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.