Stripe Domains in Coupled Magnetic Sandwiches
Ultrathin magnetic films a few atoms thick occupy a scientific "sweet spot" at the intersection of theory and application. Potentially lucrative as a medium for high-density data storage, such films are also of fundamental interest because of their low dimensionality, enabling scientists to study systems that model two-dimensional magnetic behavior. Nanostructures of several ultrathin magnetic layers can be engineered to explore many interesting phenomena, including the formation of elongated (stripe) magnetization domains. With the ALS's photoemission electron microscope, PEEM-2, researchers from the ALS, UC Berkeley, and China looked at stripe domains in magnetic sandwiches of cobalt, copper, and iron/nickel. The results revealed a hidden universal dependence of the stripe domain width on variables such as film thickness and external magnetic field.
One issue in magnetic nanostructure research has concerned the presence of long-range order in a two-dimensional magnetic material (for example, spins lined up so as to give rise to ferro- or antiferromagnetism). It has long been established that an isotropic two-dimensional system of spins (a Heisenberg system) does not carry long-range order at nonzero temperature and therefore cannot exhibit magnetic properties—as long as the system is, in fact, isotropic. The magnetic order observed in today's ultrathin films is usually attributed to the existence of magnetic anisotropy—a preferred direction for the magnetic moment. Magnetic anisotropy arises from two sources: the crystal lattice geometry (magnetocrystalline anisotropy) and the shape of a grain of the material (shape anisotropy). In an ultrathin film, the magnetocrystalline anisotropy dominates, and the magnetization is typically perpendicular to the plane of the film. As the film thickness increases, the competition between the two anisotropies results in a spin reorientation transition (SRT) point where the magnetization flips to lie in the plane of the film. Because the two magnetic anisotropies cancel each other out at the SRT, a study of the magnetic phase at the SRT is expected to provide insight into the secret of two-dimensional magnetic long-range order. Early experiments showed that the macroscopic magnetization of a thin film breaks into stripes near the SRT, raising many challenging questions about the nature of this stripe phase. Answering these questions requires observation of the magnetic stripes within an external magnetic field. However, such observations are inhibited by the difficulty of operating an electron microscope within a magnetic field. In this experiment, the issue was addressed by using magnetically coupled layers in which the interlayer coupling served as a virtual magnetic field.
A uniform thickness of ferromagnetic cobalt was separated from a wedge of ferromagnetic iron/nickel by a nonmagnetic wedge of copper. The SRT occurs in the Fe/Ni layer, with the magnetic anisotropy tuned by the Fe film thickness. The interlayer coupling between the Fe and Co layers serves as an in-plane virtual magnetic field whose strength is tuned by the Cu spacer layer thickness. The experiment was performed using PEEM-2 with circularly polarized light from ALS Beamline 7.3.1.1. Magnetic domain images were obtained by taking the ratio of L3 and L2 edges, utilizing the effects of x-ray magnetic circular dichroism. Domain imaging of the Fe/Ni layer revealed how the stripe domains change as a function of the magnetic anisotropy (Fe film thickness) and the in-plane magnetic field (Cu film thickness). 
The Fe/Ni stripe orientation was aligned with the Co in-plane magnetization, and the stripe domain width decreased exponentially with increasing interlayer coupling between the Co and Fe/Ni films. To understand the experimental observations, the researchers developed a theoretical model of the ultrathin Fe/Ni film on a two-dimensional square lattice, relating the stripe domain width to the interlayer coupling and the perpendicular anisotropy. The results agreed nicely with the experimental data, taking into account the exchange interaction, magnetic anisotropy, dipolar interaction, and magnetic Zeeman energy. Research conducted by Y.Z. Wu and C. Won (University of California, Berkeley); A. Scholl and A. Doran (ALS); H.W. Zhao (University of California, Berkeley, and Institute of Physics, Chinese Academy of Science); X.F. Jin (Fudan University, China); and Z. Q. Qiu (University of California, Berkeley, and Berkeley Lab). Research funding: National Science Foundation; U.S. Department of Energy, Office of Basic Energy Sciences (BES); Chinese National Science Foundation; Chinese Ministry of Science and Technology; and Chinese Academy of Science. Operation of the ALS is supported by BES. Publication about this research: Y.Z. Wu, C. Won, A. Scholl, A. Doran, H.W. Zhao, X.F. Jin, and Z.Q. Qiu, "Magnetic stripe domains in coupled magnetic sandwiches," Phys. Rev. Lett. 93, 117205 (2004). |
More ALS Science