Magnetic Nanoparticles
李连山
National Center for Nanoscience and Technology Email: lils@https://www.doczj.com/doc/1012198368.html, Tel: 010-********
Outline
? Introduction to Magnetism ? Property of magnetic nanoparticles ? Synthesis of magnetic nanoparticles ? Application of magnetic nanoparticles
Introduction to Magnetism
What is Magnetism?
Magnetism is the force of attraction or repulsion of a magnetic material due to the arrangement of its atoms, particularly its electrons.
Like repels like…
Opposites attract!
?Atoms themselves have magnetic properties due to the spin of the atom’s electrons. ?Groups of atoms join so that their magnetic fields are all going in the same direction ?These areas of atoms are called “domains”
When an unmagnetized substance is placed in a magnetic field, the substance can become magnetized magnetized. . This happens when the spinning electrons line up in the same direction direction. .
Magnetization, M (磁化）
? Material with a net magnetic moment is magnetized ? Magnetization is the magnetic moment （磁 矩）per unit volume within the material
?unpaired electron spins mainly ?the orbital motion of electrons within the material to a lesser extent
Magnetization depends on……..
? The arrangement of the magnetic dipoles within the material
Flux density （磁通量密度）B within material determined by both…… ? Geometry and current in solenoid ? Magnetic properties of the material ? Geometry of material
?Magnetic: concentrate flux lines: Co, Fe ?Diamagnetic materials repel flux lines weakly, H2O, Protein
B = m 0 (H + M )
m 磁导率，H 磁场强度
The H Field
? H is called the magnetic field strength ? m0 is a constant called the permeability of free space
?There is no magnetization M ?So…..
B= m H 0
Response of material to applied magnetic field strength H ? Generally, M changes in magnitude as H is varied. ? Magnitude of response is called the “magnetic susceptibility （磁化 率） ” of the material
B = m 0 (H + M )
Remanence (Mr) 剩磁
Saturation (Ms) 饱和
Coercivity (Hc) 矫顽（磁）力
Ferromagnetism
? Materials that retain a magnetization in zero field ? Quantum mechanical exchange interactions favour parallel alignment of moments ? Examples: iron, cobalt
Paramagnetic gas
? Imagine a classical gas of molecules each with a magnetic dipole moment ? In zero field the gas would have zero magnetization
Superparamagnetism
? Unblocked particles that respond to a field are known as superparamagnetic
Property of Magnetic Nanoparticles
Synthesis of Magnetic Nanoparticles
Superparamagnetism ?Magnetic Nanoparticles
? Coercive field, Hc, for a single domain particle decreases with size. When V is so small that KV (K is magnetic anisotropy constant) is comparable to the thermal energy, kT, the magnetization direction is subjected to random thermal fluctuation, which is called superparamagnetism. In this condition, Hc is zero (no hysteresis) and the particle has no stable magnetization direction. For a particle of a fixed volume, the minimum temperature required to make it superparamagneticis called the blocking temperature, TB, which is expressed in terms of K and V as; TB = KV/25K
Cullity, L. C., Introduction to Magnetic Materials, Reading, MA: Addison-Wesley, 1972.