This work focuses on the synthetic control of magnetic properties of mixed oxide magnetic nanoparticles of the general formula Fe(3-x)Co(x)O(4) (x < or = 0.33) in the protein cage ferritin. In this biomimetic approach, variations in the chemical synthesis result in the formation of single-phase Fe(3-x)Co(x)O(4) alloys or intimately mixed binary phase Fe/Co oxides, modifying the chemical structure and magnetic behavior of these particles, as characterized by static and dynamic magnetization measurements and X-ray absorption spectroscopy.

Temperature-dependent magnetic measurements are reported for 4.34 nm antiferromagnetic Co3O4 nanoparticles mineralized in the Listeria innocua Dps protein cage. ac measurements show a superparamagnetic blocking temperature of roughly 5.4 K and give an extracted anisotropy energy density of (7.6±0.4)×104J∕m3. The Néel temperature for the Co3O4 nanoparticles, determined with dc magnetometry, was determined to be roughly 15±2K.

The temperature dependent initial magnetization of ␥-Fe 2 O 3 ͑maghemite͒ mineralized inside ferritin protein cages has been investigated with a vibrating sample magnetometer up to 8 T. The data are fit to different magnetic models to extract values of the magnetic moment of each cluster. It is found that the application of a simple Langevin model with a first and second order term in the susceptibility greatly enhances the quality of the fit to the data suggesting that the inclusion of crystalline anisotropy is important in extracting the magnetic moment of each core.

Using longitudinal magneto-optical Kerr effect in combination with a variable strength rotating magnetic field, called the Rotational Magneto-Optic Kerr Effect (ROTMOKE) method, we show that the magnetic anisotropy for thin Fe82Zn18 single crystal films, grown on MgO(001) substrates, depends linearly on the strength of the applied magnetic field at low fields but is constant (saturates) at fields greater than 350 Oe. The torque moment curves generated using ROTMOKE are well fit with a model that accounts for the uniaxial and cubic anisotropy with the addition of a cubic anisotropy that depends linearly on the applied magnetic field. The field dependent term is evidence of a large effect on the effective magnetic anisotropy in Fe1−xZnx thin films by the magnetostriction.