of doctoral thesis Theory of spin-dependent transport in magnetic solids David Wagenknecht Department of Condensed Matter Physics, Faculty of Mathematics and Physics, Charles University 2019 Theoretical and ab initio description of realistic material behavior is complicated and combinations of various scattering mechanisms or temperature effects are often neglected, although experimental samples contain impurities and modern electronics work at finite temperatures. In order to remove these knowledge gaps, the alloy analogy model is worked out in this thesis and implemented within the fully relativistic tight- binding linear-muffin-tin orbital method with the coherent potential approximation. This first-principles framework is shown to be robust and computationally efficient and, consequently, employed to investigate bulk solids and their spintronic applications. Unified effect of phonons, magnons, and alloying gives agreement with literature for temperature-dependent electrical transport (longitudinal and anomalous Hall resistivities) and scattering mechanisms are explained from electronic structures. Moreover, novel data help to identify defects in real samples and experimentally hardly accessible quantities are presented, such as spin polarization of electrical current. Calculated results for both zero...

Thermal constraints and the quantum limit will soon put a boundary on the scale of new micro and nano magnetoelectronic devices. This necessitates a push into the limits of harnessable natural phenomena to facilitate a post-Moore’s era of design. Requirements for thermodynamic stability at room temperature, fast (Ghz) switching, and low energy cost narrow the list of candidates. Molecular electronic frontier orbital structure of some d-block transition metal ions in crystal fields will deform in response to their local energetic environment, giving rise to the eg and t2g suborbitals. More specifically, in an mononuclear Fe(II) complex, the energetic scale between these two orbitals yields an S=0 low spin diamagnetic state and an S=2 high spin paramagnetic state. Spin crossover complex [Fe{H2B (pz) 2 }2 (bipy)] will show locking of its spin state well above the transition temperature, with an accompanied change of conductivity, when placed in a polar environment. Here we show voltage controllable, room temperature, stable locking of the spin state, and the corresponding conductivity change, when molecular thin films of [Fe{H2B (pz) 2 }2 (bipy)] are deposited on a ferroelectric polyvinylidene fluoride hexafluropropylene substrate. This opens the door to the creation of a thermodynamically stable, room temperature, molecular multiferroic gated voltage device.

<p>Probabilistic computing has been proposed as an attractive alternative for bridging the computational gap between the classical computers of today and the quantum computers of tomorrow. It offers to accelerate the solution to many combinatorial optimization and machine learning problems of interest today, motivating the development of dedicated hardware. Similar to the ‘bit’ of classical computing or ‘q-bit’ of quantum computing, probabilistic bit or ‘p-bit’ serve as a fundamental building-block for probabilistic hardware. p-bits are robust classical quantities, fluctuating rapidly between its two states, envisioned as three-terminal devices with a stochastic output controlled by its input. It is possible to implement fast and efficient hardware p-bits by modifying the present day magnetic random access memory (MRAM) technology. In this dissertation, we evaluate the design and performance of low-barrier magnet (LBM) based p-bit realizations.<br> </p> <p>LBMs can be realized from perpendicular magnets designed to be close to the in-plane transition or from circular in-plane magnets. Magnetic tunnel junctions (MTJs) built using these LBMs as free layers can be integrated with standard transistors to implement the three-terminal p-bit units. A crucial parameter that determines the response of these devices is the correlation-time of magnetization. We show that for magnets with low energy barriers (Δ ≤ k_BT) the circular disk magnets with in-plane magnetic anisotropy (IMA) can lead to correlation-times in <i>sub-ns</i> timescales; two orders of magnitude smaller compared to magnets having perpendicular magnetic anisotropy (PMA). We show that this striking difference is due to a novel precession-like fluctuation mechanism that is enabled by the large demagnetization field in mono-domain circular disk magnets. Our predictions on fast fluctuations in LBM magnets have recently received experimental confirmation as well.<br></p> <p>We provide a detailed energy-delay performance evaluation of the stochastic MTJ (s-MTJ) based p-bit hardware. We analyze the hardware using benchmarked SPICE multi-physics modules and classify the necessary and sufficient conditions for designing them. We connect our device performance analysis to systems-level metrics by emphasizing problem and substrate independent figures-of-merit such as flips per second and dissipated energy per flip that can be used to classify probabilistic hardware. </p>

The main objective of this thesis was to bring the first experimental evidence about the spin-Hall magnetoresistance (SMR) in the Terahertz (THz) spectral range. The time-domain THz spectroscopy (TDTS) was chosen as the main method, and we presented a new experimental scheme, which allows us to observe SMR or magnetoresistive effects with similar symmetry at a wide range of THz frequencies very efficiently. We focused on the study of SMR in the bilayers consisting of either a prototypical ferrimagnetic isolator or of heavy metal (FI/HM) and metallic stacks of ferromagnetic CoFeB and heavy metal Pt layer (FM/HM). While SMR shows a rapid decrease already at the lowest THz frequencies (< 1 THz) in the FI/HM structures, the SMR in FM/HM bilayers persists above 30 THz. These observations are then explained by a different mechanism of SMR. The second part of the thesis is devoted to the construction of the new TDTS setup and an easy-to-use model of the THz setup using the Gaussian description of THz radiation. The simulated results have been compared to corresponding experimental measurements using a spintronic THz emitter (STE). The last part of the thesis describes the emission of intensive THz pulses from large-area STE.

This thesis contains detailed study of a newly discovered effect of quench switch- ing in thin films of antiferromagnetic CuMnAs. This effect can be used to induce highly reproducible resistance switching behaviour in response to electrical or optical laser pulsing. The resistance changes reach up to GMR-like values of 20 % at room temperature and 100 % at low temperatures. We attribute these changes to the nano-fragmentation of magnetic domain structure. After CuMnAs is pulsed into a high resistance state, a characteristic period of time follows, during which the resistance relaxes back to the original value. This relaxation can be described by Kohlrausch stretched exponential function. This type of relaxation is characteristic for behaviour of correlated complex systems, which goes well with the idea of highly fragmented and correlated magnetic states of quenched CuMnAs. The quench switching effect is studied in detail on devices with different geometries, for various parameters of the writing pulse, as well as growth pa- rameters of the CuMnAs films. The switching is demonstrated in CuMnAs films prepared on GaP, GaAs and Si substrates, where the quality of the film differs. This illustrates robustness and application potential of the effect. 1

The transport properties of zigzag graphene nanoribbons (zGNRs) were studied by density functional theory (DFT) in conjunction with Green’s function analysis. In particular, spin transport through a zGNR (12,0) device was investigated under the constraint of ferromagnetic coordination of the ribbon edges. Several configurations with two vacant sites in the edge and the bulk region of the zGNR device were derived from this system. For all structures, magnetocurrent ratios (MCRs) were recorded as a function of the bias as well as the amount of strain applied longitudinally to the devices. ZGNR devices with vacancies in the edge regime turn out to exhibit perfect spin-filter activity for well-defined choices of the strain and the bias, carrying completely polarized minority spin currents. In the alternative structure, characterized by vacancies in the bulk regime, spin currents with majority orientation prevail. With respect to both the sign and the size, the MCR is seen to depend sensitively on the device parameters, i.e., the vacancy locations, the bias, and the amount of strain. These results are interpreted in terms of density-of-states distributions, transmission spectra, and transmission operator eigenstates.

The spin filter capability of a (0,8) armchair graphene nanoribbon with Fe atoms at substitutional sites is investigated by density functional theory in combination with the non-equilibrium Green's function technique. For specific arrangements, a high degree of spin polarisation is achieved. These include a single substitution at an edge position or double substitution in the central sector of the transmission element. The possibility of switching between majority and minority spin polarisation by changing the double substitution geometry is predicted. Including the bias dependence of the transmission function proves to be essential for correct representation of the spin-resolved current-voltage profiles.

abstract: Graphene has been extensively researched for both scientific and technological interests since its first isolation from graphite. The excellent transport properties and long spin diffusion length of graphene make it a promising material for electronic and spintronic device applications. This dissertation deals with the optimization of magnetic field sensing in graphene and the realization of nanoparticle induced ferromagnetism in graphene towards spintronic device applications. Graphene has been used as a channel material for magnetic sensors demonstrating the potential for very high sensitivities, especially for Hall sensors, due to its extremely high mobility and low carrier concentration. However, the two-carrier nature of graphene near the charge neutrality point (CNP) causes a nonlinearity issue for graphene Hall sensors, which limits useful operating ranges and has not been fully studied. In this dissertation, a two-channel model was used to describe the transport of graphene near the CNP. The model was carefully validated by experiments and then was used to explore the optimization of graphene sensor performance by tuning the gate operating bias under realistic constraints on linearity and power dissipation. The manipulation of spin in graphene that is desired for spintronic applications is limited by its weak spin-orbit coupling (SOC). Proximity induced ferromagnetism (PIFM) from an adjacent ferromagnetic insulator (FMI) provides a method for enhancing SOC in graphene without degrading its transport properties. However, suitable FMIs are uncommon and difficult to integrate with graphene. In this dissertation, PIFM in graphene from an adjacent Fe3O4 magnetic nanoparticle (MNP) array was demonstrated for the first time. Observation of the anomalous Hall effect (AHE) in the device structures provided the signature of PIFM. Comparison of the test samples with different control samples conclusively proved that exchange interaction at the MNP/graphene interface was responsible for the observed characteristics. The PIFM in graphene was shown to persist at room temperature and to be gate-tunable, which are desirable features for electrically controlled spintronic device applications. The observation of PIFM in the MNP/graphene devices indicates that the spin transfer torque (STT) from spin-polarized current in the graphene can interact with the magnetization of the MNPs. If there is sufficient STT, spin torque oscillation (STO) could be realized in this structure. In this dissertation, three methods were employed to search for signatures of STO in the devices. STO was not observed in our devices, most likely due to the weak spin-polarization for current injected from conventional ferromagnetic contacts to graphene. Calculation indicates that graphene should provide sufficient spin-polarized current for exciting STO in optimized structures that miniaturize the device area and utilize optimized tunnel-barrier contacts for improved spin injection. / Dissertation/Thesis / Doctoral Dissertation Electrical Engineering 2019

Investigation of spin dynamics in hybrid structures based on ferromagnetic semi- conductor (Ga,Mn)As Abstract: This dissertation deals with the study of hybrid ferromagnet/semiconductor structures, which are of particular for spintronics. We focused on heterostructures that contain ferromagnetic semiconductor (Ga,Mn)As, which is the most studied model ma- terial from the group of diluted magnetic semiconductors. The main goal of this work was a detailed study of the Optical Spin Transfer Torque (OSTT) phenomenon, which is an optical equivalent of the STT effect, which is used in ferromagnetic metal layers for non-thermal switching of the direction of magnetization. In the first part of the work, we describe experiments aimed at achieving non-thermal control of the direction of mag- netization in (Ga,Mn)As with the contribution of control of magnetic anisotropy using mechanical strain induced by a piezo-transducer (PZT) in hybrid structure (Ga,Mn)As/GaAs/PZT. For this purpose, the preparation of the structure was first opti- mized, which was tested in detail by means of X-ray diffraction and magneto-optical methods. However, we were unable to achieve magnetization switching due to the OSTT phenomenon. In addition, we found that the results measured at low temperature are very poorly reproducible, despite...

This thesis is focused on a detailed investigation of the optically induced quench switching effect in different films of antiferromagnetic CuMnAs. The quench switching effect was recently discovered to be highly reproducible resistance switch- ing, which can be excited by electrical and optical laser pulses. This thesis com- pares the amplitude response to laser-induced quench switching for samples on the different substrate material, samples with different stoichiometries, and sam- ples with different thicknesses of CuMnAs film. The effects of different ratios between the laser spot and the size of the measured device are investigated, and position-dependent measurements are also presented. It is shown that resistivity change with optical excitation using a single 120 femtosecond laser pulse can, in ideal conditions, reach up to 15% at room temperature, which is comparable with the maximum signal obtained with electrical pulses. All of the measure- ments combined with current knowledge of quench switching illustrate the robust behavior of this mechanism across a wide range of conditions. 1