Half-ordered state in the anisotropic Haldane-gap antiferromagnetNi(C5D14N2)2

Abstract

Neutron diffraction experiments performed on the Haldane gap material Ni(C5D14N2)2N3(PF6)in high magnetic fields applied at an angle to the principal anisotropy axes reveal two consecutive fieldinduced phase transitions. The low-field phase is the gapped Haldane state, while at high fields the system exhibits 3-dimensional long-range Néel order. In a peculiar phase found at intermediate fields only half of all the spin chains participate in the long-range ordering, while the other half remains disordered and gapped. PACS numbers:High-field phase transitions in quantum antiferromagnets (AFs) have recently drawn a great deal of attention. Of particular interest is field-induced spin freezing exhibited by many quantum spin liquids. The massive triplet of low-lying S = 1 gap excitations (magnons) in such systems is subject to Zeeman splitting by external magnetic fields. A soft-mode quantum phase transition occurs when the gap for one member of the triplet is driven to zero. The result is a Bose condensation of magnons. In the presence of magnetic anisotropy and weak inter-chain interactions, always found in real materials, the magnetized high-field phase is a Néel-like state with AF long-range order. The phase transition itself and the peculiarities of the high-field phase have been studied experimentally in several model materials including the Haldane-gap antiferromagnets1,2,3,4,5,6 and NDMAZ (Ni (C 5 9 and dimer systems such as TlCuCl 3 10 and Cs 3 Cr 2 Br 9 . 11It has been long established that magnetic anisotropy, which is particularly important for S = 1 materials, has a strong impact on the phase transition. The value of the critical field H c depends on the relative orientation of the applied field and the anisotropy tensor.12,13,14 A very interesting case is that of NDMAP. This compound features two equivalent sets of Haldane spin chains with non-collinear local anisotropy axes. To date, all experiments were performed in magnetic fields applied along the principal axes of the orthorhombic crystal structure that are also the macroscopic magnetic anisotropy axes. In these geometries all tilts of the local anisotropy axes relative to the field direction are the same for the two chain types. In the present study we investigate a less symmetric scenario, in which the magnetic field is applied in a general direction relative to the crystal axes. We find two consecutive field-induced transitions from the quantum-disordered spin liquid to ordered Néel phase, with a novel "half-ordered" phase in-between. • relative to the c axis. The tilt direction is opposite for type-A and type-B chains. The field H is applied at an angle ψ relative to the c axis of the crystal and forms unequal angles φA and φB with the anisotropy axes zA and zB, respectively.The crystal structure of NDMAP (orthorhombic space group P nmn, a = 18.046Å, b = 8.705Å, and c = 6.139Å) was described in detail in Ref.4 . The S = 1 chains run along the crystallographic c axis. The inchain exchange constant is J = 2.6 meV. Inter-chain interactions are mu...

High-field multifrequency electron-spin-resonance study of the Haldane magnetNi(C5H14N2)Kashiwagi1, Hagiwara2, Kimura3 et al. 2009Phys. Rev. BSelf Cite71110View full textAdd to dashboardCite

“…35 This independent behavior of the two kinds of chains is very interesting because the signals around the a axis were observed in the LRO phase at 1.5 K, which means that cooperative behavior is expected.…”

“…From the neutron-scattering experiments, it was reported that the two types of chains in NDMAP behave independently against the external magnetic field. 35 In addition, Ref. 33 showed an interesting H − i ͑i =1,2͒ phase diagram of the sublattice magnetization at zero temperature, where the i stands for the angle between the external magnetic field and one of the principal axes of Ni-O 6 octahedron in the two types of chains.…”

Crossover Behavior of Magnetic Excitations in the Anisotropic Haldane Antiferromagnet under High Magnetic Fields

“…In addition, we have discussed the possibility to observe the novel consecutive field-induced phase transitions in NDMAP. Although the observation of this peculiar phenomenon has been initiated, 8) it is need further investigations to reveal the detailed structure of the order and other critical behavior both from experimental and from theoretical sides.…”

“…Thus, in NDMAP we may observe the different critical behavior just by increasing the field. It is worth mentioning that recently a trace of such behavior has been observed by the neutron elastic scattering 8) although a detailed investigation of the critical behavior has not been performed yet. §3.…”

Quantum phase transition in field-induced ordering phases of anisotropic Haldane systems

“…Note that the anisotropies, d and e, make the x-axis easy while the z-axis hard. Considering that the anisotropy z-axis in NDMAP is tilted in the crystallographic a2c plane [6], we assume h has the x and the z components: h ¼ hðsin y; 0; cos yÞ.…”