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Magnetic properties of Er2Fe17C and Er2Fe17N2.7 studied with 57Fe and 166Er Mössbauer spectroscopy
Journal of Magnetism and Magnetic Materials 101 (1991) 395-396 North-Holland
Magnetic properties of Er2Fe17C and Er2Fe17N2.7 studied with 57Fe and 166Er M6ssbauer spectroscopy A.A. M o o l e n a a r a P.C.M. G u b b e n s and K.H.J. Buschow b
a, G.J. B o e n d e r a, T.H. Jacobs b
a Interfacultair Reactor Instituut, Delft University of Technology, Mekelweg 15, 2629 JB Delft, Netherlands b Philips Research Laboratories, 5600 JA, Eindhoven, Netherlands
The 57Fe and 166Er M6ssbauer effect in Er2Fel7C and Er2Fel7N2.7 was studied in order to investigate the spin-reorientation temperature TSR and the second order field parameter A°: TSR and IA°l increases from respectively 83 K and (290_+50) Kao 2 in ErzFe17C to approximately 110 K respectively (400_+50) Kao 2 in Er2Fel7N2.7. 1. Introduction In several previous investigations it was shown that the magnetic properties of R2Fe17 compounds, with R a rare earth element, were strongly improved with interstitial solution of H, C or N [1-3]. Although there are no marked changes in the Fe sublattice magnetizations, one observes a considerable increase in Curie temperature with interstitial solution. Also strong increases in the rare earth sublattice anisotropies were reported for the carbides and nitrides, even to the extent that the corresponding Sm compounds could be considered as candidates for applications as permanent magnets. In order to study these reported changes in some more detail, we have investigated these compounds with 57Fe and 166Er M6ssbauer spectroscopy.
TSR =110 K
I E -r
, 35-
30~ Q~ ," ~N~ 25 57Fe : Er2Fe17N2.7 20 0
I
1
I
I
100
200
300
400
I
500 600 T(K) Fig. 1. The temperature dependence of the effectiv~ hyperfine field of 57Fe averaged over the 4f site (Hefff) and over the three other sites (H~f).
2. Experimental results The samples Er2F%7C and Er2Fe17N x were prepared using the procedure as described elsewhere [4]. High resolution neutron diffraction has shown that the composition of the ternary nitrides corresponds to approximately x = 2.7 [5]. This value will be used in the formula of the nitride when describing the properties below. The volume increase of Er2FeI7N2. 7 compared to Er2Fet7 was nearly 7%, the Curie temperature of Er2F%TN2. 7 is 690 K. The 57Fe M6ssbauer spectra of both compounds were obtained in the range 4.2-580 K by means of a constant acceleration type spectrometer equipped with a 57Co-Rh source. These spectra were fitted by means of a standard procedure [6] taking into account the four different Fe sites (Wyckoff notation: 4f, 6g, 12j, 16k). In ErzFe17C the relative intensities found at 4.2 K were close to the intensity ratios expected, except for the 4f site, due to a larger recoilless fraction [7]. This so-called dumb-beU site could however easily be distinguished from the other sites because of its much higher
hyperfine field. With this the situation is completely analogous to that of the compound Er2Fe17 [7]. In ErzFe17N2. 7 on the other hand we were able to resolve only the 4f site subspectrum. When one follows the temperature dependence of the hyperfine fields in ErzFea7C, one observes a sudden increase at all sites in the hyperfine field at T = 83 K. The increase is particularly pronounced for the 4f site. This increase indicates a spin-reorientation, in which the easy direction of the magnetization changes from perpendicular to the c-axis (T > TSR) to parallel to the c-axis ( T < TSR). The same transition was observed at approximately 110 K for the compound Er2Fe17N2. 7, although the transition has broadened (see fig. 1). The 166Er M6ssbauer spectra were obtained using a 166HoPd3 source. An electric quadrupole splitting of (0.49 + 0.05) c m / s was observed in ErzFelTC and a quadrupole splitting of (0.37 _+0.05) c m / s was observed in ErzFe17N2. 7.
0312-8853/91/$03.50 © 1991 - Elsevier Science Publishers B.V. All rights reserved
396
A.A. Moolenaar et al. / Magnetic properties of Er2 Fe l7C and Er2Fe l7 N2. 7
,g -200 QZ
-400
t I
Er'2 Fel7
_
I
I
Er'2 FeI7C
Er2Fet7N2. 7
Fig. 2. The second order crystal field parameters A ° for ErzFel7, Er2Fet7C and ErzFel7N2. 7.
3. Discussion The increase in the hyperfine fields at TsR in E r z F e l v C , and the comparatively strong increase in the hyperfine field at the 4f site, agrees well with the 57Fe M6ssbauer spectroscopy results obtained for T m z F e l v C x [1]. The observation of this hyperfine anomaly at TsR is also in agreement with the results of the high-field magnetization measurements, which shows that the easy magnetization direction at 4.2 K differs from that at room temperature [8]. The electric quadrupole splitting observed at the E r site in the 166Er M6ssbauer spectra, can be attributed to two contributions, the free ion contribution, i.e. the 4f electron contribution to the electric field gradient at the erbium nucleus, and a lattice contribution: QS = QS 4f + QS lan. Since the a66Er hyperfine field in these compounds has the free ion value ( J = 15/2), Q S 4f must also have the free ion value of 0.81 c m / s . Fur-
thermore the magnetization and the electric field gradient below TSR are both parallel to the c-axis, therefore: QS latt = ge~v~z 1 ~,,latt . When one assumes a proportional relation between eVzz and A°: eV~z = c A ° ~ 4 with c = 270 [9], one finds a second order crystal field parameter of ( - 4 0 0 + 50) Kao 2 for Er2FexTN2. 7 and a second order crystal field parameter of ( - 2 9 0 + 50) Ka(~ 2 for Er2Fe17C (see fig. 2). It is interesting to note that recent results in Gd2Mn17C x obtained by 155Gd M6ssbauer spectroscopy indicate that there is a strong increase in I A ° l when the interstitial hole position (Wyckoff notation: 9e) around each rare earth atom is fully occupied ( x = 3 ) [10]. In the Er2Fe17 compounds the N content is close to this maximum occupation. This suggests that the enhancement of the [ A~ [ and TSR values shown in fig. 2, is primarily the result of this higher hole occupation number, not that of a difference in nature between C and N.
References [1] P.C.M. Gubbens, A.M. van der Kraan, T.H. Jacobs and K.H.J. Buschow, J. Magn. Magn. Mater. 80 (1989) 265. [2] Hong Sun, J.M.D. Coey, Y. Otani and D.P.F. Hurley, J. Phys.: Condens. Matter. 2 (1990) 6465. [3] O. Isnard, S. Miraglia, J.L. Soubeyroux, D. Fruchart and A. Stergiou, J. Less-Common Met. 162 (1990) 273. [4] P.C.M. Gubbens, A.A. Moolenaar, G.J. Boender, A.M. van der Kraan, T.H. Jacobs and K.HJ. Buschow, J. Magn. Magn. Mater. 97 (1991) 69. [5] R.M. Ibberson, O. Moze, T.H. Jacobs and K.HJ. Buschow, J. Phys.: Con& Matter, to be published. [6] P.C.M. Gubbens, A.M. van der Kraan, J.J. van Loef and K.H.J. Buschow, J. Magn. Magn. Mater. 67 (1987) 255. [7] P.C.M. Gubbens, thesis, Delft University of Technology (1977). [8] J.P. Liu, F,R. de Boer and K.H.J. Buschow, J. Magn. Magn. Mater. 98 (1991) 291. [9] P.C.M. Gubbens, A.M. van der Kraan and K.H.J. Buschow, Phys. Rev. B 39 (1989) 17. [10] M.W. Dirken, R.C. Thiel, T.H. Jacobs and K.H.J. Buscbow, J. Magn. Magn. Mater. 94 (1991) L15.
Magnetic properties of Er2Fe17C and Er2Fe17N2.7 studied with 57Fe and 166Er M6ssbauer spectroscopy A.A. M o o l e n a a r a P.C.M. G u b b e n s and K.H.J. Buschow b
a, G.J. B o e n d e r a, T.H. Jacobs b
a Interfacultair Reactor Instituut, Delft University of Technology, Mekelweg 15, 2629 JB Delft, Netherlands b Philips Research Laboratories, 5600 JA, Eindhoven, Netherlands
The 57Fe and 166Er M6ssbauer effect in Er2Fel7C and Er2Fel7N2.7 was studied in order to investigate the spin-reorientation temperature TSR and the second order field parameter A°: TSR and IA°l increases from respectively 83 K and (290_+50) Kao 2 in ErzFe17C to approximately 110 K respectively (400_+50) Kao 2 in Er2Fel7N2.7. 1. Introduction In several previous investigations it was shown that the magnetic properties of R2Fe17 compounds, with R a rare earth element, were strongly improved with interstitial solution of H, C or N [1-3]. Although there are no marked changes in the Fe sublattice magnetizations, one observes a considerable increase in Curie temperature with interstitial solution. Also strong increases in the rare earth sublattice anisotropies were reported for the carbides and nitrides, even to the extent that the corresponding Sm compounds could be considered as candidates for applications as permanent magnets. In order to study these reported changes in some more detail, we have investigated these compounds with 57Fe and 166Er M6ssbauer spectroscopy.
TSR =110 K
I E -r
, 35-
30~ Q~ ," ~N~ 25 57Fe : Er2Fe17N2.7 20 0
I
1
I
I
100
200
300
400
I
500 600 T(K) Fig. 1. The temperature dependence of the effectiv~ hyperfine field of 57Fe averaged over the 4f site (Hefff) and over the three other sites (H~f).
2. Experimental results The samples Er2F%7C and Er2Fe17N x were prepared using the procedure as described elsewhere [4]. High resolution neutron diffraction has shown that the composition of the ternary nitrides corresponds to approximately x = 2.7 [5]. This value will be used in the formula of the nitride when describing the properties below. The volume increase of Er2FeI7N2. 7 compared to Er2Fet7 was nearly 7%, the Curie temperature of Er2F%TN2. 7 is 690 K. The 57Fe M6ssbauer spectra of both compounds were obtained in the range 4.2-580 K by means of a constant acceleration type spectrometer equipped with a 57Co-Rh source. These spectra were fitted by means of a standard procedure [6] taking into account the four different Fe sites (Wyckoff notation: 4f, 6g, 12j, 16k). In ErzFe17C the relative intensities found at 4.2 K were close to the intensity ratios expected, except for the 4f site, due to a larger recoilless fraction [7]. This so-called dumb-beU site could however easily be distinguished from the other sites because of its much higher
hyperfine field. With this the situation is completely analogous to that of the compound Er2Fe17 [7]. In ErzFe17N2. 7 on the other hand we were able to resolve only the 4f site subspectrum. When one follows the temperature dependence of the hyperfine fields in ErzFea7C, one observes a sudden increase at all sites in the hyperfine field at T = 83 K. The increase is particularly pronounced for the 4f site. This increase indicates a spin-reorientation, in which the easy direction of the magnetization changes from perpendicular to the c-axis (T > TSR) to parallel to the c-axis ( T < TSR). The same transition was observed at approximately 110 K for the compound Er2Fe17N2. 7, although the transition has broadened (see fig. 1). The 166Er M6ssbauer spectra were obtained using a 166HoPd3 source. An electric quadrupole splitting of (0.49 + 0.05) c m / s was observed in ErzFelTC and a quadrupole splitting of (0.37 _+0.05) c m / s was observed in ErzFe17N2. 7.
0312-8853/91/$03.50 © 1991 - Elsevier Science Publishers B.V. All rights reserved
396
A.A. Moolenaar et al. / Magnetic properties of Er2 Fe l7C and Er2Fe l7 N2. 7
,g -200 QZ
-400
t I
Er'2 Fel7
_
I
I
Er'2 FeI7C
Er2Fet7N2. 7
Fig. 2. The second order crystal field parameters A ° for ErzFel7, Er2Fet7C and ErzFel7N2. 7.
3. Discussion The increase in the hyperfine fields at TsR in E r z F e l v C , and the comparatively strong increase in the hyperfine field at the 4f site, agrees well with the 57Fe M6ssbauer spectroscopy results obtained for T m z F e l v C x [1]. The observation of this hyperfine anomaly at TsR is also in agreement with the results of the high-field magnetization measurements, which shows that the easy magnetization direction at 4.2 K differs from that at room temperature [8]. The electric quadrupole splitting observed at the E r site in the 166Er M6ssbauer spectra, can be attributed to two contributions, the free ion contribution, i.e. the 4f electron contribution to the electric field gradient at the erbium nucleus, and a lattice contribution: QS = QS 4f + QS lan. Since the a66Er hyperfine field in these compounds has the free ion value ( J = 15/2), Q S 4f must also have the free ion value of 0.81 c m / s . Fur-
thermore the magnetization and the electric field gradient below TSR are both parallel to the c-axis, therefore: QS latt = ge~v~z 1 ~,,latt . When one assumes a proportional relation between eVzz and A°: eV~z = c A ° ~ 4 with c = 270 [9], one finds a second order crystal field parameter of ( - 4 0 0 + 50) Kao 2 for Er2FexTN2. 7 and a second order crystal field parameter of ( - 2 9 0 + 50) Ka(~ 2 for Er2Fe17C (see fig. 2). It is interesting to note that recent results in Gd2Mn17C x obtained by 155Gd M6ssbauer spectroscopy indicate that there is a strong increase in I A ° l when the interstitial hole position (Wyckoff notation: 9e) around each rare earth atom is fully occupied ( x = 3 ) [10]. In the Er2Fe17 compounds the N content is close to this maximum occupation. This suggests that the enhancement of the [ A~ [ and TSR values shown in fig. 2, is primarily the result of this higher hole occupation number, not that of a difference in nature between C and N.
References [1] P.C.M. Gubbens, A.M. van der Kraan, T.H. Jacobs and K.H.J. Buschow, J. Magn. Magn. Mater. 80 (1989) 265. [2] Hong Sun, J.M.D. Coey, Y. Otani and D.P.F. Hurley, J. Phys.: Condens. Matter. 2 (1990) 6465. [3] O. Isnard, S. Miraglia, J.L. Soubeyroux, D. Fruchart and A. Stergiou, J. Less-Common Met. 162 (1990) 273. [4] P.C.M. Gubbens, A.A. Moolenaar, G.J. Boender, A.M. van der Kraan, T.H. Jacobs and K.HJ. Buschow, J. Magn. Magn. Mater. 97 (1991) 69. [5] R.M. Ibberson, O. Moze, T.H. Jacobs and K.HJ. Buschow, J. Phys.: Con& Matter, to be published. [6] P.C.M. Gubbens, A.M. van der Kraan, J.J. van Loef and K.H.J. Buschow, J. Magn. Magn. Mater. 67 (1987) 255. [7] P.C.M. Gubbens, thesis, Delft University of Technology (1977). [8] J.P. Liu, F,R. de Boer and K.H.J. Buschow, J. Magn. Magn. Mater. 98 (1991) 291. [9] P.C.M. Gubbens, A.M. van der Kraan and K.H.J. Buschow, Phys. Rev. B 39 (1989) 17. [10] M.W. Dirken, R.C. Thiel, T.H. Jacobs and K.H.J. Buscbow, J. Magn. Magn. Mater. 94 (1991) L15.