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General Sample preparation Crystal structure and microstructure

Rare-earth-iron nanocrystalline magnets E.Burzo 1) , C.Djega 2) 1) Faculty of Physics, Babes-Bolyai University, Cluj-Napoca 2) Universite Paris XII, France. General Sample preparation Crystal structure and microstructure Magnetic properties of R 2 Fe 17 compounds

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General Sample preparation Crystal structure and microstructure

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  1. Rare-earth-iron nanocrystalline magnetsE.Burzo1), C.Djega2)1)Faculty of Physics, Babes-Bolyai University, Cluj-Napoca2) Universite Paris XII, France General Sample preparation Crystal structure and microstructure Magnetic properties of R2Fe17 compounds Magnetic properties of Sm-Fe-Si-C alloys Mössbauer effects Technical applications

  2. Permanent magnets: • cobalt based: SmCo5, Sm2Co17 • - high Curie temperatures • - good energy product • SmCo5 (BH)max 200 kJ/m3 • Sm2Co17  240 kJ/m3 • - very expensive • iron based: Nd-Fe-B • - low Curie points, TC • - high energy product at RT • (BH)max  360 kJ/m3 • - high decrease of Energy Product with T • T<100 oC • - low cost

  3. Directions of researches: nanocrystalline systems • iron based with rare-earths • iron based without rare-earth • spring magnets RCo5/α-Fe

  4. Iron based magnets with rare-earth • R-Fe system • RFe2, RFe3, R6Fe23, R2Fe17 • no RFe5 phases are formed • R2Fe17 • Low Curie temperatures, Tc < 477 K for Gd2F17 • High magnetization MFe2.1-2.2 B/atom • Planar anisotropy • Rombohedral structure: space group • Sm:6c, Fe:6c, 9d, 18f, 18h • different local environments • Increase Tcvalues by: • replacement of iron involved in negative exchange inteactions • increase volume: interstitial atoms (C,N) • Uniaxial anisotropy

  5. 4f-5d-3d Exchange interactions Band structure calculations M5d(0)=a MFe

  6. Preparation. Crystal structure High energy ball milling and annealing Sm2Fe17-xSix; Sm2Fe17-xSixC for Ta>850 oC Metastable Sm1-s(Fe,Si)5+2s P6/mmm type structure s = 0.22 TbCu7 Ta= 650 oC-850 oC s = 0.33 Sm2Fe17 s = 0.36-0.38 SmFe9 (new) Carbonation: mixture of alloys and C14H10 powders 420 oC in vacuum Grain sizes: SmFe9-ySiy: 22-28 nm SmFe9-ySiyC: 18-22 nm Rietveld analysis C 3f sites (1/2,0,0); (0,1/2,0); (1/2,1/2,0) Sm at (0,0,0) is occupied by 0.64-0.62 atoms Si 3g sites

  7. R1-sM5+2s P6/mmm

  8. Electron microscopy: distribution of elements particle dimensions SmFe8.75Si0.25

  9. y=2 y=0 y=2 y=0 z=1 z=0 z=1 z=0

  10. Curie temperature: effect of Si • Tc increases by Si substitution in noncarbonated samples • Tc decreases in carbonated sample • Volume effects: Localized moment of iron moments 16.4 1:9 22.9 2:17 Molecular field approximation 25 1:9 30 2:17 Fe mainly localized magnetic behaviour

  11. Intrinsic magnetic properties • Magnetic measurements: H ≤ 9T T≥4.2 K • initial magnetization curves • - inflection typical for pinning effects coherent precipitates with matrix • impede the motion of domain walls • Band structure calculations: LMTO-LDA method • MSm=-0.66 B/atom • MFe(6c) > MFe(18h)  MFe(18f) > MFe(9d) • Mean magnetic moment of Fe in field of 9T • increases with Si content • Noncarbonated:1.50 B (y=0.25); 1.75 (y=1.0) • Carbonated:1.88B (y=0.25); 1.97 (y=1.0) • Asymmetric filling of Fe 3d band by Si3p electrons

  12. Mössbauer effect studies • SmFe9-xSixC P6/mmm type structure • Analysis of spectra • local environment • relationship between isomer shift, δ, and WSC volumes • x = 1.0 WSC volume in (Å3) • Sm1a (33.96); Fe2e (19.87); Fe3g (13.45); Fe6l (13.39) • Larger isomer shifts=larger WSC volumes • Statistical occupation of Si in the 3g site and random distribution in the 2e dumbbell atoms have been simulated by using an appropriate P6/mmm subgroup, P1 with a’=3a, b’=3a, c’=2c • 2e0 3g0 6l0 • Six sextets:2e ; 3g ; 6l • 2e1 3g1 6l1

  13. Mean 57Fe hyperfine fields decrease with Si content Hhf2e > Hef6l > Hhf3g correlate with number of NN Fe atoms Mean isomer shifts:δ2e>δ3g>δ6l δ2e, δ6l increase with Si substitution δ3g remains nearly constant preferred occupation of Si sites (3g)

  14. Technical parameters • Uniaxial anisotropy is induced in 1:9 phase • 2:17 phase • Coercive fields SmFe9-xSixC • X = 0.25 Hc=1.2 MA/m Ta=750 oC • x = 0.50 Hc = 1.04MA/m Ta=800 oC • The maximum in Hc values: • Too low Ta hinders the complete solid-state reaction for forming a perfect metastable phase responsible for magnetic hardening • Increasing Tc • - number of surface defects of hexagonal P6/mmm phase is reduced Hc • - the domain size increases Hc

  15. 2. Curie temperature increases Tc  700 K for SmFe8.75Si0.25C 3. Induction resonance Br  0.68 Bs High energy product is expected with a smaller temperature coefficient than Nd-Fe-B alloys

  16. Thank you very much for your attentions.

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