Ferromagnetism in an Itinerant Electron Cluster

In the present paper, we study the existence of metallic ferromagnetism in a cluster of nanometer scale,which is described by the Hubbard model defined on a complete graph. Therefore, the system is highly frustrated with respect to electron hopping. By solving the model exactly, we show that its ground state is fully spin-polarized at half-filling, even if the Coulomb interaction is finite. This conclusion is in sharp contrast to the well-known result for the Hubbard model on a bipartite lattice. As a result, our exact solution strongly suggests that frustration may play an important role in causing metallic ferromagnetism.     

The Antiferromagnetic Correlations in the Half—Filled Double—Exchange Model at Finite Temperature

We extend a previous result of ours [G.S.Tian,Phys.Rev.B63 (2001) 224413] on the antiferromagnetic spin correlations in the half-filled Hubbard model at finite temperature to the double-exchange model.To overcome the mathematical difficulty cused by the S=3/2 localized spin freedom in this model,we apply both Zener‘s argument and the finite-temperature spin-reflection-positivity method to show rigorously that,at any temperature T,the spin correlations in the half-filled double-exchange model are predominantly antiferromagnetic.This conclusion is completely consistent with the experimental observations and the previous theoretical results by approximate methods.     

Nagaoka Ferromagnetism in Two-Band Hubbard Model

In the present paper, we investigate the existence of ferromagnetism in a two-band Hubbard model, by applying a recently-introduced method by us to study ferromagnetism in metallic clusters. We prove rigorously that the ground state of this model is ferromagnetic if the intra-orbital Coulomb repulsion between electrons is infinitely strong and only one hole exists in the system. Our theorem improves a previous result. Furthermore, our method can also be applied to deal with the case of multiple holes.     

Transport,magnetic, and thermal properties of non-centrosymmetric Yb2Co12P7

We report magnetization and specific heat measurements down to 2?K and electrical resistivity down to millikelvin temperatures on polycrystalline samples of the non-centrosymmetric compound Yb₂Co₁₂P₇. In addition to the previously reported ferromagnetic ordering of the cobalt sub-lattice at T _C ?=?136?K we find a magnetic transition below T _M ?=?5?K that is likely associated with ordering of the Yb ions. The broad nature of the specific heat anomaly suggests disordered magnetism and possible short-range correlations well above T_M .     

A Rigorous Investigation on the Ground State of the Penson-Kolb Model

By using either numerical calculations or analytical methods, such as the bosonization technique, the ground state of the Penson-Kolb model has been previously studied by several groups. Some physicists argued that, as far as the existence of superconductivity in this model is concerned, it is canonically equivalent to the negative-U Hubbard model.However, others did not agree. In the present paper, we shall investigate this model by an independent and rigorous approach. We show that the ground state of the Penson-Kolb model is nondegenerate and has a nonvanishing overlap with the ground state of the negative-U Hubbard model. Furthermore, we also show that the ground states of both the models have the same good quantum numbers and may have superconducting long-range order at the same momentum q ＝ 0. Our results support the equivalence between these models.     

缺陷TiO2中铁磁性的第一性原理计算

计算钛空位（V_Ti）和氧空位（V_O）共存情况下未掺杂金红石TiO₂的铁磁性.发现V_O可以产生局域磁矩,由它引起的自旋极化比V_Ti引起的更加局域,导致V_O之间的铁磁耦合作用弱于V_Ti之间的铁磁耦合作用.V_Ti之间的铁磁耦合在引入V_O之后进一步加强.V_O引入的电子调制两个分离的V_Ti之间的长程铁磁耦合.加入V_Ti之后,两个V_O的磁矩猝灭,当V_O的数量多于V_Ti的数量二倍时,V_O会对磁矩有贡献.结果与实验发现的V_O可以提高铁磁有序,并且总的磁矩会随着V_O数量的增多而增加的结果符合很好.     

Ferromagnetism in Thermally Decomposed Polyimide Films

Magnetic resonance studies of polyimide films thermally decomposed in flowing N₂ at 520°C reveal the presence of two very different magnetic resonance spectra at room temperature. One spectra is a sharp temperature independent paramagnetic resonance line having a g value of 1.990, typical of a free radical. The other much broader line centered at lower field displays a marked broadening and shift to lower magnetic field as the temperature is lowered, characteristic of a ferromagnetic resonance (FMR) signal. Measurements of the AC susceptibility as a function of magnetic field strength confirm the existence of ferromagnetism at room temperature. Magnetic force microscope (MFM) imaging at room temperature show evidence of long thin ferromagnetic regions in the decomposed polymer.     

Ferromagnetism in ZnO with（Mn,Li） codoping

First-principles calculations were performed to investigate the magnetic properties of Zn(Mn,Li)O based on the Perdew-Burke-Ernzerhof form of generalized gradient approximation. Antiferromagnetic (AFM) ordering is the ground state in Mn-doped ZnO system without the codopant of Li, while seven different geometrical configurations of Zn(Mn,Li)O prefer stable ferromagnetic (FM) ordering. We found that dopant Li can effectively change the magnetic coupling in the ZnMnO system. The Curie temperature (T_C) of FM ordering depends on the geometric configuration, and the highest T_C is about 1388 K. The FM stabilization is greatly affected by Mn-Mn distance rather than by the position of dopant Li. We propose that dopant Li mediates FM coupling through a double exchange interaction or an RKKY interaction when Li is located, respectively, near or far from Mn ions.     

Ferromagnetism in Eu-doped ZnO films deposited by radio-frequency magnetic sputtering

This paper reports that Eu-doped ZnO films were successfully deposited on silicon (100) by radio-frequency magnetic sputtering. The x-ray diffraction patterns indicate that Eu substitutes for Zn in the lattice. Ferromagnetic loops were obtained by using superconducting quantum interference device at 10 K and room temperature. No discontinuous change was found in both of the zero-field-cooled and field-cooled curves. The observed ferromagnetism in Eu-doped ZnO can be attributed to a single magnetic phase. The saturation magnetisation decreased remarkably for the Eu-doped ZnO prepared by introducing 5% of oxygen in the sputtering gas or by the post annealing in O2, suggesting that the defects play key roles in the development of ferromagnetism in Eu-doped ZnO films.     

Ferromagnetism induced by oxygen-vacancy complex in (Mn,in) codoped ZnO

Mn doped Zinc oxide (ZnO) thin films were prepared by metal organic chemical vapor deposition (MOCVD) technique. Structural characterizations by X-ray diffraction technique (XRD) and photoluminescence (PL) indicate the crystal quality of ZnO films. PL and Raman show a large fraction of oxygen vacancies (V_O²⁺) are generated by vacuum annealed the film. The enhancement of ferromagnetism in post-annealed (Mn, In) codoped ZnO could result from V_O²⁺ incorporation. The effect of V_O²⁺ on the magnetic properties of (Mn, In) codoped ZnO has been studied by first-principles calculations. It is found that only In donor cannot induce ferromagnetism (FM) in Mn-doped ZnO. Besides, the presence of V_O²⁺ makes the Mn empty 3d-t_2g minority state broadened, and a t_2g-V_O²⁺ hybrid level at the conduction band minimum forms. The presence of V_O²⁺ can lead to strong ferromagnetic coupling with the nearest neighboring Mn cation by BMP model based on defects reveal that the ferromagnetic exchange is mediated by the donor impurity state, which mainly consists of Mn 3d electrons trapped in oxygen vacancies.     

Ferromagnetism in the single-band Hubbard model: An exact high-temperature expansion

The first ten terms of the high-temperature expansion of the susceptibility of the single-band Hubbard model in the strong correlation limit are obtained for arbitrary electron density. The series is analyzed by ratio methods and Padé approximants. A critical temperature is found for 0.2 0.8; for > 1 further terms in the series are required.Supported in part by the National Research Council of Canada.     

NH3等离子体后处理Co掺杂ZnO的薄膜结构及磁学性能

通过电化学沉积方法成功生长了Co掺杂ZnO的薄膜,但并没有实现室温下的铁磁性.通过NH3等离子体的后处理,导致有一部分N原子进入了ZnO晶格替代了一部分O格位,从而在ZnO中产生空穴.在空穴间接交换作用下,ZnCoO薄膜中产生了被束缚的磁极子,产生了室温下的铁磁性.     

Pentagon chain in external fields

We consider a pentagon chain described by a Hubbard type of model considered under periodic boundary conditions. The system (i) is placed in an external magnetic field perpendicular to the plane of the cells, and (ii) is in a site-selective manner under the action of an external electric potential. In these conditions, we show in an exact manner that the physical properties of the system can be qualitatively changed. The changes cause first strong modifications of the band structure of the system created by the one-particle part of the Hamiltonian, and second, produce marked changes of the phase diagram. We exemplify this by deducing ferromagnetic ground states in the presence of external fields in two different domains of the parameter space.     

Ferromagnetism,half-metallicity and spin-polarised electronic structures characterisation insights in Ca1−xTixO

ABSTRACT

In this study, we have computed the structural, electronic and half-metallic ferromagnetic properties of Ca_1?xTi_xO compounds at concentrations x?=?0.125, 0.25, 0.5 and 0.75 by employing the first-principle approaches of density functional theory. The generalised gradient approximation of Wu and Cohen (GGA-WC) is used to calculate the structural parameters, whereas the electronic structures and magnetic properties are characterised by the accurate Tran–Blaha-modi?ed Becke–Johnson potential (TB-mBJ). The lattice constant, bulk modulus and indirect gap of CaO are in good agreement with other theoretical and experimental results. The Ca_0.25Ti_0.75O at x?=?0.75 has metallic ferromagnetic nature. The Ca_0.875Ti_0.125O, Ca_0.75Ti_0.25O and Ca_0.50Ti_0.50O compounds have total magnetic moments of 2?μ_B per Ti atom with a half-metallic character, a spin polarisation of 100% and a large half-metallic gap of 1.345?eV for x?=?0.125. Therefore, the Ca_1?xTi_xO material with a low concentration of Ti is a true half-metallic ferromagnet and seems to be a promising candidate for semiconductor spintronics.     

Ferromagnetism on a paramagnetic host background in cobalt-doped Bi_2Se_3 topological insulator

Cobalt-doped Bi2Se3topological insulators have been grown though melt-grown reaction. The Bi2Se3matrix is diamagnetic and doped sample is a superposition of ferromagnetism(FM) and paramagnetism(PM) behavior at low temperature. The values of MSmol, Hc, and Mr increase as the Co concentration increases. Two possible explanations have been proposed for the origin of ferromagnetism in Co-doped Bi2Se3. One is the magnetic ordering from nanoclusters of Co–Se compound in the crystals, and the other is Ruderman–Kittel–Kasuya–Yosida(RKKY) interaction between magnetic impurities.     

室温下Ga1-xHoxN (x=0.0 and 0.05)稀磁半导体薄膜磁性

采用热蒸镀技术和后续氨退火制备了Ho掺杂GaN稀磁半导体薄膜. X射线衍射分析表明,所有的峰属于六角纤锌矿结构. 利用扫描电子显微镜和能量色散谱分别进行了表面形貌和成分分析. 用振动样品磁强计在室温测定了Ga_1-xHo_xN(x=0.0,0.05)的室温铁磁性. 磁性测量结果表明,未掺杂薄膜GaN具有抗磁性行为,而Ho掺杂Ga_0.95Ho_0.05N的薄膜表现出铁磁行为.     

氧化钙薄膜中缺陷诱导的高居里温度铁磁性研究

通过脉冲激光沉积在(111)取向钇稳定的氧化锆(YSZ)衬底上制备了氧化钙(CaO)薄膜.室温下磁滞回线的实验观测数据表明CaO薄膜具有明显的铁磁性.X射线衍射和X射线光电子能谱分析表明,CaO薄膜为(111)取向,没有杂质相.在高真空条件下生长和退火的CaO薄膜都表现出铁磁性磁化行为,而在相应的CaO靶材上没有检测到...     

Ferromagnetism in one-dimensional Hubbard model induced by the next-nearest-neighbor hopping at electron density 3/2

Ferromagnetism in the one-dimensional Hubbard model with the next-nearest-neighbor hopping is explored by using the exact-diagonalization method in a small cluster and the equation-of-motion method in the thermodynamic limit with electron density n = 3/2. With these two complementary methods, it is found that an intermediate value of the next-nearest- neighbor hopping amplitude tl tends to stabilize the fully polarized ferromagnetic state under the condition that the on-site coulomb interaction U is sufficiently large in our model. The ground-state phase diagram of the model is presented in the tl-U plane.     

No Intrinsic Ferromagnetism in Transition-Metal-Doped ZnO: an Electron Paramagnetic Resonance Analysis

The magnetic properties of transition-metal (TM)-doped n-type ZnTmO are theoretically analyzed and compared to the published experimental results. For the two most studied cases of TM = Mn, Co, we demonstrate the interest of magnetic resonance experiments for the investigation of the origin of sometimes observed ferromagnetism and paramagnetic properties in single-phase material. The conclusion is that ferromagnetism in ZnTMO is extrinsic and related to the second phase formation during the growth or after annealing. Substitutional TM (Mn, Co) in ZnO gives rise to antiferromagnetic nearest-neighbor interactions similar to the case of the other TM-doped II–VI compounds.     

结构相变对Fe掺杂TiO2薄膜室温铁磁性的影响

采用直流磁控溅射方法在玻璃基底上制备了不同Fe掺杂浓度的TiO₂薄膜, 并对其晶体结构和磁特性进行了研究.在所有掺杂样品中,均观察到了室温铁磁性, 磁性源于Fe离子与其近邻空间分布的空穴相互作用. 在掺杂量为7%的锐钛矿相薄膜中观察到了最大的磁化强度. 随着Fe掺杂浓度的进一步增加, TiO₂的晶体结构逐渐由锐钛矿相向金红石相转变,并且磁性减弱. 不同结构的TiO₂中Ti–O键长不同,导致替代的磁性Fe离子与空穴的作用强度发生改变, 进而使其磁性发生变化. 关键词： 稀磁半导体 结构相变 铁磁性