Ultrafast magnetism: heating magnets, freezing time

Date:
October 15, 2021
Source:
Helmholtz-Zentrum Berlin fu"r Materialien und Energie
Summary:
Magnetic solids can be demagnetized quickly with a short laser
pulse, and there are already so-called HAMR (Heat Assisted Magnetic
Recording) memories on the market that function according to
this principle.

However, the microscopic mechanisms of ultrafast demagnetization
remain unclear. Now, a team has developed a new method at BESSY
II to quantify one of these mechanisms and applied it to the
rare-earth element Gadolinium, whose magnetic properties are caused
by electrons on both the 4f and the 5d shells.



FULL STORY ==========================================================================
New materials should make information processing more efficient, for
example, through ultrafast spintronic devices that store data with less
energy input.

But to date, the microscopic mechanisms of ultrafast demagnetization
are not fully understood. Typically, the process of demagnetization
is studied by sending an ultrashort laser pulse to the sample, thereby
heating it up, and then analyzing how the system evolves in the first picoseconds afterward.


========================================================================== Snapshot of the lattice condition "Our approach is different," explains
Dr. Re'gis Decker, lead author of the study. "We keep the sample at a
certain temperature during the spectra acquisition. And we do that for
many temperatures, from -120DEGC to 450DEGC for Gd -- and much higher (1000DEGC) for previous experiments with Ni and FeNi.

This allows us to quantify the effect of the phonons for each temperature
on the ultrafast demagnetization, where the temperatures of the lattice, electrons and spins subsystems evolve with time. In other words, by
placing the system at a certain temperature, we do a capture of the
lattice condition at a given time after the ultrashort laser pulse and
we measure there." Gadolinium examined The element gadolinium has 4f
and 5d electron orbitals, which both contribute to its ferromagnetic properties. The higher the temperature, the more the crystalline sample vibrates -- and as physicists say: the more the population of phonons increases, and the more likely spin-flips are to occur due to the
scattering of electrons with phonons from the crystal lattice.

Scattering rates distinguished Using the method of inelastic X-ray
scattering (RIXS), the physicists were not only able to determine the
number of phonons at a given temperature, but also to distinguish the interactions between phonons and 4f- and 5d-electrons. Using the strict
X-ray spectroscopic symmetry selection rules, the evaluation succeeded
in distinguishing between the scattering rates of the 4f and 5d electrons.

5d electrons interact with phonons The data show that there is hardly
any scattering between the localized 4f electrons and phonons, but most
of the scattering process takes place between 5d electrons and phonons,
so that a spin-flip only occurs there. "Our approach evidences that the electron-phonon scattering, which is known to be one of the main trigger
of ultrafast demagnetization, applies to the 5d electrons only.

Interestingly, it also shows the presence of a temperature threshold,
which depends on the material, below which this mechanism does not
occur. This indicates the existence of another microscopic mechanism at
lower temperature, as predicted by theory," Decker explains.

========================================================================== Story Source: Materials provided by Helmholtz-Zentrum_Berlin_fu"r_Materialien_und_Energie.

Note: Content may be edited for style and length.


========================================================================== Journal References:
1. Re'gis Decker, Artur Born, Kari Ruotsalainen, Karl Bauer, Robert
Haverkamp, Robby Bu"chner, Annette Pietzsch, Alexander
Fo"hlisch. Spin- lattice angular momentum transfer of localized
and valence electrons in the demagnetization transient state of
gadolinium. Applied Physics Letters, 2021; 119 (15): 152403 DOI:
10.1063/5.0063404
2. Re'gis Decker, Artur Born, Robby Bu"chner, Kari Ruotsalainen,
Christian
Straahlman, Stefan Neppl, Robert Haverkamp, Annette Pietzsch,
Alexander Fo"hlisch. Measuring the atomic spin-flip scattering
rate by x-ray emission spectroscopy. Scientific Reports, 2019; 9
(1) DOI: 10.1038/ s41598-019-45242-8
3. Artur Born, Re'gis Decker, Robby Bu"chner, Robert Haverkamp, Kari
Ruotsalainen, Karl Bauer, Annette Pietzsch, Alexander Fo"hlisch.

Thresholding of the Elliott-Yafet spin-flip scattering in multi-
sublattice magnets by the respective exchange energies. Scientific
Reports, 2021; 11 (1) DOI: 10.1038/s41598-021-81177-9 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/10/211015184222.htm

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