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

Rare Earth- -Transition Metal Compounds: Transition Metal Compounds: Rare Earth Magnetism and Applications Magnetism and Applications E.Burzo E.Burzo Faculty of Physics, Babes- -Bolyai Bolyai University, Cluj University, Cluj- - Faculty of Physics, Babes Napoca, Romania , Romania Napoca Phase Diagrams Phase Diagrams 1. 1. Magnetic Properties Magnetic Properties 2. 2. Exchange enhanced paramagnets Exchange enhanced paramagnets Induced transition moments, at H H cr Induced transition moments, at cr Exchange interactions Exchange interactions

3. Technical applications 3.1 Permanent magnets SmCo 5 Sm(Co,Fe,Zr,Cu) z Nd-Fe-B Nanocrystalline magnets 3.2 Magnetostrictive materials RFe 2 3.3 Magnetocaloric materials 3.4 Hydrogen storage

1. Phase diagrams R-M, R = rare-earth, M = Mn,Fe,Co,Ni r R /r M ≅ 1.4 Fig.1 Fig.2 •Formed by peritectic reaction •At least one eutetic •Number of compounds increase from M= Mn to M= Ni

Crystal structures MgCu 2 C15 RM 2 CaCu 5 hex RM 5 3 m 43 m Fig.3 Fig.4

Fig.5 Fig.6 Hexagonal (Th 2 Ni 17 -type) R 2 M 17 P6 3 /mmc R 3 m Rhombohedral(Th 2 Zn 17 -type)R 2 M 17

2. Magnetic properties R-M compounds R: 4f electrons, small spatial extent (well localized) La,Lu, (Y) nonmagnetic M:3d electrons onset 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, H cr 2.3 Magnetic behaviour of Co,Ni,Fe at H > H cr . Magnetic coupling M R M R 4f-5d-3d type M M M M R-light rare-earth R-heavy rare earth

2.1 Exchange enhanced paramagnet Co exchange enhanced paramagnets YCo 2 LuCo 2 Y(Co 1-x Ni x ) 2 Fig.7 Fig.8

T ≤ ≤ ≤ 10 K ≤ χ χ χ χ = χ χ o (1+aT 2 ) χ χ T >T* Curie Weiss – type behaviour χ χ χ χ = C(T-θ) -1 ; θ<0 Y(Co 1-x Ni x ) 2 M eff (Co) little dependent on composition Fig.9 Fig.10

Y(Co 1-x Si x ) 2 high decrease of M eff (Co) Fig.12 Fig.11

YCo 3-x Ni x B 2 Fig.13 Fig.14

Band structure calculations: LMTO-ASA Y(Co 1-x Ni x ) 2 Fig.15

Y(Co 1-x Si x ) 2 Hybridization effect Co 3d-Si2p bands ↓ double peak is broadened by p-d hybridization Co3d band shifted to lower energy Fig.16

χ exp χ calc χ χ -2 2 ) -6 6 -2 2 ) -6 6 (K - 10 - (K - 10 - Compound a exp )· ·10 a calc )· ·10 Compound at 1.7 K a exp (K a calc (K exp at 1.7 K calc 10 3 3 10 3 3 (emu/fu)· ·10 (emu/fu)· ·10 (emu/fu) (emu/fu) YCo 2 1.90 1.80 1.2 1.1 YCo 1.90 1.80 1.2 1.1 2 YCo 1.6 Ni 0.4 2.50 2.80 1.30 1.5 YCo 1.6 Ni 2.50 2.80 1.30 1.5 0.4 χ χ χ χ = χ χ χ χ o (1+aT 2 )

Self consistent theory of spin fluctuations Wave number dependent susceptibility, χ q , for a nearly ferromagnetic alloy has a large enhancement for small q values χ χ = q − χ µ µ q 2 1 J ( ) q 0 B 1 Frequency of longitudinal spin fluctuations ω* ∝ τ τ -lifetime of LSF Low temperature ω < k T B * (thermal fluctuations-transversal slow) h π η η   2  2  χ = χ + − " '   2 2 s  1 2 1 . 2 s T  η η   p 2 6       E F

Approximation for nonmagnetic state χ∝ T 2 χ (T) as T η ” > 0 (necessary condition, not sufficient)

High temperature Average mean amplitude of LSF is temperature dependent = ∑ χ 2 S 3 k T loc B q q ( ) S S loc as T up to T* loc S loc determined by charge neutrality condition The system behaves as having local moments for temperatures T > T* where the frequency of spin fluctuations ω < k T B * h S loc

χ -1 C-W type χ∝ T 2 χ -1 ∝ T θ<0 θ T* T Crossover between low T regime governed by spin fluctuations and high T classical regime

Gaussian distribution of spin fluctuations (Yamada)

Fig.19 Y(Co x Ni 1-x ) 2 Y(Co x Ni 1-x ) 2 4 2 -2 ) 5 (K 1 b10 3 0 LuCo 2 YCo 2 YCo 1.6 Ni 0.4 1/2 2 2 > <S Fig.20 Experimental Computed 1 0 0.0 0.2 0.4 0.6 0.8 1.0 x

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. H quench ∝ T sf Specific heat (10T) external field (Ikeda et al) γ reduced by 4 % YCo 2 10 % LuCo 2

Magnetic measurements RCo 2 (R magnetic) M eff T c b = 1.7 · 10 -2 µ B T = + − 1 M a bH eff exch For ∆ H exch = 10 T ⇒ ∆ M eff decrease by 6 % Fig.21 Fig.20

LaNi 5-x Cu x ; LaNi 5-x Al x x ≤ 2 Cu CaCu 5 – type Al CaCu 5 – type x < 2 x ≥ 2 HoNi 2.6 Ga 2.4 – type LaNi 5-x Al x Fig.23 Fig.22

Fig.24 2 1.5 LaNi 5-x Al x 1.0 -2 ) 3 (K a10 0.5 3 (emu/f.u) 0.0 0.0 0.5 1.0 1.5 x 1 exp. LaNi 5-x Cu x comp. exp. Fig.25 LaNi 5-x Al x χ 10 comp. χ χ χ 0 0 1 2 3 x

RT LaNi 5-x Al x Fig.27 Fig.26

2.2 Induced transition metal moment (Gd x Y 1-x )Co 2 Lemaire, 1966 Critical field for appearance a magnetic moments ⇓ critical value of exchange interactions Gd(Co x Ni 1-x ) 2 1974 n ≥ ≥ 3 NN ≥ ≥ site 3 m Fig.29 Fig.28 Band structures M Co =1.20 µ B GdNi 2 M Ni = 0.12 µ B GdCo 2 M Co = 1.12 µ B M Ni = 0.17 µ B GdCoNi

Fig.30

Field dependence of transition metal moment ∆ M Co = V Co ∆ H exch V Co = (3·10 6 ) -1 µ B /Oe High field measurements, Amsterdam: confirmed V Co value V Fe =(18·10 6 ) -1 µ B /Oe ∆ M Fe = V Fe ∆ H exch Confirmed by high field measurements (Amsterdam) – cobalt compounds Fig.31

Gd x La 1-x Ni 5 0 K M Ni ≅ ≅ ≅ 0.20 µ ≅ µ B µ µ GdNi 5 Ni ≅ ≅ 0 ≅ ≅ LaNi 5 Fig.33 Fig.32

Fig.34 H c ≅ ≅ ≅ ≅ 40 T H c ≅ ≅ ≅ ≅ 30 T Fig.35

2.3 Exchange interactions 4f-5d-3d type (Campbell) RM 2 compounds, R-heavy rare-earth Fig.36 RM 2 Fig.37 Fig.38

M 3d ∝ G G = (g J -1) 2 J(J+1) M 5d = M 5d (0) + αG αG intra-atomic 4f-5d exchange M 5d (0) short range exchange interactions GdFe 2 0.8 GdFe 2 GdCo 2 0.5 µ B ) 0.6 µ µ M 5d ( µ 0.4 GdNi 2 0.2 0.4 0.0 0 1 2 3 4 M 3d ( µ µ µ Β µ Β /f.u.) µ B ) Β Β M 5d (0) ( µ µ µ 0.3 Fig.39 0.2 GdCo 2-x Si x GdCo 2-x Cu x 0.1 GdCo 2-x Ni x YFe 2-x V x 0.0 0 1 2 3 4 M 3d ( µ µ B /f.u.) µ µ

RM 5 M=Co, Ni α ’=1.4·10 -2 µ B M 5d =M 5d (0)+ α ’G M 5d (0) =0.32 Co =0.08 Ni β’=1·10 -2 µ B Co M 3d =M 3d (0)+β’G =1.6·10 -2 µ B Ni M 5d (0)/M 3d (0)=0.045 RCo 4 B, RM 5 H = + 2 2 J S ( 0 ) S J S S − − ∑ ∑ ∑ = = 3 d 5 d 5 d 5 d 5 d 5 d 5 d i i i 1 n i 1 3 d , n i ⇓ ∆ ∝ ∝ M ( 0 ) H n M ∑ 5 d exch i i c i Fig.40

GdLa 2 Ni 15 J 3g-3g (La) (14-15) 37 K J 3g-3g (Gd) (8-9) 40 K (14-16) 18 K (8-12) 23 K Fig.41 (15-16) 11 K (9-12) 17 K J 2c-3g (La) (4-14) 23 K J 2c-3g (Gd) (2-8) 24 K GdNi 5 (4-16) 19 K (2-12) 21 K J 3g-3g (1) = 57 K (4-15) 7 K (2-9) 9 K J 3g-3g (2) = 28 K J 2c-3g (1) = 26 K

Rhodes-Wohlfart curve

Fig.42 Fig.44 Fig.43

Both longitudinal and transverse components of the local spin fluctuations. Relative contributions given by r = S p /S o between the number of spins determined µ = + from effective moments and that obtained from saturation data g S ( S 1 ) µ s = gS o eff p p 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) RCo 5 , R 2 Co 17 - based magnets: light rare – earths T C ≅ 1000 K RCo 5 R 2 Co 17 T C ≅ 1150 K R = Sm high uniaxial anisotropy Expensive: natural abundance of Sm, Co (BH)max ≅ 200 kJ/m 3 ≅ 240 kJ/m 3

SmCo 5 Fig.46 Sm 2 (Co,M) 17 M=Fe,Zr,Cu

Pinning process Nucleation process Fig.47

• iron based: Nd-Fe-B - low Curie points, T C ≅ 580 K - high energy product at RT (BH) max ≅ 420 kJ/m 3 - high decrease of energy product with T T<100 o C - low cost Fig.48

Nd-Fe-B sintered magnets Fig.49 Fig.50

Nanocrystalline magnets Isotropic microcrystalline Nd-Fe-B ribbons B r ≅ B s /2 Alloys with low Nd content grain refinement into nanocyrstalline regime increase B r B r > B s /2 less expensive permanent magnets •High boron content Nd 4.5 Fe 77 B 18.5 :Ex Nd 2 Fe 14 B + Fe 3 B+α-Fe •Low boron content Nd 6 Fe 88 B 6 :Ex Nd 2 Fe 14 B+α-Fe Mean grain sizes Nd 2 Fe 14 B (< 30 nm) :d n Soft magnetic phase (< 15 nm) :d n Exchange coupling reduces the resistance to reversal magnetization but not lead to: - collapse of H C - deteriorated (BH) loop Condition: dimension of soft magnetic phase < exchange distance for the phase

Fig.52 Fig.51