Rare Earth- -Transition Metal Compounds: Transition Metal - - PowerPoint PPT Presentation

Rare Earth Rare Earth-

• Transition Metal Compounds:

Transition Metal Compounds: Magnetism and Applications Magnetism and Applications

E.Burzo E.Burzo Faculty of Physics, Babes Faculty of Physics, Babes-

• Bolyai

Bolyai University, Cluj University, Cluj-

• Napoca

Napoca, Romania , Romania

1. 1.

Phase Diagrams Phase Diagrams

2. 2.

Magnetic Properties Magnetic Properties Exchange enhanced paramagnets Exchange enhanced paramagnets Induced transition moments, at Induced transition moments, at H Hcr

cr

Exchange interactions Exchange interactions

• 3. Technical applications

3.1 Permanent magnets SmCo5 Sm(Co,Fe,Zr,Cu)z Nd-Fe-B Nanocrystalline magnets 3.2 Magnetostrictive materials RFe2 3.3 Magnetocaloric materials 3.4 Hydrogen storage

• 1. Phase diagrams R-M, R = rare-earth, M = Mn,Fe,Co,Ni
• Formed by peritectic reaction
• At least one eutetic
• Number of compounds increase from M= Mn to M= Ni

rR/rM ≅ 1.4

Fig.1 Fig.2

Crystal structures MgCu2 C15 RM2 CaCu5 hex RM5

Fig.3 Fig.4

m 43 m 3

Hexagonal (Th2Ni17-type) R2M17 P63/mmc Rhombohedral(Th2Zn17-type)R2M17

m 3 R

Fig.5 Fig.6

• 2. Magnetic properties R-M compounds

R: 4f electrons, small spatial extent (well localized) La,Lu, (Y) nonmagnetic M:3d electrons

• nset

well established exchange enhanced paramagnetism magnetism collapse Magnetism of transition metals in rare-earth compounds 2.1 Magnetic propertis of exchange enhanced paramagnet 2.2 Induced transition metal moment, Hcr 2.3 Magnetic behaviour of Co,Ni,Fe at H > Hcr. Magnetic coupling MR MR MM MM R-light rare-earth R-heavy rare earth

4f-5d-3d type

2.1 Exchange enhanced paramagnet Co exchange enhanced paramagnets YCo2 LuCo2 Y(Co1-xNix)2

Fig.7 Fig.8

T ≤ ≤ ≤ ≤ 10 K χ χ χ χ = χ χ χ χo(1+aT2) T >T* Curie Weiss – type behaviour χ χ χ χ = C(T-θ)-1; θ<0 Y(Co1-xNix)2 Meff(Co) little dependent on composition

Fig.9 Fig.10

Y(Co1-xSix)2 high decrease of Meff(Co)

Fig.11 Fig.12

YCo3-xNixB2

Fig.13 Fig.14

Band structure calculations: LMTO-ASA Y(Co1-xNix)2

Fig.15

Y(Co1-xSix)2 Hybridization effect Co 3d-Si2p bands ↓ double peak is broadened by p-d hybridization Co3d band shifted to lower energy

Fig.16

1.5 1.5 1.30 1.30 2.80 2.80 2.50 2.50 YCo YCo1.6

1.6Ni

Ni0.4

0.4

1.1 1.1 1.2 1.2 1.80 1.80 1.90 1.90 YCo YCo2

2

a acalc

calc (K

(K-

• 2

2)

)· ·10 10-

• 6

6

a aexp

exp (K

(K-

• 2

2)

)· ·10 10-

• 6

6

χ χcalc

calc

(emu/fu) (emu/fu)· ·10 103

3

χ χexp

exp at 1.7 K

at 1.7 K (emu/fu) (emu/fu)· ·10 103

3

Compound Compound

χ χ χ χ = χ χ χ χo(1+aT2)

Self consistent theory of spin fluctuations

Wave number dependent susceptibility, χq, for a nearly ferromagnetic alloy has a large enhancement for small q values Frequency of longitudinal spin fluctuations ω* ∝ τ-lifetime of LSF Low temperature (thermal fluctuations-transversal slow)

) ( J 1

2 B q q q

µ µ χ − χ = χ

τ 1

                η η − η η π + χ = χ

2 2 E 2 2 2 p

T s ' 2 . 1 " 2 6 1 s

F

h T k *

B

< ω

Approximation for nonmagnetic state χ∝T2 χ(T) as T η” > 0 (necessary condition, not sufficient)

High temperature

Average mean amplitude of LSF is temperature dependent Sloc as T up to T* determined by charge neutrality condition The system behaves as having local moments for temperatures T > T* where the frequency of spin fluctuations ( )

loc

S

loc

S

h T k *

B

< ω

loc

S

∑χ

=

q q B 2 loc

T k 3 S

χ∝T2 χ-1

χ-1∝T

T T* θ θ<0 C-W type

Crossover between low T regime governed by spin fluctuations and high T classical regime

Gaussian distribution of spin fluctuations (Yamada)

Fig.19 Fig.20

0.0 0.2 0.4 0.6 0.8 1.0 1 2 3 4

1 2

YCo 1.6Ni0.4 YCo2 LuCo2 b10

• 2)

Computed Experimental <S

2> 1/2

x

Y(CoxNi1-x)2 Y(CoxNi1-x)2

Quenching of spin fluctuations

• external field: Beal-Monod, Brinkman-Engelsberg

(theor. 1968) Ikeda et al: specific heat (exp. 1984)

• internal field: Burzo-Lemaire

(exp.1992) If the magnetic field is sufficiently large so that the Zeeman splitting energy of opposite spin states is comparable to, or larger than the characteristic spin fluctuation energy ⇒ paramagnons no longer have sufficient energy to flip spins and the inelastic spin flip scattering is quenched. Hquench ∝ Tsf Specific heat (10T) external field (Ikeda et al) γ reduced by 4 % YCo2 10 % LuCo2

Fig.20 Fig.21

Magnetic measurements RCo2(R magnetic) Meff Tc b = 1.7 · 10-2 µBT For ∆Hexch = 10 T ⇒ ∆Meff decrease by 6 %

1 exch eff

bH a M

−

+ =

LaNi5-xCux; LaNi5-xAlx Cu CaCu5 – type x ≤ 2 Al CaCu5 – type x < 2 HoNi2.6Ga2.4 – type x ≥ 2

Fig.22 Fig.23 LaNi5-xAlx

1 2 3 1 2

0.0 0.5 1.0 1.5 0.0 0.5 1.0 1.5

a10

• 2)

x

LaNi5-xAlx LaNi5-xCux comp. exp. comp. exp.

LaNi5-xAlx

χ χ χ χ10

3(emu/f.u)

x

Fig.24 Fig.25

Fig.26 Fig.27

LaNi5-xAlx

RT

2.2 Induced transition metal moment (GdxY1-x)Co2 Lemaire, 1966 Critical field for appearance a magnetic moments ⇓ critical value of exchange interactions Gd(CoxNi1-x)2 1974

Fig.28 Fig.29

Band structures GdCo2 MCo =1.20 µB GdNi2 MNi = 0.12 µB GdCoNi MCo = 1.12 µB MNi = 0.17 µB

n ≥ ≥ ≥ ≥ 3 NN site

m 3

Fig.30

Field dependence of transition metal moment ∆MCo = VCo∆Hexch VCo = (3·106)-1µB/Oe High field measurements, Amsterdam: confirmed VCo value ∆MFe = VFe∆Hexch VFe =(18·106)-1 µB/Oe Confirmed by high field measurements (Amsterdam) – cobalt compounds

Fig.31

GdxLa1-xNi5 0 K MNi ≅ ≅ ≅ ≅ 0.20 µ µ µ µB GdNi5 Ni ≅ ≅ ≅ ≅ 0 LaNi5

Fig.32 Fig.33

Hc ≅ ≅ ≅ ≅ 40 T

Fig.34 Fig.35

Hc ≅ ≅ ≅ ≅ 30 T

2.3 Exchange interactions 4f-5d-3d type (Campbell) RM2 compounds, R-heavy rare-earth

Fig.36 Fig.37 Fig.38 RM2

M3d ∝G G = (gJ-1)2J(J+1) M5d = M5d(0) + αG αG intra-atomic 4f-5d exchange M5d(0) short range exchange interactions

Fig.39

1 2 3 4 0.0 0.1 0.2 0.3 0.4 0.5

GdNi2 GdCo2 GdFe2 M5d(µ µ µ µB) M3d(µ µ µ µΒ

YFe2-xVx GdCo2-xNix GdCo2-xCux GdCo2-xSix GdFe2

M5d(0) (µ

µ µ µB)

M3d (µ

µ µ µB/f.u.)

RM5 M=Co, Ni M5d=M5d(0)+α’G α’=1.4·10-2µB M5d(0) =0.32 Co =0.08 Ni M3d=M3d(0)+β’G β’=1·10-2 µB Co =1.6·10-2µB Ni M5d(0)/M3d(0)=0.045 RCo4B, RM5

∑ ∑ ∑ ∑

∝ ∝ ∆ ⇓ + =

= − = − i i i exch d 5 2 1 i d 5 d 5 d 5 d 5 n , d 3 n 2 1 i d 5 d 5 d 3

c i i i

M n H ) ( M S S J S ) ( S J H

Fig.40

GdLa2Ni15 J3g-3g(La) (14-15) 37 K J3g-3g(Gd) (8-9) 40 K (14-16) 18 K (8-12) 23 K (15-16) 11 K (9-12) 17 K J2c-3g(La) (4-14) 23 K J2c-3g(Gd) (2-8) 24 K (4-16) 19 K (2-12) 21 K (4-15) 7 K (2-9) 9 K

GdNi5 J3g-3g(1) = 57 K J3g-3g(2) = 28 K J2c-3g(1) = 26 K

Fig.41

Rhodes-Wohlfart curve

Fig.42 Fig.43 Fig.44

Both longitudinal and transverse components of the local spin fluctuations. Relative contributions given by r = Sp/So between the number of spins determined from effective moments and that obtained from saturation data µs = gSo

) 1 S ( S g

p p eff

+ = µ

Mechanisms:

• increase of saturation Co and

Ni moments as exchange fields increase

• gradual quenching of spin

fluctuations by internal field, diminishing the effective moments

• 3. Technical Applications

3.1 Permanent Magnets:

• Cobalt based magnets

a) RCo5, R2Co17- based magnets: light rare – earths

RCo5 TC ≅ 1000 K R2Co17 TC ≅ 1150 K R = Sm high uniaxial anisotropy Expensive: natural abundance of Sm, Co (BH)max ≅ 200 kJ/m3 ≅ 240 kJ/m3

SmCo5 Sm2(Co,M)17 M=Fe,Zr,Cu

Fig.46

Nucleation process Pinning process Fig.47

• iron based: Nd-Fe-B
• low Curie points, TC≅580 K
• high energy product at RT

(BH)max ≅ 420 kJ/m3

• high decrease of energy product with T

T<100 oC

• low cost

Fig.48

Nd-Fe-B sintered magnets

Fig.49 Fig.50

Nanocrystalline magnets

Isotropic microcrystalline Nd-Fe-B ribbons Br≅ Bs/2 Alloys with low Nd content grain refinement into nanocyrstalline regime increase Br Br> Bs/2 less expensive permanent magnets

• High boron content

Nd4.5Fe77B18.5:Ex Nd2Fe14B + Fe3B+α-Fe

• Low boron content

Nd6Fe88B6:Ex Nd2Fe14B+α-Fe Mean grain sizes Nd2Fe14B (< 30 nm) :dn Soft magnetic phase (< 15 nm) :dn Exchange coupling reduces the resistance to reversal magnetization but not lead to:

• collapse of HC
• deteriorated (BH) loop

Condition: dimension of soft magnetic phase < exchange distance for the phase

Fig.51 Fig.52

3.2 Magnetostrictive materials

Rare-Earths (R)

• interesting properties for technical applications
• low Curie temperatures TC ≤ 300 K

= 8.8·10-3 Tb 4.2 K 8.8· 10-3 Dy 2.5·10-3Ho

• cannot be used for devices working at RT

Rare-Earth-Transition Metal Compounds: RMx M= Fe,Co On the support of exchange interactions the R useful properties translated at higher T 2 , ε

λ

K

R-Fe compounds

Fig.53 Fig.54 Fig.55

TbFe2 high anisotropy: high magnetostriction Minimizing magnetic anisotropy but maintaining a large magnetostriction

• opposite sign of anisotropy (anisotropy compensating

system) Tb0.27Dy0.73Fe2

3.3. Magnetocaloric effects The isothermal magnetic entropy changes, ∆SM (T,∆H) Discrete fields and temperature intervals From magnetic measurements ∆SM were evaluated

∫

∆

      ∂ ∂ = ∆ ∆

H H M

dH T M ) H , T ( S

∑

∆ − = ∆

+ −

+

i H i 1 i T T 1 M

H ) M M ( S

i 1 i

Fig.56

2 4 6 8 10 12 14 16 20 30 40 50 60

GdNi5

B=1.0 T B=2.0 T B=3.0 T B=4.0 T B=5.0 T B=6.0 T B=7.0 T

• ∆

∆ ∆ ∆S (J/(kg K)

T (K)

Fig.58 Fig.57

Fig.60

1 2 3 220 260 300 340

Nd17Fe74.75Si0.25B8

B=0.5T B=1.0T B=1.5T B=2.0T

• ∆

∆ ∆ ∆S (J/(kg K)

T(oC)

Fig.61

Fig.62 Fig.63

3.4 Hydrogen in Metals

LaNi5 shows an absorption plateau pressure of few bars at RT. There is a complete revesibility between formation and decomposition with a hysteresis between the corresponding equilibrium pressure. An α–phase s.s. precedes β- LaNi5H6 hydride. The system requires an activation stage which involves decrepitation into small particles.

2b 3m 2b 3m 6c m

Thank you very much for your attentions.