arXiv subjects

C. Decorse

Publications and source records attributed to C. Decorse.

15 recordsLinked to original sources

arXiv2024

Exploring possible magnetic monopoles-induced magneto-electricity in spin ices

The possibilities of combining several degrees of freedom inside a unique material have recently been highlighted in their dynamics and proposed as information carriers in quantum devices where their cross-manipulation by external parameters such as electric and magnetic fields could enhance their functionalities. An emblematic example is that of electromagnons, spin-waves dressed with electric dipoles, that are fingerprints of multiferroics. Point-like objects have also been identified, which may take the form of excited quasiparticles. This is the case for magnetic monopoles, the exotic excitations of spin ices, that have been recently proposed to carry an electric dipole although experimental evidences remain elusive. Presently, we investigate the electrical signature of a classical spin ice and a related compound that supports quantum fluctuations. Our in-depth study clearly attributes magnetoelectricity to the correlated spin ice phase distinguishing it from extrinsic and single-ion effects. Our calculations show that the proposed model conferring magnetoelectricity to monopoles is not sufficient, calling for higher order contributions.

cond-mat.str-el↗

arXiv2020

Terahertz Magneto-optical investigation of quadrupolar spin-lattice effects in magnetically frustrated Tb2Ti2O7

The significance of spin-lattice coupling in the phase diagram of the quantum spin-icepyrochlore Tb2Ti2O7 has been a topic of debate for some time. Here, we focus on the aspect of vibronic coupling, which occurs between the Tb3+ electronic levels and transverse acoustic phonons, by studying their dependence on a magnetic field applied along the cubic h111i direction. Our experimental THz spectroscopy measurements, combined with quantitative theoretical quantum calculations, show that indeed vibronic effects are observed at 3 K. An analysis incorporating quadrupolar spin-lattice effects in the Hamiltonian is therefore relevant in this compound, which is no longer optically isotropic but magnetically birefringent.

cond-mat.str-el↗

arXiv2018

Spin Textures induced by Quenched Disorder in a Reentrant Spin Glass: Vortices versus "Frustrated" Skyrmions

Reentrant spin glasses are frustrated disordered ferromagnets developing vortex-like textures under an applied magnetic field. Our study of a Ni$_{0.81}$Mn$_{0.19}$ single crystal by small angle neutron scattering clarifies their internal structure and shows that these textures are randomly distributed. Spin components transverse to the magnetic field rotate over length scales of 3-15 nm in the explored field range, decreasing as field increases according to a scaling law. Monte-Carlo simulations reveal that the internal structure of the vortices is strongly distorted and differs from that assumed for "frustrated" skyrmions, built upon a competition between symmetric exchange interactions. Isolated vortices have small non-integer topological charge. The vortices keep an anisotropic shape on a 3 dimensional lattice, recalling "croutons" in a "ferromagnetic soup". Their size and number can be tuned independently by the magnetic field and concentration x (or heat treatment), respectively. This opens an original route to understand and control the influence of quenched disorder in systems hosting non trivial spin textures.

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arXiv2017

Disorder and Quantum spin ice

We report on diffuse neutron scattering experiments providing evidence for the presence of random strains in the quantum spin ice candidate Pr2Zr2O7. Since Pr is a non-Kramers ion, the strain deeply modifies the picture of Ising magnetic moments governing the low temperature properties of this material. It is shown that the derived strain distribution accounts for the temperature dependence of the specific heat and of the spin excitation spectra. Taking advantage of mean field and spin dynamics simulations, we argue that the randomness in Pr2Zr2O7, promotes a new state of matter, which is disordered, yet characterized by short range antiferroquadrupolar correlations, and from which emerge spin-ice like excitations. This study thus opens an original research route in the field of quantum spin ice.

cond-mat.str-el↗

arXiv2017

Field induced phase diagram of the XY pyrochlore antiferromagnet Er$_2$Ti$_2$O$_7$

We explore the field-temperature phase diagram of the XY pyrochlore antiferromagnet Er$_2$Ti$_2$O$_7$, by means of magnetization and neutron diffraction experiments. Depending on the field strength and direction relative to the high symmetry cubic directions $[001], [1\bar{1}0]$ and $[111]$, the refined field induced magnetic structures are derived from the zero field $ψ_2$ and $ψ_3$ states of the $Γ_5$ irreducible representation which describes the ground state of XY pyrochlore antiferromagnets. At low field, domain selection effects are systematically at play. In addition, for $[001]$, a phase transition is reported towards a $ψ_3$ structure at a characteristic field $H_c^{001}=$ 43 mT. For $[1\bar{1}0]$ and $[111]$, the spins are continuously tilted by the field from the $ψ_2$ state, and no phase transition is found while domain selection gives rise to sharp anomalies in the field dependence of the Bragg peaks intensity. For $[1\bar{1}0]$, these results are confirmed by high resolution inelastic neutron scattering experiments, which in addition allow us to determine the field dependence of the spin gap. This study agrees qualitatively with the scenario proposed theoretically by Maryasin {\it et al.} [Phys. Rev. B {\bf 93}, 100406(R) (2016)], yet the strength of the field induced anisotropies is significantly different from theory.

cond-mat.str-el↗

arXiv2017

Double Vibronic process in the quantum spin ice candidate Tb$_2$Ti$_2$O$_7$ revealed by terahertz spectroscopy

The origin of quantum fluctuations responsible for the spin liquid state in Tb$_2$Ti$_2$O$_7$ has remained a long standing problem. By synchrotron-based terahertz measurements, we show evidence of strong coupling between the magnetic and lattice degrees of freedom that provides a path to the quantum melting process. As revealed by hybrid crystal electric field-phonon excitations that appear at 0.67 THz below 200 K, and around 0.42 THz below 50 K, the double vibronic process is unique for Tb$^{3+}$ in the titanate family due to adequate energy matching and strong quadrupolar moments. The results suggest that lattice motion can indeed be the driving force behind spin flips within the hybridized ground and first excited states, promoting quantum terms in the effective Hamiltonian that describes Tb$_2$Ti$_2$O$_7$.

cond-mat.mtrl-sci↗

arXiv2016

Antiferro-quadrupolar correlations in the quantum spin ice candidate Pr2Zr2O7

We present an experimental study of the quantum spin ice candidate pyrochlore coumpound \przr\ by means of magnetization measurements, specific heat and neutron scattering up to 12 T and down to 60 mK. When the field is applied along the $[111]$ and $[1\bar{1}0]$ directions, ${\bf k}=0$ field induced structures settle in. We find that the ordered moment rises slowly, even at very low temperature, in agreement with macroscopic magnetization. Interestingly, for $H \parallel [1\bar{1}0]$, the ordered moment appears on the so called $α$ chains only. The spin excitation spectrum is essentially {\it inelastic} and consists in a broad flat mode centered at about 0.4 meV with a magnetic structure factor which resembles the spin ice pattern. For $H \parallel [1\bar{1}0]$ (at least up to 2.5 T), we find that a well defined mode forms from this broad response, whose energy increases with $H$, in the same way as the temperature of the specific heat anomaly. We finally discuss these results in the light of mean field calculations and propose a new interpretation where quadrupolar interactions play a major role, overcoming the magnetic exchange. In this picture, the spin ice pattern appears shifted up to finite energy because of those new interactions. We then propose a range of acceptable parameters for \przr\, that allow to reproduce several experimental features observed under field. With these parameters, the actual ground state of this material would be an antiferroquadrupolar liquid with spin-ice like excitations.

cond-mat.str-el↗

arXiv2015

Mesoscopic correlations in Tb2Ti2O7 spin liquid

We have studied the spin correlations with $\bf{k}$= ($\frac12$, $\frac12$, $\frac12$) propagation vector which appear below 0.4\, K in \tbti\ spin liquid by combining powder neutron diffraction and specific heat on Tb$_{2+x}$Ti$_{2-x}$O$_{7+y}$ samples with $x$=0, 0.01, -0.01. The $\bf{k}$= ($\frac12$, $\frac12$, $\frac12$) order clearly appears on all neutron patterns by subtracting a pattern at 1.2(1)\,K. Refining the subtracted patterns at 0.07\,K yields two possible spin structures, with spin-ice-like and monopole-like correlations respectively. Mesoscopic correlations involve Tb moments of 1 to 2 \mub\ ordered on a length scale of about 20 Å. In addition, long range order involving a small spin component of 0.1 to 0.2 \mub\ is detected for the $x$= 0 and 0.01 samples showing a peak in the specific heat. Comparison with previous single crystals data suggests that the ($\frac12$, $\frac12$, $\frac12$) order settles in through nanometric spin textures with dominant spin ice character and correlated orientations, analogous to nanomagnetic twins.

cond-mat.str-el↗

arXiv2015

Spin dynamics in the presence of competing ferro- and antiferro-magnetic correlations in Yb2Ti2O7

In this work, we show that the zero field excitation spectra in the quantum spin ice candidate pyrochlore compound \ybti\ is a continuum characterized by a very broad and almost flat dynamical response which extends up to $1-1.5$ meV, coexisting or not with a quasi-elastic response depending on the wave-vector. The spectra do not evolve between 50 mK and 2 K, indicating that the spin dynamics is only little affected by the temperature in both the short-range correlated and ordered regimes. Although classical spin dynamics simulations qualitatively capture some of the experimental observations, we show that they fail to reproduce this broad continuum. In particular, the simulations predict an energy scale twice smaller than the experimental observations. This analysis is based on a careful determination of the exchange couplings, able to reproduce both the zero field diffuse scattering and the spin wave spectrum rising in the field polarized state. According to this analysis, \ybti\ lies at the border between a ferro and an antiferromagnetic phase. These results suggest that the unconventional ground state of \ybti\ is governed by strong quantum fluctuations arising from the competition between those phases. The observed spectra may correspond to a continuum of deconfined spinons as expected in quantum spin liquids.

cond-mat.str-el↗

arXiv2014

Order by Disorder and Energetic Selection of the Ground State in the XY Pyrochlore Antiferromagnet Er2Ti2O7. An Inelastic Neutron Scattering Study

Examples of materials where an "order by disorder" mechanism is at play to select a particular ground state are scarce. It has recently been proposed, however, that the antiferromagnetic XY pyrochlore Er2Ti2O7, reveals a most convincing case of this mechanism. Observation of a spin gap at zone centers has recently been interpreted as a corroboration of this physics. In this paper, we argue, however, that the anisotropy generated by the interaction-induced admixing between the crystal-field ground and excited levels provides for an alternative mechanism. It especially predicts the opening of a spin gap of about 15 micro-eV, which is of the same order of magnitude as the one observed experimentally. We report new high resolution inelastic neutron scattering data which can be well understood within this scenario.

cond-mat.str-el↗

arXiv2014

Structural and Magnetic Properties of Single Crystals of the Geometrically Frustrated Zirconium Pyrochlore, Pr2Zr2O7

We report the growth of large high quality crystals of the frustrated magnet, Pr2Zr2O7 by the floating zone technique. Several crystals have been produced starting with varying levels of excess of Pr6O11 to compensate for the loss of Pr2O3 during the growth process. X-ray diffraction analysis of the crystals shows that the Pr composition in our crystals, as determined by the Pr site occupancy in the pyrochlore structure, is always slightly less than 1. Fits to the magnetic sus-ceptibility data below 10 K using a Curie-Weiss law reveal an antiferromagnetic coupling between the Pr moments, with an estimated Weiss temperature of ΘW = -2.5 K and an effective moment for the Pr3+ of μeff = 2.5 μB. Magnetization versus field measurements show a spin-ice like anisotropy.

cond-mat.mtrl-sci↗

arXiv2013

Field-induced magnetic structures in Tb2Ti2O7 spin liquid under field H || [111]

We have studied the field-induced magnetic structures of Tb2Ti2O7 pyrochlore by single-crystal neutron diffraction under a field applied along the [111] local anisotropy axis, up to H=12T and down to T=40mK. We collected a hundred magnetic Bragg peaks for each field and temperature value and refined the magnetic structures with k=0 propagation vector by performing a symmetry analysis in the space group R-3m, reducing the number of free parameters to three only. We observe a gradual reorientation of the Tb magnetic moments towards the field direction, close to a "3-in, 1-out / 1-in, 3-out" spin structure in the whole field range 0.05-12T. We perform a quantitative comparison with mean-field calculations and we propose the presence of a low-temperature dynamic symmetry breaking of the local trigonal symmetry, akin to a dynamic Jahn-Teller effect, i.e. preserving the overall cubic symmetry. We discuss the possible origin of this off-diagonal mixing term in the crystal field hamiltonian in terms of quadrupole-quadrupole interaction or magneto-elastic effects.

cond-mat.str-el↗

arXiv2011

Evidence for impurity-induced frustration in La2CuO4

Zero-field muon spin rotation and magnetization measurements were performed in La2Cu{1-x}MxO4, for 0<x< 0.12, where Cu2+ is replaced either by M=Zn2+ or by M=Mg2+ spinless impurity. It is shown that while the doping dependence of the sublattice magnetization (M(x)) is nearly the same for both compounds, the Néel temperature (T_N(x)) decreases unambiguously more rapidly in the Zn-doped compound. This difference, not taken into account within a simple dilution model, is associated with the frustration induced by the Zn2+ impurity onto the Cu2+ antiferromagnetic lattice. In fact, from T_N(x) and M(x) the spin stiffness is derived and found to be reduced by Zn doping more significantly than expected within a dilution model. The effect of the structural modifications induced by doping on the exchange coupling is also discussed.

cond-mat.str-el↗

arXiv2010

Field evolution of the magnetic structures in Er$_2$Ti$_2$O$_7$ through the critical point

We have measured neutron diffraction patterns in a single crystal sample of the pyrochlore compound Er$_2$Ti$_2$O$_7$ in the antiferromagnetic phase (T=0.3\,K), as a function of the magnetic field, up to 6\,T, applied along the [110] direction. We determine all the characteristics of the magnetic structure throughout the quantum critical point at $H_c$=2\,T. As a main result, all Er moments align along the field at $H_c$ and their values reach a minimum. Using a four-sublattice self-consistent calculation, we show that the evolution of the magnetic structure and the value of the critical field are rather well reproduced using the same anisotropic exchange tensor as that accounting for the local paramagnetic susceptibility. In contrast, an isotropic exchange tensor does not match the moment variations through the critical point. The model also accounts semi-quantitatively for other experimental data previously measured, such as the field dependence of the heat capacity, energy of the dispersionless inelastic modes and transition temperature.

cond-mat.str-el↗

arXiv2009

Ising versus XY anisotropy in frustrated R$_{2}$Ti$_{2}$O$_7$ compounds as seen by polarized neutrons

We studied the field induced magnetic order in R$_{2}$Ti$_{2}$O$_7$ pyrochlore compounds with either uniaxial (R=Ho, Tb) or planar (R=Er, Yb) anisotropy, by polarized neutron diffraction. The determination of the local susceptibility tensor \{$χ_\parallel,χ_\perp$\} provides a universal description of the field induced structures in the paramagnetic phase (2-270 K), whatever the field value (1-7 T) and direction. Comparison of the thermal variations of $χ_\parallel$ and $χ_\perp$ with calculations using the rare earth crystal field shows that exchange and dipolar interactions must be taken into account. We determine the molecular field tensor in each case and show that it can be strongly anisotropic.

cond-mat.str-el↗