SuperparamagnetismSPM Properties and applications Kang Liu Boston University

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

Outlines n Nanomagnetism n Typical Measurements n Superparamagnetism n Applications

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

Nanomagnetism n Surface effect Spin Glass n Volume effect Superparamagnetism

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

M-T graph for hematite nanorods Blocking temperature Tb=16 K Using SQUID magnetometer Happ=90 Oe Magnetic ordering state Tb Superparamagnetic state temperature

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) R 433 -R 460

Superparamagnetism (SPM) τ=τ0 exp(E / (k. BT)) 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 τ0 – Attempt period (characteristic of the material) E – Anisotropic energy which is proportional to V E=KV K is the anisotropy energy density constant

Superparamagnetism (SPM) τ=τ0 exp(E / (k. BT)) Neel-Arrhenius equation Blocking temperature Tb E=KV=25 k. BTb T>Tb τ < <τ0 Behave like Paramagnetic particle T<Tb τ > >τ0 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

Different Hysteresis Loops Ferromagnetic state Open loop Large MS HC Paramagnetic state No open loop Small MS Superparamagnetic state No open loop Large MS

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

New Properties of SPM n Small size and larger magnetic moment for each particle like Ferromagnetism --Large MS n Response to external field like paramagnetic response---No open loop n Superparamagnetic relaxation τ=τ0 exp(E / (k. BT)) Neel-Arrhenius equation

Hyperthermia Injection of superparamagnetic nanoparticles n Translocation of the particles to the tumor n Uptake of the nanoparticles by cancerous cells n Application of an external AC magnetic field n Superparamagnetic relaxation—heat n Cancerous cells are more sensitive to temperature n No remnant magnetization n REF: J. Phys. : Condens. Matter 18(2006) S 2919 -S 2934

Thank you Any Questions?