Influence of intermartensitic transitions on transport properties of Ni2.16Mn0.84Ga alloy

Magnetic, transport, and x-ray diffraction measurements of ferromagnetic shape memory alloy Ni$_{2.16}$Mn$_{0.84}$Ga revealed that this alloy undergoes an intermartensitic transition upon cooling, whereas no such a transition is observed upon subsequent heating. The difference in the modulation of the martensite forming upon cooling from the high-temperature austenitic state [5-layered (5M) martensite], and the martensite forming upon the intermartensitic transition [7-layered (7M) martensite] strongly affects the magnetic and transport properties of the alloy and results in a large thermal hysteresis of the resistivity $\rho$ and magnetization $M$. The intermartensitic transition has an especially marked influence on the transport properties, as is evident from a large difference in the resistivity of the 5M and 7M martensite, $(\rho_{\mathrm{5M}} - \rho_{\mathrm{7M}})/\rho _{\mathrm{5M}} \approx 15$, which is larger than the jump of resistivity at the martensitic transition from the cubic austenitic phase to the monoclinic 5M martensitic phase. We assume that this significant difference in $\rho$ between the martensitic phases is accounted for by nesting features of the Fermi surface. It is also suggested that the nesting hypothesis can explain the uncommon behavior of the resistivity at the martensitic transition, observed in stoichiometric and near-stoichiometric Ni-Mn-Ga alloys.Comment: 7 pages, 6 figures, REVTEX

Ferromagnetism in (In,Mn)As Diluted Magnetic Semiconductor Thin Films Grown by Metalorganic Vapor Phase Epitaxy

In1-xMnxAs diluted magnetic semiconductor (DMS) thin films have been grown using metalorganic vapor phase epitaxy (MOVPE). Tricarbonyl(methylcyclopentadienyl)manganese was used as the Mn source. Nominally single-phase, epitaxial films were achieved with Mn content as high as x=0.14 using growth temperatures Tg>475 C. For lower growth temperatures and higher Mn concentrations, nanometer scale MnAs precipitates were detected within the In1-xMnxAs matrix. Magnetic properties of the films were investigated using a superconducting quantum interference device (SQUID) magnetometer. Room-temperature ferromagnetic order was observed in a sample with x=0.1. Magnetization measurements indicated a Curie temperature of 333 K and a room-temperature saturation magnetization of 49 emu/cm^3. The remnant magnetization and the coercive field were small, with values of 10 emu/cm^3 and 400 Oe, respectively. A mechanism for this high-temperature ferromagnetism is discussed in light of the recent theory based on the formation of small clusters of a few magnetic atoms.Comment: 5 pages, 5 figures, accepted for publication in JVST

Magnetic field dependence of galfenol elastic properties

Elastic shear moduli measurements on Fe100−xGax (x = 12–33) single crystals (via resonant ultrasound spectroscopy) with and without a magnetic field and within 4–300 K are reported. The pronounced softening of the tetragonal shear modulus c′ is concluded to be, based on magnetoelastic coupling, the cause of the second peak in the tetragonal magnetostriction constant λ100 near x = 28. Exceedingly high ΔE effects ( ∼ 25%), combined with the extreme softness in c′ (c′\u3c10 GPa), suggest structural changes take place, yet, gradual in nature, as the moduli show a smooth dependence on Ga concentration, temperature, and magnetic field. Shear anisotropy (c44/c′) as high as 14.7 was observed for Fe71.2Ga28.8

Strongly nonlinear dynamics of electrolytes in large ac voltages

We study the response of a model micro-electrochemical cell to a large ac voltage of frequency comparable to the inverse cell relaxation time. To bring out the basic physics, we consider the simplest possible model of a symmetric binary electrolyte confined between parallel-plate blocking electrodes, ignoring any transverse instability or fluid flow. We analyze the resulting one-dimensional problem by matched asymptotic expansions in the limit of thin double layers and extend previous work into the strongly nonlinear regime, which is characterized by two novel features - significant salt depletion in the electrolyte near the electrodes and, at very large voltage, the breakdown of the quasi-equilibrium structure of the double layers. The former leads to the prediction of "ac capacitive desalination", since there is a time-averaged transfer of salt from the bulk to the double layers, via oscillating diffusion layers. The latter is associated with transient diffusion limitation, which drives the formation and collapse of space-charge layers, even in the absence of any net Faradaic current through the cell. We also predict that steric effects of finite ion sizes (going beyond dilute solution theory) act to suppress the strongly nonlinear regime in the limit of concentrated electrolytes, ionic liquids and molten salts. Beyond the model problem, our reduced equations for thin double layers, based on uniformly valid matched asymptotic expansions, provide a useful mathematical framework to describe additional nonlinear responses to large ac voltages, such as Faradaic reactions, electro-osmotic instabilities, and induced-charge electrokinetic phenomena.Comment: 30 pages, 17 eps-figures, RevTe

Coexistence of ferro- and antiferromagnetic order in Mn-doped Ni2_2MnGa

Ni-Mn-Ga is interesting as a prototype of a magnetic shape-memory alloy showing large magnetic field induced strains. We present here results for the magnetic ordering of Mn-rich Ni-Mn-Ga alloys based on both experiments and theory. Experimental trends for the composition dependence of the magnetization are measured by a vibrating sample magnetometer (VSM) in magnetic fields of up to several tesla and at low temperatures. The saturation magnetization has a maximum near the stoichiometric composition and it decreases with increasing Mn content. This unexpected behaviour is interpreted via first-principles calculations within the density-functional theory. We show that extra Mn atoms are antiferromagnetically aligned to the other moments, which explains the dependence of the magnetization on composition. In addition, the effect of Mn doping on the stabilization of the structural phases and on the magnetic anisotropy energy is demonstrated.Comment: 4 pages, 3 figure

Метанопродукция мезофильных бактериальных сообществ, выделенных из отходов животноводческих комплексов

Проблематика. Розглянуто перспективні субстрати для розробки біологічних препаратів у технологіях одержання біогазу. Мета дослідження. Метою роботи є виділення з відходів тваринництва (гноївки свиноферми, курячого посліду, ферментованих відходів) накопичувальних культур мезофільних анаеробних бактеріальних угруповань, оцінка їх продуктивності за метаном, виділення та ідентифікація культур метаногенних бактерій. Методика реалізації. Культури бактерій вирощували в анаеробних умовах на рідкому та агаризованому середовищах Жиліної з додаванням ацетату натрію за температури +33 ± 2 ºC. Склад газоподібних метаболітів аналізували на газовому хроматографі ЛХМ-8МД. Зразки культури, відібраної в потоці аргону, фіксували, забарвлювали за Грамом і мікроскопували. Результати дослідження. Всі виділені накопичувальні культури достатньо ефективно продукували метан, з 12-ї доби культивування у флаконах містилось 10–15 об. % метану. Наприкінці експерименту найвищі показники продукції метану (28 об. %) мали накопичувальні культури бактеріальних угруповань, виділені з ферментованих відходів тваринництва. Культури метаногенних бактерій одержали серією пересівів на рідке та тверде живильні середовища з додаванням амоксициліну. Було ідентифіковано представників родів Methanosarcina та Methanosaeta. Висновки. Одержані культури є перспективними для розробки бактеріальних препаратів та їх подальшого використання в біогазових технологіях.Background. The perspective substrates for elaboration of biopreparations in biogas technologies were considered. Objective. The aim of the study is selection from farm wastes (pig manure, chicken excrements and fermented farm wastes) storage cultures of mesophilous bacteria communities, comparative analysis of methane producing, selection and identification of methane-producing bacteria cultures. Methods. Bacteria cultures were grown in anaerobic conditions on liquid and solid Zhylina nutrient media with addition of sodium acetate under temperature +33 ± 2 ºC. Content of gaseous metabolites was estimated by using gas chromatograph LHM-8MD. Culture patterns were taken in argon flow, were fixed, Gram stained and viewed in light microscope. Results. All selected storage cultures efficiently produced methane, about 10—15 vol. % of methane has been accumulated in bottles since the 12th day of cultivation. Cultures selected from fermented wastes had high yields of methane emission is about 28 vol. %. Cultures of methane-producing bacteria were received by passages on liquid and solid nutrient media with addition of amoxicillin. Bacteria of Methanosarcina and Methanosaeta genus were identified. Conclutions. Obtained cultures are perspective ones for elaboration of bacterial preparations and their further using in biogas technologies.Проблематика. Рассмотрены перспективные субстраты для разработки биологических препаратов в технологиях получения биогаза. Цель исследования. Целью работы было выделение из отходов животноводства (навоза свинофермы, куриного помета, ферментированных отходов) накопительных культур мезофильных анаэробных бактериальных сообществ, оценка их продуктивности по метану, выделение и идентификация культур метанобразующих бактерий. Методика реализации. Культуры бактерий выращивали в анаеробных условиях на жидкой и агаризованной средах Жилиной с добавлением ацетата натрия при температуре +33 ± 2 ºC. Состав газообразных метаболитов определяли на газовом хроматографе ЛХМ-8МД. Образцы культуры, отобранные в потоке аргона, фиксировали, окрашивали по Грамму и микроскопировали. Результаты исследования. Все выделенные накопительные культуры достаточно эффективно продуцировали метан, на 12-е сутки культивирования во флаконах содержалось около 10–15 об. % метана. В конце эксперимента самые высокие показатели продукции метана (28 об. %) имели накопительные культуры бактериальных сообществ, выделенные из ферментированных отходов животноводства. Культуры метаногенных бактерий получили путем серии пересевов на жидкую и твердую питательные среды с добавлением амоксициклина. Были идентифицированы представители родов Methanosarcina и Methanosaeta. Выводы. Полученные культуры являются перспективными для разработки бактериальных препаратов и их последующего использования в биогазовых технологиях