Annual Report 2011 / 2012 - E21 - Technische Universität München

36 E21 Annual Report 2011/2012
Neutron Depolarisation Imaging: Stress Measurements by Magnetostriction
Effects in Ni Foils
Michael Schulz 1, 2 , Philipp Schmakat 1, 2 , Christian Franz 1 , Andreas Neubauer 1 ,
Elbio Calzada 1, 2 , Burkhard Schillinger 1, 2 , Peter Böni 1 , Christian Pfleiderer 1
1 Physik-Department E21, Technische Universität München, D-85748 Garching, Germany.
2 Forschungsneutronenquelle Heinz Maier-Leibnitz (FRM II), Technische Universität München, D-85748 Garching, Germany.
Results and Discussion
Introduction
Imaging with polarized neutrons [1, 2], is a new method
which is increasingly being recognized as a powerful tool for
the study of magnetic effects. Here we present first proofof-principle
measurements of a new application of neutron
depolarization imaging. The magnetostriction effect leads to
a change of the orientation of the domains within a ferromagnetic
sample, if a uniaxial force is applied on the sample. As
a consequence, a change of the depolarization which a neutron
beam suffers after transmission of the sample is observed.
In our experiments we used this effect on high purity Ni
foils with a magnetostrictive constant of λ s ≈ −37·10 −6 [3]
as a method for the spatially resolved measurement of the
mechanical stress in the material. In the future this technique
might be used as a spatially resolved stress gauge.
In Fig. 2, the beam polarization of the neutron beam after
transmission of the sample at the position where it has its
smallest width (marked by the dashed green lines in the left
panel) is plotted vs. the load. It is clearly visible that the
polarization increases with increasing stress, which is due to
the preferred alignment of the domains in the sample parallel
to the direction of the applied mechanical stress and
thus also parallel to the polarization of the beam. A quantitative,
spatially resolved evaluation of the mechanical stress
in the foil from the presented experiment was not possible
due to the large errors in the measurements. These errors
could, however, be decreased in the future by increasing the
counting statistics.
Experiments
Our experiments were performed at the imaging beam line
ANTARES at FRM II, Munich with a one dimensional polarization
analysis setup shown in Fig. 1. The setup consists
of a collimator (C), from which the neutron beam emerges
and is monochromatized by a double crystal graphite
monochromator (M) to a wavelength of 3.2 . After the monochromator
the neutrons travel along an evacuated flight
tube (FP) of approximately 12m length to the sample area,
where a 3 He polarizer (P), a precession coil type spin flipper
(F) and an analyzer (A) were installed before and after
the sample (S), behind which the detector (D) is located.
Figure 1: Experimental setup consisting of a collimator (C), double
crystal monochromator (M), flight path (FP), polarizer (P),
spin flipper (F), sample (S), polarization analyzer (A) and a CCD
detector (D).
High purity Ni foils (99+%) with a thickness of 127µm
were cut to a concave shape as shown in Fig. 2 (left hand
side) with a maximal width of 30mm and a minimal width
of 10mm. After cutting, the foils were annealed in vacuum
at 900 ◦ C for 8h to remove any remaining strain from the
samples and then mounted in a frame, which was fixed at the
top end, while the bottom end could be loaded with weights
up to 25kg resulting in a maximum stress of σ ≈ 200MPa.
Figure 2: Radiograph of the smaple (left), beam polarization after
transmission of the sample (right).
Conclusion and Outlook
These proof-of-principle measurements have shown the possibility
to use ferromagnetic foils with a large magnetostrictive
constant as stress gauges for the spatially resolved determination
of mechanical stress. The sensitivity of the method
could be further improved by using foils with a larger magnetostrictive
constant, e.g. FeCoV (λ s ≈ 83.4·10 −6 [4]),
improving the counting statistics by using a polychromatic
beam and by performing 3D polarization analysis.
References
[1] N. Kardjilov et al., Nature Physics 4, 399 (2008).
[2] M. Schulz, PhD thesis, Technische Universität München,
http://nbn-resolving.de/urn/resolver.plurn:nbn:de:
bvb:91-diss-20100824-963430-1-2 (2012).
[3] G. Matsumomo et al., Journal of the Physical Society of Japan
21, 882 (1966).
[4] M. Senthil Kumar and P. Böni, Journal of Applied Physics 91,
3750 (2002).