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Magnetic Properties. of nanostructures. Scott Allen Physics Department University of Guelph. Outline. Types of magnetism. Ferromagnetism. terminology. Domains. nano impact on magnetism. references for further investigation. Types of Magnetism.

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Magnetic Properties

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Magnetic properties l.jpg

Magnetic Properties

of nanostructures

Scott Allen

Physics Department

University of Guelph


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Outline

Types of magnetism

Ferromagnetism

terminology

Domains

nano impact on magnetism

references for further investigation


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Types of Magnetism

constituent atoms have magnetic dipole moments

paramagnetism

moments are unaligned, but in the presence of an external magnetic field they do attempt to align

ferromagnetism

all atoms contribute positively to a spontaneous net

alignment in the absence of an external magnetic field

ferrimagnetism

net alignment in the absence of an external magnetic field, with some moments opposing, found in ionic compounds such as oxides (antiferromagnetism – special case is which there is full antialignment)


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Ferromagnetism

exchange field (BE)

strong internal interaction tending to line up the moments in a parallel manner (paramagnetism to ferromagnetism)

Curie temperature

above this temperature the spontaneous magnetization is lost due to thermal fluctuation

separates disordered paramagnetic phase from the ordered ferromagnetic phase

saturation magnetization

maximum induced magnetic moment that can be obtained in an external magnetic field


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Ferromagnetism

hysteresis

ferromagnets can have a memory of an applied field after it has been removed

http://hyperphysics.phy-astr.gsu.edu/hbase/solids/magperm.html#c1


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Ferromagnetism

in some cases a material in bulk will have a remanence of nearly zero

but if the exchange interaction between the magnetic moments is so high, why wouldn’t the material always be magnetically saturated?


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Domains

http://hyperphysics.phy-astr.gsu.edu/hbase/solids/ferro.html#c4

bulk materials are divided into domains

each domain is spontaneously magnetized to saturation, but from domain to domain the direction of magnetization can be different  thus leading to net magnetizations well below saturation

domain formation

surface charges form creating demagnetizing field

magnetostatic energy decreases through creation of second domain

http://www.irm.umn.edu/hg2m/hg2m_d/hg2m_d.html


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Domain walls

interfaces between domains having differing directions of magnetization

exchange energy acts to keep spins parallel

a thinner wall requires more energy to create and maintain as the change in spin direction must be more abrupt

anisotropy energy tends to keep spins aligned along certain crystallographic planes

a thinner wall is more energetically favorable

competition exists between anisotropy and exchange energy ( domain walls have a finite width ~ 100 nm)

http://www.irm.umn.edu/hg2m/hg2m_d/hg2m_d.html


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nano impact

nanoscale particles are so small that it is not energetically favorable to have more than one domain  single domain regime (10 - 100 nm)

in this regime the direction of magnetization can only change through rotation (not domain growth or formation)

this rotation is energetically difficult and leads to high coercivities and remanence

as the particle size decreases within the single domain regime the “superparamagnetic limit” is reached

coercivity and remanence are zero


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nano impact

http://www.irm.umn.edu/hg2m/hg2m_d/hg2m_d.html

super paramagnetism – a single domain particle that is magnetically saturated along a particular direction will overcome the anisotropy energy and reverse its direction if  the particle is sufficiently small and the temperature is high enough (thermal energy is enough)

P. Moriarty, Rep. Prog. Phys.64 (2001) 297


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nano impact

http://www.irm.umn.edu/hg2m/hg2m_d/hg2m_d.html

with no applied field, and T > 0K, the superparamagnetic particles net moment will average to zero

in an applied field, there will be a net alignment of the magnetic moments

similar to paramagnetism, except it’s the alignment of domains (many atoms) as opposed to single atoms


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nano impact  taking it further

this brief discussion of superparamagnetism was considering ideal bulk-like behaviour

due to the high surface to volume ratio in nanoparticles, one finds that surface effects start to play a dominant role

a good starting point for a more in depth look is:

R.H. Kodama, J. Magn. Magn. Mater.200 (1999) 359.


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references

magnetism

C. Kittel, “Introduction to Solid State Physics”, Wiley, (1986).

intrinsic nanoparticle properties

R.H. Kodama, J. Magn. Magn. Mater.200 (1999) 359.

R.H. Kodama, Phys. Rev. B.59 (1999) 6321.

S.A. Majetich, J.H. Scott, E.M. Kirkpatrick, K. Chowdary, K. Gallagher, M.E. McHenry, Nanostruct. Mater.9 (1997) 291.

S.A. Majetich, Y. Jin, Science284 (1999) 470.

C.P. Bean, J. Appl. Phys.26 (1955) 1381.

interactions between nanoparticles

M.F. Hansen, S.Morup, J. Magn. Magn. Mater. 184 (1998) 262.

I.M.L. Billas, A. Chatelain, W.A. de Heer, Science265 (1994) 1682.


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