Spin-polarized current of a transistor in single Mn12 molecular magnets

Focusing on the framework of how to realize the molecular spintronics in a single molecular magnet, we present theoretical studies on the spin-polarized quantum transport behavior through a single Mn12 molecular magnet. Our theoretical results were obtained by carrying out density functional theoretical calculation within the Keldysh nonequilibrium Green function formalism. The ultimate goal of the molecular spintronics is to develop single molecule transistors which generate spin-polarized currents through the molecular magnet. We obtained the density of states, the transmission coefficients and the characteristic features of the current-voltage (I-V) on the spin-polarized transport properties of Mn12 by the theoretical calculation. These results show the possibility for the realization of molecular spintroinics using single molecular magnets.     

Microstructure and magnetic properties in Mn-doped Nd-Fe-B magnets

The effect of the addition of antiferromagnetic element Mn on the local structure and the magnetic properties of Nd₉Fe₈₅B₆ magnets has been studied. It was found that the microstructure and local structure heavily depended on both the Mn addition and the quenched speed. The Mn additions could promote the order degree around Fe atom in as-spun samples. After appropriate annealing, the completely crystallized samples were obtained. It showed that the difference of the local structures around Fe atoms almost disappeared and the hard magnetic properties, especially the coercivity was enhanced. It is presumed that the enhanced hard magnetic properties were attributed to the enhancement of the exchange coupling, which is between the hard phases and the soft phases.     

Measurement of magnetic properties of cobalt-rare earth permanent magnets

Co-rare earth permanent magnets generally display a high intrinsic coercive force. In order to make magnetic measurements with such materials fully saturated, we have found it necessary to use magnetizing fields in excess of 50 kOe. The superconducting solenoid is ideally suited for generating such fields. In this study, we have used a 100 kOe Nb3-Sn superconducting solenoid for magnetizing samples and for measuring saturation magnetization. Demagnetization properties of long cylindrical samples have been measured in the superconducting solenoid, and also with a conventional hysteresigraph after the samples have been saturated with the superconducting solenoid or in some cases with a pulsed field solenoid. Short disk samples or ring magnets such as are used in traveling wave tube (TWT) designs are difficult to measure by these techniques. We have found torque magnetometry to be useful for such shapes. Open circuit magnetization is measured in this case. Results for ring magnets will be compared with peak axial field measurements for a periodic TWT structure. Reversible and irreversible changes in magnetization with temperature are also of interest. Techniques for measuring these changes are discussed.     

Selection of parameters for testing magnetic properties of permanent magnets

Conclusions The testing of the parameters B_r, H_c, B_d, and H_d does not always provide an unambiguous estimation of whether the PM whose working point lies on the regression line is suitable for utilization in a magnetic system.The evaluation of the PM efficiency by means of the induction at the working regressionline departure point and the induction at the intersection point of the working regression line with the conductance line corresponding to the condition of the magnet in an assembled product is more efficient and universal, since it is virtually independent of the working regression-line departure-point position and it serves to halve the number of tested parameters.Translated from Izmeritel'naya Tekhnika, No. 2, pp. 49–50, February, 1979.     

Effect of polymer matrices on magnetic properties of plastic magnets

Plastic magnets were prepared using commercial polyamide (Nylon 12), polystyrene and ethylene-vinylacetate copolymer, and three kinds of ferrite magnetic powders with different concentration levels (20, 35 and 50 vol%). Their magnetic properties and thermal stability are discussed comparing viscoelasticity and thermal properties of the polymers and plastic magnets. Magnetic properties of the anisotropic plastic magnets were largely dependent on the concentration of the magnetic powder and its degree of orientation in the matrix. The degree of the orientation was mainly affected by the interaction between polymer matrix and magnetic powder, and viscoelastic property of the matrix polymer. It was also affected to some extent by the shape of the magnetic particles. The order of the interaction of the polymers to the magnetic powders was as follows: PA > EVA copolymer > PS, and PA-bonded magnets gave the highest magnetic properties among the matrices examined. The decrease in magnetic properties due to the change in the orientation of magnetic powder rarely occurred during thermal treatment.     

真空退火提高薄型磁体性能的研究

机加工成一定尺寸和形状的烧结NdFeB薄型磁体,由于表面晶粒形变和缺陷的产生引起了磁性能的降低,适当的真空退火热处理后,其磁性能将会得到恢复和提髙。通过SEM扫描电镜,X射线衍射和磁性能的测量,研究了退火热处理对磁体表面层晶粒微结构、应变、亚晶粒尺寸和磁性能的影响。结果表明,在三相共晶点温度以上,900℃×2h+490℃×5h的退火可以完全消除机加工产生的应変、应力和缺陷。其次,表面层内晶界富Nd相的毛细管作用和扩散迁移,使富Nd相均匀、连续地分布在晶粒和晶界周围。这种富Nd相网状结构,消除了机加工磁体表层内的晶界裂纹和缺陷,对磁体性能的恢复和提高起着重要的作用。     

Mn含量对NdFeB磁体表面Al基防护涂层性能影响的研究

采用等离子喷涂工艺在烧结NdFeB磁体表面喷涂不同Mn含量(Mn=0～30％(质量分数))的Al基防护涂层.利用X射线衍射仪、扫描电镜、显微硬度仪和多功能材料表面性能测试仪分析涂层物相组成、表面堆积状态、硬度和结合强度;采用动电位极化曲线和中性盐雾腐蚀试验测试涂层耐腐蚀性能,研究Mn含量对涂层防护性能的影响.结果表明,...     

The microstructure and magnetic properties of NdFeB magnets directly solidified at a low cooling rate

Nd_12.5Fe₈₂B_5.5 ribbons have been prepared by a melt spinning technique at a wheel speed ranging from 5 to 22 m/s. It was found that the wheel speed v_R dramatically influences the microstructure and magnetic properties of the ribbons. At an optimized v_R, a structure with single-phase Nd₂Fe₁₄B and an enhanced coupling between grains give rise to excellent magnetic properties. For example, the coercivity, remanence ratio, and maximum energy product of the best sample quenched at a wheel speed of 13 m/s are 1227.4 kA/m, 0.76, and 182.6 kJ/m³, respectively. In comparison with the samples from under- and over-quenched conditions and all the samples with annealing, the as-quenched sample fabricated at the optimized wheel speed has the best magnetic properties.     

Synthesis and magnetic properties of one-dimensional metal oxalate networks as molecular-based magnets

The homo- and heteropolymetallic assemblies of MM′(OX)₂(H₂O)₄, where MM′ represents MnMn, CoMn, NiMn, CuMn, CoCo, NiCo, CuCo, NiNi, CuNi, and CuCu; and the respective complexes, numbered 1–10, have been prepared by reacting metal(II) salts-i.e. of Mn, Co, Ni, and Cu^- and potassium oxalate monohydrate in hot water (90–100°C). The magnetic susceptibility data of the complexes 8 and 9 in the 300 K-20 K temperature range obeys the Curie-Weiss law and exhibits Weiss constants -50 K and -100 K, respectively. On lowering the temperature, the effective magnetic moment decreases gradually and is indicative of antiferromagnetic phase transition. The complexes have also been characterized by ES mass spectrometry, infrared (IR), electronic, and electron spin resonance (ESR) spectra. Most of the work was presented at the ‘5th IUMRS International Conference in Asia’ held at IISc, Bangalore during October 13–16, 1998     

纳米晶MnFe2O4的水热法合成及其磁性能

采用水热法制备了软磁材料MnFezO4纳米晶,借助XRD、IR、SEM和VSM对产物进行了表征,着重研究了水热条件如温度和时间等对MnFe2O4纳米晶的形成及其结构和磁性能的影响。结果表明水热温度较低时的产物晶化度和纯度低,表现出较差的磁性能,温度为120℃的产物其饱和磁化强度为15．34emu／g,剩磁比为0．08,矫顽力为8lOe,而在200℃下水热产物饱和磁化强度为51．49emu／g,剩磁比达到0．14,矫顽力为121Oe。     

Preparation and magnetic properties of anisotropic bulk MnBi/NdFeB hybrid magnets

正The Mn Bi low-temperature phase(LTP)has high magnetocrystalline anisotropy and a remarkable positive temperature coefficient of coercivity below 300℃,but it is difficult to obtain the high purity.In order to obtain the Mn Bi powders with high Ms and coercivity,low-energy-ball-milling was applied.Furthermore,the anisotropic bulk Mn Bi magnets were prepared by improved sintering process.     

粘结剂对粘结钕铁硼/锶铁氧体复合磁体磁性能的影响

为获得磁性能适中的磁体,采用流动温压成型技术制备了各向同性粘结Nd Fe B/锶铁氧体复合磁体.利用振动样品磁强计(VSM)研究了不同粘结剂对粘结Nd Fe B/锶铁氧体复合磁体磁性能的影响.研究表明:环氧值适中的酚醛环氧树脂制备的磁体具有较好的磁性能;当采用环氧值为0.480 mol/100 g酚醛环氧树脂BPANE8200H做粘结剂时,粘结Nd Fe B/锶铁氧体复合磁体获得了最佳的磁性能:Br=0.55 T,Hcj=620.6 k A/m,(BH)max=45.6 k J/m3.在保证磁体磁性能的前提下兼顾力学性能,粘结磁体流动温压成型温度参数的设置必须考虑粘结剂的软化点温度.     

Luminescent properties of YAlO3:Mn single crystalline films

The YAP:Mn single crystalline films (SCF) have been crystallized by liquid phase epitaxy (LPE) method onto YAP substrates. The cathode- (CL) and photo-luminescence (PL) spectra of the YAP:Mn SCF were analyzed for determination of the preferable valence states of manganese ions which are realized in these SCF depending on Mn content in the 0.01–0.81 at.% range. The thermoluminescence (TL) properties of YAP:Mn SCF with the different Mn content above the RT range were also examined in comparison with the properties of YAP:Mn single crystal counterpart. We show that YAP:Mn (0.01 at.%) SCF possesses effective TL properties both under α-particle and γ-quanta excitation with main TSL peaks at 130 and 195 °C. We assume that the different valence states of Mn ions are responsible for their TL properties, e.g. both emission and trapping centers in YAP:Mn are formed mainly by the different valence states of Mn ions.     

Giant magnetoresistance through a single molecule

Magnetoresistance is a change in the resistance of a material system caused by an applied magnetic field. Giant magnetoresistance occurs in structures containing ferromagnetic contacts separated by a metallic non-magnetic spacer, and is now the basis of read heads for hard drives and for new forms of random access memory. Using an insulator (for example, a molecular thin film) rather than a metal as the spacer gives rise to tunnelling magnetoresistance, which typically produces a larger change in resistance for a given magnetic field strength, but also yields higher resistances, which are a disadvantage for real device operation. Here, we demonstrate giant magnetoresistance across a single, non-magnetic hydrogen phthalocyanine molecule contacted by the ferromagnetic tip of a scanning tunnelling microscope. We measure the magnetoresistance to be 60% and the conductance to be 0.26G(0), where G(0) is the quantum of conductance. Theoretical analysis identifies spin-dependent hybridization of molecular and electrode orbitals as the cause of the large magnetoresistance.     

Structural and magnetic properties of Mn nanoparticles prepared by arc-discharge

Mn nanoparticles are prepared by arc discharge technique. MnO, α-Mn, β-Mn, and γ-Mn are detected by X-ray diffraction, while the presence of Mn₃O₄ and MnO₂ is revealed by X-ray photoelectron spectroscopy. Transmission electron microscopy observations show that most of the Mn nanoparticles have irregular shapes, rough surfaces and a shell/core structure, with sizes ranging from several nanometers to 80 nm. The magnetic properties of the Mn nanoparticles are investigated between 2 and 350 K at magnetic fields up to 5 T. A magnetic transition occurring near 43 K is attributed to the formation of the ferrimagnetic Mn₃O₄. The coercivity of the Mn nanoparticles, arising mainly from Mn₃O₄, decreases linearly with increasing temperature below 40 K. Below the blocking temperature T_B ≈ 34 K, the hysteresis loops exhibit large coercivity (up to 500 kA/m), owing to finite size effects, and irreversibility in the loops is found up to 4 T, and magnetization is not saturated up to 5 T. The relationship between structure and the magnetic properties are discussed.     

First-principles determination of intergranular atomic arrangements and magnetic properties in rare-earth permanent magnets

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Surface properties of a long-chain 10:12 diynoic acid monolayer at air-liquid and solid-liquid interfaces

Monolayer stabilities of two long-chain diacetylene monocarboxylic acids at fluid interfaces have been studied and are compared with those of a conventional n-alkanoic acid. The diacetylene derivatives form highly incompressible solid monolayers at the air-liquid interface. Their surface pressure-area isotherms are shown to depend on the mode of monolayer compression, indicating the necessity of a cautious approach to experimental protocol. The coherence of deposited monolayers on hydrophobic glass surfaces was characterized by interference reflection microscopy. Birefringent crystals were obtained with UV polymerized pentacosanoyl 10:12 diynoic acid in sharp contrast to the well-defined deposition behaviour observed for monolayers of eicosanoic acid.     

放电等离子烧结制备高性能热变形Nd-Fe-B磁体

使用放电等离子烧结(SPS)可在短时间内将非晶Nd13.5Fe80.5B6粉末晶化,且可获得接近于全致密的各向同性磁体,其剩磁、矫顽力、最大磁能积分别为Br=0.8T,Hci＝1346kA/m,(BH)m=108kJ/m3.热变形后,随变形量的增加,硬磁相晶粒的取向度增加,Br及(BH)m大幅增加,在65%时达最大:B...