The modern world is ever more reliant on electronics, with devices so ubiquitous that we carry them around in our pockets. But they'll struggle to cope with the deluge of data we produce, and the search is on for their replacement. One of the options for producing faster devices that store more data but use less energy is to switch to spintronics. While electronics utilise an electron's charge, spintronics use a different fundamental property - spin - and its associated magnetic moment. In order to make these next-generation devices a reality, we need to understand a lot more about how spin behaves. In work recently published in Nature Communications, the I21 in-house team, together with international collaborators, has used Resonant inelastic X-ray scattering (RIXS) to probe higher order spin excitations in an integer spin material. Their results demonstrate the utility of RIXS to study these higher order spin excitations and uncover previously hidden magnetic behaviour.
In 2016, the Nobel Prize in Physics was awarded to three scientists who used advanced mathematical methods to study unusual phases (states) of matter. Their theoretical discoveries of topological phases of matter and topological phase transitions kickstarted experimental research to prove they existed in superconductors, superfluids and thin magnetic fields. The hope is that understanding the secrets of exotic matter will lead to future applications in materials science and electronics, such as quantum computing.
In quantum mechanics, all elementary particles - protons, neutrons, electrons, neutrinos, and quarks - have an intrinsic property known as spin and a spin value of spin-½. Particles with spin can also possess a magnetic dipole moment, analogous to an electric charge, meaning they are affected by a magnetic field.
Spins have a direction, either "up" or "down", that can be changed - "flipped". In ordinary magnetic materials, with spin-½, flipping one spin can cause collective propagation of spin excitations called magnons. It was originally thought that magnetic materials with integer spin (spin-1) would behave in the same way.
One of the 2016 Nobel Prize for Physics winners was F. Duncan M. Haldane. He theorised that spin-1 materials would behave differently and was later proved correct by neutron scattering experiments.
We observe spin excitations by flipping spins. Neutrons can flip spin by one unit, for example changing the spin on an electron from -½ to +½. Spin-1 materials have two free electrons per atom, adding two ½ spins to make an integer spin. In these materials, a neutron probe will in general again see one unit spin change propagating through the material.
But Resonant inelastic X-ray scattering (RIXS) sees more things. As well as flipping spin of one electron and seeing the change, this X-ray spectroscopy technique can flip the spins of both electrons in a spin-1 material. That means we can use RIXS to study propagation of double spin excitations in a material.
Previous studies have only observed dipolar spin excitations in spin-1 systems. In this research, scientists used RIXS on beamline I21 to study the entire picture of spin excitations in the spin-1 material Y2BaNiO5.
So what happens when you flip spins of both electrons simultaneously? Do the spin-flip excitations separate and travel independently, or do they stick together and travel as a bound excitation?
Like the neutron scattering, RIXS shows in this material a predominant curve resulting from the propagation of single spin flips. However, RIXS also probes a second, paler curve, higher up in the energy scale [see Fig. 1]. Via the assistance of the theoretical modelling, the researchers demonstrate that it is caused by double spin flips resulting in a remarkable higher order collective spin excitations. While one component propagates as non-interacting entities, the other behaves as a bound quadrupolar magnetic wave propagating in the material. The rich spin physics has been hidden until RIXS was applied to the system.
Abhishek Nag, the lead author, said:
The apparent dual nature of the double spin flip propagation, although having same origin is surprising, and an understanding of it may provide us clues to control their behaviour in these systems.
Principal Investigator Ke-Jin Zhou explains:
This is a fundamental discovery, a new fact of magnetism in integer spin systems. Although these excitations have been seen before in materials having special spin interactions, Y2BaNiO5 is devoid of these special interactions and is one of the purest integer spin materials. If these double spin flip excitations propagate in Y2BaNiO5, they will likely occur and propagate in many spin-1 materials without invoking special constraints.
Theoretically, higher order excitations could transport a higher volume of information via the spins. That would bring quantum computing a step closer, but it's still a long way off.
In the meantime, the research team will be using the RIXS technique to investigate the properties of even more exotic materials, with spins greater than one. Chemically they behave in much the same way as spin-½ materials, but on the quantum level, they're very different.
To find out more about the I21 beamline or discuss potential applications, please contact Principal Beamline Scientist Ke-Jin Zhou: kejin.zhou@diamond.ac.uk.
Nag A et al. Quadrupolar magnetic excitations in an isotropic spin-1 antiferromagnet. Nature Communications (2022). DOI:10.1038/s41467-022-30065-5.
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