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Spin Dynamics in Novel Magnetic Materials

Spin Dynamics in Novel Magnetic Materials. Srikanth Hariharan Ranko Hajndl Jeff Sanders Jessica Wilson Department of Physics University of South Florida http://chuma.cas.usf.edu/~sharihar. Support: NSF DARPA. …Material properties govern the functional characteristics of EM devices.

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Spin Dynamics in Novel Magnetic Materials

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  1. Spin Dynamics in Novel Magnetic Materials Srikanth Hariharan Ranko Hajndl Jeff Sanders Jessica Wilson Department of Physics University of South Florida http://chuma.cas.usf.edu/~sharihar Support: NSF DARPA

  2. …Material properties govern the functional characteristics of EM devices. Intrinsic: m, e, s Extrinsic: Microstructure, grain sizes, porosity, defects • Better control on a local scale • Tailored EM characteristics • Nanostructured materials 10 m The bottom line…

  3. Ultrahigh density magnetic recording • Areal density > 50Gb/in2 • Improved media and head materials • Faster switching speeds require operation at higher frequencies RF response of magnetic nanostructures is important from both fundamental and applied points of view

  4. Biomedical sensors based on magnetic nanoparticles • Biodegradable particles • Antibody “tagged” or “Bio-Stealth” technology ! • Surfactant coated magnetic particles • Superparamagnetism works for you ! DNA isolation Magnetically controlled site-specific drug delivery Novel magnetoimpedance sensors to counter bioterrorism

  5. Why Nanomaterials ? • Physical properties different from bulk and often superior • Superior mechanical properties • Better control of microstructure, porosity • Novel nanocomposite and hybrid systems • Selective enhancement of desirable parameters Why not nanomaterials ?!

  6. Materials Physics Laboratory @ USF • PPMS from Quantum Design for magnetic and transport measurements • Home-made RF probe and point contact tunneling probe integrated with PPMS • Complex impedance measurements : Z(H,T,w) • 0 < H < 7T, 2K < T < 350K, DC to 1GHz

  7. Resonant TDO Method (H. Srikanth, J. Wiggins and H. Rees, Rev. Sci. Instrum. 70, 3097 (1999) HDC HRF L C TDO • Ultrastable Tunnel Diode Oscillator • LC Tank circuit self-resonant at ~ 6 MHz • Sensitivity 1-10Hz in 10 MHz • Temperature range: • 2K < T < 300K • Variable DC field: • 0 < H < 9T t/tfres/fres

  8. Ferromagnetism Superparamagnetism Probing Dynamic Effects • Time and Temperature Dependent Experiments: • Magnetization Measurements: FC, ZFC - • AC Susceptibility • Mössbauer Spectroscopy • Transverse susceptibility: • Classical • TDO

  9. ‘Nano-onions’ synthesized by reverse micelle method 3 nm radius Gold Core acts a nucleation source for the epitaxial growth 1 nm thick Iron Shell 2 nm Gold Shell acts to passivate the surface

  10. Field and Temperature dependence of t L. Spinu, C. J. O’Connor and H. Srikanth, IEEE Trans. Magn. 37,2188 (2001) H. Srikanth et al., Mat. Sci. Engg. A 304-306, 901 (2001) L. Spinu, A. Stancu, H. Srikanth and C. J. O’Connor, Appl. Phys. Lett. 80, 276 (2002).

  11. First demonstration of mapping of switching and anisotropy fields in magnetic nanoparticles H. Srikanth et al., IEEE Trans. of Magnetics (2001)

  12. Transverse susceptibility in -Fe2O3 ‘nanorods’ synthesized by wet chemical method (J. Fang et al., UNO) Role of shape anisotropy in acicular structures

  13. Transverse Susceptibility calculation • Aharoni, 1957 • Chantrell, 1993 qK • Coherent Magnetization rotation (Stoner-Wohlfarth) • E(q) = -K1(uK · uMs)2 – Ms · H • K1 is the anisotropy constant, • is the unit vector in the easy axis direction • is the unit vector in the Ms direction • Minimize E with respect to qM and fM ; calculate transverse susceptibility z MS qM HDC uK HRF y M x K

  14. Relaxation Hrf perturbation around equilibrium Master Equation

  15. Theoretical Results L. Spinu, A. Stancu, H. Srikanth and C. J. O’Connor, Appl. Phys. Lett. (Jan. 2002) Effect of Hk Distribution Effect of Temperature

  16. Fe Nanoparticle arrays (from D. Farrell and S. Majetich, CMU) • Systematic variation of field-dependent transverse susceptibility • in the blocked and superparamagnetic regimes d = 7.5nm

  17. Transverse susceptibility of 8 nm Mn:Zn ferrite particles Samples from NRL (Everett Carpenter)

  18. Polymer coated magnetic particles • Encapsulation of individual nanoparticles • Agglomeration effects • Polymer matrix useful for potential spin coated films • and planar structures • Magnetic interactions mediated through polymer layer • Possible self assembly • Multifunctional system realized due to distinct • electromagnetic response of particles and coatings

  19. Plasma polymerization of Fe nanoparticles (MMI) • Iron pentacarbonyl • Styrene monomer 2:1 Increased carrier gas feed rate results in smaller particle size H. Srikanth et al. APL 79, 3503 (2001)

  20. Evidence for interesting magnetic and RF response • Magnetic bistability ? • Possible tuning of switching fields • Inverse Magnetostriction effect • Novel spin wave resonances 3 2 1 0 M (emu) -1 -2 -3 -0.2 -0.1 0.0 0.1 0.2 H (Tesla) (with Lloyd Engel, NHMFL)

  21. ‘Half-metallic’ epitaxial CrO2 films • Half-metallic --> complete spin polarization at Fermi level --> tunnel junctions with enhanced low-field MR • Films grown by CVD on TiO2(100) and Al2O3(0001) single crystal substrates • Film thickness in this study --> 2000 Å • Uniaxial anisotropy with easy (001) and hard (010) directions X. W. Li et al. J. Appl. Phys. 85, 5585 (1999) X. W. Li, A. Gupta and Gang Xiao APL 75, 713 (1999)

  22. RF transverse susceptibiliy data t (H, T) T=300 K=90° T=300 K=0° Hk = 600 Oe Hk = Hs = Hc = 50 Oe

  23. Evidence for Temperature Induced Anisotropy Theory Experiment L. Spinu, H. Srikanth, A. Gupta, X. W. Li and G. Xiao, Phys. Rev.B 62, 8931 (2000).

  24. Stoner-Wohlfarth Model Magnetocrystalline anisotropy + Magnetoelastic anisotropy

  25. Precise angular dependence of anisotropy in CrO2 Excellent probe of perallel and perpendicular anisotropy

  26. Dynamic magnetization in Fe3O4 thin filmsFilms grown by Laser Ablation technique @ USFHoussam Abou Mourad, Dr. S. Witanachchi, Dr. P. Mukherjee

  27. H. Srikanth et al., USF Physics M. Zaworotko et al., USF Chemistry • Metal dimers in co-ordination polymers • Nanoporous zeolite analogs • Studies of co-operative magnetic • behavior • Magnetic measurements using a PPMS MW = 6.23kDa, V = 10nm3 Intelligent Design of Nanoscale Molecular Magnets Angewandte Chemie 2002

  28. Open framework Closed framework

  29. Ballistic Injection Tunneling Spectroscopy (BITS) • Point contact system designed to fit in PPMS • DC I-V characteristics • RF modulated dynamic conductance • Variable temperature and magnetic field

  30. Andreev Reflection * Superconductor-metal junction*Increasing external DC magnetic field

  31. BSTO – Hexaferrite composite thin film ‘meta-materials’ • Possible ‘first’ development of high quality composite films of • BSTO-BaFe12O19 • Magnetron Sputtering • Co ion implantation (ORNL) • Strip-loop permeability measurements currently being set up

  32. RuSr2GdCu2O8 system • Co-existence of Magnetism and Superconductivity --> TMagnetic= 135K and Tc up to 50K • Meissner effect (recently observed !) • Spontaneous Vortex Phase ? • Pinning effects due to magnetic order ? • Unconventional pairing ? • Non-equilibrium state (FFLO) ?

  33. c c r M-H

  34. HDC HRF L C TDO Resonant Tunnel-diode Oscillator (TDO) method H. Srikanth, J. Wiggins and H. Rees, Rev. Sci. Instrum.70, 3097 (1999) • Ultrastable Tunnel Diode Oscillator • LC Tank circuit self-resonant at ~ 6 MHz • Temperature range: • 2K < T < 300K • Variable DC field: • 0 < H < 9T • Precise probe of dynamic transverse susceptibility in ferromagnets and penetration depth in superconductors fres/fres Sensitivity 1-10Hz in 6 MHz

  35. RF experiments • Complex penetration depth • Free flux flow • No trace of Hc1 • Coffey-Clem model • Transverse susceptibility • Singular peaks at anisotropy and switching fields

  36. 1.H. Srikanth et al. J. Applied Phys. 89, 7487 (2001)

  37. Note the giant peaks with spiky structure. These features scale with superconducting Tc • Inset shows low field variation with hysteresis consistent with flux entry and flux flow

  38. Features definitely related to Mixed state of the superconductor • Giant vortices or moments ? • Spin-flop transitions in the Ru-O plane ?

  39. Bipolar field scans in the normal magnetic state

  40. Acknowledgements M. Zaworotko (USF) Pritish Mukherjee (USF) Sarath Witanachchi (USF) Leonard Spinu (UNO) Charles J O’Connor (UNO) Jiye Fang (UNO) Caroline Ross (MIT) Sara Majetich (CMU) T. S. Sudarshan (MMI) Everett Carpenter (NRL) Nancy Dudney (ORNL) Arunava Gupta (IBM) Gang Xiao (Brown)

  41. Nano size, that is...

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