Onset Of Magnetic Correlations Research Articles

Magnetic order and single-ion anisotropy inTb3Ga5O12

Terbium gallium garnet (TGG), Tb$_3$Ga$_5$O$_{12}$, is well known for its applications in laser optics, but also exhibits complex low-temperature magnetism that is not yet fully understood. Its low-temperature magnetic order is determined by means of time-of-flight neutron powder diffraction. It is found to be a multiaxial antiferromagnet with magnetic Tb$^{3+}$ ions forming six sublattices of magnetic moments aligned parallel and anti-parallel to the $\langle100\rangle$ crystallographic directions of the cubic unit cell. The structure displays strong easy-axis anisotropy with respect to a two-fold axis of symmetry in the local orthorhombic environment of the Tb$^{3+}$ sites. The crystal-field splitting within the single-ion ground-state manifold is investigated by inelastic neutron scattering on powder samples. A strong temperature dependence of the quasidoublet ground-state is observed and revised parameters of the crystal-field Hamiltonian are given. The results of bulk magnetic susceptibility and magnetisation measurements are in good agreement with values based on the crystal-field model down to 20~K, where the onset of magnetic correlations is observed.

Nonlocal correlations in the orbital selective Mott phase of a one-dimensional multiorbital Hubbard model

We study non-local correlations in a three-orbital Hubbard model defined on an extended one-dimensional chain using determinant quantum Monte Carlo and density matrix renormalization group methods. We focus on a parameter with robust Hund's coupling, which produces an orbital selective Mott phase (OSMP) at intermediate values of the Hubbard U, as well as an orbitally ordered ferromagnetic insulating state at stronger coupling. An examination of the orbital and spin-correlation functions indicates that the orbital ordering occurs before the onset of magnetic correlations in this parameter regime as a function of temperature. In the OSMP, we find that the self-energy for the itinerant electrons is momentum dependent, indicating a degree of non-local correlations while the localized electrons have largely momentum independent self-energies. These non-local correlations also produce relative shifts of the hole-like and electron-like bands within our model. The overall momentum dependence of these quantities is strongly suppressed in the orbitally-ordered insulating phase.

Photoexcited states of the harmonic honeycomb iridateγ−Li2IrO3

We report equilibrium and nonequilibrium optical measurements on the recently synthesized "harmonic" honeycomb iridate \gamma-Li$_2$IrO$_3$ (LIO), as well as the layered honeycomb iridate Na$_2$IrO$_3$ (NIO). Using Fourier transform infrared microscopy we performed reflectance measurements on LIO, from which we obtained the optical conductivity below 2 eV. In addition we measured the photoinduced changed in reflectance, \Delta R, as a function of time, t, temperature, T, and probe field polarization in both LIO and NIO. In LIO, \Delta R(t,T) is anisotropic and comprised of three T dependent components. Two of these components are related to the onset of magnetic order and the third is related to a photoinduced population of metastable electronic excited states. In NIO, \Delta R(t,T) has a single T dependent component that is strikingly similar to the electronic excitation component of \Delta R in LIO. Through analysis and comparison of \Delta R(t,T) for two compounds, we extract information on the onset of magnetic correlations at and above the transition temperature in LIO, the bare spin-flip scattering rate in equilibrium, the lifetime of low-lying quasiparticle excitations, and the polarization dependence of optical transitions that are sensitive to magnetic order.

Magnetic structure determination of Ca3LiOsO6using neutron and x-ray scattering

We present a neutron and x-ray scattering investigation of Ca${}_{3}$LiOsO${}_{6}$, a 5d material predicted to host magnetic ordering solely through an extended superexchange pathway involving two anions. This contrasts with investigations of extended superexchange interactions that have been largely limited to low-dimensional 3d systems involving both superexchange and extended superexchange. Despite the apparent one-dimensional nature and triangular units of magnetic osmium ions in Ca${}_{3}$LiOsO${}_{6}$, the onset of magnetic correlations has been observed at a high temperature of 117 K in bulk measurements. We experimentally determine the magnetically ordered structure and show it to be long range and three dimensional. Our results support the model of extended superexchange interaction.

Magnetic properties and exchange integrals of the frustrated chain cuprate linarite PbCuSO4(OH)2

We present a detailed study in the paramagnetic regime of the frustrated $s$ = 1/2 spin-compound linarite, PbCuSO$_4$(OH)$_2$, with competing ferromagnetic nearest-neighbor and antiferromagnetic next-nearest-neighbor exchange interactions. Our data reveal highly anisotropic values for the saturation field along the crystallographic main directions, with $\sim$ 7.6, $\sim$ 10.5 and $\sim$ 8.5\,T for the $a$, $b$, and $c$ axes, respectively. In the paramagnetic regime, this behavior is explained mainly by the anisotropy of the \textit{g}-factor but leaving room for an easy-axis exchange anisotropy. Within the isotropic $J_1$-$J_2$ spin model our experimental data are described by various theoretical approaches yielding values for the exchange interactions $J_1$ $\sim$ -100\,K and $J_2$ $\sim$ 36\,K. These main intrachain exchange integrals are significantly larger as compared to the values derived in two previous studies in the literature and shift the frustration ratio $\alpha = J_2/|J_1|$ $\approx$ 0.36 of linarite closer to the 1D critical point at 0.25. Electron spin resonance (ESR) and nuclear magnetic resonance (NMR) measurements further prove that the static susceptibility is dominated by the intrinsic spin susceptibility. The Knight shift as well as the broadening of the linewidth in ESR and NMR at elevated temperatures indicate a highly frustrated system with the onset of magnetic correlations far above the magnetic ordering temperature $T_\mathrm{N}$ = 2.75(5)\,K, in agreement with the calculated exchange constants.

Onset of magnetic correlations in LiY1-xHoxF4 with 0.002 ≤ x ≤ 0.05 studied via μSR

We are using μSR to probe the evolution of magnetism from single-ion to spin glass behaviour in LiY1-xHoxF4 with increasing x. For x = 0.002 the Ho spin dynamics exhibit single-ion behaviour similar to that observed in molecular magnets. Near T≈ 10 K, the energy gap between the Ho ground state electronic doublet and first excited singlet energy levels, we find that the longitudinal depolarization rate exhibits a peak that depends on the Larmor frequency. At temperatures T << 10 K we find sharp reductions in the transverse field depolarization rates in the vicinity of magnetic field induced (avoided) level crossings (ALCs) in the hyperfine-split ground state energy manifold. This is attributed to a reduction in the quasistatic magnetic disorder due to resonant Ho spin fluctuations at frequencies much larger than the muon Larmor frequency. As the Ho concentration is increased toward x = 0.05, magnetic correlations are observed to become significant, changing the 10 K peak into a plateau, and significantly broadening the rate changes observed at ALCs.

Magnetic-field-induced transition inBaVS3

The metal-insulator transition (MIT) of ${\mathrm{BaVS}}_{3}$ is suppressed under pressure, and above the critical pressure of ${p}_{\mathrm{cr}}\ensuremath{\approx}2\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ the metallic phase is stabilized. We present the results of detailed magnetoresistivity measurements carried out at pressures near the critical value in magnetic fields up to $B=12\phantom{\rule{0.3em}{0ex}}\mathrm{T}$. We found that slightly below the critical pressure the structural tetramerization---which drives the MIT---is combined with the onset of magnetic correlations. If the zero-field transition temperature is suppressed to a sufficiently low value $({T}_{\mathrm{MI}}\ensuremath{\le}15\phantom{\rule{0.3em}{0ex}}\mathrm{K})$, the system can be driven into the metallic state by application of magnetic field. The main effect is not the reduction of ${T}_{\mathrm{MI}}$ with increasing $B$, but rather the broadening of the transition due to the applied magnetic field. We tentatively ascribe this phenomenon to the influence on the magnetic structure coupled to the bond order of the tetramers.

μSR studies of magnetic properties of CeRhSb and La 0.1 Ce 0.9 RhSb

Zero, longitudinal, and transverse field μSR data are reported for CeRhSband La0.1Ce0.9RhSbdown to 45 mK. The muon spin relaxation at temperatures above a few Kelvin is caused by nuclear dipoles. Only at lower temperatures the additional depolarizing effect of electronic moments is observable, signalizing the onset of magnetic correlations in the 4f moment system. These findings are compared to the corresponding data on CeNiSn, showing that the ground state properties are basically alike in the two materials, that is, magnetic correlations develop, but remain weakly dynamic and long range antiferromagnetic order does not set in. The results for La0.1Ce0.9RhSbdiffer little from those of pure CeRhSb, indicating that the magnetic ground state properties as seen by μSR have not been altered significantly. The muonic Knight shift in CeRhSb exhibits similar features as the bulk susceptibility in the temperature range between 20 and 2 K.

Onset of magnetic correlations in the dilute spin fluctuation system (La,U)Al2

Abstract We report a study of La 1- x U x Al 2 (0.006⩽ x ⩽0.2) with characterization by X-ray diffraction, susceptibility and specific heat in fields up to 14 T. Variation of the effective distance between the uranium atoms by means of lanthanum doping is seen to control the specific heat up to the onset of magnetic correlations. X-ray diffractometry and susceptibility measurements suggest that annealing of the samples causes a phase separation and formation of UAl 2 clusters, thereby lowering the susceptibility and the effective moment of the samples.

Thermodynamic behavior of the heavy-fermion compounds Ce3X (X=Al,In,Sn).

We have measured the resistivity \ensuremath{\rho}(T) and susceptibility \ensuremath{\chi}(T) of ${\mathrm{Ce}}_{3}$Al, ${\mathrm{Ce}}_{3}$In, and ${\mathrm{Ce}}_{3}$Sn in the temperature range 1--350 K, the specific heat C(T) for 1--25 K and the pressure dependence of the resistivity \ensuremath{\rho}(P,T) for 0&lt;P&lt;16 kbar and 1&lt;T&lt;300 K. These are heavy-fermion systems that show no superconductivity above 0.4 K. In the ground state the linear coefficients of the specific heat \ensuremath{\gamma} are 0.70 and 0.26 J/mol Ce ${\mathrm{K}}^{2}$ for ${\mathrm{Ce}}_{3}$In and ${\mathrm{Ce}}_{3}$Sn, respectively. The magnetic specific heat of ${\mathrm{Ce}}_{3}$In shows two separated maxima: one at 4.3 K due to the heavy fermions and a second Schottky peak at 23 K arising from a ${\ensuremath{\Gamma}}_{7}$-${\ensuremath{\Gamma}}_{8}$ crystal-field splitting of order ${T}_{\mathrm{CF}=65}$ K. For ${\mathrm{Ce}}_{3}$Sn the crystal-field splitting is comparable. From \ensuremath{\chi}(0) we obtain values of the Wilson ratio of 11.5 and 7.0 for ${\mathrm{Ce}}_{3}$In and ${\mathrm{Ce}}_{3}$Sn. We argue that these large values represent the presence of ferromagnetic correlations in the ground state.For ${\mathrm{Ce}}_{3}$In the enhancement of the susceptibility and specific-heat coefficient and the rapid decrease of the resistivity all occur below the same temperature (7 K), suggesting that the onset of the heavy mass coincides with the onset of magnetic correlations and coherence. In addition, for ${\mathrm{Ce}}_{3}$In an inflection point occurs in \ensuremath{\rho}(T) at ${T}_{\mathrm{inf}=2.2}$ K, below which \ensuremath{\rho} varies as ${T}^{2}$, and there may be a peak in C(T)/T at 2 K. Thus, it appears that there are two temperature scales for the onset of interaction effects: One coincides with the single-ion Kondo temperature ${T}_{K}$, and the other, a low-temperature scale ${T}_{L}$, obeys a rule ${T}_{L}$=${T}_{K}$/${N}_{\mathrm{deg}}$, where ${N}_{\mathrm{deg}}$ is the degeneracy of the ground-state multiplet. The ground state of ${\mathrm{Ce}}_{3}$Al is antiferromagnetic with ${T}_{N}$=2.5 K. The specific-heat anomaly makes it impossible to determine \ensuremath{\gamma} but for 10&lt;T&lt;20 K \ensuremath{\gamma}=0.085 J/mol Ce ${\mathrm{K}}^{2}$. From \ensuremath{\chi} and the magnetic entropy we estimate ${T}_{K}$=10--15 K.The magnetic entropy remains smaller than R ln2 even at 25 K; we conclude that the ordering occurs in a regime where the moment (at least on some sites) is strongly compensated by the Kondo effect. With application of pressure, the characteristic temperature increases in ${\mathrm{Ce}}_{3}$Sn; it increases initially for ${\mathrm{Ce}}_{3}$In, but for P&gt;9 kbar there is a structural transition with the unusual feature that the low-temperature phase has smaller characteristic energy than the high-temperature phase. A related transition occurs in ${\mathrm{Ce}}_{3}$Al at ambient pressure, which disappears at higher pressure.

Onset of magnetic correlations in CeAl3 below 2 K.

The results of zero-, transverse-, and longitudinal-field muon-spin rotation measurements in polycrystalline Ce${\mathrm{Al}}_{3}$ are reported. They provide evidence for static magnetic correlations in Ce${\mathrm{Al}}_{3}$ below 2 K. From the magnitude of the local magnetic field at the muon site it was estimated that the effective ${\mathrm{Ce}}^{3+}$ moments may be of the order of 0.05${\mathrm{\ensuremath{\mu}}}_{\mathrm{B}}$. The magnetic correlations appear as a spatially inhomogeneous phenomenon, and are partly coherent below 0.7 K. No evidence for a cooperative phase transition is found.