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Register a new userField-induced ferroelectric and commensurate spin state in mutiferroic HoMn$_{2}$O$_{5}$
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Neutron diffraction measurements under high magnetic fields have been performed for the multiferroic compound HoMn$_{2}$O$_{5}$. At zero field, high-temperature incommensurate magnetic (HT-ICM) -- commensurate magnetic (CM) -- low-temperature incommensurate magnetic (LT-ICM) orders occur with decreasing temperature, where ferroelectric polarization arises only in the CM phase. Upon applying a magnetic field, the LT-ICM phase completely disappears and the CM phase is induced at the lowest temperature. This field-induced CM state is completely associated with the field-induced electric polarization in this material [Higashiyama {it et al}., Phys. Rev. B {bf 72}, 064421 (2005).], strongly indicating that the commensurate spin state is essential to the ferroelectricity in the multiferroic $R$Mn$_{2}$O$_{5}$ system.
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Spin-orbit coupled honeycomb magnets with the Kitaev interaction have received a lot of attention due to their potential of hosting exotic quantum states including quantum spin liquids. Thus far, the most studied Kitaev systems are 4d/5d-based honeycomb magnets. Recent theoretical studies predicted that 3d-based honeycomb magnets, including Na2Co2TeO6 (NCTO), could also be a potential Kitaev system. Here, we have used a combination of heat capacity, magnetization, electron spin resonance measurements alongside inelastic neutron scattering (INS) to study NCTOs quantum magnetism, and we have found a field-induced spin disordered state in an applied magnetic field range of 7.5 T < B (vertical to b-axis) < 10.5 T. The INS spectra were also simulated to tentatively extract the exchange interactions. As a 3d-magnet with a field-induced disordered state on an effective spin-1/2 honeycomb lattice, NCTO expands the Kitaev model to 3d compounds, promoting further interests on the spin-orbital effect in quantum magnets.
The pyrochlore oxides Dy$_{2}$Ti$_{2}$O$_{7}$ and Ho$_{2}$Ti$_{2}$O$_{7}$ are well studied spin ice systems and have shown the evidences of magnetic monopole excitations. Unlike these, Dy$_{2}$Zr$_{2}$O$_{7}$ is reported to crystallize in a distorted fluorite structure. We present here the magnetic and heat capacity studies of La substituted Dy$_{2}$Zr$_{2}$O$_{7}$. Our findings suggest the absence of spin ice state in Dy$_{2}$Zr$_{2}$O$_{7}$ but the emergence of the magnetic field induced spin freezing near T $approx$ 10 K in ac susceptibility measurements which is similar to Dy$_{2}$Ti$_{2}$O$_{7}$. The magnetic heat capacity of Dy$_{2}$Zr$_{2}$O$_{7}$ shows a shift in the peak position from 1.2 K in zero field to higher temperatures in the magnetic field, with the corresponding decrease in the magnetic entropy. The low temperature magnetic entropy at 5 kOe field is Rln2 - (1/2)Rln(3/2) which is same as for the spin ice state. Substitution of non-magnetic, isovalent La$^{3+}$ for Dy$^{3+}$ gradually induces the structural change from highly disordered fluorite to weakly ordered pyrochlore phase. The La$^{3+}$ substituted compounds with less distorted pyrochlore phase show the spin freezing at lower field which strengthens further on the application of magnetic field. Our results suggest that the spin ice state can be stabilized in Dy$_{2}$Zr$_{2}$O$_{7}$ either by slowing down of the spin dynamics or by strengthening the pyrochlore phase by suitable substitution in the system.
We have investigated the pressure-induced spin-state transition in Co$^{2+}$ systems in terms of a competition between the Hunds exchange energy ($J$) and the crystal-field splitting ($Delta_{CF}$). First, we show the universal metastability of the low-spin state in octahedrally coordinated Co$^{2+}$ systems. Then we present the strategy to search for a Co$^{2+}$ system, for which the mechanism of spin-state and metal-insulator transitions is governed not by the Mott physics but by $J$ vs. $Delta_{CF}$ physics. Using CoCl$_{2}$ as a prototypical Co$^{2+}$ system, we have demonstrated the pressure-induced spin-state transition from high-spin to low-spin, which is accompanied with insulator-to-metal and antiferromagnetic to half-metallic ferromagnetic transitions. Combined with metastable character of Co$^{2+}$ and the high compressibility nature of CoCl$_{2}$, the transition pressure as low as 27 GPa can be identified on the basis of $J$ vs. $Delta_{CF}$ physics.
A layered triangular lattice with spin-1/2 ions is an ideal platform to explore highly entangled exotic states like quantum spin liquid (QSL). Here, we report a systematic in-field neutron scattering study on a perfect two-dimensional triangular-lattice antiferromagnet, CsYbSe$_2$, a member of the large QSL candidate family rare-earth chalcogenides. The elastic neutron scattering measured down to 70 mK shows that there is a short-range 120$^{circ}$ magnetic order at zero field. In the field-induced ordered states, the spin-spin correlation lengths along the $c$ axis are relatively short, although the heat capacity results indicate long-range magnetic orders at 3 T $-$ 5 T. The inelastic neutron scattering spectra evolve from highly damped continuum-like excitations at zero field to relatively sharp spin wave modes at the plateau phase. Our extensive large-cluster density-matrix renormalization group calculations with a Heisenberg triangular-lattice nearest-neighbor antiferromagnetic model reproduce the essential features of the experimental spectra, including continuum-like excitations at zero field, series of sharp magnons at the plateau phase as well as two-magnon excitations at high energy. This work presents comprehensive experimental and theoretical overview of the unconventional field-induced spin dynamics in triangular-lattice Heisenberg antiferromagnet and thus provides valuable insight into quantum many-body phenomena.
We investigate the low temperature magnetic properties of a $S=frac{5}{2}$ Heisenberg kagome antiferromagnet, the layered monodiphosphate Li$_9$Fe$_3$(P$_2$O$_7$)$_3$(PO$_4$)$_2$, using magnetization measurements and $^{31}$P nuclear magnetic resonance. An antiferromagnetic-type order sets in at $T_{rm N}=1.3$ K and a characteristic magnetization plateau is observed at 1/3 of the saturation magnetization below $T^* sim 5$ K. A moderate $^{31}$P NMR line broadening reveals the development of anisotropic short-range correlations within the plateau phase concomitantly with a gapless spin-lattice relaxation time $T_1 sim k_B T / hbar S$, which both point to the presence of a semiclassical nematic spin liquid state predicted for the Heisenberg kagome antiferromagnetic model.
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