Macroscopic Quantum Tunneling of Magnetization in Natural and Artificially Engineered Ferritin

Abstract

Macroscopic quantum tunneling of the Neel vector in magnetic ferritin proteins is studied as a function of particle volume and concentration.Characterization of the particles at high magnetic fields (to 27 T) reveal a spin flop transition from which the anisotropy field can be extracted and compared with the value derived from tunneling measurements. Complementary magnetization measurements in the classical regime (2 - 300 K) shows that the anisotropy in the range of particles studied depends linearly on particle volume, not surface area. In the quantum regime of very low temperatures and fields, the resonant tunneling frequency is observed to depend exponentially on the particle volume, in good agreement with theoretical predictions. Finally, the effect of interparticle interactions in the quantum regime is examined by dilution with apoferritin and is found to affect both the peak and width of the resonant frequency. Computer simulations of the dilution dependence also show an increase in the peak height and decrease in the width as the dilution is reduced.