Mechanism of Quantum Transportation and Macroscopic Quantum Tunnel Effect in Nanomagnets

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Diffusive as well as ballistic electronic transportation in nanomagnets has been investigated. The results are summed up as follows: (1) A conspicuous oscillation of magnetoresistance due to the Aharonov Bohm effect was found in a ferromagnetic NiFe nano-ring fabricated using the electron lithography. (2) The external magnetic and electric field dependence of the conductance quantization was found in Ni quantum wires. All the results indicate that the coherence of a conduction electron dominates the transport property in nanoscale ferromagnets in spite of the possible strong scattering of the conduction electron due to the local spins. Furthermore, the observed external-field dependence of the quantized conductance implies an important role of the magnetic structure in the ballistic transportation of electron in ferromagnets. On the other hand, correlations between the magnetic structures and the diffusive transportation, which has been theoretically predicted in ferromagnets, have not been observed so far in the present study. Some experiments for the investigation of the correlation using nanomagnets in which the magnetic structures can be controlled in terms of the magnetic shape anisotropy and the extra magnetic field are now under contemplation.