Home >  About ISSP >  Publications > Activity Report 2018 > Xxxx Group

Unconventional Magnetic and Thermodynamic Properties of S = 1/2 Spin Ladder with Ferromagnetic Legs

H. Yamaguchi and T. Sakakibara

Spin-ladder systems have been investigated both theoretically and experimentally in relation in particular to field-induced quantum phase transitions and high-Tc superconductivity. Among these systems, antiferromagnetic (AFM) two-leg spin ladders, which have AFM rung and leg interactions, have been most extensively studied and have turned out to show attractive behaviors originating from their strong quantum fluctuations. By contrast, a spin ladder with ferromagnetic (FM) leg interactions, which can be viewed as antiferromagnetically coupled FM chains, has not been realized experimentally. Its ground state and the magnetic behavior have been discussed extensively from a theoretical point of view as an example of complicated quantum spin systems. The experimental verification of these properties is thus quite significant for studies on complicated quantum behavior unsuspected in conventional spin systems.

Fig. 1. Temperature dependence of (a) the magnetic susceptibility and (b) Cm/T of 3-Cl-4-F-V in various magnetic fields for H//a. The arrows indicate the phase transition temperatures. For clarity, Cm/T for 1.0, 1.5, 2.0, 3.0, 4.0, and 4.5 T have been shifted up by 0.2, 0.4, 0.7, 1.3, 1.7, and 2.5 J•mol-1•K-2, respectively. The inset shows the temperature derivative of the magnetic susceptibility at 0.5 T.

Fig. 2. Magnetic field versus temperature phase diagram of 3-Cl-4-F-V for H//a The closed circles, closed, triangles, and open triangle indicate the transition temperatures determined from the magnetic specific heat, the magnetic susceptibility, and its temperature derivative, respectively. The solid vertical line indicates the saturation field.

Here, we report the first model compound of an S = 1/2 two-leg spin ladder with FM leg interaction [1]. We have succeeded in synthesizing a new verdazyl radical 3-Cl-4-F-V [3-(3-chloro-4-fluorophenyl)-1,5-diphenylverdazyl] and solved its crystal structure. The ab initio molecular orbital (MO) calculation indicated the formation of an S = 1/2 two-leg spin ladder with FM leg and AFM rung interactions. The experimental results of magnetic susceptibility, magnetization curve, and specific heat were successfully explained as the expected spin-ladder model by using the quantum Monte Carlo method.

Figures 1(a) and 1(b) show the low-temperature region of the magnetic susceptibility and magnetic specific heat in various magnetic fields, respectively. At higher magnetic fields, we observed extrema of the magnetic susceptibility, which appear almost symmetrically with respect to the curve at 3.0 T. Regarding the magnetic specific heat, the anomaly observed at zero field splits into two peaks above 1.5 T, as shown in Fig. 1(b). Figure 2 shows the temperature-field phase diagram obtained from those specific temperatures. The transition point shifts toward the high-temperature region with an increasing magnetic field up to 3.0 T. This type of phase boundary shape is often associated with the field-induced phase transition in a gapped spin system. Since the expected small energy gap of 1.06 K is considered to disappear owing to the weak interladder interactions, the present system must be in the vicinity of the quantum critical point between the gapped rung-singlet and the gapless ordered phases. Considering the magnetic specific heat, we should take into account the possibility of successive phase transitions, which is often induced by large magnetic anisotropy and/or noncollinear magnetic structure. Since organic radical systems have negligibly weak magnetic anisotropy, we can suggest the possibility of noncollinear magnetic structure induced by frustration. The MO calculation indicated that there are three kinds of possible small interladder interactions. These three interactions can form frustrated lattices, and then a noncollinear magnetic structure will appear. This unexpected field-induced successive phase transition possibly originates from the interplay of low dimensionality and frustration. The present results will stimulate studies on spin ladder with FM interactions and unconventional behavior of the complicated quantum spin systems.


References
  • [1] H. Yamaguchi, K. Iwase, T. Ono, T. Shimokawa, H. Nakano, Y. Shimura, N. Kase, S. Kittaka, T. Sakakibara, T. Kawakami, and Y. Hosokoshi, Phys. Rev. Lett. 110, 157205 (2013).
Authors
  • H. Yamaguchia, K. Iwasea, T. Onoa, T. Shimokawab, H. Nakanoc, Y. Shimura, N. Kase, S. Kittaka, T. Sakakibara, T. Kawakamic, and Y. Hosokoshia
  • aOsaka Prefecture University
  • bKobe University
  • cUniversity of Hyogo
  • dOsaka University