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Register a new userFeTi$_2$O$_5$: a spin Jahn-Teller transition enhanced by cation substitution
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We have used muon-spin rotation, heat capacity and x-ray diffraction measurements in combination with density functional theory and dipole field calculations to investigate the crystal and magnetic structure of FeTi$_2$O$_5$. We observe a long range ordered state below $T_{rm N}$=41.8(5) K with indications of significant correlations existing above this temperature. We determine candidate muon stopping sites in this compound, and find that our data are consistent with the spin Jahn-Teller driven antiferromagnetic ground state with $boldsymbol{k}$=(1/2,1/2,0) reported for CoTi$_2$O$_5$ ($T_{rm N}$=26 K). By comparing our data with calculated dipolar fields we can restrict the possible moment size and directions of the Fe$^{2+}$ ions.
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Ultrasound velocity measurements of cubic spinel GeCo$_2$O$_4$ in single crystal were performed for the investigation of shear and compression moduli. The shear moduli in the paramagnetic state reveal an absence of Jahn-Teller activity despite the presence of orbital degeneracy in the Co$^{2+}$ ions. Such a Jahn-Teller inactivity indicates that the intersite orbital-orbital interaction is much stronger than the Jahn-Teller coupling. The compression moduli in the paramagnetic state near the N$acute{e}$el temperature $T_N$ reveal that the most relevant exchange path for the antiferromagnetic transition lies in the [111] direction. This exchange-path anisotropy is consistent with the antiferromagnetic structure with the wave vector $q parallel$ [111], suggesting the presence of bond frustration due to competition among a direct ferromagnetic and several distant-neighbors antiferromagnetic interactions. In the JT-inactive condition, the bond frustration can be induced by geometrical orbital frustration of $t_{2g}$-$t_{2g}$ interaction between the Co$^{2+}$ ions which can be realized in the pyrochlore lattice of the high spin Co$^{2+}$ with $t_{2g}$-orbital degeneracy. In GeCo$_2$O$_4$, the tetragonal elongation below $T_N$ releases the orbital frustration by quenching the orbital degeneracy.
Ultrasound velocity measurements of magnesium chromite spinel MgCr$_2$O$_4$ reveal elastic anomalies in the paramagnetic phase that are characterized as due to geometrical frustration. The temperature dependence of the tetragonal shear modulus $(C_{11}-C_{12})/2$ exhibits huge Curie-type softening, which should be the precursor to spin Jahn-Teller distortion in the antiferromagnetic phase. The trigonal shear modulus $C_{44}$ exhibits nonmonotonic temperature dependence with a characteristic minimum at $sim$50 K, indicating a coupling of the lattice to dynamical molecular spin state. These results strongly suggest the coexistence of dynamical spin Jahn-Teller effect and dynamical molecular spin state in the paramagnetic phase, which is compatible with the coexistence of magnetostructural order and dynamical molecular spin state in the antiferromagnetic phase.
We examine the effect of small amounts of magnetic substituents in the $A$ sites of the frustrated spinels MgCr$_2$O$_4$ and ZnCr$_2$O$_4$. Specifically we look for the effects of spin and lattice disorder on structural changes accompanying magnetic ordering in these compounds. Substitution of Co$^{2+}$ on the non-magnetic Zn$^{2+}$ site in Zn$_{1-x}$Co$_{x}$Cr$_2$O$_4$ where 0,$<$,$x$,$leq$,0.2 completely suppresses the spin-Jahn-Teller distortion of ZnCr$_2$O$_4$ although these systems remain frustrated, and magnetic ordering occurs at very low temperatures of $T$,$<$,20,K. On the other hand, the substitution of Jahn-Teller active Cu$^{2+}$ for Mg$^{2+}$ and Zn$^{2+}$ in Mg$_{1-x}$Cu$_{x}$Cr$_2$O$_4$ and Zn$_{1-x}$Cu$_{x}$Cr$_2$O$_4$ where 0,$<$,$x$,$leq$,0.2 induce Jahn-Teller ordering at temperatures well above the Neel temperatures of these solid solutions, and yet spin interactions remain frustrated with long-range magnetic ordering occurring below 20,K without any further lattice distortion. The Jahn-Teller distorted solid solutions Mg$_{1-x}$Cu$_{x}$Cr$_2$O$_4$ and Zn$_{1-x}$Cu$_{x}$Cr$_2$O$_4$ adopt the orthorhombic $Fddd$ structure of ferrimagnetic CuCr$_2$O$_4$. Total neutron scattering studies of Zn$_{1-x}$Cu$_{x}$Cr$_2$O$_4$ suggest that there are local $A$O$_4$ distortions in these Cu$^{2+}$-containing solid solutions at room temperature and that these distortions become cooperative when average structure distortions occur. Magnetism evolves from compensated antiferromagnetism in MgCr$_2$O$_4$ and ZnCr$_2$O$_4$ to uncompensated antiferromagnetism with substitution of magnetic cations on the non-magnetic cation sites of these frustrated compounds.
We present a comprehensive structural study on perovskite-type 6H-Ba3CuSb2O9, which exhibits a spin-orbital short-range ordering on a honeycomb-based lattice. By combining synchrotron x-ray diffraction, electron spin resonance, ultrasound measurement and Raman spectroscopy, we found that the static Jahn-Teller distortion is absent down to the lowest temperature in the present material, indicating orbital ordering is strongly suppressed. We discuss such an unusual state is realized with the help of spin degree of freedom, leading to a spin-orbital entangled liquid state.
Using high resolution X-Ray diffraction (XRD) on high purity powders, we resolved the structure and $ab$ symmetry of the intriguing compound svo$ $ from room temperature down to 20 K to an unprecedented level of accuracy. Upon cooling, this new set of data unambiguously reveals a second order phase transition lowering the symmetry from tetragonal to orthorhombic at a temperature $T_{c2}=136$ K. The observation of an orthorhombic distortion of the $ab$-plane is attributed to nematic phase formation supported by local Jahn-Teller (JT) dynamical instability. At $T_{N}=105$ K, spins order and at $T_{c1}=100$ K the tetragonal structure is recovered with an elongated c-axis.
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