Prsentation PowerPoint

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Beating the Superparamagnetic Limit With Exchange
Bias
NATURE VOL 423 19 JUNE 2003
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Introduction

• Magnetic nanoparticles can be used in
  ultrahigh-density recording. However, with
  decreasing particle size the magnetic anisotropy
  energy per particle becomes comparable to the
  thermal energy. When this happens, the thermal
  fluctuations induce random flipping of the
  magnetic moment of the particles with time, and
  the nanoparticles lose their stable magnetic
  order and become superparamagnetic.
• Magnetic exchange coupling induced at the
  interface between ferromagnetic and
  antiferromagnetic systems can provide an extra
  source of anisotropy, leading to magnetization
  stability.

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Sample Preparation
Cooling water
Stepper motor
(C, CoO or Al2O3)
HV
pump
Sputtering gun
Cluster gun
ArO2 71
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Ferromagnetic nanoparticles in a non-magnetic or
antiferromagnetic matrix
Figure 1 TEM micrographs and electron diffraction
of CocoreCoOshell particles. a, Highmagnification
bright-field image, revealing the coreshell
structure. Inset, plan-view distribution of the
nanoparticles. b, High-resolution lattice image
of nanoparticle with 001f.c.c. crystallographic
orientation. c, Electron-diffraction patterns,
showing f.c.c. Co and f.c.c. CoO reflections. d,
Schematic drawing of the sample cross-section,
showing Co cores (black), and surrounding CoO
shell (white) and matrix (grey).
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M Verse T Curves in CocoreCoOshell System
Magnetic moments of 4-nm CocoreCoOshell
particles. Shown is the temperature dependence of
the zero-field cooled (ZFC filled symbols) and
field-cooled (FC 0.01 T, open symbols) magnetic
moment (m) of 4-nm CocoreCoOshell particles.
Particles were embedded in a paramagnetic (Al2O3)
matrix (diamonds), or in an AFM (CoO) matrix
(circles). The measuring field is 0.01 T.
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Magnetic Loops in Co/CoO Systems
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• Coercivity enhancement, revealing induced
  uniaxial or multiaxial anisotropy,
• (2) hysteresis loop shift along the field
  axis after field cooling, revealing
  unidirectional anisotropy.

µ0Hc 0.02 T
µ0Heb 0.92 T µ0Hc 0.59 T µ0Hc 0.39 T
Hysteresis loops at 4.2 K of 4-nm CocoreCoOshell
particles embedded in different matrices. Data
are shown after ZFC (dashed lines) and FC (FC . 5
T solid lines) procedures. a, Embedded in a
paramagnetic (Al2O3) matrix b, compacted and c,
embedded in an AFM (CoO) matrix.
µ0Heb 0.74 T µ0Hc 0.76 T
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Temperature Dependence of Hc and mR
Coupling FM nanoparticles with an AFM matrix is a
way of beating the superparamagnetic limit
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Other Evidences

• Pure Co nanoparticles (that is, without CoO
  shell) embedded in a CoO matrix shows less
  exchange anisotropy because of the poorer quality
  of the interface between the AFM matrix and the
  Co FM nanoparticles.
• Diamagnetic Ag nanoparticles embedded in a CoO
  matrix did not show FM response, thus confirming
  that these effects originate from the FM Co
  nanocores and not from a possible weak
  ferromagnetism due to the CoO matrix

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Conclution

• Exchange coupling at the Co/CoO interface gives
  enhancement of the coercivity and a shift of the
  hysteresis loop along the field axis.
• Magnetic coupling of FM nanoparticles with an AFM
  matrix is a source of an effective additional
  anisotropy, which leads to an improvement in the
  thermal stability of the FM nanoparticles