New NIST measurements address response of media to rapidly changing magnetic fields as encountered in magnetic recording. (News Briefs).

NIST scientists have made quantitative, vectorial measurements of magnetization dynamics of Ni-Fe magnetic thin films, simultaneously at the surface and approximately 50 nm in the interior. The measurements were performed using the linear and non-linear magneto-optical Kerr effects. These measurements address the problem of the inhomogeneous magnetization response of magnetic materials when subjected to rapidly changing magnetic fields.

When the rate of change of the applied field approaches the characteristic response time of the material (the precessional frequency, typically several gigahertz), the magnetizations response can become complicated. At these frequencies, the magnetization does not simply align with the field, but instead swings toward the field and oscillates (precesses) around it before finally settling into the field direction. However, it was not known whether the surface and the interior of the magnetic material reacted to the applied field in the same way. The linear effect is sensitive to the magnetization in the interior of the film, while the non-linear effect is sensitive only to the magnetization of the first few atomic layers.

NIST scientists induced rapid, near 90[degrees] rotations of the magnetization of Ni-Fe films in a geometry similar to that of the ferromagnetic cores of magnetic recording write heads. The rotation of the magnetization vector was probed with a 50 fs laser pulse. The system had an overall temporal sensitivity of 50 ps and a sensitivity to magnetization angle of about 3[degrees]. They found that, contrary to the expectations of some models for magnetic response, the surface and the interior region responded identically. Detailed measurements showed that the magnetization exhibited a fast rotational response over about one nanosecond followed by a smaller, slow response over tens of nanoseconds.

These effects will soon be important in magnetic data storage devices as their data rate increases. Disk drives store information by switching small regions of magnetic material to represent binary data. It is necessary to have a detailed understanding of the induced precessional magnetodynamics in order to optimize the recording process. Modeling of these large, rapid magnetization motions in real materials is difficult, so direct dynamic measurements are important for continued device development.

Work is ongoing to study the dependence of the response on the thickness of the film to find when the response of the surface might deviate markedly from the interior due to eddy currents. The results were presented at the Magnetic Recording Conference in Minneapolis in August 2001.

CONTACT: Ron Goldfarb, [phone]; goldfarb@ boulder.nist.gov.