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

3 Introduction. What’s permanent magnet ? Magnetic materials Large M r, high H c High (BH) max e.g: SmCo 5 (1969), Sm 2 Co 17, Nd 2 Fe 14 B (1983) B = µ 0 (M+H) B M

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.) Aim: to find a magnet and a soft magnetic material with Good mechanical properties High corrosion resistance Magnet problems Very brittle Low corrosion resistance

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

7 Experimental Sample processing Preparation Fe x Pt 100-x and (Fe 0.6 Pt 0.4 ) 100-x M x 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):  C + quench in water  fct phase.

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

9 Results. Crystallographic structure As-quenched: presence of the fct phase. Ageing: fcc-fct transformation. Fe 60 Pt 40

10 Optimal hard-magnetic properties for Fe 60 Pt 40 aged at 625  C, 1h: B r = 0.97 T, B H c = 294 kA/m, (BH) max = 118 kJ/m 3 Permanent-magnet properties: Fe x Pt 100-x

11 Microstructure: Fe 60 Pt 40 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. 625°C, 1h: 2-5 nm as-quenched: 1-3 nm

12 Magnetic properties: (Fe 0.6 Pt 0.4 ) 100-x M x (M = Nb, Al) 0.5% Nb aged at 625  C, 24h: B r = 0.98 T, B H c = 302 kA/m, (BH) max = 125 kJ/m % Al aged at 525  C, 24h : B r = 1.02 T, B H c = 300 kA/m, (BH) max = 132 kJ/m 3

13 Thermomagnetic analysis: (Fe 0.6 Pt 0.4 ) 100-x M x 0.5 at. % Nb: T c (as-quenched)  340  C T c (aged)  400  C 0.25 at. % Al: T c (as-quenched)  380  C T c (aged)  400  C

14 Temperature dependence: (Fe 0.6 Pt 0.4 ) 100-x M x Nb:  = %/K for B r,  = %/K for B H c and  = %/K for (BH) max. Al:  = %/K for B r,  = %/K for B H c and  = %/K for (BH) max.

15 Mechanical properties: (Fe 0.6 Pt 0.4 ) 100-x Nb x Hardness Tensile strength of 0.5% Nb

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

17 Microstructure: (Fe 0.6 Pt 0.4 ) 99.5 Nb 0.5 as-quenched: 1 nm 625°C,12h: 1-3 nm 625°C,24h: 3-8 nm 625°C,48h: 8-16 nm

18 Microstructure: (Fe 0.6 Pt 0.4 ) Al 0.25 as-quenched: 2 nm 525°C, 24h: 3-7 nm 100 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 m n =  g  n =  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  10 2 Å - Domain and particle size q = k f - k i I(q,  ) = A(q) + B(q)sin 2 (  /2) nuclear scattering magnetic scattering

20 2D SANS images: Fe 59.7 Pt 39.8 Nb 0.5 B = 0 (virgin) B = 1.8 T (in field)B removed (remanent) As-quenched Aged At 5m

21 Reduced SANS data: Fe 59.7 Pt 39.8 Nb 0.5 I(Q): difference in the virgin, field and remanent states I(Q): difference for the as-quenched and aged samples As-quenched Aged

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

23 Model fitting: Fe 59.7 Pt 39.8 Nb 0.5 aged Mono.: domain size ~ 100 nm. Poly.: fct particles R = 6 nm. (TEM: fct particles 3-8 nm) monodisperse Correlation length 105 nm 6 nm polydisperse

24 Conclusions Best permanent magnets obtained with (Fe 0.6 Pt 0.4 ) 100-x M x : M = 0.5 at. % Nb and 0.25 at. % Al. Good thermal stabilisation. Fe 67 Pt 33 : 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.