arXiv subjects

Karsten Holldack

Publications and source records attributed to Karsten Holldack.

10 recordsLinked to original sources

arXiv2026

Laser induced ultrafast Co 3d and Ho 4f spin dynamics in CoHo ferrimagnetic alloys

The transition metal (TM) / rare-earth (RE) alloys have received renewed interest lately as model systems to decipher the origin of all-optical helicity-independent switching (AO-HIS) by femtosecond laser pulses. Recently, a distinct single-pulse magnetization reversal mechanism, based on domain-wall motion and coalescence, has been reported in CoDy and CoHo alloys, as well as in Co/Gd ultrathin bilayers. It has been claimed that this specific toggle switching is achieved when the Co sublattice is fully demagnetized and the characteristic demagnetization times {\tau}RE of the RE (Gd, Dy or Ho) sublattices is longer than that of the Co and the angular momentum transferred to Co is reduced. Element- and time-resolved X-ray spectroscopy studies of CoDy alloys have reported characteristic demagnetization times of {\tau}Co = 0.2 ps and {\tau}Dy = 0.6 ps at room temperature. Similarly, Ho is expected to exhibit a slower response, but its ultrafast 4f spin dynamics remain experimentally unexplored. Here, we report on element- and time-resolved investigations of femtosecond laser induced ultrafast dynamics of the Co 3d and Ho 4f spins in ferrimagnetic Co_{80}Ho_{20} alloys to verify this prediction. We observed characteristic demagnetization times {\tau}Co = 0.22 +/- 0.01 ps and {\tau}Ho = 0.87 +/- 0.15 ps at room temperature. These results show that Ho demagnetizes substantially more slowly than Co in Co_{80}Ho_{20}, supporting the proposed condition for domain-wall-mediated all-optical toggle switching

cond-mat.mtrl-sci↗

arXiv2025

Non-Equilibrium Multiplet Excitations probed by the $M_{5,4}$ Branching Ratio in $3d \rightarrow 4f$ X-ray Absorption Spectroscopy

We show that ultrafast electronic $4f$ multiplet transitions in terbium metal are manifested by changes in the relative spectral weight of the $M_5$ and $M_4$ X-ray absorption resonances. Our experimental results are supported by a simulation of excited multiplet spectra with atomistic calculations; they prove that the so-called third rule of Thole and van der Laan, which relates the branching ratio of the spin-orbit split resonances to the total angular momentum $J$ of the excited ion, is also valid in non-equilibrium. The presented detection scheme allows to detect $J$-changing excitation, \textit{i.e}, alterations of spin and orbital states, even in samples without net magnetization. This makes branching-ratio spectroscopy a powerful tool for the quantitative investigation of ultrafast changes in angular momentum $J$.

cond-mat.mtrl-sci↗

arXiv2020

h-ErMnO3 absorbance, reflectivity and, emissivity in the THz to mid-infrared from 2 K to 1700 K: carrier screening, Fröhlich resonance, small polarons, and bipolarons

We report the temperature dependent THz to mid-infrared response of hexagonal-ErMnO3 using absorption, reflectivity, and emissivity techniques from 2 K to 1700 K. At low temperatures, lowest frequency vibrational modes coexist with paramagnon excitations associated with well-defined crystal field Rare Earth pure magnetic replicas in an intriguing phonon-magnetic convergence. Increasing the temperature, a number of vibrational bands close to the space group predicted undergo profile broadening and softening. In particular, a distinctive set of bands in the 288-329 cm-1 (300 K) range has a component whose profile is carrier screened becoming nearly fully blurred in the intermediate phase between ~830 K and ~1500 K. Below TC ~830 K this asymmetric band further splits as spin phonon interaction and the tripling of the unit cell takes place revealing a delicate balance of long- and short-range interactions. Ambient Raman scattering brings up evidence of a Fröhlich resonance due to Coulomb interactions between carriers and the macroscopic field of the longitudinal optical phonon mode. We found it is dynamically correlated to the hexagonal c-axis negative thermal expansion. Mid-infrared (MIR) optical conductivity show that also plays a role in small polarons and mediates in high temperature bipolarones.At higher temperatures a low frequency Drude contribution is triggered by electron hopping signaling an insulator-metal phase transition at ~1600 K while the MIR response suggests coexistence between single small polarons and bipolarons. We draw a parallel with improper ferroelectrics sustaining a lattice incommensurate intermediate phase and unit cell tripling. We argue that in the h-RMnO3 (R=Rare Earth, Y) family of compounds the intermediate phase be considered incommensurate with onset at TINC ~1500 K and ferroelectric lock-in at TC ~830 K delimiting this regime in h-ErMnO3.

cond-mat.str-el↗

arXiv2020

Laser induced ultrafast 3d and 4f spin dynamics in CoDy ferrimagnetic alloys as a function of temperature

We report on an element- and time-resolved investigation of femtosecond laser induced ultrafast dynamics of 3d and 4f spins in a ferrimagnetic Co80Dy20 alloy as a function of temperature. We observe an increase of the Co3d characteristic demagnetization time and a decrease of the Dy4f demagnetization time when the temperature is approaching the Curie temperature. It suggests that the critical slowing down regime, which affects the laser induced ultrafast dynamics in pure 3d transition metals and 4f rare-earth ferromagnetic layers, vanishes for the Dy sublattice in the CoDy alloy, in line with the theoretical predictions of the Landau-Lifshitz-Bloch model.

cond-mat.mtrl-sci↗

arXiv2018

Far- and Mid-infrared Emission and Reflectivity of Orthorhombic and Cubic ErMnO3: polarons and bipolarons

We report on the high-temperature evolution of far- and mid- infrared reflectivity and emissivity spectra of ambient orthorhombic ErMnO3 from 12 K to sample decomposition above 1800 K. At low temperatures the number of phonons agrees with the predictions for orthorhombic space group D2h16-Pbnm (Z=4) and coexists with a paramagnon spin resonance and rare earth crystal field transitions. Increasing the temperature, a number of vibrational bands undergo profile broadening and softening approaching the orbital disordered phase where the orthorhombic O' lower temperature cooperative phase coexists with cubic-orthorhombic O. O-ErMnO3 undergoes a first order order-disorder transition into the perovskite cubic phase at Tcubic ~1329 K where the three triple degenerate phonons allowed by the space group Pm-3m(Z=1) are identified. At about 800 K, a quantitative small polaron analysis of the orthorhombic mid-infrared real part optical conductivity shows that antisymmetric and symmetric breathing modes sustain the strongest electron-phonon interactions. Above Tcubic the bipolaron fingerprint profile is the mid-infrared dominant and only feature. Its appearance correlates with the localized screening of the highest vibrational mode reststrahlen band. We propose that the longitudinal optical mode macroscopic field screening is consequence of dynamically sharing δ disproportioned eg electrons hovering over the JT distorted octahedral dimer [Mn (QJT)3+δ (Mn(QJT)3-δ))O6/2]2. A thermal driven insulator-metal transition is detected with onset ~1600 K. We also address the occurrence of an inhomogeneity induced THz band result of heating the samples in dry air, triggering Mn3+-Mn4+ double exchange, under the presence of Mn4+ smaller ions stabilizing the orthorhombic lattice.

cond-mat.str-el↗

arXiv2017

Ultrafast and Energy-Efficient Quenching of Spin Order: Antiferromagnetism Beats Ferromagnetism

By comparing femtosecond laser pulse induced ferro- and antiferromagnetic dynamics in one and the same material - metallic dysprosium - we show both to behave fundamentally different. Antiferromagnetic order is considerably faster and much more efficiently manipulated by optical excitation than its ferromagnetic counterpart. We assign the fast and extremely efficient process in the antiferromagnet to an interatomic transfer of angular momentum within the spin system. Our findings do not only reveal this angular momentum transfer channel effective in antiferromagnets and other magnetic structures with non-parallel spin alignment, they also point out a possible route towards energy-efficient spin manipulation for magnetic devices.

cond-mat.mes-hall↗

arXiv2017

Identification of spin wave resonances and crystal field transitions in simple chromites RCrO3 (R=Pr, Sm, Er) at ultralow temperatures in the THz spectral region

We report on THz absorption spectroscopy combined with high magnetic fields of polycrystalline RCrO3 (R= Pr, Sm, Er) aiming understanding spin wave resonances at their low temperature magnetic phases. Our measurements show that the temperature, and the implicit anisotropies at which the Cr3+ spin reorientation at TSR takes place, are determinant on the ferromagnetic-like (FM) and the antiferromagnetic-like (AFM) spin modes being optically active. It is found that they are dependent on Rare Earth 4f moment and ion size. We also studied temperature and field dependence of crystal field levels in the same spectroscopic region. Pr3+ non-Kramers emerges at 100 K and Zeeman splits. An observed absence of spin wave resonances in PrCrO3 is attributed to Pr3+ remaining paramagnetic. In SmCrO3 near cancelation of the spin and orbital moments is proposed as the possible reason for not detecting Sm3+ ground state transitions. Here, the FM and AFM resonant modes harden when the temperature decreases and split linearly on applied fields at 5 K and below. In ErCrO3 the Er3+ Kramers doublet becomes active at about the TSR onset. Each line further experiences Zeeman splitting under magnetic fields while an spin reversal induced by a 2.5 T field, back to the Gamma4 (Fz) from the Gamma1 phase at 2 K, produces a secondary splitting. The 5 K AFM and FM excitations in ErCrO3 have a concerted frequency-intensity temperature dependence and a shoulder pointing to the Er3+ smaller ion size also disrupting the two magnetic sublattice approximation . Both resonances reduce to one when the temperature is lowered to 2 K in the Gamma1 representation.

cond-mat.str-el↗

arXiv2017

Analysis of Granular Packing Structure by Scattering of THz Radiation

Scattering methods are widespread used to characterize the structure and constituents of matter on small length scales. This motivates this introductory text on identifying prospective approaches to scattering-based methods for granular media. A survey to light scattering by particles and particle ensembles is given. It is elaborated why the established scattering methods using X-rays and visible light cannot in general be transferred to granular media. Spectroscopic measurements using Terahertz radiation are highlighted as they to probe the scattering properties of granular media, which are sensitive to the packing structure. Experimental details to optimize spectrometer for measurements on granular media are discussed. We perform transmission measurements on static and agitated granular media using Fourier-transform spectroscopy at the THz beamline of the BessyII storage ring. The measurements demonstrate the potential to evaluate degrees of order in the media and to track transient structural states in agitated bulk granular media.

cond-mat.soft↗

arXiv2014

General Magnetic Transition Dipole Moments for Electron Paramagnetic Resonance

We present general expressions for the magnetic transition rates in beam Electron Paramagnetic Resonance (EPR) experiments of anisotropic spin systems in the solid state. The expressions apply to general spin centers and arbitrary excitation geometry (Voigt, Faraday, and intermediate). They work for linear and circular polarized as well as unpolarized excitation, and for crystals and powders. The expressions are based on the concept of the (complex) magnetic transition dipole moment vector. Using the new theory, we determine the parities of ground and excited spin states of high-spin (S = 5/2) Fe(III) in hemin from the polarization dependence of experimental ground state EPR line intensities.

cond-mat.mes-hall↗

arXiv2014

Particle characterization using THz spectroscopy

THz extinction spectroscopy extends UV-Vis and NIR-spectroscopy to characterize particles from fine powders and dust to sand, grains and granulated materials. We extract particle sizes from the spectral position of the first peak of the interference structure and size distributions from the visibility of the fine ripple structure in the measured extinction spectra. As such, we can demonstrate a route for a quick determination of these parameters from single measurements.

cond-mat.dis-nn↗