Heteroepitaxial growth behavior of Mn-doped ZnO thin films (Zn1−xMnxO) on Al2O3 (0001) substrates by pulsed laser deposition was investigated particularly as a function of Mn content (0.00⩽x⩽0.35) using synchrotron x-ray scattering and atomic force microscopy. The undoped ZnO film was grown epitaxially with a 30° rotation of ZnO basal planes with respect to the Al2O3 substrate while having a small amount of grains with another epitaxial relationship, i.e., the hexagon-on-hexagon growth. A small amount of Mn doping (x=0.05) not only greatly improved the atomic alignment both in the in-plane and in the out-of-plane directions, but also led to a singly oriented film by totally suppressing the hexagon-on-hexagon growth. With further incorporation of Mn into ZnO beyond that content, the epitaxial nature again deteriorated. The superior epitaxial growth of the film with an optimum Mn content is attributed to the formation of much larger grains in it. In addition, our Mn-doped ZnO thin films showed a paramagnetic behavior.
We fabricated synthetic ferrimagnetic layers (SyFL) based on NiFe/Ru/NiFe or NiFe/V/NiFe for the free electrode of magnetic tunnel junctions (MTJ) and characterized their spin flopping, coercive and saturation fields. NiFe/Ru/NiFe SyFL exhibited a coercive field of 10 Oe, which would produce a coercive field of less than 50 Oe when incorporated into sub-micrometer sized MTJ. In comparison, NiFe/V/NiFe SyFL showed extremely low exchange coupling; only when the thickness of the V layer was ~0.8 nm, appreciable level of exchange coupling was observed. The coercive field of the NiFe/V/NiFe SyFL, however, did not have a large dependence on the thickness of V and remained at ~2 Oe. The coercive field of the V-SyFL is expected to maintain below 10 Oe even after patterned into a tunnel junction. Both NiFe/Ru/NiFe and NiFe/V/NiFe SyFLs were stable up to 525 K without showing any gross changes in microstructure and magnetic properties when thermally annealed.
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