Spin-resolved off-specular neutron scattering from magnetic domain walls using the polarized gas spin filter

Abstract

We report on the use of the polarized 3 He gas filter and neutron resonant enhancement techniques for the measurement of spin-polarized diffuse neutron scattering due to ferromagnetic domains. A CoO/Co exchange biased bilayer was grown on a Ti/Cu/Al2O3 neutron resonator template. The system is cooled in an applied magnetic field of Ha = 2000 Oe through the Néel temperature of the antiferromagnet to 10 K where the applied magnetic field is swept as to measure the magnetic hysteresis loop. After the second magnetization reversal at the coercive field Hc2 = +230 Oe, the system is supposed to approach the original magnetic configuration. In order to prove that this is not the case for our exchange biased bilayer, we have measured four off-specular maps I++, I+−, I−+, I−− at Ha ≈ +370 Oe, where the Co magnetic spins were mostly reversed. They show a striking behavior in the total reflection region: while the nonspin-flip scattering exhibits no diffuse reflectivity, the spin-flip scattering shows strong diffuse scattering at incident angles which satisfy the resonance conditions. Moreover the spin-flip off-specular part of the reflectivity is asymmetric. The I−+ intensity occurs at higher exit angles than the specularly reflected neutrons, and the I+− intensity is shifted to lower angles. Their intensities are noticeably different and there is a splitting of the resonance positions for the up and down neutron spins (αAdditionally, a strong influence of the stray fields from magnetic domains to the resonance splitting is observed.PACS numbers: 75.60.Jk, 75.70.Cn, 61.12.Ha Keywords: spin-resolved neutron reflectivity, off-specular scattering, 3 He neutron spin filter, neutron resonator, exchange biasThe exchange bias (EB) phenomenon is associated with an exchange coupling between a ferromagnetic (F) and an antiferromagnetic (AF) layer across their common interface, resulting in an unidirectional magnetic anisotropy and a shift of the magnetic hysteresis by an exchange bias field H EB [1]. A remarkable effect related to the exchange bias is the asymmetry of the magnetization reversal, which is best revealed by neutron scattering [2,3,4,5]. The first magnetization reversal after field cooling is dominated by domain wall movement, whereas all subsequent reversals proceed essentially by rotation of the magnetization. In the present work we report on neutron off-specular diffuse scattering experiments performed at applied magnetic fields close to the second coercive field H c2 supplementing the previous results at fields near the first coercive fieldThe polarized 3 He gas spin filter was recently used for the first time [6] in a reflectivity experiment to measure diffuse scattering from thin magnetic films. The device itself is a cylindrical cell of 100 mm in length and 50 mm in diameter, filled with 0.5 bar 3 He gas. After filling, * Permanent address: Department of Experimental Physics, National Institute of Physics and Nuclear Engineering, P. O. BOX MG-6, 76900, Magurele, Romania. † Corresponding author. Tel: +49-234-322-...

A broadband ytterbium-doped tunable fiber laser for 3He optical pumping at 1083�nm

“…For laser manufacturers, the main technical challenge now lies in the cost-effective production of robust, reliable and very powerful fiber sources for production of large quantities of polarized gas to support the wide dissemination of established applications of hyperpolarized 3 He gas and the emergence of new ones [45,46,47,48,49,50,51,52,53,54,55].…”

Recent advances in polarised 3He spin filters at the ILL

“…Once the neutron spin-filter (NSF) cell is detached from the filling station, the polarisation of 3 He gas starts to decay with a time constant given by the gas pressure, the magnetic field homogeneity, and interactions with the inside walls of the spin-filter cell. We performed a systematic study of the effect of the cleaning procedure on the wall-relaxation rate of the cell.…”

“…In addition, they provide a homogeneous and predictable analysing efficiency and transmission, with negligible small-angle scattering from the 3 He cell. By measuring the data from the position-sensitive detector (PSD) at different sample angles, a polarised 3 He analyser provides a two-dimensional map of the wave vector transfer in reciprocal space [3].…”

“…Low-pressure 3 He is polarised using metastability-exchange optical pumping and compressed up to several bars using a hydraulic titanium alloy compressor. After compression, the gas is filled into spin-filter cells at a polarisation of 65-70%.…”

“…The production rate is between 1 and 2 bar l/h, depending on the desired gas polarisation. In addition to the routine and reliable provision of polarised 3 He for neutron experiments, the ILL 3 He group is also making significant progress in many other areas, which are essential for the successful use of 3 He neutron spin filters. The present paper gives highlights of some of these areas.…”