Anisotropy and Ising-like transition of the S=5/2 two-dimensional Heisenberg antiferromagnet Mn-formate di-Urea

Recently reported measurements of specific heat on the compound Mn-formate di-Urea (Mn-f-2U) by Takeda et al. [Phys. Rev. B 63, 024425 (2001)] are considered. As a model to describe the overall thermodynamic behavior of such compound, the easy-axis two-dimensional Heisenberg antiferromagnet is proposed and studied by means of the 'pure quantum self-consistent harmonic approximation' (PQSCHA). In particular it is shown that, when the temperature decreases, the compound exhibits a crossover from 2D-Heisenberg to 2D-Ising behavior, followed by a 2D-Ising-like phase transition, whose location allows to get a reliable estimate of the easy-axis anisotropy driving the transition itself. Below the critical temperature T_N=3.77 K, the specific heat is well described by the two-dimensional easy-axis model down to a temperature T*=1.47 K where a T^3-law sets in, possibly marking a low-temperature crossover of magnetic fluctuations from two to three dimensions.Comment: 3 pages, 2 figures, 47th Annual Conference on Magnetism and Magnetic Materials (Tampa, FL, USA, 11-15/11/2002

Conductivity, weak ferromagnetism and charge instability in α−MnS\alpha-MnS single crystal

The temperature dependence of resistivity, magnetization and electron-spin resonance of the $\alpha- MnS$ single crystal were measured in temperature range of $5 K < T < 550 K$. Magnetization hysteresis in applied magnetic field up to 0.7 T at $T=5 K, 77 K, 300 K$, irreversible temperature behavior of magnetization and resistivity were found . The obtained data were explained in terms of degenerate tight binding model using random phase approximation. The contribution of holes in $t_{2g}$ and $e_g$ bands of manganese ions to the conductivity, optical absorbtion spectra and charge instability in $\alpha -MnS$ were studied. Charge susceptibility maxima resulted from the competition of the on-site Coulomb interaction between the holes in different orbitals and small hybridization of sub-bands were calculated at $T=160 K, 250 K, 475 K$.Comment: 6 pages, 12 figure

Spin-Glass State in CuGa2O4\rm CuGa_2O_4

Magnetic susceptibility, magnetization, specific heat and positive muon spin relaxation (\musr) measurements have been used to characterize the magnetic ground-state of the spinel compound $\rm CuGa_2O_4$. We observe a spin-glass transition of the S=1/2 $\rm Cu^{2+}$ spins below $\rm T_f=2.5K$ characterized by a cusp in the susceptibility curve which suppressed when a magnetic field is applied. We show that the magnetization of $\rm CuGa_2O_4$ depends on the magnetic histo Well below $\rm T_f$, the muon signal resembles the dynamical Kubo-Toyabe expression reflecting that the spin freezing process in $\rm CuGa_2O_4$ results Gaussian distribution of the magnetic moments. By means of Monte-Carlo simulati we obtain the relevant exchange integrals between the $\rm Cu^{2+}$ spins in this compound.Comment: 6 pages, 16 figure

Spin-Glass State in CuGa2O4\rm CuGa_2O_4

Magnetic susceptibility, magnetization, specific heat and positive muon spin relaxation (\musr) measurements have been used to characterize the magnetic ground-state of the spinel compound $\rm CuGa_2O_4$. We observe a spin-glass transition of the S=1/2 $\rm Cu^{2+}$ spins below $\rm T_f=2.5K$ characterized by a cusp in the susceptibility curve which suppressed when a magnetic field is applied. We show that the magnetization of $\rm CuGa_2O_4$ depends on the magnetic histo Well below $\rm T_f$, the muon signal resembles the dynamical Kubo-Toyabe expression reflecting that the spin freezing process in $\rm CuGa_2O_4$ results Gaussian distribution of the magnetic moments. By means of Monte-Carlo simulati we obtain the relevant exchange integrals between the $\rm Cu^{2+}$ spins in this compound.Comment: 6 pages, 16 figure

Spatially resolved variations in reflectivity across iron oxide thin films

The spin polarising properties of the iron oxide magnetite (Fe3O4) make it attractive for use in spintronic devices, but its sensitivity to compositional and structural variations make it challenging to prepare reli- ably. Infrared microspectroscopy and modelling are used to determine the spatial variation in the chem- ical composition of three thin films of iron oxide; one prepared by pulsed laser deposition (PLD), one by molecular beam epitaxy (MBE) deposition of iron whilst simultaneously flowing oxygen into the chamber and one by flowing oxygen only once deposition is complete. The technique is easily able to distinguish between films which contain metallic iron and different iron oxide phases as well as spatial variations in composition across the films. The film grown by post-oxidising iron is spatially uniform but not fully oxi- dised, the film grown by simultaneously oxidising iron showed spatial variation in oxide composition while the film grown by PLD was spatially uniform magnetite

Условия синтеза, кристаллическая структура и магнитные свойства селенидов Mn–Tm–Se

Single-phase compositions in the MnSe–TmSe quasi-binary section have been synthesized by the method of reactions in the solid phase. The crystal structure of polycrystalline powders has been studied in CuKα-radiation. It was found that the samples in the concentration range 0 x 0.7 have a cubic structure of the space group Fm m3 . An increase in the concentration of Tm cations in the Mn1–xTmxSe compositions leads to an increase in the unit cell parameter a from 0.547 nm for the Mn0.975Tm0.025Se compound to 0.566 nm for the Mn0.3Tm0.7Se composition. Thin layers of Mn1–xTmxSe solid solutions were synthesized by the flash method on optically transparent glass substrates. The film thicknesses are in the range of values from 0.8 to 3.2 µm. It has been established that Mn1–xTmxSe films also have the system NaCl, S.G.: Fm m3 . The composition of the Mn1–xTmxSe films corresponds to the chemical composition of the MnSe–TmSe charge powders. In the temperature range ~ 80–900 K, the va lues of the specific magnetization and magnetic susceptibility of the studied selenides were measured. The results obtained make it possible to determine the temperature regimes for the synthesis of new magnetic semiconductor substances, including those in the film state. The synthesized substances can be used in multifunctional microelectronic devices, as well as in the development of new materials capable of operating in wide temperature ranges and under the influence of external magnetic fields.Методом реакций в твердой фазе синтезированы однофазные составы в квазибинарном разрезе MnSe–TmSe. Кристаллическая структура поликристаллических порошков изучена в CuKα-излучении. Установлено,что образцы в интервале концентраций 0 x 0,7 имеют кубическую структуру пространственной группы Fm m3 . Изменение концентрации катионов Tm в составах Mn1–xTmxSe приводит к увеличению параметра a элементарной кристаллической ячейки от 0,547 нм для состава Mn0,975Tm0,025Se до 0,566 нм у состава Mn0,3Tm0,7Se. Методом «flash» на подложках оптически прозрачного стекла синтезированы тонкие слои твердых растворов Mn1–xTmxSe. Толщины пленок заключены в интервале значений от 0,8 до 3,2 мкм. Установлено, что пленки Mn1–xTmxSe также обладают синго-нией NaCl, S.G.: Fm m3 . Состав пленок Mn1–xTmxSe соответствует химическому составу порошков шихты MnSe–TmSe. В интервале температур ~ 80–900 К измерены величины удельной намагниченности и магнитной восприимчивости исследуемых селенидов. Полученные результаты позволяют определить температурные режимы синтеза новых магнитных полупроводниковых веществ, в том числе в пленочном состоянии. Синтезированные вещества могут быть использованы в устройствах микроэлектроники многофункционального назначения, а также при разработке новых материалов, способных работать в широких интервалах температур и при воздействии внешних магнитных полей

МАГНИТОСОПРОТИВЛЕНИЕ И ЭФФЕКТ ХОЛЛА В ТВЕРДОМ РАСТВОРЕ Mn0,55V0,45S

In the 80-300 K temperature range and magnetic fields with induction of up to 2.1 T are studied the characteristics of magnetoresistive properties and Hall effect of Mn0,55V0,45S solid solution. It was found that the Mn0,55V0,45S composition is a semiconductor with high p-type carrier concentration and low values of their mobility; a magnetoresistive effect is observed; solid solution has a noncollinear antiferromagnetic structure at temperatures ranges T &lt; TN = 130 K; in the vicinity of the temperature T ~ 180 K in Mn0,55V0,45S there is a phase transition of semiconductor-semimetal type due to delocalization of charge carriers and the formation of micro areas with ferromagnetic ordering in an antiferromagnetic matrix. Magnetoresistive effect in this case, most likely is due to the magnetic inhomogeneity and can be interpreted in the framework of the electronic and magnetic phase separation consistent with the theory of current flow in heavily doped semiconductors.В интервале температур 80-300 К и магнитных полях с индукцией до 2,1 Тл изучены особенности магнито¬резистивных свойств и эффекта Холла твердого раствора Mn0,55V0,45S. Установлено, что состав Mn0,55V0,45S является полупроводником с высокими значениями концентрации носителей заряда р-типа и низкими величинами их подвижности; обладает магниторезистивным эффектом; имеет неколлинеарную антиферромагнитную структуру в области температур Т &lt; ТN = 130 К; в окрестности температуры Т ~ 180 К в Mn0 55V0 45S имеет место фазовое превращение типа полупроводник-полуметалл, обусловленное делокализацией носителей заряда и образованием микрообластей с ферромагнитным упорядочением в антиферромагнитной матрице. Магниторезистивный эффект в этом случае, вероятнее всего, обусловлен магнитной неоднородностью и может быть проинтерпретирован в рамках модели электронного и магнитного разделения фаз, согласующейся с теорией протекания тока в сильно легированных полупроводниках