Ultrafast helicity-dependent all-optical switching in hybrid magnetic nanomaterials

Information technology has transformed our lives, connecting us and providing access to knowledge that was previously the preserve of experts and scholars. And yet today's technology is only a beginning. Embedding information gathering and remote control of objects that we either use or wear will lead to an Internet of Things that greatly extends the amount of information that we need to process and store. For this vision to be realised, the capabilities of the underlying hardware must continue to advance at breakneck speed. The principal focus of this project is on how we might store information in future.

Today we are increasingly dependent upon cloud computing that stores and retrieves information from data centres that contain enormous arrays of magnetic hard disk drives (HDDs). We have become accustomed to the idea that the capacity of each HDD will increase every year so that increased demand for storage capacity can be met by regularly replacing each HDD. However, the success of this strategy is now uncertain because the continued miniaturisation of technology needed to increase capacity has reached physical limits that are not easily overcome. Specifically, the size of the region on the surface of a disk that is used to store one "bit" of information (1 or 0), has become so small as to be unstable on the timescale of 10 years that is the industrial standard for date retention. Materials with enhanced stability have been developed, but it is not possible to switch their magnetization, i.e. write data, with the magnetic field available from a conventional magnetic recording head. This has generated intense interest in new mechanisms for magnetic switching that can bypass this seemingly unavoidable bottleneck.

This project will explore how light may be used to switch the magnetization of new magnetic materials in which the electronic and magnetic properties can be tailored to optical control. In 2004 it was shown that atomic monolayers of graphene can be peeled from a crystal of graphite, a form of carbon, by a technique known as mechanical exfoliation. The technique is effective because graphene layers are only weakly bonded to their neighbours by what are known as van der Waals forces. In fact, there are many other crystals with similar bonding from which few and monolayer films may be exfoliated. By exfoliating layers from different crystals and stacking them to form a multilayer, it is possible to create hitherto unknown hybrid materials that can combine the favourable properties of their parent crystals. Furthermore, the interface between successive layers can be extremely clean and well ordered. Here the aim is to combine 2 dimensional ferromagnetic (2dFM) layers that have permanent magnetic order with semiconducting transition metal dichalcogenide (TMDC) layers in which electrons can be optically excited with very high efficiency. Furthermore, it is possible to excite electrons that have a magnetic moment, which is associated with their quantum mechanical "spin", with direction determined by the polarization of the incident light.

Experiments will be performed in which an ultrafast laser pulse with duration less than 1 trillionth of a second is used to excite electrons in the TMDC layer so that their magnetic moments can interact with the magnetic moments in the 2dFM layer. By controlling the direction of the excited magnetic moments through the polarization of the light, the aim is to switch the magnetization of the 2dFM backwards and forwards at will. Furthermore, the manner and timescales on which the magnetization changes will be determined by using a second laser pulse to interrogate the instantaneous magnetic state at a time of our choosing. While the initial goal is to observe and understand the mechanism of all-optical switching, the ability to combine many different materials will facilitate the search for the combinations that are best suited to data storage applications.