GdAI2
4. Intermetallic compounds with 3d transition metals
92 E. DORMANN
roughly proportional to (S z ), the total 4f
spin
of the lanthanide ionH s : as(Sz) +
aL(Lz),
(19)with as = 5 1 . 7 k O e and 15.7kOe for RA12 and RZn, respectively, and a L = 4 -+ 8 kOe or 12 --+ 19 kOe for RA12 or RZn, respectively. The orbital effects in the RA12 series are present, but relatively weak - at most 30% of the spin-polarization field. Orbital effects are more important in the RZn series (for ErZn they are three times larger than the spin-polarization part). For the RA12 series Berthier and Belorizky (1986) obtained estimates for the local orbital, spin and quadrupo- lar polarization of 5d-like conduction electrons: (l~) d ~<0.06, (s~) d ~<0.09 and (lz 2 -- 2)d ~ 0.05.
NMR IN INTERMETALLIC COMPOUNDS 93 were occasionally found. Such phenomena can be very helpful, if they are understood once, but they require very careful investigations, which were not always performed. Large anisotropies for the hyperfine interaction, especially for Co, and partially different moments of the same 3d element at different sites are well known complications. On the whole, it is an important, fascinating, but a difficult area for NMR applications.
4.1. Intermetallic R - F e compounds
Rare-earth intermetallic compounds with iron are analysed more easily by NMR than those with cobalt or manganese, because the Fe moment is less sensitive to the environment. Thus many NMR studies were reported despite the disadvantageous NMR properties of the favourite M6ssbauer nucleus 57Fe (small natural abundance, small nuclear magnetic moment). 89y was the favourite 'non-magnetic' rare-earth NMR probe.
A large positive hyperfine field is transferred from the iron sublattice to the lanthanide site for heavy lanthanide ions, as can be seen from the comparison of the lanthanide hyperfine fields in RFe 2 and RAI2: Hy,v e ~- +620 kOe for R = Gd (Budnick and Skalski 1967). A much weaker difference is observed between GdFe 2 and YFe 2. A1-Assadi et al. (1984) estimated that at most one third of the difference in the hyperfine fields of about 124-138kOe is due to a direct transferred hyperfine field from the Gd ions. They investigated the 165Ho NMR in GdxY0.97_xHo0.03Fe 2 and analysed their results in terms of a model involving a non-linear dependence of the transferred hyperfine field from iron atoms on the number of Gd neighbours. Nikitin et al. (1976) showed via the NMR of Gdl_xYxFe 3 that the contribution of the Gd sublattice to the hyperfine fields at the YH and Gd n nuclei is 26 and 75 kOe, respectively. They observed resolved satellites for Y and Gd, and derived that the hyperfine interaction of the Gd nuclei with the nearest neighbours in the Gd sublattice is anisotropic. Vas- il'kovskii et al. (1983) introduced a model for Y-Fe compounds based on NMR analysis of Y hyperfine fields, where the superposition of a 'collectivized' and a 'localized' component of the moment is assumed in order to explain the observed distribution of local fields at the Y nuclei. Later on, they extended their analysis;
Vasil'kovskii et al. (1988) compared the hyperfine fields for Y in a large number of Y-Fe and Y-Co intermetallic compounds. They concluded that the local fields, induced by 3d sublattice atoms, depend on the type of the 3d atom, but are almost independent of its magnetic moment and the stoichiometry of the com- pound. Their results were again explained with the assumption that there is a system of spin-polarized, 'collectivized' electrons, whose spin polarization (which is alligned antiparallel to the 3d sublattice magnetization) determines the field at the rare-earth nuclei and contributes to the moment of the 3d atoms, but that there is also another localized contribution of the spin density that does not act at the Y nucleus. Instead, Dumelow et al. (1986) concluded from their NMR pressure analysis of YFe 2 that a moment, antiparaUel to the Fe moment (of about
-0.45 ixB) exists in YFe 2 at the Y site.
94 E. DORMANN
The influence of the lanthanide moments on the iron sublattice is in general relatively weak. The Fe moment increases from YFe 2 to GdFe 2 only by a small amount. It is generally agreed that the Gd and Fe moments in the magnetically ordered phase of GdFe 2 or Gdl_xYxFe 2 compounds are antiparallel to one another. Nitikin et al. (1975a) explained the existence of three lines for 57Fe in GdF% by the difference between the magnetic moments of the Fe atoms in different crystallographic positions. For Gdl_xY~F%, Nikitin et al. (1976) consid- ered that the magnetic moments of the iron atoms, varying with the crystallo- graphic site, depend on the exchange interaction between the rare-earth and iron sublattices.
In a detailed and comprehensive analysis, Meyer et al. (1981) concluded from a number of different investigations in PrFe2, NdFe 2 and YbFe 2 that the crystalline electric field at the lanthanides is not constant throughout the RFe 2 series. For the isotropic as well as the anisotropic parts of the Fe hyperfine field, in addition to intrinsic d contributions, contributions originating from the lanthanides could also be derived. Due to the self-polarization field and the field transferred from Fe, the hyperfine field for the lanthanide nucleus is larger than the free-ion value for the second half, but smaller for the first half of the series. Belorizky et al. (1988) analysed the dependence of the R - R exchange interactions on the nature of the R atom in many intermetallics, including R-3d compounds such as R2Fe14B or RCo 2. They concluded that the molecular field coefficient describing the exchange interactions between lanthanide spin moments decreases in a given series by almost an order of magnitude from compounds with a light lanthanide to those with heavy lanthanide elements. Simultaneously a decrease is observed in the transferred hyperfine field at the non-magnetic sites. They showed that this behaviour can be coherently understood by considering that both the R - R and R - X indirect exchange interactions in lanthanide intermetallics occur via 5d conduction electrons and that the dominant 4f-5d exchange at lanthanide sites decreases from Pr to Tm by a factor of three.
Resolved satellites of the NMR lines originating from the rare-earth nuclei could generally be observed for intermetallic compounds if iron was replaced partially by other 3d or non-magnetic elements. Very well resolved satellite structures of the 89y resonance have been reported by Oppelt and Buschow (1976) for Y(Fel_~X~) 2 with X = A1, Co, Pt. They concluded that the Y hyperfine field is mainly determined by the interactions with the nearest magnetic neighbour atoms. They analysed the transferred hyperfine fields obtained for various values of x and determined the change of the magnetic moments of the 3d atoms with composition in Y(Fea_~Cox) 2. They obtained indications of partial delocalization of the magnetic moments with increasing x. Later on, Oppelt et al. (1977) extended their analysis also to intermetallic compounds with other non-magnetic rare-earth atoms. Yamada and Ohmae (1980) investigated 59Co and 89y NMR of Y(Fea_xCOx) 2 over the whole concentration range x. They concluded that on the Fe-rich side, the Co moment first decreases slightly (x ~< 0.3) and then rapidly with increasing Co content. Furthermore, Ichinose et al. (1985) observed well resolved Y satellites, when they followed the
89y
and 27A1 hyperfine fields inNMR IN INTERMETALLIC COMPOUNDS 95 Y(FeI_~Alx) 2 in zero-field spectra with the composition x. They showed that the concentration dependences of both hyperfine fields resemble those of the mean magnetic moment of the Fe atoms, in accordance with the behaviour of fairly well localized moments of the Fe atoms. Satellites of the Y and Gd NMR lines were observed also by Vasil'kovskii et al. (1988) for Y(Fel_~Xx) 2 and Gd(Fel_xX~) 2 with X = A1 and Co. They could be extrapolated to give the hyperfine contribu- tions from Fe and Co atoms to the local fields at the Y and Gd nuclei for various compositions. However, they concluded from the pressure dependence of the 89y hyperfine field in Y(Feo.95Coo.05)2 that the variation of the Y hyperfine field with Co content, extrapolated from such a variation, might actually be caused by the variation of the interatomic distance.
In conclusion, even for the relatively 'simple' case of the R - F e intermetallic compounds, a final conclusive picture of the electronic structures and hyperfine interactions for these compounds is in general not yet established. However, NMR has given many clear pieces of evidence for a better understanding.
4.2. Intermetallic R-Co compounds
The results in this area of NMR applications do not always agree very well- experimentally as well as with respect to the interpretation. Generally the cobalt hyperfine field in R - C o intermetallic compounds is anisotropic, and on some occasions even positive, both of which are indications of the presence of an orbital moment of Co. Due to its simplicity, the Laves phase structure is most appropriate for an analysis: there is only one R site; the Co s i t e s - u p to four magnetically inequivalent, but crystallographically equivalent s i t e s - c a n be dis- tinguished, because there is an electric quadrupolar interaction in addition to the magnetic hyperfine interaction, with the E F G axis lying along the local (111) axes (Figiel and Jaszczewski 1980). Many results of NMR investigations of the R - C o series of intermetallic compounds were discussed by Figiel (1983) (in Polish). If R is a lanthanide element with a 4f moment, the 3d moment of Co is polarized below a certain temperature by a molecular field through (indirect) 4f-3d exchange interactions. The direction of the polarization is always antiparallel to that of the 4f spin. This leads to parallel coupling between 4f and 3d moments in RCo 2 for light lanthanides and an antiparallel coupling for the heavy lanthanides.
Taylor and Christopher (1969) investigated for the first time the Co hyperfine fields in ferrimagnetically ordered GdCo 2 and related compounds. They analysed the Co hyperfine field as a superposition of a contribution of the Co ion's 'own' moment and the Co- and R-neighbour contributions,
Hhf,c o = Hown,Co + HN,Co + HN, R. (20a)
For more detailed investigations Hown,Co was later generally decomposed into
How,,Co = Hcp + H4s,c o + /-/or b . (20b)
The classical dipole field and the Lorentz field also have to be included. Hirosawa and Nakamura (1982a) performed a careful and detailed analysis of the Co
96 E. DORMANN
hyperfine interaction in RCo 2. They observed that the quadrupolar interaction frequency vQ in the magnetically ordered state is always larger than in the respective paramagnetic compounds (e.g., Barnes and Lecander 1967). Hirosawa and Nakamura (1982a) used also Tbl_xYxCo 2 for the derivation of the R contribution to the Co hyperfine field and observed well resolved satellites. They indicate the primary importance of the nearest-neighbour interaction between Tb and Co. It is especially gratifying that in this investigation for both magnetically inequivalent sites (magnetization direction (111): 0 = 0 ° and 70.5 ° sites) the NMR lines corresponding to Co with six Tb, and with five Tb one Y are resolved. The hyperfine coupling constant for the total spin part of the Co hyperfine field was determined to be a s ~ - 1 3 0 k O e £ 1 (spin), whereas a L =
+650kOe ~131
(orb) was obtained for the orbital part coupling constant. The isotropic orbital part of the Co moment varies between 3% of the total Co moment in GdCo 2 and 26% in PrCo 2. The anisotropic contribution is large and reaches up to 26% for SmCo 2.Since different signs of the 59Co NMR line shifts for the magnetically inequivalent sites were observed in external fields in magnetically ordered RCo 2 by Hirosawa and Nakamura (1982a), they had clear evidence that the resonance field at Co should be extremely anisotropic. Yoshie (1978) studied the variation of the 59Co NMR in Yl_xGdxCo2 for 0.1 ~< x ~< 1. The shift by 16.8 -+ 4.0 kOe observed in this range was interpreted as the contribution of 4f electrons to the cobalt hyperfine field in GdCo 2. Yoshimura et al. (1984a) investigated the local-environment effects in the RI_~Y, Co 2 system. In Dyl_~Y~Co2, the magnetic moment of Co and the hyperfine field distribution of the Co nuclei have been synthesized by superposition of the contributions from the Co atoms with the various R nearest-neighbour configurations. The dipole field due to the lanthanide ions in the nearest-neighbour shell turned out to play an important role in analysing the Co NMR spectra. They concluded that the nearest-neighbour interaction between R and Co is of prime importance in the RCo 2 series. The most pronounced dependence of the Co moment on the number of R nearest-neighbour ions has been observed for large Y concentrations.
Cannon et al. (1975) used the Jaccarino-Walker model for the transition-metal moment in some cubic Laves phase compounds. Specifically, they showed that in the Gd(COl_xNix) 2 system the Co moment appears to be critically dependent on the number and type of its nearest neighbours. Ichinose (1987) measured the NMR of 27A1, 55Mn,
59Co
and 155/157Gd in Gd(Xa_xCOx) 2 for X = A1, Mn, Fe and Ni. He found from the analysis of the NMR spectra that the concentration dependence of the 59Co hyperfine field is proportional to the sum of the conduction electron polarization arising from nearest-neighbour transition atoms and that the Co atoms carry a magnetic moment induced by the neighbouring X atoms.For 59Co in (Gd I xY~)Co2, Hirosawa et al. (1979) observed the coexistence of two types of Co atoms in the concentration range close to the critical concen- tration (x ~0.10): one has a magnetic moment that couples antiparallel with the Gd moment, and the other has no moment. Yamada and Ohmae (1980) also showed by NMR analysis of Y(Fel ~Co~) 2 the coexistence of magnetic and
NMR IN INTERMETALLIC COMPOUNDS 97 non-magnetic Co, by observation of
59Co
N M R both in zero and external fields at the critical concentration for the destruction of ferromagnetism. The Co moment was assumed to be induced by magnetic Fe among its near neighbours. YCo 2 and LuCo 2 are exchange-enhanced Pauli paramagnets. The Knight shift of 59Co in YCo 2 is temperature dependent (Hirosawa et al. 1979), as is the magnetic susceptibility. The correlation of both quantities gave the hyperfine coupling constant a = - 8 9 kOe i.q~ I associated with the Co magnetization. Its size indicated an important role of the orbital moment on the magnetism of YCo 2 (Hirosawa and Nakamura 1982a). The orbital magnetism in the paramagnetic phase was further analysed with the help of 59Co Knight shift measurements: the anisotropy of the S9Co Knight shift in YCo 2 at 4.2K was measured by Hirosawa and Nakamura (1982c); Khl = 2.1-+ 0.2% and K± = 1.36-+ 0.015%. Their analysis of the temperature dependence of the 59Co Knight shift required the consideration of many, partially compensating contributions, as the analysis of the hyperfine field in the ordered state does. By analysis of the temperature dependence of T 1 for YCoa, Yoshimura et al. (1984b) showed that the spin fluctuations with small wave number q play an important role in determining the magnetic properties of the 'nearly ferromagnetic' compound YCo 2. The necessary condition T 1 1 ~TXd
was fulfilled with reasonable accuracy. They concluded that X(q) is enhanced only in the small-q region. Nagai et al. (1988b) analysed the Knight shift and the spin-lattice relaxation time of S9Co in Y(Nil_xCox)2; they observed x-dependent changes that could be explained by the change in the density of states, which is dominated by Y contributions for low Co content and by the unenhanced and exchange-enhanced Pauli paramagnetism for higher Co concentration. Yoshie et al. (1985a) also observed Sgco in YC%_xNi,, for x i> 0.65, whose field depen- dence indicated a nearly paramagnetic state of C o .
Streever (i979) used N M R results (i.e., the large anisotropy HII - H± for 59Co) to evaluate the spin-orbit contribution to the magnetic anisotropy of Co atoms at individual Co sites in RCo 5 compounds. The easy c-axis Co anisotropy of RCo 5 compounds has been found to arise from the 2c sites, while the 3g sites make a smaller opposing contribution. The anisotropy parameters of the individual sites have been used to calculate the variation of the anisotropy with composition in mixed R(Col_xF%) 5 compounds (and also in some related structures, e.g., R2Co17 ). Yoshie et al. (1988a, b) analysed the 59Co N M R of R C % in the presence of an applied external field. They used the fact that Ni enters preferen- tially on 2c sites in R(Col_,Ni~) 5 for the site assignment (Yoshie et al. 1987).
From the N M R study of YCo 5 they found that the Co hyperfine field at the Co 2c site has a positive sign (+15 kOe) on contrast to the negative sign ( - 9 2 k O e ) at the 3g site. This result suggested that the cobalt atoms at the 2c site in YCo 5 have large orbital moments.
Streever (1975a) was the first to analyse N M R of R2Co17 compounds. By considering the number of local Co and Nd atoms around a Co site in Nd2COl7 , he showed which 59Co N M R lines corresponded to the four different Co sites.
Inomata (1976a) redetermined the effective Co hyperfine coupling constants for own and neighbouring moments. Inomata (1981) found that in Y2(fOl_xXx)a7 Cu
98 E. DORMANN
substitution for Co was at random, while A1 mainly prefers 6c dumbbell sites- which explains changes in the anisotropy. Calculating the crystal field in YCo 5 and Y2Co17 with the point charge model, he estimated the contribution of Y or Co on the Co-site magnetic anisotropy. Kawakami (1981) derived the anisotropy of the 59Co hyperfine field for Gd2(C%_xFex)17: HFj- H a = +31 kOe for 18h sites, + 3 0 k O e for 9d sites and - 2 0 k O e for 18f sites. Since a preferential substitution of Co by Fe was observed, these results had to be correlated with the composition dependence of the magnetic anisotropy. Figiel (1982a) analysed the anisotropy of R2(COI_~Mnx)lv compounds. The corresponding changes of the local anisotropy energy and the orbital part of the Co magnetic moment for each of the Co structural sites were calculated and discussed. He concluded that only two of the four Co sites in the R2Co17 structure play a dominant role in determining the anisotropy energy of the compound. Kakol and Figiel (1986) used the single-ion anisotropy model (point-charge- crystal-field calculations with shielding effects of band electrons) also to interpret their data of Yz(Co1_xMnx)i7 and Gd2(COa_xMnx)17 pseudobinaries.
4.3. Intermetallic R-Mn compounds
Intermetallic R - M n compounds were recently rather actively studied with the help of NMR. They show interesting, occasionally puzzling, magnetic behaviour.
NMR turned out to be a useful probe for these investigations. A large part of the activities was devoted to YMn 2 and related pseudobinary compounds. YMn 2 has a first-order phase transition at about T N = 110K from a high-temperature paramagnetic state with small susceptibility (X increases with T for T > TN indicative of itinerant electron character) to a low-temperature antiferromagnetic state with an Mn moment of about 2.7 tx B.
Different Y-Mn intermetallic compounds were analysed with 55Mn NMR by Yoshimura and Nakamura (1983). They concluded that YMn 2 is an antiferromag- net with a [111] easy direction of magnetization and TN ~ 100 K. A hyperfine coupling constant of Io~l ~ 42.7 kOe ~ 1 was derived from YMn 2, the same as for YMna2 , a compound that also behaved as an itinerant electron antiferromagnet (T N = 120 K). Complex ferrimagnetic behaviour was observed for Y6Mn23. Nagai et al. (1983) analysed the complicated Sgy satellite structures in Y(F% xMnx) 2 by spin-echo NMR and concluded that two kinds of Mn should occur at low x, one with a moment of 0.5 ~x B antiparallel to the Fe moment and one with a moment of 2.7 P"B parallel to the Fe moment. Yoshimura et al. (1986b) reported that the NMR results for Y(Mnl_xXx) 2 can be described by the localized-moment model for X = A1, both in the paramagnetic and the ordered states. For X = Fe and Co, the Mn moment becomes much smaller as x increases and the results of NMR suggest that these alloys are weakly itinerant antiferromagnets. The different behaviour of the Mn moment could be related to the atomic spacing of Mn in these systems. Nakamura et al. (1988 0 studied the influence of chemical pressure on the magnetism of YMn 2 via substitution of R = Sc or La in Y~_xR~Mn 2.
Substitution of Sc for Y leads to a shrinkage of the lattice, makes the Mn moment