Rare-earth-iron nanocrystalline magnets E.Burzo 1), C.Djega 2) 1) Faculty of Physics, Babes-Bolyai...
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Transcript of Rare-earth-iron nanocrystalline magnets E.Burzo 1), C.Djega 2) 1) Faculty of Physics, Babes-Bolyai...
Rare-earth-iron nanocrystalline magnets E.Burzo1), C.Djega2)
1)Faculty of Physics, Babes-Bolyai University,
Cluj-Napoca2) Universite Paris XII, France
1. General
2. Sample preparation
3. Crystal structure and microstructure
4. Magnetic properties of R2Fe17 compounds
5. Magnetic properties of Sm-Fe-Si-C alloys
6. Mössbauer effects
7. Technical applications
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
Directions of researches: nanocrystalline systems
• iron based with rare-earths
• iron based without rare-earth
• spring magnets RCo5/α-Fe
Iron based magnets with rare-earthR-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 anisotropyRombohedral structure: space groupSm:6c, Fe:6c, 9d, 18f, 18hdifferent local environments
Increase Tcvalues by:- replacement of iron involved in negative exchange inteactions- increase volume: interstitial atoms (C,N)Uniaxial anisotropy
m3R
i j
dj5d5d5d5di3d5d5d3 S)0(SJ2)0(S)0(SJ2H
M5d(0)=a MFe
4f-5d-3d Exchange interactions
Band structure calculations
Preparation. Crystal structureHigh energy ball milling and annealingSm2Fe17-xSix; Sm2Fe17-xSixC for Ta>850 oCMetastable Sm1-s(Fe,Si)5+2s P6/mmm type structure
s = 0.22 TbCu7 Ta= 650 oC-850 oCs = 0.33 Sm2Fe17s = 0.36-0.38 SmFe9 (new)
Carbonation: mixture of alloys and C14H10 powders 420 oC in vacuumGrain sizes: SmFe9-ySiy: 22-28 nmSmFe9-ySiyC: 18-22 nmRietveld analysisC 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 atomsSi 3g sites
m3R
R1-sM5+2s
P6/mmm
SmFe8.75Si0.25
Electron microscopy: distribution of elements particle dimensions
y=2
y=0
y=2
y=0
z=0
z=0
z=1
z=1
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
cT
bI2effJ)1S(S
I2goNBk
8
5
vlndeffJlnd
23
5
Molecular field approximation
Fe mainly localized magnetic behaviour
vlndTlnd
dpdT
T1 CC
C
vlnd
Jlnd ef
vlnd
Jlnd ef
16.4 1:9
22.9 2:17
25 1:9
30 2:17
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: 1.88B (y=0.25); 1.97 (y=1.0)
Asymmetric filling of Fe 3d band by Si3p electrons
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
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)
Technical parameters
1. Uniaxial anisotropy is induced in 1:9 phase
2:17 phaseCoercive 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
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
Thank you very much for your attentions.