Shining X-rays of tunable energy on a sample and watching how strongly it absorbs as the energy sweeps past the binding energy of one element’s inner electrons. Each element absorbs at its own energies, so the method looks at one element at a time – the iron in a , the vanadium in a magnet – and the exact shape of the absorption edge tells its and surroundings. The bright, tunable X-rays it needs come from synchrotrons: large rings in which electrons circling close to the speed of light give off intense light, used also for and diffraction.
As the site uses it
Resolving the arc gap by photoemission needs high energy resolution at low temperature – laser sources showed it first, and synchrotron beamlines can with care.
Back magnetism claims with a local probe – X-ray magnetic circular dichroism, spin-polarised STM or NV magnetometry – as well as transport, and rule out magnetic impurities and oxidation; the VSe2 story shows how easily extrinsic signals appear.
At an absorption edge one element absorbs X-rays strongly: the edge and the structure just above it tell its oxidation state and surroundings, the wiggles further on the distances to its neighbours. The tunable X-rays come from synchrotrons. With circularly polarised light, the difference between two spectra measures one element’s magnetism.
One element at a time
An X-ray with enough energy can lift an electron out of a deep core level of an atom. As the X-ray energy is swept through that binding energy, absorption jumps up – an absorption edge – at an energy that belongs to one element and one core level, so the spectrum reports on that element alone, wherever it sits in the sample.
The edge carries chemistry. Its position shifts with the oxidation state, and the features just above it – the X-ray absorption near-edge structure, XANES – reflect the empty states the electron is lifted into and the symmetry of the atom’s surroundings. Further above the edge the outgoing electron wave scatters off neighbouring atoms, and the resulting wiggles, the extended X-ray absorption fine structure or EXAFS, give the distances to those neighbours and how many there are.
Synchrotrons, and catalysts at work
Absorption spectra need X-rays whose energy can be tuned finely and which are bright enough for a thin film. Synchrotrons provide them: rings, often hundreds of metres round, in which electrons travelling close to the speed of light are steered by magnets and give off intense, polarised light from the infrared to hard X-rays. Experiments run at beamlines around the ring, and the same light serves photoemission – the of Bi4Br4 were mapped with 101 eV synchrotron light – and powder diffraction of the kind used for α-In2Se3.
Because hard X-rays pass through liquids and gases, absorption can be measured on working devices – operando. On (Ni,Fe) oxyhydroxide catalysts splitting water, operando XAS found the iron as Fe3+ in sites with unusually short Fe–O bonds, and calculations made those iron sites, not the nickel, the active centres (Friebel and colleagues, 2015).
Magnetism element by element
With circularly polarised X-rays the absorption of a magnetic element depends on which way its point, and the difference between the two polarisations – X-ray magnetic circular dichroism, XMCD – measures that element’s and orbital moments separately. Because it picks out one element, it cannot be fooled by magnetic impurities or by the signal of a .
That made it decisive for VSe2. Bulk had suggested strong at room temperature; absorption spectra showed -VSe2 with vanadium in a 3d1 configuration, and XMCD found its spins correlated but without long-range order down to 2 K and up to 7 T – a frustrated magnet rather than a ferromagnet (Wong and colleagues, 2019).
For specialists
Absorption of tunable X-rays at a core-level edge of one element: the near-edge region (XANES) reflects oxidation state, empty states and local symmetry; the extended region (EXAFS) gives and coordination; circularly polarised X-rays give X-ray magnetic circular dichroism (XMCD), element-specific spin and orbital moments. It uses synchrotron radiation and works operando. Operando XAS located Fe3+ in unusually short-bonded octahedral sites as the active centres of (Ni,Fe)OOH water-oxidation catalysts; XAS and XMCD on monolayer VSe2 found vanadium 3d1 and correlated spins without long-range order, against bulk-magnetometry reports of room-temperature ferromagnetism.
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