Magnetisation processes in interacting element arrays and single layer films

The magnetic interactions in arrays of discrete magnetic elements, and ultrafast effects in both elements and continuous magnetic films were investigated in order to better understand the physics of magnetism at this fundamental level. The first theme involves the study of discrete permalloy elements of rectangular, triangular and rhombic shape, focusing on the nature of shape anisotropy and how it effects magnetic interactions between elements in large arrays. The second theme included ultrafast effects from thermal laser light on continuous permalloy films, 300 µm squares and arrays of rhombic elements, in addition to the effect of pulsed magnetic fields on 10 µm squares.

The shape anisotropy resulted in the formation of particular domain structures inside the elements, with the corresponding emanating stray fields causing the rhombic elements to exhibit the strongest interactions with their neighbours. The weaker interactions between the triangular and rectangular elements resulted in a higher energy state in the as-deposited arrays, and required thermal annealing at 300°C in order to induce the ground state. The behaviour of arrays of rhombic elements was determined as a function of the applied in-plane magnetic field and temperature. The results showed that the elements switched with a help of a rotating domain in the centre of the element. The switching field was found to be independent of the array spacing. However, the distribution of switching fields was narrowed due to interactions with neighbouring elements. The interacting arrays of rhombic elements were represented as a two-dimensional Ising model, with the energy functions plotted using the Landau approximation.

The effects from an ultrafast (100 fs) laser pulse showed thermal activation induced in magnetic film deposited on silicon, sapphire and silicon carbide substrates. Different substrates were used in order to see how the effects of the substrate changed the magnetic pattern written with the laser light. Demagnetisation effect was induced in the film on silicon substrate at low intensities 102 mJ/cm2, the intensities ≤103 mJ/cm2 induced ablation effect. The film deposited on sapphire and silicon carbide substrates required higher intensities to induce demagnetisation (~103 mJ/cm2) and ablation (>103 mJ/cm2) effects. Magnetic films deposited on silicon substrates required a lower fluence to become ablated compared to films deposited on silicon carbide or sapphire. This is due to a combination of different thermal conductivity and melting temperature of the substrates. It was found that the magnetic pattern written with the laser pulse in the film was different for continuous magnetic films (only a few domains) from that in patterned elements (a more complicated network of domains).

Simulated results of the magnetisation dynamic effects induced in 10 µm size square elements by a magnetic field pulse were performed for various applied static field strengths and directions. The direction was either parallel to an edge or the diagonal of the element. Magnetisation oscillations appeared more heavily damped with the static magnetic field applied parallel to the diagonal of the square. Dynamic images illustrated that this effect is related to static magnetisation non-uniformities at the centre of the element. The magnetisation oscillations persisted longer when a stronger static field was applied, which also resulted in an increase of the oscillation frequency. This effect can be described using the equation of Lanadau-Lifshitz for the small amplitude uniform precession frequency, proportional to the square root of the applied field. All simulations were corroborated with experimental results performed at the University of Exeter.

These thin film magnetic experiments have demonstrated how shape anisotropy in discrete elements affects the functionality of arrays of discrete interacting magnetic bits. Further, the ultrafast thermal and pulsed magnetic field effects illustrate the future impact those could have to improve writing time in magnetic media. The thin film magnetic elements were prepared using standard methods of electron beam lithography. The magnetic analysis was performed using magnetic force microscopy and transmission electron microscopy techniques. The ultrafast thermal effects utilised a single shot laser pulse, whilst the magnetic field pulse were investigated using a pump-probe system based on magneto optical scanning Kerr effect.