The Influence of Temperature on the Magnetic Behavior of Colloidal Cobalt Nanoparticles

Applications of magnetic nanoparticles, including hyperthermia for cancer treatments, require knowledge of how the colloidal environment affects the magnetic properties of the nanoparticles. Here, 10 nm diameter cobalt nanoparticles synthesized by thermodecomposition in 1,2-dichlorobenzene (DCB) are used to study the effect of the colloidal environment on the magnetic behavior of such materials. The magnetic properties are investigated by magnetization (M) versus temperature (T) measurements and vector magnetometry performed on the samples under zero-field-cooled conditions. Of particular interest in the M versus T data is a continuous rise in the magnetization observed around the DCB melting point during sample heating and a discontinuous drop around the DCB supercooling point during sample cooling. Vector magnetometer measurements quantify the portion of the sample that does not respond to the applied field. The magnitude of this unreversed component doubles with decreasing temperature as the temperature cools through the supercooling point in DCB. There is also an increase in the uniaxial anisotropy of the sample from 61.1(7)times10^-7 J to 104.2(9)times10^-7 J as the liquid-to-solid transition is traversed.