Temperature induced structural and polarization features in BaFe12O19

We report the observation of a peculiar polarization behavior of BaFe12O19 in electric field where the linear polarization is detected at temperatures below 150 K whereas at higher temperatures a hysteresis-like polarization response is observed. At the same time, the performed neutron diffraction analysis shows no variations in crystal or magnetic structures with temperature. Based on the results of ab initio calculations we suggest the mechanism able to explain the experimentally observed behavior. We show that specific Fe atoms do not occupy the positions formally assigned to them by the conventional centrosymmetric P63/mmc (#194) space group (z = 0.25; 0.75) as these positions correspond to local energy maxima. Instead, these Fe atoms are shifted along the z-axis to positions z = 0.259 (0.241) and z = 0.759 (0.741), which correspond to local energy minima. To an inversion center move between these minima Fe atoms need to overcome an energy barrier. This barrier is rather insignificant for smaller volumes but it becomes larger for expanded volumes due to coupling between the displacements of these Fe atoms. Additionally, our analysis suggests that the non-centrosymmetric and polar P63mc (#186) space group could be appropriate for the description of the BaFe12O19 structure

Кристаллическая структура и магнитные характеристики твердых растворов Ni1–xCrxMnSb

The results of an experiment on studying the features of the crystal structure and magnetic studies of substitutional solid solutions of the Ni1–xCrxMnSb system (0 m х m 0.2) are presented. It was found that an increase in the concentration of chromium in solid solutions does not lead to significant changes in the size of the unit crystal cell. It was found that solid-phase quenching can be used to expand the limit of chromium solubility in Ni1–xCrxMnSb solid solutions. The temperature and field dependences of the specific magnetization of the synthesized compositions have been studied. It was found that the substitution of chromium for nickel in the NiMnSb compound leads to a decrease in the temperature of the “magnetic order – magnetic disorder” phase transformation with an increase in the concentration x from 0 to 20 mol.%. The values of specific magnetization and Curie temperature of hardened hard rasters are higher than those of slowly cooled ones. The results obtained contribute to the creation of a physical basis for the elemental base of spintronics.Представлены результаты эксперимента по изучению особенностей кристаллической структуры и магнитных исследований твердых растворов замещения системы Ni1–xCrxMnSb (0 m х m 0,2). Выявлено, что увеличение концентрации хрома в твердых растворах не приводит к значительным изменениям размеров элементарной кристаллической ячейки. Установлено, что с помощью твердофазной закалки удается расширить границу растворимости хрома в твердых растворах Ni1–xCrxMnSb. Изучены температурные и полевые зависимости удельной намагниченности синтезированных составов. Установлено, что замещение никеля хромом в соединении NiMnSb приводит к уменьшению температуры фазового превращения «магнитный порядок – магнитный беспорядок» при увеличении концентрации x от 0 до 20 мол.%. Величины удельной намагниченности и температуры Кюри закаленных твердых растров больше, чем в медленно охлажденных. Полученные результаты вносят вклад в создание физической основы для элементной базы спинтроники

Two different charge-separation pathways in photosystem II

Charge separation is an essential step in the conversion of solar energy into chemical energy in photosynthesis. To investigate this process, we performed transient absorption experiments at 77 K with various excitation conditions on the isolated Photosystem II reaction center preparations from spinach. The results have been analyzed by global and target analysis and demonstrate that at least two different excited states, (Ch

Особенности кристаллической структуры и магнитных характеристик твердых растворов систем Ni1–xMxMnSb (M = Fe, Co)

The results of studying the effect on the crystal structure and magnetic properties of compounds based on NiMnSb when Ni cations are replaced by Fe and Co cations are presented. The solid solution Ni1–xMxMnSb (M = Fe, Co) systems were synthesized by the method of solid-phase reactions. With the help of X-ray diffraction analysis, the presence of a concentration structural transition was established. Using the ponderomotive method, it was found that the temperatures of the “magnetic order – magnetic disorder” phase transformation decrease as nickel is replaced by iron and cobalt. The field dependences of the specific magnetization demonstrate the typical behavior of a magnetically soft ferromagnet. The results of an experiment on the study of the crystal and magnetic structure of solid solutions of the Ni0.90M0.10MnSb (M = Fe, Co) systems using thermal neutron diffraction in the temperature range of ~(3–300) K are presented. It is found that the studied compositions exhibit ferromagnetic ordering along the c axis. On the spectra of Ni0.90Co0.10MnSb solid solutions in the region of 2Θ  = 28.6° the appearance of a reflection indicating the formation of antiferromagnetic ordering is observed. Within the framework of the density functional theory, an ab initio calculation of the magnetic moments for Ni1-xMxMnSb (M = Fe, Co; x = 0; 0.125; 0.250) was carried out. The results of theoretical calculations predict the existence of magnetic moments for Fe and Co ions, and they are antiferromagnetically coupled to the spins of Mn and Ni ions.Представлены результаты исследования влияния на кристаллическую структуру и магнитные свойства соединений на основе NiMnSb при замещении катионов Ni на катионы Fe и Co. Твердые растворы систем Ni1–xMxMnSb (M = Fe, Co) синтезированы методом твердофазных реакций. С помощью рентгеноструктурного анализа установлено наличие концентрационного структурного перехода. Пондеромоторным методом обнаружено, что температуры фазового превращения «магнитный порядок – магнитный беспорядок» снижаются по мере замещения никеля железом и кобальтом. Полевые зависимости удельной намагниченности демонстрируют типичное поведение магнитомягкого ферромагнетика. Приведены результаты эксперимента по изучению кристаллической и магнитной структуры твердых растворов систем Ni0,90M0,10MnSb (M = Fe, Co) с помощью дифракции тепловых нейтронов в интервале температур (3–300) K. Обнаружено, что исследуемые составы обладают ферромагнитным упорядочением вдоль оси с. На спектрах твердых растворов Ni0,90Co0,10MnSb в области 2Θ = 28,6° наблюдается появление рефлекса, указывающего на формирование антиферромагнитного упорядочения. В рамках теории функционала плотности проведен ab initio расчет магнитных моментов для Ni1–xMxMnSb (M = Fe, Co; x = 0; 0,125; 0,250). Результаты теоретических расчетов предсказывают существование магнитных моментов у ионов Fe и Co, и они антиферромагнитно связаны со спинами ионов Mn и Ni

Магнитные характеристики твердых растворов Ni1–xMxMnSb (M = Ti, V, Cr)

The results of an experiment on the study of the crystal and magnetic structures of substitutional solid solutions of the Ni0.90M0.10MnSb (M = Ti, V, Cr) systems using thermal neutron diffraction in the temperature range ~ 3–300 K are presented. It is found that all the studied compositions have ferromagnetic ordering along the c axis. In the spectra of Ni0.90V0.10MnSb and Ni0.90Cr0.10MnSb solid solutions, a reflection appears in the region 2Θ = 28.6°, which indicates the formation of antiferromagnetic ordering. It has been found that this reflection disappears at a temperature T = 75 K in Ni0.90V0.10MnSb, while it is observed in the spectrum of Ni0.90Cr0.10MnSb over the entire temperature range under study. Within the framework of density functional theory (DFT), an ab initio calculation of the crystal structure and magnetic moments for Ni1–xMxMnSb (M = Ti, V, Cr; x = 0; 0.125; 0.250) was carried out. It has been established that titanium, vanadium, and chromium ions participate in electron transfer only with Mn and Sb ions. The DFT results predict the existence of magnetic moments for Ti, V, and Cr ions. It was found that the spins of Ti, V, and Cr ions are antiferromagnetically coupled with the spins of Mn and Ni ions. The results obtained are of interest for the development of new concepts and models of structural design, which makes it possible to synthesize fundamentally new functional materials with already specified physical properties.Представлены результаты эксперимента по изучению кристаллической и магнитной структур твердых растворов замещения систем Ni0,90M0,10MnSb (M = Ti, V, Cr) с помощью дифракции тепловых нейтронов в интервале температур ~ 3–300 К. Обнаружено, что все исследуемые составы обладают ферромагнитным упорядочением вдоль оси с. На спектрах твердых растворов Ni0,90V0,10MnSb и Ni0,90Сr0,10MnSb в области 2Θ = 28,6° отмечено появление рефлекса, который указывает на формирование антиферромагнитного упорядочения. Установлено, что данный рефлекс исчезает при температуре T = 75 К в Ni0,90V0,10MnSb, а на спектре Ni0,90Cr0,10MnSb он наблюдается во всем исследуемом диапазоне температур. В рамках теории функционала плотности (DFT) проведен ab initio расчет кристаллической структуры и магнитных моментов для Ni1–xMxMnSb (M = Ti, V, Cr; x = 0; 0,125; 0,250). Установлено, что ионы титана, ванадия и хрома участвуют в переносе электрона только с ионами Mn и Sb. Результаты DFT предсказывают существование магнитных моментов у ионов Ti, V и Cr. Обнаружено, что спины ионов Ti, V и Cr антиферромагнитно связаны со спинами ионов Mn и Ni. Полученные результаты представляют интерес для разработки новых концепции и моделей структурного дизайна, при котором возможен синтез принципиально новых функциональных материалов с уже заданными физическими свойствами

Spectral hole burning: examples from photosynthesis

The optical spectra of photosynthetic pigment–protein complexes usually show broad absorption bands, often consisting of a number of overlapping, ‘hidden’ bands belonging to different species. Spectral hole burning is an ideal technique to unravel the optical and dynamic properties of such hidden species. Here, the principles of spectral hole burning (HB) and the experimental set-up used in its continuous wave (CW) and time-resolved versions are described. Examples from photosynthesis studied with hole burning, obtained in our laboratory, are then presented. These examples have been classified into three groups according to the parameters that were measured: (1) hole widths as a function of temperature, (2) hole widths as a function of delay time and (3) hole depths as a function of wavelength. Two examples from light-harvesting (LH) 2 complexes of purple bacteria are given within the first group: (a) the determination of energy-transfer times from the chromophores in the B800 ring to the B850 ring, and (b) optical dephasing in the B850 absorption band. One example from photosystem II (PSII) sub-core complexes of higher plants is given within the second group: it shows that the size of the complex determines the amount of spectral diffusion measured. Within the third group, two examples from (green) plants and purple bacteria have been chosen for: (a) the identification of ‘traps’ for energy transfer in PSII sub-core complexes of green plants, and (b) the uncovering of the lowest k = 0 exciton-state distribution within the B850 band of LH2 complexes of purple bacteria. The results prove the potential of spectral hole burning measurements for getting quantitative insight into dynamic processes in photosynthetic systems at low temperature, in particular, when individual bands are hidden within broad absorption bands. Because of its high-resolution wavelength selectivity, HB is a technique that is complementary to ultrafast pump–probe methods. In this review, we have provided an extensive bibliography for the benefit of scientists who plan to make use of this valuable technique in their future research