Controllable electron g-factors in HgMnTe quantum spheres

The electronic structure and Lande electron g-factors of manganese-doped HgTe quantum spheres are investigated, in the framework of the eight-band effective-mass model and the mean-field approximation. It is found that the electronic structure evolves continuously from the zero-gap configuration to an open-gap configuration with decreasing radius. The size dependence of electron g-factors is calculated with different Mn-doped effective concentration, magnetic field, and temperature values, respectively. It is found that the variations of electron g-factors are quite different for small and large quantum spheres, due to the strong exchange-induced interaction and spin-orbit coupling in the narrow-gap DMS nanocrystals. The electron g-factors are zero at a critical point of spherical radius R-c; however, by modulating the nanocrystal size their absolute values can be turned to be even 400 times larger than those in undoped cases. Copyright (c) EPLA, 2008.

Influence of implantation energy on the characteristics of Mn-implanted nonpolar a-plane GaN films

Nonpolar GaN Mn films have been fabricated by implanting Mn-ion into nonpolar a-plane (MO) GaN films at room temperature. The influence of implantation energy on the Structural, morphological and magnetic characteristics of samples have been investigated by means of stopping and range of ions in matter (SRIM) Simulation software, high-resolution X-ray diffraction (HRXRD), atomic force microscopy (AFM), and superconducting quantum interference device (SQUID). According to the SQUID analysis, obvious room temperature ferromagnetic properties of samples were detected. Moreover, the implantation energy has little impact on the ferromagnetic properties of samples. The XRD and AFM analyses show that the structural and morphological characteristics of samples were severely deteriorated with the increase of implantation energy. (C) 2008 Elsevier B.V. All rights reserved.

Giant Zeeman splitting in manganese-doped ZnSe quantum spheres: Eight-band effective-mass calculations

We deduce the eight-band effective-mass Hamiltonian model for a manganese-doped ZnSe quantum sphere in the presence of the magnetic field, including the interaction between the conduction and valence bands, the spin-orbit coupling within the valence bands, the intrinsic spin Zeeman splitting, and the sp-d exchange interaction between the carriers and magnetic ion in the mean-field approximation. The size dependence of the electron and hole energy levels as well as the giant Zeeman splitting energies are studied theoretically. We find that the hole giant Zeeman splitting energies decrease with the increasing radius, smaller than that in the bulk material, and are different for different J(z) states, which are caused by the quantum confinement effect. Because the quantum sphere restrains the excited Landau states and exciton states, in the experiments we can observe directly the Zeeman splitting of basic states. At low magnetic field, the total Zeeman splitting energy increases linearly with the increasing magnetic field and saturates at modest field which is in agreement with recent experimental results. Comparing to the undoped case, the Zeeman splitting energy is 445 times larger which provides us with wide freedom to tailor the electronic structure of DMS nanocrystals for technological applications.

PIXE analysis of Fe content in Fe-implanted GaN film

The diluted magnetic semiconductors (DMSs) were achieved by the ion implantation. Fe+ ions (250 keV) were implanted into n-type GaN at room temperature with doses ranging from 8 X 10(15) cm(-2) to 8 X 10(16) cm(-2) and subsequently rapidly annealed at 800 degrees C for 5 m in N-2 ambient. PIXE was employed to determine the Fe-implanted content. The magnetic property was measured by the Quantum Design MPMS SQUID magnetometer. No secondary phases or clusters are detected within the sensitivity of XRD. Apparent ferromagnetic hysteresis loops measured at 10 K were presented. The relationships between the Fe-implanted content and the ferromagnetic property are discussed. (c) 2006 Elsevier B.V. All rights reserved.

Investigation of Mn-doped Si films prepared by magnetron cosputtering

Mn-doped Si films were prepared on Si(001) substrates by magnetron cosputtering and post-annealing process. The structural, morphological and magnetic properties of the films have been investigated. X-ray diffraction results show that the as-prepared film is amorphous. By annealing at 800 degrees C, however, the film is crystallized. There is no secondary phase found except Si in the two films. Chemical mapping shows that no segregation of the Mn atoms appears in the annealed film. Atomic force microscopy images of the films indicate that the annealed film has a granular feature that covers uniformly the film surface while there is no such kind of characteristic in the as-prepared film. The field dependence of magnetization was measured using an alternating gradient magnetometer, and it has been indicated that the annealed film shows room-temperature ferromagnetism. (c) 2006 Elsevier B.V. All rights reserved.

(Ga,Mn,N) compounds growth with mass-analyzed low energy dual ion beam deposition

The (Ga,Mn,N) samples were grown by the implantation of low-energy Mn ions into GaN/Al2O3 substrate at different elevated substrate temperatures with mass-analyzed low-energy dual ion beam deposition system. Auger electron spectroscopy depth profile of samples grown at different substrate temperatures indicates that the Mn ions reach deeper in samples with higher substrate temperatures. Clear X-ray diffraction peak from (Ga,Mn)N is observed in samples grown at the higher substrate temperature. It indicates that under optimized substrate temperature and annealing conditions the solid solution (Ga,Mn)N phase in samples was formed with the same lattice structure as GaN and different lattice constant. (C) 2003 Elsevier Science B.V. All rights reserved.

ZnO基稀磁半导体的制备与性质研究

本论文利用溶胶一凝胶法和水热法制备了不同离子（Eu~3＋，Sm~3＋，Mn~2＋，Fe~3＋，Co~2＋，Ni~2＋）作为磁性杂质的ZnO基稀磁半导体，并系统地研究了材料的薄膜、粉末和纳米结构的结晶特性、结构形态和光、电、磁性质。溶胶一凝胶法制备的薄膜的晶体为c轴取向生长的六方纤维锌矿结构。薄膜的取向生长受烧结气氛、烧结温度和掺杂离子浓度的影响，其中烧结气氛是影响薄膜取向生长的最直接、最显著因素。随着烧结气氛中氧含量的减小，薄膜的沿c轴生长的趋势加强。此外，烧结温度的提高也增强薄膜沿c轴生长的趋势，但掺杂离子浓度的增加却抑制薄膜的c轴取向生长特性。通过薄膜表面形态的研究发现，在空气中烧结的薄膜由立方晶粒构成，而在真空中烧结的样品则由不规则的片状晶粒组成。组成薄膜的多晶颗粒粒径小于10Onm，15层薄膜的膜厚为357-366nm。掺杂离子在薄膜中均匀分布，成膜过程不改变掺杂离子（Eu3+，Sm3＋，Mn2+，Fe3＋，Co2＋，Ni2＋）和基质离子（Zn2+和O2-）的价态。 不同Eu3+掺杂浓度的ZnO薄膜样品的吸收光谱的吸收边出现在363nm和368nm之间，对应半导体材料的禁带宽度Eg=3.42～3.40ev。由于Eu3＋改变了薄膜的表面性质，Zn1-xEux（0.005≤x≤0.15）薄膜在可见光区出现了一系列干涉带。Zn1-xTMxO薄膜的吸收光谱的吸收边位置出现在356nm-369nm，对应半导体的禁带宽度为3.34-3.46eV，在可见光区发现了Co2＋的电子的d-d跃迁引起的吸收带。随着掺杂浓度的增加，薄膜的透光率逐渐减小。Zn1-xCoxO薄膜在近紫外与可见光区的透光率都在60％以上，Zn1-xEuxO薄膜的透光率则高达90％。在Zn1-xEuxO薄膜的激发发射光谱中，以613nm作为监控波长，激发光谱除了检测到Eu3＋的7F→5D能级的吸收跃迁外，还检测到最大值位于378nm附近的ZnO的吸收带。以394nm为激发波长，发射光谱检测到Eu3+的5D0→7FJ（J＝1，2，3，4）跃迁。以zno的带隙能量378nm作为激发波长进行激发，检测到Eu3+的5D0→7F2跃迁，说明基质zno和E矿十之间存在能量交换。薄膜磁性测试在4-400K温度范围内进行，发现在此温度范围内Zn0.9Eu0.1O薄膜表现居里一外斯顺磁性；在低温区，存在磁性增强现象。zno．gCoo．IO薄膜在23oK以下表现为铁磁性，200K的M-H曲线显示薄膜的剩磁（Br）约为0．21em侧g，矫顽力（Hc）约为327Oe。但Zn0.9Mn0.1O，Zn0.9Ni0.1O，Zn0.9Co0.1O薄膜的磁性测试则显示在80K以上三种薄膜均表现为顺磁性。Zn0.9Eu0.1O薄膜的电阻呈现典型的半导体性质，在ZT的磁场下，薄膜在110K获得最大14.53％的磁阻率。Zn1-xTMxO薄膜的电阻也表现典型的半导体特性，实验研究了薄膜在不同掺杂离子浓度、外加磁场以及温度条件下的磁阻性质。粉末样品中磁性离子的掺杂浓度均小于薄膜样品。Co，Fe，Ni，Mn掺杂的Zn1-xTMxO粉末在80以上均为顺磁性。在Co2＋掺杂的粉末样品中没有发现类似于薄膜样品的铁磁性，说明DMS的磁性与制备条件关系密切。实验证明了利用sol-gel方法，Zno：TM稀磁半导体能够有效地组装在MCM-41和AAO的孔道内。ZnO:TM材料组装进在MCM-41孔道后，不改变孔道的六方结构但使孔径变小。随着组装次数的增加，MCM-41的孔径和孔容累进减小。组装在AAo模板孔道内的材料呈单分散纳米颗粒状态，颗粒粒径小于loonm。组装材料的磁性测试显示：组装在MCM-41内的Zn0.9Co0.1O材料在80K-30OK呈现超顺磁性。而Mn，Fe，Ni掺杂的Zno在此温度范围内表现顺磁性。组装在AAO内的ZnO:TM(TM=Mn，Fe，Co，Ni）材料在SOK-30OK温度范围内都呈现顺磁性。在水热法合成ZnO:A（A＝Bu，Sm，co）纳米粒子的过程中，发现反应温度、压力、时间和溶液浓度等因素只影响Znl．xCoxO纳米粒子的的产量，而溶液的酸度却影响产物的形貌。控制溶液的酸度，可以控制产物的形貌从粒状向棒状转变。当溶液的PH＝5时，在甲醇：水体系中可以水热合成规则的棒状ZnO：RE（RE=Eu，Sm）纳米晶。所得到的Zn0.98Co0.02O纳米晶在80K呈超顺磁行为，而ZnO：RE（RE＝Eu，Sm）纳米晶在80K则表现较弱的顺磁性。 实验通过控制水热条件，制备了一种新型结构的柠檬酸锌晶体。由于利用了水热反应的非平衡合成条件，所得到的晶体的层状结构不同于目前已知的所有柠檬酸配合物的离散型分子结构。单晶衍射结果表明：化合物是一个由八面体和一个非对称单元交替相连构成的二维层状结构。

Mn2þ-induced room-temperature ferromagnetism and spin-glass behavior in hydrothermally grown Mn-doped ZnO nanorods

The magnetic properties of Mn-doped ZnO (ZnO:Mn) nanorods grown by hydrothermal process at a temperature of 200 8C and a growth time of 3 h have been studied. The samples were characterized by using powder X-ray diffraction with Rietveld refinement, scanning electron microscopy, energy-dispersive X-ray analysis and SQUID magnetometry. Mn (3 wt%) and (5 wt%)-doped ZnO samples exhibit paramagnetic and ferromagnetic behavior, respectively, at room temperature. The spin-glass behavior is observed from the samples with respect to the decrease of temperature. At 10 K, both samples exhibit a hysteresis loop with relatively low coercivity. The room-temperature ferromagnetism in 5 wt% Mn-doped ZnO nanorods is attributed to the increase in the specific area of grain boundaries, interaction between dopant Mn2þ ions substituted at Zn2þ site and the interaction between Mn2þ ions and Zn2þ ions from the ZnO host lattice

Sonochemical synthesis and magnetism in co-doped ZnO nanoparticles

The understanding and control of ferromagnetism in diluted magnetic semiconducting oxides (DMO) is a special challenge in solid-state physics and materials science due to its impact in magneto-optical devices and spintronics. Several studies and mechanisms have been proposed to explain intrinsic ferromagnetism in DMO compounds since the theoretical prediction of room-temperature ferromagnetism. However, genuine and intrinsic ferromagnetism in 3d-transition metal-doped n-type ZnO semiconductors is still a controversial issue. Furthermore, for DMO nanoparticles, some special physical and chemical effects may also play a role. In this contribution, structural and magnetic properties of sonochemically prepared cobalt-doped ZnO nanoparticles were investigated. A set of ZnO samples was prepared varying cobalt molar concentration and time of ultrasonic exposure. The obtained results showed that single phase samples can be obtained by the sonochemical method. However, cobalt nanoclusters can be detected depending on synthesis conditions. Magnetic measurements indicated a possible ferromagnetic response, associated to defects and cobalt substitutions at the zinc site by cobalt. However, ferromagnetism is depleted at higher magnetic fields. Also, an antiferromagnetic response is detected due to cobalt oxide cluster at high cobalt molar concentrations. © 2012 Springer Science+Business Media, LLC.

Estudo teórico das propriedades estruturais e eletrônicas do GaN e do semicondutor magnético Ga1-xMnxN no bulk e na superfície

Pós-graduação em Ciência e Tecnologia de Materiais - FC

Estudo das propriedades ópticas e vibracionais de filmes de GaN dopados com Mn elaborados por RF Magnetron Sputtering Reativo

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Untersuchungen zur Löslichkeit von Kobalt und Eisen in TiO 2-Bulk sowie in TiO 2-Nanoproben und deren Einfluss auf die magnetischen Eigenschaften

Co- und Fe-dotierte Rutil- und Anatas-Bulkproben wurden über einen Sol-Gel Prozess und anschließende thermische Behandlung dargestellt und auf ihre Zugehörigkeit zu der Gruppe der verdünnten magnetischen Oxide untersucht. Die Untersuchungen der dotierten Rutil-Proben mittels Röntgenbeugung, Elektronenmikroskopie und magnetischen Methoden zeigen, dass die Löslichkeit von Co und Fe in der TiO2-Modifikation Rutil sehr gering ist. Oberhalb von 1at% Co bzw. Fe wird neben Rutil die Bildung der Oxide CoTiO3 bzw. Fe2TiO5 beobachtet. Weitere thermische Behandlung im Argon-H2-Strom führte aufgrund der Bildung von metallischem Co bzw. Fe zu einem ferromagnetischen Verhalten. Die TiO2-Modifikation Anatas besitzt eine höhere Löslichkeit, so dass erst oberhalb von 4at% Co bzw. 10at% Fe die Phasen Co3O4 bzw. FeTiO3 neben Anatas auftreten. Entsprechende Proben zeigen ein paramagnetisches Verhalten. Oberhalb der Löslichkeitsgrenze führt die Reduktion im Argon-H2-Strom zu einem ferromagnetischen Verhalten, welches auf metallisches Co bzw. Fe zurückzuführen ist. Analog zu den Bulkproben wurden Co- und Fe-dotierte TiO2-Nanodrähte hergestellt. Das magnetische Verhalten der Fe-dotierten TiO2-Nanodrähte entspricht dem der Fe-dotierten Anatas-Bulkproben. Dagegen führt die Co-Dotierung nicht zu einem Einbau in die TiO2-Nanodrähte, sondern zur Bildung von CoOx-Nanopartikeln. Die entsprechenden Proben zeigen ein schwach ferromagnetisches Verhalten. Dies ist jedoch nicht auf eine ferromagnetische Dotierung der TiO2-Nanodrähte zurückzuführen, sondern auf nicht kompensierte Momente an den Oberflächen der als Verunreinigungen auftretenden CoOx-Nanopartikel. Zusammenfassend wird festgestellt, dass die Löslichkeit von Co und Fe in TiO2 für die Ausbildung eines ferromagnetischen Verhaltens zu gering ist. Der beobachtete Ferromagnetismus lässt sich eindeutig auf magnetische Verunreinigungen zurückführen. Somit können die dotierten TiO2 Proben nicht den verdünnten magnetischen Oxiden zugeordnet werden.

Special Heusler compounds for spintronic applications

This work emphasizes the potential of Heusler compounds in a wide range of spintronic applications. Using electronic structure calculations it is possible to design compounds for specific applications. Examples for GMR and TMR applications, for spin injection into semiconductors, and for spin torque transfer applications will be shown. After a detailed introduction about spintronics and related materials chapter 5 reports about the investigation of new half-metallic compounds where the Fermi energy is tuned in the middle of the gap to result in more stable compounds for GMR and TMR applications. The bulk properties of the quaternary Heusler alloy Co2Mn(1-x)Fe(x)Si with the Fe concentration ranging from x=0 to 1 will be reported and the results suggest that the best candidate for applications may be found at an iron concentration of about 50%. Due to the effect that in the Co2Mn(1-x)Fe(x)Si series the transition metal carrying the localized moment is exchanged and this might lead to unexpected effects on the magnetic properties if the samples are not completely homogeneous chapter 6 reports about the optimization of the Heusler compounds for GMR and TMR applications. The structural and magnetic properties of the quaternary Heusler alloy Co2FeAl(1-x)Si(x) with varying Si concentration will be reported. From the combination of experimental (better order for high Si content) and theoretical findings (robust gap at x = 0.5) it is concluded that a compound with an intermediate Si concentration close to x=0.5-0.7 would be best suited for spintronic applications, especially for GMR and TMR applications. In chapter 7 the detailed investigation of compounds for spin injection into semiconductors will be reported. It will be shown that the diluted magnetic semiconductors based on CoTiSb with a very low lattice mismatch among each other are interesting materials for spintronics applications like Spin-LEDs or other spin injection devices. Chapter 8 refers about the investigation of the theoretically predicted half-metallic completely compensated-ferrimagnet Mn$_3$Ga as a suitable material for spin torque transfer applications. The Curie temperature is above 730~K and the electronic structure calculations indicate a nearly half-metallic ferrimagnetic order with 88% spin polarization at the Fermi energy.}

Vacancy-induced magnetism in graphene and graphene ribbons

We address the electronic structure and magnetic properties of vacancies and voids both in graphene and graphene ribbons. By using a mean-field Hubbard model, we study the appearance of magnetic textures associated with removing a single atom (vacancy) and multiple adjacent atoms (voids) as well as the magnetic interactions between them. A simple set of rules, based on the Lieb theorem, link the atomic structure and the spatial arrangement of the defects to the emerging magnetic order. The total spin S of a given defect depends on its sublattice imbalance, but some defects with S=0 can still have local magnetic moments. The sublattice imbalance also determines whether the defects interact ferromagnetically or antiferromagnetically with one another and the range of these magnetic interactions is studied in some simple cases. We find that in semiconducting armchair ribbons and two-dimensional graphene without global sublattice imbalance, there is a maximum defect density above which local magnetization disappears. Interestingly, the electronic properties of semiconducting graphene ribbons with uncoupled local moments are very similar to those of diluted magnetic semiconductors, presenting giant Zeeman splitting.

Electronic structure and magnetism of Mn-doped GaSb for spintronic applications: A DFT study

We have carried out first-principles spin polarized calculations to obtain comprehensive information regarding the structural, magnetic, and electronic properties of the Mn-doped GaSb compound with dopant concentrations: x¼0.062, 0.083, 0.125, 0.25, and 0.50. The plane-wave pseudopotential method was used in order to calculate total energies and electronic structures. It was found that the MnGa substitution is the most stable configuration with a formation energy of 1.60 eV/Mn-atom. The calculated density of states shows that the half-metallic ferromagnetism is energetically stable for all dopant concentrations with a total magnetization of about 4.0 lB/Mn-atom. The results indicate that the magnetic ground state originates from the strong hybridization between Mn-d and Sb-p states, which agree with previous studies on Mn-doped wide gap semiconductors. This study gives new clues to the fabrication of diluted magnetic semiconductors