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Nanocrystalline alloys: I. Crystallization M. Miglierini et al. Department of Nuclear Physics and Technology Nanocrystalline alloys: I. Crystallization M. Miglierini et al. Department of Nuclear Physics and Technology Slovak University of Technology Ilkovicova 3, 812 19 Bratislava, Slovakia E-mail: marcel. [email protected] sk http: //www. nuc. elf. stuba. sk/bruno

Nanocrystalline alloys prepared by controlled annealing from rapidly quenched amorphous ribbons exhibit an interesting Nanocrystalline alloys prepared by controlled annealing from rapidly quenched amorphous ribbons exhibit an interesting class of materials from the point of view of their magnetic properties [1]. Resulting magnetic parameters, which are superior to those of conventional transformer steels and/or amorphous materials, are ensured by a presence of crystalline grains several nanometres in size embedded in the amorphous residual phase [2]. Magnetic parameters of amorphous alloys are frequently deteriorated in the process of their practical employment by elevated temperature especially during prolonged operational times. On the other hand, nanocrystalline alloys are in fact already partially crystallized and from this point of view their structure is more resistant to such external effects and that is why it is more stable. Nevertheless, because the excellent magnetic behaviour of nanocrystalline alloys depends strongly on the amount and size of the crystalline grains, the process of crystallization should be known. [1] K. Suzuki, A. Makino, A. Inoue, T. Masumoto, J. Appl. Phys. 70 (1991) 6232. [2] G. Herzer, Phys. Scr. T 49 (1993) 307. The following slide shows a comparison of some magnetic parameters (magnetic permitivity me versus saturation magnetization Bs) for different types of magnetic materials used for, e. g. the production of cores of magnetic circuits. The main three types of compositions which yield nanocrystalline alloys are also listed.

Nanocrystalline Alloys - Features • nanocrystalline alloys – good soft magnetic properties – thermal Nanocrystalline Alloys - Features • nanocrystalline alloys – good soft magnetic properties – thermal stabilization of the structure as compared to amorphous alloys nc-FINEMET NANOPERM Co-am HITPERM • 1988: FINEMET: Fe. Cu. Nb. Si. B • Yoshizawa Y, Oguma A, Yamauchi K J Appl Phys 64 (1988) 6044 • 1988: NANOPERM: Fe. MB(Cu) where M = Zr, Mo, Ti, Nb, Hf, … • Suzuki K, Kataoka N, Inoue A, et al. Mater Trans JIM 31 (1990) 743 • 1998: HITPERM: Fe. Co. Zr. B(Cu) • Willard M A, Laughlin D E, Mc. Henry M E, et al. J Appl Phys 84 (1998) 6773 Fe-am Fe-Co ferrites Si steel A. Makino, A. Inoue and T. Masumoto Mater Trans JIM 36 (1995) 924

Possible Applications of Nanocrystalline Alloys core ribbons magnetic shielding transformer sensors Possible Applications of Nanocrystalline Alloys core ribbons magnetic shielding transformer sensors

Preparation of Nanocrystalline Alloys tube • production of an amorphous precursor melt induction coil Preparation of Nanocrystalline Alloys tube • production of an amorphous precursor melt induction coil melt-spun ribbon – mixing of appropriate amounts of pure elements with subsequent melting quenching – rapid quenching of the melt ( ~106 K/min) wheel method of planar flow casting – result: ribbon up to several cm wide planar and typically about 20 mm thick flow – check of composition (OES ICP) casting and amorphicity (XRD) • (nano)crystallization amorphous ribbon – check of crystallization behaviour by DSC (onset of crystallization, first crystallization peak) – choice of temperature of annealing – annealing (in vacuum) for typically 1 hour at the selected temperature – characterization of the resulting structural and magnetic properties

Structures from a Melt Starting material (melt) Conditions (quenching rate, composition, …) crystalline quasicrystalline Structures from a Melt Starting material (melt) Conditions (quenching rate, composition, …) crystalline quasicrystalline amorphous annealing nanocrystalline • Ordered structure – periodicity – long range order • Disordered structure – short range order – no translation symmetry

Characterization of Nanocrystalline Alloys • structural characterization XRD – DSC (differential scanning calorimetry) • Characterization of Nanocrystalline Alloys • structural characterization XRD – DSC (differential scanning calorimetry) • evolution of structure with temperature – XRD (X-ray diffraction) • crystalline phases, relative fraction of crystallites and amorphous rest DSC TEM – TEM (transmission electron microscopy) • including HREM (high resolution TEM) and XTEM (cross-sectional TEM) • type and size of (nano)crystals – STM (scanning tunnelling microscopy) • including AFM (atom force microscopy) • surface features XTEM – ED (electron diffraction) • structural ordering of phases • magnetic properties STM ED – magnetic measurements • 57 Fe Mössbauer spectroscopy (TMS + CEMS) – simultaneous information on both structural arrangement and magnetic behaviour (hyperfine interactions) Miglierini M et al. J Appl Phys 85 (1999) 1014

Mössbauer spectrometry is a very sensitive tool for the study of both structural arrangement Mössbauer spectrometry is a very sensitive tool for the study of both structural arrangement and hyperfine interactions (magnetic ordering) in nanocrystalline alloys [3]. Tthe FINEMET-type alloys, which are very frequently studied because their macroscopic properties are beneficial for practical applications [4] exhibit rather complicated Mössbauer spectra. They consist of several sextets of narrow lines ascribed to different crystallographic positions in the Fe-Si lattice which are superimposed upon a broadened signal which belongs to the amorphous rest of the original precursor [5]. Evaluation of such spectra is pretty complicated and, unfortunately, prevents from acquiring more detail information related to such phenomena as for example interfacial regions [6]. In order to benefit from its diagnostic potential, it is useful to investigate such materials whose Mössbauer spectra are reasonably simple. This is the situation for example in NANOPERM-type alloys which crystallize into bcc-Fe, the latter being a calibration material for Mössbauer spectrometry. Thus, here we concentrate on the Fe. Mo-Cu-B system which belongs to the NANOPERM family. [3] H. Bremers, O. Hupe, C. E. Hofmeister, O. Michele and J. Hesse: J. Phys. : Condens. Matter 17 (2005) 3197. [4] T. Liu, Z. X. Xu and R. Z. Ma, J. Magn. Mat. 152 (1996) 365. [5] T. Pradell, N. Clavaguera, J. Zhu and M. T. Clavaguera-Mora: J. Phys. : Condens. Matter 7 (1995) 4129. [6] J. M. Grenèche and A. Slawska-Waniewska, J. Magn. Mat. 215 -216 (2000) 264.

Structural Arrangement and Mössbauer Spectra Mössbauer spectra of an ordered structure (crystallites) exhibit narrow Structural Arrangement and Mössbauer Spectra Mössbauer spectra of an ordered structure (crystallites) exhibit narrow lines which lead to single values of the spectral parameters. Due to non-unique positions of resonant atoms in a disordered structure the spectral lines are broad and, consequently, distributions P( ) and P(B) of the spectral parameters must be considered. crystalline (ordered structure) amorphous (disordered structure) hyperfine parameters AM CR non-magnetic D D AM CR magnetic B B

Mössbauer Spectra of Nanocrystalline Alloys (295 K) FINEMET Fe 73. 5 Nb 3 Cu Mössbauer Spectra of Nanocrystalline Alloys (295 K) FINEMET Fe 73. 5 Nb 3 Cu 1 Si 13. 5 B 9 Fe-Si Miglierini M J Phys Condens Matter 6 (1994) 1431 NANOPERM Fe 80 Mo 7 Cu 1 B 12 bcc- Fe Miglierini M and Grenèche J-M J Phys Condens Matter 9 (1997) 2303 Fe on A site Fe on D site Si on D site

Annealing of the Amorphous Precursor • DSC continuous heating (temperature ramp of 10 K/min) Annealing of the Amorphous Precursor • DSC continuous heating (temperature ramp of 10 K/min) • choice of annealing temperatures (B-M, A = as-quenched) => sample preparation • onset of crystallization identified at Tx 1 Fe 76 Mo 8 Cu 1 B 15 Miglierini M et al. phys stat sol (b) 243 (2006) 57 Tx 1 460 o. C structural relaxation diffusion-like precrystallization effects normal grain-growth-like formation of a-Fe nanocrystallites in amorphous matrix diffusion controlled grain-growth of already created a-Fe nanocrystallites diffusion controlled nucleation and growth-like precipitation of g-Fe(Mo)

TEM and XRD • Tx 1 = 450 o. C 550 Miglierini M et TEM and XRD • Tx 1 = 450 o. C 550 Miglierini M et al. phys stat sol (b) 243 (2006) 57 o. C Fe 76 Mo 8 Cu 1 B 15 650 o. C 470 o. C 750 o. C 450 o. C Tx 1 450 o. C

Mössbauer Spectrometry • evolution of Mössbauer spectra with temperature of annealing ta • transmission Mössbauer Spectrometry • evolution of Mössbauer spectra with temperature of annealing ta • transmission Mössbauer spectra are plotted upside-down to enable 3 D mapping • temperature of measurement 300 K and 77 K Fe 76 Mo 8 Cu 1 B 15 300 K 77 K Miglierini M et al. phys stat sol (b) 243 (2006) 57

Fitting Model Miglierini M and Grenèche J-M J Phys Condens Matter 9 (1997) 2303, Fitting Model Miglierini M and Grenèche J-M J Phys Condens Matter 9 (1997) 2303, 2321 Miglierini M and Grenèche J-M Hyperfine Interact 113 (1998) 375 crystalline interface amorphous 295 K Fe 80 Mo 7 Cu 1 B 12 440 o. C/1 h HREM 10 nm

Transmission Mössbauer Spectrometry (295 K) 550 o. C Miglierini M et al. phys stat Transmission Mössbauer Spectrometry (295 K) 550 o. C Miglierini M et al. phys stat sol (b) 243 (2006) 57 510 o. C • bulk • Tx 1 = 450 o. C (? ) 600 o. C 450 o. C Fe 76 Mo 8 Cu 1 B 15 410 o. C

Conversion Electron Mössbauer Spectrometry (295 K) 550 o. C Miglierini M et al. Hyperfine Conversion Electron Mössbauer Spectrometry (295 K) 550 o. C Miglierini M et al. Hyperfine Int 165 (2005) 75 510 o. C • surface • Tx 1 = 450 o. C 600 o. C 450 o. C Fe 76 Mo 8 Cu 1 B 15 410 o. C

XRD – Peak Decomposition 550 o. C Miglierini M et al. phys stat sol XRD – Peak Decomposition 550 o. C Miglierini M et al. phys stat sol (b) 243 (2006) 57 510 o. C • Tx 1 = 450 o. C 600 o. C 450 o. C Fe 76 Mo 8 Cu 1 B 15 410 o. C

Summary • structure of nanocrystalline alloys – (nano)crystallites – residual amorphous matrix – interface Summary • structure of nanocrystalline alloys – (nano)crystallites – residual amorphous matrix – interface = surface of crystalline grains + crystal-to-amorphous matrix region • crystallization – first at the surface – progress of crystallization is more rapid at the surface • identification of crystalline phase • amount of nanocrystals

Mössbauer spectroscopy contributes to the study of nanocrystalline alloys from several viewpoints. First, it Mössbauer spectroscopy contributes to the study of nanocrystalline alloys from several viewpoints. First, it is possible to identify the structural arrangement from a very first look at a Mössbauer spectrum (e. g. , onset and progress of crystallization). Crystalline phases are characterized by narrow and usually well separated lines whereas the amorphous residual phase exhibits broad patterns due to its disordered nature. Signal from resonant atoms located at the interfacial regions can be also distinguished. The latter two contributions are described by the help of distributions of hyperfine parameters through which information on both topological and chemical short-range order can be derived. The fraction (and/or type) of the crystalline phase(s) can be readily obtained from the spectral parameters. Second, magnetic order of the system under the study is also directly followed from changes of the spectral line shapes, viz. (broadened) doublet vs. sextet. This can be studied as a function of annealing temperature (i. e. , crystalline contents), measuring temperature, and/or composition. More details can be found in another presentation. In this presentation, we have shown that the crystallization of amorphous precursors for the preparation of nanocrystalline alloys proceeds more rapidly on the surface of the rapidly quenched ribbons than in their bulk. In doing so, we have employed CEMS and TMS, respectively. The crystalline content was determined also from XRD and the results coincide well with those from TMS. The temperature of the onset of crystallization Tx 1 determined from DSC is somewhat higher than that from XRD, TEM and MS due to different regime of annealing (continuous during DSC and isothermal during the preparation of the samples).