Temperature studies in the range 300−800 K of amorphous Fe80Nb6B14 alloy using Mössbauer spectroscopy
are presented. It is shown that at a temperature close to 700 K iron clusters with non-collinear magnetic structure are formed. The observed magnetic permeability enhancement effect in the annealed at elevated temperatures alloy, which takes place in amorphous phase, is due to the strong ferromagnetic exchange between Fe clusters via the amorphous matrix and reduction of internal stresses

Frąckowiak, Janusz

Hanc-Kuczkowska, Aneta

Haneczok, Grzegorz

Tomczyk, Adam

Nukleonika

English

Repozytorium Uniwersytetu Śląskiego RE-BUŚ

             Title: Mossbauer temperature study of the Fe80Nb6B14 amorphous alloy  Author: Janusz E. Frąckowiak, Adam Tomczyk, Aneta Hanc, Grzegorz Haneczok  Citation style: Frąckowiak Janusz E., Tomczyk Adam, Hanc Aneta, Haneczok Grzegorz. (2007). Mossbauer temperature study of the Fe80Nb6B14 amorphous alloy. "Nukleonika" (Vol. 52, suppl. 1 (2007), s. 43-46).  NUKLEONIKA 2007;52(Supplement 1):S43−S46 PROCEEDINGSIntroductionIn recent years a great interest is focused on nanocrystal-line alloys obtained by a suitable thermal annealing ofamorphous alloys at temperatures close to the crystalli-zation temperature [8]. It was shown that the softmagnetic properties of the so-called nanoperm andfinemet amorphous alloys can be significantly improvedby formation of a nanocrystalline phase, αFe and/orαFe(Si) nanograins, embedded in amorphous matrix[3−9]. An enhancement of soft magnetic properties isobserved when the after-annealing structure withrandomly oriented nanocrystallite grains of dimension(typically 10−15 nm) much smaller than the ferromagneticexchange length gives an averaging out of magneticanisotropy. Recently, it was found [1, 2] that magnetic-permeability-enhancement effect in nanoperm typeFe86−xNbxB14 (x = 5, 6) alloys takes place in amorphousphase without forming nanograins. It was proposed thatthe enhancement in permeability is due to diffusion offree volume leading to formation of small iron clusterswith different magnetic structures. The aim of the paperis to study the possible atomic-rearrangement eventsassociated with structural relaxation which takes placein the annealed at elevate temperature Fe80Nb6B14amorphous alloy. Mössbauer spectroscopy was appliedto in situ measurements in the temperature range300−800 K. The determined temperature dependencesof the hyperfine parameters derived from Mössbauerspectra are presented and discussed.Mössbauer temperature studyof the Fe80Nb6B14 amorphous alloyJanusz E. Frąckowiak,Adam Tomczyk,Aneta Hanc,Grzegorz HaneczokJ. E. Frąckowiak?, A. Tomczyk, A. Hanc, G. HaneczokInstitute of Materials Science,University of Silesia,4 Uniwersytecka Str., 40-007 Katowice, Poland,Tel.: +48 32 3591117, Fax: +48 32 2596929,E-mail: jfrack@us.edu.plReceived: 20 June 2006Accepted: 30 November 2006Abstract. Temperature studies in the range 300−800 K of amorphous Fe80Nb6B14 alloy using Mössbauer spectroscopyare presented. It is shown that at a temperature close to 700 K iron clusters with non-collinear magnetic structure areformed. The observed magnetic permeability enhancement effect in the annealed at elevated temperatures alloy, whichtakes place in amorphous phase, is due to the strong ferromagnetic exchange between Fe clusters via the amorphousmatrix and reduction of internal stresses.Key words: amorphous alloys • magnetic properties • structural relaxationS44 J. E. Frąckowiak et al.Experimental detailsThe examined alloy was fabricated by a melt-spinningtechnique in the form of strips with thickness and widthof about 25 µm and 10 mm, respectively. The results ofsystematic studies of the magnetic properties of the alloywere reported in [2] and can to be summarized asfollows. The Curie temperature of amorphous phasein as-quenched state is equal to TC = 370 K. The initialmagnetic permeability µ(Ta) determined at roomtemperature plotted versus annealing temperature Tapasses through a distinct maximum situated at theso-called 1 h optimization temperature To = 700 K. Thetransmission electron microscopy examinations ofannealed samples have shown that the first αFenanocrystallites were detected in a sample annealed atTk = 760 K. This result is confirmed by Mössbauerspectroscopy; the additional narrow lines that corre-spond to iron nanocrystallites are detected at thistemperature.In this work, transmission Mössbauer spectroscopybased on 57Fe was employed. For investigation ofstructural relaxation phenomena, a special procedureof annealing of the sample in the range 300−800 K wasapplied. The sample was mounted in a high-tempera-ture furnace and the Mössbauer spectra were measuredin situ at elevated temperatures, starting from roomtemperature and the as-quenched state. The time ofcollection for each measured spectrum was equal exactlyto 8 h. After this time, the sample was cooled to roomtemperature and the Mössbauer spectrum was recordedagain. Then, a higher temperature was applied and theprocedure of annealing was repeated. Some selectedspectra obtained are presented in Figs. 1 and 2. TheMössbauer spectra of the sample in the paramagneticstate were fitted by means of a discrete analysis (a singleline and a quadrupole doublet). The magneticallysplitted spectra were fitted by means of a hyperfine fielddistribution using the Hesse-Rubartsch method [5]. Inorder to take into account the asymmetry of lines,a linear relation between isomer shift (IS) and hyperfinemagnetic field (H) was assumed in the fitting procedure.Results and discussionAs can be seen from Fig. 1, the Mössbauer spectrarecorded above, T = 375 K, consist of asymmetricdoublets. In the work, these spectra were fitted bya single line (PL) and a quadrupole doublet (PQ). Thedependence of the isomer shift (IS) of PL componenton temperature is linear with the slope dIS/dT =0.0005 mm/s/K as a result of occurrence of the tempera-ture SOD (second order Doppler) shift. This suggeststhat the PL component represents iron atoms with thedistinct disordered amorphous neighborhood. TheFig. 1. Mössbauer spectra of Fe80Nb6B14 alloy determined: at temperature T for 8 h (a), at room temperature for the samplecooled from elevated temperatures Ta (b).S45Mössbauer temperature study of the Fe80Nb6B14 amorphous alloydependence of the IS and the quadrupole splitting (QS)of PQ component on temperature is complex and isshown in Fig. 2. Assuming that the PQ component isconnected with the existence of a local topological andchemical ordering in the vicinity of the selected Featoms, we can describe the process of structural relaxationwhich takes place in the paramagnetic state. A strongincrease in a value of QS in the range 625−700 K can beattributed to the process of formation of chemical shortrange order involved by diffusion of free volumes whichleads to redistribution of B and Nb atoms in the vicinityof the selected Fe atoms − which is confirmed by anincrease in a value of IS. An increase in values of QSand IS is observed for the optimization temperatureTo = 700 K. These results suggest that formation of Feclusters with specific configurations of magneticmoments (probably asperomagnetic, as is demonstratedin [9]), and with strong local anisotropies in the atomicFe arrangement is the source of enhancement ofmagnetic properties in the amorphous state. At highertemperatures, near Tk = 760 K, where the formationof αFe nanograins is starting, a significant decrease inIS is observed. In Fig. 3, the dependences of the meanvalue of the hyperfine magnetic field H and thedispersion DH of the distribution of the magnetic fieldsP(H), determined from the spectra measured at roomtemperature as a function of the annealing temperatureTa are presented. Two characteristic features areimportant, i.e. a minimum in H and DH observed for thetemperature 700 K and a strong increase for Ta > 750 Kdue to nanocrystallization. The significant decrease inH occurs at the optimization annealing temperature Toand is well correlated with the observed increase inIS and QS for PQ component of the Mössbauerspectrum measured at To temperature. It is worth tonotice that a very similar dependence of H vs. Ta forFe80Nb6B14 amorphous alloy after 1 h annealing attemperatures Ta was determined in [2]. This resultindicates that the described early clusters of Fe atomsformed in the paramagnetic state at a temperature closeto To are quenched and strongly influence magneticproperties of the investigated alloy at room temperaturein the amorphous state. The observed minimum in theH value for the To annealing temperature is probablyconnected with a relatively small average magneticmoment of the clusters formed with non-collinearmagnetic structure in an asperomagnetic state [7].However, the strong ferromagnetic exchange betweenclusters via the amorphous matrix and the reduction ofinternal stresses, in contrary to the case when nanograinsare formed, lead to the enhancement of magneticpermeability effect.ConclusionsThe in situ temperature study in the range 300−800 Kof the amorphous Fe80Nb6B14 alloy using Mössbauerspectroscopy showed that at a temperature close to700 K specific clusters of Fe atoms with non-collinearmagnetic structure are formed. In the annealed atelevated temperatures samples the strong ferromag-netic exchange between clusters via the amorphousmatrix leads to the enhancement of the magneticproperties in the amorphous phase without forminga nanostructure. This makes it possible to obtain a goodsoft magnetic material free of embitterment, in contraryto nanostructured materials.Fig. 2. The dependence of hyperfine parameters of PQcomponent in Mössbauer spectra recorded at elevatedtemperatures Ta: isomer shift (a), quadrupole splitting (b).The dotted line is added only for eye.Fig. 3. The mean hyperfine magnetic field H (a) and dispersionDH of the hyperfine field distribution (b) determined at roomtemperature for sample annealed for 8 h at elevated tempera-tures Ta. The dotted line is added only for eye.S46 J. E. Frąckowiak et al.References  1. Frąckowiak JE, Haneczok G, Kwapuliński P, Rasek J,Chrobak A (2004) The Mössbauer study of magneticpermeability enhancement effect in the Fe86−xNbxB14amorphous alloys. Czech J Phys D 54:109−112  2. Haneczok G, Frąckowiak JE, Chrobak A, KwapulińskiP, Rasek J (2005) Magnetic permeability enhancementeffect in the Fe86−xNbxB14 (x = 5, 6) amorphous alloys.Phys Status Solidi A 202:2574−2579  3. Herzer G (1995) Soft magnetic nanocrystalline materials.Scr Metall Mater 33:1741−1748  4. Herzer G, Varga LL (2000) Structure and coactivity ofnanocrystalline Fe-Si-B-Nb-Cu alloys. J Magn MagnMater 215/216:506−512  5. Hesse J, Rübartsch A (1974) Model independentevaluation of overlapped Mössbauer spectra. J Phys E7:526−532  6. Kwapuliński P, Rasek J, Stokłosa Z, Haneczok G (2001)Magnetic properties of Fe76X2B22 (X=Cr,Zr,Nb)amorphous alloys. J Magn Magn Mater 234:318−324  7. Lorenz R, Hafner J (1995) Noncollinear magneticstructures in amorphous iron and iron-based alloys. JMagn Magn Mater 139:209−227  8. McHenry ME, Willard MA, Laughlin DE (1999)Amorphous and nanocrystalline material for applicationsas soft magnets. Prog Mater Sci 44:291−296  9. Škorvanek I, Duhaj P, Grössinger R (2000) Low-tempera-ture magnetic behavior in amorphous and nanocrystallineFe-Nb-B alloys. J Magn Magn Mater 215/216:431−439

Mossbauer temperature study of the Fe80Nb6B14 amorphous alloy

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Mossbauer temperature study of the Fe80Nb6B14 amorphous alloy

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