Low-energy magnetic excitations in the mixed spin-($\frac{1}{2},\frac{5}{2}$) chain

We present studies on the low-energy magnetic excitations in the Lieb-Mattis (LM) ferrimagnetic phase. We investigate the low-energy magnetic excitations in the mixed spin-($\frac{1}{2},\frac{5}{2}$) chain $(4-\mathrm{Br}-o-\mathrm{Me}\mathrm{Py}-\mathrm{V})\mathrm{Fe}{\mathrm{Cl}}_4$ based on low-temperature specific heat and multifrequency electron spin resonance (ESR) measurements. A clear $\sqrt{T}$ term is observed in the specific heat, which indicates a contribution of the low-energy magnetic excitation with a ferromagnetic quadratic dispersion expected in the LM ferrimagnetic state. Through the ESR mode analysis, we examine the effects of magnetic anisotropy on the excited states in the LM plateau phase.

Figure 1

Mixed spin-($\frac{1}{2},\frac{5}{2}$) chain composed of $s=\frac{1}{2}$ on the radical and $S=\frac{5}{2}$ on the $\mathrm{Fe}{\mathrm{Cl}}_4$ anion through $J$ and $J^{\prime }$ ($J>J^{\prime }$). The enlarged illustration shows the valence bond picture of the Lieb-Mattis ferrimagnetic state. The $S=\frac{5}{2}$ spins are composed of five $s=\frac{1}{2}$ spins, and the valence bond singlet pairs are formed by two $s=\frac{1}{2}$ spins.

Figure 2

(a) Low-temperature part of the specific heat of $(4-\mathrm{Br}-o-\mathrm{Me}\mathrm{Py}-\mathrm{V})\mathrm{Fe}{\mathrm{Cl}}_4$ at zero field. The solid line indicates the $\sqrt{T}$ behavior. The inset shows the specific heat up to 30 K. (b) Magnetic field vs temperature phase diagram showing the phase boundary for the second-order phase transition between the AF order and paramagnetic state. The spin-flop transition corresponds to a first-order transition within the AF order. The circles, square, and triangle indicate the phase boundaries determined from the specific heat $C_{\mathrm{p}}$, magnetic susceptibility $\chi$, and magnetization curve $M$, respectively [31].

Figure 3

Low-field magnetization curve of $(4-\mathrm{Br}-o-\mathrm{Me}\mathrm{Py}-\mathrm{V})\mathrm{Fe}{\mathrm{Cl}}_4$ at 1.5 K [31]. The field-independent behavior above approximately 2 T corresponds to the Lieb-Mattis plateau, which is identical to the fully polarized $S_{\mathrm{tot}}=2$ spins along the field direction. The inset shows the field derivative of the magnetization curve ($dM/dH$). $H_{\mathrm{sf}}$ and $H_{\mathrm{fp}}$ indicate the spin-flop transition and transition to the fully polarized state, respectively.

Figure 4

(a) Temperature dependence of ESR absorption spectra of $(4-\mathrm{Br}-o-\mathrm{Me}\mathrm{Py}-\mathrm{V})\mathrm{Fe}{\mathrm{Cl}}_4$ at 82.8 GHz. Frequency dependence of ESR absorption spectra at 1.8 K for (b) low frequencies measured by cylindrical cavities and (c) directly detected high frequencies. The arrows indicate resonance signals.

Figure 5

Frequency-field plot of the ESR fields at 1.8 K. The solid lines indicate the calculated resonance modes for the principal axes. The discontinuous changes in the lines correspond to the spin-flop transition at $H_{\mathrm{sf}}$ and transition to the fully polarized state of $S_{\mathrm{tot}}=2$ at $H_{\mathrm{fp}}$.