Magnetic Phase Transitions in Substituted Spinels of Zn1−x Cu χ Cr2 Se4; 0.0 ≤ x ≤ 0.3

Magnetic and crystallographic properties have been studied by neutron powder diffraction and measurements of magnetization and magnetization hysteresis-loops for substituted spinels of Zn_1?xCu_xCr₂Se₄ with 0.0≤x≤0.3. It is found that the Zn_0.85Cu_0.15Cr₂Se₄ spinel has two magnetic phase transitions at 23.0 K (Néel temperature; T _N) and 410 K (Curie temperature; T _C) and that the Zn_0.70Cu_0.30Cr₂Se₄ spinel has magnetic transitions at 24.5 K (T _N) and 415 K (T _C) on heating. The low-temperature magnetic phase transition is from a spiral antiferromagnet to a ferromagnet, and the high-temperature magnetic phase transition is from a ferromagnet to a paramagnet, while ZnCr₂Se₄ shows a magnetic phase transition only from a spiral antiferromagnet to a paramagnet at about 21.0 K. From neutron powder diffraction, it is also found that the spinels of Zn_1?x Cu _x Cr₂Se₄; 0.0 ≤ x ≤ 0.3. show satellite-like magnetic reflection having indexes (h ± Q, k, l) with Q = 0.470 below T _N and short-range order of spins (spin glass-like) above T _N. The incommensurate antiferromagnetic phase below T _N results from a spiral long-range order of the spins of Cr³⁺. The intermediate ferromagnetic phase between T _N and T _C is related not to the spiral spin order but to double-exchange magnetic interaction among Cr³⁺ and Cr⁴⁺ mediated by current carriers, positive holes, which is made by the substitution of Zn²⁺ ions with Cu¹⁺ ions in Zn_1?x Cu _x Cr₂Se₄.     

On the mixed valence of the chromium ions in the noncollinear ferrimagnetic spinels Mn1−xCuxCr2S4 (Where x = 0.85, 0.90, 0.95)

For the polycrystalline samples of Mn_1?xCu_xCr₂S₄ (x = 0.85, 0.90, 0.95) the magnetization was measured in the temperature range between 77 K and the Curie temperature, T_C, using a magnetic balance (Faraday's method) and pulsed magnetic fields up to 2.0 T. The magnetic susceptibility was measured between T_C and about 600 K. The Curie temperatures were obtained using the kink point method.In the temperature range between 4.2 and 77 K the magnetization was measured in stationary magnetic fields up to 14 T. The data indicate a noncollinear ferrimagnetic structure. The compounds under investigation can be treated as CuCr₂S₄ slightly doped with Mn, with a valence distribution Mn²⁺_1?xCu¹⁺_xCr³⁺_2?xCr⁴⁺_xS^2?₄.     

Mössbauer effect study of Ni1−xCuxMnyFe2−yO4 system

The Mössbauer effect technique has been employed for the study of magnetic properties of spinel series Ni_1?xCu_xMn_yFe_2?yO₄ with 0.0≤x≤1.0, and y=0.6. The substitution of Mn³⁺ and Cu²⁺ ions results in a slight decrease of the hyperfine field at B‐ as well as A‐sites. The area ratio of Fe³⁺ ions at the A‐ and B‐site at 77 K indicates that Cu²⁺, Ni²⁺ and Mn³⁺ ions occupy the octahedral sites in an evidence for complete inverse spinel in this system. The temperature dependence of the hyperfine parameters has been studied for composition with x=0.5 where Nèel point T_N and Debye temperature θ_D are found to be 650 and 679 K, respectively. The temperature dependence of the sublattice magnetization σ(T) obeys a one‐third‐power law in the range 0.5N<0.99.     

Synthesis,X-ray,magnetic and electrical studies of substituted (Pr,Sr)MnO3 perovskites

A systematic study of the structural, magnetic and electrical properties of the manganites Pr_1-x Sr _x MnO₃(0≤ x ≤ 0.5) has been carried out. X-ray diffraction investigation shows a structural change with composition, from orthorhombic (0 ≤ x ≤ 0.2) to rhombohedral (0.25 ≤ x ≤ 0.5). The magnetic properties of Pr_1-x Sr _x MnO₃ samples could be explained on the basis of a double exchange mechanism between pairs of Mn³⁺ and Mn⁴⁺ ions. These properties are strongly dependent on the ratio of Mn³⁺/Mn⁴⁺. The maximum of the ferromagnetic transition temperature T_c is reached at x ≈ 0.35 corresponding to a value 1.85 of this ratio. The investigation of the electrical properties shows a semiconductor to metal transition as a function of temperature (0.25≤x≤0.4) with a metallic-like behaviour above a critical temperature T_p . A semiconducting-like one is observed for all the range of temperature (50–300 K) for (0 ≤ x ≤ 0.2 and x = 0.5). The evolution of activated energies with the carrier concentration has been investigated.     

Electrical properties of perovskite-type La(Cr1−xMnx)O3+δ

Perovskite-type La(Cr_1−xMn_x)O_3+δ (0.0x1.0) was synthesized using a sol–gel process. The crystal structure of La(Cr_1−xMn_x)O_3+δ changes from orthorhombic to rhombohedral at x=0.6. The Mn⁴⁺ ion content increases monotonically in the range 0.2x1.0. The magnetic measurement of La(Cr_1−xMn_x)O_3+δ indicates that a Mn³⁺ ion is a high-spin state with (d)³(dγ)¹. The variation of the average (Cr, Mn)-O distance is explained by ionic radii of the Cr³⁺, the Mn³⁺, the Mn⁴⁺ ions. Since the log σT–1/T curve is linear and the Seebeck coefficient (α) is independent of temperature, it is considered that La(Cr_1−xMn_x)O_3+δ is a p-type semiconductor and exhibits the hopping conductivity.     

Phase relationships and magnetic phase diagram in the system Mn1−xCuxCr2S4

Ternary spinel compounds have been found in the system Mn_1?xCu_xCr₂S₄ for x < 0.4 and x ? 0.8. The unit cell parameter does not follow a Vegard law. Magnetization measurements up to 150 kOe for Mn_0.95Cu_0.05Cr₂S₄ show three magnetic structures: Néel ferrimagnet, Yafet-Kittel and tridimensional. The magnetic phase diagram has been calculated within the molecular field approximation. There is strong evidence for the non-existence of Mn³⁺. Our findings are in conformity with a model proposed by Goodenough.     

Phase transitions and colossal magnetoresistance in CuVxCr1−x S2 layered disulfides

New substances CuV_xCr_1?x S₂ are synthesized in which colossal magnetoresistance (T _C = 95 K, δ_H = ?60%, H = 7 kOe), as well as the sequence of phase transitions with change in the conduction type and magnetic order, is observed as temperature is varied. The change found in the magnetic and electric properties of the CuV_xCr_1?x S₂ compounds may be a consequence of a specific disintegration into Cu⁺Cr³⁺S₂ and Cu²⁺Cr²⁺S₂ and a change in the concentration relation between these electronic phases in the substance bulk.     

Charge order melting and magnetic transition in Nd0.5(1+x)Ca0.5(1−x)Mn(1−x)CrxO3 system

The magnetic property of double doped manganite Nd_0.5(1+x)Ca_0.5(1−x)Mn_(1−x)Cr_xO₃ with a fixed ratio of Mn³⁺:Mn⁴⁺=1:1 has been investigated. For the undoped sample, it undergoes one transition from charge disordering to charge ordering (CO) associated with paramagnetic (PM)-antiferromagnetic (AFM) phase transition at T<250 K. The long range AFM ordering seems to form at 35 K, rather than previously reported 150 K. At low temperature, an asymmetrical M-H hysteresis loop occurs due to weak AFM coupling. For the doped samples, the substitution of Cr³⁺ for Mn³⁺ ions causes the increase of magnetization and the rise of T_c. As the Cr³⁺ concentration increases, the CO domain gradually becomes smaller and the CO melting process emerges. At low temperature, the FM superexchange interaction between Mn³⁺ and Cr³⁺ ions causes a magnetic upturn, namely, the second FM phase transition.     

Effect of Cu-Cr co-substitution on magnetic properties of nanocrystalline magnesium ferrite

This study deals with the temperature and composition dependence of magnetization and magnetic anisotropy of Cu²⁺-Cr³⁺ co-substituted magnesium ferrite, Mg_1−xCu_xCr_xFe_2−xO₄ (x=0.0-0.5). The synthesized materials are characterized using thermo gravimetric analysis, X-ray diffraction, scanning electron microscopy, energy dispersive X-ray fluorescence, Mössbauer spectrometer, superconducting quantum interference device magnetometer and vibrating sample magnetometer. The M-H loops measured up to 50 kOe at 300, 200 and 100 K, revealed narrow hysteresis curves with a coercive field and saturation magnetization varying for different compositions. The high field regimes of these loops are modeled using the Law of Approach to saturation to extract anisotropy information and saturation magnetization. Both the saturation magnetization and the anisotropy constant are observed to increase with the decrease in temperature while decrease with the Cu-Cr co-substituents for all the samples. Explanation of the observed behavior is proposed in terms of the preference of the co-substituent ions of Cu²⁺ and Cr³⁺ and their predominant choice to substitute into the octahedral sites of the cubic spinel lattice.     

A XPS study of Fe1-χCuχCr2S4

The X-ray photoelectron spectroscopy is applied for a chalcogen chromite system Fe_{1 ? x}Cu_xCr₂S₄ to obtain microscopic information about valence states of the constituting atoms. The features of the Cu 2p spectra show that copper is always in a monovalent state in this system. This supports a model Fe³⁺_{1 ? x}Cu¹⁺_xCr³⁺₂S^1-_{2x ? 1}S^2-_{5 ? 2x} for a compositional range 0.5 < x < 1.     

多晶Mn1-xCux(0.1≤x≤0.3)合金的磁诱发应变

采用X射线衍射分析、显微形貌观察、差示扫描量热法、标准电阻应变计法等实验方法,研究了室温下多晶Mn_1-xCu_x(0.1≤x≤0.3,原子分数)合金在低磁场中的磁诱发应变性能.结果表明,Mn_1-xCu_x合金经过长时间的固溶处理,在冷却过程中会出现fcc(γ)→fct(γ’)马氏体相变,形成(γ+γ 关键词： 磁诱发应变 MnCu合金 马氏体相变     

Al-doping-induced magnetocapacitance in the multiferroic CuCrS2

In this paper,magnetic and dielectric properties of the quasi-two-dimensional triangular-lattice system CuCrS2 and its B-site-diluted analogs CuAl1?xCrxS2(x = 0.01 and x = 0.02) are investigated.Antiferromagnetic phase transition is observed at about 38.5 K by magnetization measurement without shift induced by a small amount of doping Al.Magnetodielectric effect is found near TN in each of the three compounds.The dielectric constant decreases and the magnetocapacitance increases with the increase of substitution of nonmagnetic Al3+ ions for the magnetic Cr3+ ions.The negative magnetocapacitive effect reaches ～ 13% for CuAl0.02Cr0.98S2.     

Magnetic structure of cubic spinels MnxCr3−xO4 (x=1.0−1.6)

Neutron diffraction study on samples Mn_xCr_3?xO₄ with x=1.0, 1.2, 1.4 and 1.6 has been performed at temperatures below ～40 K where magnetic ordering is present. At 5 K the structure of the MnCr₂O₄ compound was found to be a general LKDM magnetic spiral with the propagation vector deviating by 13.3″ from the [110] axis. This results differs from previous findings. At about 16 K the structure becomes collinear. In samples higher content of manganese the magnetic structure is collinear over the whole temperature region (5–40) K, i.e. in the sense of long range ordering. However, a short range ordering of the transverse components of the spins is present both in these samples and in MnCr₂O₄ above 16 K.     

Thermal dependence of impurity induced fluorescence in the system RbMgxMn1-xF3

The absorption spectrum of RbMnF₃ and the excitation spectra of the system RbMg_xMn_1-xF₃ at 10 K as well as the fluorescence spectra and lifetimes of Mn²⁺ in the systems RbMg_xMn_1-xF₃ and KMg_xMn_1-xF₃ in the region 10–300 K were measured. The lifetime and fluorescence temperature dependence suggest that the origin of the fluorescence occurs at Mn²⁺ sites slightly perturbed by impurity ions and that a non-radiative energy transfer mechanism is responsible for the observed thermal quenching. By using different Mn²⁺ concentrations in the above systems the dependence of the energy transfer on the Mn²⁺ concentration is shown. Finally, a preliminary observation on laser stimulated Mn²⁺ luminescence in the system RbMg_xMn_1-xF₃ is reported.     

Electron hopping in Fe1−xCuxCr2S4 (0<x<0.5)

A theoretical expression for the line shape of the Mössbauer spectra in the presence of electron hopping between Fe²⁺ and Fe³⁺ is obtained by using a simple stochastic model. Analyses based upon this expression show that the origin of the complicated Mössbauer spectra observed in the magnetic semiconductors Fe_1?xCu_xCr₂S₄ (0<x<0.5) at 77 K is electron hopping between Fe⁺² and Fe³⁺ This hopping occurs at a rate of a few MHz. Quantitative estimates are given for some parameters; the isomer shifts, the internal magnetic fields, the quadrupole splittings and the proportions of Fe²⁺ and Fe³⁺. The valence distribution in this system is determined from the results. For example, the distribution Fe²⁺_0.69Fe³+_0.29Cu¹⁺_0.02Cr³⁺_1.72Cr²⁺_0.28S^2?₄ is obtained for x = 0.02. The existence of Cr²⁺ is concluded.     

Phase transition and high temperature thermoelectric properties of copper selenide Cu2-x Se （0 ≤ x ≤ 0.25）

With good electrical properties and an inherently complex crystal structure, Cu_2-xSe is a potential “phonon glass electron crystal” thermoelectric material that has previously not attracted much interest. In this study, Cu_2-xSe (0 ≤ x ≤ 0.25) compounds were synthesized by a melting-quenching method, and then sintered by spark plasma sintering to obtain bulk material. The effect of Cu content on the phase transition and thermoelectric properties of Cu_2-xSe were investigated in the temperature range of 300 K—750 K. The results of X-ray diffraction at room temperature show that Cu_2-xSe compounds possess a cubic structure with a space group of Fm3m (#225) when 0.15 < x le 0.25, whereas they adopt a composite of monoclinic and cubic phases when 0 ≤ x ≤ 0.15. The thermoelectric property measurements show that with increasing Cu content, the electrical conductivity decreases, the Seebeck coefficient increases and the thermal conductivity decreases. Due to the relatively good power factor and low thermal conductivity, the nearly stoichiometric Cu₂Se compound achieves the highest ZT of 0.38 at 750 K. It is expected that the thermoelectric performance can be further optimized by doping appropriate elements and/or via a nanostructuring approach.     

On the ferromagnetic perovskite with potential GMR properties: The manganite La1−x K x MnO3

We investigate structural features (cell distortions), magnetic and electric properties versus temperature for La_1-x K _x MnO₃ (0.05 < x < 0.2) perovskites. All the phases crystallize in a rhomboedral symmetry (R3c) and are ferromagnetic. Curie temperatures are strongly dependent of x, with a maximum around 308 K for La_0.8K_0.2MnO₃. A sharp decrease of the resistivity is observed just below the Curie temperature, accompanied by a very steep increase of the spontaneous magnetization, characteristic of a conventional Brillouin type curve. The average magnetic moment of Mn is about 88 e.m.u/g at 5 K for the composition La_0.8K_0.2MnO₃. The transition from ferromagnetic-metallic to paramagnetic-semiconducting states is explained from the suggested distribution of the cations (La³⁺ _1-x K⁺ _x )_A(Mn⁴⁺ _1-2x Mn⁴⁺ _2x )_BO₃ by the double exchange of Mn³⁺-Mn⁴⁺ pairs at the B-sublattice. Preliminary magnetoresistance measurements on bulk ceramic samples display an MR effect of the same amplitude as in the case of the alkaline earth substituted La manganites.     

Co(S1-xSex)2系统中的铁磁量子相变

针对Co(S_1-xSe_x)₂系统在x=0.11附近发生的铁磁金属到顺磁金属相变,制备了一系列不同Se替代浓度的多晶样品.通过对其结构和电阻率-温度ρ(T)关系的系统观测,结果发现,样品铁磁相变温度T_C随着Se替代浓度x值的增加,以(1-x)^1/2关系单调下降,其二级铁磁相变转变为一级相变 关键词： 量子相变 自旋量子涨落 1-xSe_x)₂')" href="#">Co(S_1-xSe_x)₂     

Preparation and characterization of manganese and cobalt based semiconducting ceramics

The spinel phases Mn_3-x Co _x O₄ are prepared directly and at low temperature (600-700°C) by means of thermal decomposition of mixed oxalate salt precursors in air or controlled atmosphere. The powders so obtained when sintered at 1200°C behave as semiconducting or insulating ceramics, depending on their cobalt content. The electrical properties of these ceramics have been found to be related to the distribution and valencies of the cations in the spinel structure. The study of their crystalline structure and stability, complemented by electrical measurements have permitted us to distinguish two types of compounds. The first type includes those given by Mn_3-x Co _x O₄ with 0 < x < 1. These are stable at high temperature, have a tetragonal structure and are insulators. The cation distributions for them can be deduced from that of hausmannite as given by Mn²⁺ [Mn³⁺ ₂]O₄, by substituting Mn²⁺ ions with Co²⁺ ions. The second type of compounds correspond to the cubic phases and occur for 1 < x < 3. These are unstable at high temperature and show a lower resistivity (e.g. 360 ° cm) which is of particular interest for their application in the design of negative temperature coefficient (N.T.C.) thermistor components.     

V替代Mn对La_(0.45)Ca_(0.55)MnO_3电荷有序相及自旋玻璃态的影响

用固相反应法制备了La0.45Ca0.55Mn1-xVxO3(x=0.00,0.10)多晶样品.通过X射线衍射谱、质量磁化强度-温度曲线、电子自旋共振谱,研究了V5+替代Mn3+/Mn4+对La0.45Ca0.55MnO3电荷有序相和自旋玻璃态的影响.实验结果表明,当x=0.10时,不仅母体的电荷有序相基本破坏,而且母体在40K左右出现的自旋玻璃态也被融化.电荷有序相被破坏的主要原因是用V5+替代Mn3+/Mn4+后,增加了Mn3+与Mn4+的比例,使铁磁双交换作用优于反铁磁超交换作用;自旋玻璃态的融化是由于V替代Mn后破坏了反铁磁背景下有少量铁磁成分的自旋玻璃态的形成条件.