arXiv subjects

H. D. Zhou

Publications and source records attributed to H. D. Zhou.

At least 91 records · Page 5Linked to original sources

arXiv2016

Canted Magnetic Ground State of Quarter-Doped Manganites $R_{0.75}$Ca$_{0.25}$MnO$_3$ ($R$ = Y, Tb, Dy, Ho, and Er)

Polycrystalline samples of the quarter-doped manganites $R_{0.75}$Ca$_{0.25}$MnO$_3$ ($R$ = Y, Tb, Dy, Ho, and Er) were studied by X-ray diffraction and AC/DC susceptibility measurements. All five samples are orthorhombic and exhibit similar magnetic properties: enhanced ferromagnetism below $T_1$ ($\sim80$ K) and a spin glass (SG) state below $T_{SG}$ ($\sim30$ K). With increasing $R^{3+}$ ionic size, both $T_1$ and $T_{SG}$ generally increase. The single crystal neutron diffraction results on Tb$_{0.75}$Ca$_{0.25}$MnO$_3$ revealed that the SG state is mainly composed of a short-range ordered version of a novel canted (i.e. noncollinear) antiferromagnetic spin state. Furthermore, calculations based on the double exchange model for quarter-doped manganites reveal that this new magnetic phase provides a transition state between the ferromagnetic state and the theoretically predicted spin-orthogonal stripe phase.

cond-mat.mtrl-sci↗

arXiv2016

Structural transition and orbital glass physics in near itinerant CoV2O4

The ferrimagnetic spinel $\mathrm{CoV_2O_4}$ has been a topic of intense recent interest, both as a frustrated insulator with unquenched orbital degeneracy and as a near-itinerant magnet which can be driven metallic with moderate applied pressure. Here, we report on our recent neutron diffraction and inelastic scattering measurements on powders with minimal cation site disorder. Our main new result is the identification of a weak ($\frac{Δa}{a} \sim 10^{-4}$), first order structural phase transition at $T^*$ = 90 K, the same temperature where spin canting was seen in recent single crystal measurements. This transition is characterized by a short-range distortion of oxygen octahedral positions, and inelastic data further establish a weak $Δ\sim 1.25 meV$ spin gap at low temperature. Together, these findings provide strong support for the local orbital picture and the existence of an orbital glass state at temperatures below $T^*$.

cond-mat.str-el↗

arXiv2015

Structural and Magnetic Phase Transitions in CeCu$_{6-x}T_x$ ($T$ = Ag, Pd)

The structural and the magnetic properties of CeCu$_{6-x}$Ag$_x$ (0 $\leq$ $x$ $\leq$ 0.85) and CeCu$_{6-x}$Pd$_x$ (0 $\leq$ $x$ $\leq$ 0.4) have been studied using neutron diffraction, resonant ultrasound spectroscopy (RUS), heat capacity, x-ray diffraction measurements and first principles calculations. The structural and magnetic phase diagrams of CeCu$_{6-x}$Ag$_x$ and CeCu$_{6-x}$Pd$_x$ as a function of Ag/Pd composition are reported. The end member, CeCu$_6$, undergoes a structural phase transition from an orthorhombic ($Pnma$) to a monoclinic ($P2_1/c$) phase at 240 K. In CeCu$_{6-x}$Ag$_x$, the structural phase transition temperature (${T_{s}}$) decreases linearly with Ag concentration and extrapolates to zero at $x_{S}$ $\approx$ 0.1. The structural transition in CeCu$_{6-x}$Pd$_x$ remains unperturbed with Pd substitution within the range of our study. The lattice constant $b$ slightly decreases with Ag/Pd doping, whereas, $a$ and $c$ increase with an overall increase in the unit cell volume. Both systems, CeCu$_{6-x}$Ag$_x$ and CeCu$_{6-x}$Pd$_x$, exhibit a magnetic quantum critical point (QCP), at $x$ $\approx$ 0.2 and $x$ $\approx$ 0.05 respectively. Near the QCP, long range antiferromagnetic ordering takes place at an incommensurate wave vector ($δ_1$ 0 $δ_2$) where $δ_1 \sim 0.62$, $δ_2 \sim 0.25$, $x$ = 0.125 for CeCu$_{6-x}$Pd$_x$ and $δ_1 \sim 0.64$, $δ_2 \sim 0.3$, $x$ = 0.3 for CeCu$_{6-x}$Ag$_x$. The magnetic structure consists of an amplitude modulation of the Ce-moments which are aligned along the $c$-axis of the orthorhombic unit cell.

cond-mat.str-el↗

arXiv2015

Low-temperature thermal conductivity of Dy_2Ti_2O_7 and Yb_2Ti_2O_7 single crystals

We study the low-temperature thermal conductivity (κ) of Dy_2Ti_2O_7 and Yb_2Ti_2O_7 single crystals in magnetic fields up to 14 T along the [111], [100] and [110] directions. The main experimental findings for Dy_2Ti_2O_7 are: (i) the low-T κ(H) isotherms exhibit not only the step-like decreases at the low-field (< 2 T) magnetic transitions but also obvious field dependencies in high fields (> 7 T); (ii) at T \le 0.5 K, the κ(H) curves show anisotropic irreversibility in low fields, that is, the κ(H) hysteresis locates at the first-order transition with H \parallel [100] and [110], while it locates between two successive transitions with H \parallel [111]; (iii) the κin the hysteresis loops for H \parallel [100] and [110] show an extremely slow relaxation with the time constant of \sim 1000 min. The main experimental findings for Yb_2Ti_2O_7 are: (i) the zero-field κ(T) show a kink-like decrease at the first-order transition (\sim 200 mK) with decreasing temperature; (ii) the low-T κ(H) isotherms show a decrease in low field and a large enhancement in high fields; (iii) the low-T κ(H) curves show a sharp minimum at 0.5 T for H \parallel [110] and [111]. The roles of monopole excitations, field-induced transitions, spin fluctuations and magnetoelastic coupling are discussed.

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arXiv2015

Ground state selection in XY pyrochlore antiferromagnets R$_{2}$Ge$_{2}$O$_{7}$ (R = Er, Yb)

Elastic neutron scattering, ac susceptibility, and specific heat experiments on the pyrochlores Er$_{2}$Ge$_{2}$O$_{7}$ and Yb$_{2}$Ge$_{2}$O$_{7}$ show that both systems are antiferromagnetically ordered in the $Γ_5$ manifold. The ground state is a $ψ_{3}$ phase for the Er sample and a $ψ_{2}$ or $ψ_{3}$ phase for the Yb sample, which suggests "Order by Disorder"(ObD) physics. Furthermore, we unify the various magnetic ground states of all known R$_{2}$B$_{2}$O$_{7}$ (R = Er, Yb, B = Sn, Ti, Ge) compounds through the enlarged XY type exchange interaction $J_{\pm}$ under chemical pressure. The mechanism for this evolution is discussed in terms of the phase diagram proposed in the theoretical study [Wong et al., Phys. Rev. B 88, 144402, (2013)].

cond-mat.str-el↗

arXiv2015

Strong competition between orbital-ordering and itinerancy in a frustrated spinel vanadate

The crossover from localized- to itinerant-electron behavior is associated with many intriguing phenomena in condensed-matter physics. In this paper, we investigate the crossover from localized to itinerant regimes in the spinel system Mn$_{1-x}$Co$_x$V$_2$O$_4$. At low Co doping, orbital order (OO) of the localized electrons on the V3+ ions suppresses magnetic frustration by triggering a tetragonal distortion. With Co doping, electronic itinerancy melts the OO and suppresses the structural phase transition while the reduced spin-lattice coupling produces magnetic frustration. Neutron scattering measurements and first-principles-guided spin models reveal that the non-collinear state at high Co doping is produced by weakened local anisotropy and enhanced Co-V spin interactions.

cond-mat.str-el↗

arXiv2015

Nature of Partial Magnetic Order in the Frustrated Antiferromagnet Gd2Ti2O7

The frustrated pyrochlore antiferromagnet Gd$_{2}$Ti$_{2}$O$_{7}$ has an unusual partially-ordered magnetic structure at the lowest measurable temperatures. This structure is currently believed to involve four magnetic propagation vectors $\mathbf{k}\in \langle \frac{1}{2} \frac{1}{2} \frac{1}{2} \rangle^*$ in a cubic 4-$\mathbf{k}$ structure, based on analysis of magnetic diffuse-scattering data [J. Phys.: Condens. Matter 16, L321 (2004)]. Here, we present three pieces of evidence against the 4-$\mathbf{k}$ structure. First, we report single-crystal neutron-diffraction measurements as a function of applied magnetic field, which are consistent with the selective field-induced population of non-cubic magnetic domains. Second, we present evidence from high-resolution powder neutron-diffraction measurements that rhombohedral strains exist within magnetic domains, which may be generated by magneto-elastic coupling only for the alternative 1-$\mathbf{k}$ structure. Finally, we show that the argument previously used to rule out the 1-$\mathbf{k}$ structure is flawed, and demonstrate that magnetic diffuse-scattering data can actually be fitted quantitatively by a 1-$\mathbf{k}$ structure in which spin fluctuations on ordered and disordered magnetic sites are strongly coupled. Our results provide an experimental foundation on which to base theoretical descriptions of partially-ordered states.

cond-mat.str-el↗

arXiv2015

High pressure floating zone growth and structural properties of ferrimagnetic quantum paraelectric BaFe$_{12}$O$_{19}$

High quality single crystals of BaFe$_{12}$O$_{19}$ were grown using the floating zone technique in flowing oxygen pressurized to 100 atm. Single crystal neutron diffraction was used to determine the nuclear and magnetic structure of BaFe$_{12}$O$_{19}$ at 4 K and 295 K. At both temperatures, there exist local electric dipoles formed by the off-mirror-plane displacements of magnetic Fe$^{3+}$ ions at the bipyramidal sites. The displacement at 4 K is about half of that at room temperature. The temperature dependence of the specific heat shows no anomaly associated with long range polar ordering in the temperature range from 1.90-300 K. The inverse dielectric permittivity, $1/\varepsilon$, along the c-axis shows a $T^2$ temperature dependence between 10 K and 20 K, with a significantly reduced temperature dependence displayed below 10 K. Moreover, as the sample is cooled below 1.4 K there is an anomalous sharp upturn in $1/\varepsilon$. These features resemble those of classic quantum paraelectrics such as SrTiO$_3$. The presence of the upturn in $1/\varepsilon$ indicates that BaFe$_{12}$O$_{19}$ is a critical quantum paraelectric system with Fe$^{3+}$ ions involved in both magnetic and electric dipole formation.

cond-mat.mtrl-sci↗

arXiv2015

Magnon spectra and strong spin-lattice coupling in magnetically frustrated MnB2O4 (B = Mn,V): Inelastic light scattering studies

The ferrimagnetic spinels MnB2O4 (B = Mn,V) exhibit a similar series of closely spaced magnetic and structural phase transitions at low temperatures, reflecting both magnetic frustration and a strong coupling between the spin and lattice degrees of freedom. Careful studies of excitations in MnB2O4 (B = Mn,V), and the evolution of these excitations with temperature, are important for obtaining a microscopic description of the role that magnetic excitations and spin-lattice coupling play in the low temperature phase transitions of these materials. We report an inelastic light (Raman) scattering study of the temperature and magnetic field dependences of one- and two-magnon excitations in MnV2O4 and Mn3O4. We observe a pair of q=0 one-magnon modes at 74 cm^{-1} and 81 cm^{-1} in MnV2O4, which is in contrast with the single 80 cm^{-1} q=0 magnon that has been reported for MnV2O4 based on previous neutron scattering measurements and spin wave calculations. Additionally, we find that the two-magnon energy of MnV2O4 decreases ("softens") with decreasing temperature below T_{N}, which we attribute to strong coupling between magnetic and vibrational excitations near the zone boundary.

cond-mat.str-el↗

arXiv2014

Incipient Ferromagnetism in Tb2Ge2O7: Application of Chemical Pressure to the Enigmatic Spin Liquid, Tb2Ti2O7

The origin of the spin liquid state in Tb$_2$Ti$_2$O$_7$ has challenged experimentalists and theorists alike for nearly 20 years. To improve our understanding of the exotic magnetism in Tb$_2$Ti$_2$O$_7$, we have synthesized a chemical pressure analog, Tb$_2$Ge$_2$O$_7$. Germanium substitution results in a lattice contraction and enhanced exchange interactions. We have characterized the magnetic ground state of Tb$_2$Ge$_2$O$_7$ with specific heat, ac and dc magnetic susceptibility, and polarized neutron scattering measurements. Akin to Tb$_2$Ti$_2$O$_7$, there is no long-range order in Tb$_2$Ge$_2$O$_7$ down to 20 mK. The Weiss temperature of $-$19.2(1) K, which is more negative than that of Tb$_2$Ti$_2$O$_7$, supports the picture of stronger antiferromagnetic exchange. Polarized neutron scattering of Tb$_2$Ge$_2$O$_7$ reveals that at 3.5 K liquid-like correlations dominate in this system. However, below 1 K, the liquid-like correlations give way to intense short-range ferromagnetic correlations with a length scale related to the Tb-Tb nearest neighbor distance. Despite stronger antiferromagnetic exchange, the ground state of Tb$_2$Ge$_2$O$_7$ has ferromagnetic character, in stark contrast to the pressure-induced antiferromagnetic order observed in Tb$_2$Ti$_2$O$_7$.

cond-mat.str-el↗

arXiv2014

Magnetic order and spin dynamics in La2O2Fe2OSe2 probed by 57Fe-Moessbauer, 139La-NMR, and muon spin relaxation spectroscopy

We present a detailed local probe study of the magnetic order in the oxychalcogenide La2O2Fe2OSe2 utilizing 57Fe Moessbauer, 139La NMR, and muon spin relaxation spectroscopy. This system can be regarded as an insulating reference system of the Fe arsenide and chalcogenide superconductors. From the combination of the local probe techniques we identify a non-collinear magnetic structure similar to Sr2F2Fe2OS2. The analysis of the magnetic order parameter yields an ordering temperature TN = 90.1 K and a critical exponent of beta = 0.133, which is close to the 2D Ising universality class as reported in the related oxychalcogenide family.

cond-mat.str-el↗

arXiv2014

Quantum phase diagram of the $S=1/2$ triangular-lattice antiferromagnet Ba$_3$CoSb$_2$O$_9$

The magnetic phases of the ideal spin-1/2 triangular-lattice antiferromagnet Ba$_3$CoSb$_2$O$_9$ are identified and studied using $^{135,137}$Ba nuclear magnetic resonance (NMR) spectroscopy in magnetic fields ranging to 30T, oriented parallel and near perpendicular to the crystallographic $ab$-plane. For both directions, the saturation field is approximately 33T. Notably, the NMR spectra provide microscopic evidence for the stabilization of an up-up-down spin configuration for in-plane fields, giving rise to an one-third magnetization plateau ($M_\text{sat}/3$), as well as for a higher field phase transition near to $\sim (3/5)M_\text{sat}$ for both field orientations. Phase transitions are signaled by the evolution of the NMR spectra, and in some cases through spin-lattice relaxation measurements. The results are compared with expectations obtained from a semi-classical energy density modeling, in which quantum effects are incorporated by effective interactions extracted from the spin-wave analysis of the two-dimensional model. The interlayer coupling also plays a significant role in the outcome. Good agreement between the model and the experimental results is achieved, except for the case of fields approaching the saturation value applied along the c-axis.

cond-mat.str-el↗

arXiv2014

Magnetoelectric coupling tuned by competing anisotropies in Mn$_{1-x}$Ni$_{x}$TiO$_3$

A flop of electric polarization from $P$$\parallel$$c$ ($P_c$) to $P$$\parallel$$a$ ($P_a$) is observed in MnTiO$_3$ as a spin flop transition is triggered by a $c$-axis magnetic field, $H_{\|c}$=7 T. The critical magnetic field $H_{\|c}$ for $P_a$ is significantly reduced in Mn$_{1-x}$Ni$_x$TiO$_3$ (x=0.33). $P_a$ and $P_c$ have been observed with both $H_{\|c}$ and $H_{\|a}$. Neutron diffraction measurements revealed similar magnetic arrangements for the two compositions where the ordered spins couple antiferromagnetically with their nearest intra- and inter-planar neighbors. In the x=0.33 system, the uniaxial and planar anisotropies of Mn$^{2+}$ and Ni$^{2+}$ compete and give rise to a spin reorientation transition at $T_R$. A magnetic field, $H_{\|c}$, aligns the spins along $c$ for $T_R$$<$$T$$<$$T_N$. The rotation of the collinear spins away from the $c$-axis for $T$$<$$T_R$ alters the magnetic point symmetry and gives rise to a new ME susceptibility tensor form. Such linear ME response provides satisfactory explanation for the behavior of the field-induced electric polarization in both compositions. As the Ni content increases to x=0.5 and 0.68, the ME effect disappears as a new magnetic phase emerges.

cond-mat.str-el↗

arXiv2014

Tuning the magnetic exchange via a control of orbital hybridization in Cr2(Te1-xWx)O6

We report the complex magnetic phase diagram and electronic structure of Cr2(Te1-xWx)O6 systems. While compounds with different x values possess the same crystal structure, they display different magnetic structures below and above xc = 0.7, where both the transition temperature TN and sublattice magnetization (Ms) reach a minimum. Unlike many known cases where magnetic interactions are controlled either by injection of charge carriers or by structural distortion induced via chemical doping, in the present case it is achieved by tuning the orbital hybridization between Cr 3d and O 2p orbitals through W 5d states. The result is supported by ab-initio electronic structure calculations. Through this concept, we introduce a new approach to tune magnetic and electronic properties via chemical doping.

cond-mat.mtrl-sci↗

arXiv2014

Magnetic structure and domain conversion of quasi-2D frustrated antiferromagnet CuCrO2 probed by NMR

We have carried out 63,65Cu NMR spectra measurements in magnetic field up to about 15.5 T on single crystal of a multiferroic triangular-lattice antiferromagnet CuCrO2. The measurements were performed for perpendicular and parallel orientation of the magnetic field with respect to the c-axis of the crystal, and the detailed angle dependence of the spectra on the magnetic field direction within ab-plane was studied. The shape of the spectra can be well described in the model of spiral spin structure proposed by recent neutron diffraction experiments. When field is rotated perpendicular to crystal c-axis, we observed, directly for the first time, a remarkable reorientation of spin plane simultaneous with rotation of the incommensurate wavevector by quantitatively deducing the conversion of less energetically favorable domain to a more favorable one. At high enough fields parallel to c-axis, the data are consistent with either a field-induced commensurate spiral magnetic structure or an incommensurate spiral magnetic structure with a disorder in the c direction, suggesting that high fields may have influence on interplanar ordering.

cond-mat.str-el↗

arXiv2014

Ground state and magnetic phase transitions of orthoferrite DyFeO_3

Low-temperature thermal conductivity (κ), as well as magnetization (M) and electric polarization (P), of multiferroic orthoferrite DyFeO_3 single crystals are studied with H \parallel c. When the crystal is cooled in zero field, M, P, and κall consistently exhibit irreversible magnetic-field dependencies. In particular, with 500 mK < T \le 2 K, all these properties show two transitions at the first run of increasing field but only the higher-field transition is present in the subsequent field sweepings. Moreover, the ultra-low-T (T < 500 mK) κ(H) shows a different irreversibility and there is only one transition when the field is swept both up and down. All the results indicate a complex low-T H-T phase diagram involving successive magnetic phase transitions of the Fe^{3+} spins. In particular, the ground state, obtained with cooling to subKelvin temperatures in zero field, is found to be an unexplored phase.

cond-mat.str-el↗

arXiv2014

Magnetic ordering induced by interladder coupling in the spin-1/2 Heisenberg two-leg ladder antiferromagnet C$_9$H$_{18}$N$_2$CuBr$_4$

We present specific-heat and neutron-scattering results for the \emph{S}=1/2 quantum antiferromagnet (dimethylammonium)(3,5-dimethylpyridinium)CuBr$_4$. The material orders magnetically at \emph{T}$_N$=1.99(2)\,K, and magnetic excitations are accompanied by an energy gap of 0.30(2) meV due to spin anisotropy. The system is best described as coupled two-leg spin-1/2 ladders with the leg exchange $J_{\rm leg}$=0.60(2)~meV, rung exchange $J_{\rm rung}$=0.64(9)~meV, interladder exchange $J_{\rm int}$=0.19(2)~meV, and an interaction-anisotropy parameter $λ$=0.93(2), according to inelastic neutron-scattering measurements. In contrast to most spin ladders reported to date, the material is a rare example in which the interladder coupling is very near the critical value required to drive the system to a Néel-ordered phase without an assistance of a magnetic field.

cond-mat.mtrl-sci↗

arXiv2014

Magnons and a two-component spin gap in FeV2O4

The spinel vanadates have become a model family for exploring orbital order on the frustrated pyrochlore lattice, and recent debate has focused on the symmetry of local crystal fields at the cation sites. Here, we present neutron scattering measurements of the magnetic excitation spectrum in $\mathrm{FeV_2O_4}$, a recent example of a ferrimagnetic spinel vanadate which is available in single crystal form. We report the existence of two emergent magnon modes at low temperatures, which draw strong parallels with the closely related material, $\mathrm{MnV_2O_4}$. We were able to reproduce the essential elements of both the magnetic ordering pattern and the dispersion of the inelastic modes with semi- classical spin wave calculations, using a minimal model that implies a sizeable single-ion anisotropy on the vanadium sublattice. Taking into account the direction of ordered spins, we associate this anisotropy with the large trigonal distortion of $\mathrm{VO_6}$ octahedra, previously observed via neutron powder diffraction measurements. We further report on the spin gap, which is an order-of-magnitude larger than that observed in $\mathrm{MnV_2O_4}$. By looking at the overall temperature dependence, we were able to show that the gap magnitude is largely associated with the ferro-orbital order known to exist on the iron sublattice, but the contribution to the gap from the vanadium sublattice is in fact comparable to what is reported in the Mn compound. This reinforces the conclusion that the spin canting transition is associated with the ordering of vanadium orbitals in this system, and closer analysis indicates closely related physics underlying orbital transitions in $\mathrm{FeV_2O_4}$ and $\mathrm{MnV_2O_4}$.

cond-mat.str-el↗