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FORMATION AND CHARACTERISATION OF FORMATION AND CHARACTERISATION OF NANOSTRUCTURED METASTABLE ALLOYS NANOSTRUCTURED METASTABLE ALLOYS Viorel Pop Faculty of Physics, Babes-Bolyai University, 3400 Cluj-Napoca, Romania Nanocrystalline materials obtained by: • vapour - inert gas condensation, sputtering, plasma processing, vapour deposition • liquid - electrodeposition, rapid solidification • solid - mechanical alloying , severe plastic deformation, spark erosion 1

hard mechanical alloying nanostructured magnetic materials and and rapid quenching soft ANNEALING modifies the structure and microstructure 2

mechanical alloying and rapid quenching F Metastable Metastable phases Instable Energy barrier ∉ Thermodynamic Stable equilibrium conditions 3

directly Nanocrystalline structures can be obtained amorphous precursors Generally, there are three empirical requirements which must be satisfied by magnetic alloys to produce amorphous precursor 1. The alloys are composed of more than three elements. 2. The constituent alloying elements have significantly different atom size. 3. The heat formation of the amorphous alloys is negative. 4

high Hard phase anisotropy + exchange exchange - spring magnets large Soft phase magnetisation 5

high Hard phase anisotropy + exchange exchange - spring magnets large Soft phase magnetisation ≈ δ D 2 h cr δ = π A / K h h h D cr = soft phase critical dimension δ h = width of domain wall in the hard phase A h and K h are the exchange and anisotropy constants 6

Large reversible A strength remanence + demagnetization curve m r > 0.5 ( m r = M r / M s ) } EXPERIMENTAL criteria for the presence of the exchange spring mechanism In hard magnetic nanocrystalline materials full or almost full crystallization is required. 7

Nanocrystalline partial crystallisation soft magnetic materials two-phase materials } + a nanocrystalline an amorphous matrix 8

Nanocrystalline partial crystallisation soft magnetic materials two-phase materials } + a nanocrystalline an amorphous matrix negative positive compensates magnetostriction magnetostriction 9

Nanocrystalline partial crystallisation soft magnetic materials two-phase materials } + a nanocrystalline an amorphous matrix negative positive compensates magnetostriction magnetostriction V cr ≈ 70-75 % for Fe 10

Nanocrystalline partial crystallisation soft magnetic materials two-phase materials } + a nanocrystalline an amorphous matrix negative positive compensates magnetostriction magnetostriction V cr ≈ 70-75 % for Fe = π 2 D < L ex ; L A / 4 M ex s D = nanocrystallite diameter L ex = magnetic exchange length 11

Nanocrystalline partial crystallisation soft magnetic materials two-phase materials } + a nanocrystalline an amorphous matrix negative positive compensates magnetostriction magnetostriction G. Herzer , IEEE Trans. Magn. MAG-25 (1989) V cr ≈ 70-75 % D <15 nm for Fe 3327; IEEE Trans. Magn. MAG-26 (1990) 1397 for a-Fe(Si) and a-Fe nanocrystals present in = π 2 D < L ex ; L A / 4 M ex s Finemet (Fe 73.5 Cu 1 Nb 3 Si 13.5 B 9 ) D = nanocrystallite diameter and respectively L ex = magnetic exchange length 12 Nanoperm (Fe 84 Zr 3.5 Nb 3.5 B8Cu 1 )

13

10 100 1000 Grain size, D (nm) 14

vanish of the coercivity in low permeability superparamagnetic regime soft magnetic nanostructures small ferromagnetic crystallites low coercivity coupled and by exchange interactions high permeability The local anisotropies are randomly averaged out by exchange interactions so that there is no anisotropy net effect on the magnetisation process. 15

Rapid quenching Nanocrystalline structure is obtained 2 steps: 1. Formation of the amorphous state by rapid quenching of liquid alloy at very high cooling rate of 10 5 -10 6 K/s. 2. Partial or complete crystallisation of the amorphous alloy by annealing. 16

Rapid quenching Nanocrystalline structure is obtained 2 steps: 1. Formation of the amorphous state by rapid quenching of liquid alloy at very high cooling rate of 10 5 -10 6 K/s. 2. Partial or complete crystallisation of the amorphous alloy by annealing. Argon Spin melting Crucible v r = 20 -30 m/s Melt r.f. Coil v r = 80 m/s !!! Copper roller Melt-spun ribbon v r 17

Table 1. General characteristics of the soft and hard magnetic materials produced by annealing of metallic glasses. Nanocrystalline Magnetically soft Magnetically hard materials (Fe-based) (Fe-based) Alloys Finemet R-Fe-B (Fe 73.5 Cu 1 Nb 3 Si 13.5 B 9 ) R = rare-earth Nanoperm (Fe 84 Zr 3.5 Nb 3.5 B 8 Cu 1 ) e.g. Nd 11.8 Fe 82.3 B 5.9 Hitperm Pr 5 Fe 88 Nb 2 B 5 (Fe 44 Co 44 Zr 7 B 4 Cu 1 ) Structure Nanocrystals (bcc-Fe)+ Nanocrystals Nd 2 F 14 B+ amorphous matrix (Fe 3 B, a-Fe, amorphous) 70 − 75 % ⇒ λ s ≈ 0 * ≤ 100 % V cr ≤ 15 nm ⇒ ≈ 0** D < 25 nm K Properties High permeability, High coercivity, low magnetic losses high remanence * λ s − saturation magnetostriction constant − averaged magnetocrystalline anisotropy K ** 18 T. Kulik , J. Non Crystalin Solids 287 (2001) 145

Spin melting SOFT Inhibits the formation of Fe boride compounds Nb Promote the formation of Cu-rich clusters Increase the density of Fe nucleation in between Cu-clusters Schematic illustration of the formation of the nanocrystalline structure in Fe-Cu-Nb-Si-B alloys based on atom probe analysis results and TEM G. Herzer, Handbook of Mag. Mater., Ed. K.H.J. Buschow, Vol 10 (1997) 415 19

Spin melting SOFT Annealed 1h/540 °C 20 G. Herzer , Handbook of Mag. Mater., Ed. K.H.J. Buschow, Vol 10 (1997) 415

Spin melting SOFT Annealed 1h/540 °C 21 G. Herzer , Handbook of Mag. Mater., Ed. K.H.J. Buschow, Vol 10 (1997) 415

Spin melting SOFT DTA – DSC – differential thermal analysis differential scanning calorimetry R 6 Fe 87 Nb 1 B 6 Fe 73.5 Cu 1 Nb 3 Si 13.5 B 9 Fe 68.5 Gd 5 Cu 1 Nb 3 Si 13.5 B 9 22 O. Crisan et al., ICM Rome 2003 G. C. Hadjipanayis , J. Magn. Magn. Mater. 200 (1999) 373 .

Nd2Fe14B/(Fe3B, Fe) melt spinning Spin melting hard Spin melting HARD and by or mechanical alloying Sm2Fe17N3/Fe high magnetisation of high magnetic anisotropy of + soft phases - Fe, Fe3B… R compounds - Nd2Fe14B, Sm 2 Fe 17 N 3 … ( BH ) max = 1090 kJ/m 3 for Sm 2 Fe 17 N 3 /Fe 65 Co 35 nanostructured multilayers R. Skomski, J. Appl. Phys. 76 (1994) 7059 Kronmuller & Coey Magnetic Materials, in European White book on Fundamentel Research in Materials Science Max Planck Inst. Metallforschung, Stuttgart, 2001, 92-96 23

Spin melting hard Spin melting HARD Behaviour connected to the microstructure changes in the melt spun evidenced by, TEM 24 H. A. Davies , J. Magn. Magn. Mater. 157-158 (1996) 11

Spin melting hard Spin melting HARD p normal 6 GPa Annealing 923 K 25 X. Y. Zhang , J. W. Zhang, W. K. Wang, J. Appl. Phys. 89 (2001) 477

Spin melting HARD p normal 6 GPa Annealing 923 K The volume fraction of Sm 2 (Fe,Si) 17 Cx phase also increases as the pressure increases. This behaviour is explained by the change of the crystallisation sequence: at low pressure a-Fe is the first crystallisation phase, while at high pressure Sm 2 (Fe,Si) 17 Cx is. 26 X. Y. Zhang , J. W. Zhang, W. K. Wang, J. Appl. Phys. 89 (2001) 477

Spin melting HARD X. Y. Zhang , J. W. Zhang, W. K. Wang, J. Appl. Phys. 89 (2001) 477 27

Mechanical alloying D 1 C 1 D 2 D 1 C 2 B 1 B 1 C 3 B 2 D - ductile component C - composite (compound) B - brittle component MA MM Mechanical milling refers to the process of milling Mechanical alloying involves pure metals or compounds which are in the synthesis of materials by 28 thermodynamical equilibrium before milling high-energy milling

Mechanical alloying Ni 3 Fe • milling of Ni and Fe powders in a high energy planetary mill • heat treatments (temperatures and duration) • X-rays diffraction (XRD) • electron microscopy morphology phase composition checked by EDX • magnetic measurements: 4 - 600 K; µ 0 H ≤ 8 T • Mössbauer spectrometry 29

The presence of the first order internal stresses acts at a macroscopic level and modifies the lattice parameters and consequently produces an angular shift of the X-ray diffraction peaks. The second-order internal stresses act at a microscopic level of the crystallites and produce a broadening of the X-ray diffraction peaks [13, 16]. L. Castex, J.L. Lebrun, G. Maeder, J.M. Sprauel, Determination de contraintes résiduelles par diffraction des rayons X, Publications scientifiques et techniques de l’ENSAM, Paris vol.22 (1981), 51-60. See also http://WWW.physiqueindustrie.com/ 30