PhD Studentship: Deterministic Recording via All-optical Magnetic Switching

Our aim is to control the magnetic state of a material using only pulses of light. “Switching” the magnetization of a magnet means that its north and south poles are interchanged without mechanically inverting the magnet.  Magnets are normally opaque so that incident light only affects ten to one hundred layers of atoms closest to the surface.  Therefore, the magnet must be sufficiently thin if light is to influence all the atoms present. The magnetization derives from the “spin” angular momentum of electrons within the material. We can visualise each electron spinning about an axis through its centre, with the axes of all electrons aligned in the same direction.   

While light interacts strongly with the charge of an electron, the interaction with its spin is very weak.  Therefore, a different approach is required if light is to strongly influence the magnetization. Two principal strategies will be explored in multi-layered thin film structures in which electrons with aligned spins can be transferred between layers.  There will be no net transfer of charge, so that only spin current flows between layers.  In a first case, trilayers will be studied in which two ferromagnetic (FM) layers are separated by a non-magnetic metal (NMM) layer.  Excitation by a laser pulse with duration ~10-13 seconds will heat the FM layers, cause their magnetization to decrease, and drive a spin current into the NMM layer.   

The FM layers will be designed so that one (FM1) demagnetizes more strongly than the other (FM2) so that a net spin current flows between the two.  By controlling the intensity of the light, the spin current will be used to switch the magnetization of FM1 into a desired state.  In a second case, a FM layer will adjoin a NMM bilayer consisting of a light metal (small atomic number), such as Cu, and a heavy metal (large atomic number), such as Pt.  Light with circular polarization will induce a circular motion of electrons in the light metal.  The associated orbital angular momentum will then be converted into spin angular momentum in the heavy metal before passing into the FM to control the orientation of the magnetization. Robust optical control of magnetism would have enormous consequences for magnetic hard disk drive (HDD) technology.  

The magnetic disk coating is used to store “bits” of information, with the two orientations of the magnetization representing binary digits “0” and “1”.  In a conventional HDD a miniature electromagnet is used to switch the magnetization of the bits.  However, as HDD capacity has increased, the size of each bit has decreased, and the energy barrier between the two magnetic states has been increased so that the magnetization remains thermally stable.  The problem is that the “write” magnetic field required to switch the magnetization has increased beyond what any electromagnet can provide. The mechanisms developed within this project will be needed if the capacity of HDDs is to continue to grow.

Funded by: EPSRC Engineering and Physical Sciences Research Council