Magnetic properties and domain structures in narrow NiFe stripes

The coercivity of narrow stripes of 81 percent Ni-19 percent Fe films has been found to increase rapidly, by up to an order of magnitude, as the stripe width decreases and approaches 1 μm. Furthermore, the coercivity increases with decreasing film thickness as it does in sheet films, but in the stripes the changes in coercivity with film thickness are much larger than in sheet films. For example, in stripes of 2 μm × 100 μm, Hcincreases from 10 Oe to 35 Oe as the thickness decreases from 1800 Å to 300 Å. The increase in coercivity with decreasing stripe width may be explained by a buckling of the magnetization perpendicular to the length of the stripe. This buckling process was made visible by decoration of domain walls with Ferrofluid, and is shown to lead to the formation of walls perpendicular to the stripe. These walls do not move, but block reverse domains from propagating down the stripe. In the narrowest stripes (2 μm) fields larger than 100 Oe are required to collapse the 360° wall segments which eventually form. A theoretical model for this buckling process is given which shows that a minimum in energy occurs when the stripe buckles with a buckling wavelength approximately equal to the width of the stripe. These findings suggest that, as structures of NiFe like those in magnetic bubble devices and in magnetic recording heads are made smaller, coercivity and dispersion will rise significantly, leading in many cases to undesirable magnetic behavior.