Poster Presentation 23rd International Society of Magnetic Resonance Conference 2023

ESR and NMR studies of S = 1/2 low-dimensional antiferromagnet Ca2Cu(OH)4[B(OH)4]2 (#306)

Kanata Hayashi 1 , Yutaka Kurachi 1 , Yuya Ishikawa 1 , Shunsuke C. Furuya 2 , Takayuki Asano 3 , Seitaro Mitsudo 3 , Hikomitsu Kikuchi 1 , Hajime Yamamoto 4 , Hideyuki Takahashi 5 , Eiji Ohmichi 6 , Susumu Okubo 5 6 , Hitoshi Ohta 5 6 , Yutaka Fujii 1
  1. Research Center for Development of Far-Infrared Region, University of Fukui, Fukui City, Fukui, Japan
  2. Saitama Medical University, Moroyama Town, Iruma District, Saitama, Japan
  3. Department of Applied Physics, University of Fukui, Fukui City, Fukui, Japan
  4. Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai, Miyagi, Japan
  5. Molecular Photoscience Research Center, Kobe University, Kobe, Hyogo, Japan
  6. Graduate School of Science, Kobe University, Kobe, Hyogo, Japan

 Ca2Cu(OH)4[B(OH)4]2 (mineral name: henmilite) is a natural mineral containing Cu2+ ions with S = 1/2 [1]. The mineral has attracted attention as a quantum spin system showing intermediate nature between a two-legged ladder system and a square lattice one [2]. The magnetic susceptibility and specific heat measurements revealed that this mineral undergoes an antiferromagnetic transition at cryogenic temperatures, 0.2 K at B = 0 T, 0.8 K at 3 T, and 0.3 K at 7 T [2]. The dome-shaped region of the antiferromagnetic ordered phase exits in the magnetic field-temperature phase diagram, indicating the presence of strong quantum fluctuations. We have investigated this interesting mineral from a microscopic viewpoint by using electron spin resonance (ESR) and nuclear magnetic resonance (NMR) techniques at temperatures down to 2 K.

 We have performed ESR measurements of Ca2Cu(OH)4[B(OH)4]2 single crystal in a wide frequency range from 10 to 300 GHz. At the frequency of 10 GHz, the angular dependence of the ESR linewidth suggests the two-dimensional interaction between Cu2+ spins [3]. In the ESR spectra at frequencies of 94 GHz, 130 GHz, and 300 GHz, multiple peaks appeared below approximately 20 K, in contrast, that at 10 GHz a single peak spectrum was obtained down to 2 K. The splitting widths of multiple peaks increased with decreasing temperature. Such a change in the spectra is possibly ascribed to the development of short-range order. However, the peak-splitting appeared at temperatures higher than the temperature at which the magnetic susceptibility reaches its maximum (~3 K). To investigate the origin of this strange splitting of the ESR spectra, 1H- and 11B-NMR measurements have been performed. Multiple-peaks structure of 1H (I = 1/2)-NMR spectrum was obtained owing to different hyperfine interactions of crystallographically inequivalent proton sites. The shifts of the peaks were proportional to the electron magnetization. However, the large splitting of several tens mT in the ESR spectrum cannot be explained quantitatively by hyperfine interactions. Results of 11B-NMR at low temperatures and the nuclear spin-lattice relaxation time T1 will also be discussed.

  1. I. Nakai et al., American Mineralogist, 71, 1234 (1986)
  2. H. Yamamoto et al., Phys. Rev. Materials, 5, 104405 (2021)
  3. K. Hayashi et al., JPS Conf. Proc., 38, 011144 (2023)