Switching Field Distribution of MnGa Bit Patterned Film Fabricated by Ion Beam Irradiation

The magnetization process and switching field distribution (SFD) of ion beam patterned MnGa films were discussed based on the first-order reversal curve (FORC) analysis and magnetic force microscope (MFM) observations. The FORC diagrams with so-called “boomerang” shape resulting from the increase of the interaction between “hysterons” that can be associated with ferromagnetic particles and/or domains were confirmed in the patterned MnGa with dot size larger than 520 nm. On the other hand, the shape of the FORC diagram became a simple Gaussian distribution, which is typical in the media comprised of well-isolated magnetic particles, for dot size smaller than 240 nm. The variation of the shape of FORC diagram was closely linked with the domain structure observed by MFM; multidomain in the dot was confirmed for dot size larger than 520 nm while single domain for dot size less than 240 nm. The average switching field of the patterned film increased with decreasing the dot size, indicating that the dot edge damage and exchange interaction through ion-irradiated regions were negligible in the ion-beam patterned MnGa. The SFD also increased with decreasing the dot size. The SFD is considered to result from the existence of the pinning centers in the as-prepared MnGa and also from the dot size fluctuation of the resist masks fabricated by electron beam lithography. The optimization of the fabrication process will be effective to reduce SFD and sufficiently low SFD in high-density MnGa bit patterned media will be achieved by these process optimizations.