Magnetic nanopantograph in the SrCu₂(BO₃)₂ Shastry–Sutherland lattice

• Published in: Proceedings of the National Academy of Sciences - PNAS Vol. 112; no. 7; pp. 1971 - 1976
• Main Authors: Radtke, Guillaume, Saúl, Andrés, Dabkowska, Hanna A., Salamon, Myron B., Jaime, Marcelo
• Format: Journal Article
• Language: English
• Published: United States National Academy of Sciences 17-02-2015; National Acad Sciences; National Academy of Sciences, Washington, DC (United States)
• Subjects: CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND SUPERFLUIDITY; density functional theory; high magnetic fields; magnetostriction; MATERIALS SCIENCE; Physical Sciences; Shastry–Sutherland; spin-lattice coupling; high magnetic fields; Shastry–Sutherland; magnetostriction; density functional theory; spin-lattice coupling
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.1421414112

Significance The spins of the unpaired electrons in a solid tend to align along an applied magnetic field. In the case of antiferromagnetic materials having competing interactions (frustration) it is common to observe that the magnetization increases, exhibiting complicated structures with discrete jumps and plateaus. SrCu ₂(BO ₃) ₂ is one of these materials, for which we find experimentally that its macroscopic physical dimensions also change with the magnetic field, mimicking the behavior in the magnetization. Using quantum mechanics, we show quantitatively that due to the orthogonal arrangement of the magnetic Cu ²⁺ dimers acting as pantographs, minute deformations allow significant reduction in the effective interactions responsible for the antiferromagnetism. This drop is sufficient to compensate the elastic energy loss in the lattice deformation.
Magnetic materials having competing, i.e., frustrated, interactions can display magnetism prolific in intricate structures, discrete jumps, plateaus, and exotic spin states with increasing applied magnetic fields. When the associated elastic energy cost is not too expensive, this high potential can be enhanced by the existence of an omnipresent magnetoelastic coupling. Here we report experimental and theoretical evidence of a nonnegligible magnetoelastic coupling in one of these fascinating materials, SrCu ₂(BO ₃) ₂ (SCBO). First, using pulsed-field transversal and longitudinal magnetostriction measurements we show that its physical dimensions, indeed, mimic closely its unusually rich field-induced magnetism. Second, using density functional-based calculations we find that the driving force behind the magnetoelastic coupling is the [Formula] superexchange angle that, due to the orthogonal Cu ²⁺ dimers acting as pantographs, can shrink significantly (0.44%) with minute (0.01%) variations in the lattice parameters. With this original approach we also find a reduction of ∼10% in the intradimer exchange integral J , enough to make predictions for the highly magnetized states and the effects of applied pressure on SCBO.