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Ultra-fast magnetisation dynamics

The magnetisation dynamics of magnetic thin films on time scales below one nanosecond is of great importance for numerous applications. It is usually characterised by inductive methods which, however, come up against their limits already when the single structures are of micrometer size. A method that has been developed at PTB now allows the ultra-fast magnetisation dynamics of single magnetic micro- and nanostructures to be measured by way of a simple measurement of the electrical resistance.

Magnetic memories and sensors are present in the most diverse areas of our everyday life. On hard disks, for instance, digital information is stored in a magnetic film that is only a few nanometers thick. Since such magnetic memories and sensors will have to work ever faster in the future, the requirements for metrology, e.g. in investigating the writing speed of magnetic memories or the reaction time of magnetic sensors, are also increasing.

Now, a method has been developed at PTB by means of which the fast magnetisation dynamics of single magnetic nanostructures can be detected by way of a simple measurement of the electrical resistance. The magnetisation of a ferromagnetic material reacts to a short magnetic field pulse with the so-called ferromagnetic precession. Thereby, the magnetisation circles around the applied magnetic field with frequencies in the GHz range. The characteristic parameters of this precession are, on the one hand, the frequency and, on the other hand, the decay behaviour, i.e. the magnetic damping.

The new measurement procedure makes use of a universal physical effect of ferromagnetic materials – the so-called anisotropic magnetio-resistance – to determine these technically relevant parameters. If the direction of the magnetisation of the ferromagnet changes, this also causes a small change in the electrical resistance. In a temporally resolved measurement of this change in the resistance, the precession motion could now be observed for the first time in this way. Thereby, the precession with a certain frequency has led to a periodical change in the resistance with the same frequency as the one that had been determined by means of an oscilloscope.

Besides simplicity, the method also has the advantage of being universally applicable. As practically all magnetic materials show a sufficiently strong anisotropic magnetio-resistance, most magnetic thin films and nanostructures can be characterised with the aid of this method.