LETTERS Three-dimensional imaging of magnetic fields with polarized neutrons NIKOLAY KARDJILOV1*, INGO MANKE1,2, MARKUS STROBL1,3, ANDRE´ HILGER1, WOLFGANG TREIMER1,4, MICHAEL MEISSNER1, THOMAS KRIST1 AND JOHN BANHART1,2 1Hahn–Meitner Institute (HMI) Berlin, 14109 Berlin, Germany 2Berlin Institute of Technology, 10623 Berlin, Germany 3Ruprecht Karls University Heidelberg, 69120 Heidelberg, Germany 4University of Applied Sciences (TFH) Berlin, 13353 Berlin, Germany *e-mail: [email protected] Published online: 30 March 2008; doi:10.1038/nphys912 Neutrons are highly sensitive to magnetic fields owing to their mechanism for neutron radiography that enables two- and magnetic moment, whereas their charge neutrality enables three-dimensional investigations of magnetic fields in matter. This them to penetrate even massive samples. The combination of method is unique not only in that it provides spatial information these properties with radiographic and tomographic imaging1–4 about the interaction of the spin with magnetic fields but also in its enables a technique that is unique for investigations of ability to measure these fields within the bulk of materials, which is macroscopic magnetic phenomena inside solid materials. not possible by any other conventional technique. Here, we introduce a new experimental method yielding two- Our concept is based on the fact that any spin wavefunction and three-dimensional images that represent changes of the corresponds to a definite spin direction and by using the quantum-mechanical spin state of neutrons caused by magnetic Schrodinger¨ equation we can keep track of the change fields in and around bulk objects. It opens up a way to the of the spin direction during passage through an arbitrary detection and imaging of previously inaccessible magnetic field inhomogeneous magnetic field. The equation of motion of the distributions, hence closing the gap between high-resolution vector S(t) = (Sx (t), Sy (t), Sz (t)) in a magnetic field B(t) is two-dimensional techniques for surface magnetism5,6 and known to be16 7–9 scattering techniques for the investigation of bulk magnetism . µ The technique was used to investigate quantum effects inside a d g N Sα (t) = [S(t) ×B(t)]α , α = x,y,z, massive sample of lead (a type-I superconductor). dt h¯ The specific interaction of neutrons with matter enables = − . µ neutron radiography to complement X-ray imaging methods for where g 3 826 is the g-factor for neutrons and N is the analysing materials1. Conventional radiography is a geometrical nuclear magneton. It can be shown that an ensemble of polarized particles with a magnetic moment and spin 1/2 behaves exactly like projection technique based on the attenuation of a beam by 17 a sample along a given ray. Quantum mechanically, neutrons a classical magnetic moment . are described by de Broglie wave packets10 whose spatial extent Thus, in a magnetic field, the spin component of neutrons may be large enough to produce interference effects similar polarized perpendicular to the field will undergo a Larmor to those known from visible laser light or highly brilliant precession according to the above equation with a frequency of synchrotron X-rays. Measurements of the neutron wave packet’s gµ ω = N B = γ B, phase shift induced by the interaction with matter have a long L h¯ L and distinguished history11–14 and were recently combined with γ neutron imaging approaches, where two- and three-dimensionally where L is the gyromagnetic ratio of the neutron − − resolved spatial information about the quantum mechanical (−1.8324×108 rad s 1 T 1 ) and B = |B|. interactions of neutrons with matter was obtained2,3,15. In addition, The fundamental idea of the method presented here is to neutrons, which from the particle-physicist’s point of view are analyse the spin states in a beam after interaction with the sample small massive particles with a confinement radius of about 0.7 fm, for each pixel of an imaging detector and to determine spatially possess another outstanding property: a magnetic moment µ resolved information about the spin rotation induced by the (µ = −9.66 × 10−27 JT−1). The magnetic moment is antiparallel magnetic field of the sample4,18,19. The precession angle ϕ for a to the internal angular momentum of the neutron described by neutron traversing a magnetic field can be written as a path integral a spin S with the quantum number s = 1/2. Consequently, the γ Z ϕ ω L , high sensitivity of neutrons to magnetic interactions has extensively = L t = Bds v been and is still being exploited in numerous experiments to path study fundamental magnetic properties and to understand basic where v is the velocity of the neutron. As the spin precession phenomena in condensed matter7–9. in a magnetic field depends on the flight time in the field, the Here, we present an experimental method that combines incident polarized beam has to be monochromatic (that is, has to spin analysis with neutron imaging and yields a new contrast contain neutrons with a single velocity v). In our case, the beam nature physics VOL 4 MAY 2008 www.nature.com/naturephysics 399 © 2008 Nature Publishing Group LETTERS Two-dimensional detecto a Spin B-field Spin r rotation Neutrons ϕ N S x z y Spin polarizer Magnet Spin analyser b c Spin Magnet orientation 1 Transmission x y 0 Superconductor 1 cm d 1.0 Experiment Calculation 0.8 0.6 Transmission 0.4 0.2 4.0 4.2 4.4 4.6 X position (cm) Figure 1 Spin-polarized neutron imaging. a, The neutron spin rotates in the magnetic field of a sample and hence typically approaches the spin analyser in a non-parallel orientation. The angle of the final spin rotation ϕ depends on the magnetic field along the neutron path. Changes in the polarization alter the transmission through the polarization analyser, which is indicated by a shorter arrow behind the analyser. b, Radiogram of a permanent magnet levitating over a YBa2Cu3O7 superconductor obtained with spin-polarized neutrons. The field of the magnet was orientated as shown in a. c, Image simulated by using the procedure described in the Methods section. The artefact in the central part of the image is discussed in Supplementary Information, Fig. S3b. d, Line profiles along the coloured arrows in b,c were used for comparison. was monochromatized by a double reflection using two graphite polarization and to absorb neutrons with antiparallel spin. The crystals as the monochromator. image of a sample detected behind the polarization analyser is The experimental set-up shown in Fig. 1a contains two determined by a superposition of conventional attenuation contrast 20 polarizers , one of which is situated in front of the sample position Ia (x,y) and the contrast variations due to spin rotation Im (x,y) to define the vertical beam polarization for the incident beam. The second polarizer called the (polarization) analyser in the following Z ( , ) ( , ) ( ) 1 ϕ( , ), is located between the sample position and the imaging detector. I x y = I0 x y ·exp − Σ s ds · 1+cos x y (1) 2 The analyser is aligned parallel to the polarizer to guarantee path | {z } | {z } Im (x,y) the passage of neutrons carrying a spin parallel to the initial Ia (x,y) 400 nature physics VOL 4 MAY 2008 www.nature.com/naturephysics © 2008 Nature Publishing Group LETTERS a b c d 1.0 A 1.0 A Experiment 1.0 Calculation 2.0 A 2.0 A 0.8 3.0 A 3.0 A Transmission 0.6 4.0 A 4.0 A 0.4 0.2 0.4 0.6 0.8 1.0 1.2 x 5.0 A x 5.0 A X position (cm) y y 1 cm Figure 2 Comparison of experimental and calculated images obtained by polarized neutron radiography of a cylindrical coil driven with various currents. a, Experimental results for five different currents. The grey scale represents the neutron spin rotation due to the magnetic field. b, Calculated images using Biot–Savart’s law. c, Schematic representation of the neutron spin rotation in the image centre. d, Line profiles along the coloured arrows in a,b for a current of 5 A. The deviation between calculation and measurement is less than 3%. where I0 (x, y) is the incident beam intensity, Σ is the placed perpendicular to the incident beam between the polarizer linear attenuation coefficient of the sample and (x, y) are and the analyser. The current was varied from 0 A up to 5 A in the coordinates in the detector plane. The cosine implies a steps of 0.1 A, and for each step an image was recorded. The periodic transmission function for the analysed precession angles measured magnetic field in the centre of the coil was 1.0 mT (examples for different configurations of permanent magnets are for an applied current of 1 A. A reference image of the field-free shown in Supplementary Information, Fig. S1) and complicates sample, representing Ia (x,y), was recorded and used to normalize a straightforward quantification with respect to the traversed the images by calculating the ratio I(x, y)/Ia (x, y). In this way, magnetic fields. However, in many cases reverse approaches starting information about the changes of the spin states, that is spin from an initial guess for the field distribution on the basis of known rotation, was extracted. These values (Fig. 2a) are compared with symmetries, boundary conditions or reference values can be used data derived from the calculated local magnetic field strength for quantitative image analyses. Figures 1 and 2 give examples of (Fig. 2b) and agree well with each other (see Supplementary how precisely the images can be calculated.