Hyperfine fields and magnetism in the Nd2Fe14B compound: a combined Mössbauer and NMR study

Journal of Magnetism and Magnetic Materials 116 (1992) 211-219 North-Holland i

Hyperfine fields and magnetism in the Nd2Fe14B compound: a combined M6ssbauer and NMR study S.H. G e , Y.D. Z h a n g , F.S. Li Department of Physics, Lanzhou Unicersity, Lanzhou, China

J.I. B u d n i c k Department of Physics, University of Connecticut, Storrs, CT 06269, USA

and P. P a n i s s o d Groupe d'Etudes des Mat~riaux M~tallique, Institut de Physique et Chimie des Mat~riaux de Strasbourg, Unicersit~ Louis Pasteur, 67070 Strasbourg, France Received 26 April 1991; in revised form 6 February 1992

A combined nuclear magnetic resonance (NMR) and M6ssbauer effect (ME) study of Nd2Fel4B has been carried out, which uses the Fe hyperfine field (HF) values obtained by N M R experiments in the M E fitting process. Taking the advantages of both N M R in determining the H F values at different Fe sites and ME in determining the intensity ratios of subspectra, the determination of the Fe H F values and their assignment can be made more precisely and more reliably. It can be deduced from the present result that the existence of a nearest Fe neighbor leads to an increase in the Fe H F of about 15 k O e / F e , while a nearest B neighbor atom reduces it at a larger rate. The nearest Nd neighbor atom seems to have no significant effect on the Fe HF. T h e analysis concerning the various contributions to the H F and their correlation with the atomic magnetic m o m e n t demonstrates that the hyperfine interaction study for the case of Fe provides a simple means to m e a s u r e Fe sublattice magnetization and relevant properties of the R2FeI4B system.

1. Introduction

In the tetragonal R2Fe14B phase, the Fe atoms locate at the six non-equivalent sites (4e, 4c, 8jl , 8j2, 16k 1 and 16k2), the Nd atoms occupy two sites (4f and 4g) and the B atoms have one site (4g) [1,2]. The Fe atoms at different sites have different atomic moments, depending very much on their local atomic environments, and consequently different hyperfine fields (HF). A large Correspondence to: Prof. Y.D. Zhang, D e p a r t m e n t of Physics, The University of Connecticut, Storrs, CI' 06269-3046, USA. Telefax: + 1-203-486-3346.

number of M6ssbauer effect (ME) and nuclear magnetic resonance (NMR) studies of this material published to date have shown the effectiveness of hyperfine interaction study to the understanding of its structural and magnetic properties [3-15]. Each technique has some advantages and weaknesses. NMR can measure H F values more directly and more precisely while in ME experiments there may exist some difficulty in decomposing the six superimposed ME sextets; but much care is needed in NMR experiments for obtaining actual intensities while intensities are more directly obtained in ME measurements. Therefore, taking advatage of NMR in determining the H F

0304-8853/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved

212

S.H. Ge et al. / Hyperfine fields and magnetism in Nd 2 Fe s4 B

value ad of M E in determining the absorption intensity, a combined N M R and M E experiment on the Nd2Fe~4B compound is useful in reaching a more reliable conclusion. In this work, we take R z F e l 4 B as a example to carry out a detailed comparison between M E ad N M R experiments. Based on the results obtained here, we discuss the correlation of the Fe H F with their local atomic environments. Since one of the major applications of H F measurements is to determine the atomic moments of Fe at each individual site, we give a discussion of the correlation of Fe H F with Fe magnetic moment.

2. Experimental The Nd2Fel4B specimen used for this work was made by arc melting using 98% l°B-enriched boron powder in order to avoid the overlap of the 57Fe N M R resonances with the ~ B one [14]. The sample was first checked by X-ray diffraction, identifying the tetragonal phase. N M R and M E experiments further confirmed that no free a - F e existed in the sample in addition to 2.6% paramagnetic phase. Note that N M R is much more sensitive in exploring the trace of e~-Fe precipitate than X-ray diffraction. The zero field 57Fe and ~°B N M R spectra and the relaxation time T 2 of this sample were measured at 4.2 K by a spin-echo N M R spectrometer ranging from 10 to 60 MHz. Fig. 1 shows the N M R spectra of the ~°B and 57Fe nuclei, which has been discussed in our previous work [14].

(÷ 5)

!1

o

/i

>ouJ

n" LL -r

CO

o.--~

~'5

40

45

50

FREQUENCY (MHz)

Fig.1. 1°B and 57Fe NMR spectra of Nd2Fel4B at 4.2 K.

55

Since the relaxation times T 2 for various Fe sites differ from one another by factors up to 3, a correction for T 2 was taken in plotting fig. 1. Obviously, the strongest resonance at 14.1 M H z originates from I°B resonance, which is consistent with the liB peak at 42.4 M H z found from the natural B abundant NdzFeI4B sample [13-16]. Attention has been paid to locate the l~B resonance as the sample contains 2% liB nuclei. In the 57Fe N M R measurement, the excitation condition was chosen to optimize the spin-echo amplitude and the echo-waveform. In this condition, no lIB was found as shown in fig. 1. When exciting the nuclei using a higher rf power, a ~ B signal was observed centered at 42.4 M H z with an amplitude weaker than that of 41.5 M H z resonance. This ruled out the possibility that the 41.5 M H z resonance shown in fig. 1 might come from the rest ~ B nuclei in the sample. From fig. 1, the 57Fe spectrum consists of seven peaks at 41.5, 44.0, 44.7, 46.0, 48.5, 52.0 and 53.0 MHz. As mentioned previously, the 52.0 and 53.0 MHz resonances come from the same Fe site and the splitting is caused by the dipolar field at this site [14]. Thus we obtained the following six Fe H F values: 302, 320, 325, 334, 352 and 378 kOe. An l°B N M R experiment performed at 77 K showed that the resonance frequency shift was less than 0.2 MHz, implying that the change in the Fe H F caused by increasing temperature from 4.2 to 77 K is about 1%. The ME experiment carried out at 77 K using a computer controlled constant acceleration ME spectrometer with a 57Co(Rh) source. The absorption was fitted by six sextet subspectra and the Fe H F values obtained from above mentioned N M R measurements were used in the fitting procedure but allowed to have small variations so as to optimize the result. Because of spin canting, the Fe H F at an identical site may further split at t e m p e r a t u r e below 148 K[8]. As shown in ref. [9], such splitting is within 4 kOe. In the present fitting process, the splittings of some subspectra were accounted for by linewidth broadening since it is small; the total spectrum still consists of six subspectra, but their linewidths are different. The relative intensities of these subspectra were not constrained; the best fit was achieved through

!e

S.H. Ge et al. / Hyperfine fields and magnetism in NdeFe14B

CHANNELNUMBER 0 z

o

co

64

128

192

1.~

z

< 0.99 I-H.I >

256

,,~ 0.98 I.IJ n" 10 - 8

-6 -4 -2 0 2 4 6 RELATIVE VELOCITY (mm/s)

8

10

Fig. 2. M6ssbauer spectrum of NdeFeI4B at 77 K.

adjusting the isomer shifts, quadruple splittings, linewidths and H F values. Fig. 2 shows the resultant spectra and table 1 lists the M E and N M R results. The resultant Fe H F values, together with their relative intensities (in the parentheses), obtained by the M E fitting are 306(6.9), 322(7.1), 334(13.9), 378(13.8), 325(28.0) and 346(27.7) kOe. The relative intensities are in good accordance with the occupancies of the Fe atoms at these six sites: 4 : 4 : 8 : 8 : 16 : 16 [1,2]. We have also carried out M E m e a s u r e m e n t s at room t e m p e r a t u r e and followed the same fitting procedure with only one exception that the linewidths of all subspectra were constrained to be equal in considering that no spin canting occurs a t room temperature. Again, a good fit was obtained and the results showed that the relative intensities remained in the ratios of 4 : 4 : 8 : 8 : 16 : 16 and that the correTable 1 The M6ssbauer parameters and N M R results of Nd2Fel4B Site

J2 k2 Jl ki c e

ME

NMR

HF (kOe)

intensity (%)

FWH (ram/s)

frequency (MHz)

HF (kOe)

378 346 334 325 322 306

13.8 27.7 13.9 28.0 7.1 6.9

0.41 0.47 0.41 0.47 0.36 0.36

52.0 48.5 46.0 44.7 44.0 41.5

378 352 334 325 320 302

paramagnetic phase

2.6

213

sponding H F values remained in the same sequence as that obtained at 77 K.

3. Discussion

3.1. Comparison between NMR and ME methodologies Regarding the decomposition of the six Fe subspectra in the Nd2FeI4B compound and the assignment of their H F values to the six Fe sites by M E experiments, the largest H F value which is ascribed to the Fe(j 2) site can be determined accurately as the outermost absorption shown in fig. 2 manifests a clear peak; while the remaining five subspectra are superimposed. On the other hand, the N M R spectrum of NdzFelaB, as shown in fig. 1, consists of six clearly separated Fe peaks from which the Fe H F can be determined precisely. One difference between N M R and M E experiments is that M6ssbauer absorption is caused by the Fe nuclei within domain while zero field N M R signal comes mainly from the nuclei in domain wall center. The difference in the Fe H F values between domain and domain wall center depends on the H F anisotropy [17] and dipolar field. Generally, the anisotropic H F component for the case of Fe is small as Fe has a very small unquenched orbital moment. A M E study of the Y2Fe14 B compound in an external magnetic fieid either perpendicular or parallel to the sample absorber has demonstrated that the H F anisotropy of Fe nuclei in the R2Fe~4B compounds is indeed very small [7]. The other evidence of the smallness of dipolar field and H F anisotropy in Nd2Fel4B compound is the fact that its N M R peak frequencies and linewidths do not vary significantly in an external magnetic field of about 6 kOe [16]. The dipolar fields for R2Fe~4B are two orders of magnitude smaller than the Fe H F [7]. Therefore, we expect the Fe H F values obtained by N M R to be very close to the values obtained from ME experiment. This enables us to take the Fe H F values obtained from the N M R experiment as a basis in the ME fitting process. In taking account of possibly small differences in the Fe H F between domain and

214

S.H. Ge et al. / Hyperfine fields and magnetism in Nd 2Fe l4B

domain wall center, we permit slight adjustments of these Fe H F values in fitting if further improvement can be achieved. It is shown in table 1 that the Fe H F values obtained by M E do not differ significantly from those obtained by the N M R experiment. Comparing fig. 1 with table 1, it has been found that the N M R intensities are, overall, compatible with the M6ssbauer absorption intensities in that the 44.7 and 48.5 M H z peaks are strong while the 41.5 and 44.0 M H z resonances are weak. However, the intensity ratios obtained by N M R measurements, either by peak amplitude or by integral intensity, do not exactly follow the occupancies of the Fe atoms at the corresponding sites. Being different from the M E absorption, the N M R signal comes from a rather complicated mechanism for ferromagnetic substances in that a nucleus couples with 3d electron moments and, as a consequence, the resonant precession of the nuclear m o m e n t also affects the 3d electrons. Since a spin magnetic m o m e n t of an electron is much larger than that of a nucleus, the observed signal originates mainly from the motion of the 3d electron moment. Thus the observed Fe N M R signal intensity for a given Fe site depends not only on the number of Fe atoms at this site but also on the strength of the nucleus-electron coupling and the mobility of the electronic magnetic moment; the latter is somewhat local atomic environment dependent. Thus the induced spin echo intensity for a given site may not follow exactly the atomic occupancy at that site in some cases. Therefore, it is valuable to use both N M R and M6ssbauer techniques in the studies of a rather complicated system like RzFe~4B. According to the ME and N M R intensities, the six Fe H F can be divided into three groups. The 325 and 346 kOe H F correspond to the strongest absorptions and should be attributed to the Fe(k~) and Fe(k 2) atoms; the 306 and 322 kOe HF, the weakest absorptions, are attributed to the Fe(e) and Fe(c) and the 334 and 378 kOe H F to the Fe(j 1) and Fe(j 2) atoms. Further assignment based on a near neighbor atomic environment consideration, which has been described in our previous work [14-16], leads to the follow-

ing results: 306(e), 322(c), 325(kl), 334(jl), 346(k 2) and 378(j 2) kOe.

3.2. Effect of nearest Fe, B, Nd neighbor atoms on Fe hyperfine field For 3d transition metal compounds or alloys, the hyperfine field at a 3d magnetic nucleus, Hhf, is often assumed to arise from three contributions [18],

Hhf(i ) = Hcp(i ) + Hs(i ) + Hsp(i ) ,

(1)

where Hop and H S are due to core electron polarization and 4s conduction electron polarization, respectively; both of them are proportional to the on-site electronic magnetic moment, lisp is a transferred hyperfine field ( T H F ) caused by neighboring magnetic atoms and may be expressed in terms of the number of magnetic atoms on the nearest neighbor shell and their magnetic moments. In writing eq. (1) we neglected dipolar hyperfine field which in magnitude is less than 1% of above three terms. Thus Hhd(i) can be written semi-quantitatively as Hcp(i ) + Hs(i) = alive(i),

H~o(i ) = c(i) Y'~n~(i)uFe(j), J Hhf(i ) = a / ~ e ( i ) + c ( i ) ~ n j ( i ) u w ( j ) ,

(2) (3)

J where t~Ve(i) is the magnetic m o m e n t of the ith Fe atom, I~v~(J) is the magnetic m o m e n t of the neighboring j-site Fe atom, a and c(i) are the hyperfine coupling constants, n~(i) is the number of the j-site Fe atoms surrounding this (ith) Fe atom. In this expression, a represents the strength of core s electron polarization and 4s conduction electron polarization due to the on-site moment and is approximately the same for different Fe sites; c(i) is environment dependent. In the study of r a r e - e a r t h - F e , transition-metal-Fe alloys it has been found that the nearest Fe neighbors appear to be an important p a r a m e t e r in describing the effect of local atomic environment on the on-site Fe moment. For instance, Gubbens et al. [19] measured the Fe H F at various sites in several

S.H. Ge et al. / Hyperfine fields and magnetism in Nd, Fe,, B

8 NEAR EJElGHBoRS 0 ---0 q -1

. -2 200

I-J

0

2

4

6

8

10

12

14

Fe COORDINATION Fig. 3. The hyperfine field at the various Fe sites in Nd,Fe,,B obtained by Miissbauer experiment at 77 K as a function of the Fe coordination.

Fe(c), Fetjr), Fe(k,) and Fe(iz) sites. About the determination of the Fe HF in R,Fe,,B and the assignment of the six Fe HF to the six non-crystalline Fe sites for each R compound, the unanimous conclusion among various ME studies is that the largest and the second largest Fe HF for all R,Fe,,B compounds originate from, respectively, the j, and k, site Fe atoms which have, respectively, the largest and the second largest nearest Fe neighbors. This result is consistent with the argument of approximate proportionality between the Fe moment and its nearest Fe neighbor atoms. The proportionality implies that the number of nearest Fe neighbor atoms may play a major factor in the environment consideration. Perhaps this is the consequence of the fact that the Fe-Fe exchange interaction is much stronger than R-Fe and R-R interactions. Based on the assumption of linearity between the Fe hyperfine field and its nearest Fe neighbors, (H,, + H,) can be written as

binary R,Fe, compounds with different structures and widely ranged Fe-Fe and R-Fe distances and found that the Fe HF is approximately proportional to the relative number of its nearest Fe neighbor atoms with an identical slope. In order to check the correlation of Fe HF with their local atomic environments in the Nd,Fe,,B phase, table 2 lists the Nd, Fe and B coordinations for each Fe site in this compound obtained based on a .Wigner-Seitz cell consideration [5,6]. Fig. 3 shows the Fe HF versus their nearest Fe neighbors for Nd,Fe,,B according to the present assignment, approximately a linear relationship appearing between the HF and the number of the Fe atoms on the nearest neighbor shell for the

Table 2 Fe, Nd and B coordinations Fe site

Fe(&) Fe&,) Fe&) Fe&,) Fe(c) Fe(e)

215

K,(i)

+4(i)

= a/-+(i) =a

= a[ /-+e(O) + A/+&)]

[p&O)++)%1,

(4)

where ~~~(0) denotes the magnetic moment of an Fe atom having no nearest Fe neighbors, n(i) = Cjnj(i) is the total number of nearest Fe neighbors surrounding the ith Fe atom. Therefore, eq. (3) can be written as

1

+-+e

1-40) + n(i) - dn +c(i)

(5)

Cnj(i)Pk(j)-

of each Fe site in Nd,Fe,,B

Fe near neighbors

Total

J2

k2

Jl

k,

C

e

Fe

B

Nd

0 2 3 2 0 2

4 3 2 3 4 0

3 1 1 1 0 2

4 3 2 2 4 4

0 1 0 1 0 0

1 0 1 1 0 1

12 10 9 10 8 9

0 0 0 1 0 2

2 2 3 2 4 2

216

S.H. Ge et aL / Hyperfinefields and magnetismin Nd2Fe14B

It will be proved below that the magnitude o f / / s o at Fe nuclei in Nd2Fel4B compound is less than 20% of the total Hhf, so we can approximately use the average Fe moment, ~Ve, to replace tzw(i) in the T H F expression. Thus

Hhf(i) =atXve(O) + n(i)[

a -dtxFe --~n + c(i)#Fe

]• (6)

It can be deduced from fig. 3 that the addition of a nearest Fe neighbor atom leads to an increase of the on-site Fe H F with a rate of about 15 k O e / F e . This amount of change is produced by the increase of both (H¢p + H.O via increasing the on-site magnetic moment and the THF. As shown in fig. 3, deviation from the proportionality occurs for the k~ and e sites. This demonstrates an influence of near neighbor B atoms on the Fe H F since Fe(k l) and Fe(e) have one and two B neighbors, respectively, while no B atoms surround the other four Fe sites. In the F e - B binary or ternary systems there is a charge transfer between Fe and B atoms which results in a reduction of the Fe magnetic m o m e n t and, hence, the Fe HF. Comparing the H F values between Fe(k~) and Fe(k 2) which have the identical number of nearest Fe neighbors (10 each), a reduction rate of Fe H F caused by the charge transfer is obtained to be about - 2 0 k O e / B . This means that the reduction in Fe magnetic m o m e n t caused by one B atom addition in its nearest neighbor shell is about 0.13/x B if taking the effective hyperfine coupling A to be 151 k O e / t x B (see below). A similar result was obtained from a M E study of Nd2Fel4B0.1C0. 9 compound [20]. Comparing Fe H F between R2FeI4B system and the R - F e alloys listed in ref. [19], we find some differences: (i) the R atom in some R - F e systems shows a negative effect on the magnetic m o m e n t of adjacent Fe atoms while in the R - F e - B system the R atom has very weak effect. When plotting fig. 3, the nearest Nd neighbors are neglected. From a comparison between the Fe H F at the J2 site, which has 12 Fe and 2 Nd nearest neighbors, and the Fe H F at the k 2, j~ and c sites, which have respectively 10, 9, 8 Fe and 2, 3, 4 Nd neighbor

atoms (see table 2), it is found that the addition of one more Nd neighbor does not lead to extra reduction of the Fe HF. Similar results obtained by M E experiments are consistent with this resuit: the variation in average Fe H F values of R2FeI4B is within 5% when R goes from Y through Th [3,7]. The smallness of the effect of R atoms on the Fe moments was also proved by magnetization m e a s u r e m e n t [21]. (2) The Fe H F in some R , F e y is approximately proportional to its nearest Fe (and R) neighbors with a rate of 30-40 k O e / F e [18]; this dependence results in a zero magnetic moment for the Fe atom having no Fe atoms adjacent. For NdzFeI4B, the rate is found to be much smaller (15 k O e / F e ) . Extrapolating the Hhf(i) versus n(i) curve shown in fig. 3 to n(i)= 0, we obtained H h f = a / X F e ( 0 ) = 210 kOe, which corresponds to a magnetic m o m e n t #re(0) of about 1.34/z s. This difference in H F behavior between these R - F e and R - F e - B systems implies that the electronic character of Fe atoms in the R - F e - B system is different from the character of Fe in R - F e compounds and ec-Fe in that it is more localized in nature.

3.3. On the determination of Fe atomic magnetic moments Hyperfine interaction experiment provides an indirect means to measure atomic magnetic moments. The applicability of H F m e a s u r e m e n t in determining the atomic magnetic m o m e n t is based on (i) the H F for a certain species is linearly dependent on its magnetic magnetic moment:

H,f(i) : A/XFe(i ) ,

(7)

where A is an effective hyperfine coupling constant, and (ii) the value of A is the same for the atoms of this species in various local atomic environments. Thus, the value of A can be determined from the material in which the atomic magnetic m o m e n t of a certain species has been known and then it can be used for determinating the magnetic m o m e n t of the same species in other materials. The reliability of H F measurement in the determination of the atomic magnetic m o m e n t depends on whether the above condi-

S.H. Ge et al. / Hyperfine fields and magnetism in Nd 2Fe14B

tions are satisfied. In the case of Fe, it was found that the values of A determined experimentally from a large variety of Fe containing materials are close to each other (140-150 k O e / t z ~ ) . Concerning the Fe atoms in 3d metallic alloys or r a r e - e a r t h - F e alloys, the contribution from core polarization to coefficient a in eq. (5) is unchanged in various Fe-containing materials and generally much greater than c(i). In this case, eq. (7) is a good approximation of eq. (5). It is just because of the dominance of the core polarization field that the proportionality between the Fe H F and the Fe atomic m o m e n t holds with approximately the same ratio A, which makes the hyperfine study a good method in determining the Fe atomic moment. In many cases of investigations, the value of A has to be taken empirically; for the Nd2Fel4B compound, we can add the following expression to expression (7) for the determination of coupling constant A ~] N//ZFe(i ) = 4 × 31.4p~ B, i

(8)

where the value 31.4/z B is the saturation magnetization of the Fe sublattice [21,23], N/ is the number of t h e / - s i t e Fe atoms per unit cell. From the H F values obtained in the present work we calculated the Fe atomic moments and the effective hyperfine coupling constant A to be 2.51(J2), 2.29(k2), 2.21(c), 2.13(c), 2.03(e) and 151 kOe//zB, respectively. Table 3 shows a comparison of the results obtained in the present work for the Nd2Fe~aB compound with the results obtained from neutron diffraction (ND) [24] and ME experiments [8]. The consistency between the ME m e a s u r e m e n t and the present work is very good. If considering that the value of the total Fe sublattice m o m e n t per unit given by N D is a little too large in comparison with the magnetic mea-

217

surement result, the result of N D experiment is consistent with the present result in overall with only one exception that magnetic moment value for the c site given by the N D is much higher than the value obtained in this work. The precision of this method depends on the weight of T H F in the total HF. For a-Fe, Hop + H s = 201 kOe, H~o = 145 kOe and/Zvc(0) = 0 [22], from eq. (5) we obtained a = 91.3 k O e / / z B, c = 8.2 k O e / / x w In the case of R2FeI4B, since /xFc(0) > 0, the weight of T H F becomes smaller. For instance, for the J2 site for which the contribution from T H F is the largest among the six Fe sites, the term d/zF,

an(i) ~n + c(i) En~(i)tXFe(J ) J contributes about 45% to the total Fe H F and the first term is much greater than the second one. Also, the relatively small differences in the Fe atomic moments makes eq. (7) more acceptable. Therefore, the accuracy of the Fe magnetic moments given above is better than 10-15%. Although the hyperfine interaction study gives a simple method for determining the atomic magnetic moment, one should be careful if there exists a factor varying the polarization hyperfine field and transferred hyperfine field in opposite directions. In some cases such as Nd2Fe14_xCOxB for example, the Fe magnetic moments increase with x for low Co composition compounds while the measured Fe H F decrease for all the Fe sites. In order to get more precise results, it is necessary to subtract the contribution of the T H F from the total HF. Although it seems impossible to measure c(i) directly, a systematic N M R study of the T H F experiments on non-magnetic nuclei T in R2Fe~4_xTxB, together with taking into ac-

Table 3 Comparison of the Fe magnetic moments (in /z B) and the effective hyperfine coupling constant (in k O e / # B) of Nd2FeI4B obtained by different means Fe moment (/x B)

A

J2

k2

Jl

kI

c

e

~Fc

(/zB)

2.55 2.85 2.51

2.30 2.60 2.29

2.21 2.30 2.21

2.24 2.60 2.15

2.17 2.75 2.13

2.00 2.10 2.03

2.27 2.57 2.29

147 151

Ref. ME[8] ND[24] this work

218

S.H. Ge et al. / Hyperfine fields and magnetism in NdeFe14B

count the change in c for 3d group elements as calculated in ref. [25] may help to solve this problem. The advantage of 57Fe H F study is that for it a >> c, leading to a simple correlation between the Fe H F and its atomic magnetic moment. However, for some species such as Co for which the transferred hyperfine field is large and the unquenched orbital moments may have a significant contribution, eq. (7) may no longer retains correct. A detailed 59Co N M R study of Nd2ColaB compound by Panissod et al. has shown that the value of A for Co obtained based on eq. (7) varied from site to site [26].

hyperfine coupling constant, the evaluation of the effect of nearest Fe, B and Nd neighbors on the Fe moment are reasonable; these results, in turn, seem to support the nearest Fe neighbor consideration. (3) With relatively large core polarization contribution to the total H F and almost quenched orbital moment for the Fe atoms in the Nd2Fet4B and, perhaps in R2Fe14B in general, Fe hyperfine interaction study provides a simple means to explore Fe sublattice magnetic properties in this system.

Acknowledgements 4. Concluding remarks (1) The investigation of R2Fe14B compounds by ND, ME and N M R is a good example showing the advantage and necessity of microscopic means in investigating crystallographically or magnetically complicated materials. On the other hand, some controversies existed among different studies reveals that each individual means is limited. To improve the capability of these microscopic exploration methods and avoid possible ambiguity, one way is to combine them. This is particularly useful in the case of H F experiments. Much care and work is required in N M R measurements in order to get correct resonance intensities while they are more directly obtained in ME experiments; however, the decomposing process for ME spectra sometimes, when the splittings among the individual sites are small, might be questionable and gives rise to controversies. As shown in the present work, if H F values are already known by NMR, the intensities ca be straightforwardly obtained by ME. (2) Whether or not the nearest Fe neighbors can be taken as a major factor in phenomenologically describing the effect of local atomic environment on the Fe moment for NdzFe14B phase, or generally for R2Fe~aB phase, is uncertain. This argument was accepted in assigning the Fe HF to the non-equivalent Fe sites. The inferences made from the present assignment about the values of the Fe magnetic moments, the value of effective

The authors wish to thank Mr. X.Z. Ding, R.J. Zhou and C.L. Yang for computation processing and many helpful discussions. This work was supported partly by China National Science Foundation under Grant no. 5870012, 84026 and by Nature Science Foundation of Gansu Province, China, under Grant no. ZC-88-26.

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S.H. Ge et al. / Hyperfine fields and magnetism in Nd eFe t4 B

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