Persistent Spin Fluctuations Research Articles

Quantum spin liquid ground state in the trimer rhodate Ba4NbRh3O12

Frustrated magnets offer a plethora of exotic magnetic ground states, including quantum spin liquids (QSLs), in which enhanced quantum fluctuations prevent a long-range magnetic ordering of the strongly correlated spins down to lowest temperature. Here we have investigated the trimer based mixed valence hexagonal rhodate Ba4NbRh3O12 using a combination of dc and ac magnetization, electrical resistivity, specific heat, and muon spin rotation/relaxation (μSR) measurements. Despite the substantial antiferromagnetic exchange interactions, as evident from the Weiss temperature (θW∼−35 to −45K), among the Rh-local moments, neither long-range magnetic ordering nor spin freezing is observed down to at least 50 mK, in ac-susceptibility, specific heat, and zero-field μSR measurements (down to 0.26 K). We ascribe the absence of any magnetic transition to enhanced quantum fluctuations as a result of geometrical frustration arising out of the edge-sharing equilateral Rh-triangular network in the structure. Our longitudinal-field μSR result evidences persistent spin fluctuations down to 0.26 K, thus stabilizing a dynamic QSL ground state in Ba4NbRh3O12. Furthermore, the magnetic specific heat data at low T reveal a significant T-linear contribution plus a quadratic T dependence, which may indicate the gapless Dirac QSL phenomenology of the spinon excitations with a linear dispersion. Published by the American Physical Society 2024

Dynamical ground state in the XY pyrochlore Yb2GaSbO7

The magnetic ground state of the pyrochlore Yb2GaSbO7 has not been established. The persistent spin fluctuations observed by muon spin-relaxation measurements at low temperatures have not been adequately explained for this material using existing theories for quantum magnetism. Here we report on the synthesis and characterisation of Yb2GaSbO7 to revisit the nature of the magnetic ground state. Through DC and AC magnetic susceptibility, heat capacity, and neutron scattering experiments, we observe evidence for a dynamical ground state that makes Yb2GaSbO7 a promising candidate for disorder-induced spin-liquid or spin-singlet behaviour. This state is quite fragile, being tuned to a splayed ferromagnet in a modest magnetic field μ0Hc ~ 1.5 T.

Pressure-induced phase transition in the J1−J2 square lattice antiferromagnet RbMoOPO4Cl

We report results of magnetization and $^{31}$P NMR measurements under high pressure up to 6.4~GPa on RbMoOPO$_4$Cl, which is a frustrated square-lattice antiferromagnet with competing nearest-neighbor and next-nearest-neighbor interactions. Anomalies in the pressure dependences of the NMR shift and the transferred hyperfine coupling constants indicate a structural phase transition at 2.6~GPa, which is likely to break mirror symmetry and triggers significant change of the exchange interactions. In fact, the NMR spectra in magnetically ordered states reveal a change from the columnar antiferromagnetic (CAF) order below 3.3~GPa to the N\'{e}el antiferromagnetic (NAF) order above 3.9~GPa. The spin lattice relaxation rate $1/T_1$ also indicates a change of dominant magnetic fluctuations from CAF-type to NAF-type with pressure. Although the NMR spectra in the intermediate pressure region between 3.3 and 3.9 GPa show coexistence of the CAF and NAF phases, a certain component of $1/T_1$ shows paramagnetic behavior with persistent spin fluctuations, leaving possibility for a quantum disordered phase. The easy-plane anisotropy of spin fluctuations with unusual nonmonotonic temperature dependence at ambient pressure gets reversed to the Ising anisotropy at high pressures. This unexpected anisotropic behavior for a spin 1/2 system may be ascribed to the strong spin-orbit coupling of Mo-4$d$ electrons.

μSR study of the triangular Ising antiferromagnet ErMgGaO4

We present a detailed magnetic study of the triangular antiferromagnet ErMgGaO$_4$. A point charge calculation under the single ion approximation reveals a crystal field ground state doublet with a strong Ising-like behavior of the Er$^{3+}$ moment along the c axis. Magnetic susceptibility and specific heat measurements indicate no presence of magnetic transitions above 0.5~K and no evidence of residual entropy as temperature approaching zero. Zero field (ZF) $\mu$SR measurements shows no sign of static uniform or random field and longitudinal field (LF) $\mu$SR measurements exhibit persistent spin fluctuations down to our lowest temperature of 25~mK. Our results provide evidence of a quantum spin liquid state in the triangular antiferromagnet ErMgGaO$_4$.

Dipolar-octupolar Ising antiferromagnetism in Sm2Ti2O7 : A moment fragmentation candidate

Over the past two decades, the magnetic ground states of all rare-earth titanate pyrochlores have been extensively studied, with the exception of ${\mathrm{Sm}}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}$. This is, in large part, due to the very high absorption cross section of naturally occurring samarium, which renders neutron scattering infeasible. To combat this, we have grown a large, isotopically enriched single crystal of ${\mathrm{Sm}}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}$. Using inelastic neutron scattering, we determine that the crystal field ground state for ${\mathrm{Sm}}^{3+}$ is a dipolar-octupolar doublet with Ising anisotropy. Neutron diffraction experiments reveal that ${\mathrm{Sm}}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}$ orders into the all-in, all-out magnetic structure with an ordered moment of $0.44(7){\ensuremath{\mu}}_{B}$ below ${T}_{N}=0.35$ K, consistent with expectations for antiferromagnetically coupled Ising spins on the pyrochlore lattice. Zero-field muon spin relaxation measurements reveal an absence of spontaneous oscillations and persistent spin fluctuations down to 0.03 K. The combination of the dipolar-octupolar nature of the ${\mathrm{Sm}}^{3+}$ moment, the all-in, all-out ordered state, and the low-temperature persistent spin dynamics make this material an intriguing candidate for moment fragmentation physics.

Evidence for a dynamical ground state in the frustrated pyrohafnate Tb2Hf2O7

We report the physical properties of Tb2Hf2O7 based on ac magnetic susceptibility \chi_ac(T), dc magnetic susceptibility \chi(T), isothermal magnetization M(H), and heat capacity C_p(T) measurements combined with muon spin relaxation (\muSR) and neutron powder diffraction measurements. No evidence for long-range magnetic order is found down to 0.1 K. However, \chi_ac(T) data present a frequency-dependent broad peak (near 0.9 K at 16 Hz) indicating slow spin dynamics. The slow spin dynamics is further evidenced from the \muSR data (characterized by a stretched exponential behavior) which show persistent spin fluctuations down to 0.3 K. The neutron powder diffraction data collected at 0.1 K show a broad peak of magnetic origin (diffuse scattering) but no magnetic Bragg peaks. The analysis of the diffuse scattering data reveals a dominant antiferromagnetic interaction in agreement with the negative Weiss temperature. The absence of long-range magnetic order and the presence of slow spin dynamics and persistent spin fluctuations together reflect a dynamical ground state in Tb2Hf2O7.

Magnetic excitations in the ground state of Yb2Ti2O7

We report an extensive study on the zero field ground state of a powder sample of the pyrochlore $\mathrm{Yb_2Ti_2O_7}$. A sharp heat capacity anomaly that labels a low temperature phase transition in this material is observed at 280 mK. Neutron diffraction shows that a \emph{quasi-collinear} ferromagnetic order develops below $T_\mathrm{c}$ with a magnetic moment of $0.87(2)\mu_\mathrm{B}$. High resolution inelastic neutron scattering measurements show, below the phase transition temperature, sharp gapped low-lying magnetic excitations coexisting with a remnant quasielastic contribution likely associated with persistent spin fluctuations. Moreover, a broad inelastic continuum of excitations at $\sim0.6$ meV is observed from the lowest measured temperature up to at least 2.5 K. At 10 K, the continuum has vanished and a broad quasielastic conventional paramagnetic scattering takes place at the observed energy range. Finally, we show that the exchange parameters obtained within the framework of linear spin-wave theory do not accurately describe the observed zero field inelastic neutron scattering data.

Spin order in the Heisenberg kagome antiferromagnet MgFe3(OH)6Cl2

Spin behaviors on kagome lattices have received increased attention because of their exotic magnetic states. Here, we report magnetic ordering in the $S=2$ kagome antiferromagnet $\mathrm{MgF}{\mathrm{e}}_{3}{(\mathrm{OH})}_{6}\mathrm{C}{\mathrm{l}}_{2}$. The compound has been synthesized by selectively replacing the magnetic ions in the triangular planes of its parent compound of deformed pyrochlore $\mathrm{F}{\mathrm{e}}_{2}{(\mathrm{OH})}_{3}\mathrm{Cl}$ with nonmagnetic Mg. Whereas $\mathrm{F}{\mathrm{e}}_{2}{(\mathrm{OH})}_{3}\mathrm{Cl}$ showed antiferromagnetic order below ${T}_{\mathrm{N}}=8.5\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ with persistent spin fluctuations, $\mathrm{MgF}{\mathrm{e}}_{3}{(\mathrm{OH})}_{6}\mathrm{C}{\mathrm{l}}_{2}$ showed an antiferromagnetic transition at an enhanced ${T}_{\mathrm{N}}$ value of 9.9 K. Susceptibility, high-field magnetization, and neutron-diffraction studies suggest that $\mathrm{MgF}{\mathrm{e}}_{3}{(\mathrm{OH})}_{6}\mathrm{C}{\mathrm{l}}_{2}$ is a Heisenberg-like spin system with a ${120}^{\ensuremath{\circ}}$ nearest-neighbor spin structure confined in the kagome plane and spin-vector chirality of $q=\ensuremath{-}1$ below ${T}_{\mathrm{N}}$. Muon spin-rotation/-relaxation studies demonstrate that the persisting spin fluctuations are suppressed relative to $\mathrm{F}{\mathrm{e}}_{2}{(\mathrm{OH})}_{3}\mathrm{Cl}$. The present paper reports the experimental realization of an $S=2$ kagome antiferromagnet, highlighting a real system with a quasiclassical Heisenberg spin, which may also be a valuable reference system for quantum Heisenberg kagome antiferromagnets.

Short-range correlations and persistent spin fluctuations in the undistorted kagome lattice Ising antiferromagnet Co3Mg(OH)6Cl2

Spin fluctuations and spin-liquid behaviors of frustrated kagome antiferromagnets have received intense recent attention. Although most severe frustration was predicted for an Ising kagome antiferromagnet, a real material system of undistorted kagome lattice has not been found so far. Here we report the frustrated magnetism of a new Ising kagome antiferromagnet, MgCo${}_{3}$(OH)${}_{6}$Cl${}_{2}$, which can be viewed as a Co version of the intensively researched quantum kagome antiferromagnet of Herbertsmithite ZnCu${}_{3}$(OH)${}_{6}$Cl${}_{2}$. Experiments of magnetization, heat capacity, \ensuremath{\mu}SR, and neutron scattering demonstrated a partially frozen state with persistent spin fluctuations below around $T$ = 2.7 K. The present study has provided a real material system to study the Ising spin behaviors on undistorted kagome lattice.

Magnetism of frustrated A-site spinels

Abstract In the A-site spinels MAl 2 O 4 ( M = Mn , Fe, Co) the A-site sublattice is subjected to strong frustration, putting all three compounds on the verge between long-range and short-range magnetic order. The μ SR spectra of the three compounds show two signals at all temperatures examined and a dramatic change in spectral shape at characteristic temperatures. Above this temperature, one signal (slowly relaxing) is typical for free paramagnetic spins, the other (fast relaxing) for highly correlated paramagnetic spins. For M = Mn the ratio of their intensities is 1:3 independent of temperature and the spectral change occurs around 40 K ( T N from neutron data) where the strongly correlated fraction enters into long-range order and the free paramagnetic fraction into a glassy spin state. For M = Co or Fe, the free paramagnetic fraction increases from ∼ 20 % to 85% with rising temperature. At the cusp temperatures observed in magnetization (5 or 12 K) the dominating, highly correlated paramagnetic fraction turns into a dynamic short-range ordered magnetic ground state. In all three compounds substantial local spin disorder and persistent spin fluctuations characterize their magnetic ground states, typical features of highly frustrated magnets.

A [formula omitted] SR magnetic study of UNiGe

We have carried out μ SR spectroscopy on polycrystalline UNiGe between 2 K and 100 K. The existence of two magnetic transitions at T N = 51 K and T 1 = 41.5 K is confirmed. The μ SR spectra clearly reveal that the magnetic state between 51 K and 41.5 K is an incommensurate spin structure ruling out a spin–slip structure which had been considered an alternative. Below 41.5 K the spectra are compatible with simple antiferromagnetic order. The local field for T → 0 is B μ = 170 mT, a comparatively low value, indicating a rather small uranium ordered moment. When going from the commensurate to the incommensurate structures at T 1 a sudden reduction in local field by 23% occurs reflecting an equal change in ordered moment. The transition at T 1 is sharp, but T N extends over roughly 5 K. The antiferromagnetic spin structure exhibits persistent spin fluctuations in the limit T → 0 , implying the presence of some additional spin interactions which tries to suppress long-range magnetic order.

Magnets frustrated by competing exchange (TbFe 6Al 6 and ErFe 6Al 6)

Frustration in the RFe 6Al 6 intermetallics originates from competition between ferromagnetic inter- and antiferromagnetic intra-sublattice exchange in conjunction with crystallographic disorder of the Fe–Al sublattice. The μSR data give no indication for the presence of a spin-glass-like state below their ferrimagnetic ordering temperature of ∼340 K. The dominant feature of frustration observed by μSR is persistent spin fluctuations well below the ordering temperature. The quasi-static limit is only reached around 50 K, which coincides with the temperature where local magnetization saturates. Furthermore, there is evidence that the magnetic ordering of the 8j sublattice with its random occupation of Al and Fe ions is retarded.

De Haas‐Van Alphen Effect in Nearly Magnetic Transition Metals

AbstractThe oscillatory part of the thermodynamic potential and magnetization are calculated for a paramagnetic system that is near to a magnetic instability. The dominant excitations of such an exchange enhanced Fermi liquid are persistent spin fluctuations. The electrons are scattered by processes in which the spin fluctuations are emitted and absorbed. The electron spin fluctuation interaction results in a many‐body resonance of the electronic density of states at the Fermi level. It is shown that the spin fluctuations manifest themselves in the de Haas‐van Alphen oscillations. The amplitude of the oscillations is reduced, the reduction is dependent on the enhancement of the linear T term in the electronic specific heat. The results are applied to the experiments on CeSn3 and on Pd.

Low-Temperature Electrical Resistivity of Pd and Pd–Ni Alloys

Persistent spin fluctuations in strongly paramagnetic metals like Pd have been shown to give rise to electron-mass renormalization and concomitant enhancement of the electronic specific-heat coefficient. In addition, recent calculations of s-electron-spin-fluctuation interactions predict a term in the low-temperature electrical resistivity which should vary as A T2, where A is predicted to be proportional to the square of the paramagnetic spin-susceptibility enhancement. To test this theory, electrical-resistivity measurements were performed from 2° to 35°K on samples of Pd containing 0, 0.5, 1.0, and 1.66 at.%Ni. All of the samples are paramagnetic with susceptibilities which increase rapidly with the Ni content. A T2 dependence in the low-temperature electrical resistivity of all samples was observed; A was found to be 26, 114, 241, and 513×10−6 μΩ·cm/deg2 for the four samples, respectively. However, in contrast to theoretical predictions, A was observed to vary less rapidly than as the square of the magnetic susceptibility.

Theory of Viscosity in Nearly Ferromagnetic Fermi Liquids

A recent theory of transport processes in nearly ferromagnetic Fermi liquids, based on the concept of particle scattering from persistent spin fluctuations, or paramagnons, is extended to calculate the viscosity coefficient $\ensuremath{\eta}$. The theoretical temperature dependence, in which $\ensuremath{\eta}$ falls off increasingly less rapidly than ${T}^{\ensuremath{-}2}$ as $T$ is increased from absolute zero, fits well that of the observed viscosity of liquid ${\mathrm{He}}^{3}$.

Theory of Transport Processes in Nearly Ferromagnetic Fermi Liquids

A theory of the low-temperature transport processes of thermal conduction and spin diffusion in Fermi liquids near the ferromagnetic state is developed on the assumption that the predominant particle scattering at low temperatures occurs from persistent spin fluctuations, or "paramagnons," of the type discussed recently by Doniach and Engelsberg and by Berk and Schrieffer. A Boltzmann equation for this type of scattering is set up and solved by the usual variational procedure to obtain expressions for the coefficients of thermal conductivity K and spin-diffusion $D$. The latter are evaluated by using a simple model approximation for the paramagnon spectral density function ${A}_{\mathrm{q}}(\ensuremath{\omega})$, based on the correct RPA result at long wavelengths. It is found that in the limit $T\ensuremath{\rightarrow}0$, $K$ varies as ${T}^{\ensuremath{-}1}$ and $D$ as ${T}^{\ensuremath{-}2}$, in accordance with the predictions of the Landau theory. As $T$ increases from absolute zero, however, $K$ and $D$ fall off increasingly less rapidly than ${T}^{\ensuremath{-}1}$ and ${T}^{\ensuremath{-}2}$, respectively, and actually increase with increasing $T$ after passing through minima. The theory can therefore account for the observed temperature dependence of the thermal conductivity and spin diffuison of liquid ${\mathrm{He}}^{3}$. In particular, the theoretical predictions are in close agreement with the low-pressure thermal-conductivity and diffusion data obtained by the Illinois group.

Low-Temperature Properties of Nearly Ferromagnetic Fermi Liquids

Effective-mass corrections due to emission and reabsorption of persistent spin fluctuations are shown to be large in Pd and to have a temperature dependence which fits qualitatively that of the low-temperature specific heat of ${\mathrm{He}}^{3}$.