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Permanent Magnets based on Fe-Pt Alloys

Permanent Magnets based on Fe-Pt Alloys. P.D. Thang, E. Brück, K.H.J. Buschow, F.R. de Boer. Financial support by STW. Introduction Permanent magnets, motivation Experimental Sample preparation, analysis techniques Results Structure, magnetic and mechanical properties

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Permanent Magnets based on Fe-Pt Alloys

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  1. Permanent Magnets based on Fe-Pt Alloys P.D. Thang, E. Brück, K.H.J. Buschow, F.R. de Boer Financial support by STW

  2. Introduction Permanent magnets, motivation Experimental Sample preparation, analysis techniques Results Structure, magnetic and mechanical properties Conclusions Contents

  3. Introduction. What’s permanent magnet ? • Magnetic materials • Large Mr, high Hc • High (BH)max e.g: SmCo5 (1969), Sm2Co17, Nd2Fe14B (1983) B M • B = µ0(M+H)

  4. Applications of permanent magnets • Automobile industry. • Computer industry. • Scientific research. • Biomedical treatment. • and many other applications.

  5. Dental applications • Denture retention system • Soft magnetic: Pd-Co • Magnet: NdFeB (DYNA Dental Engineering B.V.) • Magnet problems • Very brittle • Low corrosion resistance • Aim: to find a magnet and a soft magnetic material with • Good mechanical properties • High corrosion resistance

  6. Introduction. Fe-Pt alloys • fcc - fct phase transition fcc • statistical distribution • high magnetisation fct • layer structure • high magnetic anisotropy • Good mechanical strength and high corrosion resistance

  7. Experimental Sample processing • Preparation FexPt100-x and (Fe0.6Pt0.4)100-xMx based alloys - Arc-melting the pure elements (3N) in Ar - Casting to cylinder, disc • Heat treatment: - Homogenisation (as-quenched sample): 1325C/1h, under Ar + quenching in water  fcc phase. - Ageing (aged sample): 500-700C + quench in water  fct phase.

  8. Analysis techniques Applied force Tensile strength: tensometer Magnetic properties: hysteresis-loop and … SANS Microstructure analysis: TEM ...

  9. Results. Crystallographic structure • Fe60Pt40 • As-quenched: presence of the fct phase. • Ageing: fcc-fct transformation.

  10. Permanent-magnet properties: FexPt100-x Optimal hard-magnetic properties for Fe60Pt40 aged at 625C, 1h: Br = 0.97 T, BHc = 294 kA/m, (BH)max = 118 kJ/m3

  11. Microstructure:Fe60Pt40 625°C, 1h: 2-5 nm as-quenched: 1-3 nm • Dark field image:  Fct particle size increases during the ageing.  Fct nano-size observed. • Selected area diffraction pattern:  Degree of atomic order increases during the ageing.

  12. Magnetic properties: (Fe0.6Pt0.4)100-xMx (M = Nb, Al) • 0.5% Nb aged at 625C, 24h: Br= 0.98 T, BHc= 302 kA/m, (BH)max= 125 kJ/m3 • 0.25% Al aged at 525C, 24h : Br= 1.02 T, BHc= 300 kA/m, (BH)max= 132 kJ/m3

  13. Thermomagnetic analysis: (Fe0.6Pt0.4)100-xMx • 0.5 at. % Nb: • Tc (as-quenched)  340C • Tc (aged)  400C • 0.25 at. % Al: • Tc (as-quenched)  380C • Tc (aged)  400C

  14. Temperature dependence: (Fe0.6Pt0.4)100-xMx • Nb:  = -0.04 %/K for Br,  = -0.12 %/K for BHc and = -0.12 %/K for (BH)max. • Al:  = -0.06 %/K for Br,  = -0.15 %/K for BHc and = -0.17 %/K for (BH)max.

  15. Mechanical properties: (Fe0.6Pt0.4)100-xNbx • Hardness • Tensile strength of 0.5% Nb

  16. Mechanical properties: comparison • Hardness: comparable to file band or of cutting tools. • Tensile strength:

  17. Microstructure: (Fe0.6Pt0.4)99.5Nb0.5 as-quenched: 1 nm 625°C,12h: 1-3 nm 625°C,24h: 3-8 nm 625°C,48h: 8-16 nm

  18. Microstructure: (Fe0.6Pt0.4)99.75Al0.25 100 nm as-quenched: 2 nm 525°C, 24h: 3-7 nm • High coercivity: correlated with the magnetic anisotropy, i.e. the atomic order in the fcc/fct and fct grain growth. • High remanence: originated by the exchange coupling of the soft fcc phase with the nano-sized hard fct phase.

  19. What’s neutron and why SANS ? • Properties mn = 1.674710-24 g n = 9.6628610-27 J/T q = 0, 1/2 = 624 s • Interaction with matter - Scattering from the atomic nucleus - Magnetic scattering • SANS: Small angle neutron scattering -   10 Å  102 Å - Domain and particle size q = kf - ki I(q,) = A(q) + B(q)sin2(/2) nuclear scattering magnetic scattering

  20. 2D SANS images: Fe59.7Pt39.8Nb0.5 As-quenched B = 0 (virgin) B = 1.8 T (in field) B removed (remanent) At 5m Aged

  21. Reduced SANS data: Fe59.7Pt39.8Nb0.5 As-quenched Aged • I(Q): difference in the virgin, field and remanent states • I(Q): difference for the as-quenched and aged samples

  22. SANS analysis • Model fitting: monodisperse or polydisperse model ? • Virgin state • Field state Particles Magnetic domains SANS dominated by randomly oriented magnetic domains: monodisperse model SANS dominated by particles with different magnetisation: polydisperse model

  23. Model fitting:Fe59.7Pt39.8Nb0.5 aged monodisperse Correlation length 105 nm 6 nm polydisperse • Mono.: domain size ~ 100 nm. • Poly.: fct particles R = 6 nm. (TEM: fct particles 3-8 nm)

  24. Conclusions • Best permanent magnets obtained with (Fe0.6Pt0.4)100-xMx: M = 0.5 at. % Nb and 0.25 at. % Al. Good thermal stabilisation. • Fe67Pt33: soft magnetic properties. • Good mechanical properties.  Suitable for biomedical applications, e.g. denture retention. • Coexistent nanostructure observed by TEM and SANS:  High coercivity: correlated with the atomic order in the fcc/fct structures and the fct grain growth.  High remanence: originated by the exchange coupling of the soft fcc phase with the nano-sized hard fct phase.

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