000144900 001__ 144900 000144900 005__ 20240917143956.0 000144900 037__ $$aTESIS-2024-372 000144900 041__ $$aeng 000144900 1001_ $$aPardo Sainz, Miguel 000144900 24500 $$aNon-collinear magnetism in chiral, spinel and organic magnets 000144900 260__ $$aZaragoza$$bUniversidad de Zaragoza, Prensas de la Universidad$$c2024 000144900 300__ $$a318 000144900 4900_ $$aTesis de la Universidad de Zaragoza$$v2024-365$$x2254-7606 000144900 500__ $$aPresentado: 19 06 2024 000144900 502__ $$aTesis-Univ. Zaragoza, , 2024$$bZaragoza, Universidad de Zaragoza$$c2024 000144900 506__ $$aby-nc$$bCreative Commons$$c3.0$$uhttp://creativecommons.org/licenses/by-nc/3.0/es 000144900 520__ $$aEl magnetismo ha intrigado a los humanos durante siglos, pasando de los primeros descubrimientos de piedras imán hasta convertirse en un elemento fundamental en la física y la tecnología moderna, con un impacto significativo en la vida diaria y problemas globales como el cambio climático y la sostenibilidad energética. En los últimos años, el campo del magnetismo no colineal, que presenta alineaciones magnéticas únicas dentro de los materiales, ha emergido ofreciendo nuevas posibilidades para el desarrollo de estructuras magnéticas complejas y eficientes. En esta tesis doctoral, realizada bajo la supervisión conjunta de la Universidad de Zaragoza y la Universidad Prefectural de Osaka, consideramos tres factores que pueden contribuir al magnetismo no colineal: la interacción anti-simétrica de Dzyaloshinskii-Moriya (DM), la frustración magnética en las interacciones de intercambio de Heisenberg, y la anisotropía magnética debido al efecto del campo cristalino. Estos factores, condicionados por la simetría cristalina y la geometría, pueden dar lugar a texturas magnéticas topológicamente no triviales con una variedad de escalas de longitud y propiedades físicas. En concreto, esta tesis se centra en las fuentes de magnetismo no colineal en tres tipos clave de materiales magnéticos: quirales, espinelas y orgánicos, cada uno presentando desafíos únicos y oportunidades. El estudio de estos materiales abre la puerta no sólo a avances teóricos, sino también a revolucionarias aplicaciones tecnológicas, desde soluciones de almacenamiento de información de alta densidad y eficiencia energética hasta la vanguardia de la computación cuántica y el desarrollo de sensores de nueva generación.<br />La tesis está organizada en tres partes, cada una explorando una fuente diferente de magnetismo no colineal en materiales multifuncionales. Tras una introducción general sobre las técnicas experimentales, la primera parte está centrada en el papel de las interacciones DM en la generación de fases magnéticas no-colineales en los imanes cúbicos quirales MnSi y Fe0:75Co0:25Si.<br />- Para confirmar teorías previas sobre la existencia de nuevas fases magnéticas en MnSi, se han medido curvas de calor específico e imanación (d.c. y a.c.) en función del campo y la temperatura. Se observa claramente que la imanación a.c. presenta una anomalía, sugiriendo la existencia de una nueva fase que podría corresponder al estado desconocido predicho. Para comprender las posibles diferencias entre esta nueva fase (fase-B) y la fase de skyrmiones (fase-A), se han realizado experimentos de dispersión de neutrones a bajo ángulo (SANS) y de rotación de espín de muones (µSR). Los resultados indican que la fase-B podría ser compatible con una reorientación de las hélices magnéticas en MnSi. Además, los experimentos de µSR en la fase-A ponende manifiesto la necesidad de un estudio teórico detallado del perfil de la red de skyrmiones en MnSi.<br />- El efecto del desorden y distinta anisotropía en la existencia de nuevas fases magnéticas en el compuesto Fe0:75Co0:25Si también ha sido investigado mediante imanación d.c. y a.c., SANS y µSR. Los resultados revelan anomalías en la imanación a.c. similares a la observada en MnSi, así como una dependencia de la estabilidad de la fase-B con la historia y dirección del campo magnético, sugiriendo la influencia del desorden en el desarrollo de la fase-B. Los experimentos de SANS muestran que dicho desorden también altera tanto la estructura de la red de skyrmiones fase-A como su estabilidad. Además, permiten descartar el mismo origen para la fase-B que el propuesto en otros materiales. El análisis de los datos de µSR muestra una amplia distribución de campos locales, sugiriendo una combinación del rango de posibles sitios de implantación de muones y un aumento de las fluctuaciones magnéticas inducidas por el desorden sustitucional.<br />La siguiente parte explora el efecto combinado de la frustración y el desorden<br />en las propiedades magnéticas de las familias de espinelas Mn1 000144900 520__ $$aMagnetism has intrigued humans for centuries, transitioning from early discoveries of lodestones to a core element in modern technology and physics, impacting daily life and global issues such as climate change and energy sustainability. Recently, the field of non-collinear magnetism, featuring unique, tilted magnetic alignments within materials, has emerged, offering new potential for developing intricate and efficient magnetic structures. In this doctoral thesis, conducted in joint supervision between the University of Zaragoza and the Osaka Prefecture University, we consider three factors that can contribute to non-collinear magnetism: the antisymmetric Dzyaloshinskii-Moriya (DM) interaction, magnetic frustration in Heisenberg exchange interactions, and magnetic anisotropy due to the crystal field effect. These factors, influenced by crystal symmetry and geometry, can lead to topologically non-trivial magnetic textures with a variety of length scales and physical properties. Specifically, this thesis focuses on the sources of non-collinear magnetism in three key types of materials: chiral, spinel, and organic magnets, each presenting unique challenges and opportunities. The study of these materials opens the door not only to theoretical advances but also to revolutionary technological applications, from high-density and energy-efficient information storage solutions to the forefront of quantum computing and the development of next-generation sensors. The thesis is organized into three parts, each exploring a different source of non-collinear magnetism in multifunctional materials. After a general introduction to experimental techniques, the first part focuses on the role of antisymmetric DM interactions in generating non-collinear magnetic phases in the cubic chiral magnets MnSi and Fe0.75Co0.25Si. - To confirm previous theories about the existence of new magnetic phases in MnSi, specific heat and magnetization curves (d.c. and a.c.) were measured as a function of field and temperature. An anomaly is clearly observed in the a.c. magnetization, suggesting the existence of a new phase that could correspond to the predicted unknown state. To understand the possible differences between this new phase (B-phase) and the skyrmion phase (A-phase), experiments were carried out with neutron scattering (SANS) and muon spin rotation (muSR) techniques. The results indicate that the B-phase could be compatible with a reorientation of the magnetic helices in MnSi. Additionally, the muSR experiments in the A-phase highlight the need for a detailed theoretical study of the skyrmion lattice profile in MnSi. - The effect of disorder and different anisotropy on the presence of new magnetic phases in the compound Fe0.75Co0.25Si was also investigated using d.c. and a.c. magnetization, SANS, and muSR. The results reveal anomalies in the a.c. magnetization similar to those observed in MnSi, as well as a dependence of the stability of the B-phase on the history and direction of the magnetic field, suggesting the influence of chemical disorder in the development of the B-phase. The SANS experiments show that such disorder also alters both the structure of the skyrmion lattice (A-phase) and its stability. Additionally, they allow discarding the same origin for the B-phase as proposed in other materials. The analysis of the muSR data shows a wide distribution of local fields, suggesting a combination of the range of possible muon implantation sites and an increase in magnetic fluctuations induced by substitutional disorder. The next part explores the combined effect of frustration and disorder on the magnetic properties of the spinel families Mn1-xMgxCr2O4 and CuCr2-xSnxSe2S2. - The nuclear and magnetic structure of the spinel MnCr2O4 has been reinvestigated as a function of temperature in powder samples synthesized under different conditions, using magnetization, specific heat, and neutron powder diffraction (NPD) experiments. These experiments confirm the existence of three long-range order (LRO) magnetic phases; a ferrimagnetic (FIM) phase, an incommensurate spiral phase, and a new phase, never reported before. The symmetry of each magnetic phase has been determined using the formalism of magnetic superspace groups (MSSG). The transition temperatures of the three magnetic phases depend on the atmosphere in which the samples were synthesized. A possible explanation for these transitions has been discussed based on experimental and theoretical results. Finally, the presence of transverse conical magnetic structures allows for the existence of multiferroicity, and the direction of electric polarization has been derived using the spin current model. - Next, the effect of magnetic dilution on the frustration of the Mn1-xMgxCr2O4 family has been studied. NPD experiments similar to those performed for MnCr2O4 (x = 0) show that the nuclear symmetry is analogous to that of said compound over a wide range of temperatures. For low Mg contents, the same FIM and incommensurate spiral phases, both long-range, are observed. As the Mg content increases, the FIM phase is no longer observed, while the long-range order of the spiral phase is destroyed, forming small clusters of short-range order. The evolution of magnetic order with x agrees with previous studies, explained by the weakening of exchange interactions and the compression of Cr-O octahedra as the Mg content increases. - A structural and magnetic characterization has been carried out in solid solutions of the CuCr2-xSnxSe2S2 family using d.c. and a.c. magnetization, high-resolution transmission electron microscopy (HRTEM), and NPD experiments. The crystal structure of these spinel series has been refined, revealing a new and unexpected monoclinic secondary phase. The magnetism of these compounds is understood as the result of a random competition between ferromagnetic Cr3+ -Cr4+ interactions (double exchange process) and antiferromagnetic Cr3+- Cr3+ interactions (superexchange process). Our results allow proposing a long-range FM order for samples with x < 0.4. In samples with a relatively high Sn concentration (x > 0.4), frustration and random diamagnetic dilution by Sn suppress the long-range FM order, replacing it with spin glass-like behavior. In the last part, we examine the evolution of the magnetic properties with dimensionality in the purely organic magnets 4-F-2-NNBIP and TNN·CH3CN, which exhibit a combination of low anisotropy and frustration. - The organic compound 4-F-2-NNBIP has been characterized using X-ray diffraction, electron paramagnetic resonance (EPR), specific heat, magnetization, and susceptibility measurements at high magnetic fields and very low temperatures. Numerical calculations have also been performed to obtain the relevant magnetic parameters of the system. The joint analysis of diffraction data and EPR measurements implies that this system can be described by a two-leg antiferromagnetic Heisenberg ladder with S = 1/2. The magnetic exchange interactions have been estimated from the fit of the susceptibility data using quantum Monte Carlo (QMC) and exact diagonalization (ED) calculations. With the help of specific heat data, we propose a phase diagram with 3 distinct magnetic phases: a quantum disordered (QD) or spin liquid phase, a phase with gapless excitations (Luttinger liquid (LL) or quantum critical paramagnet (QC)), and a fully polarized (SP) phase. In addition, a region is observed where magnetization data deviate from theory, and several models to account for this deviation are proposed. - Finally, the organic compound TNN·CH3CN has been studied, focusing on the determination of its magnetic structure at low magnetic fields using single crystal neutron diffraction (SCND), polarized neutron diffraction (PND), and muSR experiments. At zero field, the magnetic signal is too weak to be observed in SCND experiments. Nevertheless, by making use of a computational model and DFT calculations, the absence of oscillations in the muSR spectra is shown to be compatible with high-symmetry magnetic structure at the muon site. The proposed magnetic configuration is consistent with previous studies, as well as with theoretical predictions of its multiferroic nature. The trimer formation is also explored at the 1/3 plateau (1.25 < B < 8.49 T). After a low-temperature magnetic and structural characterization, the spin density distribution has been obtained from PND experiments by using the wavefunction and multipole approach. The computational model used to interpret the muSR spectra agrees with the PND results and the theoretical ground state, where the magnetic moment in the TNN molecule is equally distributed among the three NN radicals. In conclusion, novel magnetic phases have been identified in chiral magnets, highlighting the critical role of DM antisymmetric interactions and the intricate interplay between disorder, anisotropy and temperature in dictating the stability and characteristics of such phases. The findings on magnetic spinels have revealed how magnetic frustration and disorder intertwine to influence the material's magnetic properties and manifest complex magnetic behaviors. The results on organic magnets have not only bridged theoretical predictions with experimental validation, but also exemplified the sophisticated interplay of structure, magnetism, and topology in low-dimensional organic systems. These findings collectively underscore the multifaceted origins of non-collinear magnetism, propelled by intrinsic and extrinsic factors, and underscore the potential for topologically non-trivial magnetic textures across different material systems.<br /> 000144900 521__ $$97076$$aPrograma de Doctorado en Física 000144900 540__ $$9info:eu-repo/semantics/openAccess 000144900 6531_ $$amagnetismo 000144900 6531_ $$apropiedades magnéticas de los sólidos 000144900 6531_ $$afísica del estado sólido 000144900 691__ $$a7 9 000144900 692__ $$aAsegurar el acceso a energías asequibles, fiables, sostenibles y modernas para todos. Desarrollar infraestructuras resilientes, promover la industrialización inclusiva y sostenible, y fomentar la innovación. 000144900 700__ $$aHosokoshi, Yuko $$edir. 000144900 700__ $$aCampo Ruiz, Jesús Javier $$edir. 000144900 7102_ $$aUniversidad de Zaragoza$$b 000144900 830__ $$9488 000144900 8560_ $$fcdeurop@unizar.es 000144900 8564_ $$s41339966$$uhttps://zaguan.unizar.es/record/144900/files/TESIS-2024-372.pdf$$zinfo:eu-repo/semantics/openAccess 000144900 8564_ $$s24765778$$uhttps://zaguan.unizar.es/record/144900/files/TESIS%202024-372%20CORREGIDA.pdf$$zinfo:eu-repo/semantics/openAccess 000144900 909CO $$ooai:zaguan.unizar.es:144900$$pdriver 000144900 909co $$ptesis 000144900 9102_ $$aCiencias$$b 000144900 980__ $$aTESIS