Cation ordering and magnetic properties of neodymium-barium manganites

Nd_0.70Ba_0.30MnO_3+δ manganites with an ordered arrangement of Nd³⁺ and Ba²⁺ cations are fabricated. The initial stoichiometric Ba-disordered Nd_0.70Ba_0.30MnO₃ solid solution is synthesized in air using a traditional ceramic technology, is characterized by an orthorhombic unit cell (space group Imma, Z = 4), and is ferromagnetic with Curie temperature T _C ≈ 151 K. The average crystallite size in the initial sample is 〈D〉 ≈ 2.169 μm. Annealing of the initial sample in a reducing atmosphere at a pressure P[O₂] ≈ 10^?4 Pa and then in air at T = 800°C leads to the formation of a material with a crystallite size 〈D〉 ≈ 440 nm. This material consists of the following two perovskite phases: a Ba-ordered stoichiometric NdBaMn₂O₆ phase with a tetragonal unit cell (P4/mmm, Z = 2) and a Curie temperature of ～301 K and a Ba-disordered hyperstoichiometric Nd_0.90Ba_0.10MnO_3+δ phase with an orthorhombic unit cell (Imma, Z = 4) and T _C ～ 121 K. Significant changes in the magnetic properties are explained in terms of chemical phase separation with allowance for cation ordering.     

Magnetocaloric properties of manganites in alternating magnetic fields

The magnetocaloric effect has been measured in manganites of various chemical compositions in weak alternating magnetic fields. The capabilities of a simple method for measuring the magnetocaloric effect by modulating the magnetic field have been demonstrated. The dependence of the magnetocaloric effect on the temperature, magnetic field, and chemical composition of samples is interpreted.     

稀磁半导体的制备与性质

报道了稀磁半导体的制备、性质与实验研究进展，介绍了稀磁半导体的应用及发展前景．     

Magnetic properties of diluted magnetic semiconductors

A review will be given of the magnetic characteristics of diluted magnetic semiconductors and the relation with the driving exchange mechanisms. II–VI as well as IV–VI compounds will be considered. The relevance of the long-range interaction and the role of the carrier concentration will be emphasized.     

Theory of spin transfer phenomena in magnetic metals and semiconductors

We propose a general theory of the spin-transfer effects that occur when current flows through inhomogeneous magnetic systems. Our theory does not rest on an appeal to conservation of total spin, can assess whether or not current-induced magnetization precession and switching in a particular geometry will occur coherently, and can estimate the efficacy of spin-transfer when spin-orbit interactions are present. We illustrate our theory by applying it to a toy-model twodimensional-electron-gas ferromagnet with Rashba spin-orbit interactions.     

Mechanical and magnetic properties of nanostructured CoNiP films

The electroplating technique is especially interesting due to its low cost, high throughput and high quality of deposit. Magnetic thin films are extensively used in various electronic devices including high-density recording media and micro electromechanical (MEMS) devices. Due to these potential applications, deposition of magnetic film draws special attention and it needs a cost-effective process. Electro-deposition being cost-effective, in the present work cobalt-based magnetic films were deposited electrochemically and deposition characteristics were studied. Effect of concentration of organic additives such as urea and thiourea in the presence of sodium hypophosphite was studied. Surface characterisation was carried out using X-ray diffractometer (XRD) and scanning electron microscope (SEM). Elemental compositions of the films were studied using atomic absorption spectrometer (AAS) and showed phosphorous content was less than 1%. Samples were subjected to vibrating sample magnetometer (VSM) and studies showed that organic additive has altered magnetic properties of these films. The reason for change in magnetic properties and structural characteristics because of the additives were discussed. Mechanical properties such as residual stress, hardness and adhesion of the films were also examined and reported.     

Thermal and magnetic properties of degenerate antiferromagnetic semiconductors

Based on the model presented by Umehara and Kasuya, specific heat, susceptibility and magnetization of degenerate antiferromagnetic semiconductors are investigated theoretically. It is suggested that the model explains very well the qualitative behaviour of the thermal and magnetic properties of non-stoichiometric gadolinium-chalcogenides of Th₃P₄ type crystal structure with small number of conduction electrons.     

Griffiths phase and the magnetic and transport properties of doped manganites

A phenomenological model is proposed to describe the magnetic and magnetoresistance properties of ferromagnetic manganites. This model is based on the methods used to describe hysteretic systems, takes into account phase separation effects, and assumes the transition of ferromagnetic manganites into the Griffiths phase at above the Curie temperature. This formalism makes it possible to describe the conducting properties of the systems in the temperature range from low temperatures to the Griffiths temperature (T _G). This approach is used to qualitatively explain the experimental laws of the behavior of ferromagnetic manganites using the temperature and field dependences of the electrical resistivity and magnetization, the hysteretic properties, and the magnetoresistive effect (MRE) and to classify manganites in the magnitude of the MRE. The parameter that is responsible for the response of a system to thermal effects is the ratio of the maximum energy barrier separating various states of a system at zero temperature W _A (0) to thermal fluctuation energy W _Cfl at T _G. The W _A (0)/W _Cfl ratio is found to determine the temperature range of the Griffiths phase. The relation between the magnitudes of the MRE and parameter W _A (0)/W _Cfl for a certain system is revealed. The behavior of the magnetization and electrical resistivity of manganites in the Griffiths phase is discussed.     

Ordering and magnetic properties of nanostructured CoPt particles

The kinetics of ordering in nanostructured CoPt particles is investigated. The low activation energy (58 ± 2 kJ/mol) of the ordering process indicates that the kinetics of ordering is not limited by bulk diffusion. Coercivity grows along with the degree of ordering and reaches its maximum value of 14.2 kOe upon the complete ordering of CoPt alloy with a crystallite size of no more than that of a single domain.     

Mn掺杂ZnO稀磁半导体材料的制备和磁性研究

采用共沉淀方法制备了名义组分为Zn_1-xMn_xO(x=0.001,0.005,0.007,0.01)的Mn掺杂的ZnO基稀磁半导体材料,并研究了在大气气氛下经过不同温度退火后样品的结构和磁性的变化.结果表明：样品在600℃的大气条件下退火后, 仍为单一的六方纤锌矿结构的ZnO颗粒材料;当样品经过800℃退火后,Mn掺杂量为0.007,0.01的样品中除了ZnO纤锌矿结构外还观察到ZnMnO₃第二相的存在.磁性测量表明,大气条件下600℃退火后的样品,呈现出室温铁磁性;而800℃退火后的样品,其室温铁磁性显著减弱,并表现为明显的顺磁性.结合对样品的光致发光谱的分析,认为合成样品的室温铁磁性是由于Mn离子对ZnO中的Zn离子的替代形成的. 关键词： ZnO 掺杂 稀磁半导体 铁磁性     

Magnetic properties of Mn-doped ZnO diluted magnetic semiconductors

A series of Mn-doped ZnO films have been prepared in different sputtering plasmas by using the inductively coupled plasma enhanced physical vapour deposition. The films show paramagnetic behaviour when they are deposited in an argon plasma. The Hall measurement indicates that ferromagnetism cannot be realized by increasing the electron concentration. However, the room-temperature ferromagnetism is obtained when the films are deposited in a mixed argon-nitrogen plasma. The first-principles calculations reveal that antiferromagnetic ordering is favoured in the case of the substitution of Mn^2＋ for Zn^2＋ without additional acceptor doping. The substitution of N for O （NO^-） is necessary to induce ferromagnetic couplings in the Zn-Mn-O system. The hybridization between N 2p and Mn 3d provides an empty orbit around the Fermi level. The hopping of Mn 3d electrons through the empty orbit can induce the ferromagnetic coupling. The ferromagnetism in the N-doped Zn-Mn-O system possibly originates from the charge transfer between Mn^2＋ and Mn^3＋ via NO^-, The key factor is the empty orbit provided by substituting N for O, rather than the conductivity type or the carrier concentration.     

Room temperature magnetic properties of ZnO nanostructured films

The ZnO nanorod array films have been epitaxially deposited on indium tin oxide (ITO) glass along 〈0001〉 direction. It is found that the film is grown in a two-step process including nanoparticle film nucleation and oriented rod growth. The as-prepared ZnO film shows a dominant diamagnetic signal and a weak ferromagnetic signal at room temperature. The room temperature ferromagnetism deteriorated by annealing in air or N₂. The photoluminescent spectra revealed that the intensity of ZnO defect band decreases after annealing. Thus, the decreased ferromagnetism is likely to have resulted from the decrease of oxygen vacancies and defects in the as-prepared film. Moreover, ZnO deposited at various times showed that defects located at or near the interface between the substrate and the film play a major role in ferromagnetism. It suggests that ferromagnetism can be tuned by changing the defects in ZnO.     

Local structure and magnetic properties of Mn substituted manganites studied by EXAFS and Dc magnetic measurements

We report extended X-ray absorption fine structure (EXAFS) measurements at the Mn K edge and magnetic measurements performed on (La_1−xCa_x)(Mn_1−yM_y)O₃ samples (M=Cr or Ni; x=0.37 and 0.75 and y=0.03 or 0.08). The Mn substitution produces important effects on both the sides of the LaMnO₃-CaMnO₃ phase diagram. For x<0.5 the ferromagnetic-metallic phase maintains its main character even after Mn substitution, but both the doping species (Ni or Cr) lower T_C and broaden the magnetic transition, and the EXAFS study evidences two Mn-O distances, suggesting the presence of zones of distorted insulating phase. For x>0.5, after the doping with Cr, the charge ordered phase persists but on a shorter scale, whereas the Jahn-Teller distortion is weakened as indicated by EXAFS measurements, and the formation of ferromagnetic clusters is evidenced by magnetic measurements.EXAFS and magnetic measurements are in mutual agreement, thus confirming the correlation between the local disorder determined by charge localization and magnetic degrees of freedom.     

Regularities in temperature,magnetic field and pressure effect on the resistive properties of magnetic semiconductors

The influence of hydrostatic pressure, magnetic field and temperature on resistivity behaviour of bulk and film samples La_0.9Mn_1.1O₃ and La_0.56Ca_0.24Mn_1.2O₃ at action of magnetic field and temperature has been analysed. It is established that the maximum of magnetoresistive and the revealed baroresistive, magnetobaroresistive effects coincide at the same temperature T_pp. This temperature is equal to the “metal-semiconductor” phase transition temperature T_ms. “Cooling” and “heating” effects of pressure and magnetic field have been revealed. A mutual correspondence of T–P–H (6.2 K, 1 kbar, 2.7 kOe) influence on polycrystalline sample La_0.9Mn_1.1O₃ resistivity has been determined. The linear change of T_ms(P) and T_ms(H) in La_0.9Mn_1.1O₃, La_0.56Ca_0.24Mn_1.2O₃ resistivity have been found. An importance of the regularities of elastic-deforming correspondence of T–H–P influence on magnetic, resistivity properties, phase transitions and effects was elucidated and explained. An alternating influence of T–H–P and its role in resistivity has been pointed. A correlation between structural, elastic and resistive properties is specified.     

Analysis of some magnetic properties of diluted magnetic semiconductors including long-range interactions

Specific heat, susceptibility and magnetization of four diluted magnetic semiconductors (Cd_1−xMn_xTe, Cd_1−xMn_xSe, Hg_1−xMn_xTe and Hg_1−x^- Mn_xSe) are analysed with a model which includes long-range interactions. Calculations based on an extended version of a nearest neighbour pair approximation yield a fair agreement with the experimental data. No adjustment of the statistical distributions of magnetic ions is found to be necessary.     

Influence of the Jahn-Teller effect on the structural, magnetic, and electrical properties of lightly doped manganites

Features of the structural, magnetic and electric properties in lightly doped La_1?x Sr _x MnO₃ (0.12 < x < 0.18) manganites are studied. Agreement between the types of orbital ordering and the local cooperative Jahn-Teller distortion octahedra is established. The interaction between the Jahn-Teller effect and magnetic ordering is described.     

Optical and dielectric properties of nanostructured photonic crystals loaded by ferroelectrics and metals

The properties of new materials produced by loading artificial opals with different ferroelectrics (sodium nitrite, barium titanate, lithium niobate. etc.) are analyzed. The possibility of modifying the ferroelectric properties of materials by introducing them into pores of artificial opals is reported. The physical properties of artificial opals with pores loaded by conducting media (mercury, amorphous carbon, silver, gold, etc.) are studied. An analysis is made of the transmission and reflection spectra of broad-band radiation, which permit one to establish the characteristics of the stop bands as a function of the globule diameter, the type of ferroelectric or metal inserted into the opal pores, thermal annealing conditions, etc. The conditions favoring emission of slow electromagnetic waves in artificial opals are specified, and their characteristic properties are reported. The possibilities for increasing the efficiency of Raman and nonlinear optical processes in photonic crystals loaded by ferroelectrics and metals are analyzed.