Магнітні та теплові властивості метал-органічних функціональних матеріалів

Abstract

Актуальність теми обумовлена тим, що дослідження низькорозмірних систем зі спіном ½ має широку зацікавленість у сучасному науковому співтоваристві завдяки своїм нетиповим магнітним властивостям. Основна мета цієї роботи полягає в розгляді фізичних властивостей квазінизькорозмірного Гайзебергового антиферомагнетику, а саме сполука [H2N-CH2-CH2-CH2-NH2][Cu(C6H2(COO)4)]·2H2O, методів та способів його отримання, аналізу кристалічної структури та основних магнітних властивостей. Окрім цього, метою є також синтезувати зразок у формі порошку та дослідити його експериментально за допомогою різних моделей та симуляцій. Під час виконання роботи було використано різні методи дослідження (наприклад апроксимація, числове інтегрування і т.д.) експериментальних даних, які були отримані на приладах PPMS та MPMS. У результаті проведених наукових досліджень установлено, що конфігурація водневих зв'язків сполуки сполука [H2N-CH2-CH2-CH2-NH2][Cu(C6H2(COO)4)]·2H2O впливає на основні магнітні та теплові властивості та призводить до того, що система набуває 3D характеристик. Окрім цього, приведенні чисельні значення для величин, що були виміряні в ході експериментів над порошковим зразком. Отримані наукові результати можна використати у вивченні квантових магнітів, які в свою чергу є невід’ємною складовою таких галузей як спінтроніка, наномедицина, квантові обчислення, зберігання інформації тощо.

The study of low-dimensional systems with spin S=1 2 plays a significant role in the scientific community because of its unconventional magnetic properties. One of the best examples of low-dimensional systems are compounds that contain pyromellitic acid in their crystal structure and a specific chemical unit that joins magnetic planes. Therefore, it can be stated that the resulting properties are mainly influenced by the anisotropy in exchange interaction and the spatial distribution of hydrogen bonds. The purpose of this work is to understand how a different type of spatial distribution of hydrogen bonds and anisotropic exchange interaction in lowdimensional structures may affect physical and magnetic properties of the studied systems. Actuality: Experimental studies of magnetic properties of low-dimensional systems with spin 𝑆= 1 2 has been attracting the attention of the scientific community for a long period of time. The interaction between quantum fluctuations associated with low spin and low spatial dimension and other phenomena leads to untypical magnetic properties. On the one hand, the presence of the exchange interaction in the Heisenberg antiferromagnet also plays an important role in the behavior of the investigated systems. On the other hand, in two-dimensional (2D) magnetic systems exchange interactions are creating different types of spatial anisotropies. As one of the recent examples of such systems is Cu[C6H2(COO)4][C2H5NH3]2. It was characterized by heat capacity, magnetization and magnetic susceptibility measurements as a Heisenberg antiferromagnet on a rectangular lattice with a spatial anisotropy value of 0.7. Furthermore, slightly different system (C2H10N2)[Cu(C10H2O8)]·3H2O was studied and it was also shown as a Heisenberg antiferromagnet on a rectangular lattice but with different value of spatial anisotropy parameter 0.44. Consequently, detailed studying similar systems is of great necessity for solving issues in modern applied physics. From the above follows the obvious relevance and importance of this work. The synthesis of such structures, a broad study of its properties, performing various experiments, detailed analysis of similar systems are all major objectives of physics as a science nowadays. That is why it is pretty evident that my diploma paper is of great scientific and practical value. The object of the study: [H3N-CH2-CH2-CH2-NH3][Cu(C6H2(COO)4)]·2H2O low-dimensional system with spin 𝑆 = 1 2 . The subject of the study: physical and magnetic properties of powder sample [H3N-CH2-CH2-CH2-NH3][Cu(C6H2(COO)4)]·2H2O. The objectives of the research:  to describe a theoretical background behind low-dimensional systems and well-known models;  to obtain a powder sample applying previously designed method;  to analyze a structure of the system and its hydrogenic bonds configuration;  to conduct several experiments on powder sample;  to compute and process the obtained results from magnetic properties’ measurements. Methods: literature review of actual publications and resources, synthesis of powder sample, performing structural analysis and description of the obtained samples’ properties. The structure and scope of diploma for the master’s degree: the paper consists of introduction, three chapters, conclusion and references, which contains 20 names. The total volume of diploma paper for the master’s degree is 50 pages, including 30 Figures and 3 Tables. The first chapter deals with the fundamental theoretical information on the studied topic. Namely, the greatest emphasis of this section is made on the overview of the main features of low-dimensional systems with low spin, various models of spatial anisotropy in the exchange interaction and various configurations that it creates are explained and described in detail. In addition, various data of quantum Monte Carlo simulations for different properties are presented in the form of figures, which were then used to analyze the real results of our experiments. The second chapter of this work consists of two main parts. The first one points out the technique of conducting main experiments, particularly, about the devices and tools that were used during our research. The conditions under which the experiments were carried out, including temperature and magnetic fields, are separately indicated for each experiment. In addition, the methods of measuring heat capacity and magnetic susceptibility using these devices are described in detail. The second part of the same chapter contains information of conducted diffraction analysis of the synthesized powder of the system. Further data, including numerical data, on the structure of this compound is also given. The third chapter presents various measurements results of the physical and magnetic properties of this sample and explanations of these results. Reasonably, they were divided into three blocks according to a certain value, that was studied: magnetic susceptibility, magnetization and heat capacity. The measurement results are presented in the form of dependence graphs (figures) and tables. The vast majority of the obtained data were compared, approximated or fitted using various models or assumptions. It is vital to note that the data obtained from various experiments interact with each other and with our initial predictions. In the course of this section, many important things were clarified and its possible explanation was provided, which then formed the basis of the conclusion of the work. CONCLUSION As part of this work, an analysis of the experimental data of magnetization, magnetic susceptibility and thermal capacity of the powder sample [H3N-CH2 CH2- CH2-NH3][Cu(C6H2(COO)4)]·2H2O was performed. The results of the measurements in the temperature range from 0.38 to 300 K in magnetic fields up to 9 T were obtained on commercial PPMS and MPMS devices. 1. The analysis of magnetic susceptibility data showed that the studied system [H3N-CH2 CH2-CH2-NH3][Cu(C6H2(COO)4)]·2H2O could be characterized using the Heisenberg model on a square lattice with the antiferromagnetic exchange interaction value J/kB=1.38 K with the corresponding value of the g-factor g=2.04. 2. At the same time, a good correlation for this model was observed based on the analysis of heat capacity data in zero magnetic field. However, the study of the temperature dependence of susceptibility and heat capacity in a zero magnetic field clearly showed the presence of a magnetic phase transition to a magnetically ordered state at a temperature of TN=1.20 K. 3. Therefore, the calculated value of the saturation magnetic field for the Heisenberg model on a square grid Bsat=4.05 T is an excellent correlation with the experimental value from the field dependence of magnetization at a temperature of 0.5 K. 4. However, it is crucial to note that based on the magnetic phase diagram in the studied system [H3N-CH2-CH2-CH2-NH3][Cu(C6H2(COO)4)]·2H2O a stronger influence of interplane interactions can be expected. 5. Compared to similar studied systems containing pyromellitic acid, it was found that a different type of spatial distribution of hydrogen bonds affects the properties of a Heisenberg antiferromagnet on a spatially anisotropic square lattice with spin S=1 2 .