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Strength of the R-T exchange coupling in R2T17 compounds
Journal of Magnetism and Magnetic Materials 104-107 (1992) 1325-1326 North-Holland
Strength of the R - T exchange coupling in R2T17compounds R. Verhoef, S. Sinnema, P.H. Quang and J.J.M. Franse Van der Waals-Zeeman Laboratorium der Unicersiteit can Amsterdam, Valckenierstraat 65, 1018 XE Amsterdam, Netherlands The R-T exchange-coupling parameter JRT has been deduced for a number of ferrimagnetic R2TI7 compounds (DY2C017, H02C017, Er2C017 and HozCo14Fe 3) from high-field magnetisation measurements on single-crystalline spheres that are allowed to orient themselves in the applied magnetic field. For JRT values ranging from 0.942 to 1.020× 10 22 j have been deduced. The magnetic properties of the hard-magnetic R - T intermetallics are governed by two types of interactions: crystalline electrical field (CEF) and exchange interactions. Among the exchange interactions that can be distinguished in R - T intermetallic compounds (the T - T , R - R and R - T interaction), the R - T exchange interaction is the most decisive one for the magnetic behaviour at low temperatures. Magnetisation measurements on oriented (single crystalline) samples have proven to be a useful technique for determining the strength of the CEF and exchange interactions. A disadvantage of this experimental technique is, however, that by an analysis of the magnetisation curves it is not always possible to derive the CEF and R - T exchange parameter in an independent and unambigious way. The experimental curves are largely determined by the balance between these two types of interactions. For ferrimagnetic compounds, however, it has recently been shown [1] in high-field magnetisation measurements performed on samples that are free to orient themselves in the applied magnetic field, that it is possible to derive directly an independent value for the strength of the R - T exchange coupling. Characteristic for this type of magnetisation experiments is that at a certain field value the magnetisation curve reveals an abrupt increase of the differential susceptibility to an almost constant value. From an analysis within a two-sublattice model [1], this abrupt increase in the differential susceptibility can be related to a critical field value at which the R and T sublattice magnetic moments start to bend towards each other under the influence of the applied magnetic field. Moreover, it can be shown that the observed constant differential susceptibility above the critical field is in first approximation directly proportional to 1 / n R v , where nRx is the inter-sublattice molecular-field coefficient, which is a measure for the strength of the R - T exchange coupling. T h e coefficient nRT can be expressed in the microscopic Heisenberg type of exchange parameter JRT via the expression [1]: gR M'2NT JRT = - - nRT, ( g R - 1) ZRT
(1)
where gR is the Land6 factor, N T is the number of T atoms per unit mass and ZRT is the number of nearest T atom neighbours of an R atom. For nearly all types of R - T intermetallic compounds this new experimental technique has been applied [2] and has proven to be a useful extension of the experimental techniques by which a value for the strength of the R - T exchange coupling can be derived. Other methods for obtaining this information are the study of the temperature dependence of the rare-earth magnetisation, the analysis of the Curie temperatures within a given series of R - T compounds and inelastic neutron scattering experiments. Here, results are reported of measurements performed on some single-crystalline RzT17 spheres of Dy2COl7 , H02C017 , Er2COl7 , Er2Fe17, ErzFel7 and HozCol4Fe 3. Dy2C017, H02Col7, Er2C017, Er2Fel7 and pseudobinary Ho2COl4Fe3 single-crystalline batches have been grown by an adapted Czochralski technique [3]. Spheres with a diameter of 3 mm have been spark eroded out of the as-grown single-crystalline batches. In case of H02Col7 , two batches have been grown, one of which turned out to be slightly off-stoichiometric and Ho rich with a composition H02.05C016.90 [4]. The experimental results of the sample with the stoichiometric composition have been reported already elsewhere [1]. Here, we focus on the results of the Ho-rich sample. At 4.2 K, magnetisation experiments have been performed at the Amsterdam High-Field Facility [5] on these spherical samples, which were free to orient themselves in the applied magnetic field. The exl~erimental results are presented in fig. 1. These results show that for all samples, except Er2Fe17, the shape of the magnetisation curve is in good agreement with the model predictions. A virtually constant magnetisation value is observed up to the critical field value and above this field value the curves show a constant differential susceptibility from which a value for the coefficient n RT can be derived. The values for nRT and the corresponding values for the parameter JRT, calculated by employing eq. (1) with ZRT = 19, are listed in table 1. The nRT values derived from an analysis of high-field magnetisation measurements performed on the same but oriented crystals are included
0312-8853/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved
R. Verhoef et al. / R-Texchange coupling in R 2TI7
1326 ~00
80
B D O ~ D O D ~ D O
~
O~
~
~
60 oJ E
40
I
b
Ho2CUl/ Ho2ColdFe o ~r2Co 17 Er'2Fe17 o Dy2Soi 7
20
,
0
I
10
0
i
I
20 BET]
30
40
Fig. 1. High-field magnetisation curves at 4.2 K of several R2TI7 single-crystalline spheres that are free to orient themselves in the applied magnetic field.
in table 1 for comparison. T h e coefficient n nT could be d e t e r m i n e d for DY2C017 , H02C017 and pseudo-binary H 0 2 C o 14Fe 3 with a fair accuracy from magnetisation m e a s u r e m e n t s on oriented samples due to the discontinuities in the m a g n e t i s a t i o n curves. T h e fields at which these discontinuities occur are largely gove r n e d by the value of the coefficient n nT [6--8]. For Er2Fe~7 no data are p r e s e n t e d in table 1. T h e large
Table 1 Values for the coefficient nnr and the parameter JRT of some ReTI7 compounds as determined by a fitting procedure of magnetisation experiments on single-crystalline samples that are free to orient themselves in the applied magnetic field. For comparison in the last column the nnT values as derived from magnetisation experiments on the same but oriented spheres are given Compound DY2COl7 Ho2Co 17 a) Ho2CoI 7 b) Er2Co ~7 Ho2CoHFe 3
nRT
JRt
nRT
[ k g T / A m 2]
[10 22]
[ k g T / A m 21
0.730(5) 0.544(3) 0.562(4) [1] 0.456(3) 0.544(3)
1.(/20(7) 0.942(5) 0.977(7) [1] 0.949(6) 0.953(5)
0.720(5) 0.545 0.54(2)
") The Ho-rich H02Co17 sphere. b) The Ho2Co17 sphere with the stoichiometric 2:17 composition.
hysteresis above the critical field value m a k e s an u n a m biguous d e t e r m i n a t i o n of the coefficient nRr impossible in this case. A p a r t from this e x p e r i m e n t a l problem, it is a question w h e t h e r a simple two-sublattice model, in which the T sublattice is assumed to have a negligible small anisotropy c o m p a r e d with the R-sublattice, is applicable for this particular c o m p o u n d . F r o m lowt e m p e r a t u r e m a g n e t i s a t i o n data, it is evident thai the R- and T-sublattice anisotropies are of nearly equal m a g n i t u d e in E r 2 F e n [9,10]. W i t h i n the experimental accuracy, the values of the coefficient nwr as derived by m a g n e t i s a t i o n e x p e r i m e n t s p e r f o r m e d on free and oriented samples are in good a g r e e m e n t . In conclusion, the e x p e r i m e n t a l m a g n e t i s a t i o n curves o b t a i n e d for single-crystalline spheres that are free to orient themselves in the applied magnetic field can be described satisfactorily except for Er2Fel7 , within the earlier developed two-sublattice model. T h e values derived tot the molecular-field coefficient hR. r are in good agreemerit with values derived from m a g n e t i s a t i o n m e a s u r e m e n t s p e r f o r m e d on oriented single crystalline samples.
References [1] R. Verhoef, R.J. Radwafiski and J.J.M. Franse, J. Magn. Magn. Mater. 89 (1990) 176. [2] F.R. de Boer, Physica B (1991) in press, and references therein. [3] A. Menovsky and J.J.M. Fransc, J. Cryst. Growth 65 (1983) 286. [4] R. Verhoef, thesis, Natuurkundig Laboratorium, University of Amsterdam (1990). [5] R. Gersdorf, F.R. de Boer, J.C. Wolfrat, F.A. Muller and L.W. Roeland, In: ttigh Field Magnetism, ed. E. Date (North-Holland, Amsterdam, 1983) p. 277. [6] R.J. Radwafiski, J.J.M. Franse and S. Sinnema, J. Magn. Magn. Mater. 51 (1985) 175. [7] S. Sinnema, J.J.M. Franse, A. Menovsky and R.J. Radwanski, J. Magn. Magn. Mater. 54-57 (1986) 1639. [8] S. Sinnema, J.J.M. Franse, R.J. Radwafiski, A. Menovsky and F.R. de Boer, J. Phys. F 17 (1987) 233. [9] S. Sinnema, thesis, Natuurkundig Laboratorium, University of Amsterdam, The Netherlands (1988). [10] R. Verhoef, F.R. de Boer, S. Sinnema, J.J.M. Franse, F. Tomiyama, M. Ono, M. Date and A. Yamagishi, Physica B (1991) in press.
Strength of the R - T exchange coupling in R2T17compounds R. Verhoef, S. Sinnema, P.H. Quang and J.J.M. Franse Van der Waals-Zeeman Laboratorium der Unicersiteit can Amsterdam, Valckenierstraat 65, 1018 XE Amsterdam, Netherlands The R-T exchange-coupling parameter JRT has been deduced for a number of ferrimagnetic R2TI7 compounds (DY2C017, H02C017, Er2C017 and HozCo14Fe 3) from high-field magnetisation measurements on single-crystalline spheres that are allowed to orient themselves in the applied magnetic field. For JRT values ranging from 0.942 to 1.020× 10 22 j have been deduced. The magnetic properties of the hard-magnetic R - T intermetallics are governed by two types of interactions: crystalline electrical field (CEF) and exchange interactions. Among the exchange interactions that can be distinguished in R - T intermetallic compounds (the T - T , R - R and R - T interaction), the R - T exchange interaction is the most decisive one for the magnetic behaviour at low temperatures. Magnetisation measurements on oriented (single crystalline) samples have proven to be a useful technique for determining the strength of the CEF and exchange interactions. A disadvantage of this experimental technique is, however, that by an analysis of the magnetisation curves it is not always possible to derive the CEF and R - T exchange parameter in an independent and unambigious way. The experimental curves are largely determined by the balance between these two types of interactions. For ferrimagnetic compounds, however, it has recently been shown [1] in high-field magnetisation measurements performed on samples that are free to orient themselves in the applied magnetic field, that it is possible to derive directly an independent value for the strength of the R - T exchange coupling. Characteristic for this type of magnetisation experiments is that at a certain field value the magnetisation curve reveals an abrupt increase of the differential susceptibility to an almost constant value. From an analysis within a two-sublattice model [1], this abrupt increase in the differential susceptibility can be related to a critical field value at which the R and T sublattice magnetic moments start to bend towards each other under the influence of the applied magnetic field. Moreover, it can be shown that the observed constant differential susceptibility above the critical field is in first approximation directly proportional to 1 / n R v , where nRx is the inter-sublattice molecular-field coefficient, which is a measure for the strength of the R - T exchange coupling. T h e coefficient nRT can be expressed in the microscopic Heisenberg type of exchange parameter JRT via the expression [1]: gR M'2NT JRT = - - nRT, ( g R - 1) ZRT
(1)
where gR is the Land6 factor, N T is the number of T atoms per unit mass and ZRT is the number of nearest T atom neighbours of an R atom. For nearly all types of R - T intermetallic compounds this new experimental technique has been applied [2] and has proven to be a useful extension of the experimental techniques by which a value for the strength of the R - T exchange coupling can be derived. Other methods for obtaining this information are the study of the temperature dependence of the rare-earth magnetisation, the analysis of the Curie temperatures within a given series of R - T compounds and inelastic neutron scattering experiments. Here, results are reported of measurements performed on some single-crystalline RzT17 spheres of Dy2COl7 , H02C017 , Er2COl7 , Er2Fe17, ErzFel7 and HozCol4Fe 3. Dy2C017, H02Col7, Er2C017, Er2Fel7 and pseudobinary Ho2COl4Fe3 single-crystalline batches have been grown by an adapted Czochralski technique [3]. Spheres with a diameter of 3 mm have been spark eroded out of the as-grown single-crystalline batches. In case of H02Col7 , two batches have been grown, one of which turned out to be slightly off-stoichiometric and Ho rich with a composition H02.05C016.90 [4]. The experimental results of the sample with the stoichiometric composition have been reported already elsewhere [1]. Here, we focus on the results of the Ho-rich sample. At 4.2 K, magnetisation experiments have been performed at the Amsterdam High-Field Facility [5] on these spherical samples, which were free to orient themselves in the applied magnetic field. The exl~erimental results are presented in fig. 1. These results show that for all samples, except Er2Fe17, the shape of the magnetisation curve is in good agreement with the model predictions. A virtually constant magnetisation value is observed up to the critical field value and above this field value the curves show a constant differential susceptibility from which a value for the coefficient n RT can be derived. The values for nRT and the corresponding values for the parameter JRT, calculated by employing eq. (1) with ZRT = 19, are listed in table 1. The nRT values derived from an analysis of high-field magnetisation measurements performed on the same but oriented crystals are included
0312-8853/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved
R. Verhoef et al. / R-Texchange coupling in R 2TI7
1326 ~00
80
B D O ~ D O D ~ D O
~
O~
~
~
60 oJ E
40
I
b
Ho2CUl/ Ho2ColdFe o ~r2Co 17 Er'2Fe17 o Dy2Soi 7
20
,
0
I
10
0
i
I
20 BET]
30
40
Fig. 1. High-field magnetisation curves at 4.2 K of several R2TI7 single-crystalline spheres that are free to orient themselves in the applied magnetic field.
in table 1 for comparison. T h e coefficient n nT could be d e t e r m i n e d for DY2C017 , H02C017 and pseudo-binary H 0 2 C o 14Fe 3 with a fair accuracy from magnetisation m e a s u r e m e n t s on oriented samples due to the discontinuities in the m a g n e t i s a t i o n curves. T h e fields at which these discontinuities occur are largely gove r n e d by the value of the coefficient n nT [6--8]. For Er2Fe~7 no data are p r e s e n t e d in table 1. T h e large
Table 1 Values for the coefficient nnr and the parameter JRT of some ReTI7 compounds as determined by a fitting procedure of magnetisation experiments on single-crystalline samples that are free to orient themselves in the applied magnetic field. For comparison in the last column the nnT values as derived from magnetisation experiments on the same but oriented spheres are given Compound DY2COl7 Ho2Co 17 a) Ho2CoI 7 b) Er2Co ~7 Ho2CoHFe 3
nRT
JRt
nRT
[ k g T / A m 2]
[10 22]
[ k g T / A m 21
0.730(5) 0.544(3) 0.562(4) [1] 0.456(3) 0.544(3)
1.(/20(7) 0.942(5) 0.977(7) [1] 0.949(6) 0.953(5)
0.720(5) 0.545 0.54(2)
") The Ho-rich H02Co17 sphere. b) The Ho2Co17 sphere with the stoichiometric 2:17 composition.
hysteresis above the critical field value m a k e s an u n a m biguous d e t e r m i n a t i o n of the coefficient nRr impossible in this case. A p a r t from this e x p e r i m e n t a l problem, it is a question w h e t h e r a simple two-sublattice model, in which the T sublattice is assumed to have a negligible small anisotropy c o m p a r e d with the R-sublattice, is applicable for this particular c o m p o u n d . F r o m lowt e m p e r a t u r e m a g n e t i s a t i o n data, it is evident thai the R- and T-sublattice anisotropies are of nearly equal m a g n i t u d e in E r 2 F e n [9,10]. W i t h i n the experimental accuracy, the values of the coefficient nwr as derived by m a g n e t i s a t i o n e x p e r i m e n t s p e r f o r m e d on free and oriented samples are in good a g r e e m e n t . In conclusion, the e x p e r i m e n t a l m a g n e t i s a t i o n curves o b t a i n e d for single-crystalline spheres that are free to orient themselves in the applied magnetic field can be described satisfactorily except for Er2Fel7 , within the earlier developed two-sublattice model. T h e values derived tot the molecular-field coefficient hR. r are in good agreemerit with values derived from m a g n e t i s a t i o n m e a s u r e m e n t s p e r f o r m e d on oriented single crystalline samples.
References [1] R. Verhoef, R.J. Radwafiski and J.J.M. Franse, J. Magn. Magn. Mater. 89 (1990) 176. [2] F.R. de Boer, Physica B (1991) in press, and references therein. [3] A. Menovsky and J.J.M. Fransc, J. Cryst. Growth 65 (1983) 286. [4] R. Verhoef, thesis, Natuurkundig Laboratorium, University of Amsterdam (1990). [5] R. Gersdorf, F.R. de Boer, J.C. Wolfrat, F.A. Muller and L.W. Roeland, In: ttigh Field Magnetism, ed. E. Date (North-Holland, Amsterdam, 1983) p. 277. [6] R.J. Radwafiski, J.J.M. Franse and S. Sinnema, J. Magn. Magn. Mater. 51 (1985) 175. [7] S. Sinnema, J.J.M. Franse, A. Menovsky and R.J. Radwanski, J. Magn. Magn. Mater. 54-57 (1986) 1639. [8] S. Sinnema, J.J.M. Franse, R.J. Radwafiski, A. Menovsky and F.R. de Boer, J. Phys. F 17 (1987) 233. [9] S. Sinnema, thesis, Natuurkundig Laboratorium, University of Amsterdam, The Netherlands (1988). [10] R. Verhoef, F.R. de Boer, S. Sinnema, J.J.M. Franse, F. Tomiyama, M. Ono, M. Date and A. Yamagishi, Physica B (1991) in press.