Yun Zhou


Spin-dependent electron transport in submicron/nano sized magnetic thin film devices fabricated using the optical lithography, e-beam lithography and focused ion beam (FIB) was investigated with the primary aim to find the ballistic magnetoresistance (BMR) in thin film nanoconstrictions. All experimental results were analysed in combination with micromagnetic simulations. The magnetisation reversal processes were investigated in a submicron half-pinned NiFe stripe with a microconstriction. An asymmetric MR curve was observed, and micromagnetic simulations verified it was due to the exchange-bias on the left side, which changed the magnetic switching mechanism. The effects of different pinning sites on the magnetisation switching and domain wall displacement were studied in NiFe film and spin-valve based nanodevices. A sign of domain wall MR was seen on the transversal MR curve of the NiFe nanodevice due to the domain wall induced electron scattering. The size effect on the magnetisation switching and interlayer magnetostatic coupling was demonstrated and characterised in synthetic antiferromagnet (SAF)-pinned spin-valve nanorings. It has been clarified by micromagnetic simulations that these nanorings exhibit a double or single magnetisation switching process, which is determined by the magnetostatic coupling as a function of the ring diameter. The interlayer magnetostatic coupling was efficiently reduced in large SAF-pinned nanorings, resulting in a small shift of the minor MR curve, which is beneficial to the magnetic memory applications. In-situ MR measurements and the investigation of domain wall properties have been carried out in FIB patterned NiFe film nanoconstrictions. Spin-valve like sharp transitions were observed on the MR curves in the 80 nm/130 nm wide nanoconstriction devices. However, our analysis of the results by micromagnetic simulations and domain observations with scanning electron microscopy with polarisation analysis (SEMPA) concluded that these sharp MR transitions originated from the anisotropic magnetoresistance (AMR) effect, due to the fast magnetisation rotation in the nanoconstriction, and not from BMR. The numerical investigation has proved that a further reduction of the constriction width/length is necessary for large MR values.

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