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In a set of papers, the authors present a comprehensive investigation of the magnetic, structural, and dynamic properties of thin film Ni-Co, Ni-Fe, and Co-Fe alloys and uncover new phenomena in systems previously thought to be well understood and commonplace. High-precision measurements of saturation magnetization, effective magnetization, spectroscopic $g$ factor, inhomogeneous linewidth broadening, and damping are performed, allowing precise testing of \textit{ab initio} calculations for the orbital magnetic moment and the intrinsic magnetic damping, which is the most integral motivation of this work. Quantitative comparison between experiment and theory is shown and discussed for magnetic damping spanning a broad range of material parameters. Furthermore, a large set of data has been created that will serve as a glossary for alloy magnetic properties in thin films.

A systematic experimental study of Gilbert damping is performed via ferromagnetic resonance for the disordered crystalline binary 3d transition-metal alloys Ni-Co, Ni-Fe, and Co-Fe over the full range of alloy compositions. After accounting for inhomogeneous linewidth broadening, the damping shows clear evidence of both interfacial damping enhancement (by spin pumping) and radiative damping. We quantify these two extrinsic contributions and thereby determine the intrinsic damping. The comparison of the intrinsic damping to multiple theoretical calculations yields good qualitative and quantitative agreement in most cases. Furthermore, the values of the damping obtained in this study are in good agreement with a wide range of published experimental and theoretical values. Additionally, we find a compositional dependence of the spin mixing conductance.

We propose a scheme for an in-line spin-torque nano-oscillator (iSTNO) composed of a single nanowire. The oscillating element is an in-plane magnetized region of an otherwise out-of-plane magnetized nanowire, which supports a spin-wave mode. Analytical exploration reveals that the nonadiabatic spin-transfer torque can cancel out the damping and thus induce sustained precession in response to a direct current. Moreover, it predicts that the frequency scales linearly with current and that the wave vector depends only on geometry. Simulations with single iSTNO cells confirm that oscillations occur, and simulations with two iSTNO cells show that they frequency lock to each other roughly in antiphase, even with mismatches in geometry. iSTNO devices can be easily fabricated by irradiating regions of a perpendicular magnetic anisotropy nanowire with ions, do not require an external field, inherently support multiple iSTNOs, and thus form an attractive alternative STNO.

Fringe fields emanating from magnetic domain structures can give rise to magnetoresistance in organic semiconductors. In this article, we explain these magnetic-field effects in terms of a $\ensuremath{\Delta}B$ mechanism. This mechanism describes how variations in magnetic-field strength between two polaron hopping sites can induce a difference in precessional motion of the polaron spins, leading to mixing of their spin states. In order to experimentally explore the fringe-field effects, polymer thin-film devices on top of a rough in-plane magnetized cobalt layer are investigated. The cobalt layer can be described by a distribution of out-of-plane magnetic anisotropies, most likely induced by thickness variations in the cobalt. With a magnetic field perpendicular to the cobalt layer, fringe fields are created because some domains are magnetized out of plane whereas the magnetization of other domains remains approximately in plane. By varying the distance between the polymer layer and the cobalt layer, we find that the magnetoresistance arising from these fringe fields reduces with the gradient in the fringe fields, in agreement with the $\ensuremath{\Delta}B$ mechanism.

Polaron traps are ubiquitous in organic semiconductors and recent evidence suggests they might be crucial for the large observed magnetic field effects (MFEs) in organic semiconductors. Here we measure MFEs in polymer thin-film devices with engineered, radiative trap sites in order to spectroscopically investigate the influence of the traps. Surprisingly, the luminescence at the trap sites and the polymer backbone is found to have an opposite response to a magnetic field. All our results are compatible with a mechanism in which spin mixing at the traps can create the large MFEs observed on the backbone. This scenario is quantitatively confirmed by numerical drift-diffusion modeling with magnetic-field-dependent exciton densities at the traps. These insights solve an outstanding controversy within the research field.

Traps are localized, deep lying energetic states and are generally considered detrimental for organic semiconducting devices. In this work, we investigate their influence on the magnetic field sensitive current in organic devices, an effect often called organic magnetoresistance. Polymer thin films were doped with materials well known to influence trap characteristics in such devices. We analyzed the observed trends in the intrinsic magnetic field effects in terms of a trapped trion model, which incorporates the spin dependent formation of excitations at these trap sites. The results provide strong evidence that the intrinsic traps in the polymer material are indeed at the origin of these magnetic field effects. This shows that traps have beneficial properties and that their control may provide an appealing route for the engineering of efficient organic magnetic field sensors.

In this work, the 55 Mn spin-echo nuclear magnetic resonance technique is used to systematically analyze the complete substitutional series Co 2 Mn 1-x Fe x Si in terms of the resonance frequencies of the satellite structure and the corresponding hyperfine fields of the 55 Mn nuclei. The frequency spacing between satellite lines is constant within a compound, but decreases with increasing Fe concentration x , which originates in changes in the band structure. These changes are also responsible for the altered resonance frequencies observed for the same third shell environment but corresponding to a different overall Fe concentration x . All modifications in the hyperfine field upon substitution of Mn by Fe can be uniquely assigned to an entirely random distribution of Mn and Fe limited to only one (4b ) out of four fcc sublattices constituting the L2 1 Heusler structure, demonstrating that the complete substitutional series Co 2 Mn 1-x Fe x Si is crystallographically very well defined. This very high degree of order sets the stage for the observation of stable half-metallic ferromagnetism which is typically very sensitive to the structural order.

We investigate a recent controversy in ultrafast magnetization dynamics by comparing the demagnetization rates from two frequently used but competing descriptions for finite temperature magnetism, namely a rigid band structure Stoner-like approach and a system of localized spins. The calculations on the localized spin system show a demagnetization rate and time comparable to experimentally obtained values, whereas the rigid band approach yields negligible demagnetization, even when the microscopic spin-flip process is assumed to be instantaneous. This shows that rigid band structure calculations will never be in quantitative agreement with experiments, irrespective of the investigated microscopic scattering mechanism.

We have observed a longitudinal nonlinear (second-harmonic) Kerr rotation up to 22\ifmmode^\circ\else\textdegree\fi{} for an Fe(2 nm)/Cr(2 nm) multilayer on an Si${\mathrm{O}}_{2}$ substrate that shows a linear Kerr rotation of only a few hundreths of a degree. The latter value excludes any linearly induced mechanism and confirms the interface sensitivity of this nonlinear Kerr technique. The enormous enhancement is shown to be in good agreement with recent predictions, based on spin dependent band structure calculations. The local optical field effects are shown to play an important role for the additional enhancements for such multilayers on a substrate.

We explore the magnetoelectroluminescence (MEL) of organic light-emitting diodes by evaluating the magnetic-field dependent fraction of singlet excitons formed. We use two- and multisite polaron-hopping models with spin mixing by hyperfine fields and different singlet and triplet exciton formation rates k(S) and k(T). A huge MEL is predicted when exciton formation is in competition with spin mixing and when k(T) is significantly larger than k(S). This competition also leads to a low-field structure in the MEL that is in agreement with recent experiments.

We studied the optical-second-harmonic generation from thin C60 films, using a combination of frequency-, rotational-, angular-, and film-thickness-dependent measurements. We present a method to resolve the phase of chi(2) by exploiting the interference between the C60 overlayer contribution and the anisotropic Si(111) substrate contributions. We find a strong and sharp resonance at 2HBARomega = 3.60 eV. The linewidth is found to be surprisingly small as compared to linear ellipsometry and third-harmonic-generation experiments. The intensity as a function of film thickness agrees with a model considering interference effects between equal second-harmonic contributions- from the inner C60/Si interface and the outer C60 surface. The possibility of nonlocal bulk contributions is discussed.

We present a theory for spin diffusion in disordered organic semiconductors, based on incoherent hopping of a charge carrier and coherent precession of its spin in an effective magnetic field, composed of the random hyperfine field of hydrogen nuclei and an applied magnetic field. From Monte Carlo simulations and an analysis of the waiting-time distribution of the carrier we predict a surprisingly weak temperature dependence, but a considerable magnetic-field dependence of the spin-diffusion length. We show that both predictions are in agreement with experiments on organic spin valves.

Argon-filled nanocavities embedded in a single crystal of copper near the surface reflect electrons and induce a quantum well (QW) between the nanocavity and the atomically flat Cu(001) surface. The spatial variation of conductance at the surface above the nanocavity was studied by scanning tunnelling microscopy and/or spectroscopy. Interference features were observed over several nanometers at some locations on the surface. In the [100] and [010] directions, the interference fringes propagate over longer distances up to tens of nanometers. In addition to these spatially resolved features, the conductance reveals an oscillatory behavior as a function of energy of injected electrons. A model taking into account the specific shape of the nanocavity, as well as the band structure of copper, allows us to simulate the spatial variation of the conductance in close agreement with the experiment. The modeling demonstrates that not only the specific shape of the subsurface nanocavity reflecting electrons is crucial to explain the observed pattern, but also the anisotropy of the band structure and the phenomenon of focusing of hot electrons connected to it. Our approach opens up opportunities to examine buried nano-objects with scanning tunneling microscopy, and also to study how the anisotropy of a crystal influences the spatial variation of QW properties.

Magnetization-induced optical second harmonic generation (MSHG) from exchange-biased $\mathrm{Mn}∕\mathrm{Co}$ thin films shows monolayer period oscillations at the $\mathrm{Mn}∕\mathrm{Co}$ interface as a function of Co thickness. Similar oscillations are found in the exchange bias $({H}_{E})$ and the coercivity $({H}_{C})$ in both the interface sensitive MSHG and the bulk sensitive magneto-optical Kerr effect, indicating that magnetic reversal in the Co bulk and at the $\mathrm{Mn}∕\mathrm{Co}$ interface is collinear. Assuming a linear relationship between the MSHG asymmetry and the magnetic moment, our results suggest that there is an enhancement of the interface net magnetic moment at the full monolayer regions.

We performed pump-probe magneto-optical Kerr spectroscopy to explore the role of excitonic effects on the electron and hole spin relaxation in Cd1˛xMnxTe-based quantum well structures. Particularly, important information is obtained from detailed interpretation of the temperature and wavelength dependence of the observed transient magneto-optics in specially engineered structures. In manganese-rich wells the electron spin relaxation is found to be strongly enhanced in the exciton due to the increased mass of the bound electron-hole pair, in agreement with earlier predictions. An even stronger enhancement of the relaxation time is found for the hole spin, which is explained by the additional mixing of heavy-hole and light-hole states in the bound state. In structures where Mn is confined to the barrier region, a different behavior is observed, indicative for a competition of various contributions.

A comprehensive experimental study on the antiferromagnetic interlayer exchange coupling in high quality epitaxial all-semiconducting EuSPbSEuS trilayers is reported. The influence of substrates, the thickness of the non-magnetic PbS spacer layer, and of temperature, was investigated by means of SQUID magnetometry. In trilayers with a PbS thickness between 4 and 12 deg A the low temperature hysteresis loops showed the signature of antiferromagnetic coupling. The value of the interlayer exchange coupling energy was determined by simulating the data with a modified Stoner model, including Zeeman, anisotropy, and exchange coupling energies. An important observation was of a strong dependence of the interlayer exchange coupling energy on temperature, consistent with a power law dependence of the exchange coupling constant on the saturation magnetization of the EuS layers. While no theoretical description is readily available, we conjecture that the observed behavior is due to a dependence of the interlayer exchange coupling energy on the exchange splitting of the EuS conduction band.

To establish the role of the barrier material in spin-polarized tunneling, the authors directly measure the sign and magnitude of the tunneling spin polarization in Al/barrier/ferromagnet junctions with different barriers using the Zeeman-split superconducting d. of states of the Al electrode. It is shown that clear Zeeman splitting is difficult to obtain with heavy metal oxide barriers, such as HfOx and TaOx, due to a large spin-orbit scattering rate most likely induced by the heavy atoms at the Al/barrier interface. Junctions with MgO barriers, however, show clear Zeeman splitting and a tunneling spin polarization of +30% with both Co and Fe as the top electrode, a no. which significantly differs from +40% found for our AlOx junctions. We claim that this barrier dependence originates from the electronic structure of the barrier/ferromagnet interface. The pos. tunneling polarization is consistent with the presumption that due to the absence of d orbitals in the MgO barrier, tunneling is dominated by electrons with s character. [on SciFinder (R)]

A novel, all-optical method to excite and detect spin waves in magnetic materials is presented. By exploiting the temperature dependence of the magnetic anisotropy, an ultrashort laser pulse is efficiently converted in a picosecond "anisotropy field" pulse that triggers a coherent precession of the magnetization. Recording the temporal evolution of the precessing spins by a time-delayed probe-pulse provides a quantitative method to study locally the magnetic anisotropy, as well as switching and damping phenomena in micromagnetic structures. Applications to nickel and permalloy ( Ni80Fe20) films are discussed, particularly showing the possibility to explore standing spin waves in thin films.

The local electron transport properties of thin aluminum oxide layers used for magnetic tunnel junctions were studied in situ by scanning tunneling microscopy (STM) and spectroscopy under ultrahigh-vacuum conditions. The STM images of the oxide films reveal a granular structure, down to atomic resolution. A reversible switching of the conductive properties of grains, attributed to a charge redistribution, is observed during scanning. We demonstrate the possibility of intentionally switching a grain to the low-resistance state by exposing it to a high current density. We conjecture that the observed switching behavior may be considered as the precursor of an electric breakdown in tunnel junctions.