Origin of transverse magnetization in epitaxial Cu/Ni/Cu nanowire arrays

Abstract

The patterning-induced changes in the magnetic anisotropy and hysteresis of epitaxial ͑100͒-oriented Cu/ Ni͑9, 10, 15 nm͒/Cu planar nanowires have been quantified. When the Ni films are patterned into lines, strain relaxation leads to a thickness-dependent net in-plane anisotropy transverse to the lines. The magnetoelastic anisotropy was found from the three-dimensional strain state measured directly by synchrotron x-ray diffraction and has a value of −21 kJ/ m 3 for 10-nm-thick nanowires. The angular dependence of the remanence of the nanowires indicates that the in-plane easy direction is the result of the competition between the cubic magnetocrystalline anisotropy and a uniaxial anisotropy that includes shape and magnetoelastic effects. The patterning-induced changes in magnetoelastic anisotropy, combined with the shape and magnetocrystalline anisotropies, quantitatively explain the net anisotropy of the nanowires. Thus by controlling the film thickness and wire orientation, the easy axis direction of the nanowires may be controlled.

Strain-induced magnetization reorientation in epitaxial Cu/Ni/Cu rings

“…Although MFM is not usually a quantitative technique, it was shown that wires with perpendicular magnetization exhibit contrast directly above the magnetic area, 10 while, if M is transverse to the wire axis, the stray field has appreciable values away from the edges of the magnetic area. 7 Comparison of the topographical and magnetic images as well as the linescan in Fig. 1(j) shows that the magnetic contrast extended outside the area of the rings.…”

“…6 For planar nanowires patterned from such films along the [100] direction, the breaking of the in-plane strain isotropy induces a ME anisotropy that produces a transverse orientation of the in-plane magnetization with respect to the stripe length. 7 Here we demonstrate the magnetic properties of Ni rings made from epitaxial Cu/Ni/Cu films as a function of the ring width and clarify the internal structure and energy of the domain walls in radially magnetized rings. The study was carried out in a structure with the Ni layer thickness set at 14 nm, a value where the net perpendicular magnetic anisotropy is small 8 and therefore effects due to nonuniform strain states are expected to be dominant.…”

“…This procedure is analogous to the process used in previous work to obtain arrays of epitaxial nanowires. 7 The magnetic microstructure was imaged by magnetic force microscopy (MFM) in the tapping-lift mode, detecting the phase shift with commercial low moment tips magnetized along the tip axis. The dark and bright contrast originate from the stray fields present at the edges of the rings or due to the formation of domains with out-of-plane magnetization.…”

“…K p depends on the strain state and a decrement with respect to the unpatterned film has been observed. 7 The value used here is roughly 75% of the value obtained for the unpatterned 10-nm-thick Ni film from the strain measured by x-ray diffraction.…”

“…9 Strain relaxation occurs normal to the ring edges and favors the in-plane orientation of M along the radial direction of the ring, as we observed previously in a narrow ring 9 and stripes. 7 For wide rings this radially magnetized structure coexists with perpendicular stripe domains in the less-relaxed regions of the ring away from the edges, whereas for ring widths below ≈400 nm a fully radial domain configuration is present. Unlike rings made from films with in-plane magnetization, the magnetization reversal proceeds by the nucleation of reverse domains that generate additional DWs in the ring.…”