Magnetism in Nanostructure

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Magnetism in Nanostructure

• Samaresh Guchhait
• Department of Physics, UT - Austin
• 4/28/2005

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Topics

• Introduction
• What is Nanomagnetism?
• Sizes and Shapes of Nanomagnets
• Fabrication of Nanomagnets
• Modeling Nanomagnets
• Magnetic Anisotropy Energy
• Technological and Medical Applications
• References

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Introduction

• Permanent magnets play critical role in
  technology and industry.
• Used in generation and distribution of electrical
  power, information processing, information
  storage.
• Nanomagnets hold promising possibility to
  increase information storage density.
• Nanomagnets may also be used for medical imaging
  and drug delivery to sensors technology and
  computing.

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What is Nanomagnetism?

• Ferromagnetic materials consist of tiny
  individual domains
• The magnetic moments of all atoms within a domain
  point same direction
• All domains remain aligned in an external field
  even after field has been switched off
• Computer disk contains 2D ferromagnetic thin film
  
• Information is stored in sub-micron-sized bits
  made of hundreds of domains

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What is Nanomagnetism?

• Magnetic moments within a domain are forced to
  align with external magnetic field
• Moments in magnetic domains remain stable-
  material remembers the information
• Nanomagnetism study of such ferromagnetic
  materials behavior when they are geometrically
  restricted in at least one dimension
• Examples 2D thin films, 1D nanowires,
  zero-dimensional magnetic islands, atomic chains

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Sizes and Shapes of Nanomagnets - 1

• Nanomagnets varies in size, shape, material
• Size varies from less than micron to few nm
• (a) Spherical nanodots about 1.4 nm diameter on
  Cu(111) substrate
• (b) Cobalt nanoplatelates- grown on Si(111)
  substrate, edge length 8.1 nm, thickness of few
  monolayers

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Sizes and Shapes of Nanomagnets -2

• (a) Co nanostripes on W(110) stepped
• (b) Co nanocrystal on NiO surface
• Nanopillars grown on Si dimension 70 nm ? 130
  nm, Fe(Cr)-Cr-Fe(Cr) (F1-N-F2 type) multilayer

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Fabrication of Nanomagnets -Fe/Cu(111) Nanodots

• Direct deposition of Fe does not form dots
• Synthesized by buffer layer assisted growth
  (BLAG) below 10-10 Torr
• Cu(111) single crystal surface prepared by Ne ion
  sputtering, annealing to 800 K, then cooling to
  15 K
• Inert Xe of 5N purity released into chamber
• Iron was evaporated from a heated wire
• After Fe deposition, sample was warmed to 300 K
• Calibrated by in situ STM, Auger spectroscopy,
  etc.

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Modeling Nanomagnets - 1

• Ferromagnets are collection of magnetic dipoles
  which are also free to rotate
• Two types of interaction between spins
• 1. Magnetostatic interaction between dipoles -
  makes each spin pair point in opposite directions
• 2. Quantum-mechanical exchange interaction that
  is due to overlap between spin wavefunctions -
  creates an effective torque on neighboring
  magnetic dipoles that causes them to line up
• Overall orientation of the dipoles in particular
  domain is balance between these two interactions

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Modeling Nanomagnets -2

• At about 10nm quantum interaction is the stronger
  
• Magnetostatic interaction decays slowly with
  distance, hence greater impact at length scales
  of 100 nm or more
• The overall orientation of dipoles also depends
  on material structure due to MAE
• One has to consider all three contributions to
  determine the magnetization distribution in a
  nanomagnet

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Magnetic Anisotropy Energy

• The property that makes magnets so useful is
  magnetic anisotropy energy (MAE)
• MAE determines stability of magnetization
• For high MAE, magnetization will point in a
  direction, rather than randomly fluctuate over
  time
• Nanoparticles offer freedom to tune the MAE by ad
  hoc modifications of particle size, shape, and
  coupling with substrate

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Super-paramagnetism

• Temperature can make spins to fluctuate
• At high temperature, magnetic order falls until
  it loses all magnetization and becomes
  paramagnetic
• Thermal effects are much stronger in low-
  dimensional materials than in the bulk
• Above blocking temperature, magnetization of
  nanoparticle fluctuates randomly at zero field
• This effect is called super-paramagnetism

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Blocking Temperature of Nanomagnet

• Below blocking temp, nanomagnet loose its
  preferred direction of magnetization
• ZFC curve peaks at the blocking temperature
• Blocking temp depends on size, shape, material
  and substrate
• Nanorods usually have higher blocking temp than
  nanodots of same volume

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Blocking Temp of Nanomagnets

• Nanoplatelates have blocking temperature of as
  high as 100 K
• Due to extremely thin and planar geometry of the
  nanoplatelates
• MAE also depends on material and substrate
• Cobalt atoms on a platinum substrate have a
  particularly large MAE of about 9.3 meV/atom
• Samarium cobalt -widely used permanent magnet-
  has a MAE of just 1.8 meV per cobalt atom

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Prospect of Data Storage

• Size of data bits fallen to 300 nm ? 15 nm in
  2002
• Signal detected by read head becomes more noisy
  as smaller data bits has fewer grains
• Solution is to make smaller grains
• If grains are too small, data are more likely be
  lost because thermal fluctuations grain
  magnetization
• One solution is increasing MAE
• Layered nanopillars (F1-N-F1 type) are more
  stable to thermal fluctuations than others

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Nanomagnets in Biology

• Nanomagnets can be used to enhance signal from
  magnetic resonance imaging (MRI)
• Iron-oxide particles (dubbed magnetic nanobeads),
  coated with a suitable neutral chemical, can be
  injected into bloodstream
• Depending on their size, chemical coating, they
  travel to different organs of the body
• By selecting particles of particular sizes,
  researchers then study specific parts of the body

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Nanomagnets in Drug-delivery

• Nanomagnets could be used for drug-delivery
• Nanobeads are first laced with drug molecules
• Steered by external magnetic-field gradients
  they reach the desired parts of the human body
• This targeted drug delivery technique limits the
  exposure of healthy tissue to the drug
• It is one of the most active areas in cancer
  research
• It is currently the subject of clinical trials

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Nanomagnets in Medicine

• It has been reported that cancer cells are more
  susceptible to high temperatures than normal
  cells
• By increasing the temperature of tissue to more
  than 42C, the cells could be selectively
  destroyed
• To achieve this, a dose of magnetic nanoparticles
  could be injected into a region of malignant
  tissue
• Then alternating magnetic field could be applied
  
• With sufficiently strong field and of optimum
  frequency, the particles absorb energy and heat
  surrounding tissue, affecting only infected cells

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Inference

• This field is driven by the search for faster,
  cheaper and high density magnetic-storage devices
  and sensors
• Many applications of nanomagnets in biomedicine
  also
• The three main challenges are to design new types
  of magnetic nanostructures, to increase the
  blocking temperature for such materials, and to
  ensure that such nanostructures can be made
  cheaply in large quantities.

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References

• J.P. Pierce et al., PRL 92, 237201 (2004).
• M. AlHajDarwish et al., PRL 93, 157203 (2004).
• M.H. Pan et al., Nano Letters 5, 87 (2005).
• P. Gambardella et al., Science 300, 1130 (2003).
• J.F. Bobo et al., Journal of Physics Condensed
  Matter 16, S471 (2004).
• D. Koltsov and M. Perry, Physics World, July
  2004.
• H. Brune et al., Nature 394, 452 (1998).
• Y. Sun et al., PRL 91, 167206 (2003).