Two-sublattice description of the dimer-trimer chain compound ${\mathrm{Li}}_2{\mathrm{Cu}}_5{\mathrm{Si}}_4{\mathrm{O}}_{14}$: High-field magnetization and ESR studies

We carried out high magnetic field magnetization and electron spin resonance (ESR) measurements for the alternating dimer-trimer chain compound ${\mathrm{Li}}_2{\mathrm{Cu}}_5{\mathrm{Si}}_4{\mathrm{O}}_{14}$. The magnetization curve measured at 2 K exhibits a spin-flop transition at 2.6 T followed by a nonlinear increase in magnetization at high field up to 55 T. The antiferromagnetic ordering at $T_{\mathrm{N}}=22\mathrm{K}$ is well characterized by the temperature-dependent ESR spectra, and the anisotropic $g$ factors at the paramagnetic phase are determined. Intriguingly, our frequency-dependent ESR spectra at 2 K reveal the development of two collinear antiferromagnetic sublattices with biaxial anisotropy, which is quite unusual because ${\mathrm{Li}}_2{\mathrm{Cu}}_5{\mathrm{Si}}_4{\mathrm{O}}_{14}$ contains five sublattices. Thus, ${\mathrm{Li}}_2{\mathrm{Cu}}_5{\mathrm{Si}}_4{\mathrm{O}}_{14}$ is three-dimensionally ordered and its magnetism might be described by a pseudo-two-sublattice model composed of antiferromagnetically coupled dimer-trimer groups (↑↑, ↑↓↑) and (↓↓, ↓↑↓).

Figure 1

(a) A sketch of dimer-trimer groups, (↑↑, ↑↓↑) and (↓↓, ↓↑↓), aligned along the chain direction ($a$ axis). The large arrows stand for the antiferromagnetically coupled pseudo-two sublattices. (b) Rietveld refinement of the XRD data showing the experimental (red open cycle), calculated (black line) and difference patterns (blue line). The green vertical bars are the expected Bragg reflection positions.

Figure 2

(a) ZFC heating χ($T$) curve measured in a field of 0.1 T and temperature dependence of the integrated intensity $I_{\mathrm{ESR}}$ of the ESR peak. The inset shows the χ($T$) curves at 0.1, 2, and 3 T for comparison. (b) High-field $M$($H$) curve at 2 K for the powder sample calibrated with the SQUID data. The inset shows the low-field $M$($H$) and dM/dH curves at 2 K. The dashed line marks the position of the spin-flop transition.

Figure 3

$X$-band (9.4 GHz) ESR spectra measured for the powder sample. The red dashed line stands for Lorentzian fit for the data at 40 K.

Figure 4

(a,b) Temperature-dependent ESR spectra measured at 95 and 285 GHz for the powder sample. The sharp absorption line of DPPH with $g=2.0$ is for the field marker. (c) ESR absorption line (black solid line) at 285 GHz and 40 K as well as the fit line (red dashed line).

Figure 5

(a) Frequency-dependent ESR spectra measured at 2 K for the powder sample. (b) Frequency-field (ω-$H$) relationships. The solid lines are the fits of the experimental data according to the two-sublattice AFM resonance modes with biaxial anisotropy. The dashed line is the EPR line with $g=2.17$.

Figure 6

$M$($H$) and dM/$\mathrm{d}H$ curves measured at 2 K for the magnetically aligned sample.

Figure 7

Temperature-dependent ESR spectra measured at 315 GHz for the magnetically aligned sample: (a) $H$ // aligned axis; (b) $H$ ⊥ aligned axis.