by Samuel Allen
An international team of researchers has discovered a new kind of magnetic state controllable by an electric field – a leap forward for developing spintronic devices1.
Many readers will be familiar with antiferromagnets – the simplest kind being a trivial “Néel” state where adjacent electron spins arrange antiparallel, leaving no net magnetisation. More exotic states with no net magnetisation also exist, like quantum spin liquids and spin density waves. One such previously predicted state was a “p-wave” state, a kind of zero-magnetisation state where the spin-polarised electronic structure breaks parity i.e., the energy is not symmetric in momentum space, meaning the energy of electrons is different if their momentum is reversed. Researchers have just found that NiI2 harbours such a state – and moreover, they can control it using electric fields.NiI2 forms a layered structure of antiferromagnetically coupled Ni2+ ions (S=1) forming a triangular lattice. This is an example of a frustrated geometry where a Néel state is impossible. Instead, the system is driven into a new helimagnetic state that is the first experimental realisation of a “p-wave” magnet.
In this state, the spins form helices in the planes, where each spin is rotated relative to the previous. Helices are chiral – they are not superimposable on their mirror images – and define a unique axis depending on which way they turn (their “handedness”). This chirality has two important physical consequences.
Firstly, virtual electron transfer due to quantum fluctuations (which is responsible for magnetic coupling), combined with the chiral spin structure, causes a small electrical polarisation to develop. This spin-current polarisation means NiI2 is ferroelectric as well as magnetic – a special kind of material called a multiferroic. Crucially, because the polarisation is coupled to the magnetic ordering, applying a voltage to switch the polarisation also reverses the handedness of the helices.
Secondly, the handedness of the helices introduces a preferred spin orientation for free electrons depending on the direction they conduct in. This is the characteristic that breaks parity and makes these “p-wave” magnets.
Combined, these effects allow electron spins to be flipped by applying a small voltage. This is a major goal of spintronics, which seeks to use electron spin instead of charge to store and manipulate information. Spintronic devices could allow an order of magnitude increase in data density and huge reductions in energy use compared to conventional devices. This arises because spins can be flipped without moving, whereas changing electric polarisation involves moving charges, which scatter and cause dissipative loss.
Now that a switchable p-wave magnet has been identified, the next challenge is to make one that works at room temperature and apply it to a real-world device.
