Abstract
Epitaxial Cu/Ni/Cu (001) films exhibit perpendicular anisotropy over an exceptionally wide thickness range (30 Å⩽Ni⩽145 Å).12 Magnetic force microscopy (MFM) has been used to reveal new details of the magnetization process;34 here we focus on two features. (1) the forces of attraction and repulsion between different segments of domain walls and (2) a bimodal distribution of magnetization response times to perpendicular applied fields. Domain images were taken using high-resolution scanning MFM in perpendicular fields up to 500 Oe. The following observations are explained. As high energy domains are reduced in area by a perpendicular applied field, the domain patterns (whose wall orientations in zero field have no correlation with the easy in-plane 〈110〉 directions) evolve to a serpentine pattern. Some lengths of the serpentine domains collapse completely while others shrink in width with opposite walls failing to annihilate each other even in fields up to 500 Oe. These “hard domains” show a preference for alignment with the easy 〈110〉 directions. We believe the annihilation and hard domain behavior to be due to the combination of opposing short-range exchange and long-range dipole interactions between lengths of domain wall having the same or opposite chirality; chirality along a domain wall changes at a Bloch line. The alignment of hard threadlike domains with the 〈110〉 directions is due to the magnetoelastic interaction between the misfit dislocation strain field and the in-plane magnetization of the domain wall. Films of Ni thickness up to 85 Å can be saturated and show a remanence ratio of unity by MFM, vibrating sample magnetometry and magneto-optic Kerr effect in perpendicular fields. On the other hand, for 100 Å of Ni, the remanence ratio is smaller and the MFM images at 500 Oe show a small fractional area of unreversed domains; thicker films show larger unsaturated fractions. The magnetization process in films of 75–100 Å Ni is revealed in the MFM images to have an instantaneous field response and a slower time response (in constant field) over a period of several minutes. The instantaneous response is due to the motion of glissile domain wall segments; the after effect appears to be due to the thermally activated motion of Bloch lines along the hard wall segments.
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