Spin Dynamics in Novel Magnetic MaterialsSrikanth HariharanRanko HajndlJeff SandersJessica Wilson

Department of PhysicsUniversity of South Florida

http://chuma.cas.usf.edu/~sharihar

Support:NSFDARPA

The bottom line

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

Biodegradable particles Antibody tagged or Bio-Stealth technology ! Surfactant coated magnetic particles Superparamagnetism works for you !

Biomedical sensors based on magnetic nanoparticlesDNA isolationMagnetically controlled site-specific drug deliveryNovel magnetoimpedance sensors to counter bioterrorism

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 parametersWhy not nanomaterials ?!

Materials Physics Laboratory @ USF Complex impedance measurements : Z(H,T,w)0 < H < 7T, 2K < T < 350K, DC to 1GHz

PPMS from Quantum Design for magnetic and transport measurementsHome-made RF probe and point contact tunneling probe integrated with PPMS

Resonant 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

Sensitivity 1-10Hz in 10 MHz

Temperature range:2K < T < 300K

Variable DC field:0 < H < 9T

t/tfres/fres

Probing Dynamic EffectsTime and Temperature Dependent Experiments:Magnetization Measurements: FC, ZFC -AC SusceptibilityMssbauer Spectroscopy Transverse susceptibility:ClassicalTDO

3 nm radius Gold Core acts a nucleation source for the epitaxial growth1 nm thick Iron Shell2 nm Gold Shell acts to passivate the surfaceNano-onions synthesized by reverse micelle method

Field and Temperature dependence of t L. Spinu, C. J. OConnor 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. OConnor, Appl. Phys. Lett. 80, 276 (2002).

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

Transverse susceptibility in -Fe2O3 nanorods synthesized by wet chemical method (J. Fang et al., UNO)

Role of shape anisotropy in acicular structures

Transverse Susceptibility calculationzxyHDCHRFMSuKKMqMqKCoherent 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 Aharoni, 1957 Chantrell, 1993

RelaxationMaster EquationHrf perturbation around equilibrium

Theoretical Results Effect of Hk DistributionEffect of TemperatureL. Spinu, A. Stancu, H. Srikanth and C. J. OConnor, Appl. Phys. Lett. (Jan. 2002)

d = 7.5nmFe Nanoparticle arrays (from D. Farrell and S. Majetich, CMU)

Systematic variation of field-dependent transverse susceptibility in the blocked and superparamagnetic regimes

Transverse susceptibility of 8 nm Mn:Zn ferrite particlesSamples from NRL (Everett Carpenter)

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

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

(with Lloyd Engel, NHMFL)

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

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)

Half-metallic epitaxial CrO2 films

RF transverse susceptibiliy data t (H, T)T=300 K =0T=300 K =90Hk = 600 OeHk = Hs = Hc = 50 Oe

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

Stoner-Wohlfarth Model Magnetocrystalline anisotropy + Magnetoelastic anisotropy

Precise angular dependence of anisotropy in CrO2

Excellent probe of perallel and perpendicular anisotropy

Dynamic magnetization in Fe3O4 thin films Films grown by Laser Ablation technique @ USF Houssam Abou Mourad, Dr. S. Witanachchi, Dr. P. Mukherjee

Intelligent Design of Nanoscale Molecular Magnets Angewandte Chemie 2002

Supramolecular Systems:

Dimetal tetracarboxylates [L2M2(O2CR4)] L = Coordinated ligand M= transition metal

Self-assembled infinite structures with predictable topology and high thermal stability

Cu-Cu dimers

Open frameworkClosedframework

Effective susceptibility in dimeric compounds(Bleaney-Bowers model)

J Intramolecular interactionJ Inter-molecular interaction

(includes correction foruncompensated Moments)

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

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

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

XRD image of the composite on Al2O3 substrates with 50%-50% BSTO-Ba ferrite ratio

wasp-waisted loop

RuSr2GdCu2O8 systemCo-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) ?

rcM-Hc

Resonant Tunnel-diode Oscillator (TDO) method

H. Srikanth, J. Wiggins and H. Rees, Rev. Sci. Instrum. 70, 3097 (1999)fres/fres

Sensitivity 1-10Hz in 6 MHzUltrastable 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

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

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

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

Features definitely related to Mixed state of the superconductorGiant vortices or moments ? Spin-flop transitions in the Ru-O plane ?

Bipolar field scans in the normal magnetic state

M. Zaworotko (USF)Pritish Mukherjee (USF)Sarath Witanachchi (USF)Leonard Spinu (UNO)Charles J OConnor (UNO)Jiye Fang (UNO)

Acknowledgements

Caroline Ross (MIT)Sara Majetich (CMU)T. S. Sudarshan (MMI)Everett Carpenter (NRL)Nancy Dudney (ORNL)Arunava Gupta (IBM)Gang Xiao (Brown)