NMR in metallic glasses

Journal of Magnetism and Magnetlc Matermls 31-34 (1983) 1567-1570 NMR

IN METALLIC

J. D U R A N D

1567

GLASSES

* a n d P. P A N I S S O D

LMSES (LA 306) 4, rue Blaise Pascal 67070 Strasbourg Cedex, France

NMR measurements can y~eld some insight into the electronic structure of metalhc glasses through the Korrmga ratio in non-magnetic alloys and through the mean value of the hyperfine field in amorphous ferromagnets Some reformation on the local symmetry around a gwen nuclear site is provided by analys~s of the quadrupolar NMR spectra Spin echo NMR spectra m amorphous ferromagnets exhibit structures whose origin is &scussed for each family of alloy These structures originate either from quadrupolar interactions, or from different magnetic states of the resonant species, or from well-defined regions of different local enwronments with different domain wall regimes

While the N M R has been widely used in investigating b o t h the static a n d the dynamic manifestations of the atomic-scale structure of n o n m e t a l h c glasses, the application of this technique to the study of metallic glasses has developed only in the recent past However, some interesting insights into the electronic and atomic structure of a m o r p h o u s metallic alloys was already obtained through N M R measurements. We summarize the most significant aspects of this information concerning the paramagnetlc a n d ferromagnetic metalhc glasses (for a more complete review see ref. [1]). The static properties that can be investigated by N M R In metallic glasses can be summarized as follows T h r o u g h their magnetic moment, some nuclei can be used as probes of the internal magnetic fields (Knightshift a n d hyperfine field in paramagnetlc a n d ferromagnetic materials, respectively). M e a n values of these internal fields are related to the electronic structure, whde the structural disorder inherent to the a m o r p h o u s state IS expected to result in a distribution of the internal fields. Those nuclei having a nuclear spin larger than 1 / 2 can be used through their electric quadrupole mom e n t as probes of the internal electric fields W h e n e v e r possible, a complete determination of the c o m p o n e n t s (mean values and distribution) of the electric-fieldgradient ( E F G ) tensor allows one to characterize b o t h the local symmetry and the effects of the topological & s o r d e r Even for nuclei with n o quadrupole moment, some structural i n f o r m a t i o n can be extracted from the field-Independent N M R hnewxdth originating in dip o l e - d i p o l e interactions Besides the time-averaged values of the internal fields in a material, the N M R can yield the values of the relaxation times T I and T2 whose inverse values are related to the fluctuating c o m p o n e n t s of the internal fields parallel and transverse, respectively, to the quantizatlon direction Preliminary T2 m e a s u r e m e n t s in metalhc glasses have been reported in the literature, namely on 59Co in ferromagnetic C o P alloys [2] a n d on * LA 155, Umverslt8 de Nancy l, B P 239, 54506 Vandoeuvre les Nancy, France 0304-8853/83/0000-0000/$03.00

© 1983 N o r t h - H o l l a n d

3]p in paramagnetic NIP alloys [3]. The interpretation of these T2 data is not straightforward. The temperature d e p e n d e n c e of the s p i n - l a t t i c e relaxation time T I can be more readily analyzed. A great deal of i n f o r m a t i o n can then be o b t a i n e d concerning various dynamic aspects of the physics of metalhc glasses, namely structural instablhties involving groups of atoms [4], e l e c t r o n - p h o n o n c o u p h n g in a m o r p h o u s superconductors [5], the magnetic impurity relaxation [6,7] and the b a n d structure of the electrons through which the t h e r m o d y n a m i c equih b r i u m occurs. This review is organized as follows. First, we summarize the N M R data related to the electronic structure of metallic glasses. Then we present the information o b t a i n e d by N M R on the local symmetry a r o u n d some nuclear sites in a m o r p h o u s alloys. Finally, we analyze the structures observed on some spin-echo N M R spectra in a m o r p h o u s ferromagnets. Some insight into the electronic structure of metallic glasses can be o b t a i n e d by analyzing either the Knightshift K and the span-lattice relaxation time T I in paramagnetic alloys or the m e a n value of the hyperflne field in a m o r p h o u s ferromagnets. In paramagnetlc alloys where K is temperature indep e n d e n t a n d T 1 verifies the Korrlnga relation T I T = constant, one can obtain a qualitative idea a b o u t the m e c h a n i s m responsible for b o t h the Knight-shift a n d the s p i n - l a t t i c e relaxation by estimating the Korrlnga ratio k = K 2 T I T / S The Korrlnga constant S is defined by S = (h/8~r2k~)(ye/Vn) 2, with 7e and 7n being the electronic a n d nuclear gyromagnetlc ratios, respectively If K and T l originate from pure s electrons, k is equal to 1 In fact, if the Korringa ratio was found to be rather c l o s e to u n i t y for 31p in ( a m o r p h o u s ) a(Moo 5 Ruo 5)80P20 [5], larger values of k were obtained on 31p and l i b in a-NIPB alloys [6] as well as m NIP [7-9], (Ni05Pd05)10o_xP x [10] and (Ni020Pt080))00.xP x alloys [11] For Hines et al [9-11], K and T~ o n g l n a t e only from s-electrons owing to a charge transfer of the p-electrons of P to the unoccupied d b a n d s of the transition metal. The e n h a n c e d values for k are then attributed to exchange and correlation effects which are,

1568

J Durand, P Pantssod / NNR m rnetalh~ gla~se~

neglected in the free s-electron estimate. More consistently with the photoemlss~on results, A h a g a - G u e r r a et al [6] think that the aUoylng effect m alloys of the NIPB type results in a p - d hybridization rather than in a charge transfer The larger values for k are attributed mainly to p-electrons contributions Reasonable values for the local susceptlblhtles are then estimated [6] Values of K a n d TaT measured for 31p and aaB m crystalline N13P and N13B crystalhne c o m p o u n d s were analyzed u n d e r the latter assumptions a n d compared with those obtained for a-NIPB alloys [12]. The concentratlon dependence of K and T I T on 3ap and a95Pt m the a m o r p h o u s (Pda.~Cux)s0P20, (N1Pd)ao0_~,P ~ and (NI,, Pt a- ~)75 P25 series, respectively, was studied in detad a n d discussed in terms of electronic structure [10,13] Similar attempts have been recently presented to interpret the concentration dependence of the 63Cu (6~Cu) a n d 91Zr Knight-shifts in a m o r p h o u s ZrxCUl_ x alloys [14] as well as for the 9Be K n i g h t - s h i f t in Be32 5Nb~Zr67 5-x a n d Be32 5MoxZr67 5-, metalhc glasses m relatton with their superconducting properties [15] In conclusion, K and T~T N M R studies in non-magnetic metallic glasses can yield very valuable information a b o u t their electronic structure Owing to its local character, such information is quite complementary to that obtained through photoemxsslon or soft X-ray measurements A q u a n t l t a t w e analysis of the different contributions to the hyperflne fields (hff) m ferromagnetic metals is a very difficult task, for it would reqmre an exact knowledge of the electronic densities at the nuclear sites for b o t h spin directions. This is made even more difficult m a m o r p h o u s ferromagnets, where little is k n o w n about the atomic structure However, the various contributions to the hff can be semi-quantitatively estimated by using some phenomenologlcal approaches e m p h a s m n g the role of local a t o n u c e n v i r o n m e n t s Moreover, comparison can be m a d e with hff values in crystalline c o m p o u n d s of Slnular composition for which the electronic structure is well d o c u m e n t e d (for a review on hyperflne fields in metallic glasses see ref [16]) The 59C0 hff has been measured by N M R in a m o r p h o u s CoP [2,17], CoB [18], (Fe10o_xCOx)79P13B8 [19] alloys a n d in a series of Fe(Co)SIB metallic glasses [20,21] The 57Fe hff has been studied by Mossbauer spectroscopy in m a n y Fe-based metalhc glasses N M R measurements on 57Fe are rather scarce (see review in ref [22]). The 59Co hyperflne coupling constant was found to be the same as for pure crystalhne cobalt. The 57Fe hyperflne coupling constant is the same in Fe-based a m o r p h o u s alloys with B, C, P as in Fe-rlch interstitial crystalhne compounds, such as FeaN, FeaP, Fe3B, Fe3C, while it is slgmflcantly larger for b o t h crystalhne and a m o r p h o u s Fe-based c o m p o u n d s with non-interstitial elements Analysis of these various data leads one to conclude that the various contributions to the 59C0 hff are the same in a m o r p h o u s alloys as in crystalline Co, wtule for

57Fe, the hff contributions (core polarization, local and non-local conduction-electron polarization) are about the same as in crystalline eqmvalent compounds, but significantly different from those estimated for a - F e Very similar conclusions can be drawn by studying thc mapurity hff m a m o r p h o u s ferromagnets The SgCo hfl has been determined by N M R on diluted Co in a m o r p h o u s G d 2 N I [23] a n d m a m o r p h o u s Fe7~PI~B8 [19] The hffs on M n in a m o r p h o u s FePB [24] and on NI in a m o r p h o u s FeB alloys [25] were determined by N M R a n d Mossbauer spectroscopy, respectively It as interesting to note that the hff values for N1, M n and Co m anaorphous alloys of the FeB type are practically the same as in crystalhne FerMI [26] It was found also that the laB and 31p hffs in a m o r p h o u s FeB [22,27,28], FePB a n d FePC [29] alloys are practically identical to those measured in crystalline Fe3B and Fe~P compounds. respectively The l m p h c a t l o n s of these results in term~ of electromc structure are self-conclusive M a n y experimental observations of quadrupole mtera~ttons in metallic glasses have been reported (mainly from Mossbauer spectroscopy), before these results were first analyzed m terms of atomic-scale structure [30] However, systematic N M R investigations of E F G m metalhc glasses w~th respect to their s~gmficance lor structural i n f o r m a t i o n are rather recent (for a review see ref. [31]) In a few cases, such as a m o r p h o u s evaporated G a films studied by the perturbed angular correlatmn technique [32] or sputtered a m o r p h o u s GdNa alloys investigated by 15eGd Mossbauer spectroscopy [33], b r o a d distributions for the c o m p o n e n t s of the E F G tensor ( V.~ a n d r / = IV, ~ - V~ I/V.: ) were o b t a m e d which were found to be compatible with model pred~ctnms deduced from a d e n s e - r a n d o m packing model of hard spheres ( D R P H S ) or analytically calculated for r a n d o m ionic coordinations. These model calculations yield zero probability for both V._.= 0 and rl = 0 In addition, the p r o b a b l h t y would be nearly the same to obtain 1/._ values of b o t h signs, while the probability would be high to have large values for ~/ Actually, in most cases investigated so far, the local symmetry was found to be rather well-defined with narrow distributions for I~;_ a n d v/. This Is the case, for example, of the EusoAu2. a m o r p h o u s alloy studied by both N M R [34] and Mossbauer [35] spectroscoples, where V.: has a well-defined positive sign wLthout significant distribution on ~ : and with a value for rt close to zero Recent N M R mvesugatlons of E F G in metallic glasses made of t r a n s m o n metals with s - p elements showed that the average local symmetry around the s - p elements tend to reproduce to some extent the site symmetry prevailing m the crystalline counterparts [36] Thus, the site symmetry around G a in a m o r p h o u s La3Ga is spherical on average ( 1~: = 0) as in the metastable crystalline c o m p o u n d . Similar b . the s~te symmetry around B m the a m o r p h o u s sputtered Mo2B as well as around A1 in the La~A1 metallic glass Is axial (7/= 0) as in the hexagonal corresponding corn-

J Durand, P Pantssod / NNR m metalhc glasses

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Fig 1 NMR spectra on 71Ga m (amorphous) a-LavsGa25 ( T = 10 K), on lIB m (crystalline) c-Mo2B, m a-MoToB30, m a-Mo48RuuB20 ( T = 1 0 K ) and on liB m c-NbB and m aNI78PlaB 8 ( T = 42 K) (after ref [36]) p o u n d s (see fig l). As illustrated by the concentration d e p e n d e n c e of the E F G parameters in the a m o r p h o u s NIB alloys [37], there is not a one-to-one correspondence between the local symmetry a r o u n d a given constItuent m a crystalline c o m p o u n d a n d in its a m o r p h o u s modification. But, m any case, the average symmetry a r o u n d the different atomm sites in an a m o r p h o u s alloy has to be understood in relationship with the basic motifs of the corresponding crystalhne c o m p o u n d s of the phase diagram. This is well-documented for these a m o r p h o u s alloys whose formation is related to a deep-lying eutectlc in the phase diagram. This is true also, but p r o b a b l y to a smaller extent, for bimetallic a m o r p h o u s alloys of the CuZr-type [14]. In conclusion, the D R P H S - t y p e models might represent an adequate a p p r o a c h to the atomm-scale structure only for those a m o r p h o u s alloys where the short-range order is loosely defined But such models are unable to account for the large variety of local symmetries a n d E F G distributions encountered in metallic glasses [38]. The study of the q u a d r u p o l a r interactions has thus proved to be a unique tool in investigating the atomic-scale structure of metalhc glasses. In comparison, the structural mforma-

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Fig 2 Concentration dependence (with no correction for the

enhancement factor from the domain walls) of the relative lntensltms of the 59Co NMR subspectra (see text) m Como_ ~B, metalhc glasses Inset wmght fractmns of fcc Co, Co2B and Co3B phases m crystallized CoB samples as a functmn of B content (after ref [18])

1569

u o n that can be obtained from analysis of dipole interactions [39] remains rather vague Finally, as an example of the N M R a p p h e d to the study of the d y n a m i c structural properties of metallic glasses, let us m e n t i o n the determination of the activation energies for the diffusion of hydrogen In a m o r p h o u s Pd35Zr65 through the study of the temperature dependence of the p r o t o n N M R h n e w l d t h [40] Practically all the spin-echo N M R spectra obtained so far In a m o r p h o u s ferromagnets exhibited structures Some structures were clearly identified as q u a d r u p o l a r in origin, such as 59Co spectra for dilute Co m a m o r p h o u s Fe79P13Bs [19] or Eu in a m o r p h o u s Eus0Au20 [34]. Structures m 55Mn subspectra for F e ( M n ) P B alloys were also attributed to q u a d r u p o l a r interactions [24], while the subspectra themselves are thought to originate from the magnetic state of M n being strongly d e p e n d e n t of the local e n v i r o n m e n t Different in nature are the structures observed in the S9Co spin-echo spectra for a m o r p h o u s CoP [17] and CoB [18] alloys. This latter family of a m o r p h o u s alloys has been the subject of a detailed N M R study for B concentrations varying between 14 and 27 at% The experimental Co spectra were c o m p u t e r analyzed as the sum of three subspectra. For an alloy of a given c o m p o s m o n , the relative intensities of the subspectra were found to vary with the rf field strength, with the sample orientation a n d with the sub-Tg a n n e a h n g treatments But, in any case, neither the positions of the subspectra centrolds nor the position of the centroid for the whole spectrum were significantly affected by changing the experimental conditions The concentration dependence of the 59Co hff in Com0_xB~ alloys is, therefore, firmly established It is then possible to assess a well-defined average composition to each subspectrum for a given alloy The centrold of the high-frequency subspectrum is concentration d e p e n d e n t Its frequency decreases from a b o u t 200 M H z for x = 14 down to about 170 M H z for x = 27 A h n e a r extrapolation to zero B content yields a frequency close to that of fcc Co This subspectrum arises then from Co e n v i r o n m e n t s resembling a supersaturated CoB solid solution. The centrold of the central s u b s p e c t r u m is practmally i n d e p e n d e n t of concentrau o n It corresponds to Co atoms lying in zones where the B content is 18-20 at% on average, 1 e a r o u n d the eutectlc c o m p o s m o n . The centrotd of the low-frequency s u b s p e c t r u m remains roughly constant when changing the concentration of the whole alloy Its position (about 110 MHz) is close to that of the centrold of 59Co hf in c-Co2B. The concentration dependence of the relative intensities of these subspectra at fixed excitation con&Uons mimics somewhat the concentration dependence of the weight fractions of crystalline phases in crystalhzed CoB samples (see fig 2) F r o m systematic variations of the excitation c o n d m o n s and from various annealing treatments, it is concluded that the zones of Co e n v i r o n m e n t with an effecuve B concentration of

t570

J Durand, P Pamssod / NNR m metalh~ glasses

18 20% (2 B a t o m s , o n average, in t h e first a t o m i c shell a r o u n d a C o a t o m ) c o r r e s p o n d to r e g i o n s w h e r e t h e d e n s i t y of d e f e c t s is h i g h a n d w h e r e t h e m a g n e t i z a t i o n is p e r p e n d i c u l a r to t h e p l a n e o f t h e r i b b o n s T h e h i g h - a n d l o w - f r e q u e n c y s u b s p e c t r a arise f r o m C o n u c l e i l o c a t e d in r e g i o n s w h e r e t h e d e n s i t y of d e f e c t s as low a n d w h e r e t h e m a g n e t a z a t a o n d o m a i n s are i n - p l a n e . S u b s p e c t r a very s l m d a r to t h o s e a n a l y z e d m C o B m e t a l l i c g l a s s e s were also o b s e r v e d in e l e c t r o d e p o s a t e d C o P alloys T h e q u e s tion is raised w h e t h e r t h e s e i n h o m o g e n e i t i e s are i n t r i n sic to a m o r p h o u s s t a t e or d e p e n d e n t u p o n t h e fabric a t i o n t e c h n i q u e A n o t h e r q u e s t a o n arises as to t h e role o f the eutectlc c o m p o s i t i o n in t h e a t o m i c s t r u c t u r e a n d m the physical properties of these amorphous materials F u r t h e r e x p e r a m e n t a l w o r k a l o n g t h e s e lanes is e x p e c t e d m t h e n e a r future.

References [1] J Durand, m Application of Nuclear Techmques to the stud,es of Amorphous Metals, ed U Gonser (Atomic Energy Review Suppl No. 1, Vienna. 1981) p 143 [2] K Raj, J I Budmck, R Alben, G C C h l a n d G S Cargdl, AIP Conf Proc 31 (1976) 390 [3] I Bakonyi, K Tompa, E Toth-Kadar and A Lovas, m Magnetic Resonance and Related Phenomena, XX Congress Ampere, eds E Kundla, E Llppmaa and T Saluvere (Springer Verlag, Berlin, 1979) p 437 [4] D Ahaga-Guerra, P Panissod and J Durand, J de Phys 41 Colloq 8 (1980) 674 [5] D Allaga-Guerra, J Durand, W L Johnson and P Pantssod, Solid State C o m m u n 31 (1979) 487 [6] D Ahaga-Guerra, P Pamssod and J Durand, Solid State C o m m u n 28 (1978) 745 [7] L Varga and K Tompa, in Proc Int Conf Amorphous Systems InvesUgated by Nuclear Methods, Vol 1, eds Zs Kajcsos et al (Central Research Institute for Physics, Budapest, 1981) p 569 [8] L H Bennett, H E Schone and P Gustafsom Phys Rev B18 91978) 2027 [9] W A Hines, C U Modzelewski, R N Paohno and R Hasegawa, Sohd State C o m m u n 39 (1981) 699 [10] W A Hmes, K Glover, W G Clark, L T Kabacoff, C U Modzelewskl, R Hasegawa and P Duwez, Phys Rev B 21 (1980) 3771 [11] W A Hines, C U Modzelewskl, R N Paohno and H S Chen, J Appl Phys 52 (1981)1914 [12] A Amamou, D Ahaga-Guerra, P Panlssod, G Krdl and R Kuentzler, J de Phys 41 Colloq 8 (1980) 396 [13] D Ahaga-Guerra, Th~:se d'Etat, Umverslte de Strasbourg, France (1980) [14] H J Elfert, B Elschner and K H J Buschow, Phys Rev B25 (1982) 7441

[15] J Goebbels, K Luders, H C Freyhardt and J Relcheh, m ref [7], Vol 1, p 479 [16[ P Pamssod, J Durand and J 1 Budmck, Nucl Instr and Meth 199 (1982) 99 [17] J Durand and M F Laplerre, J Phys F 6 (1976) 1185 [18] P Panlssod, A Qachaou, J Durand and R Hasegawa, m ref [7], vol 1, p 543 [19] J Durand, D Ahaga-Guerra, P Pamssod and R Hasegawa, J Appl. Phys 50 (1979) 7668 J Durand, B Ldmms, R Hasegawa, D Ahaga-Guerra and P Panlssod, J Magn Magn Mat 15 18 (1980)1373 [201 V S Pokatllov, Y u A Gratslanov and B N Kujahn, Soy Phys Dokl 25 (1980) 206 [21] K lnomata, m Rapidly Quenched Metals IV, Vol 2, eds T Masumoto and K Suzuki (The Japan Instttute for Metals, Sendal, 1982) p 547 [22] K Raj, J Durand, J I Budnlck and S Skalskt, J Appl Phys 49 (1978) 1671 [23] T Mizoguchl, J I Budmck, P Panlssod, J Durand and H J Guntherodt, in ref [21], Vol 2, p 1149 [24] A Qachaou, P Panlssod and J Durand, J Magn Magn Mat 31-34 (1983) 1525 [25] M Sostarlch, S Dey, P Deppe, M Rosenberg, G Czjzek, V Oestretch, H Schmldt and F E Luborsky, IEEE Trans Magn MAG-17 (1981) 2612 [26] T J Burch, J I Budnlck, V A Ntculescu, K Raj and T Lttrenta, Phys Rev B 23 (1981) 3866 [27] V S PokatllOV, Sov Pbys Dokl 26 (1981) 327 [28] H Lerchner, K Erdmann, D Welz, M Rosenberg and F E Luborsky, l E E E T r a n s Magn MAG-17 (1981) 2609 [29] K Raj, J Durand, J I Budnlck, C C Tsuet and S Skalskt Solid State C o m m u n 24 (1977) 189 [30] D Sarkar, R Segnan, E K Cornell, E Callem R H a m s , M Phschke and M J Zuckermann, Phys Lett 32 (1974) 542 R W Cochrane, R Harris, M Phschke, D Zobm and M J Zuckermann, Phys Rev B5 (1975)1969 [31] J Durand and P Panlssod, IEEE Trans Magn MAG-17 (1981) 2595 [32] P Heubes, D Korn, G Schatz and G Ztbold, Phys Lett 74A (1979) 267 [33] G Czjzek, J Fmk, F Gotz, H Schmldt. J M D Coey, J P Rebomllat and A Ll6nard, Phys Rev B23 (1981) 2513 {34] J M Fnedt, M Maurer. J P Sanchez, A Berrada, A Qachaou, P Pamssod and J Durand, J de Phys 41 Colloq 8 (1980) 638 [35] J M Fnedt, M Maurer, J P Sanchez and J Durand, 1 Phys F 12 (1982) 821 [36] P Pamssod, D Ahaga-Guerra, A Amamou. J Durand, W L Johnson. W L Carter and S J Poon. Phys Rev Lett 44 (1980) 1465 [37] P Pamssod, I Bakonyl and R Hasegawa, J Magn Magn Mat 31 34 (1983) 1523 [38] M Maurer, J M. Fnedt and J P Sanchez, in ref [7] p 517 [39] I Bakonyl, L Takacs and K Tompa, Phys Stat Sol (b)t03 (1981) 489 [40} P Pamssod and T Mlzoguchl. m ref [21], Vol 2, p 1621