Neutron diffraction investigation of theH−Tphase diagram above the longitudinal incommensurate phase ofBaCo2V2O8

Abstract

The quasi-one-dimensional antiferromagnetic Ising-like compound BaCo2V2O8 has been shown to be describable by the Tomonaga-Luttinger liquid theory in its gapless phase induced by a magnetic field applied along the Ising axis. Above 3.9 T, this leads to an exotic field-induced low-temperature magnetic order, made of a longitudinal incommensurate spin-density wave, stabilized by weak interchain interactions. By single-crystal neutron diffraction we explore the destabilization of this phase at a higher magnetic field. We evidence a transition at around 8.5 T towards a more conventional magnetic structure with antiferromagnetic components in the plane perpendicular to the magnetic field. The phase diagram boundaries and the nature of this second field-induced phase are discussed with respect to previous results obtained by means of nuclear magnetic resonance and electron spin resonance, and in the framework of the simple model based on the Tomonaga-Luttinger liquid theory, which obviously has to be refined in this complex system.

Magnetic phase diagram of the quantum spin chain compound SrCo2V2O8: a single-crystal neutron diffraction study

“…We note that the rate of this drop is overestimated due to the fact that only the partial double peak profiles could be resolved above 5.0 T ( figure 5(a)). The intensity reaches its minimum at 7.0 T, after which it increases monotonously with the magnetic field until 12.5 T. The reemergence of the scattering intensity above 7.0 T ( figure 5(b)) is also consistent with the transverse AFM order observed in BaCo 2 V 2 O 8 [24]. At 7.0 T, we performed two additional measurements with a much longer counting time.…”

“…However, the IC-AFM order in SrCo 2 V 2 O 8 turns out to be more fragile ( figure 5(b)). We note that the experimental critical field for the LSDW to transverse AFM order transition in BaCo 2 V 2 O 8 [24] already deviates from the mean-field prediction in [22]. The even more significant deviation in SrCo 2 V 2 O 8 revealed in our study further stresses the inadequacy of the interchain mean-field theory in addressing the complex magnetism in these systems.…”

“…At high fields, additional neutron counts can be resolved on top of (2, 0, 0), while (4, 0, 0) is not affected up to 14.9 T (not shown here). This indicates the ferromagnetic origin of the weak field-induced intensity at (2, 0, 0) [21,24]; the non-resolvable change at (4, 0, 0) is presumably due to the reduction of the magnetic form factor. Interestingly, our measurements on the (2, 0, 0) reflection at 7.5 T clearly reveal an intensity drop below ∼0.3 K ( figure 7(b)).…”

“…At very low temperatures, they obtained a LSDW phase that replaces the Néel order above an intermediate critical field, and another LSDW to transverse AFM phase transition in the high field region. So far, the exotic magnetic-field-induced phases have only been confirmed in the Ba-compound by single-crystal neutron diffraction [20,21,23,24]. These studies have revealed that the critical field for the Néel to LSDW transition (∼3.9 T) agrees well with the mean-field prediction, while the one for the LSDW to transverse AFM transition (∼9.0 T) appears to be much lower than the predicted value (∼15.1 T).…”

“…These studies have revealed that the critical field for the Néel to LSDW transition (∼3.9 T) agrees well with the mean-field prediction, while the one for the LSDW to transverse AFM transition (∼9.0 T) appears to be much lower than the predicted value (∼15.1 T). The discrepancy might indicate that the inter-and intra-chain exchange parameters used in [22] are not correct [24] or that the meanfield theory is inadequate. As pointed out by a recent nuclear magnetic resonance (NMR) study, the intra-and inter-couplings in BaCo 2 V 2 O 8 are rather complicated and could be strongly field-dependent [25].…”