We report the results of dc and ac magnetization measurements as a function of temperature (1.8 -300 K) for the spin chain compound, Ca3CoRhO6, which has been recently reported to exhibit a partially disordered antiferromagnetic (PDAF) structure in the range 30 -90 K and spin-glass freezing below 30 K. We observe an unexpectedly large frequency dependence of ac susceptibility in the T range 30 -90 K, typical of superparamagnets. In addition, we find that there is no difference in the isothermal remanent magnetization behavior for the two regimes below 90 K. These findings call for more investigations to understand the magnetism of this compound.

CaV 2 O 4 is a spin-1 antiferromagnet, where the magnetic vanadium ions have an orbital degree of freedom and are arranged on quasi-one-dimensional zigzag chains. The first-and second-neighbor vanadium separations are approximately equal suggesting frustrated antiferromagnetic exchange interactions. High-temperature susceptibility and single-crystal neutron-diffraction measurements are used to deduce the dominant exchange paths and orbital configurations. The results suggest that at high temperatures CaV 2 O 4 behaves as a Haldane chain, but at low temperatures, it is a spin-1 ladder. These two magnetic structures are explained by different orbital configurations and show how orbital ordering can drive a system from one exotic spin Hamiltonian to another.The magnetism of low-dimensional and frustrated systems is a current research topic. Often, such phenomena arise from the crystal structure, e.g., well-separated chains or planes of magnetic ions can lead to low-dimensional magnetism while triangular structures combined with antiferromagnetic interactions give rise to frustration ͑e.g., Ref. 1͒. More recently the role of orbitals on exchange paths has been investigated and materials where direct exchange is the dominant magnetic interaction are particularly sensitive to orbital occupation and overlap. In some cases the structure gives rise to orbital configurations and exchange interactions that could not be deduced from a simple inspection of magnetic ion separations. Recent studies of vanadium spinels suggest that geometrically frustrated systems with an orbital degree of freedom can relax frustration via orbital ordering which gives rise to preferred exchange pathways. 2-6 In some cases there is a lowering of the dimensionality of the magnetism with dominant antiferromagnetic interactions coupling the magnetic moments into spin-1 chains ͑Haldane chains͒ 2,3 or into pairs ͑dimers͒. 6 Here we investigate CaV 2 O 4 where the vanadium ions interact predominantly via direct exchange and are arranged on zigzag chains. The intrachain V-V distances are approximately equal, suggesting competing firstand second-neighbor antiferromagnetic exchange interactions. However, we find that the magnetism is more complex than this simple picture suggests and is strongly influenced by the orbital ordering of the vanadium ions.Unlike many similar AV 2 O 4 systems that are spinels, CaV 2 O 4 crystallizes in the CaFe 2 O 4 -type structure with orthorhombic space group Pnam at room temperature. The magnetic V 3+ ions possess spin S = 1 due to two unpaired electrons in the 3d shell and are arranged in two inequivalent ͑although similar͒ zigzag chains of slightly distorted edgesharing VO 6 octahedra running along the crystallographic c direction ͑see Fig. 1͒.The octahedral environment partially removes the degeneracy of the 3d orbitals and both electrons occupy the lowerlying t 2g levels. The combination of edge-sharing octahedra which allows the overlap of t 2g orbitals and V-V distances of Ϸ3 Å favors direct exchange interactions. 7 S...

SrYb 2 O 4 is a geometrically frustrated rare-earth magnet, which presents a variety of interrelated magnetic phenomena. The magnetic Yb 3+ ions (J = 7/2) form potentially frustrated "zigzag" chains along the c axis, arranged in a honeycomb fashion in the ab plane. Heat capacity reveals a magnetic phase transition at T N = 0.9 K. The magnetic structure was solved by polarized neutron diffraction and found to be noncollinear with a reduction of the ordered spin moment from the full ionic moment. The low-energy excitations, which were measured by inelastic neutron scattering reveal diffuse scattering both above and below T N. Heat capacity and magnetocaloric effect were performed to map out the magnetic phase diagram as a function of magnetic field and temperature and show a complicated series of states. Altogether, the results suggest that the magnetic interactions in SrYb 2 O 4 compete with each other and with the single-ion anisotropy to produce a highly degenerate ground state manifold that suppresses the magnetic order, broadens the excitations and gives rise to a complex phase diagram.

The compound CaV 2 O 4 contains V+3 cations with spin S=1 and has an orthorhombic structure at room temperature containing zigzag chains of V atoms running along the c axis. We have grown single crystals of CaV 2 O 4 and report crystallography, static magnetization, magnetic susceptibility χ, ac magnetic susceptibility, heat capacity C p , and thermal expansion measurements in the temperature T range of 1.8-350 K on the single crystals and on polycrystalline samples. An orthorhombic-to-monoclinic structural distortion and a longrange antiferromagnetic (AF) transition were found at sample-dependent temperatures T S ≈108-145 K andT N ≈51-76 K, respectively. In two annealed single crystals, another transition was found at ≈200 K. In one of the crystals, this transition is mostly due to V 2 O 3 impurity phase that grows coherently in the crystals during annealing. However, in the other crystal the origin of this transition at 200 K is unknown. The χ(T) shows a broad maximum at ≈300 K associated with short-range AF ordering and the anisotropy of χ above T N is small. The anisotropic χ(T→0) data below T N show that the (average) easy axis of the AF magnetic structure is the b axis. The C p (T) data indicate strong short-range AF ordering above T N , consistent with the χ(T) data. We fitted our χ data by a J 1 -J 2 S=1 Heisenberg chain model, where J 1 ( J 2 ) is the (next)-nearestneighbor exchange interaction. We find J 1 ≈230 K and surprisingly, J 2 /J 1 ≈0 (or J 1 /J 2 ≈0). The interaction J ⊥ between these S=1 chains leading to long-range AF ordering at T N is estimated to be J ⊥ /J 1 ≳0.04. Keywords Physics and Astronomy Disciplines Condensed Matter Physics | Metallurgy CommentsThis article is from Physical Review B 79 (2009) The compound CaV 2 O 4 contains V +3 cations with spin S = 1 and has an orthorhombic structure at room temperature containing zigzag chains of V atoms running along the c axis. We have grown single crystals of CaV 2 O 4 and report crystallography, static magnetization, magnetic susceptibility , ac magnetic susceptibility, heat capacity C p , and thermal expansion measurements in the temperature T range of 1.8-350 K on the single crystals and on polycrystalline samples. An orthorhombic-to-monoclinic structural distortion and a long-range antiferromagnetic ͑AF͒ transition were found at sample-dependent temperatures T S Ϸ 108-145 K and T N Ϸ 51-76 K, respectively. In two annealed single crystals, another transition was found at Ϸ200 K. In one of the crystals, this transition is mostly due to V 2 O 3 impurity phase that grows coherently in the crystals during annealing. However, in the other crystal the origin of this transition at 200 K is unknown. The ͑T͒ shows a broad maximum at Ϸ300 K associated with short-range AF ordering and the anisotropy of above T N is small. The anisotropic ͑T → 0͒ data below T N show that the ͑average͒ easy axis of the AF magnetic structure is the b axis. The C p ͑T͒ data indicate strong short-range AF ordering above T N , consistent with the ͑T͒ data. We fitt...