What is the susceptibility?

Transcription

1 What is the susceptibility? Answer which one? M Initial susceptibility Mean susceptibility M st M 0 0 m High field susceptibility i dm = dh H =0 H st H

2 M M st M 0 0 m i H st H H

3 What is the susceptibility? Answer which one? Initial susceptibility = M H Susceptibility often express / mass or / volume or / molecule Mean susceptibility High field susceptibility Tensor of susceptibility r M M M2 = M Differential susceptibility d ij r = H 13 H1 23 H2 33 H3 Mij = Hij H M dc or ac susceptibility = ' i '' material = f ( ω, T, H )

4 What is the physical meaning of real and imaginary part of susceptibility? H ( t) = H + Hac cosωt = ' i '' M ( t) = M + Mac cos( ωt φ) M ( t) = M + M ac (cosωt cosφ sinωt sinφ ) = ' i '' Phase shift H( t) = H + Hac exp( iωt) Mac φ ' cos = and Hac '' Mac sinφ = Hac M ( t) = M + ac Mac exp( iωt) power absorbed per unit volume 1 P = ω '' ac 2 H 2 ac

5 What is the physical meaning of real and imaginary part of susceptibility? Real part Mac φ ' cos = Hac Reflects sensitivity of material to H applied Eventually in the different directions (tensor rank 2) Tm 2 Fe 17 D 3 '' Imaginary part Mac sinφ = H ac Reflects : phase shift of H applied dissipation energy lost absorption of energy Tm 2 Fe 17 F. Grandjean, O. Isnard, G.J. Long Non zero values of related to dispersion centered at a frequency of ω τ =1/τ τ being a relaxation time ie: domain wall motions, relaxation irreversibility, magnetic phase transition, Tc, spin reorientation Remember material = f ( ω, T, H ) Ac susceptibility is a very sensitive technique!! From Hz to 10 KHz

6 After Lemoine thesis Univ. Nancy Signal at the ordering of the rare earth

7 What is the susceptibility? Differential susceptibility d ij = M ij H ij In ferromagnetic materials the relation between M and H is not linear Differential susceptibility also referred to M M st M 0 0 m dm = dh H =0 i H st H

8 In which measurement can you see difference between superparamagnetism and spin glass behaviour? What is spin glass? What is superparamagnetism? Similar behaviour from magnetization measurements M Field cooling Zero Field cooling T critical = T blocking or T freezing T superparamagnetism

9 Superparamagnetism Ferromagnetic particles => J>0 Small particles => single domain (cf lecture Olivier) Anisotropy M preferentially aligned along one direction energy KV The particle behaves like a macrospin Competition between kt and KV => limit on particle size If V decreases then anisotropy energy also and Magnetization of the particle no longer force to be along the easy direction because of kt Fluctuation of the magnetization direction intrinsically ferromagnetic particles but appearing like para (superpara) k T blocking ~ K V Coherence length larger than the particle size at T> T blocking Superpara Fe particles 20 nm

10 Spin glasses ie: Fe diluted in Au or Cu 1-x Mn x and x 0 The term "glass" comes from an analogy between the magnetic disorder in a spin glass and the positional disorder of a conventional, chemical glass FC ZFC Diluted system Mixed interaction competing J>0 and J<0 (RKKY int.) Random T spin freezing Transition from not ordered to low T metastable state (no long range order) Opposite limit percolation between clusters => ordering

11 In which measurement can you see difference between superparamagnetism and spin glass behaviour? superparamagnetism and spin glass exhibit similar behavior from magnetization measurements ac susceptibility in particular frequency dependence For more details J.A. Mydosh, Spin Glasses : An experimental Introduction, Taylor & Francis : London, B. Idzikowski et al 1999 Europhys. Lett Specific heat (magnetic one) independent of T below T freezing Local probe of the internal magnetic field Mössbauer spectroscopy s Muon spectroscopy s Neutron spectroscopies s NMR Access to relaxation time ie: Relaxation time of the moment in the superparamagnet

12 Signature of Spin Glass from M(T) and Xac '' ' Follow the frequency dependence = ' + i '' For more details J.A. Mydosh, Spin Glasses : An experimental Introduction, Taylor & Francis : London, 1993.

13 Usuallly of interest for Crystalline materials Large anisotropy What is superparamagnetism? CoFe2O4 Fe2O3 Fe3O4 FeCo FePt D sp D sd CoPt Ni Co Nanoparticle diameter (nm)

14 alloy system Minimal Stable Grain Diameter material K: anisotropy in 1e7 erg/cc Ms: saturation magnetizati on in emu/cc D P Hk: anisotropy field in koe k = 60 K Dp: minimal stable diameter in nm B T CoCrPtX Co-alloys Co hex Co3Pt A few nm L10 phases FePd FePt CoPt MnAl RE-TM Nd2Fe14B Co5Sm

15 Ideally In practise YCo 4 Si 20 M 2 (u.a.) 15 µ 0 H=1T 10 T c =320(5)K T(K)

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17 Origin of this behavior? Short range ordering = correlation between neighbouring spins even just above Tc Existence of a typical correlation length which is infinite in the ordered state Tends to zero at high temperature Typical scale where there is correlation between the magnetic moments No longer independent => not strictly paramagnetic Reminiscent of magnetic order for a certain T range How to observe it experimentally Neutron scattering since short range order => diffuse neutron scattering Small angle scattering depending upon coherence length

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