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Register a new userAntiferromagnetism of Double Molybdate LiFe(MoO$_4$)$_2$
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The magnetic properties of the spin-5/2 double molybdate LiFe(MoO$_4$)$_2$ have been characterized by heat capacity, magnetic susceptibility, and neutron powder diffraction techniques. Unlike the multiferroic system LiFe(MoO$_4$)$_2$ which exhibits two successive magnetic transitions, LiFe(MoO$_4$)$_2$ undergoes only one antiferromagnetic transition at $T_N$ ~ 23.8 K. Its antiferromagnetic magnetic structure with the commensurate propagation vector k = (0, 0.5, 0) has been determined. Density functional theory calculations confirm the antiferromagnetic ground state and provide a numerical estimate of the relevant exchange coupling constants.
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Tungstates $A$WO$_4$ with the wolframite structure characterized by the $A$O$_6$ octahedral zigzag chains along the $c$-axis, can be magnetic if $A$=Mn, Fe, Co, Cu, Ni. Among them, MnWO$_4$ is a unique member with a cycloid Mn$^{2+}$ spin order developed at low temperature, leading to an interesting type-II multiferroic behavior. However, so far no other multiferroic material in the tungstate family has been found. In this work, we present the synthesis and the systematic study of the double tungstate LiFe(WO$_4$)$_2$. Experimental characterizations including structural, thermodynamic, magnetic, neutron powder diffraction, and pyroelectric measurements, unambiguously confirm that LiFe(WO$_4$)$_2$ is the secondly found multiferroic system in the tungstate family. The cycloidal magnetism driven ferroelectricity is also verified by density functional theory calculations. Although here the magnetic couplings between Fe ions are indirect, namely via the so-called super-super-exchanges, the temperatures of magnetic and ferroelectric transitions are surprisingly much higher than those of MnWO$_4$.
Macroscopic magnetic properties and microscopic magnetic structure of Rb$_2$Mn$_3$(MoO$_4$)$_3$(OH)$_2$ (space group $Pnma$) are investigated by magnetization, heat capacity and single-crystal neutron diffraction measurements. The compounds crystal structure contains bond-alternating [Mn$_3$O$_{11}$]$^{infty}$ chains along the $b$-axis, formed by isosceles triangles of Mn ions occupying two crystallographically nonequivalent sites (Mn1 site on the base and Mn2 site on the vertex). These chains are only weakly linked to each other by nonmagnetic oxyanions. Both SQUID magnetometry and neutron diffraction experiments show two successive magnetic transitions as a function of temperature. On cooling, it transitions from a paramagnetic phase into an incommensurate phase below 4.5~K with a magnetic wavevector near ${bf k}_{1} = (0,~0.46,~0)$. An additional commensurate antiferromagnetically ordered component arises with ${bf k}_{2} = (0,~0,~0)$, forming a complex magnetic structure below 3.5~K with two different propagation vectors of different stars. On further cooling, the incommensurate wavevector undergoes a lock-in transition below 2.3~K. The experimental results suggest that the magnetic superspace group is $Pnma.1(0b0)s0ss$ for the single-${bf k}$ incommensurate phase and is $Pnma(0b0)00s$ for the 2-${bf k}$ magnetic phase. We propose a simplified magnetic structure model taking into account the major ordered contributions, where the commensurate ${bf k}_{2}$ defines the ordering of the $c$-axis component of Mn1 magnetic moment, while the incommensurate ${bf k}_{1}$ describes the ordering of the $ab$-plane components of both Mn1 and Mn2 moments into elliptical cycloids
Superconductors and multiferroics are two of the hottest branches in condensed matter physics. The connections between those two fields are fundamentally meaningful to unify the physical rules of correlated electrons. Recently, BaFe$_2$Se$_3$, was predicted to be multiferroic [Phys. Rev. Lett. 113, 187204 (2014)] due to its unique one-dimensional block-type antiferromagnetism. Here, another iron-selenide KFe$_2$Se$_2$, a parent state of iron-based superconductor, is predicted to be multiferroic. Its two-dimensional block-type antiferromagnetism can generate a moderate electric dipole for each Fe-Se layer via the Fe-Se-Fe exchange striction. Different stacking configurations of these magnetic blocks give closely proximate energies and thus the ground state of KFe$_2$Se$_2$ may be switchable between antiferroelectric and ferroelectric phases.
We report pressure-dependent reflection and transmission measurements on ZnCr$_2$Se$_4$, HgCr$_2$S$_4$, and CdCr$_2$O$_4$ single crystals at room temperature over a broad spectral range 200-24000 cm$^{-1}$. The pressure dependence of the phonon modes and the high-frequency electronic excitations indicates that all three compounds undergo a pressure-induced structural phase transition with the critical pressure 15 GPa, 12 GPa, and 10 GPa for CdCr$_2$O$_4$, HgCr$_2$S$_4$, and ZnCr$_2$Se$_4$, respectively. The eigenfrequencies of the electronic transitions are very close to the expected values for chromium crystal-field transitions. In the case of the chalcogenides pressure induces a red shift of the electronic excitation which indicates a strong hybridization of the Cr d-bands with the chalcogenide bands.
We report synthesis, crystal structure and physical properties of a quinary iron-arsenide fluoride KCa$_2$Fe$_4$As$_4$F$_2$. The new compound crystallizes in a body-centered tetragonal lattice (with space group $I4/mmm$, $a$ = 3.8684(2) {AA}, c = 31.007(1) {AA}, and $Z$ = 2), which contains double Fe$_2$As$_2$ conducting layers separated by insulating Ca$_2$F$_2$ layers. Our measurements of electrical resistivity, dc magnetic susceptibility and heat capacity demonstrate bulk superconductivity at 33 K in KCa$_2$Fe$_4$As$_4$F$_2$.
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