Picosecond Dynamics of Magnetic Exchange Springs

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Ferromagnetic materials are found throughout the electromagnetic technology upon which modern life depends. They range from the bulk materials found in motors and dynamos to thin films used to store data in hard disk drives. Within a ferromagnet each atom has a magnetic moment, like planet earth, with north and south poles. The magnetic moments of adjacent atoms are forced to point in the same direction by the exchange interaction (EI), a purely quantum-mechanical effect, which is the most powerful force in magnetism, generating effective magnetic fields up to one hundred million times as strong as the earth's magnetic field.Our everyday experience is that some ferromagnets remain permanently magnetized while others do not. In the latter case, the magnetic moments have parallel alignment within microscopic regions known as domains, but different domains have magnetic moments pointing in different directions, so that there is no net magnetic moment overall. Neighbouring domains are separated by domain walls, about 10 nm (100 atomic diameters) wide, through which the orientation of the magnetic moments gradually rotates in a helical structure. The finite width of the domain wall is a consequence of the EI and the wall stores exchange energy like a spring. The proposed project is concerned with exchange spring (ES) structures that form through the thickness of multilayered thin films. Alternate layers are termed hard and soft because it is easier to form the helical structure in the latter. The helical structure is induced either by applying a magnetic field or by changing the relative alignment of the magnetic moments in different hard layers so as to twist the magnetic moments in the soft layers in between. By studying the form of the ES structure, and its response to external stimuli, we can obtain information about how the strength of the EI varies through the structure.The EI present in perfect crystals can already be calculated accurately. However, the magnetic materials used in the strongest permanent magnets, or as recording media in hard disk drives, are far from perfect and consist of nanoscale crystallites that interact with each other through the EI at their grain boundaries. Furthermore, the next generation of magnetic recording technology will use the combined influence of a magnetic field and a short laser pulse to switch the orientation of the magnetic moments so as to represent binary information. Rather little is known about the EI within the grain boundary regions, or how the EI is modified immediately after application of a laser pulse. The aim of this project is to use ES spring structures to obtain new information about the EI in such circumstances.State of the art thin film deposition will be used to fabricate ES structures in which the atomic scale structure can be carefully controlled so that the relationship between magnetic and structural properties can be better understood. Microwave radiation will be used to excite the ES so that magnetic moments oscillate with characteristic frequencies that allow the strength of the EI within different regions of the ES to be deduced. In particular, x-rays will be used to detect the motion, since by tuning the energy of the x-ray photons obtained from a synchrotron, the response of different atomic species can be separately determined, providing more detailed information of the mode of oscillation. Finally, the ES will be excited with an ultrafast laser pulse to soften the magnetic moments within one or more hard layer so that the ES can unwind. This unwinding motion will provide information about how the magnetic parameters of the material, including the EI, are modified by the laser pulse, and the conditions required for the magnetic moments of the hard layer to switch their orientation will be explored. The potential of ESs as laser assisted recording media will hence be determined.