Digital information plays a central and ubiquitous role within our modern society. Within hard-disk-drives, data is most commonly encoded in the form of “bits” with magnetization pointing up or down (corresponding to ones or zeros). To therefore write data, one needs to switch these magnetic bits from one polarity to the other, most commonly achieved using a strong and localized magnetic field. However, data centers are now struggling to cope with the already-enormous and exponentially-growing amounts of data in the world. There is therefore a very pressing need to develop faster approaches for reversing magnetization with much better energy-efficiency.
In recent years, breakthrough experiments have demonstrated that circularly-polarized femtosecond-long pulses of light can slightly “push” magnetization via the so-called inverse Faraday effect. Depending on the handedness of the light’s corkscrewing electric field, the deflection can act either upwards or downwards. However, the fundamental pathway of energy flow (from light to electrons to spins) is very lossy and restricts the functional usefulness of the push to the sub-picosecond lifetime of the photo-excited electrons. Put simply, this approach is too fast for its own good to switch magnetization.
Davies’ project aims to reveal an alternative and potentially superior method for selectively switching magnetic order, based instead on exploiting vibrations of the crystal lattice. Specifically, the project will explore how circularly-polarized oscillations of the lattice can switch magnetization. “A major advantage of this approach comes from the fact that the crystal lattice – a geometrical network of many different elements - has many different frequencies of resonance, corresponding to so-called optical phonons. By driving these phonons at resonance, the resulting push that acts on the magnetization may even become strong enough to switch it”, Davies explains. “This approach benefits hugely from the longer lifetime of the phonons, which are usually on the order of picoseconds. Moreover, we should be able to steer the direction of the magnetic reversal via the helicity of the chiral lattice vibrations.”