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PGI Colloquium:

Prof. Dr. Stephen J. Blundell,
University of Oxford, UK

PGI Lecture Hall, Building 04.8, 2nd Floor, Room 365

08 Dec 2017 11:00

Frustrated Magnetism Studied with Muons: the Effect of Muons on Crystal Fields

BlundellCopyright: Prof. Dr. Blundell

Implanted muons provide a very powerful local probe for studying frustrated magnetism. In this talk, I will explain why this is the case and review various experimental studies in which muons have been effective in revealing the way in which a magnetic system fails to order when frustrated interactions are present. However, when a muon is implanted into a magnetic insulating compound it will stop close to an anion and will pull that anion away from its equilibrium position.

The resulting distortion introduced into the crystal structure is anisotropic and can alter the crystal field around the transition metal or lanthanide cations to which the anion is bonded. Quantifying this effect is very important since it can, on occasion, substantially and dramatically affect the local physics being probed [1]. This has been demonstrated in Pr-based pyrochlores in which a hyperfine enhancement of nuclear moments [2] arises from the splitting of the non-Kramers doublet ground states for Pr ions nearby to the muon, an effect that overwhelms the expected spin ice behaviour [1].

Our approach to studying this problem is to use DFT+μ (density functional theory with an implanted muon) [3] to identify the muon site and characterise the local distortion. We then calculate the effect on the crystal field levels. We have repeated these calculations for other rare earth pyrochlores and develop some general arguments for understanding when these effects in μSR experiments will become crucially important and when they can be safely ignored. Our results are compared with experimental data and provide new insight into the nature of the muon as a probe in magnetic systems.


[1] F. R. Foronda, F. Lang, J. S. Möller, T. Lancaster, A. T. Boothroyd, F. L. Pratt, S. R. Giblin, D. Prabhakaran and S. J. Blundell, Phys. Rev. Lett. 114, 017602 (2015).
[2] B. Bleaney, Physica 69, 317 (1973).
[3] D. E. MacLaughlin, Y. Ohta, Y. Machida, S. Nakatsuji, G. Luke, K. Ishida, R. Heffner, L. Shu, and O. Bernal, Physica 404B, 667 (2009).
[4] J. S. Möller, D. Ceresoli, T. Lancaster, N. Marzari, and S. J. Blundell, Phys. Rev. B 87, 121108(R) (2013); F. Bernardini, P. Bonfà, S. Massidda, and R. De Renzi, Phys. Rev. B 87, 115148 (2013).


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