Field-induced incommensurate phase in the strong-rung spin ladder with ferromagnetic legs

Abstract

We report magnetization, specific heat, and NMR measurements of 3-Br-4-F-V [= 3-(3-bromo-4fluorophenyl)-1,5-diphenylverdazyl], strong-rung S=1/2 Heisenberg spin ladder with ferromagnetic leg interactions. We explain the magnetic and thermodynamic properties based on the strong-rung regime. Furthermore, we find a field-induced successive phase transition in the specific heat and the nuclear spin-lattice relaxation rate 1/T 1 . 19 F-NMR spectra for higher and lower temperature phases indicate partial magnetic order and incommensurate long-range order, respectively, evidencing the presence of frustration due to weak interladder couplings.

Three-dimensional Bose-Einstein condensation in the spin- 12 ferromagnetic-leg ladder 3-Br-4-F-V

“…A sharp peak indicative of the 3D ordering can be observed. On the other hand, the shoulderlike anomaly inferred in the previous report [17] to be a partial ordering is weak (only the one at 7 T is indicated by an open arrow) or even indiscernible within the experimental resolution. This weakness of the partial-ordering anomaly might be due to a difference of the sample quality from the previous one.…”

“…We also note that the anomaly in d(T χ)/dT in Fig. 4 (b) is broader than that in dχ/dT reported in the previous experiment [17]. Considering these, we assess that the phase boundary determined from C/T is more reliable in this study.…”

“…Theoretically, such extremum in (quasi-)1D gapped spin systems can be interpreted as a crossover to the low-temperature TLL region [22]. Then, a 3D ordering exists slightly below T ex [17]. In a previous report [17], 3D ordering temperature, T c , from χ-T was determined from a kink anomaly of the temperature derivative of χ, and it was in good agreement with T c determined from a peak of the temperature dependence of the specific heat C. This correspondence is considered to be associated with Fisher's relation for antiferromagnets,…”

“…) and only this compound has a spin gap (∼ 5 T) [16,17]. The other two compounds are antiferromagnetically ordered in the ground state due to 3D interactions [16].…”

“…The other two compounds are antiferromagnetically ordered in the ground state due to 3D interactions [16]. Compared with BPCB [11], which has a similar rung-coupling constant (J || /k B = 3.3 K, J ⊥ /k B = 12.9 K, and µ 0 H c2 ∼ 14.5 T), 3-Br-4-F-V actually has the smaller saturation field, µ 0 H c2 ∼ 9 T. In the magnetic field range between H c1 and H c2 , 3-Br-4-F-V exhibits a 3D incommensurate ordering due to interladder couplings below ∼0.6 K as revealed by nuclear magnetic resonance (NMR) measurements [17]. A TLL region was expected to exist above the 3D ordering temperatures as is the case for (quasi-)1D gapped spin systems [10,11,22], but, in reality, most of the anticipated TLL region is replaced by a partial ordering phase in 3-Br-4-F-V [17].…”