In this thesis, we use Muon Spin Rotation, Relaxation, and Resonance (μSR) as a probe for three frustrated pyrochlore systems; Gd2ScNbO7 (GSNO), Nd2ScNbO7 (NSNO) and Sm2Ti2O7 (STO), as well as the type-I superconductor BeAu. We grew all of the pyrochlore samples at McMaster using the Optical Floating Zone method. We make use of Direct Current (DC) and Alternating Current (AC) susceptibility, powder x-ray diffraction and Laue x-ray diffraction to characterize our samples. We make use of AC susceptibility measurements to explore the dynamics of the classical spin-ice Dy2Ti2O7 (DTO) and find that the system acts as a supercooled magnetic liquid, analogous to glassforming dielectric liquids. We find GSNO is a dense spin-glass based on our μSR and AC susceptibility measurements. NSNO is a moment fragmentation candidate where spin-ice, as well as all-in all-out magnetic ordering, are observed simultaneously. Our μSR measurements on this material show a strong similarity to another moment fragmentation candidate, Nd2Zr2O7, suggesting NSNO may be in a similar state. STO is a closely related compound that fully orders into a magnetic state which we study using μSR. We find subtle evidence of this magnetic transition along with persistent spin dynamics which we suggest has a common, but as of yet unexplained, origin as other frustrated pyrochlores measured in μSR. Finally, we use μSR to measure the temperature dependence of the critical field in the type-I superconductor BeAu. Using an ellipsoid of BeAu and a pressure cell, we study the magnetic properties of the sample under pressure. / Thesis / Candidate in Philosophy / In this thesis, we use Muon Spin Rotation, Relaxation, and Resonance (μSR) as a probe for three frustrated pyrochlore systems; Gd2ScNbO7 (GSNO), Nd2ScNbO7 (NSNO) and Sm2Ti2O7 (STO), as well as the type-I superconductor BeAu. We grew all of the pyrochlore samples at McMaster using the Optical Floating Zone method. We make use of Direct Current (DC) and Alternating Current (AC) susceptibility, powder x-ray diffraction and Laue x-ray diffraction to characterize our samples. We make use of AC susceptibility measurements to explore the dynamics of the classical spin-ice Dy2Ti2O7 (DTO) and find that the system acts as a supercooled magnetic liquid, analogous to glassforming dielectric liquids. We find GSNO is a dense spin-glass based on our μSR and AC susceptibility measurements. NSNO is a moment fragmentation candidate where spin-ice, as well as all-in all-out magnetic ordering, are observed simultaneously. Our μSR measurements on this material show a strong similarity to another moment fragmentation candidate, Nd2Zr2O7, suggesting NSNO may be in a similar state. STO is a closely related compound that fully orders into a magnetic state which we study using μSR. We find subtle evidence of this magnetic transition along with persistent spin dynamics which we suggest has a common, but as of yet unexplained, origin as other frustrated pyrochlores measured in μSR. Finally, we use μSR to measure the temperature dependence of the critical field in the type-I superconductor BeAu. Using an ellipsoid of BeAu and a pressure cell, we study the magnetic properties of the sample under pressure.

In the present thesis the superconducting properties of the Iron-based Ba_{1-x}Rb_{x}Fe_{2}As_{2} arsenides, and A_{x}Fe_{2-y}Se_{2} (A = Cs, Rb, K) chalcogenides are investigated by means of Muon Spin Rotation Spectroscopy. The temperature and pressure dependence of the magnetic penetration depth is obtained form muSR experiments and analyzed to probe the superconducting gap-symmetries for each samples. The Ba_{1-x}Rb_{x}Fe_{2}As_{2} system is described within the multi-gap s+s-wave scenario and results are discussed in the light of the suppression of inter-band processes upon hole doping. Due to the lowered upper critical field Bc2 and reduced Tc, a large section of B-T-p phase diagram is studied for the hole-overdoped x=1 case. By applying hydrostatic pressure, the RbFe_{2}As_{2} system exhibits a classical BCS superconducting characteristics. The A_{x}Fe_{2-y}Se_{2} chalcogenide represents a system containing magnetically ordered and superconducting phases simultaneously. In all investigated chalcogenide samples, about 90% of the total volume show the strong antiferromagnetic phase and 10% exhibit a paramagnetic behavior. Magnetization measurements reveal a 100% Meissner effect, while muSR clearly indicates that the paramagnetic phase is a perfect superconductor. Up to now, there is no clear evidence whether the antiferromagnetic phase is also superconducting. The microscopic coexistence and/or phase separation of superconductivity and magnetism is discussed. Moreover, a new hydrostatic double-wall pressure cell is developed and produced, satisfying the demands of muSR experiments. The designs and characteristics of the new pressure cell are reviewed in the present thesis.

Cette thèse est divisée en trois parties. Une première section présente les résultats de l'étude de la formation de polarons magnétiques liés (BMP) dans le ferroaimant EuB6 par diffusion de neutrons à petits angles (SANS). La nature magnétique du système ferromagnétique est observée sous une température critique de 15K. La signature des BMP n'apparaît pas dans la diffusion de neutrons, mais ces mesures permettent de confirmer une limite inférieure de 100\AA à la longueur de cohérence des BMP (xi_{Lower}). Dans un second temps, l'étude du LaRhSi3, un supraconducteur sans symétrie d'inversion, par muSR et ZF-muSR nous permet de sonder le comportement magnétique du système dans la phase supraconductrice. Aucun champ magnétique interne n'a été détecté en ZF-muSR sous la température critique (T_c = 2.2K). Cela indique que la phase supraconductrice ne porte pas de moment cinétique intrinsèque. L'analyse du spectre d'asymétrie sous l'application d'un champ magnétique externe nous apprend que le système est faiblement type II par l'apparition de la signature de domaines magnétiques typique d'un réseau de vortex entre H_{c1}(0) et H_{c2}(0), respectivement de 80+/- 5 et 169.0 +/- 0.5 G. Finalement, la troisième section porte sur l'étude du champ magnétique interne dans l'antiferroaimant organique NIT-2Py. L'observation d'une dépendance en température des champs magnétiques internes aux sites d'implantation muonique par ZF-muSR confirme la présence d'une interaction à longue portée entre les moments cinétiques moléculaires. Ces valeurs de champs internes, comparées aux calculs basés sur la densité de spins obtenue par calculs de la théorie de la fonctionnelle de la densité, indiquent que la moitié des molécules se dimérisent et ne contribuent pas à l'ordre antiferromagnétique. La fraction des molécules contribuant à l'ordre antiferromagnétique sous la température critique (T_c = 1.33 +/- 0.01K) forme des chaines uniformément polarisées selon l'axe (1 0 -2). Ces chaines interagissent antiferromagnétiquement entre elles le long de l'axe (0 1 0) et ferromagnétiquement entre les plan [-1 0 2]. / This thesis is divided in three sections. The first section presents the results from a small angle neutron scattering (SANS) investigation of the formation of bound magnetic polarons in the ferromagnet EuB6. While the magnetic nature of the system was observed below 15K, we could not resolve the q dependent signature of the polarons, thus putting a lower limit of 100\AA to the coherence length of the phenomenon (xi_{Lower}). Secondly, we investigated the non-centrosymmetric superconductor LaRhSi3 by muSR. The absence of an internal field below T_c = 2.2 K in ZF-muSR, indicates that the superconducting wave function does not carry an intrinsic magnetic moment. The asymmetry spectrum taken under external magnetic field shows the magnetic signature associated with vortices between H_{c1}(0) and H_{c2}(0), respectively 80 +/- 5 and 169.0 +/- 0.5 G, suggesting the system is weakly type-II. Finally, the third section presents the zero field muSR study of internal magnetic fields in the organic antiferromagnet NIT-2Py. The temperature dependent oscillating signal in the ZF-muSR spectrum confirms the presence of a long-range magnetic interaction between the molecules. By comparing the measured internal magnetic fields to calculated values based on density fonctional theory calculations, we confirm that half the molecules dimerizes while the other half forms the antiferromagnetic order under the critical temperature (T_c = 1.33 +/- 0.01K). In this antiferromagnetic order, the moments on the magnetic molecules are uniformly aligned along the (1 0 -2) axis. They interact antiferromagnetically along the (0 1 0) axis and ferromagnetically between the [-1 0 2] planes.

S'inscrivant dans le cadre général des matériaux fortement corrélés, les cobaltates Na_xCoO2 présentent de riches propriétés de magnétisme, supraconductivité et thermoélectricité. Elles associent une grande gamme de dopage (0<=x<=1) à une structure quasi-2D avec un réseau triangulaire susceptible d'interactions géométriquement frustrées. Le dopage des plans CoO2 peut y être inhomogène, si bien que des effets d'ordre de charge et de spin du cobalt sont possibles, avec l'influence éventuelle de l'ordre structural des dopants Na. Par RMN, MuSR et SQUID, cette thèse aborde le problème de la nature des corrélations magnétiques et des états de charge et de spin du cobalt qui en sont responsables, ainsi que l'origine du magnétisme ordonné parfois observé. Nous montrons qu'au dopage extrémal x=1, le système est non-magnétique comme attendu, avec seulement l'état Co3+ bas-spin S=0. En dopant alors en trous (0.67<=x<=0.87), un excès apparent de corrélations magnétiques est mis en évidence. À partir de x~0.6, la susceptibilité de spin est brutalement bien moins dépendante de la température, et présente un maximum compatible avec un pseudogap à basse température. Parallèlement, les fluctuations de spin acquièrent un caractère non-ferromagnétique. Les sites Co3+ cèdent progressivement la place à des sites 3.5+, seuls présents à x=0.5 à toute température. Tant à x=0.5 qu'à x~0.8, les caractéristiques du magnétisme ordonné suggèrent une nature itinérante, potentiellement liée à une reconstruction de la surface de Fermi.

Dans les composés antiferromagnétiques Heisenberg à géométrie kagomé, la frustration des interactions est à l'origine d'un état liquide de spins à T=0. Un état RVB, originalement proposé par Anderson dans les réseaux triangulaires puis dans les cuprates, décrit probablement ce fondamental, dont l'état singulet est dégénéré exponentiellement dans un faible gap singulet-triplet. Peu de composés expérimentaux se rapprochent aujourd'hui du système idéal et l'existence de perturbations ou d'anisotropie lève souvent la dégénérescence du fondamental lorsque T->0. Une étude par RMN, MuSR et SQUID, a été réalisée dans les composés à géométrie de bicouches kagomé de spin 3/2, Ba2Sn2ZnGa10-7pCr7pO22 et SrCr9pGa12-9pO19, considérés comme les archétypes d'un hamiltonien de spins purement Heisenberg sur un réseau kagomé. Outre l'absence caractéristique de transition jusqu'à une température Tg~2 K bien plus basse que la température de Curie-Weiss thetaCW~250 K, des propriétés physiques semblables dans ces deux systèmes sont mesurées malgré des défauts radicalement différent. Les propriétés intrinsèques de cette géométrie en sont déduites, à savoir: - une décroissance de la susceptibilité en dessous de 45 K (RMN) interprétée par de très faibles longueurs de corrélations magnétiques, au moins pour T>10 K, malgré les fortes interactions antiferromagnétiques (thetaCW~250 K). Ce maximum de la susceptibilité reste compatible avec l'existence d'un gap de spin; - l'existence de fluctuations quantiques pour T>30 mK, beaucoup plus bas que Tg (MuSR); - la corrélation entre l'apparition de ces fluctuations et celle d'un état de type verre de spin à Tg. Un modèle phénoménologique décrivant la relaxation des muons est présenté, pour la première fois dans ces systèmes, et suggère la stabilisation d'un état de type RVB en dessous de Tg.

In the present thesis the superconducting properties of the Iron-based Ba_{1-x}Rb_{x}Fe_{2}As_{2} arsenides, and A_{x}Fe_{2-y}Se_{2} (A = Cs, Rb, K) chalcogenides are investigated by means of Muon Spin Rotation Spectroscopy. The temperature and pressure dependence of the magnetic penetration depth is obtained form muSR experiments and analyzed to probe the superconducting gap-symmetries for each samples. The Ba_{1-x}Rb_{x}Fe_{2}As_{2} system is described within the multi-gap s+s-wave scenario and results are discussed in the light of the suppression of inter-band processes upon hole doping. Due to the lowered upper critical field Bc2 and reduced Tc, a large section of B-T-p phase diagram is studied for the hole-overdoped x=1 case. By applying hydrostatic pressure, the RbFe_{2}As_{2} system exhibits a classical BCS superconducting characteristics. The A_{x}Fe_{2-y}Se_{2} chalcogenide represents a system containing magnetically ordered and superconducting phases simultaneously. In all investigated chalcogenide samples, about 90% of the total volume show the strong antiferromagnetic phase and 10% exhibit a paramagnetic behavior. Magnetization measurements reveal a 100% Meissner effect, while muSR clearly indicates that the paramagnetic phase is a perfect superconductor. Up to now, there is no clear evidence whether the antiferromagnetic phase is also superconducting. The microscopic coexistence and/or phase separation of superconductivity and magnetism is discussed. Moreover, a new hydrostatic double-wall pressure cell is developed and produced, satisfying the demands of muSR experiments. The designs and characteristics of the new pressure cell are reviewed in the present thesis.

In this thesis the superconducting and magnetic phases of LiOH(Fe,Co)(Se,S), CuFeAs/CuFeSb, and LaFeP_1-xAs_xO - belonging to the 11, 111 and 1111 structural classes of iron-based arsenides and chalcogenides - are investigated by means of 57Fe Mössbauer spectroscopy and muon spin rotation/relaxation (μSR). Of major importance in this study is the application of high magnetic fields in Mössbauer spectroscopy to distinguish and characterize ferro- (FM) and antiferromagnetic (AFM) order. A user-friendly Mössbauer data analysis program was developed to provide suitable model functions not only for high field spectra, but relaxation spectra or parameter distributions in general. In LaFeP_1-xAs_xO the reconstruction of the Fermi surface is described by the vanishing of the Γ hole pocket with decreasing x. The continuous change of the orbital character and the covalency of the d-electrons is shown by Mössbauer spectroscopy. A novel antiferromagnetic phase with small magnetic moments of ~ 0.1 μ_B state is characterized. The superconducting order parameter is proven to continuously change from a nodal to a fully gapped s-wave like Fermi surface in the superconducting regime as a function of x, partially investigated on (O,F) substituted samples. LiOHFeSe is one of the novel intercalated FeSe compounds, showing strongly increased T_C = 43 K mainly due to increased interlayer spacing and resulting two-dimensionality of the Fermi surface. The primary interest of the samples of this thesis is the simultaneously observed ferromagnetism and superconductivity. The local probe techniques prove that superconducting sample volume gets replaced by ferromagnetic volume. Ferromagnetism arises from magnetic order with T_C = 10 K of secondary iron in the interlayer. The tendency of this system to show (Li,Fe) disorder is preserved upon (Se,S) substitution. However, superconductivity gets suppressed. The results of Mössbauer spectroscopy indicate that the systems tends to a secondary structural phase, where the local iron environment observed in pure FeS is absent. Moreover, two interlayer positions of the iron are identified. The absence of enhanced superconducting T_C in LiOHFeS thus is related to a structural instability. Also, in CuFeAs the role of secondary iron at the Cu position turns out to be decisive for the observed magnetic behaviour. As in LiOHFeSe, it orders ferromagnetically at T_C ~ 11 K and superimposes with the magnetic instability of the main iron site. It is shown that a small charge doping of 0.1e/Fe, which is expected from (Cu,Fe) disorder, is sufficient to switch the system between a paramagnetic and an AFM ground state. Both magnetic orders are indistinguishable, because the magnetic order parameters are strongly coupled. This coupling was observed in the structurally identical CuFeSb, where the magnetic order parameters of both iron sites scale perfectly. The magnetically unstable CuFeAs and the ferromagnetic CuFeSb can be classified according to the theory of As height driven magnetism, predicting a change from paramagnetism to AFM and finally FM with increasing As height.

In this thesis the superconducting and magnetic phases of LiOH(Fe,Co)(Se,S), CuFeAs/CuFeSb, and LaFeP_1-xAs_xO - belonging to the 11, 111 and 1111 structural classes of iron-based arsenides and chalcogenides - are investigated by means of 57Fe Mössbauer spectroscopy and muon spin rotation/relaxation (μSR). Of major importance in this study is the application of high magnetic fields in Mössbauer spectroscopy to distinguish and characterize ferro- (FM) and antiferromagnetic (AFM) order. A user-friendly Mössbauer data analysis program was developed to provide suitable model functions not only for high field spectra, but relaxation spectra or parameter distributions in general. In LaFeP_1-xAs_xO the reconstruction of the Fermi surface is described by the vanishing of the Γ hole pocket with decreasing x. The continuous change of the orbital character and the covalency of the d-electrons is shown by Mössbauer spectroscopy. A novel antiferromagnetic phase with small magnetic moments of ~ 0.1 μ_B state is characterized. The superconducting order parameter is proven to continuously change from a nodal to a fully gapped s-wave like Fermi surface in the superconducting regime as a function of x, partially investigated on (O,F) substituted samples. LiOHFeSe is one of the novel intercalated FeSe compounds, showing strongly increased T_C = 43 K mainly due to increased interlayer spacing and resulting two-dimensionality of the Fermi surface. The primary interest of the samples of this thesis is the simultaneously observed ferromagnetism and superconductivity. The local probe techniques prove that superconducting sample volume gets replaced by ferromagnetic volume. Ferromagnetism arises from magnetic order with T_C = 10 K of secondary iron in the interlayer. The tendency of this system to show (Li,Fe) disorder is preserved upon (Se,S) substitution. However, superconductivity gets suppressed. The results of Mössbauer spectroscopy indicate that the systems tends to a secondary structural phase, where the local iron environment observed in pure FeS is absent. Moreover, two interlayer positions of the iron are identified. The absence of enhanced superconducting T_C in LiOHFeS thus is related to a structural instability. Also, in CuFeAs the role of secondary iron at the Cu position turns out to be decisive for the observed magnetic behaviour. As in LiOHFeSe, it orders ferromagnetically at T_C ~ 11 K and superimposes with the magnetic instability of the main iron site. It is shown that a small charge doping of 0.1e/Fe, which is expected from (Cu,Fe) disorder, is sufficient to switch the system between a paramagnetic and an AFM ground state. Both magnetic orders are indistinguishable, because the magnetic order parameters are strongly coupled. This coupling was observed in the structurally identical CuFeSb, where the magnetic order parameters of both iron sites scale perfectly. The magnetically unstable CuFeAs and the ferromagnetic CuFeSb can be classified according to the theory of As height driven magnetism, predicting a change from paramagnetism to AFM and finally FM with increasing As height.:1 Acronyms and Symbols 2 Introduction 3 Iron-based arsenides and chalcogenides 3.1 Structural properties 3.2 Electronic properties 3.2.1 Magnetism 3.2.2 Superconductivity 3.2.3 Nematic phase 3.3 Investigated samples 4 Moessfit - a free Mössbauer fitting program 4.1 Aspects of program design 4.2 Errors 4.2.1 Uncorrelated 4.2.2 Hesse 4.2.3 MonteCarlo 4.2.4 Minos 4.3 Fitting algorithm 4.4 Maximum entropy method (MEM) 4.5 Kolmogorov-Smirnov confidence 5 Mössbauer spectroscopy 5.1 Mössbauer effect 5.2 Relativistic Doppler effect 5.3 Full static Hamiltonian 5.3.1 Quadrupole interaction 5.3.2 Isomer shift. 5.3.3 Zeeman splitting 5.3.4 Combined interaction 5.3.5 Transition probabilities 5.3.6 The magic angle 5.4 Transmission integral 5.4.1 Absorption area 5.4.2 Ideal thickness 5.4.3 Line width and line shape 5.4.4 Levelling 5.5 Applied field measurements of powder samples 5.5.1 Paramagnet, axial symmetric EFG in transverse field geometry 6 5.5.2 Uniaxial antiferromagnet, axial symmetric EFG in transverse field geometry 6 5.5.3 Paramagnet, axial symmetric EFG in longitudinal field geometry 6 5.5.4 Uniaxial ferromagnet, axial symmetric EFG in transverse field geometry 6 5.5.5 Polarised photons 5.5.6 Total absorption cross section 5.5.7 Polarised sources 5.6 Blume line shape model 6 μSR 6.1 Muon decay and detection 6.2 Magnetic order and dynamic relaxation 6.2.1 Magnetic order 6.2.2 Time dependent field distributions 6.2.3 Aspects of μSR in iron-based arsenides and chalcogenides 6.2.4 Weak transverse field (WTF) 6.3 Superconductivity - transverse field (TF) experiments 7 Intercalated FeSe 7.1 Bulk properties: XRD, susceptibility, resistivity 7.2 Structural characterization 7.3 LiOHFeSe - Mössbauer spectroscopy 7.3.1 Applied transverse field 7.4 LiOHFeSe - μSR 7.4.1 Zero field (ZF) 7.4.2 Pinning experiment 7.4.3 Transverse field (TF) 7.5 Mössbauer investigation of LiOHFe_1-yCo_ySe_1-xS_x. 7.6 Discussion 8 LaFeO(As,P) 8.1 Preliminary measurements and electronic structure calculations 8.2 Mössbauer spectroscopy 8.3 μSR 8.3.1 Magnetic characterization 8.3.2 Spin dynamics 8.3.3 Superconductivity 8.4 Discussion 9 CuFeAs and CuFeSb 9.1 Preliminary results of CuFeAs and CuFeSb 9.2 CuFeAs: Mössbauer spectroscopy 9.2.1 Zero field (ZF) 9.2.2 Longitudinal field (LF) 9.2.3 Transverse field (TF) 9.3 CuFeAs: μSR 9.3.1 Zero field (ZF) 9.3.2 Weak transverse field (WTF) 9.4 Further investigations on CuFeAs 9.4.1 Neutron scattering 9.4.2 Theoretical calculation 9.4.3 Local element analysis with EDX/WDX 9.5 CuFeSb: Mössbauer spectroscopy 9.5.1 Zero Field (ZF) 9.5.2 Transverse field (TF) 9.6 Discussion 10 Conclusion 11 Appendix 11.1 Derivation of the quadrupole interaction and isomer shift 11.2 Matrix form of the static nuclear Hamiltonian 11.3 Mössbauer line intensities 11.4 Blume line shape model 11.4.1 Special case: two states with diagonal Hamiltonians 11.5 Moessfit models 11.5.1 FeSe_1-xS_x(Li_1-zFe_zOH) ZF, standard 11.5.2 FeSe_1-xS_x(Li_1-zFe_zOH) ZF, 4 fractions 11.5.3 FeSe_1-xS_x(Li_1-zFe_zOH) Pinning 11.5.4 FeSe_1-xS_x(Li_1-zFe_zOH) TF 11.5.5 FeSe_1-xS_x(Li_1-zFe_zOH) CS-Vzz-MEM 11.5.6 LaFeP_1-xAs_x+ ferrocene, ZF 11.5.7 LaFeP_1-xAs_x+ ferrocene, LF 11.5.8 LaFeP_1-xAs_x+ iron foil, ZF 11.5.9 LaFeAsO ZF 11.5.10 LaFeAsO TF 11.5.11 CuFeAs + ferrocen, ZF 11.5.12 CuFeAs + ferrocen, ZF, high statistics 11.5.13 CuFeAs + ferrocen, LF 11.5.14 CuFeAs + ferrocen, TF 11.5.15 CuFeSb ZF 11.5.16 CuFeSb TF

La déstabilisation de l'ordre antiferromagnétique de Néel au profit de nouvelles phases quantiques à température nulle à deux dimensions est envisageable grâce au phénomène de frustration magnétique. Le modèle théorique de spins Heisenberg S=1/2 répartis sur le réseau bidimensionnel frustré kagome, constitué de triangles joints uniquement par leurs sommets, est susceptible de stabiliser des phases quantiques originales de liquides de spin, qui ne présentent aucune brisure de symétrie à T = 0. Cette thèse a été consacrée à l'étude expérimentale de deux types de composés de spins S=1/2 (Cu2+) à géométrie kagome à l'aide de techniques spectroscopiques locales, la RMN et la μSR, ainsi que de mesures thermodynamiques (susceptibilité magnétique, chaleur spécifique). Dans Mg-herbertsmithite, la frustration est générée par une interaction d'échange premiers voisins antiferromagnétique J et est responsable d'un comportement liquide de spin jusqu'à des températures de l'ordre de J/10000. Par rapport au composé isostructural antérieur, Zn-herbertsmithite, nous avons montré qu'il possédait des propriétés physiques similaires tout en permettant une caractérisation fine du taux de défauts de substitutions Cu/Mg. Nos expériences réalisées à partir d'échantillons contrôlés permettent d'étudier finement l'origine des plateaux de relaxation observés en μSR à basse température en lien avec l'existence des défauts de spins interplans. La kapellasite et l'haydéite possèdent des interactions ferromagnétiques (J1) et antiferromagnétiques (Jd), offrant la possibilité d'explorer le diagramme de phases générées par la compétition de ces interactions sur le réseau kagome. Pour la kapellasite, nos mesures de μSR démontrent le caractère liquide de spin jusqu'à T ≈ J1/1000. La dépendance en température de la susceptibilité magnétique sondée par RMN du 35Cl ainsi que de la chaleur spécifique permettent d'évaluer le rapport Jd/J1 = 0.85, qui localise classiquement son fondamental au sein d'une phase originale de spins non coplanaires à 12 sous-réseaux appelée cuboc2. Les interactions présentes dans l'haydéite localisent son fondamental au sein de la phase ferromagnétique, en bon accord avec nos mesures qui indiquent une transition partielle à caractère ferromagnétique à T = 4 K. Cette étude confirme la pertinence du réseau kagome frustré pour la stabilisation de phases quantiques originales et démontre l'existence d'une nouvelle phase liquide de spin sur ce réseau, distincte de celle attendue pour des spins couplés antiferromagnétiquement.

La déstabilisation de l’ordre antiferromagnétique de Néel au profit de nouvelles phases quantiques à température nulle à deux dimensions est envisageable grâce au phénomène de frustration magnétique. Le modèle théorique de spins Heisenberg S=1/2 répartis sur le réseau bidimensionnel frustré kagome, constitué de triangles joints uniquement par leurs sommets, est susceptible de stabiliser des phases quantiques originales de liquides de spin, qui ne présentent aucune brisure de symétrie à T = 0. Cette thèse a été consacrée à l’étude expérimentale de deux types de composés de spins S=1/2 (Cu2+) à géométrie kagome à l’aide de techniques spectroscopiques locales, la RMN et la μSR, ainsi que de mesures thermodynamiques (susceptibilité magnétique, chaleur spécifique). Dans Mg-herbertsmithite, la frustration est générée par une interaction d’échange premiers voisins antiferromagnétique J et est responsable d’un comportement liquide de spin jusqu’à des températures de l’ordre de J/10000. Par rapport au composé isostructural antérieur, Zn-herbertsmithite, nous avons montré qu’il possédait des propriétés physiques similaires tout en permettant une caractérisation fine du taux de défauts de substitutions Cu/Mg. Nos expériences réalisées à partir d’échantillons contrôlés permettent d’étudier finement l’origine des plateaux de relaxation observés en μSR à basse température en lien avec l’existence des défauts de spins interplans. La kapellasite et l’haydéite possèdent des interactions ferromagnétiques (J1) et antiferromagnétiques (Jd), offrant la possibilité d’explorer le diagramme de phases générées par la compétition de ces interactions sur le réseau kagome. Pour la kapellasite, nos mesures de μSR démontrent le caractère liquide de spin jusqu’à T ≈ J1/1000. La dépendance en température de la susceptibilité magnétique sondée par RMN du 35Cl ainsi que de la chaleur spécifique permettent d’évaluer le rapport Jd/J1 = 0.85, qui localise classiquement son fondamental au sein d’une phase originale de spins non coplanaires à 12 sous-réseaux appelée cuboc2. Les interactions présentes dans l’haydéite localisent son fondamental au sein de la phase ferromagnétique, en bon accord avec nos mesures qui indiquent une transition partielle à caractère ferromagnétique à T = 4 K. Cette étude confirme la pertinence du réseau kagome frustré pour la stabilisation de phases quantiques originales et démontre l’existence d’une nouvelle phase liquide de spin sur ce réseau, distincte de celle attendue pour des spins couplés antiferromagnétiquement. / Magnetic frustration helps destabilizing conventional Néel order at T = 0 in dimensions 2, and therefore allows the emergence of new original quantum phases. The S=1/2 Heisenberg Hamiltonian on the highly frustrated kagome lattice, which is made of corner-sharing triangles, is expected to stabilize such quantum states, including the spin liquid ones which do not break any symmetry even at T = 0. This thesis work focuses on the experimental study of two kinds of S=1/2 (Cu2+) kagome compounds using NMR and μSR local probes as well as thermodynamic measurements (magnetic susceptibility, specific heat).In Mg-herbertsmithite magnetic frustration occurs thanks to a first nearest-neighbor antiferromagnetic interaction J and is responsible for the spin liquid behavior observed down to T = J/10000. In comparison with the formerly known isostructural counterpart Zn-herbertsmithite, we showed that it shares similar physical magnetic properties while allowing sensitive structural refinements and therefore a control of the level of Cu/Mg substitutions defects. Our experiments performed on such well controlled materials allow us to investigate the origin of the dynamical relaxation in these compounds in relation with the existence of interplane spins defects. Kapellasite and haydeite possess both ferromagnetic (J1) and antiferromagnetic (Jd) interactions. They offer the possibility to explore the phase diagram generated by such competing interactions on the kagome lattice. For kapellasite, our μSR experiments evidenced a spin liquid character down to T ≈ J1/1000. We tracked the temperature dependence of the magnetic susceptibility probed by 35Cl-NMR as well as of the specific heat, from which the ratio Jd/J1 = 0.85 can be evaluated. This ratio locates the ground-state of kapellasite to be within an original non-coplanar spin phase described by 12 magnetic sublattices and called cuboc2. Magnetic exchanges in haydeite locate its ground-state within the ferromagnetic phase. Both our local and thermodynamic measurements point to a partial ferromagnetic transition at T = 4 K. This study confirms the relevance of the frustrated quantum kagome lattice to stabilize original quantum phases and suggests the existence of a new spin liquid phase, distinct from the one expected for antiferromagnetically coupled spins.