Transport of spin angular momentum (spin currents) in magnetic thin films is important for non-volatile spin-based memory devices and other emerging information technology applications. It is especially important to understand how a spin current propagates across interfaces and how a spin current interacts with magnetic moments. The great interest in devices based on ferromagnetic metals generated intensive theoretical and experimental studies on the basic physics of spin currents for the last few decades. Of particular interest recently is the so-called "pure" electronic spin current, which is carried by electrons and yet unaccompanied by net charge flow, in part because of the prospect of transporting spin with minimal Joule heating. However, in contrast to ferromagnetic metals, spin transport in antiferromagnetic metals, which are promising materials for next-generation magnetic information technology, is not well understood yet. This dissertation addresses the mechanisms of transport by pure spin current in thin-film multilayers incorporating metals with antiferromagnetic order. We focus on two specific materials: (1) CoGd alloys with ferrimagnetic sublattices, which resemble antiferromagnets near the compensation composition, and (2) elemental antiferromagnetic Cr, which can be grown as epitaxial films and hence serve as a model system material. For both the CoGd and Cr studies, spin-valve-like structures of NiFe/Cu/CoGd and NiFe/Cu/Cr/CoFe are prepared to conduct ferromagnetic resonance spin pumping experiments. Precessing magnetization in the NiFe "spin source" pumps a transverse spin current to the adjacent layers. We measure the loss of the spin angular momentum in the "spin sink" layer by measuring the broadening of the resonance linewidth, i.e., the non-local damping enhancement, of the spin source. The antiparallel magnetic moments of Co and Gd sublattices partially cancel out the dephasing of a transverse spin current, thereby resulting in a long spin dephasing length of ≈ 5-6 nm near the magnetic compensation point. We find evidence that the spin current interacts somewhat more strongly with the itinerant transition-metal Co magnetism than the localized rare-earth-metal Gd magnetism in the CoGd alloy. We also examine spin transport via structurally clean antiferromagnetic Cr, epitaxially grown with BCC crystal order. We observe strong spin reflection at the Cu/Cr interface, which is surprising considering that thin layers of Cu and Cr individually are transparent to spin currents carried by electrons. Further, our results indicate other combinations of electrically conductive elemental metals (e.g., Cu/V) can form effective spin-reflecting interfaces. Overall, this thesis advances the basic understanding of spin transport in metallic thin films with and without magnetic order, which can aid the development of next generations of efficient spintronic devices. This work was supported in part by the National Science Foundation, Grant No. DMR-2003914. / Doctor of Philosophy / Manipulation of electronic flow, i.e., net charge flow, underlies modern electronic devices such as computers, mobile phones, and electric cars. However, the conventional charge transport inevitably results in wasted energy, due to resistive (Joule) heating in the devices. A new research area which uses the electron's spin has recently emerged, namely spintronics. Spintronics uses the spin of electrons rather than just the charge, thereby reducing the dependence on charge flow. The flow of spin angular momentum carried by electrons, i.e., "electronic spin current," underpins numerous phenomena in condensed matter physics. An important example is switching and excitation of magnetic order driven electrically by spin current rather than external magnetic field. Spin currents can interact not only with ferromagnetic order consisting of parallel magnetic moments – but also with antiferromagnetic order consisting of alternating magnetic moments that cancel the net magnetization of the material. Indeed, experiments from the last few years demonstrate the ability to rotate antiferromagnetic order (a.k.a. Néel vector) by spin current, which offers new physics not achievable in ferromagnets, such as ultrafast spin dynamics in the THz regime and superfluid spin transport analogous to superconducting electronic transport. However, interaction of a spin current with antiferromagnetic order is not well understood yet. The aim of this thesis is to build a better understanding of spin currents in antiferromagnetic metals. Specifically, we experimentally study basic spin-current physics in a ferrimagnet (CoGd) and an antiferromagnet (Cr). We choose CoGd because adjusting its chemical composition allows us to easily tune its magnetism from ferromagnet-like (uncompensated magnetization) to antiferromagnet-like (compensated magnetization). In antiferromagnet-like CoGd, we find that the oppositely oriented Co and Gd magnetic moments partially cancel the scrambling (dephasing) of spins, so that the spin current is able to propagate over a longer distance - about 3-4 times more than in ferromagnetic metals. The mechanisms behind the longer spin propagation is somewhat akin to the spin "rephasing" technique for lengthening the lifetime of spin-based qubits for quantum computers, but what is remarkable is that we observe this effect in rather disordered magnetic alloys at room temperature. In the other major project of this thesis, we investigate spin transport through multilayers that contain Cr, a structurally and chemically clean antiferromagnetic material. We find that Cr by itself is a good spin transmitter, i.e., effectively allowing a pure spin current to pass through. Surprisingly, when Cr and Cu (another good spin transmitter) are stacked together, we observe strong reflection of a pure spin current at the interface of Cr and Cu. We find that the antiferromagnetic order in Cr is not responsible for this peculiar spin reflection and that other pairs of spin-transmitting metals (for example, V and Cu) can form spin-reflecting interfaces as well. Our work shows an interesting example of "emergent" phenomena where the interface behaves in a way that is not intuitively expected from the properties of the constituent materials. The basic scientific findings from this thesis may help the development of more efficient information-technology devices that run on spin currents.

A new family of molecule-based magnets related to the V[PTCE]x · yCH2Cl2 magnet (PTCE = phenyltricyanoethylene, x = ~2, y = ~0.2) was synthesized utilizing new reversible one-electron acceptors with the general form MexPTCE (PTCE = phenyltricyanoethylene, x is the number of methyl groups on the phenyl ring, Me = -CH3). These new acceptors were synthesized for the purpose of studying electronic and steric effects of the substitution of the phenyl ring with electron donating groups on the overall magnetic properties of the solid, specifically to contrast their behavior with materials that contain similar trifluoromethyl (-CF3) substituted acceptors. These electron-poor olefins react with V(CO)6 in dichloromethane (CH2Cl2) under N2 to yield air-sensitive, amorphous magnetic coordination polymers that exhibit ordering temperatures ranging from 160 K to 250 K and display anhysteretic behavior at all temperatures. The magnets synthesized in this project were all characterized and studied using magnetic measurements, infrared spectroscopy, and elemental analysis. The neutral acceptors used were characterized using NMR spectroscopy, infrared spectroscopy, mass spectrometry, cyclic voltammetry, and modeled using density functional theory (DFT) calculations. / Master of Science

Un nouveau matériau spintronique, Mn4N, a été étudié. Les couches minces ferrimagnétiques Mn4N possèdent une aimantation spontanée Ms relativement petite et une forte anisotropie magnétique perpendiculaire (PMA) et conviennent donc aux dispositifs à mémoire à couple de rotation. De plus, Mn4N est composé uniquement d’éléments bon marché, légers et abondants, sans terres rares ni métaux nobles, et donc exempt de criticité matérielle. Dans ce travail, les propriétés magnétiques et de magnéto-transport de Mn4N développé sur un substrat de SrTiO3 (STO) ont été évaluées.Tout d'abord, les propriétés magnétiques et magnéto-transport des films minces de Mn4N sont évaluées, ce qui permet de constater leur amélioration spectaculaire en remplaçant les substrats classiques en MgO par des substrats en STO. Ce système Mn4N / STO présente des propriétés étonnantes: une structure de domaine de taille millimétrique, une aimantation totalement rémanente à champ nul et une commutation d’aimantation nette provoquée par une faible nucléation de domaine inversé et une propagation en douceur de DW. Ces propriétés, associées à un très petit Ms et à un grand PMA, soulignent son potentiel pour les applications spintroniques.Deuxièmement, l’efficacité de génération du couple de transfert de spin (STT) dans le film mince Mn4N a été mesurée en mesurant la vitesse de la paroi de domaine (DW) entraînée par des impulsions de courant. La vitesse DW atteint des valeurs record de 900 m / s pour une densité de courant de 1,3 × 10 12 A / m2. Cette valeur est la plus élevée de tous les systèmes pilotés par le STT et est comparable à la vitesse la plus élevée obtenue avec les SOT. La mobilité DW η est également très grande, la plus élevée de tous les systèmes basés sur STT. L'ajustement de nos données à l'aide d'un modèle analytique 1D permet d'extraire une polarisation de spin des électrons de conduction de 0,81, ce qui suggère que Mn4N pourrait convenir à l'obtention de grandes magnétorésistances. De plus, ces propriétés étonnantes ont été obtenues sans aucun élément de terre rare, aucune structure d’empilement, ni assistance extérieure telle que des champs magnétiques / électriques ou des contraintes mécaniques.Enfin, les propriétés magnétiques ont été ajustées par une petite quantité d'introduction de Ni dans Mn4N. L'aimantation spontanée de Mn4N sur STO a été réduite par l'introduction de Ni avec maintien d'un PMA fort et rémanence totale. Ce résultat indique que le système ferrimagnétique Mn4N pourrait être compensé en substituant des atomes de Ni. Récemment, la compensation du ferrimagnet a été activement étudiée car le ferrimagnet compensé fournit une efficacité infinie en spin-couple. Les trois évidences de la compensation ont également été démontrées, l'inversion de l'angle de Hall anormal, la chiralité de rotation de Kerr et la dépendance de l'aimantation en fonction de la température. Le point de compensation de la composition a été estimé autour de Mn3.82Ni0.18N. Nous avons suggéré le modèle de compensation de Mn4N par l'introduction de Ni, qui est compatible avec la réduction par MS, l'inversion des courbes AHE, Kerr et M-T.En résumé, un potentiel de films Mn4N et Mn4-xNixN a été démontré comme un candidat prometteur pour les applications spintroniques telles que les dispositifs de mouvement DW induits par le courant avec de grandes propriétés: nucléation de domaine et propagation DW lisse, efficacité de génération de STT ultra-haute et accordabilité de la magnétisation par Ni-introduction. Ces propriétés étonnantes ont été réalisées sans terres rares ni métaux nobles, ce qui peut constituer une étape importante dans le remplacement des matériaux à base de terres rares par des éléments abondants. / A new spintronic material Mn4N has been investigated. Ferrimagnetic Mn4N thin films possess relatively small spontaneous magnetization Ms and strong perpendicular magnetic anisotropy (PMA) and thus are suitable for spin-torque based memory devises. In addition, Mn4N is composed of only cheap, light and abundant elements without any rare-earth nor noble metals., thus free from material criticality. In this work, magnetic and magneto-transport properties of Mn4N grown on SrTiO3 (STO) substrate have been evaluated.First, the magnetic and magneto-transport properties of Mn4N thin films are evaluated, resulting in finding out dramatically improvement of them by replacing conventional MgO substrates by STO substrates. This Mn4N/STO system exhibits astonishing properties: a millimeter-sized domain structure, fully remnant magnetization at zero field and a sharp magnetization switching caused by scarce nucleation of reversed domain and smooth DW propagation. These properties, associated to a very small Ms and a large PMA, underline its potential for spintronic applications.Second, the generation efficiency of spin-transfer torque (STT) in Mn4N thin film has been measured by measuring the speed of domain wall (DW) driven by current pulses. The DW velocity reaches record values of 900 m/s for a current density of 1.3×10^12 A/m2. This value is the highest in all STT-driven systems and is comparable to the highest speed obtained using SOTs. The DW mobility η is also very large, the highest in all STT-based systems too. Fit of our data using a 1D analytical model allows extracting a spin polarization of the conduction electrons of 0.81, suggesting that Mn4N could be suitable to obtain large magnetoresistances. In addition, these amazing properties have been achieved without any rare earth elements, stack structures, nor external assistance such as magnetic/electric field or mechanical stress.At last, the magnetic properties have been tuned by a small amount of Ni-introduction to Mn4N. The spontaneous magnetization of Mn4N on STO has been reduced by Ni-introduction with keeping strong PMA and full remanence. This result indicates the ferrimagnetic Mn4N system might be compensated by substituting Ni atoms. Recently compensation of ferrimagnet has been actively studied because the compensated ferrimagnet provides infinite spin-torque efficiency. The three evidences of the compensation have also been demonstrated, the reversal of anomalous Hall angle, Kerr rotation chirality, and the temperature dependence of magnetization. The compensation point of composition has been estimated around Mn3.82Ni0.18N. We suggested the compensation model of Mn4N by Ni introduction which is consistent with the MS reduction, the reversal of AHE, Kerr and M-T curves.In summary, a potential of Mn4N and Mn4-xNixN films has been demonstrated as a promising candidate for spintronic applications such as current induced DW motion devices with great properties: scares domain nucleation and smooth DW propagation, ultrahigh STT generation efficiency, and tunability of magnetization by Ni-introduction. These amazing properties have been achieved without rare-earth nor noble metal, which can be a milestone for replacement of rare-earth-based materials by abundant elements.

In the geometrical spin frustrated systems, order-disorder phenomena are interesting, for their intrinsic fluctuation, complicated completing interactions, and lattice structure leading to many intrigue physical behavior. To clarify the mechanism of such systems, experiments under extreme conditions (diverse magnetic fields and hydrostatic pressures) are powerful tools to meet the needs. In this dissertation, two kind of interesting materials are investigated. One is quasi-1D double chain antiferromagnet NaV2O4, the other is cubic like ferrimagnet Cu2OSeO3. In the polycrystalline compound NaV2O4, it exhibits an antiferromagnetic transition TN at 140 K, together with two field dependence subphases at 1 T≤H≤5 T. The two characteristic temperature TN1 and TN2 are associated to two subphases, which is determined by the peak position in the derivative of magnetization with respect to temperature. Under magnetic field, TN and TN1 remain almost unchanged (linear behavior), while TN2 acts in a nonlinear behavior with the application of magnetic field. Further, TN1 and TN2 are found to decrease roughly linear with applied pressure, while TN2 follows a nonlinear relation with applied pressure. On the other hand, the cubic single crystal Cu2OSeO3 exhibits a ferrimagnetic transition at 58 K, which is shifting to high temperature range with increasing magnetic fields. The peak values (from the mutual inductance measurements) associated with the ferrimagnetic transitions also increase with applied hydrostatic pressures. Moreover, the spin-flipped transitions are observed below transition temperature at ambient and applied pressure (12.67 kbar). The measurements above strongly suggested the ferrimagnetic spin configurations order earlier, i.e. transition temperatures increase with applied magnetic fields and pressures. In summary, the investigated frustrated spin systems (NaV2O4 and Cu2OSeO3) behave with the same trend with applied magnetic fields and hydrostatic pressures. It is possibly induced by the external DC magnetic field and the structure change and/or deformation under pressure.

Les nano-oscillateurs à transfert de spin (STNO) sont des candidats prometteurs pour la réalisation de composants radiofréquence (RF) intégrés, du à leur taille nanométrique, l'importante gamme de fréquences de base qu'ils peuvent couvrir, ainsi qu'à leur accordabilité autour de ces fréquences de base. Le signal RF est obtenu grâce à l'effet de transfert de spin (STT) qui donne lieu à une oscillation non-linéaire de l'aimantation dans un élément magnétorésistif. Jusqu'ici, ces excitations ont été examinées dans le cadre d'une couche magnétique isolée, c'est-à-dire sans prendre en compte le couplage entre couches. Cependant, nombreux aspects du spectre d'excitation ne peuvent pas être expliqués si l'on considère une couche isolée. Dans cette thèse nous nous attacherons à répondre à la question importante du couplage dynamique entre couches magnétiques dans un nanopilier magnétorésistif, afin de développer une meilleure compréhension des spectres d'excitation, et en particulier la dépendance en courant et champ magnétique appliqué des caractéristiques du pic d'émission, telles que la largeur de raie et la fréquence. Une première étude est réalisée pour un système composé de deux couches ferromagnétiques, couplées entre elles par le couplage RKKY (ce système est appelé un ferrimagnétique synthétique (SyF)). Le couplage induit des différences importantes dans la dépendance en courant de la fréquence par rapport aux excitations d'une couche isolée. Ces différences sont expliquées par l'important couplage dynamique RKKY. Une seconde étude prend en considération une interaction plus complexe, ayant lieu dans un nano-pilier STNO standard basé sur jonctions tunnel ou vannes de spin. Ce dispositif est composé d'un SyF ainsi que d'une couche libre(FL) magnétique, séparés par une fine couche métallique ou isolante. Pour ce système, en plus du couplage dynamique RKKY propre au SyF, nous prenons en compte le couplage dynamique généré par le champ dipolaire ainsi que le spin-torque mutuel (MSTT) entre la couche libre et le SyF. Ce couplage multiple donne lieu à deux signatures distinctes. La première est l'apparition d'un « saut » dans le spectre d'excitation dû à l'hybridation des modes SyF and FL dans le régime atténué. Le second est dû à l'interaction entre les excitations en régime entretenu, éventuellement via leurs composantes harmoniques, avec les excitations en régime atténué. Cette interaction donne lieu à des discontinuités dans la dépendance fréquence – champ, ce lorsque les excitations FL sont prédominantes. Il est intéressant de noter que cela mène à des régions ou la largeur de raie est diminuée. De plus, lorsque les excitations SyF sont prédominantes, la largeur de raie est diminuée par rapport aux cas ou les excitations FL sont prédominantes. Partant de ces observations, nous proposons une structure plus complexe, où un seconde couche de type SyF remplace la couche libre. Les résultats obtenus par une combinaison d'expériences, de simulations numériques et d'analyse analytique, montrent le rôle important des interactions dynamiques dans un nano-pilier. Ils ouvrent de nouvelles voies pour la conception de configurations STNO qui mèneront à des améliorations des performances du signal ainsi synthétisé. / Spin-torque nano-oscillators (STNOs) are promising candidates for integrated radiofrequency (RF) components due to their nanoscale size, the large range of base frequencies that can be covered, as well as the large achievable tuning ranges around the base frequency. The RF signal is obtained due to the spin transfer torque (STT) generating a non-linear magnetization oscillation in a magnetoresistive device. In the past, these excitations were investigated using the picture of a single (or independent) layer. However, many features of the excitation spectra observed experimentally in nanopillar devices cannot be explained considering a single layer. In this thesis we address the important question on the dynamical coupling between the magnetic layers inside a magneto-resistive nanopillar device, to gain a better understanding of the excitation spectra, i.e. the dependence of the frequency and the linewidth on current and applied magnetic field. A first study is realized for a coupled system, composed by two ferromagnetic layers, coupled by the interlayer RKKY coupling (so called Synthetic Ferrimagnet SyF). Due to the coupling the frequency dependence versus current is different as compared to excitations of a single layer. This is explained by the strong dynamical RKKY coupling. A second study considers a more complex interaction, occurring within standard STNO nanopillar spin valves or tunnel junctions. They are composed by a SyF separated by a metallic or insulating spacer respectively from the single free layer (FL). For this system we take into account besides the RKKY coupling within the SyF, also the dynamical dipolar field coupling and the mutual spin torque (MSTT) between the SyF and the free layer. We find two definite signatures arising from this coupling. The first is a gap in the steady state excitation spectra that is due to the hybridization of the SyF and FL modes in the damped regime. The second is the possibility of the spintorque driven excitation or its harmonics with the damped modes leading to discontinuities in the frequency field dependence when the free layer is dominantly excited. Interestingly this leads to a region of reduced linewidth. Furthermore for SyF layer dominated excitations, the linewidth is lower than in the FL dominated excitations. From these observations we propose a more complex structure, composed by two SyF layers where the single FL is replaced by a SyF. The results obtained by a combination of experiments, numerical simulations and analytical analysis, demonstrate the important role of the dynamic interactions in nanopillar STNOs and provide routes for designing novel STNO configurations that should lead to improved microwave performances.

<div>The response time of magnetization switching in current spintronic devices is limited to nanosecond timescales due to the precessional motion of the magnetization during reversal. To overcome this limit two routes of investigation leading to novel recording and logic devices are considered in this thesis: 1) Magnetic tunnel junction structures where the recording and reference layers are replaced by synthetic ferrimagnets and switching is induced by spin transfer torque and 2) Hybrid magneto-photonic devices where switching is induced by plasmon-enhanced all-optical switching. To circumvent limitations of the materials and magnetic properties of CoFeB, the most utilized alloy in spintronics, hcp-CoCrPt, a material that exhibits superior perpendicular anisotropy and thermal stability, is chosen as the ferromagnetic electrode in this work. Whereas actual devices based on the two schemes aforementioned are still in the process of being fabricated, through collaborative work with our international collaborators, this thesis describes fundamental magnetic and structural characterization needed for the realization of said ultrafast switching devices. The magnetic switching behavior of CoCrPt-Ru-CoCrPt synthetic ferrimagnets with perpendicular magnetic anisotropy have been studied in the temperature range from 2K to 300K. It was found that two sets of magnetic transitions occur in the CoCrPt-Ru-CoCrPt ferrimagnet systems studied. The first set exhibits three magnetization states in the 50K – 370K range, whereas the second involves only two states in the 2K and 50K range. The magnetic hysteresis curves of the synthetic ferrimagnet are assessed using an energy diagram technique which accurately describes the competition between interlayer exchange coupling energy, Zeeman energy, and anisotropy energy in the system. This energy diagram analysis is then used to predict the changes in the magnetic hysteresis curves of the synthetic ferrimagnet from 200K to 370K. This represents the potential operation temperature extrema that a synthetic ferrimagnet could be expected to operate at, were it to be utilized as a free layer in a memory or sensor spintronic device in the device configuration described in this dissertation.</div><div>Circularly polarized fs laser pulses generate large opto-magnetic fields in magnetic materials, through the inverse Faraday effect. These fields are attributed to be largely responsible for achieving ultrafast all-optical magnetization switching (AOS). All experimental demonstrations of AOS thus far have been realized on thin films over micron-sized irradiated regions. To achieve magnetization switching speeds in the ps and potentially fs time regimes, this work proposes the use of surface plasmon resonances at the interface of hybrid magneto-photonic heterostructures. In addition to the ability of plasmon resonances to confine light in the nm scale, the resonant excitation can largely enhance induced opto-magnetic fields in perpendicular magnetic anisotropy materials. This requires strong spin-photon coupling between the plasmonic and the magnetic materials, which thus requires the minimization of seed layers used for growth of the magnetic layer. This work reports on the development of ultrathin (1 nm thick) interlayers to control the growth orientation of hcp-Co alloys grown on the refractory plasmonic material, TiN, to align the magnetic axis out-of-plane. CoCrPtTa seed layers down to 1 nm were developed to seed the growth of CoCrPt, and the dependence of the quality of the CoCrPt is investigated as Ta composition is varied in the seed layer. Whereas bismuth iron garnet (BIG) meets the magneto-optical requirements for a hybrid magneto-photonic material, its magnetic and structural properties are highly sensitive to the Bi:Fe ratio and must be grown epitaxially on single crystalline substrates. Therefore, in this work we have investigated alternative materials that offer superior magnetic properties and are amenable to growth on inexpensive substrates. Opto-magnetic field enhancements up to 2.6x in Co-ferrite magneto-photonic heterostructures have been obtained via finite element analysis modelling. Alternative materials for plasmon-enhanced all-optical switching such as Co/Pd multilayers have also been investigated. Successful growth of Co/Pd multilayers on TiN using ultrathin Ti interlayers has been achieved. </div><div><br></div>

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