Urs Staub (left) and Steven Johnson at the FEMTO beamline at SLS. Image: Paul Scherrer Institute/M. Fischer
first-of-their-kind experiments performed at the American X-ray laser
LCLS, a collaboration led by researchers from the Paul Scherrer
Institute in Switzerland has been able to precisely follow how the
magnetic structure of a material changes. The study was carried out on
cupric oxide (CuO). The change of structure was initiated by a laser
pulse, and then, with the help of short X-ray pulses, near-instantaneous
images were obtained at different points in time for individual
intermediate steps during the process. It appears as if the structure
begins to change 400 femtoseconds after the laser pulse strikes.
the fundamental magnets within the material need that much time to
communicate with each other and then react. In addition to this
scientific result, the work proves that it is actually possible with
X-ray lasers to follow certain types of extremely rapid magnetic
processes. This is another milestone, because such investigations will
also be a major focus of research at the planned Swiss X-ray Laser,
SwissFEL, at PSI. The results could contribute to the development of new
technologies for magnetic storage media for the future. The researchers
have reported on their work in the latest edition of the technical
journal Physical Review Letters.
with particular magnetic properties are the basis of many current
technologies, in particular, data storage on hard discs and in other
media. For this, the magnetic orientation in the material is most often
used: the atoms in the material behave to some extent like tiny rod
magnets (“spins”). These mini-magnets can be oriented in different ways
and information can be stored through their orientation. For efficient
data storage, it is crucial that old data can be rapidly overwritten.
This is possible if the magnetic orientation in a material can be
altered in a very short time. To develop innovative materials which can
store data quickly, it is therefore important to understand exactly how
this change occurs as a function of time.
Detail of the structure of cupric oxide (CuO). The copper atoms (green) carry a magnetic moment, behaving like small compass needles. The direction of the magnetic moment is illustrated by a red arrow. A point means that the arrow is pointing out of the surface (we are looking at its sharp end), a cross shows that the arrow is pointing into the surface (we are looking at its tail end). The magnetic structure changes significantly as the temperature increases above 213 Kelvin (around -60 C). One aspect of this change is a difference in the period of the magnetic order. Unlike the ordering at low temperatures, the magnetic structure in the temperature range 213 K to 230 K is incommensurate: its period does not ‘fit’ with the period of the crystal structure of copper and oxygen atoms. To be precise, a full rotation of the direction of the magnetic moment does not require exactly four atomic separations, but a little more or a little less, depending on the direction.
experiments performed at the X-ray laser LCLS at Stanford University, a
collaboration led by researchers from the Paul Scherrer Institute have
been able to study the magnetic orientation in cupric oxide, CuO. This
material demonstrates completely different magnetic orientations
depending on temperature: Below -60 C, the spins, which function in the
copper atoms like magnets, point periodically in one direction and then
the opposite; between -60 C and -43 C, they are arranged helically, as
if they were forming a spiral staircase. Although the spin orientations
for the two arrangements have been known for some time, the time
required to move from one arrangement to the other has only now been
shown by the experiment.
our investigation, we began with a “cold” sample and then heated it
with an intense flash of light from an optical laser,” explains Steven
Johnson, spokesman for the PSI experiment. “Shortly after this, we
determined the structure of the sample by illuminating it with an
extremely short pulse from an X-ray laser. When we repeated this at
different time intervals between the flash of light and the X-ray pulse,
we were able to reconstruct the course of the change in the magnetic
results show that it takes about 400 femtoseconds before the magnetic
structure begins to alter visibly. Then the structure gradually reaches
its final state. The more intense the initiating flash of light, the
faster the change of state.
“The spins of all copper atoms are involved
in the magnetic structure. Thus the atoms at opposite ends of the
material must be coordinated before the structure can change. This takes
400 femtoseconds,” explains Urs Staub, one of the PSI researchers
responsible. “For cupric oxide, that is the fundamental limit; it simply
cannot happen faster than that. This depends upon how strongly the
spins are coupled between neighbouring atoms.”
Change in the magnetic structure of CuO with time—at the initial value 1, significantly more of the low-temperature structure (CM) is present; as the value rises, so does the proportion of the second structure (ICM). The brown curve shows the actual measured values, the black one is a guide to the eye. The zero of time indicates when the laser pulse hits the sample. The transition to the new structure starts 400 femtoseconds later (tp).
is a good reason why the researchers were particularly interested in
cupric oxide. Along with the screw-like magnetic orientation that occurs
between -60 C and -43 C, the material is also “multiferroic”, a
material where electrical and magnetic processes mutually influence one
another. These materials have many different potential areas of
application where magnetism and electronics interact.
LCLS facility went into operation in 2009 as the first Free-Electron
X-ray Laser in the world, and the PSI researchers were amongst the first
to perform experiments at it. The extremely short pulses of only a few
femtoseconds that it produces make it possible to follow the course of
very rapid changes in materials, of which only the initial and final
states have until now been known. The experiment on cupric oxide
demonstrated that X-ray lasers can truly fulfil these expectations in
studies on magnetic materials. Such investigations will also be a major
focus of research at SwissFEL, the X-ray laser which is planned for PSI
and which will begin operation in the year 2016, in the close vicinity
of the existing PSI site.