Calculations of the Structure of Néel, Bloch, and Intermediate Walls and the Influence of Their Stray Fields on Bitter Powder Patterns

Abstract

A comprehensive computer investigation of Néel, Bloch, and intermediate walls in films 200 Å to 2000 Å thick was made by letting the computer adjust the magnetization direction in small segments of the wall until the sum of the torques on the magnetization in each segment was minimal. Many parallel walls lying parallel to the easy axis were assumed to exist as in a real film during hard axis remagnetization. The Néel wall width was found to be approximately proportional to φ_f⁻¹, where φ_f is the angle between the magnetization in the center of the domain and the hard axis. Intermediate walls were found to exist for 90° > φ_f > φ_c, where φ_c is a critical angle dependent on thickness. For φ_f ≤ φ_c pure Néel walls exist and only for φ_f = 90° can pure Bloch walls exist. Once the magnetization distribution was known, the stray field from the wall was calculated. For Néel walls, the magnitude of the stray field was found to increase slightly as the film thickness increased (500 < T < 2000 Å) for a fixed‐angle wall and to increase linearly as the wall angle increases for a fixed film thickness. For intermediate walls, the magnitude of the Bloch component of the stray field was found to be much less than the magnitude of the Néel component. This is due to differences in the Néel and Bloch wall magnetization tails, i.e., that portion of the wall where the magnetization angle changes slowly as opposed to the central portion of the wall where the magnetization angle changes rapidly. The binding energy of the Bitter powder colloid at the wall is proportional to the stray field; Néel walls are experimentally observed to have greater visibility.