Low-Energy Muons : Research Topics
Thin films and multilayers are of increasing scientific and technological
importance in contemporary condensed matter science, with the reduced
dimensionality providing insights into fundamental and emergent physical
behavior and novel applications. The development of a full understanding of
observed properties requires the use of experimental probes that access the
physical quantities of concern on a local scale within the material. Muon spin
rotation is a technique that has proven extremely useful to measure the spatial
and temporal properties of local magnetic fields within bulk materials.
The muon, as a local sensitive probe with complementary observational time
window to other probes or techniques, can also offer new insights into new
objects of investigation. However, it has previously been unsuitable for
studies of thin films and multilayers, because of the long stopping distance of
muons in matter and noticeable straggle in the implantation depth (typically,
0.3 mm with a full width half maximum straggle of 0.07 mm).
Based on a moderation technique of surface muons in cryocrystals, the LEM
group has developed a beam of ~ 100% polarized muons with tunable energy
between ~0 and 30 keV. At these energies implantation depths in matter
typically extend from the subnanometer region to 200-300 nm. The ultrahigh
vacuum apparatus includes a µSR spectrometer and sample environment. This
development, allowing all the advantages of µSR to be obtained in thin
samples, near surfaces, and as a function of depth below and above surfaces,
has set the basis of the LE-µSR method and opened new fields of µSR
The LE-µSR technique has been recently used to investigate the
microscopic magnetic field distribution in the vortex state of a thin epitaxial
film of a high-temperature superconductor. By varying the energy of the muons
and using films with a thin normal layer deposited at the surface, the muons
could be implanted below, across and above the surface of the superconducting
film to monitor the spatial evolution of the magnetic field distribution as the
flux lines emerge through the surface .
In another experiment the spatial variation on a scale of some nm of the
magnetic flux penetration beneath the surface of a high-temperature
superconductor in the Meissner state was observed for the first time. The
determination of the spatial dependence of the field is a microscopic test of
the London equations and has made possible the first absolute
model-independent measurement of the magnetic penetration depth (an important
quantity directly related to the superconducting carrier density), since no
assumption about the functional form of the magnetic field needs to be made in
this experiment . This experiment is perhaps the most powerful demonstration
of the new capabilities offered by a local, depth-dependent magnetic probe and
has been covered by reports in Physical Review Focus and in Physics News
Update . The possibility to measure local field distributions on some nm
scale demonstrated on a single layer superconductor will be used to investigate
more complex structures. For instance, studies of the influence of increasing
anisotropy on the vortex state in artificially grown multilayers consisting of
superconducting, metallic and insulating materials are planned (in
collaboration with the Universities of Geneva, Zürich, Birmingham, and
Columbia University). Another example is the search for spontaneous
magnetization below the surface of a HTc-superconductor as a
consequence of broken time reversal symmetry, where the local probe character
of the muons implanted a few nm below the surface can be used to give a direct
proof of the magnetic field and to quantify its magnitude (in collaboration
with the Universities of Urbana-Illinois and Zürich).
Investigations of magnetization reversal in nanometer size clusters of
ferromagnetic materials such as iron are an example of the application of
LE-µ+ to measure properties of samples that cannot be made
thick enough to stop the normally available surface muons. Assemblies of iron
nanoclusters with a very tight size distribution embedded in a silver thin film
matrix and only 500 nm thick were used in these studies  as a first step
toward investigations of the activated magnetic behavior in monodispersed
cluster with controlled anisotropy. Measurement of finite size effects in the
freezing character of spin glasses (in collaboration with the University of
Leiden)  and determination of magnetic order in thin magnetic Cr multilayers
further demonstrate the potential of LE-µSR .
For a correct analysis of the data it is often essential to know accurately
the implantation depth and implantation profiles of LE-muons in multilayers and
their behavior at interfaces and surfaces. We have developed Monte Carlo codes
to simulate the behavior of low energy muons stopped in multilayered materials
of variable composition and performed experiments to test their reliability
. The development of these codes, along with the ongoing program of
implantation depth measurements and diffusion studies in multilayers is an
important component in the systematic application and extension of LE-µSR
techniques. These measurements have also shed new light onto the understanding
of the behavior of particles implanted in matter.
We have closely collaborated with the University of Birmingham group on
development and testing of the Maximum Entropy method of analysis into a form
capable of extracting the maximum information on field distributions from
low-statistics LE-µSR data . Without this, the experiments on
superconductors described above would not have been analyzable in detail.
 E. Morenzoni et al.,
Rev. Lett. 72 (1994) 2793.
 E. Morenzoni et al.,
B289-290 (2000) 653.
 C. Niedermayer et al.,
Rev. Lett. 83 (1999) 3932.
 T. Jackson et al.,
Rev. Lett. 84 (2000) 4958.
 Phys. Rev. Focus 5
Story 22 15 May (2000)
News Update, May 2000
 T.J. Jackson et al.,
Phys.: Cond. Matt. 12 (2000) 1399.
 H. Luetkens et al.,
B 289-290 (2000) 326.
 G.J. Nieuwenhuys et al., in preparation.
 H. Glückler et al.,
B 289-290 (2000) 658 and in preparation.
 T.M. Riseman, E.M. Forgan,
Physica B 289 (2000) 718.
of publications about development and use of Low Energy Muons (since 1992)