Tunnel magnetoresistance of granular superparamagnetic and ferrimagnetic structures
Applicable magnetic sensors based on nanogranular ferromagnetic materials were developed already more than 25 years ago. Since the then, nanotechnology has advanced significantly. New methods for manufacturing agglomerated core-shell structures have emerged. This opens up new possibilities of sensor fabrication and an opportunity for reassessment of the electric and magnetic properties of ideal granular structures. This work represents a comprehensive study of the intergranular resistivity, tunnel magnetoresistance and magnetic field sensibility of superparamagnetic and ferro(ferri)magnetic granular materials. Starting with the tunnel resistance of a granular metal network in which the grains are interconnected by insulating barriers, the tunnel magnetoresistance is calculated under consideration of the temperatures dependencies of magnetization, spin polarization and the magnetic flux dependencies of magnetization and tunnelling barrier height. Granular, superparamagnetic materials show a higher magnetic field sensitivity than ferromagnetic ones. They show a lower temperature coefficient of the tunnel magnetoresistance. Owing to their small magnetic response a higher temperature, superparamagnetic ferrimagnetic oxides are not suitable for application at room temperature. Ferromagnetic nanoparticles possess a high field sensitivity only in a small region of 0.1 to 0.5 T.
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