半导体极性膜中束缚磁极化子的自陷能

采用线性组合算符法和幺正变换方法,研究极性晶体膜中束缚磁极化子的自陷能随膜厚d的变化关系。得出束缚磁极化子的自陷能由两部分组成：第一部分是由于电子—体LO声子相互作用所引起的（ ）极化子效应;第二部分则是电子－SO声子相互作用引起的。后者又包含两部分,分别是电子与极性膜中两支表面声子相互作用的贡献（ ）。通过对KCl半导体膜的数值计算表明, 和磁极化子的振动频率 随膜厚d的增加而减少;当膜厚大于5nm时,总自陷能 趋于一稳定值。另外,由于稳恒磁场的存在,使磁极化子的自陷能增大,这主要是由于稳恒磁场的存在,使电子—声子间的相互作用增强,极化子效应增大而引起的。     

自旋对强耦合二维磁极化子基态能量的影响

采用线性组合算符法研究电子自旋对强耦合二维磁极化子基态能量的影响．对ＫＩ晶体所作的数值计算结果表明,随着磁场的加强,不同取向的电子自旋使强耦合二维磁极化子基态能量表现为增加和减少两种截然相反的情形．电子自旋能量与磁极化子基态能量之比随磁场的增加而增加．     

束缚磁极化子的有效质量与共振频率的关系

采用改进的线性组合算符法和幺正变换方法,研究极性晶体膜中束缚磁极化子的有效质量与振动频率λ的变化关系.得出束缚磁极化子的有效质量均由两部分组成:第一部分是由于电子-体LO声子相互作用所引起的;第二部分则是电子-SO声子相互作用引起的.后者又包含两部分,分别是电子与极性膜中两支表面声子相互作用的贡献.而且当λ＜20×10...     

自旋对量子点中束缚磁极化子性质的影响

采用线性组合算符和幺正变换方法,研究了自旋对量子点中弱耦合束缚磁极化子性质的影响。考虑自旋影响时,讨论了束缚磁极化子的基态能量随量子点受限长度和磁场的变化关系。结果表明:在库仑束缚势保持不变时,磁极化子的基态能量、自旋向上(向下)分裂能都随量子点受限长度的增大而减小,随磁场的增大而增大;在磁场保持不变时,磁极化子基态能量、自旋向上(向下)分裂能都随库仑束缚势的增大而减小。     

自旋对非对称量子点中弱耦合束缚磁极化子基态能量的影响

采用Tokuda修正的线性组合算符和幺正变换方法,研究了非对称量子点中弱耦合束缚磁极化子的性质。推导出了磁极化子基态能量随量子点横向、纵向受限长度和磁场的变化关系。进行了数值计算,计算结果表明：考虑自旋影响时,束缚磁极化子基态能量分裂成自旋向上（向下）两个分支。并且磁极化子基态能量、自旋向上（向下）分裂能随量子点横向、纵向受限长度的增加而减少,随磁场强度的增加而增大。     

非对称抛物限制势量子点中强耦合束缚磁极化子的性质

采用Pekar类型的变分方法研究了非对称抛物限制势作用下的量子点中强耦合束缚磁极化子的性质.导出了束缚磁极化子的基态束缚能、光学声子平均数.通过数值计算表明,量子点中强耦合束缚磁极化子的基态束缚能和光学声子平均数均随量子点的横向受限强度、纵向受限强度的增加而增大,随回旋频率的增加而增大.基态束缚能随库仑束缚势的增加而增大.     

三角量子阱中磁极化子的Zeeman效应和Rashba效应

采用Pekar变分法研究了三角量子阱中磁极化子的Zeeman效应和Rashba效应,理论推导得到磁极化子基态能量的表达式。结果表明,磁极化子的基态能量不仅是波矢和磁场回旋共振频率的函数,而且还是电子面密度和电子-声子耦合强度的函数;由于晶体结构反演非对称性及磁场的影响,磁极化子能量发生Rashba自旋轨道分裂和Zeeman分裂;在强、弱磁场下,分别讨论了Rashba效应和Zeeman效应在能量分裂中所占的主导地位;由于声子的存在降低了极化子的总能量,使得极化子态比裸电子态更稳定。     

非对称抛物限制势量子点中强耦合束缚磁极化子的性质

采用Pekar类型的变分方法研究了非对称抛物限制势作用下的量子点中强耦合束缚磁极化子的性质.导出了束缚磁极化子的基态束缚能、光学声子平均数.通过数值计算表明,量子点中强耦合束缚磁极化子的基态束缚能和光学声子平均数均随量子点的横向受限强度、纵向受限强度的增加而增大,随回旋频率的增加而增大.基态束缚能随库仑束缚势的增加而增大.     

声子之间相互作用对磁场中表面极化子性质的影响

研究了在稳定磁场作用下极性晶体中表面电子与表面光学声子耦合弱的表面极化子性质，采用微扰法和线性组合算符法导出表面磁极化子的基态能量，讨论了反冲效应中不同波矢声子间相互作用对表面磁极化子基态能量的影响     

极化子速率对量子点中磁极化子性质的影响

采用改进的线性组合算符及么正变换的方法研究了极化子速率对量子点中强耦合磁极化子的性质的影响。在考虑Rashba自旋轨道耦合相互作用影响下,导出了量子点中强耦合磁极化子的振动频率、基态能量、有效质量和相互作用能随极化子速率的变化关系。对RbCl晶体进行数值计算,结果表明：量子点中强耦合磁极化子的振动频率、基态能量、有效质量随极化子速率的增加而增大,相互作用能随极化子速率的增加而减少。     

Effect of spin excited states on electron transport through an organic ferromagnetic device

Spin excited states in an organic ferromagnet are proposed and investigated on the basis of an extended SSH + Heisenberg (SSH = Su-Schrieffer-Heeger) model. It is found that a spin excited state will form a local distortion of the spin density wave (SDW) of π-electrons while the lattice configuration of main chain has no obvious change. Then the spin-polarized transport properties through an organic ferromagnetic device are investigated with the Landauer-Büttiker formula and Green’s function method. It is obtained that the current will be spin polarized due to the existence of SDW in the ferromagnetic molecule. Both the total current and the spin-polarized current will be modulated when the SDW is excited. The total current through the device is suppressed by the spin excitation of side radicals, through which a conductance switch function may be realized. Compared with ground state, the spin polarization has no obvious change in a low spin excited state and the device still has spin-filter function. Finally, spin excitations induced by temperature is studied and we find that an organic ferromagnetic device can hold a high spin polarization when temperature is not too high.     

A rotating ferromagnetic sphere as a source of long-wavelength electromagnetic radiation

A ferromagnetic sphere rotating with constant angular velocity ω₁ is considered when the directions of magnetization vector M applied external magnetic field H and rotation frequency ω₁ do not coincide. The intensity of electromagnetic waves radiated by this sphere is found. It is shown that the radiation is strongly anisotropic and nonstationary.     

Magnetic properties of different temperature treated Co- and Ni-doped ZnO hollow nanospheres

The effect of different annealing temperatures on magnetic properties of Co- and Ni-doped ZnO hollow nanospheres was investigated. It was found that the hollow structures and room-temperature ferromagnetism were kept when the Co- and Ni-doped ZnO samples were annealed at low temperature of 550 °C. When the temperature was elevated to 700 °C, the hollow structures partially collapsed and the samples still exhibited ferromagnetic behavior. The hollow structures were completely broken with annealing temperature above 1200 °C. The ferromagnetic behavior of Co-doped ZnO disappeared, while the Ni-doped ZnO still exhibited reduced ferromagnetism. However, the ferromagnetism in high-temperature annealed Ni-doped ZnO nanospheres was extrinsic and probably originated from secondary phases.     

Influence of Superexchange Interaction on the Ferromagnetic Properties of Manganites and Cobaltites

The role of superexchange interaction in the formation of the ferromagnetic state of cobaltites in the La_0.5Sr_0.5Co_1–xMexO₃ (Me = Cr, Ga, Fe) systems and manganites La_0.7Sr_0.3Mn_0.85M_0.15O₃ (M–Nb, Mg) with a perovskite structure has been studied. It was found that the ferromagnetic state in cobaltites can be implemented in some compositions without the mixed-valence effect of cobalt ions. The initial compound (x = 0) is ferromagnetic (Т_С = 247 K) with the saturation magnetization close to 2μB (at T = 30 K) per formular unit. It has been shown that the chemical substitution of cobalt by chromium reduces the spontaneous magnetization to 0.3μB (at х = 0.2), while the substitution of cobalt by iron ions (х = 0.2) does not alter the magnetization. The obtained data are interpreted in the model of positive superexchange interactions between cobalt and iron ions and negative ones between cobalt and chrome. It has been shown that the La_0.7Sr_0.3Mn_0.85Nb_0.15O₃ composition is ferromagnetic with ТС=145K, with a magnetic moment of 3.1 μB/Mn at 10 K, and no evidence of a cooperative orbital ordering in the manganite compounds has been revealed. Partial chemical substitution of Nb⁵⁺ ions by Mg²⁺ ones leads to the formation of Mn⁴⁺ ions, while it does not strengthen the ferromagnetic state. The strengthening of the structural distortions reduces the ferromagnetic component. It is assumed that the ferromagnetic state is caused by a significant hybridization of eg-orbitals of the manganese and oxygen ions, which strengthens the positive component of the superexchange interactions.     

Room-temperature ferromagnetic ordering in Mn-doped ZnO thin films grown by pulsed laser deposition

The ferromagnetic ordering in Mn-doped ZnO thin films grown by pulsed laser deposition (PLD) as a function of oxygen pressure and substrate temperature has been investigated. Room-temperature ferromagnetic behaviors in the Mn-doped ZnO films grown at 700°C and 800°C under 10⁻¹ torr in oxygen pressure were found, whereas ferromagnetic ordering in the films grown under 10⁻³ torr disappeared at 300 K. The large positive magnetoresistance (MR), ∼10%, was observed at 5 K at low fields and small negative MR was observed at high fields, irrespective of oxygen pressure. In particular, anomalous Hall effect (AHE) in the Mn-doped ZnO film grown at 700°C under 10⁻¹ Torr has been observed up to 210 K. In this work, the observed AHE is believed to be further direct evidence demonstrating that the Mn-doped ZnO thin films are ferromagnetic.     

Symmetry-Mismatch-Induced Ferromagnetism in the Interfacial Layers of CaRuO3/SrTiO3 Superlattices

By modifying the entangled multi-degrees of freedom of transition-metal oxides, interlayer coupling usually produces interfacial phases with unusual functionalities. Herein, a symmetry-mismatch-driven interfacial phase transition from paramagnetic to ferromagnetic state is reported. By constructing superlattices using CaRuO₃ and SrTiO₃, two oxides with different oxygen octahedron networks, the tilting/rotation of oxygen octahedra near interface is tuned dramatically, causing an angle increase from ≈150° to ≈165° for the Ru O Ru bond. This in turn drives the interfacial layer of CaRuO₃, ≈3 unit cells in thickness, from paramagnetic into ferromagnetic state. The ferromagnetic order is robust, showing the highest Curie temperature of ≈120 K and the largest saturation magnetization of ≈0.7 µ_B per formula unit. Density functional theory calculations show that the reduced tilting/rotation of RuO₆ octahedra favors an itinerant ferromagnetic ground state. This work demonstrates an effective phase tuning by coupled octahedral rotations, offering a new approach to explore emergent materials with desired functionalities.     

MOCVD制备过掺杂GaN基稀磁半导体微结构研究

应用MOCVD方法我们在c轴取向的蓝宝石衬底上生长出Fe掺杂和Mn掺杂GaN薄膜。通过改变前驱物的通入量,我们制备出不同掺杂浓度的样品。应用高分辨透射电镜,我们对样品的微结构进行了分析。对于Fe过掺杂GaN样品,我们发现了六角结构的Fe3N团簇的存在,并且Fe3N(0002)面平行于GaN(0002)面;对于Mn过掺杂GaN样品,我们发现了六角结构的Mn6N2.58相的存在,并且Mn6N2.58(0002)面平行于GaN(0002)面。同时,由于晶格中掺入了大量掺杂离子,GaN晶格取向遭到了破坏,导致了部分GaN(0002)面的倾斜。磁学测量表明均一相的Fe掺杂GaN显现铁磁性,而均一相Mn掺杂GaN没有铁磁性。由于铁磁性Fe单晶和Fe3N团簇的存在,相比于均一性Fe掺杂GaN,过掺杂GaN样品的磁性大幅度增强,而Mn过掺杂GaN样品显现出很弱的铁磁性,这有可能来源于Mn6N2.58相。     

Long‐Range Nonvolatile Electric Field Effect in Epitaxial Fe/Pb(Mg1/3Nb2/3)0.7Ti0.3O3 Heterostructures

One of the ideal candidates of using electric field to manipulate magnetism is the recently developed multiferroics with emergent coupling of magnetism and electricity, particularly in synthesizing artificial nanoscale ferroelectric and ferromagnetic materials. Here, a long‐range nonvolatile electric field effect is investigated in Fe/Pb(Mg_1/3Nb_2/3)_0.7Ti_0.3O₃ heterostructure using the dependence of the magnon‐driven magnetoelectric coupling on the epitaxial Fe thin film (4–30 nm) thickness at room temperature using measurements based on the ferromagnetic resonance. The magnon‐driven magnetoelectric coupling tuning of the ferromagnetic resonance field shows a linear response to the electric field, with a resonance field shift that occurs under both positive and negative remanent polarizations, and demonstrates nonvolatile behavior. Moreover, the spin diffusion length of the epitaxial Fe thin film of ≈9 nm is obtained from the results that the change of the cubic magnetocrystalline anisotropy field under different electric fields varies with Fe thickness. These results are promising for the design of future multiferroic devices.     

Proximity-Induced Fully Ferromagnetic Order with Eightfold Magnetic Anisotropy in Heavy Transition Metal Oxide CaRuO3

Ferromagnetic materials with a strong spin-orbit coupling (SOC) have attracted much attention in recent years because of their exotic properties and potential applications in energy-efficient spintronics. However, such materials are scarce in nature. Here, a proximity-induced paramagnetic to ferromagnetic transition for the heavy transition metal oxide CaRuO₃ in (001)-(LaMnO₃/CaRuO₃) superlattices is reported. Anomalous Hall effect is observed in the temperature range up to 180 K. Maximal anomalous Hall conductivity and anomalous Hall angle are as large as ∼15 Ω⁻¹ cm⁻¹ and ∼0.93%, respectively, by one to two orders of magnitude larger than those of the typical 3d ferromagnetic oxides such as La_0.67Sr_0.33MnO₃. Density functional theory calculations indicate the existence of avoid band crossings in the electronic band structure of the ferromagnetic CRO layer, which enhances Berry curvature thus strong anomalous Hall effects. Further evidences from polarized neutron reflectometry show that the CaRuO₃ layers are in a fully ferromagnetic state (∼0.8 μ_B/Ru), in sharp contrast to the proximity-induced canted antiferromagnetic state in 5d oxides SrIrO₃ and CaIrO₃ (∼0.1 μ_B/Ir). More than that, the magnetic anisotropy of the (001)-(LaMnO₃/CaRuO₃) superlattices is eightfold symmetric, showing potential applications in the technology of multistate data storage.     

Controlling Ferromagnetic Ground States and Solitons in Thin Films and Nanowires Built from Iron Phthalocyanine Chains

Iron phthalocyanine (FePc) is a molecular semiconductor whose building blocks are 1D ferromagnetic chains. It is shown that its optical and magnetic properties are controlled by the growth strategy, obtaining extremely high coercivities of over 1 T and modulating the exchange constant between 15 and 29 K through switching from thin films with controlled orientations, to ultralong nanowires. Magnetization measurements are analyzed using concepts and formulas with broad applicability to all 1D ferromagnetic chains. They show that FePc is best described by a xy model with moments preferentially lying in the molecular planes. The chain Hamiltonian is very similar to that for the classic inorganic magnet CsNiF₃, but with ferromagnetic rather than antiferromagnetic interchain interactions. The dominant degrees of freedom are topological excitations called solitons, namely moving 1D magnetic domain walls, and at low temperatures and fields a “super‐Curie–Weiss” law characteristic of nearly 1D xy and Heisenberg ferromagnets, where susceptibility scales as 1/(T²–θ²), is observed. The ability to control the molecular orientation and ferromagnetism of FePc systems, and produce them on flexible substrates, together with excellent transistor characteristics reported previously for phthalocyanine analogues, makes them potentially useful for magneto‐optical and spintronic devices.