Magnetism and spintronics
Can magnetic disorder make a metal shrink when it is heated?
Open access · cc by · source: Europe PMC
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.
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.
Key findings
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.
Methodology
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.
Limitations
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.
How this study connects
Role on claims
Each row is a claim on a concept or method page where this paper supports, challenges, or qualifies the statement. Roles are hand-checked — not a model guess.
Spins shape excitons and the lattice, not only magnetisation.
Magnetic order feeds back on other degrees of freedom: in CrSBr the excitonic 1s-2p transition shifted from about 50 to 15 meV across the magnetic transition, and in granular FePt films laser demagnetisation caused a transient lattice contraction that returned only after remagnetisation (about 100 ps).
Evidence for the claim as stated.
Mechanisms are inferred in most control experiments: the Mn3Sn seed layer is not imaged, the hydrogen Fermi-surface mechanism in RKKY multilayers is not measured, and the FePt spin-stress model reproduces data only qualitatively.
Evidence for the claim as stated.
Open questions
Tensions this paper is part of
From concept pages' “where studies disagree.” Disagreement means the same question; scope means different assays, populations, or outcomes.
Mechanisms are inferred in most control experiments: the Mn3Sn seed layer is not imaged, the hydrogen Fermi-surface mechanism in RKKY multilayers is not measured, and the FePt spin-stress model reproduces data only qualitatively.
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