Physica B 406 (2011) 2412–2414

Contents lists available at ScienceDirect

Physica B

0921-45

doi:10.1

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Univers

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journal homepage: www.elsevier.com/locate/physb

Neutron depolarisation imaging: Stress measurements by magnetostrictioneffects in Ni foils

Michael Schulz a,b,�, Philipp Schmakat b, Christian Franz b, Andreas Neubauer b, Elbio Calzada a,b,Burkhard Schillinger a,b, Peter Boni b, Christian Pfleiderer b

a Forschungsneutronenquelle FRM II, Technische Universitat Munchen, Lichtenbergstrasse 1, 85748 Garching, Germanyb Physik Department E21, Technische Universitat Munchen, James-Franck-Strasse, 85748 Garching, Germany

a r t i c l e i n f o

Available online 5 November 2010

Keywords:

Radiography

Polarized neutrons

Magnetostriction

26/$ - see front matter & 2010 Elsevier B.V. A

016/j.physb.2010.10.079

esponding author at: Forschungsneutronenq

itat Munchen, Lichtenbergstrasse 1, 85747 Ga

ail address: michael.schulz@frm2.tum.de (M.

a b s t r a c t

In this article we present first proof-of-principle neutron depolarization imaging measurements on Ni

foils under mechanical stress. The magnetostrictive effect in Ni leads to a reorientation of the magnetic

domains in the material depending on the applied force. This in turn leads to a change of the

depolarization a neutron beam suffers from transmission of the sample. We propose to use this method

as a new technique for the spatially resolved measurement of mechanical stress.

& 2010 Elsevier B.V. All rights reserved.

1. Introduction

Imaging with polarized neutrons, is a new method which isincreasingly being recognized as a powerful tool for the study ofmagnetic effects. The precession of the neutron spin in a magneticfield can i.e. be used to determine the field integral along theneutron flight path and thus to map out magnetic field arrange-ments [1]. Moreover, the method allows to probe internal magneticfields in various materials which may for instance arise fromsuperconducting vortices [1] or in magnetized thin ferromagneticfilms [2]. Neutron depolarization imaging (NDI), which is based onthe measurement of the depolarization of a neutron beam aftertransmission of unmagnetized samples has been used for thecharacterization and investigation of inhomogeneous ferromag-nets [3,4].

Here we will present first proof-of-principle measurements of afurther interesting application of neutron depolarization imaging.The magnetostriction effect leads to a change of the orientation ofthe domains within a sample, if a uniaxial force is applied on thesample. As a consequence, a change of the depolarization which aneutron beam suffers after transmission of the sample is observed.In our experiments we used this effect on high purity Ni foils as amethod for the spatially resolved measurement of the mechanicalstress in the material. Ni has a negative magnetostrictive constantof ls ��37� 10�6 [5], which results from averaging over theanisotropic values of �45:9� 10�6 along the [1 0 0] axis and�24:3� 10�6 along the [1 1 1] axis for a polycrystalline sample

ll rights reserved.

uelle FRM II, Technische

rching, Germany.

Schulz).

[6]. Non-spatially resolved 3D neutron depolarization measure-ments of the magnetostrictive effect in Ni foils have already shownthe possibility to use this effect as a measure of the stress applied tothe foil [7]. In the future this technique might be used as a spatiallyresolved stress gauge.

2. Experimental techniques

Our experiments were performed at the imaging beam lineANTARES at FRM II, Munich with a one dimensional polarizationanalysis setup shown in Fig. 1. The setup consists of a collimator (C),from which the neutron beam emerges and is monochromatized bya double crystal graphite monochromator (M) [8] to a wavelengthof 3.2 A. After the monochromator the neutrons travel along anevacuated flight tube (FP) of approximately 12 m length to thesample area, where a 3He polarizer (P) and analyzer (A) wereinstalled before and after the sample (S). The advantage of using3He polarizers is that the beam geometry is not altered by thepolarizer and thus a good spatial resolution is achieved. For adetailed discussion of the different polarizing options for neutronimaging we refer to [3]. Additionally, a precession coil type spinflipper (F) was installed before the sample for selection of thedesired polarization direction. As a detector (D) we used athermoelectrically cooled 2048 �2048 pixel CCD camera, whichrecords the image of a LiF:ZnS scintillator with a thickness of200 mm. The size of the polarized beam at the sample position wasapproximately 100� 100 mm2 and the pixel size of the detectorwas 74 mm.

High purity Ni foils (99+%) with a thickness of 127 mm were cutto a concave shape as shown in Fig. 2 (right hand side) with amaximal width of 30 mm and a minimal width of 10 mm. After

M. Schulz et al. / Physica B 406 (2011) 2412–2414 2413

cutting, the foils were annealed in vacuum at 900 1C for 8 h toremove any remaining strain from the samples and then mountedin a frame, which was fixed at the top end, while the bottom endcould be loaded with weights up to 25 kg resulting in a maximumstress ofs� 200 MPa. For the experiment, the foil was placed in thepolarized neutron beam between the 3He polarizer and analyzer ofthe setup shown above with the direction of the stress applied bythe weights being parallel to the beam polarization. Spatiallyresolved measurements of the beam polarization after transmis-sion of the Ni foils were performed with three different loads from5 to 25 kg. A loading with 30 kg leads to the destruction of thesample, which is equivalent to an ultimate tensile strength of thematerial of 240 MPa.

3. Results and discussion

Five spin-up and spin-down images with an exposure time of60 s each were acquired per applied load, from which the trans-mitted beam polarization was calculated.

The beam polarization after transmission of the foil is plotted inthe left panel of Fig. 2 along the yellow arrow shown in the rightimage. However, due to the strong depolarization of the beam bythe Ni foil ðP� 0Þ and the low neutron flux which results from usinga monochromatic beam, the statistical error in these measure-ments is very large. To reduce the statistical error, the polarizationwas averaged over the horizontal extension of the arrow, as weexpected the stress to be relatively constant along the x-direction.For better visibility, the red and blue curves were shifted by a value

Fig. 1. Experimental setup used for the experiments described in this article. The

components include 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).

Fig. 2. Beam polarization after transmission of a Ni foil along the line shown in the image o

to modify the mechanical stress. Increasing stress leads to a reorientation of the domains

value of 0.02 along the y-axis as indicated by the red and blue baselines. (For interpretat

version of this article.)

of 0.02 with respect to each other along the y-axis as indicated bythe dashed lines. It can be seen that the beam polarization increaseswith decreasing sample width and reaches its maximum atx� 28 mm. The asymmetric shape of the curve might result fromthe clamps used to hold the sample which were mounted atdifferent positions at the upper and lower end of the sample aty� 10 mm and y� 60 mm.

The height of the peak in the beam polarization, which is locatedat the position where the sample has its smallest width (marked bythe dashed green lines in Fig. 2) is plotted in Fig. 3 vs. the load. Eventhough the statistical errors in these measurements are very high, itis still visible that the peak height increases with increasing stress.The beam polarization increases due to the preferred alignment ofthe domains in the sample parallel to the direction of the appliedmechanical stress and thus also parallel to the polarization of thebeam. In general, the beam polarization after transmission of thesample also depends on the average domain size and the samplethickness, which were both assumed to be constant for theseexperiments. The latter assumption is justifiable due to therelatively small deformation of the sample by the applied stress.

n the right hand side (from top to bottom). The foil was loaded with different weights

and to increasing beam polarization. For better visibility, the curves are shifted by a

ion of the references to color in this figure legend, the reader is referred to the web

Fig. 3. Beam polarization after transmission of the region where the sample has its

smallest width as indicated by the green dashed line in Fig. 2. The transmitted beam

polarization increases with increasing stress due to the preferred orientation of the

domains in parallel to the direction of the incoming beam polarization.

M. Schulz et al. / Physica B 406 (2011) 2412–24142414

To distinguish a change in the domain size from the formation ofmagnetic texture, a 3D depolarization analysis would be requiredwhich was not feasible with the current setup. A quantitative,spatially resolved evaluation of the mechanical stress in the foilfrom the presented experiment was not possible due to the largeerrors in the measurements. These errors could, however, bedecreased in future measurements by acquiring more imagesand also by using a polychromatic beam in combination with anadiabatic fast passage (AFP) type spin flipper integrated in the 3Hepolarizer [9] giving much improved counting statistics.

4. Conclusion and outlook

These proof-of-principle measurements have shown the possi-bility to use ferromagnetic foils with a large magnetostrictiveconstant as stress gauges for the spatially resolved determinationof mechanical stress. The sensitivity of the method could be furtherimproved by using foils with a larger magnetostrictive constant,e.g. FeCoV, for which the magnitude of the magnetostrictiveconstant ls � 83:4� 10�6 is approximately twice as high as forNi [10,6]. Moreover, the use of a polychromatic neutron beamwould lead to a large increase in the neutron flux and thus improvethe counting statistics. Stress sensors could be built by mountingannealed Ni foils on the surface of a (nonmagnetic) sample andmeasuring the depolarization of the beam after transmission of thefoil as a spatially resolved replacement of standard resistance strain

gauges. More detailed calibration measurements in a dedicatedsetup would have to be performed to determine the dependence ofthe beam polarization on the applied stress more accurately.Furthermore, 3D depolarization analysis experiments are plannedwith a modified setup which will allow to distinguish betweenchanges of the domain size and the introduction of magnetictexture by the applied stress.

References

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