Magnetic media: faster and smaller

Conventional magnetic recording works by changing the magnetisation states of different domains. An in-plane applied magnetic field takes a few nanoseconds to ‘switch’ the domain state. But Back and colleagues were surprised recently to find that a magnetic field applied perpendicular to a cobalt film can also flip magnetisation states (C H Back et al 1999 Science 285 864). The team believed that a new mechanism must be at work.

When a magnetic dipole is placed at an angle inside a magnetic field, it feels a pull – or torque – that tries to align it with the applied field. Because the dipole has angular momentum, the torque makes the dipole spin – or precess – around its axis until it lines up with the field. Little is known about the dynamics that govern this precession, but Back and colleagues devised the new experiment to establish whether the phenomenon was responsible for reversing the magnetisation in their cobalt film. By imaging the film under an applied perpendicular magnetic field, the team confirmed that precession does lift the dipoles out of the plane of the film, and, with a sufficiently strong magnetic field, will reverse their directions. Crucially, the team found that this ‘precessional switching’ takes place about a thousand times more quickly than conventional domain switching.

“Our observations give clear indications that precessional motion is the quickest way to switch a magnetic state”, ETH team member Danilo Pescia told PhysicsWeb. The dipoles are also smaller than the domains used in conventional recording, and this could lead to higher-density recording media. “Nature might have provided us with a new law”, says Pescia. “Small is not only beautiful, it is also quick”.