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- ItemStrong out-of-plane magnetic anisotropy in ion irradiated anatase TiO2 thin films(New York, NY : American Inst. of Physics, 2016) Stiller, M.; Barzola-Quiquia, J.; Esquinazi, P.; Spemann, D.; Meijer, J.; Lorenz, M.; Grundmann, M.The temperature and field dependence of the magnetization of epitaxial, undoped anatase TiO2 thin films on SrTiO3 substrates was investigated. Low-energy ion irradiation was used to modify the surface of the films within a few nanometers, yet with high enough energy to produce oxygen and titanium vacancies. The as-prepared thin film shows ferromagnetism which increases after irradiation with low-energy ions. An optimal and clear magnetic anisotropy was observed after the first irradiation, opposite to the expected form anisotropy. Taking into account the experimental parameters, titanium vacancies as di-Frenkel pairs appear to be responsible for the enhanced ferromagnetism and the strong anisotropy observed in our films. The magnetic impurities concentrations was measured by particle-induced X-ray emission with ppm resolution. They are ruled out as a source of the observed ferromagnetism before and after irradiation.
- ItemPhase transition and anomalous low temperature ferromagnetic phase in Pr 0.6Sr 0.4MnO 3 single crystals(New York, NY : Springer Science + Business Media B.V., 2009) Rößler, S.; Harikrishnan, S.; Naveen Kumar, C.M.; Bhat, H.L.; Elizabeth, S.; Rößler, U.K.; Steglich, F.; Wirth, S.We report on the magnetic and electrical properties of Pr 0.6Sr 0.4MnO 3 single crystals. This compound undergoes a continuous paramagnetic-ferromagnetic transition with a Curie temperature T C301 K and a first-order structural transition at T S64 K. At T S, the magnetic susceptibility exhibits an abrupt jump, and a corresponding small hump is seen in the resistivity. The critical behavior of the static magnetization and the temperature dependence of the resistivity are consistent with the behavior expected for a nearly isotropic ferromagnet with short-range exchange belonging to the Heisenberg universality class. The magnetization (M-H) curves below T S are anomalous in that the virgin curve lies outside the subsequent M-H loops. The hysteretic structural transition at T S as well as the irreversible magnetization processes below T S can be explained by phase separation between a high-temperature orthorhombic and a low-temperature monoclinic ferromagnetic phase.
- ItemSub-15-fs X-ray pump and X-ray probe experiment for the study of ultrafast magnetization dynamics in ferromagnetic alloys(Washington, DC : Soc., 2021) Liu, Xuan; Merhe, Alaaeldine; Jal, Emmanuelle; Delaunay, Renaud; Jarrier, Romain; Chardonnet, Valentin; Hennes, Marcel; Chiuzbaian, Sorin G.; Légaré, Katherine; Hennecke, Martin; Radu, Ilie; Von Korff Schmising, Clemens; Grunewald, Særen; Kuhlmann, Marion; Lüning, Jan; Vodungbo, BorisIn this paper, we present a new setup for the measurement of element-specific ultrafast magnetization dynamics in ferromagnetic thin films with a sub-15-fs time resolution. Our experiment relies on a split and delay approach which allows us to fully exploit the shortest X-rays pulses delivered by X-ray Free Electrons Lasers (close to the attosecond range), in an X-ray pump – X-ray probe geometry. The setup performance is demonstrated by measuring the ultrafast elemental response of Ni and Fe during demagnetization of ferromagnetic Ni and Ni80Fe20 (Permalloy) samples upon resonant excitation at the corresponding absorption edges. The transient demagnetization process is measured in both reflection and transmission geometry using, respectively, the transverse magneto-optical Kerr effect (T-MOKE) and the Faraday effect as probing mechanisms.
- ItemStrong effects of uniaxial pressure and short-range correlations in Cr2Ge2Te6(College Park, MD : APS, 2022) Spachmann, S.; Elghandour, A.; Selter, S.; Büchner, B.; Aswartham, S.; Klingeler, R.Cr2Ge2Te6 is a quasi-two-dimensional semiconducting van der Waals ferromagnet down to the bilayer with great potential for technological applications. Engineering the critical temperature to achieve room-temperature applications is one of the critical next steps on this path. Here, we report high-resolution capacitance dilatometry studies on Cr2Ge2Te6 single crystals which directly prove significant magnetoelastic coupling and provide quantitative values of the large uniaxial pressure effects on long-range magnetic order (∂TC/∂pc=24.7 K/GPa and ∂TC/∂pab=−15.6 K/GPa) derived from thermodynamic relations. Moderate in-plane strain is thus sufficient to strongly enhance ferromagnetism in Cr2Ge2Te6 up to room temperature. Moreover, unambiguous signs of short-range magnetic order up to 200 K are found.
- ItemVisualization of localized perturbations on a (001) surface of the ferromagnetic semimetal EuB6(College Park, MD : American Physical Society, 2020) Rößler, S.; Jiao, L.; Seiro, S.; Rosa, P.F.S.; Fisk, Z.; Rößler, U.K.; Wirth, S.We performed scanning tunneling microscopy (STM) and spectroscopy on a (001) surface of the ferromagnetic semimetal EuB6. Large-amplitude oscillations emanating from the elastic scattering of electrons by the surface impurities are observed in topography and in differential conductance maps. Fourier transform of the conductance maps embracing these regions indicate a holelike dispersion centered around the Γ point of the two-dimensional Brillouin zone. Using density functional theory slab calculations, we identify a spin-split surface state, which stems from the dangling pz orbitals of the apical boron atom. Hybridization with bulk electronic states leads to a resonance enhancement in certain regions around the Γ point, contributing to the remarkably strong real-space response around static point defects, which are observed in STM measurements.
- ItemSpin pumping at interfaces with ferro- and paramagnetic Fe60Al40films acting as spin source and spin sink(Melville, NY : American Inst. of Physics, 2022) Strusch, T.; Lenz, K.; Meckenstock, R.; Bali, R.; Ehrler, J.; Lindner, J.; Fassbender, J.; Farle, M.; Potzger, K.; Semisalova, A.We present a study of spin pumping efficiency and determine the spin mixing conductance and spin diffusion length in thin bilayer films based on 3d transition metal alloy Fe60Al40. Due to its magnetostructural phase transition, Fe60Al40 can be utilized as a ferromagnetic (FM) or paramagnetic (PM) material at the same temperature depending on its structural order; thus a thin Fe60Al40 film can act as a spin source or a spin sink when interfaced with a paramagnet or a ferromagnet, respectively. Ferromagnetic resonance measurements were performed in a frequency range of 5-35 GHz on bilayer films composed of FM-Fe60Al40/Pd and PM-Fe60Al40/Ni80Fe20 (permalloy). The increase in damping with the thickness of the paramagnetic layer was interpreted as a result of spin pumping into the paramagnet. We determine the spin mixing conductance g P d ↑↓ = (3.8 ± 0.5) × 10 18 m - 2 at the FM-Fe60Al40/Pd interface and the spin diffusion length λ P d = 9.1 ± 2.0 nm in Pd. For the PM-Fe60Al40/permalloy interface, we find a spin mixing conductance g F e A l ↑↓ = (2.1 ± 0.2) × 10 18 m - 2 and a spin diffusion length λ F e A l = 11.9 ± 0.2 nm for PM-Fe60Al40. The demonstrated bi-functionality of the Fe60Al40 alloy in spin pumping structures may be promising for spintronic applications.
- ItemMechanical properties and twin boundary drag in Fe-Pd ferromagnetic shape memory foils-experiments and ab initio modeling(Bristol : IOP, 2011) Claussen, I.; Mayr, S.G.We report on vibrating reed measurements combined with density functional theory-based calculations to assess the elastic and damping properties of Fe-Pd ferromagnetic shape memory alloy splats. While the austenite-martensite phase transformation is generally accompanied by lattice softening, a severe modulus defect and elevated damping behavior are characteristic of the martensitic state. We interpret the latter in terms of twin boundary motion between pinning defects via partial 'twinning' dislocations. Energy dissipation is governed by twin boundary drag, primarily due to lattice imperfections, as concluded from the temperature dependence of damping and related activation enthalpies.
- ItemMagnetic hysteresis and strong ferromagnetic coupling of sulfur-bridged Dy ions in clusterfullerene Dy2S@C82(Cambridge : RSC, 2020) Krylov, Denis; Velkos, Georgios; Chen, Chia-Hsiang; Büchner, Bernd; Kostanyan, Aram; Greber, Thomas; Avdoshenko, Stanislav M.; Popov, Alexey A.Two isomers of metallofullerene Dy2S@C82 with sulfur-bridged Dy ions exhibit broad magnetic hysteresis with sharp steps at sub-Kelvin temperature. Analysis of the level crossing events for different orientations of a magnetic field showed that even in powder samples, the hysteresis steps caused by quantum tunneling of magnetization can provide precise information on the strength of intramolecular Dy⋯Dy interactions. A comparison of different methods to determine the energy difference between ferromagnetic and antiferromagnetic states showed that sub-Kelvin hysteresis gives the most robust and reliable values. The ground state in Dy2S@C82 has ferromagnetic coupling of Dy magnetic moments, whereas the state with antiferromagnetic coupling in Cs and C3v cage isomers is 10.7 and 5.1 cm-1 higher, respectively. The value for the Cs isomer is among the highest found in metallofullerenes and is considerably larger than that reported in non-fullerene dinuclear molecular magnets. Magnetization relaxation times measured in zero magnetic field at sub-Kelvin temperatures tend to level off near 900 and 3200 s in Cs and C3v isomers. These times correspond to the quantum tunneling relaxation mechanism, in which the whole magnetic moment of the Dy2S@C82 molecule flips at once as a single entity. © the Partner Organisations.
- ItemThe impact of surface morphology on the magnetovolume transition in magnetocaloric LaFe11.8Si1.2(New York : American Institute of Physics, 2016) Waske, A.; Lovell, E.; Funk, A.; Sellschopp, K.; Rack, A.; Giebeler, L.; Gostin, P.F.; Fähler, S.; Cohen, L.F.First order magnetocaloric materials reach high entropy changes but at the same time exhibit hysteresis losses which depend on the sample’s microstructure. We use non-destructive 3D X-ray microtomography to understand the role of surface morphology for the magnetovolume transition of LaFe11.8Si1.2. The technique provides unique information on the spatial distribution of the volume change at the transition and its relationship with the surface morphology. Complementary Hall probe imaging confirms that on a morphologically complex surface minimization of strain energy dominates. Our findings sketch the way for a tailored surface morphology with low hysteresis without changing the underlying phase transition.
- ItemElectrodeposition of Fe70Pd30 nanowires from a complexed ammonium-sulfosalicylic electrolyte with high stability(Amsterdam : Elsevier, 2010) Haehnel, V.; Fähler, S.; Schultz, L.; Schlörb, H.A highly stable plating bath for the electrodeposition of Fe-Pd nanowires into nanoporous alumina templates has been developed. Complexing of both metal ions and exchanging Fe2+ by Fe3+ avoid chemical reduction of Pd ions and, therefore, undesirable deposition. By using a pulse potential mode and appropriate adjustment of deposition potentials homogeneously filled templates without surface deposits and nanowires close to the desired composition of Fe70Pd30 have been achieved. These alloy nanowires represent a key step towards nanoactuators based on magnetic shape memory alloys. © 2010 Elsevier B.V. All rights reserved.
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