g) 390 Oe and (h) 0 Oe. The arrow(b) indicates the direction of the applied field H.
domains with a magnetization opposite to the applied field direction begin to disappear during this process. As the field is decreased newly created spike domains (in the nucleation field H,, = 1080 Oe for this grain) begin to grow (Fig. 3(f), H= 730 Oe) and on further decreasing the field these domains are recombined into one domain with 180” domain walls (Fig. 3(g), H= 390 Oe). At zero field (the sample in remanent state, Fig. 3(h)) there are no distinct differences in domain structure in comparison with the initial state, which might be one of the reasons for the very low values of coercivity and remanence in the UCo,,Si, and UFe,,Si, compounds. In Fig. 4 the magnetization process on the grain surface of the thermally demagnetized UCo,,Si, compound is shown (H= 0 + 2.5 kOe -, 0 -, - 0.73 kOe-0). It has been observed that the magnetization process in the left and middle part of the grain takes place by shrinking of the spike domains which are magnetized antiparallel to the applied field direction. However, domains with 180” domain walls in which spike domains are apparent (see Fig. 4(b), upper grain boundary in the right part of the grain) are more difficult to magnetize. At a field of H= 2500 Oe the grain is single domain up to the maximum field (Fig. 4(c)). On decreasing the field spike domains reappeared spontaneously (in the nucleation field H, = 1250 Oe for this grain) almost at the same positions as those at which these spike domains disappeared upon saturation (Fig. 4(d), H= 390 Oe). Domain structures of the sample in the remanent (Fig. 4(e), H= 0 Oe) and initial states are similar and differ only in details. An increase in the applied field in the opposite
120
directiun causes new spike domains with reverse magnet~ation to be nucleated as a result of a disruption of the existing spike domains with magnetization parallel to the applied field direction and further convergence of adjacent domain walls which surround the area with reverse magnetization (compare middle part of Figs. 4(e)
Fig. 4. The influence of the magnetic field Han the domain structure of the UCo,,Si,: (a) H-0 Oe, (b) 730 Oe, (c) 2500 Oe, (d) 390 Qe, (e) 0 Oe, (f) - 390 Oe, (g) - 730 Oe and (h) 0 Oe. The arrows(b) and (ff indicate the direction of the appiied field H.
121
and 4(f)). On further increasing the field these newly created spike domains shrink (Fig. 4(g), H= 730 Oe) and finally disappear (when saturation is achieved). Decreasing the field to zero causes rising of the remanent domain structure, which is shown in Fig. 4(h). From the comparison of the remanent (Figs. 4(e) and 4(h)), and initial (Fig. 4(a)) domain structures it follows that these structures differ only in details. It is not possible to suggest any preferable sites for domain nucleation from the domain structure observations undertaken in the present paper.
4. Concluding
remarks
(1) The domain structures observed in UFe,,Si, and UCo,,Si, compounds are of the same type as those for crystals with high uniaxial magnetocrystalline anisotropy. (2) Using the values of average surface domain width D, domain wall energies y were calculated for UFe,,Si, and UCo,,,Si, and found to be equal to 18 erg cm-? and 5 erg cm- 2 respectively. (3) The magnetization process of the UFe,,,Si, and UCo,,,Si, compounds showed that there are no distinct differences between domain structures in the remanent and the initial state, which might be one of the reasons for the very low values of the coercivity and the remanence in these compounds.
References I K. H. J. Buschow, Mater. Sci. Rep., 1 (1986) 1. 2 J. M. Fournier and R. Trod, in A. J. Freeman and G. H. Lander (eds.), Handbook on the Physics and Chemisrry ofthe Actinides, Vol. 2, North-Holland, Amsterdam, 1985, p. 29. 3 W. Suski, A. Baran and T. Mydlarz, Whys. Lett. A, I36 (1989) 89. 4 A. Baran, M. kwkasiak, W. Suski, J. Suwalski, H. Figiel, J. Opti, K. Turek and T. Mydlarz, J. Map. Map. Mater., 83 (1990) 262. 5 R. Bodenberger and A. Hubert, Phys. Status Solidi, A, 44 (1977) K7. 6 D. Phrsa, J. J. Wyskxki, B. Wystocki and R. Pfranger, J. Less-Common Met., 133 (1987) 231. 7 R. Szymczak, D. Givord and H. S. Li, Acra Phys. Pal. A, 72 ( 1987) 113.