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NMR-analysis of indirect magnetic interactions in intermetallic rare-earth compounds
NMR-ANALYSIS O F I N D I R E C T MAGNETIC I N T E R A C T I O N S IN I N T E R M E T A L L I C RARE-EARTH COMPOUNDS E. D O R M A N N Physikalisches Institut, Unit~rsitiJt Bayreuth, D-8580 Bayreuth, Fed. Rep. Germany
U. GOEBEL, H. K R O P P Institut fiJr Festki~rperphysik, Technisehe Hochschule Darmstadt, D-6100 Darmstadt, Fed. Rep. Germany
and K. H. J. B U S C H O W Philips Research Laboratories, Eindhoven, The Netherlands
NMR Spectroscopy has given several pieces of information with regard to the indirect magnetic interactions in intermetallic compounds of gadolinium or europium. These comprise information on the contribution of non-s conduction electrons, on the predominance of short-range interactions, on the anisotropy of the transferred hyperfine interaction and on the range of d electron spin polarization.
An interesting problem to do with indirect magnetic interactions in intermetallic rare earth (RE) compounds is the question whether coupling proceeds predominantly via conduction electrons (ce) of s-character or not. A qualitative decision can be made with the help of zero field N M R in the magnetically ordered state of these compounds. The total transferred hyperfine field at the RE site can be determined, for instance, by means of 139La N M R in La doped compounds (a small proportion of La replacing the trivalent RE moment). It can also be determined by studying the concentration dependence of the rare earth hyperfine field BRE(X) in isoelectronic series like (REj_xNMx)nMe,, where N M is a non-magnetic element. The value of the transferred hyperfine field can be compared with the value of the magnetic ordering temperature. Both quantities depend on the transferred ce spin polarization at the RE site, the hyperfine field by way of the hyperfine coupling constants and the ordering temperature via the respective exchange coupling constants. These constants are roughly known, in general. Both quantities should be positive when only scharacter ce are mediating the indirect interaction. They can have different signs when non-s ce, especially d-like ce are of importance. Such an analysis in ferromagnetic Gd-intermetallics (with a nonmagnetic partner element) [1] revealed that the s-electrons predominate only in those compounds investigated that have a weak coupling (GdRh, G d R h 2, G d P t 2, GdNi, G d N i 2, Gdlr2). 658
In the compounds that have a high ordering temperature (GdA12, GdZn) 5d-like ce clearly are essential. A corresponding analysis of the antiferromagnet GdAg [2] proved that a predominant contribution of d-like ce can also lead to antiferromagnetic coupling. In Eu intermetallics (Eu2+; [3]) we found that coupling via non-s ce is important in many compounds including even those with low magnetic ordering temperatures (EuZn 5, EuCua, EuSn 3, EuCu 5, EuA12). N M R spectroscopy also makes it possible to determine the contribution of distinct neighbours to the total transferred hyperfine field at the RE sites in intermetallic compounds. Despite the general occurrence of a large broadening of the zero field 139La N M R lines, we were able to observe the appearence of satellites to the main 139La resonance line in (RE~_xLax)mM % for increasing x (e.g. in (Gd l_xLax)Rh 2 or (Gd I _xLax)Ir: [1]). This, in fact, means that the predominant neighbour contributions can be read directly from the N M R spectrum. Similar results could be obtained for the N M R of the nonmagnetic partner in RE intermetallics (e.g. for 67Zn in G d Z n [4]). The decision, which RE neighbours are responsible for the dominant contributions observed is not always trivial, at least if it is based only on the concentration dependence of the line intensities in the zero field spectra of powder samples. Rather convincing is the analysis of the angular dependence of the N M R spectrum in an external magnetic field, obtained on a single crystal sample. From investigations on paramagnetic La I _xGdxAg
Journal of Magnetism and Magnetic Materials 15-18 (1980) 658-659 ©North Holland
E. Dormann et al./ NMR-analysis of indirect magnetic interactions
with a small Gd concentration (x = 0.005), it could be derived, via the dipolar part of the anisotropy that a third nearest G d neighbour gives the largest contribution to the ~a9La hyperfine field [2]. Actually in this example the splitting of the respective La resonance lines is four times larger than could be explained by a classical dipolar interaction. This is a further piece of experimental evidence for the presence of orbital contributions in the indirect magnetic interactions, even in intermetfillic compounds containing only S-state RE-ions [4]. Finally the combination of results obtained by means of N M R and neutron scattering makes it possible to prove that indirect magnetic interaction via 5d-like ce in intermetallic compounds is not restricted to nearest neighbours. Pierre et al. [5] derived the exchange parameters between distinct RE neighbours in H o Z n from inelastic neutron scattering data. The dominant contributions stem from the 1st, 2nd, and 3rd nearest neighbours, 11 -- - 79 mK, 1 2 - ~ - - 7 m K , and 13 = - 19 mK, all being ferromagnetic. The decomposition of the transferred hyperfine field at the RE site made by means of N M R data obtained on concentrated ferromagnetically ordered Gdl_x139LaxZn [4] and on dilute paramagnetic ~39Lal _xGdxZn [6] leads to the following neighbour contributions to the total field: AB l = - 0.69 T, A B 2 = - - 0.06 T and AB 3 = - 0.28 T. (Several more distant neighbourtypes (n > 4) also contribute about - 0 . 0 6 T). A comparison of sign and magnitude of the total (negative) transferred hyperfine fields at the RE sites with the (ferromagnetic) ordering temperatures in case of H o Z n and G d Z n [1, 4] indicates that the
659
prevailing coupling is via d-like ce. Furthermore, taking into account that the hyperfine coupling constants of s- and d-like ce differ in sign and also differ in size by an order of magnitude, the correlation between the indirect exchange parameters I i and the hyperfine field contributions AB i demonstrates clearly that this d-like ce spin polarization extends at least to the third nearest magnetic neighbour. The examples summarized show that N M R analysis can give detailed information on the indirect magnetic interactions in intermetallic REcompounds. We are indebted to Prof. B. Elschner for discussions. The N M R equipment used at Darmstadt was provided by the Bundesministerium f/Jr Bildung und Wissenschaft and by the Deutsche Forschungsgemeinschaft (SFB 65-Festk6rpers p e k t r o s k o p i e - D a r m s t a d t / F r a n k f u r t).
References [!] E. Dormann, J. Magn. Magn. Mat. 6 (1977) 87, and refs. therein. [2] U. Goebel and E. Dormann, J. Magn. Magn. Mat. 13 (1979) 219. [3] H. Kropp, W. Zipf, E. Dormann and K. H. J. Buschow, J. Magn. Magn. Mat. 13 (1979) 224. [4] K. Eckrich, E. Dormann, A. Oppelt and K. H. J. Buschow, Z. Phys. B23 (1976) 157. [5] J. Pierre, D. Schmitt, P. Morin and B. Hennion, J. Phys. F 7 (1977) 1965. [6] U. Goebel, PhD Thesis, Technische Hochschule Darmstadt (1979).
U. GOEBEL, H. K R O P P Institut fiJr Festki~rperphysik, Technisehe Hochschule Darmstadt, D-6100 Darmstadt, Fed. Rep. Germany
and K. H. J. B U S C H O W Philips Research Laboratories, Eindhoven, The Netherlands
NMR Spectroscopy has given several pieces of information with regard to the indirect magnetic interactions in intermetallic compounds of gadolinium or europium. These comprise information on the contribution of non-s conduction electrons, on the predominance of short-range interactions, on the anisotropy of the transferred hyperfine interaction and on the range of d electron spin polarization.
An interesting problem to do with indirect magnetic interactions in intermetallic rare earth (RE) compounds is the question whether coupling proceeds predominantly via conduction electrons (ce) of s-character or not. A qualitative decision can be made with the help of zero field N M R in the magnetically ordered state of these compounds. The total transferred hyperfine field at the RE site can be determined, for instance, by means of 139La N M R in La doped compounds (a small proportion of La replacing the trivalent RE moment). It can also be determined by studying the concentration dependence of the rare earth hyperfine field BRE(X) in isoelectronic series like (REj_xNMx)nMe,, where N M is a non-magnetic element. The value of the transferred hyperfine field can be compared with the value of the magnetic ordering temperature. Both quantities depend on the transferred ce spin polarization at the RE site, the hyperfine field by way of the hyperfine coupling constants and the ordering temperature via the respective exchange coupling constants. These constants are roughly known, in general. Both quantities should be positive when only scharacter ce are mediating the indirect interaction. They can have different signs when non-s ce, especially d-like ce are of importance. Such an analysis in ferromagnetic Gd-intermetallics (with a nonmagnetic partner element) [1] revealed that the s-electrons predominate only in those compounds investigated that have a weak coupling (GdRh, G d R h 2, G d P t 2, GdNi, G d N i 2, Gdlr2). 658
In the compounds that have a high ordering temperature (GdA12, GdZn) 5d-like ce clearly are essential. A corresponding analysis of the antiferromagnet GdAg [2] proved that a predominant contribution of d-like ce can also lead to antiferromagnetic coupling. In Eu intermetallics (Eu2+; [3]) we found that coupling via non-s ce is important in many compounds including even those with low magnetic ordering temperatures (EuZn 5, EuCua, EuSn 3, EuCu 5, EuA12). N M R spectroscopy also makes it possible to determine the contribution of distinct neighbours to the total transferred hyperfine field at the RE sites in intermetallic compounds. Despite the general occurrence of a large broadening of the zero field 139La N M R lines, we were able to observe the appearence of satellites to the main 139La resonance line in (RE~_xLax)mM % for increasing x (e.g. in (Gd l_xLax)Rh 2 or (Gd I _xLax)Ir: [1]). This, in fact, means that the predominant neighbour contributions can be read directly from the N M R spectrum. Similar results could be obtained for the N M R of the nonmagnetic partner in RE intermetallics (e.g. for 67Zn in G d Z n [4]). The decision, which RE neighbours are responsible for the dominant contributions observed is not always trivial, at least if it is based only on the concentration dependence of the line intensities in the zero field spectra of powder samples. Rather convincing is the analysis of the angular dependence of the N M R spectrum in an external magnetic field, obtained on a single crystal sample. From investigations on paramagnetic La I _xGdxAg
Journal of Magnetism and Magnetic Materials 15-18 (1980) 658-659 ©North Holland
E. Dormann et al./ NMR-analysis of indirect magnetic interactions
with a small Gd concentration (x = 0.005), it could be derived, via the dipolar part of the anisotropy that a third nearest G d neighbour gives the largest contribution to the ~a9La hyperfine field [2]. Actually in this example the splitting of the respective La resonance lines is four times larger than could be explained by a classical dipolar interaction. This is a further piece of experimental evidence for the presence of orbital contributions in the indirect magnetic interactions, even in intermetfillic compounds containing only S-state RE-ions [4]. Finally the combination of results obtained by means of N M R and neutron scattering makes it possible to prove that indirect magnetic interaction via 5d-like ce in intermetallic compounds is not restricted to nearest neighbours. Pierre et al. [5] derived the exchange parameters between distinct RE neighbours in H o Z n from inelastic neutron scattering data. The dominant contributions stem from the 1st, 2nd, and 3rd nearest neighbours, 11 -- - 79 mK, 1 2 - ~ - - 7 m K , and 13 = - 19 mK, all being ferromagnetic. The decomposition of the transferred hyperfine field at the RE site made by means of N M R data obtained on concentrated ferromagnetically ordered Gdl_x139LaxZn [4] and on dilute paramagnetic ~39Lal _xGdxZn [6] leads to the following neighbour contributions to the total field: AB l = - 0.69 T, A B 2 = - - 0.06 T and AB 3 = - 0.28 T. (Several more distant neighbourtypes (n > 4) also contribute about - 0 . 0 6 T). A comparison of sign and magnitude of the total (negative) transferred hyperfine fields at the RE sites with the (ferromagnetic) ordering temperatures in case of H o Z n and G d Z n [1, 4] indicates that the
659
prevailing coupling is via d-like ce. Furthermore, taking into account that the hyperfine coupling constants of s- and d-like ce differ in sign and also differ in size by an order of magnitude, the correlation between the indirect exchange parameters I i and the hyperfine field contributions AB i demonstrates clearly that this d-like ce spin polarization extends at least to the third nearest magnetic neighbour. The examples summarized show that N M R analysis can give detailed information on the indirect magnetic interactions in intermetallic REcompounds. We are indebted to Prof. B. Elschner for discussions. The N M R equipment used at Darmstadt was provided by the Bundesministerium f/Jr Bildung und Wissenschaft and by the Deutsche Forschungsgemeinschaft (SFB 65-Festk6rpers p e k t r o s k o p i e - D a r m s t a d t / F r a n k f u r t).
References [!] E. Dormann, J. Magn. Magn. Mat. 6 (1977) 87, and refs. therein. [2] U. Goebel and E. Dormann, J. Magn. Magn. Mat. 13 (1979) 219. [3] H. Kropp, W. Zipf, E. Dormann and K. H. J. Buschow, J. Magn. Magn. Mat. 13 (1979) 224. [4] K. Eckrich, E. Dormann, A. Oppelt and K. H. J. Buschow, Z. Phys. B23 (1976) 157. [5] J. Pierre, D. Schmitt, P. Morin and B. Hennion, J. Phys. F 7 (1977) 1965. [6] U. Goebel, PhD Thesis, Technische Hochschule Darmstadt (1979).