Some magnetic data memories contain countless microscopically small magnetic particles. The permanent magnetization of such a particle can point in two opposite directions and thus store one bit of data. With a sufficiently strong magnetic field, the magnetization direction can be reversed, whereby a hysteresis as known from macroscopic magnets can occur. In order to increase storage density, the magnetic particles need to be decreased in size, which can change their behaviour during the magnetization reversal.
Dr. Martin Heumann and Dr. Thomas Uhlig from Prof. Josef Zweck's working group in Regensburg were able to record – with the aid of a modified transmission electron microscope – the shape of hysteresis curves of single magnetic nanoparticles. They were awarded the 2005 Helmholtz Prize for their work entitled "Measurement of hysteresis curves of single magnetic particles in the nanometre range" which resulted from their doctoral theses.
The two young researchers conducted their investigations on disk-shaped nanomagnets made of permalloy, which were 6 to 8 nm thick and had a diameter down to 100 nm. They used an electron hologram to visualize the magnetization pattern in such nanoparticles. To do this, they placed the particle in the electron beam of a specially modified transmission electron microscope so that it was traversed by an electron wave which was then superimposed with a reference wave and was brought to interference. The result was a light-dark pattern with areas of high and low electron beam intensity, the so-called electron hologram, which is essentially an interference pattern.
Such a hologram contains the information about the phase shifts of the electron wave. These are caused by the Lorentz force, which is due to the interaction of the electron wave with the magnetization of the particle. Hence, from these holograms it was possible to calculate the magnetization and measure the hysteresis loop of a remagnetization cycle.
In this way, the two scientists were able to obtain the magnetic configuration of a nanoparticle with a high spatial resolution of less than 10 nm which had so far been unprecedented. They investigated the magnetization reversal of the nanoparticles by applying an external magnetic field to the particles. When this field reached the so-called switching field, the direction of the magnetization was reversed, as the researchers could see from the microscope images. Thereby, each particle exhibited an individual hysteresis curve which could show several magnetization steps.
Even if two nanomagnets had been manufactured in the same way, their hysteresis curves and their switching fields could differ considerably from each other. One reason for this was that even small deviations in the shape of the nanoparticles have major effects on their magnetic behaviour. Apart from this, Heumann and Uhlig were able to observe the magnetization reversal of a single magnetic domain in very small particles for the very first time. The individual behaviour of nanomagnets must be taken into account if magnetic memories are to be produced on the basis of nanoparticles.
