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DC and AC Measurements of Magnetite Nanoparticulates and Implications for

Nonlinear ResponseUltra Wideband Center of Excellence

(UWBTech.gatech.edu)

Drs. Silvia. Liong1,3 and Ricky L. Moore2; 1Materials Science and Engineering, Georgia Institute of Technology,

Atlanta, GA 30332, USA; 2Signature Technology Laboratory, Georgia Tech Research Institute, Atlanta, GA

30332, USA; 3 currently at Intel Corporation, Hillsboro, OR, 97124

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AbstractOverview: This paper discusses preparation, characterization and measurement of linear

DC and AC magnetic properties of magnetite (Fe3O4) nanoparticles (size ranges of 7-50 nm and 5 microns) and polymer composites of those particulates. Selected data and analysis are taken from the PhD thesis of Liong.

Goal: Obtain magnetic data, specifically magnetization, anisotropy and coercivity as functions of nano magnetite particle size. Selections are taken from the Dr. Silvia Liong’s, PhD Thesis- Materials Science and Engineering School of Georgia Institute of Technology, December 2005.

Summary Results: Measured DC saturation magnetization and coercivity decreased with particle dimension. Anisotropy was calculated to increase. Magnetization data are consistent with model of nanoparticle magnetization as a volumetric average of a spherical bulk material core and a passive outer shell. The shell thickness was calculated at 0.84 nm, very near one lattice constant of bulk Fe3O4, 0.8394 nm. Composites containing particulate volume fractions less than 20% were fabricated. Effective media theory was applied to measured AC composite permeability to extract particle magnetic properties and thereby anisotropy field, which increased by an order of magnitude from the bulk. Anisotropy increase consistent with observation of decreased paramagnetic critical size.

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Outline

• Motivation and Background on magnetite and superparamagnetism

• Synthesis by Coprecipitation• DC Magnetic Characterization of Fe3O4 Nanoparticles• AC Magnetic Characterization of Fe3O4 Nanoparticles

− Frequency dispersion− Effective Media Derived: DC Permeability and Anisotropy

vs. particle size− Anisotropy and superparamagnetic critical volume

agreement• Conclusions and Future Work

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Motivation

• Study Magnetization, Coercivity and Anisotropy fields as a function of scale to facilitate micromagnetic simulations

• Application in biological and medical science

• Application in electrical engineering; radio frequency filed suppression

• Preparation for future AC nonlinear magnetic measurments as function of particle scale (Presentation in Section K. 2008 MRS)

• Sponsorship through the Ultrawideband Center of Excellence, Ga Tech Research Institute, Georgia Institute of Technology (UWBTech@gtri.gatech.edu)

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Background on Fe3O4

• Found in nature

• Magnetic properties− Ferrimagnetic, 4 μB per molecule

− Ms (bulk) = 92-100 emu/g, Hc(bulk) ~ 150 Oe

• Crystal structure− Inverse spinel

° 8 Fe3+ in Tetrahedral

° 8 Fe3+, 8 Fe2+ in Octahedral

− FCC lattice constant, a = 8.394Å

• Net moment of each single domain particle fluctuates randomly from thermal energy, kT

• Coercive field, Hc = 0 , at T>TB, TB is blocking temperature

• Magnetization curve is not function of temperature at T>TB

• \Predicted critical size for Fe3O4 is ~ 20 nm at RT and bulk anisotropy field

• Critical size can increase if anisotropy increases as size decrease

25 : is particle volume;

is anisotropy field

p pkTV V

KK

≤

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Synthesis by Coprecipitation

• Advantages

− Simple procedure and inexpensive precursors

− 1-2 grams of nanoparticles per batch

• Accelerated hydrolysis of ferric and ferrous ions

− Requires alkaline conditions and elevated temperature

− Reaction under inert atmosphere

Fe2+ + 2Fe3+ + 8OH- → Fe3O4 + 4H2O

• Particle sizes range from 2 – 51 nm

• Determine process parameters that control particle size

• Systematic approach

− Experiment matrix

− Particle size analyzed using TEM and XRD

− DC magnetic properties measured using VSM

• FeCl2 and FeCl3 as precursors

• NaOH as precipitating agent

• Reaction time 30 minutes

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Phase Analysis• Analysis of XRD peaks match JCPDS card for Fe3O4• Broadening of peak (440) used to calculate particle size

D1 = 8.9 nm

D6 = 10.7 nm

θλ

cosBktD ==

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VSM For DC Magnetizaton Measurements

• Hysteresis of samples collected using VSM at RT

• Determine Ms and Hc from hysteresis

• Low applied field for Hc

± 1,000 Oe, ramp 1.66 Oe/sec

• High applied field for Ms

± 10,000 Oe, ramp 16.6 Oe/sec

High field ± 10,000 OeMs(1) = 49.61 emu/gMs(6) = 54.05 emu/g

Ms

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DC Magnetic Characterization of Fe3O4 NanoparticlesMeasured Magnetization and Coercivity vs. Particle Size

Liong

Magnetization and Coercivity decrease with particle size

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Theory of Magnetization Reducion:Dead zone Δr and Core with Ms,bulk

r

bulksbulksp

peffectives M

RrRM

RVrV

M ,3

3

,,)(

)()(

⋅Δ−

=⋅=

⎥⎦⎤

⎢⎣⎡ Δ−=

RrMM bulkseffectives 13/1

,3/1

,

R = r + Δr = total physical radius of particleΔr = thickness of magnetically dead layer, Ms=0r = magnetic radius of particle with Ms > 0Berkowitz et al., J. App. Phys., 39 (1968) p. 1261

particle

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AC Characterization of Magnetite-Polymer Composites and Particles

• AC permeability of composites measured using coaxial line reflection and transmission [Agilent applications note 1369-1] and Permeameter [J.App.Phys. V.85,n8, p.4311, 15Apr1999]

− Permeabilty from ~ 10 MHz to 8 GHz

− Thin Film composites with magnetite volume concentrations from 1 to 20 percent

− Four sizes: 5 microns, 25 nm, 12 nm and 7nm

25 nm

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Derivation of Magnetite DC Permeability and Anisotropy:5 micron, 25 nm, 12 nm and 7 nm sizes

• Fit measured magnetic susceptibility (χ = μ -1) data to Lorentzian

• Extract DC component of composite, χc

• Apply effective media and volume fraction, Vp, to calculate DC particle susceptibility χp

• Apply DC particle susceptibility and measured magnetization to calculate Hk for each particle size

1131.2p s kM Hχ ρ −=

( )11 1 .

3p c p c pV Vχ χ χ−

⎧ ⎫= − −⎨ ⎬⎩ ⎭

2( )

1

c

df rf

ff fj

f f

χχ =⎛ ⎞

+ − ⎜ ⎟⎜ ⎟⎝ ⎠

5 micron, 25 nm, 12 nm and 7 nm sizes

L. Bickford, Phy.Rev. v99,n4, 1955

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Calculated DC Magnetic Susceptibility and Anisotropy vs Particle Size

Hk

pχ

Log10(1/diameter)

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Hc vs. Particle Diameter (Size) Bulk and Measured Anisotropy Increase

• Bulk anisotropy predicts diameter for onset of superparamagnetism between 14.5 to 18.5 nm at room temperature

• Measured near zero Coercivity at superparamagnetic particles are < 8 nm

• Anisotropy calculated from DC magnetization and particle susceptibility 1 order of magnitude higher than bulk

−

25p

kTVK

≤

4 3 4 3

298 , Bulk Anisotropy constant range3.9 10 / 8.0 10 /

T KK x erg cm x erg cm=

= −

Vp volume at paramagnetic onset

Measured 57 10 /K erg cm≈ × Predicted diameter ~ 8 nm

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Conclusions and Future Work

• Measured Four magnetic parameters: Ms, Coer, Hkand paramagnetic onset

− Magnetite particles with diameter from 5 nm to 5 microns

• Magnetization decreased from 97 to ~ 29 emu/g

• Anisotropy increase by 1 order in magnitude

• Shift of Parametric onset to ~ 8 nm diameter

• Continuing dispersive spectra measurements of composites and particles in preparation for future nonlinear susceptibility measurements

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Referenced Publications

− Dr. Silvia Liong, PhD Thesis for the Materials Science and Engineering School of Georgia Institute of Technology, December 2005

− M. Dikeakos, L. Tung, , 2003 Mat. Res. Soc. Proc., Vol. 734, B9.45.1− Y. Raiker, V. Stepanov, Microelectronics Engineering, V 69, pp. 317-323, (2003).− G. Mohler, A. W. Harter, R. L. Moore , J. Appl. Phys., V93 n 10, p 7456, (15 May 2003) − Sensors-Actuators B, 121 (2007) p. 330 authors?− M. Bailleul, e,t, al., Phys. Rev. B, 76, 224401, (Dec 2007).− K.N. Rozanov et.al., J.Appl.Phys., 97, 013905 (2005)− G. F. Goya, T. S. Berquo, F. C. Fonseaca, and M. P. Morales, J.Appl.Phys. 94, 3520 (2003)− Wen X., et.al, Current Applied Physics, 8, (2008), 535-541− T. Kim and M. Shima, J.Appl.Phys., 101, 09M516 (2007)− A. E. Berkowitz, et.al., , ,"J.Appl.Phys.,. 39, 1261 (1968).− L.Bickford, Phys.Rev. v99, no.4, Aug.15, 1955− C. Caizer, C. Savii, M. Popovici, Materials Science and Engineering, B97, 129 (2003)− J. Mazo-Zuluaga, et.al., J.Appl.Phys., 103, 113906 (2008)

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Synthesized Particle Examples

• Liu et al. observed that higher reaction temperature produced more faceted particles

• Nanoparticles for composites were synthesized at T = 65 °C

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Synthesized Particles, Magnetizationand Coercivity

Variables: Total iron salt concentration; Base Concentration; TemperatureSample ID correlated with Liong Thesis