Interface-enhanced Magnetization Damping

The piece of material representing a "bit" in a magnetic storage medium is designed to have two stable states that can represent a one or a zero. To switch the orientation of the magnetization from "up" to "down" or vice versa, the system is taken out of its stable state by supplying energy. To return to a stable state, the surplus energy and angular momentum must be removed by dissipation. This so-called "magnetization damping" represents a limit to how fast information can be written onto a magnetic hard disk. For bulk materials, it is described using an equation called the Landau-Lifshitz-Gilbert equation in which the damping is described by a dimensionless constant α introduced by Gilbert and called after him.

In a bulk magnetic material, α is taken to be just that - constant. However, experiments performed on thin layers of the ferromagnetic alloy Permalloy (Py: Ni80Fe20) sandwiched between layers of nonmagnetic metals (NM) show that the damping increases as the thickness d of the magnetic film is reduced (Fig.1) and also that the damping depends on the NM metal used in the sandwich (NM = Cu, Pd, Ta and Pt). The approximately 1/d behaviour of the damping can be explained in terms of energy and angular momentum being "pumped" out of the magnetic material through the interface into the NM material where dissipation occurs; for heavy elements like Pt with large spin-orbit coupling, this dissipation is very large. The damping in a layer of magnetic material can be expressed in terms of the microsopic, quantum mechanical scattering matrix for the layer. We used a method developed in Twente to calculate scattering matrices from "first-principles" to determine the damping (dashed lines). Not only do we reproduce the observed 1/d behaviour, we also reproduce the dependence on NM. Further analysis shows that spin-flipping at the interface plays an essential role.

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Figure 1. Calculated (solid lines) Gilbert damping of NM|Py|NM (NM = Cu, Pd, Ta, and Pt) compared to experimental measurements (dotted lines) as a function of the Py thickness d. Inset: sketch of the structure used in the calculations. The dashed frame denotes one structural unit consisting of a Py film between two NM films.