Magnetization and spin dynamics of the spin S=12 hourglass nanomagnet Cu5(OH)2(NIPA)4·10H2O
journal contributionposted on 14.08.2018, 15:38 by Ramesh Nath, Alexander A. Tsirlin, P. Khuntia, Oleg Janson, T. Forster, M. Padmanabhan, J. Li, Yu Skourski, M. Baenitz, Helge Rosner, Ioannis RousochatzakisIoannis Rousochatzakis
We report a combined experimental and theoretical study of the spin S=12 nanomagnet Cu5(OH)2(NIPA)4·10H 2O (Cu5-NIPA). Using thermodynamic, electron spin resonance, and 1H nuclear magnetic resonance measurements on one hand, and ab initio density-functional band-structure calculations, exact diagonalizations, and a strong-coupling theory on the other, we derive a microscopic magnetic model of Cu5-NIPA and characterize the spin dynamics of this system. The elementary fivefold Cu2+ unit features an hourglass structure of two corner-sharing scalene triangles related by inversion symmetry. Our microscopic Heisenberg model comprises one ferromagnetic and two antiferromagnetic exchange couplings in each triangle, stabilizing a single spin S=12 doublet ground state (GS), with an exactly vanishing zero-field splitting (by Kramers' theorem), and a very large excitation gap of Δâ‰ 68 K. Thus, Cu5-NIPA is a good candidate for achieving long electronic spin relaxation (T1) and coherence (T2) times at low temperatures, in analogy to other nanomagnets with low-spin GS's. Of particular interest is the strongly inhomogeneous distribution of the GS magnetic moment over the five Cu2+ spins. This is a purely quantum-mechanical effect since, despite the nonfrustrated nature of the magnetic couplings, the GS is far from the classical collinear ferrimagnetic configuration. Finally, Cu5-NIPA is a rare example of a S=12 nanomagnet showing an enhancement in the nuclear spin-lattice relaxation rate 1/T1 at intermediate temperatures.
R.N. was funded by MPG-DST (Max Planck Gesellschaft, Germany, and Department of Science and Technology, India) fellowship. A.T. was supported by the Alexander von Humboldt Foundation and the Mobilitas program of the ESF (Grant No. MTT77). O.J. acknowledges partial support by Mobilitas Grant No. MJD447. I.R. was funded by the Deutsche Forschungsgemeinschaft (DFG) under the EmmyNoether program. The high-field magnetization measurements were supported by EuroMagNET II under EC Contract No. 228043.