An experimental Nernst effect measuring system is designed and constructed. The ability to measure the Nernst effect allows completion of a thermoelectric suite of measurements consisting of electrical conductivity, the Seebeck effect, the Hall effect, and the Nernst effect. This suite of measurements gives information about electron transport, carrier concentration, and electron scattering within a thermoelectric sample. Programs were designed in LabView to control the various instruments in the measuring system. Measurements of the Nernst effect were taken on two thermoelectric samples, bismuth nickel telluride and bismuth antimony telluride. These measurements were taken at both constant temperature and constant magnetic field. An error analysis of the Nernst effect measuring system is also presented, with consideration as to future work that can be done to improve the quality of Nernst effect measurements taken from the system.

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The focus of this thesis lies on the thermoelectric transport properties of rare-earth compounds containing Ce, Eu, and Yb. These systems have been investigated either to study fundamental problems or to evaluate their potential for thermoelectric applications.

Dans cette thèse, j'ai étudié l'évolution de la surface de Fermi sous l'influence d'un champ magnétique dans des systèmes massifs facilement polarisables à basse température. La première partie est dévouée aux cas du supraconducteur ferromagnétique UCoGe et du supraconducteur paramagnétique CeIrIn5, où la surface de Fermi peut être modifiée sans transition magnétique. Dans UCoGe, plusieurs anomalies successives ont été détectées dans l'effet Seebeck, la résistivité et l'effet Hall, sans transition nette dans l'aimantation. L'observation d'oscillations quantiques montre que ces anomalies sont reliées à des changements de topologie de la surface de Fermi, aussi appelés transitions de Lifshitz. Dans CeIrIn5, une anomalie est détectée dans l'effet Seebeck à HM = 28 T et les oscillations quantiques observées en magnétométrie torque montrent qu'une transition de Lifshitz à lieu à ce champ.Dans la deuxième partie, j'ai étudié comment varie la surface de Fermi à travers une transition magnétique du premier ordre induite par le champ magnétique dans le supraconducteur ferromagnétique URhGe avec le champ selon l'axe de difficile aimantation b et le supraconducteur antiferromagnétique UPd2Al3 avec le champ dans le plan basal. Dans URhGe, l'effet Seebeck permet d'observer un changement de la surface de Fermi à la transition de réorientation des spins à HR = 11.75 T et avec la résistivité confirme le caractère premier ordre de la transition en plus de fournir la localisation dans le diagramme de phase du point tricritique. Dans UPd2Al3, une nouvelle branche de la surface de Fermi est observée dans les oscillations quantiques de de Haas-van Alphen dans l'état antiferromagnétique et l'effet Seebeck montre que la surface de Fermi change à la transition métamagnétique à HM = 18 T. En outre, quatre nouvelles branches sont observées dans la phase polarisée au delà de HM et qui ne peuvent être associées à celles calculées dans les états paramagnétique et antiferromagnétique. / In this thesis, we have studied the evolution of the Fermi surface under the influence of a magnetic field in bulk materials that can be easily polarized at low temperature. The first part was devoted to the cases of the ferromagnetic superconductor UCoGe with a magnetic field applied along the easy magnetization c-axis and the paramagnetic superconductor CeIrIn5 with the field along the c-axis. In UCoGe, several successive anomalies were detected in resistivity, Hall effect and thermoelectric power, without any thermodynamic transition being detected in magnetization. The direct observation of quantum oscillations showed that these anomalies are related to topological changes of the Fermi surface, also known as Lifshitz transitions. In CeIrIn5, the thermoelectric power detected an anomaly at HM = 28 T and the quantum oscillations observed in torque magnetometry showed that a Lifshitz transition occurs at this field.In the second part of this thesis, we studied the evolution of the Fermi surface through first order magnetic transitions induced by magnetic field. In the ferromagnetic superconductor URhGe with the field applied along the hard magnetization b-axis and the antiferromagnetic superconductor UPd2Al3 with the field in the basal plane. In URhGe, the thermoelectric power allowed to observe a change in the Fermi surface at the spin reorientation transition at HR = 11.75 T defining the ferromagnetic state and along with resistivity confirmed the first order character of the transition as well as give a location of the tricritical point. In UPd2Al3, a new branch was observed in de Haas-van Alphen experiment in the antiferromagnetic phase and the thermoelectric power showed that the Fermi surface is reconstructed at the metamagnetic transition at HM = 18 T where the antiferromagnetic state is suppressed and could suggest that the Fermi surface changes before this transition. Additionally, four new branches were observed in the polarized paramagnetic phase, above HM, that cannot be associated with calculated branches in the paramagnetic of antiferromagnetic states.

Popis magnetodynamických vlastností nanomagnetů a nanostrukturovaných magnetických materiálů vyžaduje metody vhodné pro zkoumání typické časové odezvy těchto systémů, tj. v řádu nanosekund a méně. Nedostatek technik, vhodných právě pro charakterizaci v časové doméně, je spojen s možnostmi současné elektroniky. Další možný přístup, jak popsat vlastnosti nanomagnetů, je charakterizace ve frekvenční doméně v pásmu GHz. Nejrozšířenější technikou charakterizace ve frekvenční doméně je měření feromagnetické rezonance (FMR). Ze spekter FMR lze získat cenné informace o systému: parametr tlumení, saturační magnetizace atd. Metoda, kterou využíváme k detekci excitací spinových vln, má za cíl zjednodušení charakterizace. Využíváme termoelektrickou detekci spinových vln v magnetických drátech prostřednictvím anomálního Nernstova jevu. Metoda je založena na disipaci tepla uvnitř magnetické vrstvy v důsledku útlumu spinových vln, a proto dochází k vytvoření teplotního gradientu směrem k substrátu (kolmo k povrchu). To vede k vytvoření elektrického pole kolmého jak na teplotní gradient, tak na směr magnetizace. Napětí je obvykle v řádu V, proto může být měřeno obvyklým laboratorním vybavením. Navzdory své jednoduchosti poskytuje tato metoda velmi zajímavé výsledky a může být použita pro charakterizaci magnonických vlnovodů, magnonických metamateriálů, emitorů spinových vln a dalších zařízení, pracujících se spinovými vlnami.

The focus of this thesis lies on the thermoelectric transport properties of rare-earth compounds containing Ce, Eu, and Yb. These systems have been investigated either to study fundamental problems or to evaluate their potential for thermoelectric applications.

Cette thèse porte sous l'étude sous conditions extrêmes (basse température, fort champ magnétique et haute pression) des composés fortement corrélés du type fermions lourds. Trois composés ont été analysés UCoAl, UGe$_2$ et CeRh$_2$Si$_2$, en utilisant principalement de mesures thermoélectriques; une technique récente et très sensible dans le domaine des fermions lourds. À cette fin, de nouvelles dispositives de mesures de pouvoir thermoélectricité sous pression ont été développés au cours de cette thèse.Concernant le composé d'UCoAl, notre étude a permis d'analyser précisément la transition metamagnétique, induite par le champ magnétique, entre la phase paramagnétique (PM) et la phase ferromagnétique (FM) ainsi que, son évolution sous pression. Ainsi, nos mesures ont permis de compléter le diagramme de phase $(T,P,H)$ et notamment, de mettre en évidence la structure magnétique originale qui apparaît sous pression en forme de ```wings" pas des mesures thermoélectriques.Une fine analyse de la surface de Fermi de la phase FM$2$ d'UGe$_2$ a été réalisée grace à l'observation des oscillations quantiques du pouvoir thermoélectrique. Les résultats obtenues ont été comparés aux études conventionnelles des oscillations quantiques comme de ``de Haas-van Alphen" (dHvA) et de ``Suhbnikov-de Hass" (SdH) effets. Une très bonne accord entre les trois techniques a été constatée. % et montre les avantages d'utilisation des mesures du pouvoir thermoélectrique pour analyser les paramètres microscopiques des fermions lourds.% Les inconvénients de cette technique sont aussi présentés.Finalement, dans le système CeRh$_2$Si$_2$, la suppression du domaine antiferromagnétique (AF) sous champ magnétique $H_c sim 26$T et sous pression $Psim 1$GPa a été étudiée. Un très fort changement de la surface de Fermi à $H_c$ correspondant à la transition de l'ordre AF vers une phase paramagnétique polarisée (PPM), a été observé. Sous pression, des fluctuations magnétiques et une reconstruction de la surface de Fermi apparaissent autour de $P_c$. Ces fluctuations cachent la nature de la suppression de l'ordre AF vers un ordre paramagnétique (PM). L'étude du diagramme de phase $(T,H,P)$ révèle que les phases PM et PPM sont différentes, cependant des points en commun demeurent. / Thermopower is a technique whose importance is related to the possibility of directly measuring electronic properties of the systems, as it is sensitive to the derivative of the density of states. In this work, the low temperature regime of strongly correlated electron systems has been studied using this technique. For that, a new pressure-field thermopower device was developed, and used, to determine $(T,P,H)$ phase diagrams of the itinerant ferromagnets UCoAl and UGe$_2$, and of the weak antiferromagnet CeRh$_2$Si$_2$.For example, in the case of UCoAl, this same technique was used to analyze the metamagnetic transition from paramagnetic (PM) to ferromagnetic (FM) phases and to study its evolution towards the quantum critical end point. The existence of exotic magnetic excitations in the ground state and around the critical end point were also evidenced.On the compound CeRh$_2$Si$_2$, the suppression of the antiferromagnetic (AF) order by magnetic fields and pressures was explored. A strong change of the Fermi surface at $H_c$, the field at which the suppression of the AF into the paramagnetic polarized (PPM) phase, was observed. We show that under pressure, the magnetic fluctuations around the critical pressure $P_c$ masked the Fermi surface reconstruction of the AF phase into the PM phase. The analysis of the $(T,P,H)$ phase diagram revealed that the non-ordered phases of this compound (PM and PPM) are different, therefore pressure and field behave as different suppressor mechanisms.In the UGe$_2$ compound, the analysis of its Fermi surface by thermopower quantum oscillations was performed as a last example of the utility and of the importance of this technique. To the best of the author knowledge, this is the first time that this technique was used in heavy fermion systems. A comparison to traditional probes such as de Haas-van Alphen and Shubnikov–de Haas effects was done. We observed a good agreement between them and we explain the advantages and the disadvantages of thermopower quantum oscillations technique over the traditional probes.

It was the purpose of this thesis to evaluate two-dimensional (2D) magneto-thermoelectric (MTE) phenomena in thinner regime. Mostly this work was motivated by the recent discovery of MTE properties in transition metal dichalcogenides (TMD). In general, TMD thin films have attracted much attention due to their very good electrical, optical, and electrochemical properties. However the total amount of studies of the MTE in TMD is rather small compared to the other properties, such as electric, opto-electric, and catalyst. Hence, in this thesis, we aimed to evaluate the MTE properties in TMD materials. Before we started to measure TMDs, we established a measurement platform and studied MTE properties in ferromagnetic CoFeB, and Weyl semimetal Co2MnGa.:1. Introduction a. Physical background i. Seebeck Effect ii. Anomalous Hall Effect and Anomalous Nernst Effect iii. Mott relation 2. Sample Preparation and evaluation a. Physical vapor deposition b. Mechanical Exfoliation c. Patterning Process 3. Data Evaluation 4. State of the art in Transition Metal Dichalcogenids a. Introduction b. TMD in use c. Magneto-thermoelectric properties in TMD 5. Magneto-thermoelectrical properties in CoFeB thin film a. Introduction b. Results and Discussion c. Conclusion 6. Anomalous Nernst and Anomalous Hall effect in Co2MnGa thin film a. Introduction b. Results and Discussion c. Summary 7. Anomalous Hall effect in exfoliated VS2 flake a. Introduction b. Experiment c. Results and Discussion d. Summary 8. Summary Acknowledgement and References

Magnetoresistive effects are powerful tools for studying the intricate structure of solid state electronic systems, and have many applications in our current information technology. In particular, the electronic system reflects the crystal symmetry and the orbital structure of the atoms of a given solid, and thus is crucial to understanding magnetism, superconductivity and many other effects which are of key interest to current solid state research. Consequently, studies of the electrical transport properties of solid state matter allow to evaluate this imprint and in turn draw conclusions about the interactions within a material. In this thesis, we will exploit the capabilities of magnetotransport measurements to infer the properties of a multitude of magnetic systems. In turn, this allows us to push the understanding of transport phenomena in magnetic materials. The first part of this work is focused on the magnetoresistance observed in spin Hall active metals in contact with a magnetic insulator. In such bilayers, the interfacial spin accumulation caused by the spin Hall effect in the metal can interact with the magnetic insulator, giving rise to interesting magnetoresistive effects. In the framework of this thesis, bilayers with several magnetic insulators are studied, including antiferomagnets, ferrimagnets and paramagnets (disordered magnets). For the disordered magnetic insulators, we find that the established spin Hall magnetoresistance framework does not allow to consistently describe the observed transport response. Consequently, we propose an alternative explanation of the magnetoresistance in such heterostructures, using the Hanle magnetoresistance and assuming an interface which has a finite electrical conductivity. This alternative model can serve to generalize the theory of the spin Hall magnetoresistance, providing addition information on the microscopic picture for the loss of the transverse spin component. Additionally, by partly removing the magnetic insulator and studying the ensuing changes, we verify that magnons are crucial for the observation of a non-local magnetoresistance in bilayers of a magnetic insulator and a metal. Finally, the local and non-local spin Seebeck effect (i.e. the electric field generated by a thermally driven pure spin current) is investigated in bilayers of Cr2O3 and Pt where the occurrence of a spin superfluid ground state was reported. In our sample, however, the transport response is consistent with the antiferromagnetic spin Seebeck effect mediated by the small magnetic field induced magnetization also reported for other antiferromagnet/metal heterostructures. As such, we cannot verify the presence of a spin superfluid ground state in the system. In the second part of this thesis, the topological properties of the electronic system and the related changes of the magnetoelectric and magnetothermal transport response are investigated. To that end, we first demonstrate a novel measurement technique, the alternating thermal gradient technique, allowing to separate the relevant thermovoltages from spurious other voltages generated within the measurement setup. We employ this novel technique for measuring the topological Nernst effect in Mn 1.8 PtSn and show the possibility to combine the magnetoelectric and magnetothermal transport response to evaluate the presence of topological transport signatures without requiring magnetization measurements. Additionally, we show that the anomalous Nernst effect in the non-collinear antiferromagnet Mn3Sn is connected to the antiferromagnetic domain structure: Using spatially resolved measurements of the anomalous Nernst effect, direct access to the antiferromagnetic domain structure is demonstrated. Additionally, a thermally assisted domain writing scheme is implemented, allowing the preparation of Mn3Sn into a defined antiferromagnetic domain state.