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Antiferromagnetic spintronics is a new promising field in applied magnetism. Antiferromagnetic materials display a staggered arrangement of magnetic moments so that they exhibit no overall magnetization while possessing a local magnetic order. Unlike ferromagnets that possess a homogeneous magnetic order, the spin-dependent phenomena occur locally upon the interaction between the itinerant electron and the localized magnetic moments. In fact, unique spin transport properties such as anisotropic magnetoresistance, anomalous Hall effect, magnetooptical Kerr effect, spin transfer torque and spin pumping have been predicted and observed, proving that antiferromagnetic materials stand out as promising candidates for spin information control and manipulation, and could potentially replace ferromagnets as the active part of spintronic devices. As a matter of fact, owing to their vanishing net magnetization, they produce no parasite stray fields, hence, they are mostly insensitive to external magnetic fields perturbations and displaying ultrafast magnetic dynamics. When a spin current is sent into an antiferromagnet, it experiences spin-dependent scattering, a mechanism that controls the spin transfer torque as well as the spin transmission across the antiferromagnet. The fully compensated antiferromagnetic interfaces are full of intriguing properties. For example, itinerant electron impinging on such an interface experiences a spin-flip associated with the sub-lattices interchange. This process, associated with Umklapp scattering, gives rise to a non-vanishing spin mixing conductance that governs spin transfer torque, spin pumping, and spin transmission. The thesis explores the mechanism of Umklapp scattering at a staggered antiferromagnetic interface and its associated spin mixing conductance. In this project we consider two systems of bilayer and trilayer antiferromagnetic (L-type, G-type) heterostructures. We first study the scattering coeffcients at the interface implemented by adopting the tight-binding model and proper boundary conditions. Then, in the trilayer case, we study the spin mixing conductance and the dephasing length associated with the transition from ferromagnetic order to antiferromagnetic order.

This work is dedicated to the study of two antiferromagnetic materials that are suitable for use in spintronic devices. In series of FeRh samples we studied the transition temperature between the antiferromagnetic and ferromagnetic phases. We developed a method based on material optical response for a quick determination of this temperature, which enabled us to study with a spatial resolution of 1 μm a magnetic inhomogeneity of prepared samples.We also developed a method for a determination of the Néel temperature and the magnetization easy axis position in thin films prepared from compensated antiferromagnetic metal. We successfully applied this method on an uniaxial sample of CuMnAs and we discussed its applicability for a research of samples with a biaxial magnetic anisotropy.

In the past two decades, the interest in materials with strong spin-orbit coupling has attracted substantial attention because of the novel physical mechanisms they display and their potential for applications. The interface displaying large spin-orbit coupling has been recognized as a powerful platform to investigate the spin transport in ferromagnetic, antiferromagnetic, and non-magnetic materials, as well as their interfaces. Besides its rich physics, the related applications are also worth studying. The current-induced spin-orbit-torque arising from angular momentum transfer from the lattice to the spin system has substantial potential in recent state-of-art spin-orbit torque magnetic random access memory. In this dissertation, we have been interested in better understanding and characterizing the spin-orbit torque and spin Hall transport in various heterostructures of interest. We used the second harmonic method to determine the magnitude of the spin currents generation and transmission in Cu-Au alloy and Ir-Mn compound, respectively. We also characterized the device performance in selected heterostructures displaying either perpendicular MgO-based tunnel magnetoresistance or unusual surface states. Finally, we used these properties to approach spin-orbit torque magnetic random access memory through designing, fabricating, and characterizing the devices that focused on current-induced spin-orbit-torque magnetization switching.

Thesis advisor: Stephen D. Wilson / Spin-orbit intertwined order gives rise to many novel phenomena with a broad phase space spanned by the competing energy scales within a system. This dissertation synthesized and studied two such systems demonstrating different manifestations of spin-orbit interactions, originating from orbital degeneracy effects, on geometrically frustrated magnetic lattices. Firstly, strong spin-orbit coupling in the hyperkagome lattice, Na4Ir3O8, and secondly, the layered material, α-NaMnO2, where single-ion anisotropy and a cooperative Jahn-Teller distortion drive magnetism to the quasi-1D limit. The magnetic ground state of the Jeff = 1/2 spin-liquid candidate, Na4Ir3O8, is explored via combined bulk magnetization, muon spin relaxation, and neutron scattering measurements. A short-range, frozen, state comprised of quasi-static moments develops below a characteristic temperature of TF = 6 K, revealing an inhomogeneous distribution of spins occupying the entirety of the sample volume. Quasi-static, short-range, spin correlations persist until at least 20 mK and differ substantially from the nominally dynamic response of a quantum spin liquid. Much of this dissertation focuses on the second spin-orbit intertwined system, α-NaMnO2, where a cooperative Jahn-Teller distortion of the MnO6 octahedra arising from an orbital degeneracy in the Mn3+ cations directly affects the electronic (ferro-orbital) and magnetic (antiferromagnetic) order, which results in an intriguing study of low-dimensional magnetism. Intricacies of the structure, static magnetic order, and magnon dynamics are presented, which heavily relied on neutron scattering techniques. In particular, a longitudinally polarized bound magnon mode is characterized through the use of polarized neutron scattering. / Thesis (PhD) — Boston College, 2018. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Physics.

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In this PhD thesis, the physics of vacancies in two-dimensional ordered Heisenberg antiferromagnets is investigated. We use semi-classical methods to study the influence of a single vacancy in long-range ordered states, with a focus on non-collinear order. Here, on a classical level, a magnetic distortion is created as the spins readjust in response to the vacancy. We use the non-collinear $120^\\circ$ state on the frustrated triangular lattice as an example, where we determine the impurity contributions to the magnetization and susceptibility. An important discovery is the vacancy moment not being quantized due to non-universal partial screening. The resulting effective moment $m_0 \\ll S$ can be observed as a fractional prefactor to an impurity-induced Curie response $m_0^2/(3k_BT)$ at finite temperature. This is in sharp contrast to collinearly ordered states. Here the moment is always quantized to the bulk spin value, $m_0=S$. Furthermore, we present a detailed analysis of the vacancy-induced distortion cloud. Due to Goldstone modes, it decays algebraically as $r^{-3}$ with distance $r$ to the vacancy. Using leading-order $1/S$-expansion, we determine the quantum corrections to both size and direction of the distorted magnetic moments. Secondly, we study the same problem in the presence of an external magnetic field $h$, both for the square and triangular lattice. For the triangular lattice we use a biquadratic exchange term $K$ to stabilize a unique ground state from a degenerate manifold. The finite-field vacancy moment $m(h)$ is generated by field-dependent screening clouds, as different non-collinear bulk states evolve with increasing field. These distortion clouds decay exponentially on a magnetic length scale $l_h\\propto 1/h$. Most importantly, we find that the magnetic-field linear-response limit $h \\rightarrow 0^+$ is generically singular for $SU(2)$ ordered local-moment antiferromagnets, as the vacancy moment in zero field differs fundamentally from even an infinitesimal but finite field, $m(h \\rightarrow 0^+)\\neq m_0$. Moreover, a part of the screening cloud itself becomes universally singular. Particularly for spin-flop states, this leads to a semi-classical version of perfect screening. We present general arguments to support these claims, as well as microscopic calculations. Another remarkable result is an impurity-induced quantum phase transition for overcompensated vacancies in the $M=1/3$ plateau phase on the triangular lattice with $K<0$. We close our analysis with a discussion about important limits for finite vacancy concentrations, as well as a possible experimental verification of our predictions.

Static techniques to measure different magnetic properties of coupled magnetic nanostructured systems is researched and documented with an extensive analysis of the tunnel diode oscillator (TDO). The VSM was used to obtain the major hysteresis loop for the samples and the TDO was used to measure the magnetic susceptibility. The magnetic susceptibility was employed to conceive the static critical curve. The thesis describes both equipments, VSM and TDO, that were used to obtain data for our experiments. Albeit a more comprehensive outlook on the TDO is provided. The theoretical functionality of TDO, previous successful applications for experiments, and the physical setup in the laboratory is explored. The novel addition of the double Helmholtz coil in this setup is described. The viability of replacement of the big electromagnet and the advantages of the Helmholtz coil are discussed. Magnetization dynamics in a series of FeCoB/Ru/FeCoB synthetic antiferromagnetic samples were investigated via reversible susceptibility measurements acquired through the TDO. The major hysteresis loop generated by the VSM were used to calculate the coercivity and magnetic saturation of the sample. The VSM and TDO were subsequently used to explore the magnetization switching in a di_erent coupled magnetic system, the exchange bias samples. A range of NiFe/FeMn samples were studied with varying thickness of the antiferromagnetic layer.

This work presents a detailed investigation of magnetization dynamics in synthetic antiferromagnets (SAFs), which has been studied both experimentally, using electrically-detected ferromagnetic resonance (ED-FMR) and vector-network analyzer-based ferromagnetic resonance (VNA-FMR), and theoretically. Two modes, one with in-phase and one with 180° out-of-phase precessing magnetizations of the layers, are identified in all applied field regimes, namely, a low-field antiferromagnetically coupled regime (when magnetizations of the layers have opposite directions), a spin-flop regime at intermediate field values (when magnetizations are non-collinear) and a high-field saturation regime (when both magnetazations are collinear to each other and the external magnetic field direction). The qualitative theoretical description, found to be in good agreement with the experimental data, is given using a system of coupled Landau-Lifshitz equations. In this work, for the first time to our knowledge, it is shown that for SAFs with different magnetic moments of the individual layers, dynamic, and not static, Zeeman and interlayer exchange coupling energies are solely resposible for the frequency-field dependence in the antiferromagnetically coupled regime. The changes in the dynamical energy terms lead to the changes in the dynamical components of the precessing magnetizations. As the external magnetic field is varied, the amplitude of the components vary continiously in different ways for different modes, dropping to zero in the spin-flop regime, which is reflected in the amplitudes of ED-FMR and VNA-FMR. This effect appears only in SAFs with different magnetic moments of the layers and is related to the different increase in the Zeeman energy of the layers as the externally applied field increases. The variation of the dynamical components leads to the modulation of an exchange of spin-angular momentum between the layers (so called spin-pumping effect).This is directly reflected in the measured linewidths for the modes in both ED-FMR and VNA-FMR. As was shown before in the literature and is also confirmed here, spin-pumping leads to a constant difference in linewidths between two modes for SAFs in the saturated case. Here, we extend the previous findings to non-saturated regimes. We show that in non-saturated states, the linewidth difference does not remain constant and varries with the external magnetic field. This linewidth difference changes can be qualitatively explained using the modulation of the dynamical magnetization components. In order to directly model the spin-pumping effects, additional terms are introduced in the coupled Landau-Lifshitz equations, related to the intrinsic damping in the magnetic layers and spin-pumping induced effects. The calculations of linewidth dependences using the extended model are not in full agreement with the experiments, suggesting that additional effects must be added to the model (for example, potential domain formations). Additionly, due to the dependence of the effect on the Zeeman energy asymmetry between the layers of the SAF, SAFs with different ratio of thicknesses are studied. Although the trends described above are common to all the samples, no significant enhancement of spin-pumping effects are observed. Modeling shows that, although the evolution of the dynamical magnetization components does depend on the magnetic moment ratio between the layers, the difference between maximum and minimum values remains almost constant and does not alter the observed linewidth-field dependence.:1 Fundamentals 1 1.1 Magnetic moment 1 1.2 Magnetic energy contributions 6 1.2.1 Zeeman energy 6 1.2.2 Demagnetization energy 7 1.2.3 Magnetic anisotropy 9 1.2.4 Direct Exchange Energy 13 1.2.5 Indirect Exchange. Interlayer Exchange Coupling Energy 15 1.3 Magnetoresistance 17 1.3.1 Ordinary Magnetoresistance 17 1.3.2 Anisotropic Magnetoresistance 20 1.3.3 Giant Magnetoresistance 22 1.4 Magnetization dynamics 25 1.4.1 Classical motivation 25 1.4.2 Quantum mechanical justification 27 1.5 Spin-currents and Spin-pumping 29 2 Experimental methods 33 2.1 Vibrating Sample Magnetometry 33 2.2 Ferromagnetic resonance 34 2.2.1 Cavity-FMR 35 2.2.2 VNA-FMR 36 2.3 Electrically detected ferromagnetic resonance 39 3 Synthetic antiferromagnets. Theoretical model 47 3.1 Static model 47 3.2 Dynamical model 54 4 Sample fabrication and characterization 58 4.1 Fabrication 58 4.2 Static characterization 62 5 Magnetization dynamics in asymmetric SAFs 68 5.1 Dynamical measurements on Py(3nm)/ Ru(0.85 nm)/Py(9 nm) 68 5.2 Theoretical explanation 71 5.3 Dynamics in SAFs with varying asymmetry of the layers 80 6 High-frequency spin-pumping in SAFs 86 6.1 Spin-pumping in magnetic trilayers 86 6.2 Spin-pumping in SAFs 88 6.3 Dependence on the asymmetry between the layers 94 7 Conclusions and outlook 99 A General description of the trilayer system without damping and spin-pumping contributions 102 A.1 Effective fields 103 A.2 Final equations 107 B Mathematica program used for the theoretical modeling 108 B.1 Matrix Elements 108 B.2 Frequency-Modes 109 B.3 Dynamical components and corresponding dynamical energies 112 B.4 Equilibrium condition 114 B.5 Dynamical Energies 118 B.6 Dynamical Trajectories 121 Bibliography 125

The study of magnons is an essential part of combining quantum information science and spintronics, allowing for the investigation of quantum properties such as entanglement in solid-state devices. Magnons are commonly described using the theory of T. Holstein and H. Primakoff, associating the spin operators with bosonic creation and annihilation operators. The quantum mechanical properties inherent to this description are the commutation relations. These relations must be conserved under transformation of the basis. This requires the application of pseudo-unitary transformations U (p, q) when studying the magnon eigenspectrum for example. Depending on the system at hand, the groups U (1, 1) and U (2, 2) are of particular interest and will be the focus of this work. We present a general formalism that leads to a representation of pseudo-unitary matrices via their self-adjoint elements. We apply this representation in examples involving magnons in antiferromagnets to find an explicit picture in connection to material properties. Finally, we explore numerical methods for determining magnon energies and compare them to the analytical counterpart.