Speaker
Description
Nuclear magnetic resonance exploits the delicate sensitivity of nuclear spin to characterise the local dynamics and structure of the surrounding chemical and magnetic environment inside materials. β-NMR is an extension of this technique, similar to muon spectroscopy, which relies on the detection of anisotropic beta emission produced by a polarised radioactive isotope such as ⁸Li. This allows for the measurement of local magnetic susceptibility, electric field gradients, and ionic transport processes.
In the past, this technique was successfully deployed as a bulk probe of matter using the capture of polarised thermal neutrons at Grenoble [1]. More recently, low-energy spin-polarised beams have been prepared at TRIUMF, Canada, enabling depth profiling of thin films and surfaces at depths ranging from 1 to 100 nm [2,3]. The ability to study surfaces and buried interfaces makes low-energy β-NMR a local real-space probe that complements reciprocal space techniques like polarised neutron reflectometry, offering different sensitivity to low-moment magnetism and the inelastic regime of dynamics and fluctuations.
We present a specific example of β-NMR in antiferromagnetic epitaxial α-Fe₂O₃ [4] and and 2D antiferromagnet FePS₃. This study shows that the nuclear spin-lattice relaxation rate of the implanted ⁸Li spin provides a sensitive measure of surface-localised spin dynamics and spin reorientation of the antiferromagnetic interface near the Morin transition. A variant of the technique using polarised neutron capture at ANSTO is also discussed.
[1] P. Heitjans, Solid State lonics 18 & 19, 50-64 (1986)
[2] Z. Salman et al., Phys. Rev. Lett. 109, 257207 (2012)
[3] I. McKenzie, J. Am. Chem. Soc. 136, 7833 (2014)
[4] D.L Cortie et al, Physical Review Letters 116, 106103 (2016)
