arXiv subjects

Katharina Ollefs

Publications and source records attributed to Katharina Ollefs.

27 records · Page 2Linked to original sources

arXiv2020

Role of intermediate 4$f$ states in tuning the band structure of high entropy oxides

High entropy oxides (HEOs) are single phase solid solutions consisting of 5 or more cations in approximately equiatomic proportions. In this study, we show reversible control of optical properties in a rare-earth (RE) based HEO-(Ce$_{0.2}$La$_{0.2}$Pr$_{0.2}$Sm$_{0.2}$Y$_{0.2}$)O$_{2-δ}$ and subsequently utilize a combination of spectroscopic techniques to derive the features of the electronic band structure underpinning the observed optical phenomena. Heat treatment of the HEO under vacuum atmosphere followed by reheat-treatment in air results in a reversible change of the band gap energy, from 1.9 eV to 2.5 eV. The finding is consistent with the reversible changes in the oxidation state and related $f$-orbital occupancy of Pr. However, no pertinent changes in the phase composition or crystal structure is observed upon the vacuum heat treatment. Further annealing of this HEO under H$_2$ atmosphere, followed by reheat-treatment in air, results in even larger but still reversible change of the band gap energy from 1.9 eV to 3.2 eV. This is accompanied by a disorder-order type crystal structure transition and changes in the O 2$p$-RE 5$d$ hybridization evidenced from X-ray absorption near edge spectra (XANES). The O $K$ and RE ${M_{4,5}}$/$L_{3}$ XANES indicate that the presence of Ce and Pr (in 3+/4+) state leads to the formation of intermediate 4$f$ energy levels between the O 2$p$ and RE 5$d$ gap in HEO. It is concluded that heat treatment under reducing/oxidizing atmospheres affects these intermediate levels, thus, offering the possibility to tune the band gap energy in HEO.

cond-mat.mtrl-sci↗

arXiv2020

Magnetic properties of rare earth and transition metal based perovskite type high entropy oxides

High entropy oxides (HEO) are a recently introduced class of oxide materials, which are characterized by a large number of elements (i.e. five or more) sharing one lattice site which crystallize in a single phase structure. One complex example of the rather young HEO family are the rare-earth transition metal perovskite high entropy oxides. In this comprehensive study, we provide an overview over the magnetic properties of three perovskite type high entropy oxides. The compounds have a rare-earth site which is occupied by five different rare-earth elements, while the transition metal site is occupied by a single transition metal. In this way a comparison to the parent binary oxides, namely the orthocobaltites, -chromites and -ferrites is possible. X-ray absorption near edge spectroscopy (XANES), magnetometry and Mössbauer spectroscopy are employed to characterize these complex materials. In general, we find surprising similarities to the magnetic properties of the binary oxides, despite the chemical disorder on the rare-earth site. However distinct differences and interesting magnetic properties are also observed such as noncollinearity, spin reorientation transitions as well as large coercive fields of up to 2\,T at ambient temperature. Both the chemical disorder on the RE A-site, and the nature of the TM on the B-site play an important role in the physical properties of these high entropy oxides.

cond-mat.mtrl-sci↗

arXiv2020

Interface-related magnetic and vibrational properties in Fe/MgO heterostructures from nuclear resonant spectroscopy and first-principles calculations

We combine $^{57}$Fe Mössbauer spectroscopy and $^{57}$Fe nuclear resonant inelastic x-ray scattering (NRIXS) in nanoscale polycrystalline [bcc-$^{57}$Fe/MgO] multilayers with various Fe layer thicknesses and layer-resolved density-functional-theory (DFT) based first-principles calculations of a (001)-oriented [Fe(8 ML)/MgO(8 ML)](001) heterostructure to unravel the interface-related atomic vibrational properties of a multilayer system. In theory and experiment, we observe consistently enhanced hyperfine magnetic fields compared to bulk which are associated with the Fe/MgO interface layers. NRIXS and DFT both reveal a strong reduction of the longitudinal acoustic (LA) phonon peak in combination with an enhancement of the low-energy vibrational density of states (VDOS) suggesting that the presence of interfaces and the associated increase in the layer-resolved magnetic moments results in drastic changes in the Fe-partial VDOS. From the experimental and calculated VDOS, vibrational thermodynamic properties have been determined as a function of Fe thickness and are found to be in excellent agreement.

cond-mat.mtrl-sci↗

arXiv2019

Magnetic response of FeRh to static and dynamic disorder

Changes of the magnetic and crystal structure on the microscopic scale in 40 nm FeRh thin films have been applied to investigate the phenomena of a disorder induced ferromagnetism at room temperature initiated through light ion-irradiation with fluences up to 0.125 Ne$^+$/nm$^{-2}$. Magnetometry shows an increase of magnetic ordering at low temperatures and a decrease of the transition temperature combined with a broadening of the hysteresis with rising ion fluence. $^{57}$Fe Mössbauer spectroscopy reveals the occurrence of an additional magnetic contributions with an hyperfine splitting of 27.2 T - identical to that of ferromagnetic B2-FeRh. The appearance of an anti-site Fe-contribution can be assumed to be lower than 0.6 Fe-at%, indicating that no change of the chemical composition is evident. The investigation of the local structure shows an increase of the static mean square relative displacement determined by X-ray absorption fine structure spectroscopy, while an increase of the defect-concentration has been determined by positron annihilation spectroscopy. From the changes of the microscopic magnetic structure a similarity between the temperature induced and the structural disorder induced ferromagnetic phase can be observed. These findings emphasize the relationship between magnetic ordering and the microscopic defect structure in FeRh.

cond-mat.mtrl-sci↗

arXiv2019

Influence of hydrogenation on the vibrational density of states of magnetocaloric $\mathrm{LaFe}_\mathrm{11.4}\mathrm{Si}_\mathrm{1.6}\mathrm{H}_{1.6}$

We report on the impact of magnetoelastic coupling on the magnetocaloric properties of LaFe$_{11.4}$Si$_{1.6}$H$_{1.6}$ in terms of the vibrational density of states, which we determined with $^{57}$Fe nuclear resonant inelastic X-ray scattering measurements and with density-functional-theory based first-principles calculations in the ferromagnetic low-temperature and paramagnetic high-temperature phase. In experiments and calculations, we observe pronounced differences in the shape of the Fe-partial VDOS between non-hydrogenated and hydrogenated samples. This shows that hydrogen does not only shift the temperature of the first-order phase transition, but also affects the elastic response of the Fe-subsystem significantly. In turn, the anomalous redshift of the Fe VDOS, observed by going to the low-volume PM phase, survives hydrogenation. As a consequence, the change in the Fe specific vibrational entropy $ΔS_\mathrm{lat}$ across the phase transition has the same sign as the magnetic and electronic contribution. DFT calculations show that the same mechanism, which is a consequence of the itinerant electron metamagnetism associated with the Fe subsystem, is effective in both the hydrogenated and he hydrogen-free compounds. Although reduced by 50 % as compared to the hydrogen-free system, the measured change $ΔS_\mathrm{lat}$ of 3.2\pm1.9 J/kgK across the FM to PM transition contributes with 35 % significantly and cooperatively to the total isothermal entropy change $ΔS_\mathrm{iso}$. Hydrogenation is observed to induce an overall blueshift of the Fe-VDOS with respect to the H-free compound; this effect, together with the enhanced Debye temperature observed, is a fingerprint of the hardening of the Fe sublattice by hydrogen incorporation. In addition, the mean Debye velocity of sound of LaFe$_{11.4}$Si$_{1.6}$H$_{1.6}$ was determined from the NRIXS and the DFT data.

cond-mat.mtrl-sci↗

arXiv2019

Imaging non-standing spin-waves in confined micron-sized ferromagnetic structures under uniform excitation

A non-standing character of directly imaged spin-waves in confined micron-sized ultrathin permalloy (Ni\textsubscript{80}Fe\textsubscript{20}) structures is reported along with evidence of the possibility to alter the observed state by modifications to the sample geometry. Using micromagnetic simulations the presence of the spin-wave modes excited in the permalloy stripes along with the quasi-uniform modes were calculated. The predicted spin-waves were imaged in direct space using time resolved scanning transmission X-ray microscopy, combined with a ferromagnetic resonance excitation scheme (STXM-FMR). STXM-FMR measurements revealed a non-standing character of the spin-waves. Also it was shown by micromagnetic simulations and confirmed with STXM-FMR results that the observed character of the spin-waves can be influenced by the local magnetic fields in different sample geometries.

cond-mat.mes-hall↗

arXiv2019

Evaluation protocol for revealing magnonic contrast in STXM-FMR measurements

We present a statistically motivated method to extract magnonic contrast from STXM-FMR measurement with microwave frequencies of the order of \unit[10]{GHz}. With this method it is possible to generate phase and amplitude profiles with a spatial resolution of about \unit[30]{nm} given by the STXM resolution, furthermore this method allows fo a rigoros transformation to reciprocal \vec{k}-space, revealing \vec{k}-dependent magnon properties.

cond-mat.str-el↗

arXiv2015

Microwave soft x-ray microscopy for nanoscale magnetization dynamics in the 5-10 GHz frequency range

We present a scanning transmission x-ray microscopy setup combined with a novel microwave synchronization scheme in order to study high frequency magnetization dynamics at synchrotron light sources. The sensitivity necessary to detect small changes of the magnetization on short time scales and nanometer spatial dimensions is achieved by combination of the developed excitation mechanism with a single photon counting electronics that is locked to the synchrotron operation frequency. The required mechanical stability is achieved by a compact design of the microscope. Our instrument is capable of creating direct images of dynamical phenomena in the 5-10 GHz range, with 35 nm resolution. When used together with circularly polarized x-rays, the above capabilities can be combined to study magnetic phenomena at microwave frequencies, such as ferromagnetic resonance (FMR) and spin waves. We demonstrate the capabilities of our technique by presenting phase resolved images of a 6 GHz nanoscale spin wave generated by a spin torque oscillator, as well as the uniform ferromagnetic precession with ~0.1 deg amplitude at 9 GHz in a micrometer-sized cobalt strip.

cond-mat.mes-hall↗

arXiv2013

Comment on "Pt magnetic polarization on Y3Fe5O12 and magnetotransport characteristics"

In a recent Letter [Y.M. Lu et al., Phys. Rev. Lett. 110, 147207 (2013)], Lu et al. reported on "ferromagneticlike transport properties" of thin films of Pt, deposited ex situ via sputtering on the ferrimagnetic insulator Y3Fe5O12. The authors found a magnetoresistance in Pt displaying a hysteresis corresponding to the coercive field of Y3Fe5O12, consistent with the findings of other groups. While the latter interpreted their data in terms of the recently proposed spin-Hall magnetoresistance, Lu et al. attributed their observation to a magnetic proximity effect. To support this interpretation, they measured the X-ray magnetic circular dichroism (XMCD) at the Pt L2,3 edges from a Pt/Y3Fe5O12 sample with a Pt thickness of 1.5 nm and derived an average induced magnetic moment of 0.054 Bohr magnetons per Pt atom. This is contradictory to the results of our previous comprehensive XMCD study of three different Pt/Y3Fe5O12 samples with Pt thicknesses of 3, 7, and 10 nm from which we identified an upper limit of (0.003 +/- 0.001) Bohr magnetons per Pt [Geprägs et al., Appl. Phys. Lett. 101, 262407 (2012), arXiv:1211.0916].

cond-mat.mtrl-sci↗