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Register a new userCollinear order in a frustrated three-dimensional spin-$frac12$ antiferromagnet Li$_2$CuW$_2$O$_8$
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Magnetic frustration in three dimensions (3D) manifests itself in the spin-$frac12$ insulator Li$_2$CuW$_2$O$_8$. Density-functional band-structure calculations reveal a peculiar spin lattice built of triangular planes with frustrated interplane couplings. The saturation field of 29 T contrasts with the susceptibility maximum at 8.5 K and a relatively low Neel temperature $T_Nsimeq 3.9$ K. Magnetic order below $T_N$ is collinear with the propagation vector $(0,frac12,0)$ and an ordered moment of 0.65(4) $mu_B$ according to neutron diffraction data. This reduced ordered moment together with the low maximum of the magnetic specific heat ($C^{max}/Rsimeq 0.35$) pinpoint strong magnetic frustration in 3D. Collinear magnetic order suggests that quantum fluctuations play crucial role in this system, where a non-collinear spiral state would be stabilized classically.
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We report thermodynamic properties, magnetic ground state, and microscopic magnetic model of the spin-1 frustrated antiferromaget Li$_{2}$NiW$_{2}$O$_{8}$ showing successive transitions at $T_{rm N1}simeq 18$ K and $T_{rm N2}simeq 12.5$ K in zero field. Nuclear magnetic resonance and neutron diffraction reveal collinear and commensurate magnetic order with the propagation vector $mathbf k=(frac12,0,frac12)$ below $T_{rm N2}$. The ordered moment of 1.8 $mu_B$ at 1.5 K is directed along $[0.89(9),-0.10(5),-0.49(6)]$ and matches the magnetic easy axis of spin-1 Ni$^{2+}$ ions, which is determined by the scissor-like distortion of the NiO$_6$ octahedra. Incommensurate magnetic order, presumably of spin-density-wave type, is observed in the region between $T_{rm N2}$ and $T_{rm N1}$. Density-functional band-structure calculations put forward a three-dimensional spin lattice with spin-1 chains running along the $[01bar 1]$ direction and stacked on a spatially anisotropic triangular lattice in the $ab$ plane. We show that the collinear magnetic order in Li$_2$NiW$_2$O$_8$ is incompatible with the triangular lattice geometry and thus driven by a pronounced easy-axis single-ion anisotropy of Ni$^{2+}$.
Spin density waves, based on modulated local moments, are usually associated with metallic materials, but have recently been reported in insulators which display coupled magnetic and structural order parameters. We discuss one such example, the multiferroic Cu$_3$Nb$_2$O$_8$, which is reported to undergo two magnetic phase transitions, first to a spin density wave phase at $T_N approx 26.5K$, and then to a helicoidal structure coupled to an electric polarization below $T_2 approx 24K$ [R. D. Johnson, et al., Phys. Rev. Lett., 107, 137205 (2011)] which breaks the crystallographic inversion symmetry. We apply spherical polarimetry to confirm the low-temperature magnetic structure, yet only observe a single magnetic phase transition to helicoidal order. We argue that the reported spin density wave originates from a decoupling of the components of the magnetic order parameter, as allowed by symmetry and driven by thermal fluctuations. This provides a mechanism for the magnetic, but not nuclear, structure to break inversion symmetry thereby creating an intermediate phase where the structure imitates a spin density wave. As the temperature is reduced, this intermediate structure destabilizes the crystal such that a structural chirality is induced, as reflected by the emergence of the electric polarization, and the imitation spin density wave relaxes into a generic helicoid. This provides a situation where the magnetic structure breaks inversion symmetry while the crystal structure remains centrosymmetric.
We present a novel hydrated layered manganate MgMn$_3$O$_7$$cdot$3H$_2$O as a maple-leaf-lattice (MLL) antiferromagnet candidate. The MLL is obtained by regularly depleting 1/7 of the lattice points from a triangular lattice so that the magnetic connectivity $z = 5$ and is thus intermediately frustrated between the triangular ($z = 6$) and kagome ($z = 4$) lattices. In MgMn$_3$O$_7$$cdot$3H$_2$O, the Mn$^{4+}$ ions, carrying Heisenberg spin 3/2, form a regular MLL lattice in the quasi-two-dimensional structure. Magnetization and heat capacity measurements using a hydrothermally-prepared powder sample reveal successive antiferromagnetic transitions at 5 and 15 K. A high-field magnetization curve up to 60 T at 1.3 K exhibits a multi-step plateau-like anomaly. We discuss the unique frustration of the MLL antiferromagnet in which the chiraldegree of freedom may play an important role.
Magnetocaloric effect in {[Fe(pyrazole)$_4$]$_2$[Nb(CN)$_8$]$cdot$4H$_2$O}$_n$ molecular magnet is reported. It crystallizes in tetragonal I4$_1$/a space group. The compound exhibits a phase transition to a long range magnetically ordered state at $T_mathrm{c}approx$8.3 K. The magnetic entropy change $Delta S_mathrm{M}$ as well as the adiabatic temperature change $Delta T_mathrm{ad}$ due to applied field change $mu_0Delta H$=0.1, 0.2, 0.5, 1, 2, 5, 9 T as a function of temperature have been determined by the relaxation calorimetry measurements. The maximum value of $Delta S_mathrm{M}$ for $mu_0Delta H=5$ T is 4.9 J mol$^{-1}$ K$^{-1}$ (4.8 J kg$^{-1}$ K$^{-1}$) at 10.3 K. The corresponding maximum value of $Delta T_mathrm{ad}$ is 2.0 K at 8.9 K. The temperature dependence of the exponent $n$ characterizing the field dependence of $Delta S_mathrm{M}$ has been estimated. It attains the value of 0.64 at the transition temperature, which is consistent with the 3D Heisenberg universality class.
Magnetic excitations of the recently discovered frustrated spin-1/2 two-leg ladder system Li$_2$Cu$_2$O(SO$_4$)$_2$ are investigated using inelastic neutron scattering, magnetic susceptibility and infrared absorption measurements. Despite the presence of a magnetic dimerization concomitant with the tetragonal-to-triclinic structural distortion occurring below 125 K, neutron scattering experiments reveal the presence of dispersive triplet excitations above a spin gap of $Delta = 10.6$ meV at 1.5 K, a value consistent with the estimates extracted from magnetic susceptibility. The likely detection of these spin excitations in infrared spectroscopy is explained by invoking a dynamic Dzyaloshinskii-Moriya mechanism in which light is coupled to the dimer singlet-to-triplet transition through an optical phonon. These results are qualitatively explained by exact diagonalization and higher-order perturbation calculations carried out on the basis of the dimerized spin Hamiltonian derived from first-principles.
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