DSpace at VNU: Magnetic and Mossbauer studies of the DyFe11Mo compound

Journal of Magnetism and Magnetic Materials 237 (2001) 10–16 Magnetic and Mossbauer studies of the DyFe11Mo compound V.T Hiena, J.M Le Bretonb,*, N.T Hiena,c, L.T Taia,c, N.P Thuya,c, N.H Duca, N.P Duongc, J Teilletb a Cryogenic Laboratory, Faculty of Physics, National University of Hanoi, 334 Nguyen Trai, Thanh Xuan, Hanoi, Viet Nam b ! Groupe de Physique des Materiaux, UMR CNRS 6634, Universite! de Rouen, 76821 Mont-Saint-Aignan Cedex, France c International Training Institute for Materials Science (ITIMS), Dai Co Viet, DHBK Hanoi, Hanoi, Viet Nam Received June 2001; received in revised form 19 July 2001 Abstract A detailed magnetic and Mossbauer study focusing on intrinsic magnetic and anisotropy properties of the DyFe11Mo compound is reported The compound shows a spin reorientation phase transition at Tsr ¼ 220 K Anomalies in physical properties such as saturation magnetization, AC-susceptibility and hyperfine field at Tsr were identified, analysed and are discussed in terms of the individual site anisotropy model r 2001 Elsevier Science B.V All rights reserved PACS: 75.30.Àm; 75.30.Cr; 76.80.+y; 75.50.Bb Keywords: Rare earth-transition metal intermetallics; Spin reorientation; Mossbauer spectrometry Introduction The pseudobinary intermetallics RFe12ÀxMx (R=rare earths; M=Ti, V, Cr, Mn, Mo, W, Al or Si and x varying from 0.5 to 4.0 depending on the type of the stabilizing element M) have been intensively studied in the last decade [1–5] These compounds crystallize in the ThMn12 structure and exhibit many interesting magnetic and electronic properties It is known that in the compounds containing R elements with a negative second-order Stevens factor aJ ; the rare-earth (4f) *Corresponding author Tel.: +33-2-35-14-67-66; fax: +332-35-14-66-52 E-mail address: jean-marie.lebreton@univ-rouen.fr (J.M Le Breton) sublattice shows a planar anisotropy, whereas the Fe (3d) sublattice has a uniaxial easy axis anisotropy The competitive anisotropy contributions from the two sublattices can lead to spin reorientation phase transitions as the temperature changes, inducing rotations of the resultant magnetic moments with respect to the crystallographic directions This phenomenon can be evidenced by an abrupt variation of the hyperfine fields at the 3d site [6] In the DyFe12ÀxMox compounds, where Dy has a negative Stevens factor aJ ; the sign of the overall magnetic anisotropy constant changes at a temperature Tsr leading to a spin reorientation phase transition at this temperature These compounds, therefore, can serve as an outstanding example for studying the above-mentioned observable phenomena 0304-8853/01/$ - see front matter r 2001 Elsevier Science B.V All rights reserved PII: S - 8 ( ) 0 - V.T Hien et al / Journal of Magnetism and Magnetic Materials 237 (2001) 10–16 In general, spin reorientation phase transitions are macroscopically studied by means of magnetic property investigations, e.g magnetization and susceptibility measurements Mossbauer effect measurements, on the other hand, can provide useful information on microscopic properties of the system, e.g the discontinuity in the hyperfine fields at the 3d site at Tsr which reflects the orbital contribution to the 3d magnetic moment at a given site This information, therefore, enables one to consider the local anisotropy at each 3d site in this type of compounds [6] In this paper, we present a study of the intrinsic magnetic and anisotropy properties of the DyFe11Mo compound Attention is focused on the magnetization and hyperfine field changes around the spin reorientation phase transition temperature Tsr : The results are discussed in terms of the individual site anisotropy (ISA) model Experimental A DyFe11Mo compound was prepared by arcmelting the constituents in the nominal stoichiometric composition in a protective atmosphere of pure argon (99.99%) Pure metals (Dy of 99.9%, Fe and Mo both of 99.99% purity) were used In order to ensure its homogeneity, the as-melt sample was several times turned and melted again We have added about wt% excess of Dy to compensate the rare earth lost caused by evaporation during the repeated melting procedure The ingot obtained was then annealed at 10001C for 70 h in a pure argon atmosphere At the end of the annealing procedure the sample was quenched in water down to room temperature and then used for the measurements Samples were characterized by X-ray diffraction using a Philips APD 1700 diffractometer with a copper anticathode (lKa ðCuÞ D0:1540 nm) For the study of the magnetocrystalline anisotropy, oriented powder samples were fabricated by mixing the sample powder with a non-magnetic epoxy resin, and orienting the mixture along the easy direction of magnetization in a magnetic field of about T Magnetic measurements were carried out by means of different types of magnetometers, such as 11 extraction magnetometer (EM), vibrating sample magnetometer (VSM), SQUID-magnetometer and AC-susceptometer in varying temperatures and fields up to 600 K and T, respectively Mossbauer spectra were recorded in the temperature range from 77 to 300 K in transmission geometry using a 57Co source in a rhodium matrix The Mossbauer sample contains about 10 mg/cm2 of natural ion Results and discussion Fig presents the X-ray diffraction pattern of the as-prepared sample The pattern shows typical reflections of the well-known ThMn12 structure As can be seen, an impurity phase in form of a-Fe is present in the sample The lattice parameters were determined from the diffraction pattern and yielded values of a ¼ 0:8513 and c ¼ 0:4775 nm for the tetragonal unit cell These parameters are in very good agreement with published data [7] In Fig 2, we show the magnetization as a function of the temperature for DyFe11Mo measured in the temperature range from 77 to 600 K in an applied field of 0.1 T As one can see in this figure, the MðTÞ curve exhibits two distinguished features at about 220 and 460 K The 220 K anomaly is ascribed to the spin reorientation phase transition whereas the 460 K anomaly can be interpreted as the Curie temperature of this Fig X-ray diffraction pattern of the DyFe11Mo sample The peaks of the DyFe11Mo compound are indexed in the figure The main peak of the a-Fe phase is also indicated 12 V.T Hien et al / Journal of Magnetism and Magnetic Materials 237 (2001) 10–16 Fig Thermal variation of the magnetization measured in a field of 0.1 T for DyFe11Mo compound The magnetization measured above 460 K does not completely diminish clearly confirming the presence of the a-Fe phase in the sample although it is not a significantly large contribution The spin reorientation phase transition of DyFe11Mo is also confirmed by the AC susceptibility (wAC ) measurements (see Fig 3) From this figure, another anomaly is visible at about 35 K The origin of this anomaly, however, is still unknown and its characterization is beyond the scope of this article The magnetization isotherms MðHÞ measured at 5, 77 and 300 K on a free powder DyFe11Mo sample is shown in Fig It is clearly seen that the high field magnetic susceptibility is rather large, in particular at T ¼ and 77 K In addition, the magnetization measured at 77 K is larger than that at both and 300 K The inset shows the MðTÞ curve measured from to 300 K in an applied field of T The curve exhibits a broad maximum around 200 K Again, this may originate from the thermal variation of the spin configuration in the sample From the K curve, a value of 10.0 mB was determined for the spontaneous magnetization Assuming a value of mDy ¼ 10 mB for the free Dy3+ ion-moment at low temperatures, we derived a value of mFe E1:82 mB for the moment of the Fesublattice at T This value is well below that of metallic iron (mFe E2:2 mB) This indeed is a Fig Temperature dependence of AC susceptibility for DyFe11Mo Fig Magnetization isotherms of DyFe11Mo The inset shows the magnetization measured in an applied field of T as a function of the temperature common behaviour observed for the Fe-moment in RFe12ÀxMx compounds [2], which is influenced by the additional 3d(Fe)–3d(M) hybridization The magnetization as a function of the angle between the applied field and the aligned direction of an oriented-powder sample, measured at liquid nitrogen and room temperature using a field of V.T Hien et al / Journal of Magnetism and Magnetic Materials 237 (2001) 10–16 13 Fig Magnetization as a function of the angle between the applied field and the alignment direction of a DyFe11Mo aligned powder sample, measured at 300 and 77 K in a field of 0.3 T 0.3 T, is presented in Fig It can be seen from this figure that at T ¼ 77 K (oTsr ), the easy magnetization direction (EMD) makes a coneangle (around 451) with respect to the c-axis which is the EMD of the compound at T ¼ 300 K (> Tsr ) This is in good agreement with the observation reported by Yang et al [8] and again confirms the spin reorientation phenomenon evidenced in the other measurements mentioned above The Mossbauer spectra of the as-prepared DyFe11Mo compound at different temperatures are reported in Fig The spectra were fitted consistently in the whole temperature range according to the following assumptions: (i) Like the magnetic moment, the hyperfine field at a given Fe site is mainly governed by the Fe–Fe exchange interactions, depending on both the number of Fe nearest neighbours and the corresponding interatomic Fe–Fe distances [2] The order of Bð8iÞ hf > ð8fÞ Bð8jÞ > B is thus assumed for the hyperfine fields hf hf of the three Fe sites of the ThMn12 structure, in agreement with Mossbauer spectrometry and neutron diffraction data [2,9] (ii) The Mo atoms are substituted for Fe on the 8i site only, as supported by experimental data [10,11] This is in agreement with the positive enthalpy contribution associated with R and Mo, the R atoms having Fig Mossbauer spectra of the DyFe11Mo compound in the temperature range from 77 to 300 K four nearest 8i-site neighbours compared with eight nearest 8j- and 8f-sites neighbours [11] The Mossbauer relative intensities of the different Fe sites in the DyFe11Mo compound were thus constrained to the following values: 27.2% for 8i, 36.4% for 8j and 8f sites (iii) Each site contribution is fitted with a discrete distribution of magnetic sextets, in order to account for the distribution of environments around the Fe atoms, due to the presence of Mo The hyperfine field corresponding to each site is the mean value of the corresponding distribution (iv) As evidenced by 14 V.T Hien et al / Journal of Magnetism and Magnetic Materials 237 (2001) 10–16 X-ray diffraction and thermomagnetic analysis, some small amount of a-Fe is present in the sample Its Mossbauer relative intensity can be deduced from the fitting of the room temperature spectrum, as its contribution is clearly distinguishable from that of the pure DyFe11Mo phase: the obtained value is 5% Thus, the contribution of aFe is fitted in each spectrum with a relative intensity fixed to 5% At each temperature, the contribution of the DyFe11Mo phase thus represents 95% of the intensity of the spectrum From these fittings, the mean hyperfine field of the DyFe11Mo compound and the hyperfine fields (Bhf ) of each Fe 3d site contribution were obtained Their temperature dependence is presented in Figs 7a and b, respectively As shown in Fig 7a, the average hyperfine field gradually decreases as the temperature rises In the temperature region around the spin reorientation phase transition (between 220 and 240 K) an obvious discontinuity is evidenced This discontinuity is also apparent in the curves corresponding to the Fig Temperature dependence of (a) the mean hyperfine field /Bhf S and (b) the hyperfine field Bhf at each Fe site of the DyFe11Mo compound The full lines are guides for the eye temperature evolution of the hyperfine fields at each single Fe site (see Fig 7b) The average hyperfine field extrapolated to K (see Fig 7a) reaches a value of /Bhf S ¼ 28:6 T corresponding to /mFe S ¼ 1:83 mB/Fe by using the conversion factor of 15:6 T/mB from hyperfine field to magnetic moment [1] Indeed, this is in good agreement with the value deduced from magnetization data The values for the individual site hyperfine fields extrapolated to K are calculated from the Mossbauer data as Bhf iị ẳ 34:5; 28.5, and 23.5 T for the 8i, 8j and 8f sites, respectively Using the same conversion factor as above, the corresponding Fe-moment at the corresponding sites are miị Fe ẳ 2:21; 1.83 and 1.51 mB The order sequence of the magnitudes of these individual site Fe ð8jÞ ð8fÞ moments mð8iÞ Fe > mFe > mFe is the same as that observed by Mossbauer effect on other RFe12ÀxMx compounds This order can be understood as the consequence of the order of the average Fe–Fe distances for each Fe ion site ð8iÞ ð8jÞ ð8fÞ (dFe2Fe > dFe2Fe > dFe2Fe ) [2] It is worthwhile to mention that among the Fe–Fe distances around the (8i) site, the mean Fe(8f)–Fe(8i) distance is the shortest one Consequently, the 3d(Fe(8f))– 3d(Mo(8i)) hybridization would be the strongest and the density of the negative 3d(Mo) spin around Fe(8f) site would be the highest [12] As the change of hyperfine field is related to a change in the spin density, this leads to a strong reduction of Bhf ð8fÞ as well as mð8fÞ Fe at the 8f site The discontinuity in the temperature dependence of both the average and the individual Fe site hyperfine field observed at the spin reorientation temperature Tsr is closely related to the second-order anisotropy constant which is determined by the residual orbital moment quenched by the crystal field [3,4] At Tsr ; the sign of the hyperne-eld change is positive: DBhf ẳ ẵBhf M8c Þ À Bhf ðMconical ފ > in case DK1 ẳ ẵK1 T > Tsr ị K1 ToTsr ị40: This is in good agreement with what was found in many other reports concerning the discontinuity of the hyperfine field at Tsr : For example, we can compare this result with that obtained for Er2Fe13Mn4 [3] and Tm2Fe17 [4] compounds, where the rotation of the magnetization from the basal plane to the easy axis V.T Hien et al / Journal of Magnetism and Magnetic Materials 237 (2001) 10–16 with decreasing temperature is accompanied by an obvious increase of the hyperfine field As far as the magnitude of the discontinuity in the average (DBhf ) and in the individual site (DBðiÞ hf ) hyperfine field is concerned, this should be proportional to the overall Kl and the individual site KlðiÞ anisotropy, respectively, as it was suggested by Thuy et al [3] A close inspection of the data presented in Figs 7a and b shows that DBhf E0:6 T whereas DBð8iÞ DBð8jÞ hf E0:2; hf E0:4; ð8fÞ DBhf E1:7 T This means that the most prominent contribution to the overall anisotropy in the 3dsublattice in this compound should be from the Fe ion at the 8f site (if one takes 1.7 T/3=0.57 T, so rather closely to 0.6 T) The fact that the Fe ion at the 8f site gives the largest contribution to the overall 3d anisotropy in the DyFe11Mo compound is quite contradictory to the result reported by Thang et al [5], who suggested that the 8i site plays a leading role when analysing the M.ossbauer data on the YFe12ÀxVx compound It is also contradictory to the M.ossbauer results reported by Hu et al [1] and Li et al [2] for RFe12ÀxTix compounds Among the YFe11M compounds with M=Cr, Al, Si, Ti, V, W, Mn and Mo, the smallest Fe magnetic moment at 4.2 K is 1.35 mB in YFe11Mo and the largest one is 1.77 mB in YFe11Ti (see e.g [2]) Our values of individual site hyperfine fields at 77 K for DyFe11Mo are Bhf iị ẳ 33:5; 27.5 and 22.2 T corresponding to miị Fe ẳ 2:15; 1.76 and 1.42 mB for the 8i, 8j and 8f sites, respectively, and /mFe S ¼ 1:74 mB Comparing these values with those reported in Ref [1] at 77 K for DyFe11Ti, namely 33.7, 28.8, 25.5 T [1], we find mðiÞ Fe ¼ 2:16; 1.85 and 1.63 mB for the 8i, 8j and 8f sites, respectively, and /mFe S ¼ 1:85 mB It is clear that for the DyFe11Ti and DyFe11Mo counterparts, the Fe magnetic moments are enhanced (because of the 3d-band splitting effect due to the molecular field of the Dy-sublattice) but the order sequence of magnitude of the Fe moments remains the same as in the Y-based compounds (i.e the Femoment in the Ti-containing compound is larger than that in the Mo-containing one) In closer details, it is interesting to compare the ratio ðiÞ mðiÞ Fe ðMoÞ=mFe ðTiÞ between the Fe-moment at the corresponding sites in the two compounds This ratio equals to 99.5%, 95.5% and 87% for 15 the 8i, 8j and 8f sites, respectively The strongest reduction of the Fe-moment is thus observed at the 8f site This might be well associated with the preferential substitution of Mo for Fe at the 8i site and suggests that the 3d(Fe(8f))– 3d(Mo(8i)) hybridization may be stronger than the 3d(Fe(8f))–3d(Ti(8i)) hybridization Concluding remarks It is well known that compared to other elements, Mo substituting for Fe in RFe12ÀxMox compounds leads to many interesting anomalies such as a stronger dependence of both Curie temperature and saturation magnetization on the Mo concentration, an irregularity in the spin reorientation phenomenon, the magneto-history effect, the first-order magnetization process (FOMP) and a higher capability of nitrogenation, etc Our present study has brought by additional evidences to this series of anomalies, namely the Mo element strongly reduces the Fe magnetic moment at the 8f site, while making Fe in the 8i, 8j sites more sensitive to the band splitting effect caused by the molecular field of the Dy-sublattice At the same time, Mo preferentially occupying the 8i site notably suppresses the contribution to the overall anisotropy at this site while causing it to increase at the 8f site Under these circumstances, the 3d(Fe)–3d(Mo) hybridization may play a quite important role here Acknowledgements This work is partly supported by the Program of the Fundamental Research of Vietnam References [1] Bo-Ping Hu, Hong-Shuo Li, J.P Gavigan, J.M.D Coey, J Phys.: Condens Matter (1989) 755 [2] Hong-Shuo Li, J.M.D Coey, in: K.H.J Buschow (Ed.), Handbook of Magnetic Materials, Vol 6, Elsevier, Amsterdam, 1991, p [3] N.P Thuy, J.J.M Franse, N.M Hong, T.D Hien, J Phys 49 (1988) 499 16 V.T Hien et al / Journal of Magnetism and Magnetic Materials 237 (2001) 10–16 [4] P.C.M Gubbens, A.M van der Kraan, K.H.J Buschow, Hyperfine Interactions 40 (1988) 389 [5] C.V Thang, N.P Thuy, N.M Hong, T.D Hien, N.S Almodova, R Grossinger, J Magn Magn Mater 140– 144 (1995) 1017 [6] N.P Thuy, J Zukrowski, H Figied, J Przewoznik, K Krop, Hyperfine Interactions 40 (1988) 441 [7] C.P Yang, Y.Z Wang, B.P Hu, J.L Wang, Z.X Wang, J Magn Magn Mater 192 (1999) 105 [8] C.P Yang, Y.Z Wang, B.P Hu, J.L Wang, Z.X Wang, Z.L Jiang, C.L Ma, J Zhu, J Alloys Compounds 290 (1999) 144 [9] H Fujii, H Sun, in: K.H.J Buschow (Ed.), Handbook of Magnetic Materials, Vol 9, Elsevier, Amsterdam, 1995, p 303 [10] W.B Yelon, G.C Hadjipanayis, IEEE Trans Magn MAG-28 (1992) 2316 [11] D.B de Mooij, K.H.J Buschow, J Less-Common Met 136 (1988) 207 [12] N.H Duc, A Fnidiki, J Teillet, J Ben Youssef, H Le Gall, J Appl Phys 88 (2000) 1265  ... as the Curie temperature of this Fig X-ray diffraction pattern of the DyFe11Mo sample The peaks of the DyFe11Mo compound are indexed in the figure The main peak of the a-Fe phase is also indicated... 95% of the intensity of the spectrum From these fittings, the mean hyperfine field of the DyFe11Mo compound and the hyperfine fields (Bhf ) of each Fe 3d site contribution were obtained Their temperature... et al / Journal of Magnetism and Magnetic Materials 237 (2001) 10–16 Fig Thermal variation of the magnetization measured in a field of 0.1 T for DyFe11Mo compound The magnetization measured above