Superparamagnetism(SP)M) Properties and applications

1
Superparamagnetism(SP)M)Properties and
applications

• Kang Liu
• Boston University

2
Outlines

• Nanomagnetism
• Typical Measurements
• Superparamagnetism
• Applications

3
Nanomagnetism

• (1) An overwhelming variety of structures
• (2) The involvement of nanoscale effects
• Superparamagnetic limit of hard drive
• (3) New technologies
• Hyperthermia

4
Nanomagnetism

• Surface effect
• Spin Glass
• Volume effect
• Superparamagnetism

5
Typical Measurements

• Magnetization vs Temperature
• A ZFC-zero field cooling
• B FC-field cooling
• 1 Susceptibility (?M/H)
• 2 Phase transition
• 3 Blocking temperature (Peak in ZFC curve)

Magnetic ordering state Tb
Superparamagnetic state
temperature
6

• M-T graph for hematite nanorods

Blocking temperature Tb16K
Using SQUID magnetometer
Happ90Oe
Magnetic ordering state Tb
Superparamagnetic state
temperature
7
Typical Measurements

• Magnetization vs Field (hysteresis loops)
• 1 Saturated magnetization MS
• 2 Coercivity HC --Open loop (magnetic order)

MS
HC
REF J Phys Condens. Matter 13(2001) R433-R460
8
Superparamagnetism (SPM)
tt0exp(E / (kBT)) Neel-Arrhenius equation
t Average length of time that it takes for a
ferromagnetic cluster to randomly flip directions
as a result of thermal fluctuations t0 Attempt
period (characteristic of the material) E
Anisotropic energy which is proportional to V
EKV K is the anisotropy energy density constant
9
Superparamagnetism (SPM)
tt0exp(E / (kBT)) Neel-Arrhenius equation

• Blocking temperature Tb EKV25kBTb
• TgtTb t lt ltt0 Behave like Paramagnetic
  particle
• TltTb t gt gtt0 Magnetic ordering and open
  loops
• If V? then t ? SPM limit of hard drives

REF IEEE Transaction on Magnetics Vol 33, No.
1(1997)978-983 An upper bound of about
36 Gbit/in.2
10
Different Hysteresis Loops
Ferromagnetic state Open loop Large
MS Paramagnetic state No open loop Small
MS Superparamagnetic state No open loop Large MS
HC
11
20nm40nm100nmTb16K
Using SQUID magnetometer
Data for hematite nanorods
16K
Magnetic ordering state Tb
Superparamagnetic state
temperature
12
New Properties of SPM

• Small size and larger magnetic moment for each
  particle like Ferromagnetism --Large MS
• Response to external field like paramagnetic
  response---No open loop
• Superparamagnetic relaxation

tt0exp(E / (kBT)) Neel-Arrhenius equation
13
Hyperthermia

• Injection of superparamagnetic nanoparticles
• Translocation of the particles to the tumor
• Uptake of the nanoparticles by cancerous cells
• Application of an external AC magnetic field
• Superparamagnetic relaxationheat
• Cancerous cells are more sensitive to temperature
• No remnant magnetization

REF J. Phys. Condens. Matter 18(2006)
S2919-S2934
14
Thank you
Any Questions?