Neutron magnetic imaging

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. 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. In a few cases, Polarized Grazing Incidence SANS was performed on magnetic systems

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.

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

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

Precession techniques

The neutron precession in an induction field is expressed as d M → d t = γ n M → × B → ( r ) where M → is the neutron magnetic moment, B → is the local magnetic induction at the neutron position r ( t ) and γ n = g n μ N / ℏ is the neutron gyromagnetic ratio. For neutrons, the gyromagnetic ratio is γ n = g n μ N ℏ = 183 × 10 6 r a d . s − 1 . T − 1 = 29.2 M H z . T − 1 (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 B → ( r → ) in space. 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 polaarized with supermirrors or with polarized 3He gaz

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 systemis 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. A very thorough description of the measurement process can be found in.

Phase imaging

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

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. It is however only possible to produce 1 dimensional microbeams dur to the intrinsics limitation in neutron flux. Hence this technique can presently be applied only for 1 dimensional problems.