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- 1. Athanasios T. Giannitsis Dynamic Properties of Nanoparticle Magnetic Fluids Measured by Impedance Spectroscopy
- 2. What is a magnetic fluid? Colloidal suspension of magnetic nanoparticles in carrier liquid 1. Origin of magnetism is due to the orbital and spin motion of the electrons. 2. A ferroparticle consists of single domain ferromagnetic or ferrimagnetic material having anisotropy. 3. Bulk magnetization, M s , of a magnetite ferroparticle is 0.4 T . 4. Average radius of magnetic nanoparticles spans between 2-10 nm. Surfactant is 2nm. 5. The carrier liquid can be either water, or oil such as kerosene, isoparaffin, diester, or toluene.
- 3. Applications Engineering - stepper motors - electromagnets - transformers - loudspeakers - ink jet printing - memories - lubrication - sealing <ul><li>Science </li></ul><ul><li>- theoretical mechanics </li></ul><ul><li>- magnetism </li></ul><ul><li>- material science </li></ul><ul><li>- statistical physics </li></ul><ul><li>spectroscopy </li></ul><ul><li>chemical physics </li></ul><ul><li>Medicine </li></ul><ul><li>- bioelectronics </li></ul><ul><li>separation </li></ul><ul><li>targeting cells </li></ul><ul><li>Space technology: </li></ul><ul><li>- controllable flows </li></ul><ul><li>thermal agent </li></ul><ul><li>microgravity </li></ul>
- 4. Impedance Spectroscopy Methods Autobalancing bridge Audio frequency measurements 0.1 Hz - 100 MHz Radio frequency vector analyzer Radio frequency measurements 1 MHz -1 GHz Network analyzer Microwave frequency measurements 50 MHz – 20 GHz
- 5. Measurement setup The dynamic magnetic susceptibility χ (ω) = χ΄(ω) - i χ΄΄(ω) can be measured by impedance spectroscopy, using alternating magnetic field, h (t) , in combination with a static magnetic field, H s . Autobalancing bridge method for low frequency measurements Δ L( ω )/L( ω ) , Δ R( ω )/ ω L, L= inductance of coil
- 6. Coaxial line method for radio and microwave frequency measurements
- 7. Magnetostatic properties Magnetic fluids demonstrate: 1. Paramagnetic behavior 2. No hysteresis Langevin law of magnetization: linear region nonlinear region Curie law of static susceptibility:
- 8. Magnetodynamic properties <ul><li>Magnetic relaxation is due to the rotation of the magnetic moment vector </li></ul><ul><li>- Brownian relaxation is due to Brownian motion. </li></ul><ul><li>- Néel relaxation is due to the rotation of in the magnetic core, due to </li></ul><ul><li> thermal activity. </li></ul><ul><li>Resonance is due to precession of about the axis of the effective field </li></ul><ul><li> + + (internal anisotropy field + external fields) and occurs when </li></ul><ul><li>the frequency of the precession of become equal to the frequency of a </li></ul><ul><li>probing alternating field which is used to excite the system. </li></ul>
- 9. Dynamic magnetic susceptibility and L( ξ ) is the Langevin function <ul><li>Dynamic magnetic susceptibility χ( ω) =χ΄ (ω) -i χ΄΄ (ω) is frequency and field dependent . </li></ul><ul><li>The magnetic susceptibility χ( ω) =χ΄ (ω) -i χ΄΄ (ω) can be decomposed in </li></ul><ul><li>The components follow Debye and Landau-Lifshitz dispersions . </li></ul><ul><li>Debye: , , , </li></ul><ul><li>Landau-Lifshitz: , </li></ul><ul><li>The field, , dependence of follows Langevin profile: </li></ul><ul><li>The relaxation time is field, ξ , dependent: </li></ul>
- 10. Theoretical spectrum of magnetic susceptibility Susceptibility according to Debye and Landau-Lifshitz dispersions. Debye term : (relaxation term) Landau-Lifshitz: (resonance term) relaxation resonance
- 11. Magnetic relaxation Magnetite Fe 3 O 4 magnetic fluid Estimation of the particle size distribution from the fit Estimation of the average particle size from the relaxation time
- 12. Field dependent susceptibility Field dependence of the relaxation time
- 13. Cole-Cole parameter α accounts for size distribution Cole-Cole
- 14. Nonlinear increment of susceptibility Susceptibility difference between polarized and non polarized. Anomalous rotational diffusion
- 15. Spectrum of χ΄ ( ω) and χ΄΄(ω) for frequency range 1 MHz-6 GHz oil-based magnetic fluid consisting of magnetite Fe 3 O 4 nano particles Magnetic resonance relaxation
- 16. The slops define the gyromagnetic ratio γ Polarizing field 0-100 kA/m Field dependence
- 17. Characterization of various magnetic fluids Ferrofluid type gyromagnetic ratio γ (m/A sec) anisotropy field (A/m) Anisotropy constant (J/m 3 ) Magnetite in water or isoparaffin 220,000-235,000 40,000 7,500-11,000 Cobalt ferrite in isoparaffin ** ** 5,000 Cobalt in diester 200,000 126,000 63,000 Manganese ferrite in isoparaffin 230,000-255,000 20,000-40,000 4,000-6,000 Nickel-zinc ferrite in isoparaffin 250,000 5,000 3,000-3,500 ** Have not been measured because its resonant frequency is outside the experimental frequency range.
- 18. susceptibility fits as function of frequency The polydispersion of the radius and anisotropy constant of the nanoparticles is accounted using distributions such as Normal (Gaussian), log-normal and Nakagami. unpolarized polarized (60 kA/m) Fits obtained using Landau-Lifshitz model
- 19. χ΄ versus χ΄΄ for various fields χ΄ versus χ΄΄ for various frequencies χ΄ versus field and frequency χ΄΄ versus field and frequency Cole-Cole
- 20. susceptibility fits as function of field Fits obtained using Landau-Lifshitz model Cole-Cole diagrams ease the fitting Each curve corresponds to a specific frequency and each point of the curve corresponds to a particular polarizing field. The parameters gained from the fit are γ , K and τ 0 .
- 21. Time domain Decay of the magnetization of a magnetic fluid obtained from the susceptibility spectrum by Inverse Fourier Transform. Time interval is of the order of nanoseconds
- 22. Other resonance models Landau-Lifshitz model versus Coffey’s model: Coffey’s model Coffey’s equation covers the whole range of σ values. Debye equation and Landau-Lifshitz are the two limits for very low and very high σ respectively. , ,
- 23. Application Estimation of losses in magnetic fluids Loss factor:
- 24. Microwave absorption in magnetic fluids - Energy loss Also possible to estimate energy losses in Joule/m 3 by applying the definitions: (frequency dependent) (time average)
- 25. Application Signal to noise ratio in magnetic nanoparticle systems Magnetite Fe 3 O 4 in isoparaffin Cobalt ferrite CoFe 2 O 4 in diester The imposition of an external alternating field together with random noise, influences the switching of the magnetic moment within the nanoparticle. Definition : SNR=
- 26. Application Potential use of alloys of ferrite-based magnetic fluids, as adjustable magnetic resonators Nickel-zinc ferrites of different stoichiometries Ni 0.3 Zn 0.7 Fe 2 O 4 and Ni 0.5 Zn 0.5 Fe 2 O 4 mixed Ni 0.5 Zn 0.5 Fe 2 O 4 Ni 0.3 Zn 0.7 Fe 2 O 4 ratio1:1
- 27. Application Use of magnetic fluids in microgravity Thermomagnetic convection: A flow driven by magnetic forces in a fluid under influence of thermal gradients. Gravity free conditions enhance the thermomagnetic convection effect. ESA has scientific interest to study the effect of thermomagnetic convection in the International Space Station and in parabolic flights. ZARM drop tower in the University of Bremen was also employed. Magnetization of a magnetic fluid saturates earlier in a gravity free environment than on ground. Ferrofluid T 1 T 2
- 28. Potential use of magnetic fluids in microfluidics Application Pipe diameter 0.004 m Magnetic strength 300 Gauss Ferrofluid type oil based Surfactant hydrophobic coils
- 29. Thanks are due to <ul><li>Professor Paul Fannin, Trinity College Dublin (TCD), Ireland </li></ul><ul><li>Professor William Coffey, Trinity College Dublin (TCD), Ireland </li></ul><ul><li>Professor Brendan Scaife, Trinity College Dublin (TCD), Ireland </li></ul><ul><li>Professor Yury Kalmykov, University Perpignan (UPDV), France </li></ul>
- 30. Academics Technological Institute Crete Greece Ireland OPEN UNIVERSITY OF GREECE www.eap.gr 25.000 students 4 Faculties www.teicrete.gr 10.000 students 4 Faculties www.tcd.ie 15.500 students 3 Faculties

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