Boer, F.R. de

Buschow, K.H.J.

de Groot, C.H.

Giovanelli, L.

Kockelmann, W.

Moze, O.

English

Southampton (e-Prints Soton)

Journal of Magnetism and Magnetic Materials 189 (1998) 329Ð334Structure and magnetic propertiesof Nd2Co17~xGax compounds studied by magneticmeasurements and neutron di¤ractionO. Moze!, L. Giovanelli",1, W. Kockelmann#,$, C.H. de Groot%, F.R. de Boer%,K.H.J. Buschow%,*!Istituto Nazionale per la Fisica della Materia, Dipartimento di Fisica, UniversitÕa di Modena,Via G. Campi 213/a, I-41100 Modena, Italy"Dipartimento di Fisica, UniversitÕa di Parma, Viale delle Scienze, I-43100 Parma, Italy#Mineralogical-Petrological Institute, University of Bonn, D-53115, Bonn, Germany$ISIS, Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire OX11 0QX, UK%Van der Waals-Zeeman Institute, University of Amsterdam, Valckenierstraat 65, 1018 XE Amsterdam, NetherlandsReceived 18 May 1998; received in revised form 6 July 1998AbstractThe structural properties of Nd2Co17~xGaxcompounds (x"5, 6 and 7) were studied by means of neutron di¤raction.The reÞnement results of the neutron powder di¤raction patterns show that the Ga atoms completely avoid the 9d site inthe rhombohedral Th2Zn17structure. Furthermore, the Ga atoms increasingly occupy the 6c and 18f sites at the cost ofthe18h site that showsa decreasing Gaoccupancyin this concentrationrange.At 4.2 K, theeasy magnetisationdirectionis perpendicular to the c-axis for x"5 and 6, but parallel to the c-axis for x"7. From magnetic measurements andneutron di¤raction it is derived that the Co and Nd moments at 4.2 K strongly decrease with increasing Ga concentra-tion.There is also a strongdecrease of theCurie temperaturewith Ga increasingconcentration. ( 1998 ElsevierScienceB.V. All rights reserved.PACS: 75.25; 75.30; 75.30.GKeywords: Rare earth cobaltintermetallics; Crystal structure; Magnetic properties; Magnetic structure; Neutron di¤raction1. IntroductionIn a previous investigation, the magnetic proper-ties of several series R2Co17~xGax compounds*Corresponding author. Tel.: #31-20-525-5788; e-mail:buschow@nunleus.phys.uva.nl.1Present address: Max-Planck-Institut fu ¬ r Metallforschung,Heisenbergstrasse 1, D-70569 Stuttgart, Germany.were studied by means of magnetic measurementsand X-ray di¤raction on magnetically aligned pow-ders, including series in which the rare earth com-ponent is Pr [1,2], Sm, Gd [3], Tb [4], Y, Dy [5,6]and Ho [7]. These measurements have shown thatincreasing Ga concentration may lead in somecases to a sign reversal of the magnetocrystal-line Co-sublattice anisotropy from easy-planeanisotropy for low Ga concentration to easy-axis0304-8853/98/$19.00 ( 1998 Elsevier Science B.V. All rights reserved.PII: S0 3 0 4 -8 85 3 (9 8 ) 00 2 9 1- 1anisotropy for higher Ga concentration. Froma neutron di¤raction study performed on severalTb2Co17~xGax compounds [4] we have obtainedevidence that also the rare earth sublattice anisot-ropy may give rise to a sign change with increasingGa concentration. In the present study we haveextended the investigation of R2Co17~xGax to com-pounds with R"Nd which we studied by magneticmeasurements and also by neutron di¤raction.2. ExperimentalThe Nd2Co17~xGax samples were prepared byarc melting starting materials of at least 99.9%purity. After arc melting the samples were wrappedinto Ta foil, sealed into an evacuated quartz tubeand annealed for three weeks at 900¡C. The X-raydi¤raction diagrams showed that the annealed sam-ples were single phase within the detection limit of theX-ray di¤raction, their crystal structure correspond-ing to the rhombohedral Th2Zn17 structure type.The neutron powder di¤raction experimentswere made on the ROTAX high-intensity powderdi¤ractometer, a facility present at the ISIS spalla-tion neutron source [8]. Data were collected in thebackscattering mode (2H"124.5¡) and forwardscattering mode (2H"37.5¡) over the entire 20 mstime frame between ISIS pulses, providing a rangeof accessed d-spacings from 0.4 to 15 A s . For mea-surements in the paramagnetic regime, data weretaken at room temperature for the Nd2Co17~xGaxcompounds with x"6 and 7. For the compoundwith x"5 the data were taken at 400 K, which issu¦ciently high above the Curie temperature (seebelow). All compounds were investigated also at 4.2 K.The magnetic measurements were made on pow-dered material on a SQUID magnetometer in thetemperature range 5Ð300 K in magnetic Þelds up to6 T. The measurements above 300 K were made ona home-built (Amsterdam) magnetometer based onthe Faraday principle, using polycrystalline lumpsin order to avoid oxidation at elevated temper-atures as far as possible.3. ResultsThe Nd2Co17~xGax compound with x"5 hasits Curie temperature above room temperature sothat it was possible to take X-ray di¤raction dataon a magnetically aligned sample. From the en-hancement of the 3 0 0 and 2 2 0 reßections and thesimultaneous disappearence of the other reßectionsit was derived that this compound has an easymagnetisation direction perpendicular to the c-axisat room temperature.Examples of neutron di¤raction diagrams areshownin Figs. 1 and 2. Thepowder neutron di¤rac-tion data were analysed by the Rietveld method[8]. The nuclear scattering lengths employed inthe reÞnement were bN$"0.769]10~12 cm, bC0"0.249]10~12 cm and bG!"0.7288]10~12 cm.The results of the Rietveld reÞnements are pre-sented in Table 1 for the data taken in the para-magnetic regime, and in Table 2 for the data takenat 4.2 K.Fig. 1. Observed and calculated neutron di¤raction pattern at300 K for Nd2Co10Ga7 for forward scattering (a) and backscat-tering (b) detector banks.330 O. Moze et al. / Journal of Magnetism and Magnetic Materials 189 (1998) 329Ð334Fig.2. Sectionsof the longd-spacing reÞneddi¤ractionpatternsfor Nd2Co12Ga5 at 400 and 4 K. The presence of a (0 0 3)reßection implies that this compound is planar at 4 K.Results of magnetic measurements showing thetemperature dependence of the magnetisation aredisplayed in Fig. 3. It is seen that the magneticordering temperatures of the three Nd2Co17~xGaxcompounds investigated decrease strongly with in-creasing Ga concentration.The Þeld dependence of the magnetisation at4.2 K of the three Nd2Co17~xGax compounds in-vestigated is shown in Fig. 4. The spontaneousmoments, M0, obtained by extrapolation of themagnetic isotherms to zero Þeld strength, havebeen listed in the last row of Table 2. The Gasubstitution is seen to give rise to a fairly strongdecrease of the magnetisation.4. DiscussionIn several previous investigations [9Ð13], neu-tron powder di¤raction was used to study the siteoccupation of the non-magnetic elements M inrare earth compounds of the type R2Fe17~xMx(M"Al, Ga and Si). In all examples studied, quiteTable 1ReÞned neutron di¤ractionfor Nd2Co17~xGax data obtained at400 K for x"5, and at 300 K for x"6 and 7. Listed are thelattice constants, positional parameters and site occupancies.The Curie temperatures are given in the last rowCompound Nd2Co12Ga5 Nd2Co11Ga6 Nd2Co10Ga7a (A s ) 8.60111(15) 8.62115(9) 8.69658(8)c (A s ) 12.5103(5) 12.48840(27) 12.44132(23)V#%-- (A s 3) 801.509(5) 803.837(21) 814.882(19)Nd, 6c, z 0.3414(5) 0.34428(28) 0.34913(24)Co/Ga, 6c, z 0.0961(8) 0.09922(37) 0.10901(29)Co/Ga, 18f, x 0.2888(5) 0.29549(0) 0.30328(23)Co/Ga, 18h, x 0.5023(4) 0.50220(23) 0.50215(25)Co/Ga, 18h, z 0.1531(4) 0.15242(0) 0.15214(29)%Ga, 6c, z 14.7(15) 39.7(10) 73.4(10)%Ga, 9d 0.0 0.0 0.0%Ga, 18f 34.3(12) 42.9(7) 62.3(8)%Ga, 18h 38.5(11) 37.8(6) 27.5(2)R81 0.0166 0.0188 0.0290R%91 0.0111 0.0286 0.0205s2 1.553 1.269 1.303¹C (K) 350 175 45Table 2ReÞned neutron di¤ractiondata obtained for Nd2Co17~xGax at4.2 K: lattice constants, positional parameters and magneticmoments. At the bottom of the table the reÞned values of themoment per formula unit (M) are compared with the spontan-eous moments (M0) derived from the magnetic isotherms at 5 KCompound Nd2Co12Ga5 Nd2Co11Ga6 Nd2Co10Ga7a (A s ) 8.56478(0) 8.60842(0) 8.684266(0)c (A s ) 12.46233(36) 12.46833(48) 12.41713(58)»#%-- (A s 3) 791.70 800.21 810.99Nd, 6c, z 0.3429(12) 0.3444(12) 0.35060(79)Co/Ga, 6c, z 0.0976(14) 0.0983(14) 0.1080(10)Co/Ga, 8f, x 0.2896(10) 0.29003(98) 0.30569(78)Co/Ga, 18h, x 0.50151(94) 0.49972(92) 0.4985(10)Co/Ga, 18h, z 0.1535(10) 0.1529(11) 0.1471(10)R81 0.0172 0.0178 0.0165R%91 0.0168 0.0191 0.0188s2 1.05 0.87 0.77k, Nd, 6c, lB 2.99(25)! 2.11(30)! 1.42(26)"k, Co, 6c, lB 1.85(31)! 1.5# 1.5#k, Co, 9d, lB 0.85(19)! 1.07(18)! 0.45(19)"k, Co, 18f, lB 0.78(25)! 1.22(33)! 0.15(50)"k, Co, 18h, lB 1.08(31)! 0.60(32)! 0.78(15)"M/f.u., lB 18.8 15.6 8.5M0/f.u., lB 17.0 12.4 7.9!Magnetic moments oriented along [1 0 0]."Magnetic moments oriented along [0 0 1].#Co moments Þxed during reÞnements.O. Moze et al. / Journal of Magnetism and Magnetic Materials 189 (1998) 329Ð334 331Fig. 5. Concentration dependence of Ga atom occupation of the four available sites in the Th2Zn17-type structure of theNd2Co17~xGax compounds.Fig. 3. Temperature dependence of the magnetisation ofNd2Co17~xGax compounds measured in a Þeld of 0.1 T.substantial deviations from a statistical site distri-bution of the non-magnetic atoms over the fouravailable Fe sites (6c, 9d, 18f and 18h) were ob-served. A similar strong deviation from a statisticaldistribution is observed also in the compoundsstudied in the course of the present investigation.This can be seen more clearly in Fig. 5 where weFig. 4. Field dependence of the magnetic moment ofNd2Co17~xGax compounds measured at 4.2 K with decreasingÞeld.have plotted the site occupations derived from thereÞnement of the neutron data listed in Table 1.Generally, it is to be expected that the Ga occu-pancy of a given site increases with increasing Gaconcentration. This behaviour is followed by the 6cand 18f sites, indeed. By contrast, the 9d site ap-pears to be extremely reluctant to accept any Ga332 O. Moze et al. / Journal of Magnetism and Magnetic Materials 189 (1998) 329Ð334atoms. The by far most prominent feature of theresults shownin Fig. 5 is thatthe Ga atomsoccupa-tion for the 18h site decreases with increasing Gaconcentration. In fact, the results shown in Fig.5 can be taken as a further conÞrmation of thetendency of R2Co17~xGax systems to form a trueternary compound at the composition R2Co9Ga8in which the Ga occupancy is restricted to the 6cand 18f sites.The tendency to form a ternary compound ofthe type mentioned above is not restricted to theCo compounds but was also observed for Fe com-pounds. Neutron di¤raction studies made onTb2Fe17~xGax compounds [13] showed that theGa atoms avoid the 9d site completely and that the6c and 18f sites become almost fully occupied byGa atoms for Ga concentrations exceeding x"8.In all R2Fe17~xGax compounds investigated thus-far the concentration dependence of the Curie tem-perature forms a maximum at about x"3 anddecreases for higher Ga concentrations. However,beyond x"7 further Ga substitution leads againto an increase in Curie temperature. This surprisingfeature and the decrease in line broadening of the57Fe Mo ¬ ssbauer spectra in this concentrationrange are further indications of the tendency toternary compound formation [13].Results of the reÞnements pertaining to the mag-netically ordered regime are given in Table 2. Themoment of the Co atoms occupying the 6c siteswere Þxed in this reÞnement because of the lowmultiplicity of the 6c sites. For the other Co mo-ments one may notice a tendency to decrease withincreasing Ga concentration within experimentalerror. This is reßected also in the values of the totalmoment per formula unit, listed at the bottom of thetable. The latter values are in fair agreement with thespontaneous moments derived from the magneticmeasurements, given in the last row of Table 2.The decrease in moment per formula unit is seenin the Table 2 to be partly caused by the strongdecrease of the Nd moment. These results showthat crystal Þeld interactions are presumably fairlystrong in these materials. In the compound withx"5 the comparatively large Co moments andlarge number of Co atoms probably lead to anexchange splitting of a magnitude comparable tothe crystal Þeld splitting so that the Nd momentsare only slightly below the free ion value. Evidently,in the compound with x"7 the crystal-Þeld split-ting dominates.The Co sublattice anisotropy in R2Co17~xGaxsystems is still a matter of controversy. For in-stance, the preferred moment direction in Gd2Co17is perpendicular to the c-axis [14], but results ofX-ray di¤raction made on magnetically alignedGd2Co17~xGax samples with x*2 showed thatthe easy magnetisation direction at room temper-ature corresponds to the c-axis in these com-pounds, meaning that the Co sublattice anisotropychanges sign from negative to positive with increas-ing Ga concentration [3]. By contrast, similar stud-ies made on magnetically aligned Y2Co17~xGaxsamples showed that no such sign reversal of theCo sublattice anisotropy occurs in this system [6].It is therefore di¦cult to make any predictionregarding the Co sublattice anisotropy in theNd2Co17~xGax compounds. Anyhow, the X-raydi¤raction observation of an easy-plane anisotropyin the compound with x"5 at room temperatureindicates that even if the Co sublattice has uniaxialanisotropy for this composition, it is dominated bythe planar anisotropy of the Nd sublattice. Fromthe reÞnement of the neutron data shown in Table2 it can be derived that this situation persists downto 4.2 K. The upturn in the temperature depend-ence of the magnetisation seen below about 45 Kin Fig. 3 is probably due a weakening of the Ndsublattice anisotropy below this temperature. Thisupturn disappears completely when the magnetisa-tion is measured in much higher Þelds than appliedin Fig. 3.It can be seen in Table 2 that the easy magnetisa-tion direction is perpendicular to the c-axis at 4.2 Kalso in the compound with x"6. Surprisingly,however, the easy magnetisation direction is paral-lel to the c-axis in the compoundwith x"7. Owingto the fact that the rare earth sublattice alwayspredominates the 3d sublattice anisotropy in thisclass of materials at low temperatures, the concen-tration dependent change in easy magnetisationdirection implies a change in the nature of thecrystal Þeld with increasing Ga concentration. Asimilar observation was recently made in neutrondi¤raction studies of Tb2Co17~xGax compounds[4] and Tb2Fe17~xGax compounds [13]. In bothO. Moze et al. / Journal of Magnetism and Magnetic Materials 189 (1998) 329Ð334 333systems increasing Ga concentration leads toa change from easy-plane anisotropy to easy-axisanisotropy. Noting that the second-order Stevensfactor for Tm is di¤erent from that of Nd and Tb,this change in anisotropy is consistent with thatobserved at low temperatures in Tm2Fe17~xGaxcompounds where it changes fom easy axis to easyplane with increasing Ga concentration [15].5. Concluding remarksUsing neutron di¤raction and exploiting thelargedi¤erencein scattering length betweenCo andGa we have obtained reliable experimental in-formation of the preferential site occupation of Gain Nd2Co17~xGax compounds (5)x)7). Withincreasing Ga concentration the 18h site becomeslessoccupiedby Ga atoms,a fact that indicatesthatthere is a tendency in this system to form a ternarycompound of the composition Nd2Co9Ga8.W ehave conÞrmed results obtained in previous studiesthat increasing Ga concentration leads to drasticchanges in the crystal-Þeld interaction in 3d com-pounds of the type R2T17~xGax to the extent thata sign reversal occurs in the rare earth sublatticeanisotropy.AcknowledgementsThis work was partly Þnanced by support fromthe TMR (Training and Mobility of Researchers)programme of the European Community. Finan-cial support from the Italian National ResearchCouncil, Consiglio Nazionale delle Ricerche, andthe Dutch Technology Foundation STW is alsogratefully acknowledged.References[1] C. Zhang, J.C.P. Klaasse, E. Bru ¬ ck, F.R. de Boer, K.H.J.Buschow, J. Alloys Compounds 266 (1998) 43.[2] S.-Y. Zhang, B.-G. Shen, B. Liang, Z.-H. Cheng,F.-W. Wang, H.-W. Zhang, Solid State Commun. 104(1998) 732.[3] D. 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Structure and magnetic properties of Nd2Co17-xGax compounds studied by magnetic measurements and neutron diffraction

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