Ultrafast demagnetization in metallic ferromagnetic/antiferromagnetic heterostructures

This thesis summarizes experimental work performed on thin-film metallic and magnetic systems by time-resolved soft X-ray reflectivity and magneto-optical Kerr effect measurements. The delay-time-dependent magnetization dynamics in Co, Mn, and Ni after laser excitation are investigated in layered systems. The work aims to study the effects of antiferromagnetic coupling on de- and remagnetization dynamics in an adjacent ferromagnetic film on the ultrafast timescale. The presented work bridges the timescale from the first femtoseconds to macroscopic dynamics after several microseconds. The thesis is divided into a literature review covering the treatment and scientific context of the investigated phenomena and experimental reports where the findings of this work are presented and discussed. The presentation of the experiments is divided into four chapters. First, differences between the ultrafast demagnetization in a Cu/Co/NiMn/Cu(001) system recorded at high and low sample base temperatures are investigated. The report shows that the antiferromagnetic alignment in the NiMn system, present at low temperature, facilitates a faster demagnetization of the adjacent Co layer compared to a magnetically disordered NiMn layer at high temperatures. The second investigation covers the timescale of laser excitation and presents experimental evidence of the optically induced intersite spin transfer in a (Co/Mn)x6/Cu(001) multilayer system. An increase in the magnetic contrast in the antiferromagnetically ordered Mn layer is observed. The magnetic contrast lives for the time of excitation and indicates a realignment of the magnetic moments of the interface layers of Mn in the direction of Co. The third chapter treats simulations of the X-ray reflectivity after laser excitation. The multilayer system investigated in the aforementioned experiment is analyzed with respect to its magneto-optical properties, and the observations recorded with the X-rays are directly correlated with the transient optical response function of the elements. Furthermore, we compare the results of the simulations of the X-ray reflectivity considering structural, electronic, and magnetic effects. The last presented study is concerned with time-dependent magnetization dynamics recorded in Pt/Co multilayers. We investigate the delay-time-dependent Kerr effect and present experimental evidence for different dynamics in a Pt/Co bilayer system compared to a Pt/MnPt/Pt/Co system. Lastly, we investigate the effects of the spatial pump-probe overlap on the magnetization dynamics and link the observation to thermal and lateral transport phenomena

Effect of ligand methylation on the spin-switching properties of surface-supported spin-crossover molecules

X-ray absorption spectroscopy investigations of the spin-state switching of spin-crossover (SCO) complexes adsorbed on a highly-oriented pyrolytic graphite (HOPG) surface have shown so far that HOPG is a promising candidate to realize applications such as spintronic devices because of the stability of SCO complexes on HOPG and the possibility of highly efficient thermal and light-induced spin-state switching. Herein, we present the spin switching of several Fe(II) SCO complexes adsorbed on an HOPG surface with particular emphasis on the thermally induced spin transition behaviour with respect to different structural modifications. The complexes of the type [Fe(bpz)2(L)] (bpz  =  dihydrobis(pyrazolyl)borate, L  =  1,10-phenanthroline, 2,2'-bipyridine) and their methylated derivatives exhibit SCO in the solid state with some differences regarding cooperative effects. However, in the vacuum-deposited thick films on quartz, complete and more gradual spin transition behavior is observable via UV/vis spectroscopy. In contrast to that, all complexes show large differences upon direct contact with HOPG. Whereas the unmodified complexes show thermal and light-induced SCO, the addition of e.g. two or four methyl groups leads to a partial or a complete loss of the SCO on the surface. The angle-dependent measurement of the N K-edge compared to calculations indicates that the complete SCO and HS-locked molecules on the surface exhibit a similar preferential orientation, whereas complexes undergoing an incomplete SCO exhibit a random orientation on the surface. These results are discussed in the light of molecule-substrate interactions

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

Influence of magnetic domain walls on all-optical magnetic toggle switching in a ferrimagnetic GdFe film

We present a microscopic magnetic domain imaging study of single-shot all-optical magnetic toggle switching of a ferrimagnetic Gd(26)Fe(74) film with out-of-plane easy axis of magnetization by X-ray magnetic circular dichroism photoelectron emission microscopy. Individual linearly polarized laser pulses of 800 nm wavelength and 100 fs duration above a certain threshold fluence reverse the sample magnetization, independent of the magnetization direction, the so-called toggle switching. Local deviations from this deterministic behavior close to magnetic domain walls are studied in detail. Reasons for nondeterministic toggle switching are related to extrinsic effects, caused by pulse-to-pulse variations of the exciting laser system, and to intrinsic effects related to the magnetic domain structure of the sample. The latter are, on the one hand, caused by magnetic domain wall elasticity, which leads to a reduction of the domain-wall length at features with sharp tips. These features appear after the optical switching at positions where the line of constant threshold fluence in the Gaussian footprint of the laser pulse comes close to an already existing domain wall. On the other hand, we identify the presence of laser-induced domain-wall motion in the toggle-switching event as a further cause for local deviations from purely deterministic toggle switching

Accelerating the laser-induced demagnetization of a ferromagnetic film by antiferromagnetic order in an adjacent layer

We study the ultrafast demagnetization of Ni/NiMn and Co/NiMn ferromagnetic/antiferromagnetic bilayer systems after excitation by a laser pulse. We probe the ferromagnetic order of Ni and Co using magnetic circular dichroism in time-resolved pump-probe resonant x-ray reflectivity. Tuning the sample temperature across the antiferromagnetic ordering temperature of the NiMn layer allows us to investigate effects induced by the magnetic order of the latter. The presence of antiferromagnetic order in NiMn speeds up the demagnetization of the ferromagnetic layer, which is attributed to bidirectional laser-induced superdiffusive spin currents between the ferromagnetic and the antiferromagnetic layer

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

Thermal- and Light-Induced Spin-Crossover Characteristics of a Functional Iron(II) Complex at Submonolayer Coverage on HOPG

Studies on the spin-state switching characteristics of surface-bound thin films of spin-crossover (SCO) complexes are of interest to harness the device utility of the SCO complexes. Molecule–substrate interactions govern the SCO of surface-bound films in direct contact with the underlying substrates. In this study, we elucidate the role of molecule–substrate interactions on the thermal- and light-induced spin-state switching characteristics of a functional SCO-complex—[Fe(H2B(pz)2)2COOC12H25-bipy] (pz = pyrazole, C12-bpy = dodecyl[2,2′-bipyridine]-5-carboxylate) deposited at a submonolayer coverage on a highly oriented pyrolytic graphite (HOPG) substrate. A spin-state coexistence of 42% low-spin (LS) and 58% high-spin (HS) is observed for the 0.4 ML deposit of the complex at 40 K, in contrast to the complete spin-state switching observed in the bulk and in SiOx-bound 10 nm thick films. Cooling the sample to 10 K results in a decrease of the LS fraction to 36%, attributed to soft-X-ray-induced excited spin-state trapping (SOXIESST). Illumination of the sample with a green light (λ = 520 nm) at 10 K caused the LS-to-HS switching of the remaining (36%) LS complexes, by a process termed light-induced excited spin-state trapping (LIESST). The mixed spin-state in the submonolayer coverage of [Fe(H2B(pz)2)2COOC12H25-bipy] highlights the role of molecule–HOPG substrate interactions in tuning the thermal SCO characteristics of the complex. The 100% HS state obtained after light irradiation indicates the occurrence of efficient on-surface light-induced spin switching, encouraging the development of light-addressable molecular devices based on SCO complexes

Epitaxial monolayers of the magnetic 2D semiconductor FeBr2 grown on Au(111)

Magnetic two-dimensional (2D) semiconductors have attracted a lot of attention because modern preparation techniques are capable of providing single-crystal films of these materials with precise control of thickness down to the single-layer limit. It opens up a way to study a rich variety of electronic and magnetic phenomena with promising routes toward potential applications. We have investigated the initial stages of epitaxial growth of the magnetic van der Waals semiconductor FeBr2 on a single-crystal Au(111) substrate by means of low-temperature scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), X-ray photoemission spectroscopy (XPS), low-energy electron emission microscopy (LEEM), and X-ray photoemission electron microscopy (XPEEM). Magnetic properties of the one- and two-layer thick films were measured via X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD). Our findings show a striking difference in the magnetic behavior of the single layer of FeBr2 and its bulk counterpart, which can be attributed to the modifications in the crystal structure due to the interaction with the substrate.C.G.-O. and M.P.-D. acknowledge funding of the Ph.D. fellowship from the MPC Foundation. S.E.H. thanks the whole AG Kuch and in particular J. Gördes for help during the BESSY measurements and also the local IT/electronics workshop/fine mechanics workshop teams for their continuous support. In particular, he is very thankful for the possibility to perform some last STM measurements at the PEARL beamline at SLS thanks Dr. Matthias Muntwiler. J.N. thanks the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) for his funding under project 277101999 - CRC 183. S.T. acknowledges financial support by BMBF through project VEKMAG (BMBF 05K19KEA). P.G. acknowledges funding from PID2020-116181RB-C32 and FlagEraSOgraphMEM PCI2019-111908-2 (AEI/FEDER). D.G.O. acknowledges funding by the Spanish MCIN/AEI/10.13039/501100011033 and by the European Union “NextGenerationEU”/PRTR (PID2019-107338RB-C63 and TED2021-132388B–C43). C.R., M.I., C.G.-O., and M.P.-D. acknowledge funding by the European Union’s Horizon 2020 research and innovation program (grant agreement No 800923), the Spanish MCIN/AEI/10.13039/501100011033 (PID2020-114252GB-I00, PID2019-107338RB-C63, TED2021-130292B–C42), the Basque Goverment IT1591-22, and by the IKUR Strategy under the collaboration agreement between Ikerbasque Foundation and MPC on behalf of the Department of Education of the Basque Government. C.R., M.I., C.G.-O., and S.E.H. are very thankful for the help during the XMCD and STM measurements of Samuel Kerschbaumer, Andrea Aguirre Baños, and Amitayush Jha Thakur.Peer reviewe

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