Magnetic structure and interactions in the quasi-one-dimensional antiferromagnetCaV2O4

Abstract

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...

Ground State of S = 1 Zigzag Spin–Orbital Chain

“…However, recent NMR results gave an evidence for an antiferromagnetic transition at T N =78 K [4], consistent with previous neutron diffraction measurements [5]. Based on susceptibility and neutron diffraction measurements, it has been suggested that CaV 2 O 4 behaves as weakly coupled Haldane chains at high temperatures above a structural phase transition temperature T S =141 K, while it changes to a spin ladder at low temperatures below T S [6]. In an orbital-based senario, these two spin systems are explained by different orbital configurations, caused by the structural distortion.…”

“…Regarding the spin exchange interaction, we have an AFM interaction through the second-order hopping process between the adjacent xy (yz) orbitals along the u (v) direction, while there is no spin exchange interaction along the w direction. That is, the system is a spin S =1 AFM ladder, corresponding to the low-temperature phase of CaV 2 O 4 [6,9]. Now we move to the DMRG results.…”

“…On the other hand, due to the (yz/zx)-type AFO configuration along the zigzag path, the second-order hopping process between the adjacent yz (zx) orbitals along the v (w) direction leads to a ferromagnetic (FM) interaction. Therefore, the system is considered as a spin S =1 zigzag chain with nearest-neighbor FM and next-nearest-neighbor AFM interactions, corresponding to the high-temperature phase of CaV 2 O 4 [6,9].…”

Synthesis and structural, magnetic, thermal, and transport properties of several transition metal oxides and aresnides

“…[10] Our magnetic neutron diffraction studies of the antiferromagnetic structure of CaV 2 O 4 single crystals are qualitatively consistent with the NMR analyses; these results together with high-temperature (T ≤ 1000 K) magnetic susceptibility measurements and their analysis are presented elsewhere. [151] We also find that CaV 2 O 4 exhibits a weak orthorhombic to monoclinic structural distortion upon cooling below a sample-dependent temperature T S = 108-147 K, discovered from our neutron and x-ray diffraction measurements to be reported in detail elsewhere. [152] In our two annealed single crystals only, anomalies in the heat capacity and thermal expansion are also found at T S1 ≈ 200 K. From high-energy x-ray diffraction measurements reported here, we find that in one of the crystals the anomaly is most likely primarily due to the metal-insulator and structural transitions in V 2 O 3 impurity phase that grows coherently in the crystal when it is annealed.…”

“…The Ca atoms are located in between these chains. The system undergoes an orthorhombic to monoclinic structural distortion at a sample-dependent T S ≈ 108 − 145 K and an antiferromagnetic transition with noncollinear spin structure at a sample-dependent T N ≈ 50 − 70 K. [40][41][42] A peak in the heat capacity versus temperature at T ∼ 200 K has been observed which might be the long-sought chiral phase transition. A detailed study involving synthesis of polycrystalline and single crystal samples of CaV 2 O 4 , and investigation of magnetic, structural, and thermal properties of the samples, is given in Appendix A of this thesis.…”