arXiv subjects

R. Mathieu

Publications and source records attributed to R. Mathieu.

At least 55 records · Page 3Linked to original sources

arXiv2010

Isothermal remanent magnetization and the spin dimensionality of spin glasses

The isothermal remanent magnetization is used to investigate dynamical magnetic properties of spatially three dimensional spin glasses with different spin dimensionality (Ising, XY, Heisenberg). The isothermal remanent magnetization is recorded vs. temperature after intermittent application of a weak magnetic field at a constant temperature $T_h$. We observe that in the case of the Heisenberg spin glasses, the equilibrated spin structure and the direction of the excess moment are recovered at $T_h$. The isothermal remanent magnetization thus reflects the directional character of the Dzyaloshinsky-Moriya interaction present in Heisenberg systems.

cond-mat.dis-nn↗

arXiv2009

Electronic self-organization in the single-layer manganite $\rm Pr_{\it 1-x}Ca_{\it 1+x}MnO_4$

We use neutron scattering to investigate the doping evolution of the magnetic correlations in the single-layer manganite $\rm Pr_{\it 1-x}Ca_{\it 1+x}MnO_4$, away from the $x=0.5$ composition where the CE-type commensurate antiferromagnetic (AF) structure is stable. We find that short-range incommensurate spin correlations develop as the system is electron doped ($x<0.5$), which coexist with the CE-type AF order. This suggests that electron doping in this system induces an inhomogeneous electronic self-organization, where commensurate AF patches with $x=0.5$ are separated by electron-rich domain walls with short range magnetic correlations. This behavior is strikingly different than for the perovksite $\rm Pr_{\it 1-x}Ca_{\it x}MnO_3$, where the long-range CE-type commensurate AF structure is stable over a wide range of electron or hole doping around $x=0.5$.

cond-mat.str-el↗

arXiv2009

A prototype liquid Argon Time Projection Chamber for the study of UV laser multi-photonic ionization

This paper describes the design, realization and operation of a prototype liquid Argon Time Projection Chamber (LAr TPC) detector dedicated to the development of a novel online monitoring and calibration system exploiting UV laser beams. In particular, the system is intended to measure the lifetime of the primary ionization in LAr, in turn related to the LAr purity level. This technique could be exploited by present and next generation large mass LAr TPCs for which monitoring of the performance and calibration plays an important role. Results from the first measurements are presented together with some considerations and outlook.

physics.ins-det↗

arXiv2009

Magnetic Superstructure and Metal-Insulator Transition in Mn-Substituted Sr3Ru2O7

We present a temperature-dependent resonant elastic soft x-ray scattering (REXS) study of the metal-insulator transition in Sr3(Ru1-xMnx)2O7, performed at both Ru and Mn L-edges. Resonant magnetic superstructure reflections, which indicate an incipient instability of the parent compound, are detected below the transition. Based on modelling of the REXS intensity from randomly distributed Mn impurities, we establish the inhomogeneous nature of the metal-insulator transition, with an effective percolation threshold corresponding to an anomalously low x<0.05 Mn substitution.

cond-mat.str-el↗

arXiv2008

Crystal-field level inversion in lightly Mn-doped Sr3Ru2O7

Sr3(Ru1-xMnx)2O7, in which 4d-Ru is substituted by the more localized 3d-Mn, is studied by x-ray dichroism and spin-resolved density functional theory. We find that Mn impurities do not exhibit the same 4+ valence of Ru, but act as 3+ acceptors; the extra eg electron occupies the in-plane 3dx2-y2 orbital instead of the expected out-of-plane 3d3z2-r2. We propose that the 3d-4d interplay, via the ligand oxygen orbitals, is responsible for this crystal-field level inversion and the material's transition to an antiferromagnetic, possibly orbitally-ordered, low-temperature state.

cond-mat.str-el↗

arXiv2007

The nanoscale phase separation in hole-doped manganites

A macroscopic phase separation, in which ferromagnetic clusters are observed in an insulating matrix, is sometimes observed, and believed to be essential to the colossal magnetoresistive (CMR) properties of manganese oxides. The application of a magnetic field may indeed trigger large magnetoresistance effects due to the percolation between clusters allowing the movement of the charge carriers. However, this macroscopic phase separation is mainly related to extrinsic defects or impurities, which hinder the long-ranged charge-orbital order of the system. We show in the present article that rather than the macroscopic phase separation, an homogeneous short-ranged charge-orbital order accompanied by a spin glass state occurs, as an intrinsic result of the uniformity of the random potential perturbation induced by the solid solution of the cations on the $A$-sites of the structure of these materials. Hence the phase separation does occur, but in a more subtle and interesting nanoscopic form, here referred as ``homogeneous''. Remarkably, this ``nanoscale phase separation'' alone is able to bring forth the colossal magnetoresistance in the perovskite manganites, and is potentially relevant to a wide variety of other magnetic and/or electrical properties of manganites, as well as many other transition metal oxides, in bulk or thin film form as we exemplify throughout the article.

cond-mat.mtrl-sci↗

arXiv2007

Coexistence of long-ranged charge and orbital order and spin-glass state in single-layered manganites with weak quenched disorder

The relationship between orbital and spin degrees of freedom in the single-crystals of the hole-doped Pr$_{1-x}$Ca$_{1+x}$MnO$_4$, 0.3 $\leq$ $x$ $\leq$ 0.7, has been investigated by means of ac-magnetometry and charge transport. Even though there is no cation ordering on the $A$-site, the quenched disorder is extremely weak in this system due to the very similar ionic size of Pr$^{3+}$ and Ca$^{2+}$. A clear asymmetric response of the system to the under- (respective over-) hole doping was observed. The long-ranged charge-orbital order established for half doping ($x$=0.5) subsists in the over-doping case ($x$ $>$ 0.5), albeit rearranged to accommodate the extra holes introduced in the structure. The charge-orbital order is however destabilized by the presence of extra localized electrons (under-doping, $x$ $<$ 0.5), leading to its disappearance below $x$=0.35. We show that in an intermediate under-doped region, with 0.35 $\leq$ $x$ $<$ 0.5, the ``orbital-master spin-slave'' relationship commonly observed in half-doped manganites does not take place. The long-ranged charge-orbital order is not accompanied by an antiferromagnetic transition at low temperatures, but by a frustrated short-ranged magnetic state bringing forth a spin-glass phase. We discuss in detail the nature and origin of this spin-glass state, which, as in the half-doped manganites with large quenched disorder, is not related to the macroscopic phase separation observed in crystals with minor defects or impurities.

cond-mat.mtrl-sci↗

arXiv2007

Absence of Conventional Spin-Glass Transition in the Ising Dipolar System LiHo_xY_{1-x}F_4

The magnetic properties of single crystals of LiHo_xY_{1-x}F_4 with x=16.5% and x=4.5% were recorded down to 35 mK using a micro-SQUID magnetometer. While this system is considered as the archetypal quantum spin glass, the detailed analysis of our magnetization data indicates the absence of a phase transition, not only in a transverse applied magnetic field, but also without field. A zero-Kelvin phase transition is also unlikely, as the magnetization seems to follow a non-critical exponential dependence on the temperature. Our analysis thus unmasks the true, short-ranged nature of the magnetic properties of the LiHo_xY_{1-x}F_4 system, validating recent theoretical investigations suggesting the lack of phase transition in this system.

cond-mat.dis-nn↗

arXiv2007

Effect of antiferromagnetic spin correlations on lattice distortion and charge ordering in Pr$_{0.5}$Ca$_{1.5}$MnO$_{4}$

We use neutron scattering to study the lattice and magnetic structure of the layered half-doped manganite Pr$_{0.5}$Ca$_{1.5}$MnO$_4$. On cooling from high temperature, the system first becomes charge- and orbital- ordered (CO/OO) near $T_{CO}=300$ K and then develops checkerboard-like antiferromagnetic (AF) order below $T_{N}=130$ K. At temperatures above $T_{N}$ but below $T_{CO}$ ($T_N<T<T_{CO}$), the appearance of short-range AF spin correlations suppresses the CO/OO induced orthorhombic strain, contrasting with other half-doped manganites, where AF order has no observable effect on the lattice distortion. These results suggest that a strong spin-lattice coupling and the competition between AF exchange and CO/OO ordering ultimately determines the low-temperature properties of the system.

cond-mat.str-el↗

arXiv2007

Logarithmic growth law in the two-dimensional Ising spin glass state resulting from the electron doping in single-layered manganites

The ac-susceptibility of the electron doped single-layered manganite La$_{1.1}$Sr$_{0.9}$MnO$_4$ is analyzed in detail. A quasi two-dimensional (2$D$) antiferromagnetic (AFM) order with Ising anisotropy is stabilized below $T_N$ $\sim$ 80K. We show that below $T_N$, a rare 2$D$ spin-glass (SG) correlation develops with the same Ising anisotropy as the AFM state. Using simple scaling arguments of the droplet model, we derive a scaling form for the ac-susceptibility data of a 2$D$ SG, which our experimental data follows fairly well. Due to simplifications in this 2$D$ case, the proposed scaling form only contains two unknown variables $ψν$ and $τ_0$. Hence, the logarithmic growth law of the SG correlation predicted by the droplet model is convincingly evidenced by the scaling of our experimental data. The origin and nature of this 2$D$ SG state is also discussed.

cond-mat.mtrl-sci↗

arXiv2007

Variation of charge/orbital ordering in layered manganites Pr1-xCa1+xMnO4 investigated by transmission electron microscopy

Structural features of the charge/orbital ordering (CO/OO) in single-layered manganites Pr1-xCa1+xMnO4 have been investigated systematically by transmission electron microscopy. Analyses of electron diffraction patterns as well as dark-field images have revealed that the CO/OO shows a striking asymmetric behavior as the hole doping x deviates from x = 0.5. The modulation wavenumber linearly decreases with increasing x in the over-hole-doped (x > 0.5) crystals, while much less dependent on x in the under-hole-doped (x < 0.5) crystals. A temperature-induced incommensurate-commensurate crossover is observed in 0.35 < x < 0.5 and x = 0.65. The correlation length of CO/OO in x = 0.3 was proven to become shorter than that in x > 0.3.

cond-mat.str-el↗

arXiv2006

Bandwidth-disorder phase diagram of half doped layered manganites

Phase diagrams in the plane of $r_A$ (the average ionic radius, related to one-electron bandwidth $W$) and $σ^2$ (the ionic radius variance, measuring the quenched disorder), or ``bandwidth-disorder phase diagrams'', have been established for perovskite manganites, with three-dimensional (3$D$) Mn-O network. Here we establish the intrinsic bandwidth-disorder phase diagram of half-doped layered manganites with the two-dimensional (2$D$) Mn-O network, examining in detail the ``mother state'' of the colossal magnetoresistance (CMR) phenomenon in crystals without ferromagnetic instability. The consequences of the reduced dimensionality, from 3$D$ to 2$D$, on the order-disorder phenomena in the charge-orbital sectors are also highlighted.

cond-mat.mtrl-sci↗

arXiv2005

Impurity-induced transition to a Mott insulator in Sr$_3$Ru$_2$O$_7$

The electrical, magnetic, and structural properties of Sr$_3$(Ru$_{1-x}$Mn$_x$)$_2$O$_7$ (0 $\leq x \leq$ 0.2) are investigated. The parent compound Sr$_3$Ru$_2$O$_7$ is a paramagnetic metal, critically close to magnetic order. We have found that, with a Ru-site doping by only a few percent of Mn, the ground state is switched from a paramagnetic metal to an antiferromagnetic insulator. Optical conductivity measurements show the opening of a gap as large as 0.1 eV, indicating that the metal-to-insulator transition is driven by the electron correlation. The complex low-temperature antiferromagnetic spin arrangement, reminiscent of those observed in some nickelates and manganites, suggests a long range orbital order.

cond-mat.mtrl-sci↗

arXiv2005

Determination of the intrinsic anomalous Hall effect of SrRuO$_3$

The anomalous Hall effect (AHE) of epitaxial SrRuO$_3$ films with varying lattice parameters is investigated, and analyzed according to the Berry-phase scenario. SrRuO$_3$ thin films were deposited on SrTiO$_3$ substrates directly, or using intermediate buffer layers, in order to finely control the epitaxial strain. The AHE of the different films exhibits intrinsic features such as the sign change of the Hall resistivity with the temperature, even for small thicknesses of SrRuO$_3$. However, the anomalous Hall conductivity is greatly reduced from its intrinsic value as the carrier scattering is increased when the epitaxial strain is released. We argue that the AHE of fully strained SrRuO$_3$ film with low residual resistivity represents the intrinsic AHE of SrRuO$_3$.

cond-mat.mtrl-sci↗

arXiv2005

Eu$_{0.5}$Sr$_{1.5}$MnO$_4$: a three-dimensional XY spin glass

The frequency, temperature, and dc-bias dependence of the ac-susceptibility of a high quality single crystal of the Eu$_{0.5}$Sr$_{1.5}$MnO$_4$ layered manganite is investigated. Eu$_{0.5}$Sr$_{1.5}$MnO$_4$ behaves like a XY spin glass with a strong basal anisotropy. Dynamical and static scalings reveal a three-dimensional phase transition near $T_g$ = 18 K, and yield critical exponent values between those of Heisenberg- and Ising-like systems, albeit slightly closer to the Ising case. Interestingly, as in the latter system, the here observed rejuvenation effects are rather weak. The origin and nature of the low temperature XY spin glass state is discussed.

cond-mat.mtrl-sci↗

arXiv2004

Colossal magnetoresistance without phase separation: Disorder-induced spin glass state and nanometer scale orbital-charge correlation in half doped manganites

The magnetic and electrical properties of high quality single crystals of $A$-site disordered (solid solution) Ln$_{0.5}$Ba$_{0.5}$MnO$_3$ are investigated near the phase boundary between the spin glass insulator and colossal-magnetoresistive ferromagnetic metal, locating near Ln = Sm. The temperature dependence of the ac-susceptibility and the x-ray diffuse scattering of Eu$_{0.5}$Ba$_{0.5}$MnO$_3$ are analyzed in detail. The uniformity of the random potential perturbation in Ln$_{0.5}$Ba$_{0.5}$MnO$_3$ crystals with small bandwidth yields, rather than the phase separation, an homogeneous short ranged charge/orbital order which gives rise to a nearly-atomic spin glass state. Remarkably, this microscopically disordered ``CE-glass'' state alone is able to bring forth the colossal magnetoresistance.

cond-mat.mtrl-sci↗

arXiv2004

Spin Glasses: A Ghost Story

Extensive experimental and numerical studies of the non-equilibrium dynamics of spin glasses subjected to temperature or bond perturbations have been performed to investigate chaos and memory effects in selected spin glass systems. Temperature shift and cycling experiments were performed on the strongly anisotropic Ising-like system {\ising} and the weakly anisotropic Heisenberg-like system {\AgMn}, while bond shift and cycling simulations were carried out on a 4 dimensional Ising Edwards-Anderson spin glass. These spin glass systems display qualitatively the same characteristic features and the observed memory phenomena are found to be consistent with predictions from the ghost domain scenario of the droplet scaling model.

cond-mat.mtrl-sci↗

arXiv2004

Scaling of the anomalous Hall effect in Sr$_{1-x}$Ca$_x$RuO$_3$

The anomalous Hall effect (AHE) of ferromagnetic thin films of Sr$_{1-x}$Ca$_{x}$RuO$_3$ (0 $\leq x \leq$ 0.4) is studied as a function of $x$ and temperature $T$. As $x$ increases, both the transition temperature $T_c$ and the magnetization $M$ are reduced and vanish near $x \sim$ 0.7. For all compositions, the transverse resistivity $ρ_{H}$ varies non-monotonously with $T$, and even changes sign, thus violating the conventional expression $ρ_{H}=R_o B + 4πR_s M(T)$ ($B$ is the magnetic induction, while $R_o$ and $R_s$ are the ordinary and anomalous Hall coefficients). From the rather complicated data of $ρ_H$, we find a scaling behavior of the transverse conductivity $σ_{xy}$ with $M(T)$, which is well reproduced by the first-principles band calculation assuming the intrinsic origin of the AHE.

cond-mat.mtrl-sci↗