arXiv subjects

Christian Franz

Publications and source records attributed to Christian Franz.

15 recordsLinked to original sources

arXiv2023

Resonant elastic X-ray scattering of antiferromagnetic superstructures in EuPtSi$_{3}$

We report resonant elastic X-ray scattering (REXS) of long-range magnetic order in EuPtSi$_{\text{3}}$, combining different scattering geometries with full linear polarization analysis to unambiguously identify magnetic scattering contributions. At low temperatures, EuPtSi$_{\text{3}}$ stabilizes type A antiferromagnetism featuring various long-wavelength modulations. For magnetic fields applied in the hard magnetic basal plane, well-defined regimes of cycloidal, conical, and fan-like superstructures may be distinguished that encompass a pocket of commensurate type A order without superstructure. For magnetic field applied along the easy axis, the phase diagram comprises the cycloidal and conical superstructures only. Highlighting the power of polarized REXS, our results reveal a combination of magnetic phases that suggest a highly unusual competition between antiferromagnetic exchange interactions with Dzyaloshinsky--Moriya spin--orbit coupling of similar strength.

cond-mat.str-el↗

arXiv2023

Active learning-assisted neutron spectroscopy with log-Gaussian processes

Neutron scattering experiments at three-axes spectrometers (TAS) investigate magnetic and lattice excitations by measuring intensity distributions to understand the origins of materials properties. The high demand and limited availability of beam time for TAS experiments however raise the natural question whether we can improve their efficiency and make better use of the experimenter's time. In fact, there are a number of scientific problems that require searching for signals, which may be time consuming and inefficient if done manually due to measurements in uninformative regions. Here, we describe a probabilistic active learning approach that not only runs autonomously, i.e., without human interference, but can also directly provide locations for informative measurements in a mathematically sound and methodologically robust way by exploiting log-Gaussian processes. Ultimately, the resulting benefits can be demonstrated on a real TAS experiment and a benchmark including numerous different excitations.

physics.data-an↗

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 Néel temperature $T_{\rm N}$ = 5.6 K. The specific heat at zero-field shows a clear $λ$-type anomaly with a broad shoulder at $T_{\rm N}$. A conservative estimate of the Sommerfeld coefficient of the electronic specific heat, $γ= 121~\mathrm{mJ~K^{-2}~mol^{-1}}$, 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↗

arXiv2023

Evolution of the spin dynamics during freezing in the spin-glass Fe$_{x}$Cr$_{1-x}$

In the iron--chromium system, Fe$_{x}$Cr$_{1-x}$, a wide dome of spin-glass behavior emerges when the ferromagnetism of iron is suppressed and the antiferromagnetism of chromium emerges as a function of increasing iron content $x$. As both, the high-temperature state and the characteristic cluster size vary as a function of $x$, different regimes of spin-glass behavior may be compared in a single, isostructural material system. Here, we report a study of the spin dynamics across the freezing process into the spin-glass state for different iron concentrations ($x = 0.145$, $0.175$, $0.21$) using Modulation of IntEnsity with Zero Effort (MIEZE) spectroscopy. In the parameter range studied, the relaxation process observed experimentally may be described well in terms of a stretched exponential. In the reentrant cluster-glass regime, $x = 0.145$, this behavior persists up to high temperatures. In comparison, in the superparamagnetic regime, $x = 0.175$ and $x = 0.21$, a single relaxation time at elevated temperatures is observed. For all samples studied, the spin relaxation exhibits a momentum dependence consistent with a power law, providing evidence of a dispersive character of the spin relaxation.

cond-mat.str-el↗

arXiv2022

Extending MIEZE spectroscopy towards thermal wavelengths

We propose a Modulation of intensity with zero effort (MIEZE) set-up for high-resolution neutron spectroscopy at momentum transfers up to 3Å$^{-1}$,energy transfers up to ~ 20 meV, and an energy resolution in the $μ$eV-range using both thermal and cold neutrons. MIEZE has two prominent advantages compared to classical neutron spin-echo. The first one is the possibility to investigate spin-depolarizing samples or samples in strong magnetic fields without loss of signal amplitude and intensity. This allows for the study of spin fluctuations in ferromagnets, and facilitates the study of samples with strong spin-incoherent scattering. The second advantage is that multi-analyzer setups can be implemented with comparatively small effort. The use of thermal neutrons increases the range of validity of the spin-echo approximation towards shorter spin-echo times. In turn, the thermal MIEZE option for greater ranges (TIGER) closes the gap between classical neutron spin-echo spectroscopy and conventional high-resolution neutron spectroscopy techniques such as triple-axis, time-of-flight, and back-scattering. To illustrate the feasibility of TIGER we present the details of an implementation at the beamline RESEDA at FRM II by means of an additional velocity selector, polarizer and analyzer.

cond-mat.str-el↗

arXiv2021

Autonomous Experiments for Neutron Three-Axis Spectrometers (TAS) with Log-Gaussian Processes

Autonomous experiments are excellent tools to increase the efficiency of material discovery. Indeed, AI and ML methods can help optimizing valuable experimental resources as, for example, beam time in neutron scattering experiments, in addition to scientists' knowledge and experience. Active learning methods form a particular class of techniques that acquire knowledge on a specific quantity of interest by autonomous decisions on what or where to investigate next based on previous measurements. For instance, Gaussian Process Regression (GPR) is a well-known technique that can be exploited to accomplish active learning tasks for scattering experiments as was recently demonstrated. Gaussian processes are not only capable to approximate functions by their posterior mean function, but can also quantify uncertainty about the approximation itself. Hence, if we perform function evaluations at locations of highest uncertainty, the function can be "optimally" learned in an iterative manner. We suggest the use of log-Gaussian processes, being a natural approach to successfully conduct autonomous neutron scattering experiments in general and TAS experiments with the instrument PANDA at MLZ in particular.

physics.data-an↗

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

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↗

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

Local atomic arrangement in LaCuAl3 and LaAuAl3 by NMR and density functional theory

CeCuAl3 and CeAuAl3, crystallizing in the non-centrosymmetric BaNiSn3 tetragonal structure, are known mainly for their unusual neutron scattering spectra involving additional excitations ascribed to vibron quasi-bound quantum state in CeCuAl3 and anti-crossing of phonon and crystal field excitations in CeAuAl3. In this work, we present results of nuclear magnetic resonance experiments on their lanthanum analogues - LaCuAl3 and LaAuAl3. The character of nuclear magnetic resonance spectra of 139La, 27Al, and 65Cu measured in LaAuAl3 and LaCuAl3 is dominated by electric quadrupole interaction. The spectral parameters acquired from experimental data are confronted with values obtained from the electronic structure calculations. The results show remarkable diffrences for the two compounds. The 139La spectrum in LaAuAl3 can be interpreted by a single spectral component corresponding to uniform environment of La atoms in the crystal structure, whereas for LaCuAl3 the spectrum decomposition yields a wide distribution of spectral parameters, which is not possible to explain by a single La environment, and multiple non-equivalent La positions in the crystal structure are required to interpret the spectrum.

cond-mat.mtrl-sci↗

arXiv2014

On/off switching of bit readout in bias-enhanced tunnel magneto-Seebeck effect

Thermoelectric effects in magnetic tunnel junctions are currently an attractive research topic. Here, we demonstrate that the tunnel magneto-Seebeck effect (TMS) in CoFeB/MgO/CoFeB tunnel junctions can be switched on to a logic 1 state and off to 0 by simply changing the magnetic state of the CoFeB electrodes. We enable this new functionality of magnetic tunnel junctions by combining a thermal gradient and an electric field. This new technique unveils the bias-enhanced tunnel magneto-Seebeck effect, which can serve as the basis for logic devices or memories in a green information technology with a pure thermal write and read process. Furthermore, the thermally generated voltages that are referred to as the Seebeck effect are well known to sensitively depend on the electronic structure and therefore have been valued in solid-state physics for nearly one hundred years. Here, we lift Seebeck's historic discovery from 1821 to a new level of current spintronics. Our results show that the signal crosses zero and can be adjusted by tuning a bias voltage that is applied between the electrodes of the junction; hence, the name of the effect is bias-enhanced tunnel magneto-Seebeck effect (bTMS). Via the spin- and energy-dependent transmission of electrons in the junction, the bTMS effect can be configured using the bias voltage with much higher control than the tunnel magnetoresistance (TMR) and even completely suppressed for only one magnetic configuration, which is either parallel (P) or anti-parallel (AP). This option allows a readout contrast for the magnetic information of -3000% at room temperature while maintaining a large signal for one magnetic orientation. This contrast is much larger than the value that can be obtained using the TMR effect. Moreover, our measurements are a step towards the experimental realization of high TMS ratios, which are predicted for specific Co-Fe compositions.

cond-mat.mes-hall↗

arXiv2013

Band Structure and Effective Masses of ZnMgO

We analyze the influence of the Mg concentration on several important properties of the band structure of ZnMgO alloys in wurtzite structure using ab initio calculations. For this purpose, the band structure for finite concentrations is defined in terms of the Bloch spectral density, which can be calculated within the coherent potential approximation. We investigate the concentration dependence of the band gap and the crystal-field splitting of the valence bands. The effective electron and hole masses are determined by extending the effective mass model to finite concentrations. We compare our results with experimental results and other calculations.

cond-mat.mtrl-sci↗

arXiv2013

Influence of the magnetic material on tunneling magnetoresistance and spin-transfer torque in tunnel junctions: Ab initio studies

The dependence of tunneling magnetoresistance and spin-transfer torque in FeCo/MgO/FeCo tunnel junctions on the Co concentration and the bias voltage are investigated ab initio. We find that the tunneling magnetoresistance decreases with the Co concentration in contradiction with previous calculations but in agreement with recent experiments. This dependence is explained from bulk properties of the alloys. By using a realistic description of the disorder in the alloys we can show that even small amounts of disorder lead to a drastic drop in the tunneling magnetoresistance. This provides a quantitative explanation of the difference between calculated and measured values. The spin-transfer torque shows a linear voltage dependence for the in-plane component and a quadratic for the out-of-plane component for all concentrations at small bias voltages. In particular, the linear slope of the in-plane torque is independent of the concentration. For high bias voltages the in-plane torque shows a strong nonlinear deviation from the linear slope for high Co concentrations. This is explained from the same effects which govern the tunneling magnetoresistance.

cond-mat.mes-hall↗

arXiv2013

Implementation of non-equilibrium vertex corrections in KKR: transport through disordered layers

The theoretical description of modern nanoelectronic devices requires a quantum mechanical treatment and often involves disorder, e.g. form alloys. Therefore, the ab initio theory of transport using non-equilibrium Green's functions is extended to the case of disorder described by the coherent potential approximation. This requires the calculation of non-equilibrium vertex corrections. We implement the vertex corrections in a Korringa-Kohn-Rostoker multiple scattering scheme. In order to verify our implementation and to demonstrate the accuracy and applicability we investigate a system of an iron-cobalt alloy layer embedded in copper. The results obtained with the coherent potential approximation are compared to supercell calculations. It turns out that vertex corrections play an important role for this system.

cond-mat.mes-hall↗

arXiv2013

Ab initio studies of the tunneling magneto-Seebeck effect: influence of magnetic material

We found a strong influence of the composition of the magnetic material on the temperature dependence of the tunneling magneto-Seebeck effect in $MgO$ based tunnel junctions. We use \textit{ab initio} alloy theory to consider different $Fe_xCo_{1-x}$ alloys for the ferromagnetic layer. Even a small change of the composition leads to strong changes in the magnitude or even in the sign of the tunneling magneto-Seebeck effect. This can explain differences between recent experimental results. In addition, changing the barrier thickness from six to ten monolayers of $MgO$ leads also to a non-trivial change of the temperature dependence. Our results emphasize that the tunneling magneto-Seebeck effect depends very crucially and is very sensitive to material parameters and show that further experimental and theoretical investigations are necessary.

cond-mat.mes-hall↗