arXiv subjects

C. Pfleiderer

Publications and source records attributed to C. Pfleiderer.

At least 19 recordsLinked to original sources

arXiv2025

Spin-density wave of ferrimagnetic building blocks masking the ferromagnetic quantum-critical point in NbFe2

In the metallic magnet NbFe2, the low temperature threshold of ferromagnetism can be investigated by varying the Fe concentration within a narrow homogeneity range. NbFe2 is one of a number of compounds where modulated order is found to mask the ferromagnetic quantum critical point. However, here we report the rare case where the masking modulated magnetic order has been fully refined. Spherical neutron polarimetry and high-intensity single-crystal neutron diffraction reveal the first case of a longitudinal spin-density wave masking the ferromagnetic quantum critical point. The spin-density wave is characterised by a large-wavelength incommensurate modulation of its low average moment. It is formed from ferrimagnetic building blocks with antiparallel ferromagnetic sheets. The existence of ferromagnetic sheets and cancellation of the magnetisation only over mesoscopic length scales show local similarity between the spin-density wave and the ferromagnetic parent phase and indicate the spin-density wave's unconventional nature as emerging from underlying ferromagnetic quantum criticality.

cond-mat.str-el↗

arXiv2024

Long-range magnetic order in CePdAl$_3$ enabled by orthorhombic deformation

We investigate the effect of structural deformation on the magnetic properties of orthorhombic CePdAl$_3$ in relation to its tetragonal polymorph. Utilizing x-ray and neutron diffraction we establish that the crystal structure has the $Cmcm$ space group symmetry and exhibits pseudo-tetragonal twinning. According to density-functional calculations the tetragonal-orthorhombic deformation mechanism has its grounds in relatively small free enthalpy difference between the polymorphs, allowing either phase to be quenched and fully accounts for the twinned microstructure of the orthorhombic phase. Neutron diffraction measurements show that orthorhombic CePdAl$_3$ establishes long-range magnetic order below $T_\mathrm{N}$=5.29 (5) K characterized by a collinear, antiferromagnetic arrangement of magnetic moments. Magnetic anisotropies of orthorhombic CePdAl$_3$ arise from strong spin-orbit coupling as evidenced by the crystal-field splitting of the $4f$ multiplet, fully characterised with neutron spectroscopy. We discuss the potential mechanism of frustration posed by antiferromagnetic interactions between nearest neighbours in the tetragonal phase, which hinders the formation of long-range magnetic order in tetragonal CePdAl$_3$. We propose that orthorhombic deformation releases the frustration and allows for long-range magnetic order.

cond-mat.str-el↗

arXiv2024

Spin-Energy Entanglement of a Time-Focused Neutron

Intra-particle entanglement of individual particles such as neutrons could enable another class of scattering probes that are sensitive to entanglement in quantum systems and materials. In this work, we present experimental results demonstrating quantum contextuality as a result of entanglement between the spin and energy modes (i.e., degrees of freedom) of single neutrons in a beam using a pair of resonant radio-frequency neutron spin flippers in the MIEZE configuration (Modulated IntEnsity with Zero Effort). We verified the mode-entanglement by measuring a Clauser-Horne-Shimony-Holt (CHSH) contextuality witness $S$ defined in the spin and energy subsystems, observing a clear breach of the classical bound of $|S| \leq 2$, obtaining $S = 2.40 \pm 0.02$. These entangled beams could enable alternative approaches for directly probing dynamics and entanglement in quantum materials whose low-energy excitation scales match those of the incident entangled neutron.

quant-ph↗

arXiv2024

Influence of the magneto-volume effect on the transient reflectivity of MnSi

The magneto-volume effect is a well-established yet frequently overlooked phenomenon in magnetic materials that may affect a wide range of physical properties. Our study explores the influence of the magneto-volume effect on the transient reflectivity of MnSi, a well-known chiral magnet with strong magnetoelastic coupling. We observe a unipolar reflectivity transient in the paramagnetic phase, contrasting with a bipolar response in phases with magnetic long-range order. Comparing our findings with thermal expansion from literature, we establish that the bipolar response originates in the magneto-volume effect which dominates the thermal expansion and influences the optical reflectivity. Our results highlight not only that the magneto-volume effect must be considered when discussing transient reflectivity measurements of magnetic materials but also that such measurements permit to study the characteristic time scales of the magneto-volume effect itself, contributing to a deeper understanding of this often-neglected phenomenon.

cond-mat.mes-hall↗

arXiv2023

Comparison of time-of-flight with MIEZE spectroscopy of H$_2$O: Necessity to go beyond the spin-echo approximation

Here, we discuss the comparability of data acquisitioned with the Modulation of IntEnsity with Zero Effort data, a neutron spin-echo (NSE) technique to neutron Time-of-Flight (ToF) spectroscopy data. As a NSE technique MIEZE records the intermediate scattering function $\mathcal{I}(Q, \tau)$ making it necessary to perform a Fourier transform to directly compare it to $S(Q, E)$, measured by ToF spectroscopy. Transforming either data set into the complementary parameter space requires detailed knowledge of detector efficiency, instrumental resolution, and background. We discuss these aspects by comparing measurements on pure water performed on the spectrometers RESEDA and TOFTOF under the same experimental conditions. Additionally, we discuss the data evaluation of spin-echo data beyond the SE approximation, which limits these techniques to small energy transfers. Furthermore, computational methods like molecular dynamics simulations are essential for understanding these processes and will become increasingly important as we study more complex systems.

cond-mat.soft↗

arXiv2023

Low-temperature antiferromagnetic order in orthorhombic CePdAl$_{3}$

We report the magnetization, ac susceptibility, and specific heat of optically float-zoned single crystals of CePdAl$_{3}$. In comparison to the properties of polycrystalline CePdAl$_{3}$ reported in the literature, which displays a tetragonal crystal structure and no long-range magnetic order, our single crystals exhibit an orthorhombic structure ($Cmcm$) and order antiferromagnetically below a N\'eel temperature $T_{\rm N}$ = 5.6 K. The specific heat at zero-field shows a clear $\lambda$-type anomaly with a broad shoulder at $T_{\rm N}$. A conservative estimate of the Sommerfeld coefficient of the electronic specific heat, $\gamma = 121~\mathrm{mJ~K^{-2}~mol^{-1}}$, indicates a moderately enhanced heavy-fermion ground state. A twin microstructure evolves in the family of planes spanned by the basal plane lattice vectors $a_{\rm o}$ and $c_{\rm o}$, with the magnetic hard axis $b_{\rm o}$ common to all twins. The antiferromagnetic state is characterized by a strong magnetic anisotropy and a spin-flop transition induced under magnetic field along the easy direction, resulting in a complex magnetic phase diagram. Taken together our results reveal a high sensitivity of the magnetic and electronic properties of CePdAl$_{3}$ to its structural modifications.

cond-mat.str-el↗

arXiv2022

MIASANS at the longitudinal neutron resonant spin-echo spectrometer RESEDA

The RESEDA (Resonant Spin-Echo for Diverse Applications) instrument has been optimized for neutron scattering measurements of quasi-elastic and inelastic processes over a wide parameter range. One spectrometer arm of RESEDA is configured for the MIEZE (Modulation of Intensity with Zero Effort) technique, where the measured signal is an oscillation in neutron intensity over time prepared by two precisely tuned radio-frequency (RF) flippers. With MIEZE, all spin-manipulations are performed before the beam reaches the sample, and thus the signal from sample scattering is not disrupted by any depolarizing conditions there (i.e. magnetic materials and fields). The MIEZE spectrometer is being further optimized for the requirements of small angle neutron scattering (MIASANS), a versatile combination of the spatial and dynamical resolving power of both techniques. We present the current status of (i) the newly installed superconducting solenoids as part of the RF flippers to significantly extend the dynamic range (ii) the development and installation of a new detector on a translation stage within a new larger SANS-type vacuum vessel for flexibility with angular coverage and resolution, and (iii) the efforts to reduce background.

physics.ins-det↗

arXiv2022

Neutron Depolarization due to Ferromagnetism and Spin Freezing in CePd$_{1-x}$Rh$_x$

We report neutron depolarization measurements of the suppression of long-range ferromagnetism and the emergence of magnetic irreversibilities and spin freezing in CePd$_{1-x}$Rh$_x$ around $x^*\approx0.6$. Tracking the temperature versus field history of the neutron depolarization, we find clear signatures of long-range Ising ferromagnetism below a Curie temperature $T_{\rm C}$ for $x=0.4$ and a spin freezing of tiny ferromagnetic clusters below a freezing temperature $T_{\rm F1}$ for $x>x^*$. Under zero-field-cooling/field-heating and for $x>x^*$ a reentrant temperature dependence of the neutron depolarization between $T_{\rm F2}<T_{\rm F1}$ and $T_{\rm F1}$ is microscopically consistent with a thermally activated growth of the cluster size. The evolution of the depolarization as well as the reentrant temperature dependence as a function of Rh content are consistent with the formation of a Kondo-cluster glass below $T_{\rm F1}$ adjacent to a ferromagnetic quantum phase transition at $x^*$.

cond-mat.str-el↗

arXiv2022

Magnetocaloric properties of $R_3$Ga$_5$O$_{12}$ ($R$ = Tb,Gd,Nd,Dy)

We report the characteristic magnetic properties of several members of the rare-earth garnet family, Gd$_3$Ga$_5$O$_{12}$ (GGG), Dy$_3$Ga$_5$O$_{12}$ (DGG), Tb$_3$Ga$_5$O$_{12}$ (TGG), and Nd$_3$Ga$_5$O$_{12}$ (NGG), and compare their relative potential utility for magnetocaloric cooling, including their minimal adiabatic demagnetization refrigeration (ADR) temperatures and relative cooling parameters. A main objective of this work concerns the identification of potential improvements over the magnetocaloric properties of GGG for use in low temperature ADR cryostats. Using Tb$^{+3}$ and Dy$^{+3}$ in the rare-earth site offers, in principle, a higher saturation magnetization and Nd$^{+3}$ gives a lower de Gennes factor and therefore potentially reduced magnetic transition temperatures, limiting the useful temperature range. Our results show that Dy$_3$Ga$_5$O$_{12}$ yields an optimal relative cooling parameter ($RCP$) at low applied fields and low limiting temperatures, which would allow for the design of more efficient ADR cryostats.

cond-mat.str-el↗

arXiv2022

Magnetic properties of the noncentrosymmetric tetragonal antiferromagnet EuPtSi$_{3}$

We report a comprehensive study of single crystals of the noncentrosymmetric rare-earth compound EuPtSi$_{3}$ grown by the optical floating-zone technique. Measurements of the magnetization, ac susceptibility, and specific heat consistently establish antiferromagnetic order of localized Eu$^{2+}$ moments below the N\'{e}el temperature $T_{\mathrm{N}} = 17~\mathrm{K}$, followed by a second magnetic transition at $T_{\mathrm{N1}} = 16~\mathrm{K}$. For a magnetic field along the easy $[001]$ axis, the magnetic phase diagram is composed of these two phases. For fields applied in the magnetically hard basal plane, two additional phases emerge under magnetic field, where the in-plane anisotropy is weak with $[100]$ being the hardest axis. At the phase transitions, the magnetic properties exhibit hysteresis and discrepancies between differential and ac susceptibility, suggesting slow reorientation processes of mesoscale magnetic textures. Consistently, powder and single-crystal neutron diffraction in zero field identify magnetic textures that are modulated on a length scale of the order of $100~\textrm{\r{A}}$, most likely in the form of N\'{e}el-type antiferromagnetic cycloids.

cond-mat.str-el↗

arXiv2022

Topological magnon band structure of emergent Landau levels in a skyrmion lattice

The motion of a spin excitation across topologically non-trivial magnetic order exhibits a deflection that is analogous to the effect of the Lorentz force on an electrically charged particle in an orbital magnetic field. We used polarized inelastic neutron scattering to investigate the propagation of magnons (i.e., bosonic collective spin excitations) in a lattice of skyrmion tubes in manganese silicide. For wave vectors perpendicular to the skyrmion tubes, the magnon spectra are consistent with the formation of finely spaced emergent Landau levels that are characteristic of the fictitious magnetic field used to account for the nontrivial topological winding of the skyrmion lattice. This provides evidence of a topological magnon band structure in reciprocal space, which is borne out of the nontrivial real-space topology of a magnetic order.

cond-mat.str-el↗

arXiv2021

Incommensurate antiferromagnetic order in CePtAl$_3$

We report a neutron diffraction study of single-crystal CePtAl$_3$ complemented by measurements of the specific heat under applied magnetic field. Below $T_\mathrm{N}$=3 K CePtAl$_3$ develops incommensurate antiferromagnetic order with a single modulation vector $\vec{k}$=$(0.676 \, 0 \, 0)$. Residual magnetic scattering intensity above $T_\mathrm{N}$ and a broadening of the specific heat anomaly at $T_\mathrm{N}$ may be consistently described in terms of a Gaussian distribution of transition temperatures with a standard deviation $\sigma\approx0.5\,{\rm K}$. The distribution of $T_\mathrm{N}$ may be attributed to the observation of occupational and positional disorder between the Pt and Al sites. Measurements under magnetic field reveal unusual changes of the domain populations when the field is applied along the $[0\,1\,0]$ direction consistent with a transition from cycloidal to amplitude modulated magnetic order around 2.5 T.

cond-mat.str-el↗

arXiv2021

Observation of two independent skyrmion phases in a chiral magnetic material

Magnetic materials can host skyrmions, which are topologically non-trivial spin textures. In chiral magnets with cubic lattice symmetry, all previously-observed skyrmion phases require thermal fluctuations to become thermodynamically stable in bulk materials, and therefore exist only at relatively high temperature, close to the helimagnetic transition temperature. Other stabilization mechanisms require a lowering of the cubic crystal symmetry. Here, we report the identification of a second skyrmion phase in Cu$_{2}$OSeO$_{3}$ at low temperature and in the presence of an applied magnetic field. The new skyrmion phase is thermodynamically disconnected from the well-known, nearly-isotropic, high-temperature phase, and exists, in contrast, when the external magnetic field is oriented along the $\langle100\rangle$ crystal axis only. Theoretical modelling provides evidence that the stabilization mechanism is given by well-known cubic anisotropy terms, and accounts for an additional observation of metastable helices tilted away from the applied field. The identification of two distinct skyrmion phases in the same material and the generic character of the underlying mechanism suggest a new avenue for the discovery, design, and manipulation of topological spin textures.

cond-mat.str-el↗

arXiv2021

Thermodynamic evidence of a second skyrmion lattice phase and tilted conical phase in Cu$_2$0SeO$_3$

Precision measurements of the magnetization and ac susceptibility of Cu$_2$0SeO$_3$ are reported for magnetic fields along different crystallographic directions, focussing on the border between the conical and the field-polarized state for a magnetic field along the $\langle 100 \rangle$ axis, complemented by selected specific heat data. Clear signatures of the emergence of a second skyrmion phase and a tilted conical phase are observed, as recently identified by means of small-angle neutron scattering. The low-temperature skyrmion phase displays strongly hysteretic phase boundaries, but no dissipative effects. In contrast, the tilted conical phase is accompanied by strong dissipation and higher-harmonic contributions, while the transition fields are essentially nonhysteretic. The formation of the second skyrmion phase and tilted conical phase are found to be insensitive to a vanishing demagnetization factor. A quantitative estimate of the temperature dependence of the magnetocrystalline anisotropy may be consistently inferred from the magnetization and the upper critical field and agrees well with a stabilization of the low-temperature skyrmion phase and tilted conical state by conventional cubic magnetic anisotropies.

cond-mat.str-el↗

arXiv2021

Optimized signal deduction procedure for the MIEZE spectroscopy technique

We report a method to determine the phase and amplitude of sinusoidally modulated event rates, binned into four bins per oscillation, based on data generated at the resonant neutron spin-echo spectrometer RESEDA. The presented algorithm relies on a reconstruction of the unknown parameters. It omits a calculation intensive fitting procedure and avoids contrast reduction due to averaging effects. It allows the current data acquisition bottleneck at RESEDA to be relaxed by a factor of four and thus increases the potential time resolution of the detector by the same factor. We explain the approach in detail and compare it to the established fitting procedures of time series having four and 16 time bins per oscillation. In addition we present the empirical estimates of the errors of the three methods and compare them to each other. We show that the reconstruction is unbiased, asymptotic, and efficient for estimating the phase. Reconstructing the contrast increases the error bars by roughly 10% as compared to fitting 16 time binned oscillations. Finally, we give heuristic, analytical equations to estimate the error for phase and contrast as a function of their initial values and counting statistics.

cond-mat.str-el↗

arXiv2020

Oscillatory magnetic fields for neutron resonance spin-echo spectroscopy

The generation of high frequency oscillatory magnetic fields represents a fundamental component underlying the successful implementation of neutron resonant spin-echo spectrometers, a class of instrumentation critical for the high-resolution extraction of dynamical excitations (structural and magnetic) in materials. In this paper, the setup of the resonant circuits at the longitudinal resonant spin-echo spectrometer RESEDA is described in comprehensive technical detail. We demonstrate that these circuits are capable of functioning at frequencies up to 3.6 MHz and over a broad bandwidth down to 35 kHz using a combination of signal generators, amplifiers, impedance matching transformers, and a carefully designed cascade of tunable capacitors and customized coils.

cond-mat.str-el↗

arXiv2019

The 2020 Skyrmionics Roadmap

The notion of non-trivial topological winding in condensed matter systems represents a major area of present-day theoretical and experimental research. Magnetic materials offer a versatile platform that is particularly amenable for the exploration of topological spin solitons in real space such as skyrmions. First identified in non-centrosymmetric bulk materials, the rapidly growing zoology of materials systems hosting skyrmions and related topological spin solitons includes bulk compounds, surfaces, thin films, heterostructures, nano-wires and nano-dots. This underscores an exceptional potential for major breakthroughs ranging from fundamental questions to applications as driven by an interdisciplinary exchange of ideas between areas in magnetism which traditionally have been pursued rather independently. The skyrmionics roadmap provides a review of the present state of the art and the wide range of research directions and strategies currently under way. These are, for instance, motivated by the identification of the fundamental structural properties of skyrmions and related textures, processes of nucleation and annihilation in the presence of non-trivial topological winding, an exceptionally efficient coupling to spin currents generating spin transfer torques at tiny current densities, as well as the capability to purpose-design broad-band spin dynamic and logic devices.

cond-mat.str-el↗

arXiv2019

Orientation Dependence of the Magnetic Phase Diagram of Yb$_2$Ti$_2$O$_7$

In the quest to realize a quantum spin liquid (QSL), magnetic long-range order is hardly welcome. Yet it can offer deep insights into a complex world of strong correlations and fluctuations. Much hope was placed in the cubic pyrochlore Yb$_2$Ti$_2$O$_7$ as a putative U(1) QSL but a new class of ultra-pure single crystals make it abundantly clear the stoichiometric compound is a ferromagnet. Here we present a detailed experimental and theoretical study of the corresponding field-temperature phase diagram. We find it to be richly anisotropic with a critical endpoint for $\vec{B}\,\parallel\,\langle 100\rangle$, while field parallel to $\langle 110 \rangle$ and $\langle 111 \rangle$ enhances the critical temperature by up to a factor of two and shifts the onset of the field-polarized state to finite fields. Landau theory shows that Yb$_2$Ti$_2$O$_7$ in some ways is remarkably similar to pure iron. However, it also pinpoints anomalies that cannot be accounted for at the classical mean-field level including a dramatic enhancement of $T_{\mathrm{C}}$ and reentrant phase boundary by fields with a component transverse to the easy axes, as well as the anisotropy of the upper critical field in the quantum limit.

cond-mat.str-el↗