Magnetism in Nanostructure - PowerPoint PPT Presentation

1 / 20
About This Presentation
Title:

Magnetism in Nanostructure

Description:

Magnetic Anisotropy Energy. Technological and Medical Applications. References. Introduction ... Magnetic Anisotropy Energy. The property that makes magnets so ... – PowerPoint PPT presentation

Number of Views:1417
Avg rating:3.0/5.0
Slides: 21
Provided by: Tri94
Category:

less

Transcript and Presenter's Notes

Title: Magnetism in Nanostructure


1
Magnetism in Nanostructure
  • Samaresh Guchhait
  • Department of Physics, UT - Austin
  • 4/28/2005

2
Topics
  • Introduction
  • What is Nanomagnetism?
  • Sizes and Shapes of Nanomagnets
  • Fabrication of Nanomagnets
  • Modeling Nanomagnets
  • Magnetic Anisotropy Energy
  • Technological and Medical Applications
  • References

3
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.

4
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

5
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

6
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

7
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

8
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.

9
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

10
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

11
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

12
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

13
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

14
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

15
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

16
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

17
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

18
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

19
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.

20
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).
Write a Comment
User Comments (0)
About PowerShow.com