arXiv subjects

Clemens Ulrich

Publications and source records attributed to Clemens Ulrich.

10 recordsLinked to original sources

arXiv2026

Raman spectroscopic signature of Kitaev magnetism and complex spin-lattice coupling in S = 1/2 antiferromagnet SrLaCoNbO$_6$ double perovskite

We report a detailed analysis of temperature-dependent Raman spectroscopy and Co $K$-edge extended x-ray absorption fine structure (EXAFS) for a pseudospin-$\tilde{S}=1/2$ insulating antiferromagnet SrLaCoNbO$_6$, a B-site--ordered double perovskite hosting Co$^{2+}$ ($3d^7$) ions on an {\it f.c.c.} sublattice. Notably, pronounced anomalies in the phonon frequency, linewidth and spectral weight are observed around 60~K, well above the long-range antiferromagnetic transition at $T_{\rm N} \approx 15$~K. These renormalizations indicate a significant coupling between lattice and spin degrees of freedom, although a purely structural contribution cannot be excluded. Additional modifications of both high- and low-energy Raman modes are detected near 160-180 K, including changes in linewidth and intensity, variations of the Fano asymmetry parameter, and the emergence of an additional low-energy feature. The asymmetric Fano line shape of selected low-energy modes, together with a broad low-energy continuum and quasielastic response, suggests coupling between discrete phonons and fluctuating magnetic excitations. Moreover, the EXAFS analysis reveals correlated changes in bond distances and Debye-Waller factors around the Co ions near 60~K and 160~K, evidencing subtle local structural distortions and possible magnetostrictive effects. The persistence of anomalous lattice dynamics far above $T_{\rm N}$, combined with the excitation-energy--independent continuum, is consistent with fluctuating bond-directional interactions and proximate Kitaev-like correlations.

cond-mat.str-el↗

arXiv2024

Continuous Collapse of the Spin Cycloid in BiFeO3 Thin Films under an Applied Magnetic Field probed by Neutron Scattering

Bismuth ferrite (BiFeO3) is one of the rare materials that exhibits multiferroic properties already at room-temperature. Therefore, it offers tremendous potential for future technological applications, such as memory and logic. However, a weak magnetoelectric coupling together with the presence of a noncollinear cycloidal spin order restricts various practical applications of BiFeO3. Therefore, there is a large interest in the search for suitable methods for the modulation of the spin cycloid in BiFeO3. By performing neutron diffraction experiments using a triple-axis instrument we have determined that the spin cycloid can be systematically suppressed by applying a high magnetic field of 10 T in a BiFeO3 thin film of about 100 nm grown on a (110)-oriented SrTiO3 substrate. As predicted by previous theoretical calculations, we observed that the required critical magnetic field to suppress the spin cycloid in a BiFeO3 thin film was lower as compared to the previously reported critical magnetic field for bulk BiFeO3 single crystals. Our experiment reveals that the spin cycloid continuously expands with increasing magnetic field before the complete transformation into a G-type antiferromagnetic spin order. Such tuning of the length of the spin cycloid up to a complete suppression offers new functionalities for future technological applications as in spintronics or magnonics.

cond-mat.mtrl-sci↗

arXiv2023

Giant shifts of crystal-field excitations in ErFeO3 driven by internal magnetic fields

Due to the complex interactions between rare-earth elements and transition metals, as well as/or themselves, rare-earth transition-metal oxides are likely to exhibit highly intriguing and novel magnetic structures and dynamic behaviours. Rare-earth elements in these compounds frequently demonstrate unusual behaviours in their crystal-field (CF) excitations, which necessitate thorough research for in-depth comprehensions. When cooling from 10 K to 1.5 K via the magnetic ordering temperature of Er3+ at 4.1 K, we observed a significant energy shift of the low-lying CF excitation of Er3+ in ErFeO3 from 0.35 meV to 0.75 meV utilizing the inelastic neutron-scattering technique. A sound CF model was proposed for Er3+ in ErFeO3 by fitting to the observed CF excitation peaks, which enables to explain all the observed experimental results in a very consistent manner. According to the model, the ground crystal field level of Er3+, which corresponds to the lowest Kramers doublet supposed to be at zero energy, has been shifted by the internal magnetic fields induced by both Er3+ and Fe3+ spin orders below and above the Er3+ ordering temperature, respectively. Additional measurements in various magnetic fields offer compelling evidence in favour of this hypothesis. The measured external field dependence of the CF excitation energy led to the derivation of the internal field of Er3+ as 0.33 meV, which is strongly corroborated by theoretical modelling. Additionally, the effective g-factor for Er3+ in ErFeO3 showed an exceptionally significant anisotropy.

cond-mat.str-el↗

arXiv2023

Unveiling the magnetic structure and phase transition of Cr$_2$CoAl using neutron diffraction

We report the detailed analysis of temperature dependent neutron diffraction pattern of the Cr$_2$CoAl inverse Heusler alloy and unveil the magnetic structure up to the phase transition as well as its fully compensated ferrimagnetic nature. The Rietveld refinement of the diffraction pattern using the space group I$\bar4${\it m}2 confirm the inverse tetragonal structure over the large temperature range from 100~K to 900~K. The refinement of the magnetic phase considering the wave vector $k=$ (0, 0, 0) reveals the ferrimagnetic nature of the sample below 730$\pm$5~K. This transition temperature is obtained from empirical power law fitting of the variation in the ordered net magnetic moment and intensity of (110) peak as a function of temperature. The spin configuration of the microscopic magnetic structure suggests the nearly fully compensated ferrimagnetic behavior where the magnetic moments of Cr2 are antiparallel with respect to the Cr1, and Co moments. Moreover, the observed anomaly in the thermal expansion and lattice parameters at 730$\pm$5~K suggest that the distortion in crystal structure may play an important role in the magnetic phase transition.

cond-mat.str-el↗

arXiv2022

Investigation of lattice dynamics, magnetism and electronic transport in $β$-Na$_{0.33}$V$_2$O$_{5}$

We investigate the electronic and magnetic properties as well as lattice dynamics and spin-phonon coupling of $β$-Na$_{0.33}$V$_2$O$_5$ using temperature-dependent Raman scattering, dc-magnetization and dc-resistivity, x-ray photoemission, and absorption spectroscopy. The Rietveld refinement of XRD pattern with space group C2/m confirms the monoclinic structure. The analysis of temperature-dependent Raman spectra in a temperature range of 13--673~K reveals an anharmonic dependence of the phonon frequency and full width at half maximum, which is accredited to the symmetric phonon decay. However, below about 40 K, the hardening of the phonon frequency beyond anharmonicity is attributed to the spin-phonon coupling. Interestingly, the estimated effective magnetic moment $μ_{\rm eff}=$ 0.63~$μ_B$ from the magnetization data manifests a mixed-valence state of V ions in 4+ (18$\pm$1\%) and 5+ (82$\pm$1\%). A similar ratio of V$^{4+}$ to V$^{5+}$ is also observed in the x-ray photoemission and x-ray absorption near-edge spectra and that is found to be consistent with the sample stoichiometry. In addition, the V$^{4+}$ ions are distributed between different vanadium (V1 and V3) sites. The analysis of extended x-ray absorption fine structure at different V-sites gives the corresponding V--O bond lengths, which are utilized in the assignment of Raman modes. Moreover, the temperature-dependent resistivity resembles a semiconducting behavior where the charge carrier transport is facilitated by the band conduction at higher temperatures and via hopping $\le$260~K.

cond-mat.mtrl-sci↗

arXiv2021

Giant shifts of crystal-field excitations with temperature as consequence of internal magnetic exchange fields

Crystal-field excitations, for example in transition-metal oxides where a rare-earth element is used as a spacer between the transition-metal-oxide tetrahedra and octahedra, are assumed to be extremely robust with respect to external perturbations such as temperature. Using inelastic neutron scattering experiments, a giant shift of the energy of the lowest crystal-field excitation of Er3+ (4I15/2) in ErFeO3 from 0.30(2) meV to 0.75(2) meV was measured below the magnetic-ordering temperature of erbium at 4.1 K. Quantum-mechanical point-charge calculations of the crystal-field levels indicate that the shift is caused by the internal magnetic field created by the erbium spins themselves, which causes a Zeeman splitting of the erbium 4f electronic levels, and therefore a change in the energies of crystal-field transitions. To verify this explanation, the effect of an external magnetic field on the crystal-field excitations was measured by inelastic neutron scattering and compared to the field-dependent point-charge calculations. The existence of an internal magnetic exchange interaction will have implications for a deeper understanding of a broader group of phenomena such as multiferroic properties or spin frustration, which are a consequence of various competing electronic and magnetic exchange interactions.

cond-mat.str-el↗

arXiv2019

Neutron diffraction and magnetic properties of Co$_2$Cr$_{1-x}$Ti$_x$Al Heusler alloys

We report the structural, magnetic, and magnetocaloric properties of Co$_2$Cr$_{1-x}$Ti$_x$Al ($x=$ 0--0.5) Heusler alloys for spintronic and magnetic refrigerator applications. Room temperature X-ray diffraction and neutron diffraction patterns along with Rietveld refinements confirm that the samples are of single phase and possess a cubic structure. Interestingly, magnetic susceptibly measurements indicate a second order phase transition from paramagnetic to ferromagnetic where the Curie temperature (T$_{\rm C}$) of Co$_2$CrAl increases from 330~K to 445~K with Ti substitution. Neutron powder diffraction data of the $x=$ 0 sample across the magnetic phase transition taken in a large temperature range confirm the structural stability and exclude the possibility of antiferromagnetic ordering. The saturation magnetization of the $x=$ 0 sample is found to be 8000~emu/mol (1.45~$μ_{\rm B}$/{\it f.u.}) at 5~K, which is in good agreement with the value (1.35$\pm$0.05~$μ_{\rm B}$/{\it f.u.}) obtained from the Rietveld analysis of the neutron powder diffraction pattern measured at temperature of 4~K. By analysing the temperature dependence of the neutron data of the $x=$ 0 sample, we find that the change in the intensity of the most intense Bragg peak (220) is consistent with the magnetization behavior with temperature. Furthermore, an enhancement of change in the magnetic entropy and relative cooling power values has been observed for the $x=$ 0.25 sample. Interestingly, the critical behavior analysis across the second order magnetic phase transition and extracted exponents ($β\approx$ 0.496, $γ\approx$ 1.348, and $δ\approx$ 3.71 for the $x=$ 0.25 sample) suggest the presence of long-range ordering, which deviates towards 3D Heisenberg type interactions above T$_{\rm C}$, consistent with the interaction range value $σ$.

cond-mat.mtrl-sci↗

arXiv2019

Spin dynamics of edge-sharing spin chains in SrCa13Cu24O41

The low-energy magnetic excitation from the highly Ca-doped quasi-one-dimensional magnet SrCa13Cu24O41 was studied in the magnetic ordered state by using inelastic neutron scattering. We observed the gapless spin-wave excitation, dispersive along the a and c axes but nondispersive along the b axis. Such excitations are attributed to the spin wave from the spin-chain sublattice. Model fitting to the experimental data gives the nearest-neighbour interaction Jc as 5.4 meV and the interchain interaction Ja = 4.4 meV. Jc is antiferromagnetic and its value is close to the nearest-neighbour interactions of the similar edge-sharing spin-chain systems such as CuGeO3. Comparing with the hole-doped spin chains in Sr14Cu24O41, which shows a spin gap due to spin dimers formed around Zhang-Rice singlets, the chains in SrCa13Cu24O41 show a gapless excitation in this study. We ascribe such a change from gapped to gapless excitations to holes transferring away from the chain sublattice into the ladder sublattice upon Ca doping.

cond-mat.str-el↗

arXiv2018

Investigation of the structural, electronic, transport and magnetic properties of Co$_2$FeGa Heusler alloy nanoparticles

We report the structural, transport, electronic, and magnetic properties of Co$_2$FeGa Heusler alloy nanoparticles. The Rietveld refinements of x-ray diffraction (XRD) data with the space group Fm$\bar {3}$m clearly demonstrates that the nanoparticles are of single phase. The particle size (D) decreases with increasing the SiO$_2$ concentration. The Bragg peak positions and the inter-planer spacing extracted from high-resolution transmission electron microscopy image and selected area electron diffraction are in well agreement with data obtained from XRD. The coercivity initially increases from 127~Oe to 208~Oe between D = 8.5~nm and 12.5~nm, following the D$^{-3/2}$ dependence and then decreases with further increasing D up to 21.5~nm with a D$^{-1}$ dependence, indicating the transition from single domain to multidomain regime. The effective magnetic anisotropic constant behaves similarly as coercivity, which confirms this transition. A complex scattering mechanisms have been fitted to explain the electronic transport properties of these nanoparticles. In addition we have studied core-level and valence band spectra using photoemission spectroscopy, which confirm the hybridization between $d$ states of Co/Fe. Further nanoparticle samples synthesized by co-precipitation method show higher saturation magnetization. The presence of Raman active modes can be associated with the high chemical ordering, which motivates for detailed temperature dependent structural investigation using synchrotron radiation and neutron sources.

cond-mat.mtrl-sci↗

arXiv2018

Spin Dynamics and Magnetoelectric Coupling Mechanism of Co4Nb2O9

Neutron powder diffraction experiments reveal that Co4Nb2O9 forms a noncollinear in-plane magnetic structure with Co2+ moments lying in the ab plane. The spin-wave excitations of this magnet were measured by using inelastic neutron scattering and soundly simulated by a dynamic model involving nearest and next-nearest neighbour exchange interactions, in-plane anisotropy and the Dzyaloshinskii-Moriya interaction. The in-plane magnetic structure of Co4Nb2O9 is attributed to the large in-plane anisotropy while the noncollinearity of the spin configuration is attributed to the Dzyaloshinskii-Moriya interaction. The high magnetoelectric coupling effect of Co4Nb2O9 in fields can be explained by its special in-plane magnetic structure.

cond-mat.mtrl-sci↗