POSY I - Polarized Neutron Reflectometer

POSY I Website

Instrument Scientist - Suzanne G.E. te Velthuis

Purpose

The primary purpose of reflectometers is to measure the chemical and/or magnetic depth profile in thin films and multilayers deposited over flat substrates. This is accomplished in the following way. First, the reflectivity is measured as a function of the neutron momentum transfer perpendicular to the surface q = 4p sinq / l , where q is the angle of the incident beam with the surface and l is the neutron wavelength. Second, the measured reflectivity is compared with that calculated for model depth profiles containing parameters (representing the thickness and "nature" of the layers) to be fitted. By "nature" is meant the nuclear scattering amplitude density, that can be calculated from available tables, and the magnetic induction B. The spatial resolution that can be reached is of the order of 10 Å. The maximum thickness of the layers over which a profile can be obtained is 5000 Å.

Operation

The primary pulsed beam contains all wavelengths in the spectrum moderated to 20 K by a solid methane moderator. The intensity reflected at one angle q is measured as a function of the wavelength. The reflectivity is calculated by normalizing the intensities reflected to those of the direct beam. This measurements covers a range of q's defined by the largest and the shortest wavelengths available. To extend the range of q measurements are taken at different angles. Reflectivity curves taken at different angles are spliced together by fitting the data in the partially overlapping q ranges. The data are put on an absolute basis by assuming that at the lowest q part of the q range covers the region of total reflection (R=1). The procedure assumes that the resolution function D q/q is constant.

Geometry

POSY I has been used to measure the magnetic profile of thin ferromagnetic films, either by themselves or stacked in multilayers with ferromagnetic or antiferromagnetic coupling. For a review, see Ref. 1.

Sample Environment

In the normal mode of operation the instrument measures the reflectivities R+, R- for neutrons polarized parallel and opposite to an applied magnetic field, switching between the two at every pulse. The position sensitive detector measures not only the specularly reflected neutrons (for which the angle of the reflected beam is the same as the angle of incidence on the reflecting surface) but also those scattered (for which the two angles are different).

Insertion of an analyzer on the reflected beam allows the definition of the spin state of the exit beam ( the neutrons may flip their spin upon reflection if the magnetization of the sample is not parallel to the applied field). Thus four reflectivities can be measured, R++, R-+, R+- and R--, depending upon the orientation of the spins before and after reflection. Description of the working of the instrument is given in Ref. 2.

Controlled temperature cryostat (2-300K)

Samples can be inserted from top; maximum size 25 x 25 mm. The cryostat is not computer controlled.

Electromagnet

JANIS XYZ Triple Axis superconducting magnet system. Details are available here.

Data handling and data analysis: see user guide. Screen shots of analyzed data are also available here.

References:

1. Magnetic Depth Profiling Studies by Polarized Neutron Reflection, G. P. Felcher, Physica B 192, 137 (1993).

2. The Polarized Neutron Reflectometer, a New Instrument to Measure Magnetic Depth Profiles, G. P. Felcher, R. O. Hilleke, R. K. Crawford, J. Haumann, R. Kleb and G. Ostrowski, Review of Scientific Instruments, 58, 609 (1987).