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Register a new userRealising square and diamond lattice $S=1/2$ Heisenberg antiferromagnet models in the $alpha$ and $beta$ phases of the coordination framework, KTi(C$_2$O$_4$)$_2cdot$textit{x}H$_2$O
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We report the crystal structures and magnetic properties of two psuedo-polymorphs of the $S=1/2$ Ti$^{3+}$ coordination framework, KTi(C$_2$O$_4$)$_2cdot$xH$_2$O. Single-crystal X-ray and powder neutron diffraction measurements on $alpha$-KTi(C$_2$O$_4$)$_2cdot$xH$_2$O confirm its structure in the tetragonal $I4/mcm$ space group with a square planar arrangement of Ti$^{3+}$ ions. Magnetometry and specific heat measurements reveal weak antiferromagnetic interactions, with $J_1approx7$ K and $J_2/J_1=0.11$ indicating a slight frustration of nearest- and next-nearest-neighbor interactions. Below $1.8$ K, $alpha$ undergoes a transition to G-type antiferromagnetic order with magnetic moments aligned along the $c$ axis of the tetragonal structure. The estimated ordered moment of Ti$^{3+}$ in $alpha$ is suppressed from its spin-only value to $0.62(3)~mu_B$, thus verifying the two-dimensional nature of the magnetic interactions within the system. $beta$-KTi(C$_2$O$_4$)$_2cdot$2H$_2$O, on the other hand, realises a three-dimensional diamond-like magnetic network of Ti$^{3+}$ moments within a hexagonal $P6_222$ structure. An antiferromagnetic exchange coupling of $Japprox54$ K -- an order of magnitude larger than in $alpha$ -- is extracted from magnetometry and specific heat data. $beta$ undergoes Neel ordering at $T_N=28$ K, with the magnetic moments aligned within the $ab$ plane and a slightly reduced ordered moment of $0.79~mu_B$ per Ti$^{3+}$. Through density-functional theory calculations, we address the origin of the large difference in the exchange parameters between the $alpha$ and $beta$ psuedo-polymorphs. Given their observed magnetic behaviors, we propose $alpha$-KTi(C$_2$O$_4$)$_2cdot$xH$_2$O and $beta$-KTi(C$_2$O$_4$)$_2cdot$2H$_2$O as close to ideal model $S=1/2$ Heisenberg square and diamond lattice antiferromagnets, respectively.
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We present the synthesis and a detail investigation of structural and magnetic properties of polycrystalline [VO(HCOO)$_2cdot$(H$_2$O)] by means of x-ray diffraction, magnetic susceptibility, high-field magnetization, heat capacity, and electron spin resonance measurements. It crystallizes in a orthorhombic structure with space group $Pcca$. It features distorted VO$_6$ octahedra connected via HCOO linker (formate anions) forming a two-dimensional square lattice network with a bilayered structure. Analysis of magnetic susceptibility, high field magnetization, and heat capacity data in terms of the frustrated square lattice model unambiguously establish quasi-two-dimensional nature of the compound with nearest neighbour interaction $J_1/k_{rm B} simeq 11.7$~K and next-nearest-neighbour interaction $J_2/k_{rm B} simeq 0.02$~K. It undergoes a Neel antiferromagnetic ordering at $T_{rm N} simeq 1.1$~K. The ratio $theta_{rm CW}/T_{rm N} simeq 10.9$ reflects excellent two-dimensionality of the spin-lattice in the compound. A strong in-plane anisotropy is inferred from the linear increase of $T_{rm N}$ with magnetic field, consistent with the structural data.
We report the magnetization ($chi$, $M$), specific heat ($C_{text{P}}$), and neutron powder diffraction results on a quasi-two-dimensional $S$ = 2 square lattice antiferromagnet Ba$_2$FeSi$_2$O$_7$ consisting of FeO$_4$ tetragons with a large compressive distortion (27%). Despite of the quasi-two-dimensional lattice structure, both $chi$ and $C_{text{P}}$ present three dimensional magnetic long-range order below the Neel temperature $T_{text{N}}$ = 5.2 K. Neutron diffraction data shows a collinear $Q_{m}$ = (1,0,0.5) antiferromagnetic (AFM) structure with the in-plane ordered magnetic moment suppressed by 26% below $T_{text{N}}$. Both the AFM structure and the suppressed moments are well explained by the Monte Carlo simulation with a large single-ion ab-plane anisotropy $D$ = 1.4 meV and a rather small in-plane Heisenberg exchange $J_{text{intra}}$ = 0.15 meV. The characteristic two dimensional spin fluctuations can be recognized in the magnetic entropy release and diffuse scattering above $T_{text{N}}$. This new quasi-2D magnetic system also displays unusual non-monotonic dependence of the $T_{text{N}}$ as a function of magnetic field $H$.
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