Magnetic anisotropy in permalloy antidot square array investigated by ROTMOKE and MTXM

Magnetic anisotropy of Permalloy (Py) antidot square array was investigated systematically by a torquemetry method using Rotation Magneto-Optic Kerr Effect (ROTMOKE). In addition to the expected 4-fold magnetic anisotropy, we find that there also exists an unexpected uniaxial magnetic anisotropy in ROTMOKE result. By a careful analysis, we show that this uniaxial anisotropy is a general artifact of torquemetry methods due to non-uniform magnetizations which usually exist in patterned artificial magnetic structures. We show that both the 4-fold anisotropy and the artifact of the uniaxial anisotropy are due to the periodic wiggling of the magnetization in space which is confirmed by Magnetic Transmission Soft X-ray Microscopy (MTXM) and supported by micromagnetic simulations via MuMax3. Simulations also reproduce the artifact of the uniaxial anisotropy in ROTMOKE result. A simplified model is then developed based on the periodic wiggling of the magnetizations and successfully explores the physical origin of the field-dependent 4-fold anisotropy and the artifact of the uniaxial anisotropy.