arXiv subjects

Christian Pfleiderer

Publications and source records attributed to Christian Pfleiderer.

At least 37 records · Page 2Linked to original sources

arXiv2022

Emergence of mesoscale quantum phase transitions in a ferromagnet

Mesoscale patterns as observed, e.g., in ferromagnets, ferroelectrics, superconductors, mono-molecular films, or block-copolymers, reflect spatial variations of a pertinent order parameter at length- and time-scales that may be described classically. This raises the question for the relevance of mesoscale patterns near zero temperature phase transitions, also known as quantum phase transitions (QPTs). Here we report the magnetic susceptibility of LiHoF$_4$ -- a dipolar Ising ferromagnet -- near a well-understood transverse-field quantum critical point (TF-QCP). When tilting the magnetic field away from the hard axis such that the Ising symmetry is always broken, a line of well-defined phase transitions emerges from the TF-QCP characteristic of an additional symmetry breaking, in stark contrast to a crossover expected microscopically. We show that a continuous suppression of ferromagnetic domains, representing a breaking of translation symmetry on mesoscopic scales in an environment of broken magnetic Ising symmetry on microscopic scales, is in excellent qualitative and quantitative agreement with the field- and temperature dependence of the susceptibility and the magnetic phase diagram of LiHoF$_4$ under tilted field. This identifies a new type of phase transition that may be referred to as mesocale quantum criticality, which emanates from the text-book example of a microscopic ferromagnetic TF-QCP. Our results establish the surroundings of QPTs as a regime of mesoscale pattern formation, where non-analytical quantum dynamics and materials properties without classical analogue may be expected.

cond-mat.str-el↗

arXiv2022

Optically excited spin dynamics of thermally metastable skyrmions in Fe$_{0.75}$Co$_{0.25}$Si

We investigate the microwave spin excitations of the cubic chiral magnet Fe$_{0.75}$Co$_{0.25}$Si as driven by the thermal modulation of magnetic interactions via laser heating and probed by time-resolved measurements of the magneto-optical Kerr effect. Focusing on the topologically nontrivial skyrmion lattice state, the dynamic properties in thermodynamic equilibrium are compared with those of a metastable state prepared by means of rapid field cooling. In both cases, we find precessional and exponential contributions to the dynamic response, characteristic of a breathing mode and energy dissipation, respectively. When taking into account the universal scaling as a function of temperature, the precession frequencies in the equilibrium and metastable skyrmion state are in excellent quantitative agreement. This finding highlights that skyrmion states far from thermal equilibrium promise great flexibility, for instance with respect to temperature and field scales, both for possible microwave applications and the study of fundamental properties.

cond-mat.mes-hall↗

arXiv2021

Compositional Studies of Metals with Complex Order by means of the Optical Floating-Zone Technique

The availability of large high-quality single crystals is an important prerequisite for many studies in solid-state research. The optical floating-zone technique is an elegant method to grow such crystals, offering potential to prepare samples that may be hardly accessible with other techniques. As elaborated in this report, examples include single crystals with intentional compositional gradients, deliberate off-stoichiometry, or complex metallurgy. For the cubic chiral magnets Mn$_{1-x}$Fe$_{x}$Si and Fe$_{1-x}$Co$_{x}$Si, we prepared single crystals in which the composition was varied during growth from $x = 0 - 0.15$ and from $x = 0.1 - 0.3$, respectively. Such samples allowed us to efficiently study the evolution of the magnetic properties as a function of composition, as demonstrated by means of neutron scattering. For the archetypical chiral magnet MnSi and the itinerant antiferromagnet CrB$_{2}$, we grew single crystals with varying initial manganese (0.99 to 1.04) and boron (1.95 to 2.1) content. Measurements of the low-temperature properties addressed the correlation between magnetic transition temperature and sample quality. Furthermore, we prepared single crystals of the diborides ErB$_{2}$, MnB$_{2}$, and VB$_{2}$. In addition to high vapor pressures, these materials suffer from peritectic formation, potential decomposition, and high melting temperature, respectively.

cond-mat.str-el↗

arXiv2021

Symmetry-enforced topological nodal planes at the Fermi surface of a chiral magnet

Following over a decade of intense efforts to enable major progress in spintronics devices and quantum information technology by means of materials in which the electronic structure exhibits non-trivial topological properties, three key challenges are still unresolved. First, the identification of topological band degeneracies that are generically rather than accidentally located at the Fermi level. Second, the ability to easily control such topological degeneracies. And third, to identify generic topological degeneracies in large, multi-sheeted Fermi surfaces. Combining de Haas - van Alphen spectroscopy with density functional theory and band-topology calculations, we report here that the non-symmorphic symmetries in ferromagnetic MnSi generate nodal planes (NPs), which enforce topological protectorates (TPs) with substantial Berry curvatures at the intersection of the NPs with the Fermi surface (FS) regardless of the complexity of the FS. We predict that these TPs will be accompanied by sizeable Fermi arcs subject to the direction of the magnetization. Deriving the symmetry conditions underlying topological NPs, we show that the 1651 magnetic space groups comprise 7 grey groups and 26 black-and-white groups with topological NPs, including the space group of ferromagnetic MnSi. Thus, the identification of symmetry-enforced TPs on the FS of MnSi that may be controlled with a magnetic field suggests the existence of similar properties, amenable for technological exploitation, in a large number of materials.

cond-mat.str-el↗

arXiv2021

Network of topological nodal planes, multifold degeneracies, and Weyl points in CoSi

We report the identification of symmetry-enforced nodal planes (NPs) in CoSi providing the missing topological charges in an entire network of band-crossings comprising in addition multifold degeneracies and Weyl points, such that the fermion doubling theorem is satisfied. In our study we have combined measurements of Shubnikov-de Haas (SdH) oscillations in CoSi with material-specific calculations of the electronic structure and Berry curvature, as well as a general analysis of the band topology of space group (SG) 198. The observation of two nearly dispersionless SdH frequency branches provides unambiguous evidence of four Fermi surface sheets at the R point that reflect the symmetry-enforced orthogonality of the underlying wave functions at the intersections with the NPs. Hence, irrespective of the spin-orbit coupling strength, SG198 features always six- and fourfold degenerate crossings at R and $Γ$ that are intimately connected to the topological charges distributed across the network.

cond-mat.str-el↗

arXiv2021

Confined dipole and exchange spin waves in a bulk chiral magnet with Dzyaloshinskii-Moriya interaction

The Dzyaloshinskii-Moriya interaction (DMI) has an impact on excited spin waves in the chiral magnet Cu$_2$OSeO$_3$ by means of introducing asymmetry on their dispersion relations. The confined eigenmodes of a chiral magnet are hence no longer the conventional standing spin waves. Here we report a combined experimental and micromagnetic modeling study by broadband microwave spectroscopy we observe confined spin waves up to eleventh order in bulk Cu$_2$OSeO$_3$ in the field-polarized state. In micromagnetic simulations we find similarly rich spectra. They indicate the simultaneous excitation of both dipole- and exchange-dominated spin waves with wavelengths down to (47.2 $\pm$ 0.05) nm attributed to the exchange interaction modulation. Our results suggest DMI to be effective to create exchange spin waves in a bulk sample without the challenging nanofabrication and thereby to explore their scattering with noncollinear spin textures.

cond-mat.mtrl-sci↗

arXiv2021

MIEZE Neutron Spin-Echo Spectroscopy of Strongly Correlated Electron Systems

Recent progress in neutron spin-echo spectroscopy by means of longitudinal Modulation of IntEnsity with Zero Effort (MIEZE) is reviewed. Key technical characteristics are summarized which highlight that the parameter range accessible in momentum and energy, as well as its limitations, are extremely well understood and controlled. Typical experimental data comprising quasi-elastic and inelastic scattering are presented, featuring magneto-elastic coupling and crystal field excitations in Ho2Ti2O7, the skyrmion lattice to paramagnetic transition under applied magnetic field in MnSi, ferromagnetic criticality and spin waves in Fe. In addition bench marking studies of the molecular dynamics in H2O are reported. Taken together, the advantages of MIEZE spectroscopy in studies at small and intermediate momentum transfers comprise an exceptionally wide dynamic range of over seven orders of magnitude, the capability to perform straight forward studies on depolarizing samples or under depolarizing sample environments, as well as on incoherently scattering materials.

cond-mat.str-el↗

arXiv2021

Compact susceptometer for studies under transverse field geometries at very low temperatures

We present the design of a compact AC susceptometer for studies under arbitrarily oriented static magnetic fields, in particular magnetic fields oriented transverse to the AC excitation field. The small size of the susceptometer permits versatile use in conventional cryostats with superconducting magnet systems. The design of the susceptometer minimizes parasitic signal contributions while providing excellent thermal anchoring suitable for measurements in a wide range down to very low temperatures. The performance is illustrated by means of measurements of the transverse susceptibility at the magnetic field tuned quantum phase transition of the dipolar-coupled Ising ferromagnet LiHoF$_4$.

cond-mat.str-el↗

arXiv2021

The longitudinal neutron resonant spin echo spectrometer RESEDA

The instrumental layout and technical realisation of the neutron resonant spin echo (NRSE) spectrometer RESEDA at the Heinz Maier-Leibnitz Zentrum (MLZ) in Garching, Germany, is presented. RESEDA is based on a longitudinal field configuration, boosting both dynamic range and maximum resolution of the spectrometer compared to the conventional transverse layout. The resonant neutron spin echo technique enables the realisation of two complementary implementations: A longitudinal NRSE (LNRSE) option comparable to the classical neutron spin echo (NSE) method for highest energy resolution and large momentum transfers as well as a Modulation of Intensity with Zero Effort (MIEZE) option for depolarising samples or sample environments such as high magnetic fields, and strong incoherent scattering samples. With their outstanding dynamic range, exceeding nominally seven orders of magnitude, both options cover new fields for ultra-high resolution neutron spectroscopy in hard and soft condensed matter systems. In this paper the concept of RESEDA as well as the technical realisation along with reference measurements are reported.

physics.ins-det↗

arXiv2021

Surface pinning and triggered unwinding of skyrmions in a cubic chiral magnet

In the cubic chiral magnet Fe$_{1-x}$Co$_{x}$Si a metastable state comprising of topologically nontrivial spin whirls, so-called skyrmions, may be preserved down to low temperatures by means of field cooling the sample. This metastable skyrmion state is energetically separated from the topologically trivial ground state by a considerable potential barrier, a phenomenon also referred to as topological protection. Using magnetic force microscopy on the surface of a bulk crystal, we show that certain positions are preferentially and reproducibly decorated with metastable skyrmions, indicating that surface pinning plays a crucial role. Increasing the magnetic field allows an increasing number of skyrmions to overcome the potential barrier, and hence to transform into the ground state. Most notably, we find that the unwinding of individual skyrmions may be triggered by the magnetic tip itself, however, only when its magnetization is aligned parallel to the external field. This implies that the stray field of the tip is key for locally overcoming the topological protection. Both the control of the position of topologically nontrivial states as well as their creation and annihilation on demand pose important challenges in the context of potential skyrmionic applications.

cond-mat.mtrl-sci↗

arXiv2021

Field-induced reorientation of helimagnetic order in Cu$_2$OSeO$_3$ probed by magnetic force microscopy

Cu$_2$OSeO$_3$ is an insulating skyrmion-host material with a magnetoelectric coupling giving rise to an electric polarization with a characteristic dependence on the magnetic field $\vec H$. We report magnetic force microscopy imaging of the helical real-space spin structure on the surface of a bulk single crystal of Cu$_2$OSeO$_3$. In the presence of a magnetic field, the helimagnetic order in general reorients and acquires a homogeneous component of the magnetization, resulting in a conical arrangement at larger fields. We investigate this reorientation process at a temperature of 10~K for fields close to the crystallographic $\langle 110\rangle$ direction that involves a phase transition at $H_{c1}$. Experimental evidence is presented for the formation of magnetic domains in real space as well as for the microscopic origin of relaxation events that accompany the reorientation process. In addition, the electric polarization is measured by means of Kelvin-probe force microscopy. We show that the characteristic field dependency of the electric polarization originates in this helimagnetic reorientation process. Our experimental results are well described by an effective Landau theory previously invoked for MnSi, that captures the competition between magnetocrystalline anisotropies and Zeeman energy.

cond-mat.mtrl-sci↗

arXiv2021

Tunable Cooperativity in Coupled Spin--Cavity Systems

We experimentally study the tunability of the cooperativity in coupled spin--cavity systems by changing the magnetic state of the spin system via an external control parameter. As model system, we use the skyrmion host material Cu$_2$OSeO$_3$ coupled to a microwave cavity resonator. In the different magnetic phases we measure a dispersive coupling between the resonator and the magnon modes and model our results by using the input--output formalism. Our results show a strong tunability of the normalized coupling rate by magnetic field, allowing us to change the magnon--photon cooperativity from 1 to 60 at the phase boundaries of the skyrmion lattice state.

cond-mat.mes-hall↗

arXiv2020

Interplay of itinerant magnetism and reentrant spin-glass behavior in Fe$_{x}$Cr$_{1-x}$

When suppressing the itinerant antiferromagnetism in chromium by doping with the isostructual itinerant ferromagnet iron, a dome of spin-glass behavior emerges around a putative quantum critical point at an iron concentration $x \approx 0.15$. Here, we report a comprehensive investigation of polycrystalline samples of Fe$_{x}$Cr$_{1-x}$ in the range $0.05 \leq x \leq 0.30$ using x-ray powder diffraction, magnetization, ac susceptibility, and neutron depolarization measurements, complemented by specific heat and electrical resistivity data for $x = 0.15$. Besides antiferromagnetic ($x < 0.15$) and ferromagnetic regimes ($0.15 \leq x$), we identify a dome of reentrant spin-glass behavior at low temperatures for $0.10 \leq x \leq 0.25$ that is preceded by a precursor phenomenon. Neutron depolarization indicates an increase of the size of ferromagnetic clusters with increasing $x$ and the Mydosh parameter $ϕ$, inferred from the ac susceptibility, implies a crossover from cluster-glass to superparamagnetic behavior. Taken together, these findings consistently identify Fe$_{x}$Cr$_{1-x}$ as an itinerant-electron system that permits to study the evolution of spin-glass behavior of gradually varying character in unchanged crystalline environment.

cond-mat.str-el↗

arXiv2019

Ferromagnetic resonance with magnetic phase selectivity by means of resonant elastic x-ray scattering on a chiral magnet

Cubic chiral magnets, such as Cu$_{2}$OSeO$_{3}$, exhibit a variety of non-collinear spin textures, including a trigonal lattice of spin whirls, so-called skyrmions. Using magnetic resonant elastic x-ray scattering (REXS) on a crystalline Bragg peak and its magnetic satellites while exciting the sample with magnetic fields at GHz frequencies, we probe the ferromagnetic resonance modes of these spin textures by means of the scattered intensity. Most notably, the three eigenmodes of the skyrmion lattice are detected with large sensitivity. As this novel technique, which we label REXS-FMR, is carried out at distinct positions in reciprocal space, it allows to distinguish contributions originating from different magnetic states, providing information on the precise character, weight and mode mixing as a prerequisite of tailored excitations for applications.

cond-mat.mtrl-sci↗

arXiv2019

Polarized inelastic neutron scattering of non-reciprocal spin waves in MnSi

We report spin-polarized inelastic neutron scattering of the dynamical structure factor of the conical magnetic helix in the cubic chiral magnet MnSi. We find that the spectral weight of spin-flip scattering processes is concentrated on single branches for wavevector transfer parallel to the helix axis as inferred from well-defined peaks in the neutron spectra. In contrast, for wavevector transfers perpendicular to the helix the spectral weight is distributed among different branches of the magnon band structure as reflected in broader features of the spectra. Taking into account the effects of instrumental resolution, our experimental results are in excellent quantitative agreement with parameter-free theoretical predictions. Whereas the dispersion of the spin waves in MnSi appears to be approximately reciprocal at low energies and small applied fields, the associated spin-resolved spectral weight displays a pronounced non-reciprocity that implies a distinct non-reciprocal response in the limit of vanishing uniform magnetization at zero magnetic field.

cond-mat.str-el↗

arXiv2018

High-resolution neutron depolarization microscopy of the ferromagnetic transitions in Ni$_3$Al and HgCr$_2$Se$_4$ under pressure

We performed neutron imaging of ferromagnetic transitions in Ni$_3$Al and HgCr$_2$Se$_4$ crystals. These neutron depolarization measurements revealed bulk magnetic inhomogeneities in the ferromagnetic transition temperature with spatial resolution of about 100~$μ$m. To obtain such spatial resolution, we employed a novel neutron microscope equipped with Wolter mirrors as a neutron image-forming lens and a focusing neutron guide as a neutron condenser lens. The images of Ni$_3$Al show that the sample does not homogeneously go through the ferromagnetic transition; the improved resolution allowed us to identify a distribution of small grains with slightly off-stoichiometric composition. Additionally, neutron depolarization imaging experiments on the chrome spinel, HgCr$_2$Se$_4$, under pressures up to 15~kbar highlight the advantages of the new technique especially for small samples or sample environments with restricted sample space. The improved spatial resolution enables one to observe domain formation in the sample while decreasing the acquisition time despite having a bulky pressure cell in the beam.

cond-mat.str-el↗

arXiv2018

Magnetoelastic hybrid excitations in CeAuAl$_3$

The interactions between elementary excitations such as phonons, plasmons, magnons, or particle-hole pairs, drive emergent functionalities and electronic instabilities such as multiferroic behaviour, anomalous thermoelectric properties, polar order, or superconductivity. Whereas various hybrid excitations have been studied extensively, the feed-back of prototypical elementary excitations on the crystal electric fields (CEF), defining the environment in which the elementary excitations arise, has been explored for very strong coupling only. We report high-resolution neutron spectroscopy and ab-initio phonon calculations of {\ceaual}, an archetypal fluctuating valence compound. The high resolution of our data allows us to quantify the energy scales of three coupling mechanisms between phonons, CEF-split localized 4f electron states, and conduction electrons. Although these interactions do not appear to be atypically strong for this class of materials, we resolve, for the first time, a profound renormalization of low-energy quasiparticle excitations on all levels. The key anomalies of the spectrum we observe comprise (1) the formation of a CEF-phonon bound state with a comparatively low density of acoustic phonons reminiscent of vibronic modes observed in other materials, where they require a pronounced abundance of optical phonons, (2) an anti-crossing of CEF states and acoustic phonons, and (3) a strong broadening of CEF states due to the hybridization with more itinerant excitations. The fact that all of these features are well resolved in CeAuAl$_3$ suggests that similar hybrid excitations should also be dominant in a large family of related materials. This promises a predictive understanding towards the discovery of new magneto-elastic functionalities and instabilities.

cond-mat.str-el↗

arXiv2018

Search for pressure-induced tricriticality in Cr

The antiferromagnetic ordering of chromium has long been known for its peculiar physical properties. One of them is the observation of the weak first-order character of the N'eel transition that is explained by the lack of a stable fixed point by Bak and Mukamel. Barak et al. predicted that by lowering the symmetry of the order parameter by the application of uniaxial pressure along the [110] direction changes the N'eel transition to second-order. In a previous experiment by Fawcett et al., irreversible broadening of the Néel transition was already observed between 160 bar and 300 bar, which could, however, be caused by plastic deformation. Using an improved setup with reduced stress inhomogeneities we succeeded to increase the pressure range until irreversible broadening is observed above 450 bar. Despite the observed tripling of the intensity of the magnetic Bragg peak $[0, 0, 1-δ]$ at $p_{[110]}\geq450\,$bar, indicating a single $\bf{\text{Q}}_\pm$-domain state of the sample, no hints for a tricritical point were observed.

cond-mat.str-el↗