Magnetic ripple in Permalloy narrow wires investigated by Lorentz microscopy

We investigate the magnetic and crystalline structures of Permalloy narrow wires by means of Lorentz microscopy and standard transmission electron microscopy. Permalloy wires with highly anisotropic cross section and fine grains less than 10 nm fabricated by electron beam heating evaporation method exhibits uniformly-magnetized state along the wire direction due to strong magnetic shape anisotropy. On the other hand, Permalloy wires fabricated by resistance heating evaporation method are found to present magnetic ripple or uniformly-magnetized state depending on the aspect ratio of the wire. In the latter case, the magnetic ripple appears when in-plane magnetic anisotropy perpendicular to the wire direction induced by large crystalline grains, crystalline and interstitial defects, and strain in the substrate is comparable to shape anisotropy. Controlling the crystalline structures is useful to tune the effective in-plane magnetic anisotropy and induce magnetic ripple in the wire, where the magnetic domain and domain wall dynamics using the spin current will exhibit interesting features reflecting inhomogeneous magnetic state which favors the spin transfer torque.