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Solid State Sciences 3 (2001) 301–308 59 Co NMR study in Co–Fe alloys/Co magnetite composites Jean Philippe Jay a,1 , Ioan-Sorin Jurca a , Genevie ve Pourroy b, *, Nathalie Viart b , Christian Me ´ny a , Pierre Panissod a a Groupe dEtudes des Mate ´riaux Me ´talliques, IPCMS, UMR 046 du CNRS, 23 Rue du Loess, F -67037 Strasbourg Cedex, France b Groupe des Mate ´riaux Inorganiques, IPCMS, UMR 046 du CNRS, 23 Rue du Loess, F -67037 Strasbourg Cedex, France Received 31 July 2000; accepted 25 September 2000 Abstract Iron – cobalt compounds containing 20 – 30 at.% Fe with unusual crystallographic structure have been observed in metal-oxide composite synthesized in an aqueous media at 120 – 140°C. The oxide is a cobalt-containing magnetite. The metallic component is found in two crystallographic structures depending on the preparation conditions: a b.c.c. (a-Fe) structure and an a-Mn, also called Re 24 Ti 5 , structure (space group I -43m). The a-Mn-like CoFe alloy is encountered for the first time and transforms into the b.c.c. structure during annealing at temperatures as low as 160°C. In the b.c.c. structure itself, the cobalt and iron atoms present a chemical short-range order totally different from the regular B2 phase of the ordered bulk CoFe compound. This particular order is similar to the one encountered in high vacuum co-deposited thin films. For annealing temperatures larger than 400°C, the metallic component loses some Fe and transforms into nearly pure f.c.c. cobalt. This study shows that chemical as well as physical low temperature preparation techniques favor original CoFe ordered phases not achievable by conventional metallurgy. © 2001 E ´ ditions scientifiques et me ´dicales Elsevier SAS. All rights reserved. Keywords: Composite; FeCo alloys; Isomorphous to a-Mn; NMR spectroscopy www.elsevier.com/locate/ssscie 1. Introduction Nowadays, composite materials give rise to nu- merous studies with a view to improving mechanical, thermal, optical, and other properties, which hold promise for novel technological applications. Vari- ous methods of synthesis have been used — high-en- ergy ball milling, reduction or decomposition of metallic precursors in polymers, oxides or gels [1 – 4]. Nanocomposites made of an iron – cobalt alloy and Co-containing magnetite have been recently synthe- sized in an aqueous medium by using the dispropor- tionation of Fe(OH) 2 [5]. The metal is embedded in the oxide within octahedral particles and is protected by a thin amorphous layer about 3.5-nm thick [6]. Three structures have been encountered for the metal owing to the synthesis temperature and the following thermal treatment — a b.c.c. structure and an a-Mn like structure in the as-prepared composites and a f.c.c. structure that appears after a thermal treatment [7,8]. The b.c.c. and f.c.c. structures are classical structures for iron and cobalt-based alloys. Indeed, the iron – cobalt phase diagram is characterized be- low 700°C by a wide solid solution of b.c.c. structure expending from pure iron up to about 75 at.% Co [9]. For larger cobalt concentrations, the structure is f.c.c. and, close to pure cobalt, becomes hexagonal at * Correspondence and reprints. E-mail address: [email protected] (G. Pourroy). 1 Present address: LEM, De ´p. de Physique, UBO, 6 Av. Le Gorgeu, BP 809, F-29285 Brest, France. 1293-2558/01/$ - see front matter © 2001 E ´ ditions scientifiques et me ´dicales Elsevier SAS. All rights reserved. PII: S1293-2558(00)01100-6

59Co NMR study in Co–Fe alloys/Co magnetite composites

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Page 1: 59Co NMR study in Co–Fe alloys/Co magnetite composites

Solid State Sciences 3 (2001) 301–308

59Co NMR study in Co–Fe alloys/Co magnetite composites

Jean Philippe Jay a,1, Ioan-Sorin Jurca a, Genevie' ve Pourroy b,*, Nathalie Viart b,Christian Meny a, Pierre Panissod a

a Groupe d’Etudes des Materiaux Metalliques, IPCMS, UMR 046 du CNRS, 23 Rue du Loess, F-67037 Strasbourg Cedex, Franceb Groupe des Materiaux Inorganiques, IPCMS, UMR 046 du CNRS, 23 Rue du Loess, F-67037 Strasbourg Cedex, France

Received 31 July 2000; accepted 25 September 2000

Abstract

Iron–cobalt compounds containing 20–30 at.% Fe with unusual crystallographic structure have been observed inmetal-oxide composite synthesized in an aqueous media at 120–140°C. The oxide is a cobalt-containing magnetite. Themetallic component is found in two crystallographic structures depending on the preparation conditions: a b.c.c. (a-Fe)structure and an a-Mn, also called Re24Ti5, structure (space group I-43m). The a-Mn-like CoFe alloy is encountered for thefirst time and transforms into the b.c.c. structure during annealing at temperatures as low as 160°C. In the b.c.c. structureitself, the cobalt and iron atoms present a chemical short-range order totally different from the regular B2 phase of theordered bulk CoFe compound. This particular order is similar to the one encountered in high vacuum co-deposited thinfilms. For annealing temperatures larger than 400°C, the metallic component loses some Fe and transforms into nearly puref.c.c. cobalt. This study shows that chemical as well as physical low temperature preparation techniques favor original CoFeordered phases not achievable by conventional metallurgy. © 2001 Editions scientifiques et medicales Elsevier SAS. Allrights reserved.

Keywords: Composite; FeCo alloys; Isomorphous to a-Mn; NMR spectroscopy

www.elsevier.com/locate/ssscie

1. Introduction

Nowadays, composite materials give rise to nu-merous studies with a view to improving mechanical,thermal, optical, and other properties, which holdpromise for novel technological applications. Vari-ous methods of synthesis have been used — high-en-ergy ball milling, reduction or decomposition ofmetallic precursors in polymers, oxides or gels [1–4].Nanocomposites made of an iron–cobalt alloy and

Co-containing magnetite have been recently synthe-sized in an aqueous medium by using the dispropor-tionation of Fe(OH)2 [5]. The metal is embedded inthe oxide within octahedral particles and is protectedby a thin amorphous layer about 3.5-nm thick [6].Three structures have been encountered for the metalowing to the synthesis temperature and the followingthermal treatment — a b.c.c. structure and an a-Mnlike structure in the as-prepared composites and af.c.c. structure that appears after a thermal treatment[7,8]. The b.c.c. and f.c.c. structures are classicalstructures for iron and cobalt-based alloys. Indeed,the iron–cobalt phase diagram is characterized be-low 700°C by a wide solid solution of b.c.c. structureexpending from pure iron up to about 75 at.% Co[9]. For larger cobalt concentrations, the structure isf.c.c. and, close to pure cobalt, becomes hexagonal at

* Correspondence and reprints.E-mail address: [email protected] (G.

Pourroy).1Present address: LEM, Dep. de Physique, UBO, 6 Av. LeGorgeu, BP 809, F-29285 Brest, France.

1293-2558/01/$ - see front matter © 2001 Editions scientifiques et medicales Elsevier SAS. All rights reserved.

PII: S1293-2558(00)01100-6

Page 2: 59Co NMR study in Co–Fe alloys/Co magnetite composites

J.P. Jay et al. / Solid State Sciences 3 (2001) 301–308302

temperatures lower than 400°C. On the contrary, thea-Mn structure has been stabilized only in this typeof composite and transforms into b.c.c. by annealingabove 160°C [8]. The crystallographic positions havebeen calculated and refined from the X-ray diffrac-tion pattern by means of the FullProf code [10].While preliminary Mossbauer spectroscopy measure-ments have given us information on the site occupa-tion in the spinel phase, we had no access to thelocal environment in the metal due to the closeatomic factor of cobalt and iron [11]. Therefore, inorder to get more information on the structuralarrangement in the composite and particularly in themetal phase, we have performed 59Co NMRspectroscopy.

2. Experimental

In order to obtain the composite, aqueous solu-tions of Co(II) and Fe(II) chlorides, FeC12·4H2Oand CoC12·6H2O with a Co to Fe ratio of 0.5 wereintroduced by means of a peristaltic pump intoboiling KOH solutions (1205T5140°C) in a stain-less steel vessel equipped with a mechanical stirrer ona hot plate. The temperature was measured bymeans of a stainless steel thermometer placed in thecenter of the vessel. During the following maturationtime (20 min), the heating and stirring were main-tained. The black precipitate formed was thenfiltered from the hot solution, washed with boilingwater and ethanol and dried at 40°C in air. Threesamples corresponding to KOH concentrations of 13N (T=140°C, sample A), 11 N (T=130°C, sampleB) and 9 N (T=120°C, sample C) were preparedand submitted to further annealings in air or argon.

The samples were analyzed by the ICP method inthe ‘Laboratoire Central d’ Analyse de VernaisonCNRS’, which gave the global (Co/Fe) ratio in thefinal powder. The (Co/Fe) ratio in the metallic com-ponent was determined by analysis of the solutionobtained after dissolution of the metal in a solutionof copper sulfate. Thermal gravimetric (TG) anddifferential thermal analysis (DTA) measurementswere carried out in platinum crucibles in air or invacuum immediately after the synthesis by using aSetaram TG92 apparatus. The heating rate was 2 Kmin−1. The weight variation was calculated afterremoving the signal of the empty crucible, whichgave the mass of the metallic part assuming a full

oxidation of Co and Fe into Co-ferrite (checked byX-ray diffraction).

X-ray diffraction data were collected at room tem-perature with a D500 Siemens diffractometerequipped with a primary beam quartz monochroma-tor (Co Ka1,=1.78897 A, ).

59Co NMR spectra were recorded at 4.2 K in zeroexternal field, i.e. only in the internal (hyperfine) fieldarising from the spontaneous magnetization of thematerial. The frequency of the automated broadband spectrometer was swept from 100 to 350 MHzin steps of 1 MHz and the integral intensity of thespin echo (Hahn’s echo) measured at each frequency.

3. Results and discussion

3.1. As-prepared samples

Fig. 1 displays the X-ray diffraction patterns thatwere observed in the as-prepared composites. Thethree samples contain a spinel phase for which thelattice parameter is close to that of magnetite (a=0.8397 nm) and that of cobalt ferrite (a=0.8392 nm)[12](Table 1). Sample A contains only a b.c.c. metal-lic phase (space group Im3m) while samples B and Cshow an admixture of the two metallic structures. Insample C, the structure of the metallic component ismostly a-Mn like (space group I-43m). Unfortu-nately, the attempts to obtain only the a-Mn likestructure for the metallic structure have failed untilnow. The composition of the metal given by chemi-cal analysis is reported in Table 1. It is in agreementwith the measured lattice parameter of the b.c.c.phase when compared with that of the b.c.c. CoFesolid solution [13]. Thus, the two metallic phaseshave the same composition. TG measurements in airexhibit an oxidation of the phases above 300°C. Theweight increase, in addition to the metal compositionand the Co/Fe values for the whole composite, al-lows to determine the chemical formula (Table 1).

3.2. NMR spectra of the composites

The NMR spectra of the as-prepared samples areshown in Fig. 1. They arise only from the metalliccomponent of the composites. Indeed, all the at-tempts to observe a signal in samples that containedonly the spinel phase have failed. This is due to thefact that the metallic part of the composite is a soft

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magnet, while the spinel phase is a hard magnet. Asthe enhancement of the observed signal is propor-tional to the local electronic susceptibility, the signalof the metal is greatly enhanced compared with thatof the spinel.

The NMR spectra reflect the distribution of hy-perfine field on 59Co which is characteristic of themagnetic state of the ion and its immediate struc-tural and chemical environment (essentially thenearest neighbor shell, hereafter NN). Therefore,such a zero-field NMR spectrum is a signature of thevarious chemical coordination shells present in thesample. For pure metallic cobalt, the base NMRfrequencies are 217 MHz in the high temperaturef.c.c. phase, 219–228 MHz (depending on the mag-netization orientation) in the anisotropic stable h.c.p.phase and 200 MHz in the b.c.c. metastable phase[14,15]. The modification of this frequency due to thepresence of Fe in the NN shell of a Co atom hasbeen thoroughly studied in CoFe alloys and com-pounds with f.c.c. [14] and b.c.c. [15] structure: itshows that every Fe atom substituted for Co in thecoordination shell of cobalt shifts up its NMR fre-quency by 9 MHz (f.c.c. case) or by 11 MHz (b.c.c.case).

3.2.1. The b.c.c. phaseFrom the shape of the spectrum of sample A, one

can immediately conclude that, in the b.c.c. phase,two dominant chemical coordination shells arepresent: (i) a NN shell containing only eight Coatoms as in pure b.c.c. Co, which gives rise to a lineat 200 MHz, and (ii) a NN shell containing six Coand two Fe (line at 222 MHz). Note that the ratherbroad 222 MHz line could also arise from h.c.p. Cobut this possibility is totally excluded by the X-raydiffraction study. However, there is a large similaritybetween that spectrum and those observed in co-de-posited b.c.c. thin film alloys grown in ultra-highvacuum [16]. As can be seen in Fig. 2, the CoFe thinfilms deposited at 175°C show the same two-peakedspectrum as the as-prepared composites with a b.c.c.metallic phase. Note that such a comparison indi-cates that the composition of the metallic componentin sample A lies between Co0.80 Fe0.20 andCo0.76Fe0.24 in fair agreement with the chemical anal-ysis (Table 1). The peculiar short-range order(mostly Co surrounded either by eight Co and zeroFe atoms or Co surrounded by six Co and two Featoms) observed in sample A and in co-deposited

Fig. 1. X-ray diffraction patterns and NMR spectra observed forthe three samples (as prepared). The X-ray diffraction lines thatare not indexed belong to the spinel phase.

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Table 1Co to Fe ratios in the composites, lattice parameters, weight increase observed in air and formula of composites

Sample A B C

0.5Co/Fe 0.50.50.2839(2) 0.2841(2)a (nm) b.c.c. 0.2838(2)0.8738(3) 0.8740(3)a (nm) a-Mn like structure

0.8396(2)a (nm) spinel 0.8397(2) 0.8396(2)6.1 6.4Dm/m in air (90.1%) 6.1(Co0.78(1)Fe0.22(1))0.80(1)(Co0.80(1)Fe0.20(1))0.80(1) (Co0.71(1)Fe0.29(1))0.85(1)Chemical formula

(Fe2.37(1)Co0.63(1)O4) (Fe2.37(1)Co0.63(1)O4) (Fe2.32(1)Co0.68(1)O4)

thin film alloys has been studied [17]. It is compatiblewith a mixture of ordered phases that may be stabi-lized on a b.c.c. lattice as predicted by Finel [18].

3.2.2. The a-Mn like phaseAs mentioned above, the a-Mn phase could not be

obtained without some degree of admixture with theb.c.c. phase. So, its NMR spectrum can only beextracted by a linear regression on spectra withdifferent ratios of the two phases. Previous workshave shown that the a-Mn like phase transforms intothe b.c.c. phase by heating. Thus, we have performedannealing treatments of various durations at 160°Con sample C. One observes a considerable evolutionof the X-ray diffraction and of the NMR spectrumas a function of the annealing time (Fig. 3). Thea-Mn phase progressively transforms into the b.c.c.phase for which no variation of the b.c.c. latticeparameter has been observed. This confirms that thetwo phases have the same composition. After 7 days,the a-Mn phase gets totally converted into the b.c.c.phase. Because of the similar packing of the atoms inthe b.c.c. and in the a-Mn phases, one can under-stand the low temperature needed for the transfor-mation, which does not necessitate any masstransport. The mass fractions of the various phaseshave been calculated by refinement of the X-raypowder patterns using the FullProf code and thediffraction patterns recorded after each annealingstage (Table 2) [5,6].

An example of simulation with the spectral shapesdeduced from the linear regression is presented inFig. 4a (for the sample annealed during 1 h). As canbe seen the reconstructed spectrum is very close tothe experimental one. Since the a-Mn to the bccphase ratios are known from the X-ray analyses andsince the shape of the bcc CoFe contribution is alsoknown (the sample annealed during 166 h has no

a-Mn phase left), the shape of the a-Mn phasecontribution is deduced unambiguously by the fittingprocedure. The deduced NMR spectrum of the a-Mn phase is compared with that of the b.c.c. phasein Fig. 4b. As in the b.c.c. phase, the shape of theNMR spectrum observed in the composite samplesshows that the chemical environment of Co is verydifferent from the one prevailing in b.c.c. CoFerandom alloys (solid solution) and ordered com-pounds (B2 phase) prepared by standard metallurgi-cal ways with a comparable composition [16].

The analysis of the NMR data is difficult owing tothe existence of four inequivalent sites, 2a, 8c, 24g1and 24g2 in the space group I-43m and of largelydifferent coordination numbers and nearest neighbordistances [5]. The 59Co spectrum shown in Fig. 4bexhibits three very badly resolved lines at high fre-quency and a much better resolved line at 201 MHz

Fig. 2. 59Co NMR spectra (normalized to unit area) observed forthe b.c.c. metallic component in the composites (sample A) ascompared with those observed in molecular beam deposited Col−

xFex films at 175°C.

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Fig. 3. X-ray diffraction patterns and NMR spectra observedafter different annealing treatments of sample C (1–166 h at160°C and 20 h at 200°C). The X-ray diffraction lines that are notindexed belong to the spinel phase.

on top of the low frequency tail. The narrow lineindicates that one of the sites occupied by cobalt hasa well-defined chemical coordination and that its sitesymmetry has to be weakly anisotropic only. Amongthe four sites of the structure, only site 2a with 12 Conearest neighbors of the same kind (24g2) and atequal distances can possibly satisfy the criteria.Moreover, the relative intensity of the 201 MHz lineis very close to the atomic fraction expected in thecase of a site fully occupied by cobalt atoms. Despitethe fact that no direct comparison can be made withb.c.c. or f.c.c. alloys, it seems unrealistic that thenearest neighbor shell of this site contains a lot ofiron: this is supported (i) by the low resonancefrequency and (ii) by the fact that there is littlechemical disorder among its 12 NNs. Therefore, onecan rather safely conclude that site 2a is mostlyoccupied by cobalt. A second conclusion, thoughweaker, is that site 24g2 is also mostly occupied bycobalt. As regards the distribution of Co and Fe onthe two other sites (8c and 24g), the only informa-tion that can be drawn out of the NMR observationsis that there must be some degree of chemical order:this is evidenced by the fact that the remaining partof the spectrum exhibits some structures and not abroad Gaussian line as observed in concentratedsolid solutions.

3.3. Thermal behavior in air or in vacuum

Further transformations have been studied byisochronal (2 h) annealing at 200, 300, 400, 600 and800°C either in air or in vacuum. Fig. 5 displays theevolution of the X-ray diffraction spectra of sampleA in air. At 300°C, the metallic phase is still in ab.c.c. structure although a small f.c.c. peak begins toappear. At 400°C, the b.c.c. phase has been replacedby the high temperature f.c.c. phase of Co.

The shape of the NMR spectra shows that thechemical order evolves considerably. Indeed, the in-tensity of the line at 200 MHz (Co surrounded by Coonly) decreases strongly leaving a largely dominantline at 222 MHz. This means that at 300°C, in theb.c.c. phase, most of the Co atoms (more than 60%)are bound to six Co and two Fe. At 400°C, a narrowpeak at 217 MHz corresponding to f.c.c. Co peakappears. The metallic component disappears totallyabove 600°C due to oxidation. During oxidation, the59Co NMR peak intensity decreases down to zero(Fig. 5).

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Table 2The abundance of the various phases observed by X ray diffraction in sample C after different annealing treatments in vacuum (analysiswith the FullProf Code [10])

Duration (h)Annealing temperature (°C) Mass fractions (90.1%)

Ferrite b.c.c. a-Mn

80.7 4.8 14.5As prepared sample1160 80.4 6.8 12.75160 80.4 8.2 11.4

80.3 9.720 10.0160160 80.3166 19.7 0.0

The same evolution is observed for annealingtreatments in vacuum but the various stages of thetransformation take place at higher temperatures(Fig. 6). At 600°C, the NMR spectrum does in-deed show a narrow f.c.c. Co peak at 217 MHz.The shoulder on the high frequency side of thisline can be attributed either to h.c.p. stackingfaults (twins) in the f.c.c. structure or to a smallamount of Fe (at most 5%) into f.c.c. Co. In anycase, this shows that the metallic phase after high-temperature annealing contains only a very smallamount of Fe. Considering that the initial Fe con-tent in the metallic component of the as-preparedsamples is larger than 20 at.%, this means that,during annealing, the metallic component getsricher in cobalt while iron moves from the metalto the oxide.

4. Conclusion

The disproportionation of Fe(OH)2 when Co(II)is involved leads to a Fe–Co/Co containing mag-netite composite for which the metal presents par-ticular arrangements. 59Co NMR spectroscopy iswell adapted to give insights on the chemical orderbetween Co and Fe in the metal. First, it givesinformation on the a-Mn structure, that we couldnot obtain by X-ray diffraction measurements. Coand Fe atoms are not distributed at random onthe four sites of the a-Mn structure, i.e. sites 2aand, probably, 24g2 are occupied mostly by Co.Secondly, it shows that the arrangement is com-pletely different from that encountered in bulkb.c.c. CoFe alloys prepared by classical metallurgi-cal routines, but similar to that observed in CoFe

Fig. 4. (a) Example of simulation obtained with the spectralshapes deduced from the linear regression. The sample was an-nealed during 1 h; (b) 59Co NMR spectra (normalized to unitarea) observed for the b.c.c. phase (sample A) and the a-Mn phaseas compared with those observed in ordered (B2 phase) anddisordered bulk Co0.7Fe0.3 alloys of comparable composition.

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Fig. 5. X-ray diffraction patterns and NMR spectra observedafter isochronal (2 h) annealing at 200–400°C in air. No metallicphase was observed at 600°C. Fig. 6. X-ray diffraction patterns and NMR spectra observed

after isochronal (2 h) annealing at 200–800°C in an argon atmo-sphere.

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thin films grown by co-deposition in ultra-highvacuum. It may seem surprising that so differentpreparation techniques (chemical, and ultra-highvacuum deposition) yield similar, though uncom-mon, structures for the CoFe compound. Actually,for both techniques, metallic ions or atoms con-dense at low temperature, 120–140°C, in the chem-ical bath and 175°C on the substrate for the vapordeposition. Since there is no or little bulk diffusionin the metal at such temperatures, one may specu-late that, in both cases, the chemical short-rangeorder is driven mostly by diffusion and interactionsat the surface of the growing grain or film, respec-tively. There is a significant difference between thetwo kinds of samples though, contrary to the caseof the composites, annealing treatments up to500°C did not modify the spectra of the thin filmalloys, even after 4 weeks [16]. This observationshows that atomic diffusion in the bulk of themetallic compounds is very slow, even at 500°C.Therefore, the rapid changes that are observed inthe composites result most probably from a chemi-cal reaction between the metallic and oxide compo-nents.

It would be interesting to know the a-Mn struc-ture better. Neutron diffraction measurementswould be adapted to separate the iron and cobaltcontributions. However, the number of phases inthe composite makes such a study difficult. There-fore, we are now working to obtain the a-Mn likestructure by co-deposition techniques or as theunique metal phase in the composite by using hy-drothermal synthesis.

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