Physics:Neutron magnetic imaging

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Neutrons are spin 1/2 particles that interact with magnetic induction fields via the Zeeman interaction. This interaction is both rather large and simple to describe. Several neutron scattering techniques have been developed to use thermal neutrons to characterize magnetic micro and nanostructures.

Polarized small-angle neutron scattering (SANS)

Small-angle neutron scattering is a technique which is especially suited for the study of nanoparticles. It has for example been used extensively for the study of ferrofluids. More recently, polarized SANS has become more widely available and a wide range of study have been performed.[1][2][3] Polarized SANS allows either to probe the internal structure of magnetic nanoparticles via the measurement of the magnetic form factor or the magnetic interactions between magnetic nanoparticles via the structure factor.[4] In a few cases, Polarized Grazing Incidence SANS was performed on magnetic systems [5][6]

A few polarized neutrons SANS spectrometers are available across the world:

Polarized neutron reflectometry

Polarized neutron reflectometry allows probing magnetic thin films and ultra-thin films. The polarized reflectivity measurements allow measuring the magnitude and directions of the magnetic induction in magnetic heterostructures with a depth resolution on the order of 2-3 nm for films with thicknesses ranging from 5 to 100 nm.[7]

A number of polarized neutrons reflectometers are available across the world:

A catalogue of neutron reflectometers is available at www.reflectometry.net.

Polarized Neutron Radiography and Tomography

Precession techniques

The neutron precession in an induction field is expressed as [math]\displaystyle{ \frac{d\overrightarrow{M}}{dt}=\gamma_{n}\overrightarrow{M}\times\overrightarrow{B}(r) }[/math] where [math]\displaystyle{ \overrightarrow{M} }[/math] is the neutron magnetic moment, [math]\displaystyle{ \overrightarrow{B} }[/math] is the local magnetic induction at the neutron position [math]\displaystyle{ r(t) }[/math] and [math]\displaystyle{ \gamma_{n}=g_{n}\mu_{N}/\hbar }[/math] is the neutron gyromagnetic ratio. For neutrons, the gyromagnetic ratio is [math]\displaystyle{ \gamma_{n} = g_{n} \mu_{N} \hbar=183\times10^{6}rad.s^{-1}.T^{-1}=29.2 MHz.T^{-1} }[/math] (note that for neutrons g factor is negative and equal to -3.83).

Bulk systems

Neutron radiography can be used to map the distribution of an induction field [math]\displaystyle{ \vec{B}(\vec{r}) }[/math] in space.[8] In order to perform such experiments, the neutron beam is initially polarized, it interacts with the induction field of interest and the neutron precession is measured with a neutron analyzer in front of the 2D detector. The beam can be either polarized with supermirrors[9] or with polarized 3He gaz[10]

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Radiographic projections of trapped flux in a polycrystalline lead cylinder at different temperatures below Tc=7.2 K. b, Trapped flux at 7.0 K (yellow) visualized in different tomographic views. The magnetic field strength in the sample can be analysed to be about 1.00.2 mT (yellow parts) (N. Kardjilov et al., Nature Physics 4, 399 - 403 (2008) doi:10.1038/nphys912)

Thin film structures

The neutron precession in an induction field is rather small. Thus in the case of thin films (~1 µm thick) the neutron interaction is rather small. Thus in order to obtain a measurable signal, it has been proposed that a grazing incidence geometry could be used. In such a geometry, the interaction is enhance since the neutron travels a longer path inside the induction field. Such measurements however assume that the planar structure of the system is homogeneous and that the induction varie only through the depth of the magnetic film. The magnetisation depth profile was measured in thick CoZr films in which the magnetic anisotropy field was "engineered" during deposition.[11] A very thorough description of the measurement process can be found in.[12]

Phase imaging

Phase contrast (or dark field) imaging has recently been developed for neutron radiography and tomography. It has been applied to visualize magnetic domains in several types of systems:

  • soft magnetic alloys [13]
  • magnetic vortices in low Tc superconductors [14][15] superconductors [16]
Nb Mixed phase neutron radiography

Scanning magnetic neutron imaging

Magnetic neutron radiography is currently limited in spatial resolution due to the need of analyzing the neutron polarization which results in losses in spatial resolution. It has been proposed that neutron scanning imaging could be performed by using micro beams.[17][18] It is however only possible to produce 1 dimensional microbeams due to the intrinsic limitation in neutron flux. Hence this technique can presently be applied only for 1 dimensional problems.

See also

References

  1. ↑ Maurer, Thomas (2014). "Ordered arrays of magnetic nanowires investigated by polarized small-angle neutron scattering". Physical Review B 89 (18): 184423. doi:10.1103/physrevb.89.184423. Bibcode: 2014PhRvB..89r4423M. 
  2. ↑ Perigo, E.A. (2016). "Magnetic microstructure of a textured Nd-Fe-B sintered magnet characterized by small-angle neutron scattering". Journal of Alloys and Compounds 661: 110–114. doi:10.1016/j.jallcom.2015.11.167. 
  3. ↑ Krycka, K.L. (2014). "Origin of Surface Canting within Fe3O4 Nanoparticles". Phys. Rev. Lett. 113 (14): 147203. doi:10.1103/physrevlett.113.147203. PMID 25325655. Bibcode: 2014PhRvL.113n7203K. 
  4. ↑ Michels, A. (2014). "Magnetic small-angle neutron scattering of bulk ferromagnets". Journal of Physics: Condensed Matter 26 (38): 383201. doi:10.1088/0953-8984/26/38/383201. PMID 25180625. 
  5. ↑ Fermon, C (1999). "Towards a 3D magnetometry by neutron reflectometry". Physica B: Condensed Matter 267 (1–4): 162–167. doi:10.1016/S0921-4526(99)00014-9. Bibcode: 1999PhyB..267..162F. 
  6. ↑ Pannetier, M. (2003). "Surface diffraction on magnetic nanostructures in thin films using grazing incidence SANS". Physica B: Condensed Matter 335 (1–4): 54–58. doi:10.1016/s0921-4526(03)00190-x. Bibcode: 2003PhyB..335...54P. 
  7. ↑ Zabel, H (2006). "Neutron reflectivity of spintronic materials". Materials Today 9 (1–2): 42–49. doi:10.1016/S1369-7021(05)71337-7. 
  8. ↑ Treimer, Wolfgang (2014). "Radiography and tomography with polarized neutrons". Journal of Magnetism and Magnetic Materials 350: 188–198. doi:10.1016/j.jmmm.2013.09.032. Bibcode: 2014JMMM..350..188T. 
  9. ↑ "Swiss Neutronics". http://www.swissneutronics.ch/fileadmin/user_upload/Dokumente/PDF/Product_Flyers/Flyer_ESS_-_polarising_devices_-_V3.pdf. 
  10. ↑ Dawson, M. (2011). "Polarized neutronimagingusinghelium-3cellsandapolychromaticbeam". Nuclear Instruments and Methods in Physics Research Section A (654): 144. 
  11. ↑ Thibaudeau, Pascal (2011). "Probing magnetic domain wall profiles by neutron spin precession". Europhysics Letters 93 (3): 37003. doi:10.1209/0295-5075/93/37003. Bibcode: 2011EL.....9337003T. 
  12. ↑ Rekveldt, T. (2013). "Single domain wall chirality studies using polarised neutrons". Journal of Magnetism and Magnetic Materials 329: 105–117. doi:10.1016/j.jmmm.2012.10.001. Bibcode: 2013JMMM..329..105R. 
  13. ↑ Lee, Seung Wook (2010). "Observation of Magnetic Domains in Insulation-Coated Electrical Steels by Neutron Dark-Field Imaging". Applied Physics Express 3 (10): 106602. doi:10.1143/apex.3.106602. Bibcode: 2010APExp...3j6602L. 
  14. ↑ Kardjilov, N. (2008). "Three-dimensional imaging of magnetic fields with polarized neutrons". Nature Physics 4 (5): 399–403. doi:10.1038/nphys912. Bibcode: 2008NatPh...4..399K. 
  15. ↑ Treimer, W. (2012). "Polarized neutron imaging and three-dimensional calculation of magnetic flux trapping in bulk of superconductors". Physical Review B 85 (18): 184522. doi:10.1103/PhysRevB.85.184522. Bibcode: 2012PhRvB..85r4522T. 
  16. ↑ Reimann, T. (2015). "Visualizing the morphology of vortex lattice domains in a bulk type-II superconductor". Nature Communications 6: 8813. doi:10.1038/ncomms9813. PMID 26522610. Bibcode: 2015NatCo...6.8813R. 
  17. ↑ Ott, Frédéric (2015). "Shaping micron-sized cold neutron beams". Nuclear Instruments and Methods in Physics Research Section A 788: 29–34. doi:10.1016/j.nima.2015.03.057. Bibcode: 2015NIMPA.788...29O. 
  18. ↑ Kozhevnikov, S.K. (2015). "System of neutron microbeams from a planar waveguide.". JETP Letters 102 (1): 1–6. doi:10.1134/s0021364015130068. Bibcode: 2015JETPL.102....1K. 



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