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Crystal field parameters in the RE Be13 intermetallics
227
Journal of Magnetism and Magnetic Materials 31-34 (1983) 227-228 C R Y S T A L F I E L D P A R A M E T E R S I N T H E R E Bel3 I N T E R M E T A L L I C S M.J. B E S N U S , P. P A N I S S O D , J.P. K A P P L E R , G. H E I N R I C H a n d A. M E Y E R LMSES, lnstitut de Physique, 3, rue de l'Universitk, 67084 Strasbour~ France
New results on CEF effects in cubic RE Be]3 compounds with RE = Nd, Sm, Er are obtained from magnetic measurements. The crystal field parameter A4(r 4) shows an unusual variation through the series with a change of sign for RE = Tb.
1. Introduction The RE Bel3 intermetallics crystallize in the cubic NaZnl3 structure where the RE are in a cubic arrangement. Each RE is closely surrounded quasi-spherically by 24 Be atoms. These compounds are characterized by small R E - i o n interactions, e.g. TN(GdBe13 ) = 25 K, ensuring near single ion behaviour. Due to these properties, small interactions and high crystal symmetry, allowing the description of CEF in terms of only two parameters, the RE Bel3 appear as good candidates for studying CEF effects. While detailed information is available concerning the magnetic properties throughout the series [1,2], comparatively few attempts have been made to determine the CEF properties. CEF data concern mainly PrBel3, TmBel3 [1], ErBel3 [3,4] and TbBej3 [5]. The aim of this paper is to present new data on RE Be]3 with Sm, Nd, Er which complete our previous studies ob YbBe13 [6], PrBel3 [7] and TbBel3 [5]. In these series the RE including Sm, Eu and Yb are in the trivalent ionic state; in contrast CeBet3 has been well established to be a mixed valence system, but an integral 3 valency is demonstrated for the dilute (CeLa)Bel3 alloys. 2. Experimental High-field (up to 150 kOe) magnetization measurements were performed on polycrystalline samples in the temperature range 0.07-4.2 K. Susceptibility measurements were carried out between 1.5 and 800 K. Additional experiments (resistivity, NMR, EPR) concern the Yb and Pr compounds and have been reported elsewhere [6,7]. Details regarding sample preparation are given in refs. [2] and [6].
change field. The crystal field parameters (CFP) were determined by fitting the magnetization curves and x ( T ) variations. The results were analyzed in the usual way, through a diagonalization of a Hamiltonian molecular, applied and crystalline fields. The magnetization obtained from the eigenfunctions and eigenvalues is averaged by weighing along the three major symmetry axes. The crystal field parameters extracted from these analyses are listed in table 1 and shown in fig. 1. The overall splittings range from 10 K (TbBe13) to 135 K (PrBe13). Except PrBel3 and TmBel3 which show Van Vleck paramagnetism and ThBe13, an induced moment system, the other compounds investigated have magnetic ground states. For NdBel3 and SmBe~3 the best fit to our experimental results yields the respective level sequence F 2, F6 (36 K), F8l (70 K) and/'7, F 8 (30 K). For ErBel3 specific heat data [3] and inelastic neutron scattering experiments [4] lead to sets of parameters (table 1) differing mainly in the sign of x; these sets yield however the same F 7 ground state separated by only 10 K from the first excited level F8I, the overall splitting being - 40-50 K. This case is somewhat complicated due to the large degeneracy of the lowest J multiplet, and if one looks at the possible level arrangements for J = 15/2 one remarks that in the - 0 . 3 to
A4~r4~
50
As REBe ~
O 0
(K) ~
O
3. Results and discussion Our experiments were performed on the binary RE Bel3 and on La diluted samples, to avoid difficulties arising from the low-temperature ordered states. Indeed the effect of substituting La for RE does not significantly change the crystal field in these systems, which can be explained by assuming that the CEF is predominantly due to the Be neighbours, but reduces the ex0304-8853/83/0000-0000/$03.00
O
O
i
Pr
i
Nd
i
i
Sm
i
i
I
Tb
i
Dy
I
Ho
i
Er
[]
T~II~
[]
I
Yb
Fig. 1. CFP versus RE; open symbols A,,(r4), full symbols A6(r6); I":1, G, X from refs. [1,3,4]; point charge model: dashed line (A4), dotted fine (Ar).
© 1983 N o r t h - H o l l a n d
228
M.J. Besnus et al. / Crystal field parameters in R E Bet3
Table 1 LLW parameters x, w; CFP A4, A 6 and overall splitting A Com pound
x
PrBel3 [7] [1] NdBel3 SmBets TbBe13 [5] ErBe 13
--0.8 --0.7 0.6 1 -0.8 - 0.1 - 0.276 -0.276 0.318 -0.35 0.45
[4] [3] TmBel3 [1] YbBe~3 [6]
w
A4
A6
(K)
(K)
(K)
2.45 2.08 - 1.2 5 0.05 0.08 0.12 0.094 0.099 -0.134 1.90
44 6.4 33 8.1 41 5.0 33 -5.4 - 1.2 - 3 2.5 - 12 3 -10 2.4 12 2.4 -4.8 2.06 -8.1 5.6
(K) 135 125 70 30 10 42 49 38 48 30 46
+ 0.3 x range the only difference coonsists in the relative positions of the F6 and F~ levels. Analysis of our results show that only the solutions with x < 0 give satisfactory agreement for both high and low field data. Two sets of x, w values give the best fits which equal M (within 0.5%). The corresponding level schemes are: F7, F~ (10 K), F6 (17 K), F 2 (29 K), F83 (42 K) for x = - 0 . 1 , w = 0.08 and F 7, F~ (10 K), F 6 (13 K) F 2 (40 K),/"83 (49 K) for x = - 0 . 2 7 6 , w = 0.12. This experimental method of determining crystal fields thus does not permit, in this complicated case, the determination of a unique set of x, w values. However we must notice that the different solutions with x < 0 yield nearly the same forth and sixth order crystal fields. For YbBe13 our results evidence a magnetic transition at T N = 1.28 K, the ground state being the F 7 magnetic doublet. The stable trivalent ionic state of Yb is confirmed by the MiSssbauer isomer shift at 50 m K [8]. If CeBel3 is well known to be a IV compound, the (CeLa)Bej3 system exhibits K o n d o like behaviour [9]. No well-defined C E F effects have been observed in such IV systems. However the magnetic contribution to the resistivity versus temperature curves with large logarithmic decreases can be interpreted in the C o r n u t - C o q b l i n model [10] by combined influences of Kondo and crystal field effects. In this model the
ratio of the observed logarithmic slopes is indicative of a splitting of - 130 K between the/'8 and F 7 levels. The C F P extracted from our measurements (fig. l) in agreement with neutron and specific heat data, when available, confirm and complete the noticeable variation of A 4 throughout the series, in contrast to the A 6 values which (except for Tb) scatter between 2 and 8 K. The most striking result is the surprising change of sign of A 4 for R E = T b resulting in two distinct domains determined by the light R E for which the point charge model is roughly satisfied (if Z = 2) and the heavy R E which experience a small C E F with nearly constant and negative A 4 values. A significant change of the C F P due to lattice parameter variation and to the effect of the further neighbourhood as well as that of its atomic number dependence is unlikely. As proposed by Williams [11] one may invokve a 5d contribution to only the fourth order C E F terms, which partly cancels the Coulomb contribution, this 5d contribution being then strongly 4f n dependent.
References [1] E. Bucher, J.P. Malta, G.W. Hull, R.C. Fulton and A.S. Cooper, Phys. Rev. Bll (1975)440. [2] A. Herr, M.J. Besnus and A. Meyer, Coll. Int. CNRS 242 (1975) 47. [3] H.W.M. Van der Linden, G.J. Nieuwenhuys, H.D. Dokter, D. Davidov and I. Felner, J. Magn. Magn. Mat. 15-18 (1980) 42. [4] F. Vigneron, M. Bonnet and R. Kahn, in: Crystalline Electric Fields and Structural Effects in f Electron Systems Plenum, New York (1980) p. 513. [5] J.M. Bouton, R. Clad, A. Herr, P. M6riel, A. Meyer and F. Vigneron, J. Magn. Magn. Mat. 15-18 (1980) 49. [6] G. Heinrich, J.P. Kappler and A. Meyer, Phys. Lett. 74A (1979) 121. [7] M.J. Besnus, J.P. Kappler, B. Lemius, M. Benakki and A. Meyer, J. Appl. Phys. 53 (1982) 2158. [8] P. Bonville, private communication. [9] J.P. Kappler, G. Krill, M.F. gavet, M.J. Besnus and A. Meyer, in: Valence Fluctuations in Solids (North-Holland, Amsterdam, 1981) p. 271. [10] B. Cornut and B. Coqblin, Phys. gev. B5 (1972) 4541. [11] G. Williams and L.L. Hirst, Phys. Rev. 185 (1969) 407.
Journal of Magnetism and Magnetic Materials 31-34 (1983) 227-228 C R Y S T A L F I E L D P A R A M E T E R S I N T H E R E Bel3 I N T E R M E T A L L I C S M.J. B E S N U S , P. P A N I S S O D , J.P. K A P P L E R , G. H E I N R I C H a n d A. M E Y E R LMSES, lnstitut de Physique, 3, rue de l'Universitk, 67084 Strasbour~ France
New results on CEF effects in cubic RE Be]3 compounds with RE = Nd, Sm, Er are obtained from magnetic measurements. The crystal field parameter A4(r 4) shows an unusual variation through the series with a change of sign for RE = Tb.
1. Introduction The RE Bel3 intermetallics crystallize in the cubic NaZnl3 structure where the RE are in a cubic arrangement. Each RE is closely surrounded quasi-spherically by 24 Be atoms. These compounds are characterized by small R E - i o n interactions, e.g. TN(GdBe13 ) = 25 K, ensuring near single ion behaviour. Due to these properties, small interactions and high crystal symmetry, allowing the description of CEF in terms of only two parameters, the RE Bel3 appear as good candidates for studying CEF effects. While detailed information is available concerning the magnetic properties throughout the series [1,2], comparatively few attempts have been made to determine the CEF properties. CEF data concern mainly PrBel3, TmBel3 [1], ErBel3 [3,4] and TbBej3 [5]. The aim of this paper is to present new data on RE Be]3 with Sm, Nd, Er which complete our previous studies ob YbBe13 [6], PrBel3 [7] and TbBel3 [5]. In these series the RE including Sm, Eu and Yb are in the trivalent ionic state; in contrast CeBet3 has been well established to be a mixed valence system, but an integral 3 valency is demonstrated for the dilute (CeLa)Bel3 alloys. 2. Experimental High-field (up to 150 kOe) magnetization measurements were performed on polycrystalline samples in the temperature range 0.07-4.2 K. Susceptibility measurements were carried out between 1.5 and 800 K. Additional experiments (resistivity, NMR, EPR) concern the Yb and Pr compounds and have been reported elsewhere [6,7]. Details regarding sample preparation are given in refs. [2] and [6].
change field. The crystal field parameters (CFP) were determined by fitting the magnetization curves and x ( T ) variations. The results were analyzed in the usual way, through a diagonalization of a Hamiltonian molecular, applied and crystalline fields. The magnetization obtained from the eigenfunctions and eigenvalues is averaged by weighing along the three major symmetry axes. The crystal field parameters extracted from these analyses are listed in table 1 and shown in fig. 1. The overall splittings range from 10 K (TbBe13) to 135 K (PrBe13). Except PrBel3 and TmBel3 which show Van Vleck paramagnetism and ThBe13, an induced moment system, the other compounds investigated have magnetic ground states. For NdBel3 and SmBe~3 the best fit to our experimental results yields the respective level sequence F 2, F6 (36 K), F8l (70 K) and/'7, F 8 (30 K). For ErBel3 specific heat data [3] and inelastic neutron scattering experiments [4] lead to sets of parameters (table 1) differing mainly in the sign of x; these sets yield however the same F 7 ground state separated by only 10 K from the first excited level F8I, the overall splitting being - 40-50 K. This case is somewhat complicated due to the large degeneracy of the lowest J multiplet, and if one looks at the possible level arrangements for J = 15/2 one remarks that in the - 0 . 3 to
A4~r4~
50
As
O 0
(K) ~
O
3. Results and discussion Our experiments were performed on the binary RE Bel3 and on La diluted samples, to avoid difficulties arising from the low-temperature ordered states. Indeed the effect of substituting La for RE does not significantly change the crystal field in these systems, which can be explained by assuming that the CEF is predominantly due to the Be neighbours, but reduces the ex0304-8853/83/0000-0000/$03.00
O
O
i
Pr
i
Nd
i
i
Sm
i
i
I
Tb
i
Dy
I
Ho
i
Er
[]
T~II~
[]
I
Yb
Fig. 1. CFP versus RE; open symbols A,,(r4), full symbols A6(r6); I":1, G, X from refs. [1,3,4]; point charge model: dashed line (A4), dotted fine (Ar).
© 1983 N o r t h - H o l l a n d
228
M.J. Besnus et al. / Crystal field parameters in R E Bet3
Table 1 LLW parameters x, w; CFP A4, A 6 and overall splitting A Com pound
x
PrBel3 [7] [1] NdBel3 SmBets TbBe13 [5] ErBe 13
--0.8 --0.7 0.6 1 -0.8 - 0.1 - 0.276 -0.276 0.318 -0.35 0.45
[4] [3] TmBel3 [1] YbBe~3 [6]
w
A4
A6
(K)
(K)
(K)
2.45 2.08 - 1.2 5 0.05 0.08 0.12 0.094 0.099 -0.134 1.90
44 6.4 33 8.1 41 5.0 33 -5.4 - 1.2 - 3 2.5 - 12 3 -10 2.4 12 2.4 -4.8 2.06 -8.1 5.6
(K) 135 125 70 30 10 42 49 38 48 30 46
+ 0.3 x range the only difference coonsists in the relative positions of the F6 and F~ levels. Analysis of our results show that only the solutions with x < 0 give satisfactory agreement for both high and low field data. Two sets of x, w values give the best fits which equal M (within 0.5%). The corresponding level schemes are: F7, F~ (10 K), F6 (17 K), F 2 (29 K), F83 (42 K) for x = - 0 . 1 , w = 0.08 and F 7, F~ (10 K), F 6 (13 K) F 2 (40 K),/"83 (49 K) for x = - 0 . 2 7 6 , w = 0.12. This experimental method of determining crystal fields thus does not permit, in this complicated case, the determination of a unique set of x, w values. However we must notice that the different solutions with x < 0 yield nearly the same forth and sixth order crystal fields. For YbBe13 our results evidence a magnetic transition at T N = 1.28 K, the ground state being the F 7 magnetic doublet. The stable trivalent ionic state of Yb is confirmed by the MiSssbauer isomer shift at 50 m K [8]. If CeBel3 is well known to be a IV compound, the (CeLa)Bej3 system exhibits K o n d o like behaviour [9]. No well-defined C E F effects have been observed in such IV systems. However the magnetic contribution to the resistivity versus temperature curves with large logarithmic decreases can be interpreted in the C o r n u t - C o q b l i n model [10] by combined influences of Kondo and crystal field effects. In this model the
ratio of the observed logarithmic slopes is indicative of a splitting of - 130 K between the/'8 and F 7 levels. The C F P extracted from our measurements (fig. l) in agreement with neutron and specific heat data, when available, confirm and complete the noticeable variation of A 4 throughout the series, in contrast to the A 6 values which (except for Tb) scatter between 2 and 8 K. The most striking result is the surprising change of sign of A 4 for R E = T b resulting in two distinct domains determined by the light R E for which the point charge model is roughly satisfied (if Z = 2) and the heavy R E which experience a small C E F with nearly constant and negative A 4 values. A significant change of the C F P due to lattice parameter variation and to the effect of the further neighbourhood as well as that of its atomic number dependence is unlikely. As proposed by Williams [11] one may invokve a 5d contribution to only the fourth order C E F terms, which partly cancels the Coulomb contribution, this 5d contribution being then strongly 4f n dependent.
References [1] E. Bucher, J.P. Malta, G.W. Hull, R.C. Fulton and A.S. Cooper, Phys. Rev. Bll (1975)440. [2] A. Herr, M.J. Besnus and A. Meyer, Coll. Int. CNRS 242 (1975) 47. [3] H.W.M. Van der Linden, G.J. Nieuwenhuys, H.D. Dokter, D. Davidov and I. Felner, J. Magn. Magn. Mat. 15-18 (1980) 42. [4] F. Vigneron, M. Bonnet and R. Kahn, in: Crystalline Electric Fields and Structural Effects in f Electron Systems Plenum, New York (1980) p. 513. [5] J.M. Bouton, R. Clad, A. Herr, P. M6riel, A. Meyer and F. Vigneron, J. Magn. Magn. Mat. 15-18 (1980) 49. [6] G. Heinrich, J.P. Kappler and A. Meyer, Phys. Lett. 74A (1979) 121. [7] M.J. Besnus, J.P. Kappler, B. Lemius, M. Benakki and A. Meyer, J. Appl. Phys. 53 (1982) 2158. [8] P. Bonville, private communication. [9] J.P. Kappler, G. Krill, M.F. gavet, M.J. Besnus and A. Meyer, in: Valence Fluctuations in Solids (North-Holland, Amsterdam, 1981) p. 271. [10] B. Cornut and B. Coqblin, Phys. gev. B5 (1972) 4541. [11] G. Williams and L.L. Hirst, Phys. Rev. 185 (1969) 407.