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Register a new userField driven magnetostructural transitions in GeCo$_2$O$_4$
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In the spinel compound GeCo$_2$O$_4$, the Co$^{2+}$ pyrochlore sublattice presents remarkable magnetic field-induced behaviors that we unveil through neutron and X-ray single-crystal diffraction. The Neel ordered magnetic phase is entered through a structural lowering of the cubic symmetry. In this phase, when a magnetic field is applied along a 2-fold cubic direction, a spin-flop transition of one fourth of the magnetic moments releases the magnetic frustration and triggers magnetostructural effects. At high field, these ultimately lead to an unusual spin reorientation associated to structural changes.
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The spinel NiCr$_2$O$_4$ is known to show a ferrimagnetic transition at $T_c = 70$~K, and magneto-structural transitions at $T_s = 30$~K and $T_o = 20$~K@. We present a detailed magnetic and magnetocaloric effect (MCE $= -Delta S_{M}(T)$) study across these transitions. The $-Delta S_{M}(T)$ shows a positive anomaly at $T_c$, $T_s$, and $T_o$. In addition to these anomalies, we report a new unreported feature at $T approx 8.5$~K where $-Delta S_{M}(T)$ shows a negative anomaly or the inverse MCE. An Arrot plot of the isothermal magnetization data reveals important information about the nature of the possible phases revealed in $-Delta S_{M}(T)$. We have also made a scaling analysis of the $-Delta S_{M}(T)$ data around these transitions. This analysis suggests that the transition at $T_c$ is a second-order Mean field like transition, the transition at $T_s$ is not second order and is non-mean field like, while the new transition at $T = 8.5$~K is non-mean field like but is second order in nature. Our study demonstrates that magnetocaloric effect is sensitive to magneto-structural changes in materials and can be used for the identification of new phases and transitions.
We describe powder and single-crystal inelastic neutron scattering experiments on a spinel-type antiferromagnet GeCo$_2$O$_4$, represented by an effective total angular momentum J_eff = 1/2. Several types of non-dispersive short-range magnetic excitations were discovered. The scattering intensity maps in $vec{Q}$ space are well reproduced by dynamical structure factor analyses using molecular model Hamiltonians. The results of analyses strongly suggest that the molecular excitations below T_N arise from a hidden molecular-singlet ground state, in which ferromagnetic subunits are antiferromagnetically coupled. The quasielastic excitations above T_N are interpreted as its precursor. A combination of frustration and J_eff = 1/2 might induce these quantum phenomena.
Ultrasound velocity measurements of the orbitally-frustrated GeCo$_2$O$_4$ reveal unusual elastic instabilities due to the phonon-spin coupling within the antiferromagnetic phase. Shear moduli exhibit anomalies arising from the coupling to short-range ferromagnetic excitations. Diplike anomalies in the magnetic-field dependence of elastic moduli reveal magnetic-field-induced orbital order-order transitions. These results strongly suggest the presence of geometrical orbital frustration which causes novel orbital phenomena within the antiferromagnetic phase.
We report single-crystal neutron diffraction studies on a spinel antiferromagnet GeCo$_2$O$_4$, which exhibits magnetic order with a trigonal propagation vector and tetragonal lattice expansion ($c/asimeq1.001$) below $T_{rm N}=21$ K. For this inconsistency between spin and lattice in symmetry, magnetic Bragg reflections with a tetragonal propagation vector were discovered below $T_{rm N}$. We discuss spin and orbital states of Co$^{2+}$ ion underlying the new magnetic component.
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.
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