The study of the topology and structure of magnetic domains is one of the active topics in modern research in nanomagnetism. Beamline 9 Mistral offers the possibility of high resolution imaging of domains using the microscope and the circular magnetic dichroic absorption contrast. Thanks to the stability of the microscope and beamline optics, it is possible to achieve detailed information on the changes of a specific magnetic domain when a pulsed magnetic field is applied.
The samples analyzed were thin films (80 nm thickness) of Co5Nd alloys grown by sputtering techniques at the University of Oviedo and were deposited on silicon nitride membranes fabricated at the Centro Nacional de Microelectrónica (CNM-CSIC), located in Bellaterra, that fit exactly the geometry of the microscope sample holder. The pulses of magnetic fields were created with a microcoil developed at ALBA. They could achieve +/- 1 T as a maximum amplitude and had a duration of about 15 µs.
The figure at the top displays the characteristic labyrinth domains which are the most common magnetic structure at remanence after applying saturation pulses either positive or negative. Black and white domains correspond to inward and outward perpendicular magnetizations. The inset is the Fourier transform which shows a ring pattern indicating azimuthal disorder. The inverse of the thickness of the ring gives the lateral correlation length of the distribution which is about 120 nm.
The new and somewhat surprising result observed is displayed in the lower figure. Applying pulses of selected amplitudes below the saturation of the perpendicular magnetization, resulted in a bubble domain structure with a very narrow distribution of the dimensions of the bubbles which had a diameter of about 60 nm. The bubbles have mostly 6 fold and also 5 fold coordination and do not display long range order as shown by the annular shape of the Fourier transform in the inset.
These are the initial results of a systematic exploration of the different domain topologies that will be carried on the near future.
Fig. Images of the experiment showing, in the first image, the characteristic labyrinth domains and, in the second image, the bubble domain structure. The insets are the corresponding Fourier transforms.
Reference: Universidad de Oviedo: Carlos Quiros, Cristina Blanco, María Velez and José M. Alameda Centro Nacional de Microelectrónica (CNM-CSIC) : Jaume Esteve, Marta Duch and Nuria Torras ALBA: Ricardo Valcárcel, Jose Ávila and Óscar Matilla for the development of the microcoil, Eva Pereiro, Andrea Sorrentino and Salvador Ferrer for performing the measurements.
