Can magnetic disorder make a metal shrink when it is heated?
In granular FePt, laser heating first makes the crystal contract because disordering the spins pulls the lattice in, and this contraction disappears if the spins are already disordered.
Source
Spin stress contribution to the lattice dynamics of FePt
Study at a glance
- Design
- Other — Lab pump-probe experiment: femtosecond laser pulses (single and double) excited granular and continuous FePt films, ultrafast X-ray diffraction tracked out-of-plane strain, MOKE tracked magnetisation, and finite-element models tested the stress picture.
- N
- No sample count; two thin films (one granular, one continuous) measured over a range of fluences and pulse delays.
- Population
- About 9.5 nm thick L10-ordered FePt films on MgO: a granular film of FePt grains in carbon and a continuous epitaxial film
- Outcome
- Time-resolved out-of-plane lattice strain, and transient magnetisation
Structured fields used in claim comparison tables when every cited study has a complete layer.
What they did
The researchers heated FePt films with 100-femtosecond laser pulses and used ultrashort X-ray pulses to measure how the out-of-plane lattice spacing changed over picoseconds, comparing a granular film with a continuous film across fluences. In double-pulse experiments, a strong first pulse disordered the spins and a weaker second pulse arrived after a chosen delay. They measured magnetisation recovery with time-resolved magneto-optical Kerr effect and modelled the nanograins' elastic response with finite elements.
What they found
The granular film contracted during the first 2 ps before expanding, the contraction was largest at medium fluence and vanished at high fluence, and the continuous film barely contracted at all. A second pulse 13 ps after a strong first pulse caused no contraction, whereas with delays near 200 ps the contraction returned, matching the roughly 100-ps timescale of remagnetisation. Static heating showed negative thermal expansion out of plane for the granular film and invar-like behaviour for the continuous film, and the modelling attributes the film difference to the Poisson effect allowed by the granular geometry.
The limits
What it doesn't show
The link between spin energy and stress is a first-order estimate using mean-field heat capacity and assumes near-full demagnetisation from MOKE saturation. The simulations reproduce the data only qualitatively, underestimate expansion beyond 3 ps, and need an electron-phonon anisotropy less than half the predicted value. Only two films of one thickness were studied, and grain-size dispersion and heat flow into the carbon matrix were not fully modelled.
Key terms
- Invar effect
- Near-zero thermal expansion in some magnetic alloys because magnetic stress cancels normal thermal expansion.
- Negative thermal expansion
- A material shrinking along some direction as it is heated.
- Spin stress
- Mechanical stress on the lattice caused by changes in magnetic order, here contractive when spins disorder.
- Ultrafast X-ray diffraction
- Pump-probe technique using sub-picosecond X-ray pulses to track lattice spacing after excitation.
- Poisson effect
- A material expanding in one direction tends to contract in the perpendicular directions.
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Quiz yourself
What drives the brief lattice contraction in granular FePt?
Common questions
Why does the double-pulse scheme separate spin effects from phonon effects?
The first pulse saturates spin disorder, so the second pulse can only add phonon and electron stress; any missing contraction therefore measures the spin contribution.
Why does the continuous film not contract?
On picosecond timescales it cannot expand sideways by symmetry, so there is no Poisson contraction to add to the spin stress; grains in carbon can expand sideways.
Why does the contraction recover over about 100 ps?
Heat must flow out of the grains so the spins reorder, and this thermal transport sets the same timescale as remagnetisation.
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