Pulsed-field broadband NMR of SrCu2(BO3)2
The spin-dimer antiferromagnet SrCu2(BO3)2 was investigated in great detail over the past two decades, as it represents the most prominent realization of the Shastry-Sutherland lattice model.
In this material, electronic spins of Cu2+ ions within the Cu2(BO3)2 layers form a lattice of mutually orthogonal spin-singlet dimers with significant interdimer interaction, giving rise to pronounced magnetic frustration. At high magnetic fields, triplet states with reduced kinetic energy condense, resulting in a field-driven sequence of magnetic superlattices with corresponding plateaus in the macroscopic magnetization. The microscopic detection of these superlattice structures by means
of NMR as a local probe is of great interest. To study all magnetization plateaus up to half of the saturation value, pulsed magnetic fields up to the regime of 100 T are required. A team of Estonian, Canadian, and German scientists from Leipzig University, the NICPB (Tallinn), McMaster University (Hamilton), and the HLD has
performed NMR measurements on SrCu2(BO3)2 in pulsed magnetic fields. The results are in very good agreement with a transition from a high-temperature, paramagnetic state to a low-temperature, commensurate superstructure of field-induced spin-dimer triplets in the 1/3 magnetization plateau. Moreover, the technical approach to measure broadband NMR in pulsed fields, that was developed in the course of this work, opens the door not only to the exploration of the higher-field ground states of SrCu2(BO3)2, but also to studies of many other quantum magnets with complex interactions that stabilize new phases of matter in very strong magnetic fields.
(a) Macroscopic magnetization of SrCu2(BO3)2 at 2 K (from Matsuda et al.). (b) Unit cell of the Cu2(BO3)2 plane and (c) the corresponding 11B NMR spectrum at 6 T and 5 K. (d) Magnetic superlattice in the 1/3 magnetization plateau with three different 11B sites (red spheres). White and black spheres represent negative and positive spin polarization, their size the magnitude. (e) 11B NMR spectrum at 54 T and 2 K.
Field-stepped broadband NMR in pulsed magnets and application to SrCu2(BO3)2 at 54 T
T, J. Kohlrautz, J, Haase, E.L. Green, Z. T. Zhang, J. Wosnitza, T. Herrmannsdörfer, H. A. Dabkowska, B. D. Gaulin, R. Stern, and H. Kühne, J. Magn. Reson. 271, 52 (2016).