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 1.General 2.Sample preparation 3.Crystal structure and microstructure 4.Magnetic properties of R 2 Fe 17 compounds 5.Magnetic properties of Sm-Fe-Si-C alloys 6.Mössbauer effects 7.Technical applications

Permanent magnets: cobalt based: SmCo 5, Sm 2 Co 17 - high Curie temperatures - good energy product SmCo 5 (BH) max 200 kJ/m 3 Sm 2 Co kJ/m 3 - very expensive iron based: Nd-Fe-B - low Curie points, T C - high energy product at RT (BH) max 360 kJ/m 3 - high decrease of Energy Product with T T<100 o C - low cost

Directions of researches: nanocrystalline systems iron based with rare-earths iron based without rare-earth spring magnets RCo 5 /α-Fe

Iron based magnets with rare-earth R-Fe system RFe 2, RFe 3, R 6 Fe 23, R 2 Fe 17 no RFe 5 phases are formed R 2 Fe 17 -Low Curie temperatures, T c < 477 K for Gd 2 F 17 -High magnetization M Fe B /atom -Planar anisotropy Rombohedral structure: space group Sm:6c, Fe:6c, 9d, 18f, 18h different local environments Increase T c values by: - replacement of iron involved in negative exchange inteactions - increase volume: interstitial atoms (C,N) Uniaxial anisotropy

M 5d (0)=a M Fe 4f-5d-3d Exchange interactions Band structure calculations

Preparation. Crystal structure High energy ball milling and annealing Sm 2 Fe 17-x Si x ; Sm 2 Fe 17-x Si x C for T a >850 o C Metastable Sm 1-s (Fe,Si) 5+2s P6/mmm type structure s = 0.22TbCu 7 T a = 650 o C-850 o C s = 0.33Sm 2 Fe 17 s = SmFe 9 (new) Carbonation: mixture of alloys and C 14 H 10 powders 420 o C in vacuum Grain sizes: SmFe 9-y Si y : nm SmFe 9-y Si y C: 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 atoms Si 3g sites

R 1-s M 5+2s P6/mmm

SmFe 8.75 Si 0.25 Electron microscopy: distribution of elements particle dimensions

y=2 y=0 y=2 y=0 z=0 z=1

Curie temperature: effect of Si T c increases by Si substitution in noncarbonated samples T c decreases in carbonated sample Volume effects: Localized moment of iron moments Molecular field approximation Fe mainly localized magnetic behaviour : : :9 30 2:17

Intrinsic magnetic properties Magnetic measurements: H 9T T4.2 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 M Sm =-0.66 B /atom M Fe (6c) > M Fe (18h) M Fe (18f) > M Fe (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

Mössbauer effect studies SmFe 9-x Si x C P6/mmm type structure Analysis of spectra local environment relationship between isomer shift, δ, and WSC volumes x = 1.0 WSC volume in (Å 3 ) 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 2e 0 3g 0 6l 0 Six sextets:2e ; 3g; 6l 2e 1 3g 1 6l 1

Mean 57 Fe hyperfine fields decrease with Si content H hf 2e > H ef 6l > H hf 3g correlate with number of NN Fe atoms Mean isomer shifts: δ 2e >δ 3g >δ 6l δ 2e, δ 6l increase with Si substitution δ 3g remains nearly constant preferred occupation of Si sites (3g)

Technical parameters 1.Uniaxial anisotropy is induced in1:9 phase 2:17 phase Coercive fields SmFe 9-x Si x C X = 0.25H c =1.2 MA/mT a =750 o C x = 0.50H c = 1.04MA/mT a =800 o C The maximum in H c values: Too low T a hinders the complete solid-state reaction for forming a perfect metastable phase responsible for magnetic hardening Increasing T c - number of surface defects of hexagonal P6/mmm phase is reduced H c - the domain size increases H c

2. Curie temperature increases T c 700 K for SmFe 8.75 Si 0.25 C 3. Induction resonance B r 0.68 B s High energy product is expected with a smaller temperature coefficient than Nd-Fe-B alloys

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