Wall energy and wall thickness of exchange-coupled rare-earth transition-metal triple layer stacks

Abstract

The room-temperature wall energy σw=4.0×10−3 J/m2 of an exchange-coupled Tb19.6Fe74.7Co5.7/Dy28.5Fe43.2Co28.3 double layer stack can be reduced by introducing a soft magnetic intermediate layer in between both layers exhibiting a significantly smaller anisotropy compared to Tb–FeCo and Dy–FeCo. σw will decrease linearly with increasing intermediate layer thickness, dIL, until the wall is completely located within the intermediate layer for dIL⩾dw, where dw denotes the wall thickness. Thus, dw can be obtained from the plot σw versus dIL. We determined σw and dw on Gd–FeCo intermediate layers with different anisotropy behavior (perpendicular and in-plane easy axis) and compared the results with data obtained from Brillouin light-scattering measurements, where exchange stiffness, A, and uniaxial anisotropy, Ku, could be determined. With the knowledge of A and Ku, wall energy and thickness were calculated and showed an excellent agreement with the magnetic measurements. A ten times smaller perpendicular anisotropy of Gd28.1Fe71.9 in comparison to Tb–FeCo and Dy–FeCo resulted in a much smaller σw=1.1×10−3 J/m2 and dw=24 nm at 300 K. A Gd34.1Fe61.4Co4.5 with in-plane anisotropy at room temperature showed a further reduced σw=0.3×10−3 J/m2 and dw=17 nm. The smaller wall energy was a result of a different wall structure compared to perpendicular layers.