Magnetism and structure of MnxAu1-x on Cu(001) and Ag(001)

In this thesis, ultra-thin films of MnxAu1−x have been studied structurally and magnetically. We prepared the thin films with electron-beam evaporators in ultrahigh vacuum (UHV). The growth of the films was monitored by medium-energy electron diffraction (MEED). The films’ structures were investigated by means of low-energy electron diffraction (LEED) and scanning tunneling microscopy (STM), and the chemical composition was checked by Auger electron spectroscopy (AES). The magnetic property of the antiferromagnetic (AFM) films in contact with FM films was studied by longitudinal magneto-optic Kerr effect (L-MOKE), X-ray magnetic circular dichroism (XMCD), and X-ray resonant magnetic reflectivity (XRMR) spectroscopy at L2,3 absorption edge. AFM Mn2Au might be very significant for future spin-electronic applications. We studied MnxAu1−x films in detail to investigate their structure for a variety of thicknesses on Cu(001) and Ag(001) single crystals. First, we studied Au/Mn/Co on Cu(001) to define the growth rate for Au, Mn, and Co using MEED. The vertical interlayer distances for Co on Cu(001) and Mn on Co/Cu(001) and surface topography of Au on Mn/Co/Cu(001) were investigated by LEED-I(V) (00 spot intensity was recorded vs. electron beam energy) and STM, respectively. After that, we studied from sub-ML (surface coverage of less than one monolayer) to more than 1ML of MnxAu1−x on Cu(001) structurally by LEED and STM. We observed a c(2×2) superstructure for coverages between 0.5 and 1ML of MnxAu1−x on Cu(001). MnxAu1−x revealed MEED oscillations during growth on Cu(001), but no LEED patterns could be observed for thicker films (> 1 ML). For studying the magnetic properties of AFM MnxAu1−x on Cu(001), we grew Co on top, then we employed MOKE after zero-field cooling as well as after field cooling. We observed coercivity changes with temperature, however, we did not observe any exchange bias. Finally, we studied Mn and MnxAu1−x growth on Ag(001) to define the growth rate and structure of the resulting films. Fe growth on Ag(001) was also studied structurally. We deduced vertical interlayer distances for single-layer and bilayer films (Fe, MnxAu1−x) on Ag(001) from a kinematic analysis of LEED-I(V) curves because both show LEED patterns. The MnxAu1−x showed MEED oscillations during growth on Ag(001). Fe/MnxAu1−x bilayers on Ag(001) were studied magnetically by MOKE.We did not observe any significant change in coercivity to confirm the antiferromagnetism of the MnxAu1−x films

Growth, Structure, and Magnetic Properties of Artificially Layered NiMn in Contact to Ferromagnetic Co on Cu3Au(001)

Single-crystalline antiferromagnetic artificially layered [Ni/Mn] films of different thicknesses, covered by ferromagnetic Co layers, are deposited on Cu3Au(001).Their structural and magnetic properties are characterized by low-energy electron diffraction (LEED) and magneto-optical Kerr effect, respectively, and compared with disordered NixMn100-x alloy films with the same Ni/Mn ratio and the same film thickness. LEED intensity-versus-energy curves show that the perpendicular inter-atomic lattice distance is decreased in the artificially layered [Ni/Mn] samples incomparison to the disordered NixMn100-x alloy films.At the same time, the artificially layered [Ni/Mn] films exhibit higher coercivity and exchange bias of the adjacent Co layer compared to those of NixMn100-x/Co. This is discussed as a consequence of the different interatomic lattice distance, presumably caused by an ordered buckling in the artificially layered [Ni/Mn] samples, leading to a stronger interlayer exchange coupling

Growth of MnxAu1−x Films on Cu(001) and Ag(001) Single-Crystal Substrates

The growth, morphology, and structure of MnxAu1-x films on Cu(001) and Ag(001) are studied by means of low-energy electron diffraction (LEED), medium-energy electron diffraction, Auger electron spectroscopy, and scanning tunnelling microscopy. Different concentrations x from about 0.5 to 1 and thicknesses from0.2 to 12.9 ML of MnxAu1-x are examined. For several values of x, MnxAu1-x exhibits a c(2 x 2) superstructure pattern on Cu(001) when the total thickness is around or above 0.5 ML. Above 1 ML, LEED patterns of MnxAu1-x can be only observed on Ag(001), but not on Cu(001). LEED-I(V) is employed to deduce the vertical interlayer distance for as-grown and post-annealed films on Ag(001). Above 500 K, Ag from the substrate segregates into thefilms

Ultrafast laser-induced magneto-optical changes in resonant magnetic x-ray reflectivity

We investigate the magneto-optical response of Co to an ultrashort laser excitation by x-ray resonant magnetic reflectivity (XRMR) employing circular polarization. The time-resolved reflectivities detected for opposite sample magnetization are separated into magnetic and nonmagnetic contributions, which contain information about the structural, electronic, and magnetic properties of the sample. Different response times of the different contributions are observed. The experimental results are reproduced numerically by two different simulation approaches. On the one hand, we use a purely thermal model, a time-dependent heat-induced loss of macroscopic magnetization, and an inhomogeneous laser-induced strain profile. On the other hand, we employ time-dependent density-functional theory to calculate the transient optical response to the laser-induced excitation and from that the reflected intensities. While both methods are able to reproduce the time dependence of the magnetic signal, the ultrafast nonmagnetic change in reflectivity is captured satisfactorily only in simulations of the transient optical response function and has thus to be assigned to electronic effects. The energy dependence of the magnetic circular dichroism is investigated in the simulations, highlighting a dependence of the observable on the probing energy. Finally, a phenomenological explanation of the dynamics measured in dichroic x-ray reflectivity in the different channels is offered

Bulk and interfacial effects in the Co/NixMn100-x exchange-bias system due to creation of defects by Ar+ sputtering

A series of experiments is carried out to identify the contribution of interface and bulk antiferromagnetic (AFM) spins to exchange bias (EB) in ultrathin epitaxial ferromagnetic (FM)/AFM bilayer samples. These are single-crystalline AFM NiMn100− and ferromagnetic Co layers on Cu3Au(001), deposited under ultrahigh vacuum conditions, in which structural or chemical defects are deliberately introduced by controlled Ar ion sputtering at the surface of the AFM layer or at a certain depth inside the AFM layer. Comparison of the magnetic properties measured by magneto-optical Kerr effect for sputtered and nonsputtered parts of the same sample then allows a precise determination of the influence of sputtering on the AFM layer during the sample preparation, whereas all other parameters are kept identical. The results show that the creation of defects in the bulk of the AFM layer enhances the magnitude of EB and its blocking temperature, but not the creation of defects at the interface. It is also observed that the deeper the insertion of defects in the AFM layer, the higher the value of the EB field and the larger the coercivity, These findings are discussed as the effect of additional pinning centers in the bulk of the AFM layer

Ultrafast laser-induced magneto-optical changes in resonant magnetic x-ray reflectivity

<p>Datasets for the publication "Ultrafast laser-induced magneto-optical changes in resonant magnetic x-ray reflectivity", published in Physical Review B <strong>108</strong>, 054439 (2023).</p><p> </p&gt