1. Superparamagnetism(SP)M) Properties and applications Kang Liu Boston University

2. Outlines Nanomagnetism Nanomagnetism Typical Measurements Typical Measurements Superparamagnetism Superparamagnetism Applications Applications

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

4. Nanomagnetism Surface effect Surface effect Spin Glass Spin Glass Volume effect Volume effect Superparamagnetism Superparamagnetism

5. Typical Measurements Magnetization vs Temperature A ZFC-zero field cooling A ZFC-zero field cooling B FC-field cooling B FC-field cooling 1 Susceptibility (χ=M/H) 1 Susceptibility (χ=M/H) 2 Phase transition 2 Phase transition 3 Blocking temperature (Peak in ZFC curve) 3 Blocking temperature (Peak in ZFC curve) Magnetic ordering state T b Superparamagnetic state temperature

6. M-T graph for hematite nanorods H app =90Oe Blocking temperature T b =16K Using SQUID magnetometer Magnetic ordering state T b Superparamagnetic state temperature

7. Typical Measurements Magnetization vs Field (hysteresis loops) 1 Saturated magnetization M S 1 Saturated magnetization M S 2 Coercivity H C --Open loop (magnetic order) 2 Coercivity H C --Open loop (magnetic order) MSMS HCHC REF: J Phys: Condens. Matter 13(2001) R433-R460

8. Superparamagnetism (SPM) τ=τ 0 exp(E / (k B T)) Neel-Arrhenius equation τ – Average length of time that it takes for a ferromagnetic cluster to randomly flip directions as a result of thermal fluctuations τ 0 – Attempt period (characteristic of the material) E – Anisotropic energy which is proportional to V E=KV K is the anisotropy energy density constant

9. Superparamagnetism (SPM) Blocking temperature T b E=KV=25k B T b T>T b τ < <τ 0 Behave like Paramagnetic particle T >τ 0 Magnetic ordering and open loops If V↓ then τ ↓ 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 An upper bound of about 36 Gbit/in.2 τ=τ 0 exp(E / (k B T)) Neel-Arrhenius equation

10. Different Hysteresis Loops HCHC Ferromagnetic state Open loop Large M S Paramagnetic state No open loop Small M S Superparamagnetic state No open loop Large M S

11. 20nm*40nm*100nm T b =16K Data for hematite nanorods Using SQUID magnetometer Magnetic ordering state T b Superparamagnetic state temperature 16K

12. New Properties of SPM Small size and larger magnetic moment for each particle like Ferromagnetism --Large M S Small size and larger magnetic moment for each particle like Ferromagnetism --Large M S Response to external field like paramagnetic response---No open loop Response to external field like paramagnetic response---No open loop Superparamagnetic relaxation Superparamagnetic relaxation τ=τ 0 exp(E / (k B T)) Neel-Arrhenius equation

13. Hyperthermia Injection of superparamagnetic nanoparticles Injection of superparamagnetic nanoparticles Translocation of the particles to the tumor Translocation of the particles to the tumor Uptake of the nanoparticles by cancerous cells Uptake of the nanoparticles by cancerous cells Application of an external AC magnetic field Application of an external AC magnetic field Superparamagnetic relaxation — heat Superparamagnetic relaxation — heat Cancerous cells are more sensitive to temperature Cancerous cells are more sensitive to temperature No remnant magnetization No remnant magnetization REF: J. Phys.: Condens. Matter 18(2006) S2919-S2934

14. Thank you Any Questions?