In a magnetic material, the magnetization usually prefers to point in certain "easy" crystallographic directions because the energy is lower. In a thin film, the easy directions typically lie in the plane of the film. PMA refers to the tendency in some ultrathin magnetic layers for the magnetization to point out of the plane, especially if the magnetic layer is also bounded by top and bottom metallic layers. PMA is one of several phenomena, such as giant magnetoresistance, exchange bias, and spin tunneling, that make devices based on nanolayer structures attractive for magnetic data storage, memory, and related applications.

 

X rays reflecting from the W/B4C multilayer generate a standing wave pattern that extends into the overlying Pd/Co/Pd trilayer (above). The intensity distribution of the standing wave varies with angle of incidence q, thereby providing a way to examine how magnetic behavior varies with depth z in the Co (below).

The Buried Face of Magnetism Revealed

Interest in the novel behaviors that can arise as magnetic materials become thinner and thinner arises in part because they form the basis for advanced magnetic data storage and other devices. Some years ago, researchers discovered the curious phenomenon of perpendicular magnetic anistropy (PMA) in very thin magnetic layers only a few atoms thick that are sandwiched between layers of nonmagnetic metals. Usually, the magnetic field direction lies in the layer, but PMA causes it to sometimes point up or down.

Understanding just why thinness causes these effects requires the ability to peer into each layer and especially at the boundaries (interfaces) between the magnetic layer and its neighbors, which scientists suspect may hold the keys to what is going on. Combining two established x-ray techniques, one that zeroes in on any selected depth below the surface and one that probes magnetism, two Berkeley Lab researchers have used the ALS to demonstrate that magnetic behavior at the interface between a layer of the PMA material cobalt only 2 nanometers thick and bounding palladium metal layers is indeed different from that in the center of the cobalt.

To explain the basics of PMA, researchers often use a phenomenological model due to Néel in which a surface term inversely proportional to the layer thickness lowers the energy of the perpendicular relative to the in-plane orientation as the thickness decreases. However, researchers are not able to rule out other microstructural effects, such as anisotropic strain or chemical intermixing at the interface between the magnetic and metal layers, in part because experimental techniques average over depths of two to three nanometers and thus cannot resolve physical effects localized at an interface.

To address this issue, the Berkeley Lab team adapted the established technique of x-ray standing wave spectroscopy by combining it with circularly polarized synchrotron radiation. Standing waves build up within multilayer mirrors owing to the constructive and destructive interference of the waves reflected from each interface in the multilayer. Such a standing wave will extend through a PMA trilayer grown on top of the standing-wave generator. The difference in the absorption of left and right circularly polarized x rays (magnetic circular dichroism or MCD) at the cobalt L edges probes the cobalt magnetic properties. Since the depths of the periodic intensity maxima depend on the reflection (incidence) angle, it is possible to scan the standing wave vertically through the trilayer and to resolve the depth dependence of the dichroism by varying the angle.

Absorption by Co of a standing wave consisting of circularly polarized x rays varies not only with photon energy and the helicity of the polarization but on the scattering vector Q (related to the reflection angle q). Q scans show that the absorption and, hence, the magnetic properties, are different in the middle of the Co layer (Q = 0.16) and at the interface with Pd (Q = 0.17).

The team chose to investigate a palladium/cobalt/palladium trilayer on a W/B₄C multilayer standing-wave generator. With the cobalt layer thickness used (2 nm), PMA does not occur, but precursor interfacial effects were expected to become prominent. The experimental results showed exactly that, as the researchers obtained cobalt MCD spectra over a range of angles that probed from one palladium interface to the center of the cobalt layer. From the MCD data, they extracted values of the number of missing electrons (holes) in the magnetically active cobalt d states and of the associated spin and orbital magnetic moments. The team interpreted increases in the number of d holes and the orbital moment near the interface in terms of hybridization of cobalt with palladium states at the interface and concluded that the two-term surface magnetocrystalline anisotropy model of Néel is oversimplified.

Analysis of the depth-dependent magnetic circular dichroism spectra yield the number of holes nh in d states and their orbital and spin magnetic moments in the Co layer. The changing values with depth suggest a model attributing the behavior to chemical modifications near the Co/Pd interface.

Research conducted by S.-K. Kim (Berkeley Lab and Korea Advanced Institute of Science and Technology) and J.B. Kortright (Berkeley Lab).

Research funding: U.S. Department of Energy, Office of Basic Energy Sciences (BES). Operation of the ALS is supported by BES.

Publication about this research: S.-K. Kim and J.B. Kortright, "Modified Magnetism at a Buried Co/Pd Interface Resolved with X-Ray Standing Waves," Phys. Rev. Lett. 86, 1347 (2001).

ALSNews Vol. 188, November 14, 2001