Magnetic light scattering in low-dimensional quantum spin systems

We have investigated magnetic properties of ${\mathrm{Rb}}_{2}{\mathrm{Cu}}_{2}{\mathrm{Mo}}_{3}{\mathrm{O}}_{12}$ powder. Temperature dependence of magnetic susceptibility and magnetic-field dependence of magnetization have shown that this cuprate can be considered as a compound of a one-dimensional spin-$1∕2$ Heisenberg system with ferromagnetic first-nearest-neighbor (1NN) and antiferromagnetic second-nearest-neighbor (2NN) competing interactions (competing system). Values of the 1NN and 2NN exchange interactions are estimated as ${J}_{1}=\ensuremath{-}138\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and ${J}_{2}=51\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ $(\ensuremath{\alpha}\ensuremath{\equiv}{J}_{2}∕{J}_{1}=\ensuremath{-}0.37)$. This value of $\ensuremath{\alpha}$ suggests that the ground state is a spin-singlet incommensurate state. In spite of relatively large ${J}_{1}$ and ${J}_{2}$, no magnetic phase transition appears down to $2\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, while an antiferromagnetic transition occurs in other model compounds of the competing system with ferromagnetic 1NN interaction. For that reason, ${\mathrm{Rb}}_{2}{\mathrm{Cu}}_{2}{\mathrm{Mo}}_{3}{\mathrm{O}}_{12}$ is a suitable compound to study properties of the incommensurate ground state that are unconfirmed experimentally.

https://arxiv.org/pdf/cond-mat/0404463.pdf

Magnetic properties of Rb 2 Cu 2 Mo 3 O 12 including a one-dimensional spin- 1 ∕ 2 Heisenberg system with ferromagnetic first-nearest-neighbor and antiferromagnetic second-nearest-neighbor exchange interactions

The structural and magnetic properties of the vanadate family with the chemical formula AV{sub 2}O{sub 5} (A = Li, Na, Cs, Mg, and Ca) are reviewed in terms of low-dimensional quantum-spin systems. These compounds have layered structures with A cations lying between layers. A structural element common to them is a VO{sub 5} square pyramid formed by five oxygens surrounding the V ion. Each compound has a structure with a characteristic arrangement of VO{sub 5} square pyramids formed by the sharing of edges and/or corners. All of these compounds are low-dimensional, spin-1/2 magnets and are spin-gap systems except {gamma}-LiV{sub 2}O{sub 5}. {alpha}{prime}-NaV{sub 2}O{sub 5} is a spin-Peierls system with the transition temperature T{sub SP} = 35 K and the energy gap {Delta} = 114 K. The parameter 2{Delta}/k{sub B}T{sub SP} = 3.53 holds well for the all organic and inorganic spin-Peierls compounds except {alpha}{prime}-NaV{sub 2}O{sub 5} for which the measure is 6.44. This difference suggests that the transition observed in {alpha}{prime}-NaV{sub 2}O{sub 5} is not a conventional spin-Peierls transition but an exotic one. Both MgV{sub 2}O{sub 5} and CaV{sub 2}O{sub 5} are spin-ladder systems with a similar two-leg ladder structure but the gap energy observed in MgV{sub 2}O{sub 5} ismore » very small compared with that in CaV{sub 2}O{sub 5}. CsV{sub 2}O{sub 5} is a dimer system with a spin gap of {approximately}160 K. {gamma}-LiV{sub 2}O{sub 5}, which has one-dimensional zigzag chains in the structure, shows neither spin gap nor magnetic order down to 0.5 K.« less

Vanadate Family as Spin-Gap Systems

A recent study of the magnetic susceptibility in single crystals of the spin-Peierls material CuGeO 3 doped with Zn indicates antiferromagnetic order at low temperatures. In the present publication, we report a neutron scattering investigation of the static and dynamic magnetic properties of single-crystal Cu 0.966 Zn 0.034 GeO 3 . Below the transition temperature of 4.2 ± 0.3 K, sharp magnetic Bragg peaks are observed at the magnetic superlattice positions (0 k l /2) with both k and l odd. However, the intensities of these peaks are not saturated even at 1.4 K, and the long range ordered magnetic moment is only ∼0.2 µ B per Cu site. Moreover, no well-defined spin-wave-like excitations are seen. Instead, overdamped magnetic excitations appear around the magnetic Bragg peaks at low temperatures. These results suggest an unusual state of magnetic order incorporating considerable magnetic and possibly structural disorder. We speculate that this unusual order is caused by a spatial distribution of spin states...

Neutron-scattering study of magnetism in single-crystal Cu1-xZnxGeO3

We report magnetic properties of ${\mathrm{Cu}}_{3}{({\mathrm{P}}_{2}{\mathrm{O}}_{6}\mathrm{O}\mathrm{H})}_{2}$. A spin-$1∕2$ trimer chain with ${J}_{1}\ensuremath{-}{J}_{2}\ensuremath{-}{J}_{2}$ interactions exists, where ${J}_{1}$ and ${J}_{2}$ denote two antiferromagnetic (AF) exchange interactions. A $1∕3$ magnetization plateau was observed above $12\phantom{\rule{0.3em}{0ex}}\mathrm{T}$ in a magnetization curve at $1.6\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The appearance of the plateau is consistent with the theorem of Oshikawa, Yamanaka, and Affleck [Phys. Rev. Lett. 78, 1984 (1997)]. Experimental results of magnetic susceptibility and magnetization agree well with quantum Monte Carlo results for the trimer chain with ${J}_{1}=95$ and ${J}_{2}=28\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. To our knowledge, ${\mathrm{Cu}}_{3}{({\mathrm{P}}_{2}{\mathrm{O}}_{6}\mathrm{O}\mathrm{H})}_{2}$ is the first model compound of trimer chains with only AF interactions showing a magnetization plateau.

1 ∕ 3 magnetization plateau observed in the spin- 1 ∕ 2 trimer chain compound Cu 3 ( P 2 O 6 O H ) 2

The magnetization was obtained as a function of temperature and magnetic field of a Cu 1− x Zn x GeO 3 single-crystal with x ∼ 0.04. The temperature dependence of the magnetic susceptibility in 0.1 T parallel to the c axis shows a clear cusp around 4.5 K and decreases rapidly below 4.5 K. There is no hysteresis between the data measured in zero-field-cooling and field-cooling processes. The field dependence of the magnetization parallel to the c axis at 2.0 K exhibits an apparent increase around 1.0 T, which corresponds to a spin-flop transition. The temperature and field dependences, the anisotropy, and the absence of the hysteresis undoubtedly prove the occurrence of an antiferromagnetic long-range order below 4.5 K instead of the spin-glass-like transition reported previously. Spins below the transition temperature are ordered almost parallel to the c axis. This is the first report of magnetic long-range order caused by nonmagnetic impurities.

Kenzou Manabe

Papers

Antiferromagnetic long-range order caused by nonmagnetic impurities; magnetization of single-crystal Cu1−xZnxGeO3