We discuss fundamental aspects of laser-induced ultrafast demagnetization probed by the time-resolved magneto-optical Kerr effect (MOKE). Studying thin Fe films on MgO substrate in the absence of electronic transport, we demonstrate how to disentangle pump-induced variations of magnetization and magneto-optical coefficients. We provide a mathematical formalism for retrieving genuine laser-induced magnetization dynamics and discuss its applicability in real experimental situations. We further stress the importance of temporal resolution achieved in the experiments and argue that measurements of both time-resolved MOKE rotation and ellipticity are needed for the correct assessment of magnetization dynamics on sub-picosecond timescales. The framework developed here sheds light onto the details of the timeresolved MOKE technique and contributes to the understanding of the interplay between ultrafast laser-induced optical and magnetic effects.

Large area (A = 6 cm 2), thin tantalum films (5 nm < d < 100 nm) are accomplished by evaporation from tantalum rods using small pocket e-beam evaporators. Using a sample to source distance of ≈20 cm, homogeneous amorphous films with a small surface roughness (<1 nm) can be prepared on glass. Films are characterized by scanning electron microscope images, atomic force microscopy, electrochemical oxidation and resistivity measurements as a function of film thickness. The samples show high resistivities of 200-2000 µ cm. The temperature coefficient of the resistivity (TCR) is negative, as characteristic for highly disordered metals. A theoretical description of the thickness distribution (evaporation from plane and hemispherical sources on plane targets) is given in the appendix.

We demonstrate a novel method for the excitation of sizable magneto-optical effects in Au by means of the laser-induced injection of hot spin-polarized electrons in Au/Fe/MgO(001) heterostructures. It is based on the energy-and spin-dependent electron transmittance of Fe/Au interface which acts as a spin filter for non-thermalized electrons optically excited in Fe. We show that after crossing the interface, majority electrons propagate through the Au layer with the velocity on the order of 1 nm/fs (close to the Fermi velocity) and the decay length on the order of 100 nm. Featuring ultrafast functionality and requiring no strong external magnetic fields, spin injection results in a distinct magneto-optical response of Au. We develop a formalism based on the phase of the transient complex MOKE response and demonstrate its robustness in a plethora of experimental and theoretical MOKE studies on Au, including our ab initio calculations. Our work introduces a flexible tool to manipulate magneto-optical properties of metals on the femtosecond timescale that holds high potential for active magneto-photonics, plasmonics, and spintronics. arXiv:1810.12237v1 [cond-mat.mes-hall]

Electrochemical cyclovoltammetry experiments were carried out on the platinum surface of a platinum–silicon thin‐film device in sulfuric acid. The internal electronic barrier of the device allows the separation of ground state and excited state charge transfer events released by the electrochemical surface reaction. The traces of the device current show similar features as the cyclovoltammogram and can be discussed in terms of small deviations from the equilibrium state of the thin platinum film. The device current plotted as a function of the electrode potential shows a well‐defined shift of the baseline when hydrogen is adsorbed. This baseline shift may be originated by an adsorbate‐induced change of the chemical potential of the platinum film, thus influencing the internal field in the device (up to values of 23 mV). Plot of the electrochemical current and the device current as functions of the electrode potential.

Thin amorphous tantalum films are prepared on Si(111) substrates in a metallic glassy state. The amorphous monoatomic state of the film is characterized by X‐ray diffraction studies. The glassy state leads to a negative temperature coefficient of the resistivity (TCR) for low sample temperatures <200 K which is attributed to incipient localization. Above 200 K a positive TCR is observed as expected for a normal Boltzmann transport regime. Upon heating the Si substrate to 1200 K TaSi2 is formed out of the amorphous tantalum film and the silicon substrate. The TaSi2 layer is crystalline as evident from X‐ray diffraction data.magnified imageSchematic drawing of the evaporation setup on either glass or silicon samples. Scheme of annealing effects.(© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Propagation dynamics of spin-dependent optical excitations is investigated by back-pump front-probe experiments in Au/Fe/MgO(001). We observe a decrease for all pump-probe signals detected at the Au surface, if the Fe thickness in increased. Relaxation processes within Fe limit the emission region of ballistic spins at the Fe/Au interface to ~1 nm.Recently, we have established magneto-optical femtosecond back-pump frontprobe experiments [1 ] in order to provide insight into spin-dependent (i) transport contributions in ultrafast magnetization dynamics and (ii) non-equilibrium transport in metallic films in general. Fig. 1. Scheme of back-pump front-probe experiment. The fs pump pulse at 800 nm is absorbed in the metallic bilayer after transmission through MgO(001) depending on the investigated Fe layer thickness dFe, here 2, 11, and 17 nm. The dynamics of excitations propagating through the bilayer stack to the Au surface is probed by magneto-induced second harmonic generation (mSHG) as a function of time delay between pump and probe pulses.

We make a step towards the understanding of spin dynamics induced by spin-polarized hot carriers in metals. Exciting the Fe layer of Au/Fe/MgO(001) structures with femtosecond laser pulses, we demonstrate the ultrafast spin transport from Fe into Au using time-resolved MOKE and mSHG for depthsensitive detection of the transient magnetization.Ultrafast spin dynamics is the key for development of data storage and spintronics devices. Modern all-optical techniques provide ultimate time resolution of sub-10 fs for the related studies. However, recent findings on laser-induced magnetization dynamics [1] still challenge the understanding of ultrafast magnetism. With this contribution, we make a step towards the understanding of ultrafast spin dynamics in metallic multi-layers, which is (i) spatially non-uniform due to interfaces and strong absorption of the pump pulse and (ii) non-local due to highly mobile hot carriers (HC) that are spin-polarized if excited in a ferromagnet.The transient magnetization M was monitored by the magneto-optical Kerr effect (MOKE) and magneto-induced second harmonic (SH) generation (mSHG) using the 800 nm, 14 fs output of a cavity-dumped Ti:sapphire oscillator. The SH intensity where t is the pump-probe delay, Eeven(t) is independent of M and Eodd(t) ∝ M(t).The interface spin dynamics is analyzed by ∆odd(t) ≈ Eodd(t)/Eodd(t0) −1≈M(t)/M0 −1 defined from I 2ω (±M,t) and I 2ω (±M0,t0); t0 represents the absence of excitation [2]. The bulk M is probed by ∆MR, ML ≡ θ(t)/θ0 −1 ≈ M(t)/M0 −1, where θ is the MOKE rotation (MR) or ellipticity (ME). The results obtained in epitaxial Au/Fe/MgO(001) [3] serving as a model system, show pronounced dependence of ∆MR and ∆odd on the Fe thickness dFe (Fig.1 a, b). This is explained by an increase of the Fe/Au interface contribution with reducing dFe due to absorption (at 800 nm the light penetration depth in Fe ≈20 nm), which is corroborated by the transient reflectivity and Eeven(t) (not shown).

Stepwise potentiostatic oxidation is used to reduce the thickness of thin aluminum and tantalum films from an initial thickness of 10 nm down to 2 nm. The thicknesses of the oxide and the residual metal are adjusted by the finite potential of an electrochemical oxidation procedure which consumes the initially 10 nm thick metal films. The metal-metal oxide interfaces are smooth and sharply defined. The metal consumption and oxide formation are proportional to each other by the ratio of their specific densities. This enables the derivation of a metal consumption factor for the residual metal film. Residual aluminum films show a significant increase of the specific resistivity with decreasing film thicknesses. This can be explained by modified electronic transport in the residual aluminum for example by changed electronic scattering processes at the metal-metal oxide interface or in the metal. Residual tantalum films show a weaker dependence of the specific resistivity down to 3 nm pointing to only slightly changed transport properties for electrons in the thin tantalum layers.