Magnetic properties of nitrogen doped ZnO

Using the first principle method based on density functional theory,this paper studies the electronic structure and the ferromagnetic stability in N-doped ZnO.The calculated results based on local density approximation(LDA) and LDA+U method show that ferromagnetism coupling between N atoms is more energetically favourable for eight geometrically distinct configurations.The dominant ferromagnetic interaction is due to the hybridization between O 2p and N 2p.The origin of the ferromagnetic state in N doped ZnO is discussed by analysing coupling between N 2p levels.We also analyse N dopant concentration and lattice strain effect on ferromagnetism.     

Local structures of Mn in dilute magnetic semiconductor ZnMnO

The local structures of Mn atoms in (Zn, Mn)O have been investigated by extended x-ray absorption fine structure (EXAFS). For samples with lower Mn concentrations (<11%), EXAFS results show that the majority of Mn atoms are incorporated at Zn substitutional sites. Ferromagnetic behavior has been observed in those ZnMnO samples. It is the direct evidence to suppose that the ferromagnetism in (Zn, Mn)O is not due to Mn-related secondary phases but is intrinsic. When the Mn concentration is increased to 20% or higher, the EXAFS simulation shows the formation of MnO secondary phases and the films show a change from ferromagnetic (FM) to antiferromagnetic (AFM) state. We suggest that Mn-related secondary phases might be responsible for the transformation from FM to AFM state.     

First-Principles Study on Magnetic Properties of V-Doped ZnO Nanotubes

Electronic and magnetic properties of V-doped ZnO nanotubes in which one of Zn^2＋ ions is substituted by V^2＋ ions are studied by the first-principles calculations of plane wave ultra-soft pseudo-potential technology based on the spin-density function theory. The computational results reveal that spontaneous magnetization in Vdoped （9,0） ZnO nanotubes can be induced without p-type or n-type doping treatment, and the ferromagnetism is isotropic and independent of the chirality and diameter of the nanotubes. It is found that V-doped ZnO nanotubes have large magnetic moments and are ferromagnetic half-metal materials. Moreover, the ferromagnetic coupling among V atoms is generated by O 2p electron spins and V 3d electron spins localized at the exchanging interactions between magnetic transitional metal （TM） impurities. The appearance of ferromagnetism in V-doped ZnO nanotubes gives some reference to fabrication of a transparent ferromagnet which may have a great impact on industrial applications in magneto-optical devices.     

Evidence of a critical Mn-Mn distance for the onset of ferromagnetism in NiAs type compounds

Recently, the technique of spin wave resonance has been used to measure the exchange coupling constants as a function of composition in several solid solutions of MnSb. From an examination of the results we show that in those alloys in which the linear chains of magnetic atoms are not disturbed, the exchange coupling constants become positive leading to ferromagnetic coupling when the distance between Mn atoms becomes equal to 2.83 Å or more. This experimental value of the Mn-Mn distance for the onset of ferromagnetism is equal to the critical distance derived by Forrer after examining a large number of materials which owe their ferromagnetism predominantly to the direct exchange interaction.     

Structures and magnetic behaviours of TiO2-Mn-TiO2 multilayers

The TiO2-Mn-TiO2 multilayers are successfully grown on glass and silicon substrates by alternately using radio frequency reactive magnetron sputtering and direct current magnetron sputtering. The structures and the magnetic behaviours of these films are characterised with x-ray diffraction, transmission electron microscope (TEM), vibrating sample magnetometer, and superconducting quantum interference device (SQUID). It is shown that the multi-film consists of a mixture of anatase and rutile TiO2 with an embedded Mn nano-film. It is found that there are two turning points from ferromagnetic phase to antiferromagnetic phase. One is at 42 K attributed to interface coupling between ferromagnetic Mn3O4 and antiferromagnetic Mn2O3, and the other is at 97 K owing to the interface coupling between ferromagnetic Mn and antiferromagnetic MnO. The samples are shown to have ferromagnetic behaviours at room temperature from hysteresis in the M-H loops, and their ferromagnetism is found to vary with the thickness of Mn nano-film. Moreover, the Mn nano-film has a critical thickness of about 18.5 nm, which makes the coercivity of the multi-film reach a maximum of about 3.965×10 2 T.     

基于XPS与XAS的稀磁半导体GaMnN电子结构研究

采用基于同步辐射技术的X射线光电子能谱(XPS)与X射线吸收谱(XAS)测试由金属有机化学气相沉积(MOCVD)技术制备的不同Mn掺杂浓度的稀磁半导体GaMnN薄膜的电子结构,探究Mn掺杂浓度对磁性原子Mn周围的局域环境和电子态等方面的影响,并阐述材料铁磁性变化的机理. XPS和XAS图谱分析表明：Mn²⁺和Mn³⁺共存于薄膜样品内,样品D中Mn²⁺占比高达70%-80%,N空位随Mn掺杂浓度增加而增多且N空位能够使空穴浓度降低,导致Mn 3d和N 2p轨道间的相互交换作用减小,从而减弱体系铁磁性.此外,Mn不同的掺杂浓度会影响GaMnN薄膜p-d耦合杂化能力的强弱,当掺Mn 1.8%时具有较强的p-d耦合杂化能力.     

Zn1-xMnxS(001)稀磁半导体薄膜的能量稳定性、磁性和电子结构

通过基于广义梯度近似的总能密度泛函理论研究不同Mn掺杂浓度的ZnS(001)薄膜的电学和磁学特性. 计算单个Mn原子和两个Mn原子处于各种掺杂位置及不同的磁耦合状态时的能量稳定性.计算了单个Mn原子掺杂和两个Mn原子掺杂的ZnS(001)薄膜的态密度. 不同掺杂组态的p-d杂化的程度不同. 不同掺杂组态,Mn原子所处的晶场环境不同,所以不同掺杂组态的Mn的3d分波态密度峰的劈裂有很大的不同. 掺杂两个Mn原子时,得到三种稳定组态的基态都是反铁磁态. 分析了以上三种能量稳定的组态中,两个Mn原子在不同磁耦合状态下的3d态密度图. 当两原子为铁磁耦合时,由于d-d电子相互作用,使反键态的态密度峰明显加宽. 随着Mn掺杂浓度的增加,Mn原子有相互靠近,并围绕S原子形成団簇的趋势. 对于这样的组态,Mn原子之间为反铁磁耦合能量更低.     

V,Cr,Mn掺杂MoS2磁性的第一性原理研究

基于第一性原理的自旋极化密度泛函理论分别研究了过渡金属V, Cr, Mn掺杂单层MoS₂的电子结构、 磁性和稳定性. 结果表明: V和Mn单掺杂均能产生一定的磁矩, 而磁矩主要集中在掺杂的过渡金属原子上, Cr单掺杂时体系不显示磁性. 进一步讨论双原子掺杂MoS₂ 体系中掺杂原子之间的磁耦合作用发现, Mn掺杂的体系在室温下显示出稳定的铁磁性, 而V掺杂则表现出非自旋极化基态. 形成能的计算表明Mn掺杂的MoS₂体系相对V和Cr 掺杂结构更稳定. 由于Mn掺杂的MoS₂ 不仅在室温下可以获得比较好的铁磁性而且其稳定性很高, 有望在自旋电子器件方面发挥重要的作用. 关键词： 2')" href="#">单层MoS₂ 掺杂 铁磁态 第一性原理     

Zn1-xMnxO纳米薄膜磁有序性的Monte Carlo模拟

基于Zn_1-xMn_xO纳米薄膜磁性研究的实验结果及相关理论,建立了一个包含多种交换作用的Ising多层膜模型,采用Monte Carlo模拟的Metropolis算法对于其铁磁序的成因进行了模拟研究.结果表明,Mn掺杂浓度(x)越低越有利于铁磁序的形成,但是x越低,系统的磁化强度越小,居里温度越低.载流子对铁磁序的形成所起的调节作用随着x的增大而增强,又随着磁各向异性常数(K)的增大而弱化.本 关键词： 稀磁半导体(DMS) 1-xMn_xO纳米薄膜')" href="#">Zn_1-xMn_xO纳米薄膜 Ising多层膜 Monte Carlo模拟     

Stabilization of ferromagnetism in Mn doped ZnO with C co-doping

We present a method for stabilizing ferromagnetism in Mn doped ZnO. We find that Mn doped ZnO show anti-ferromagnetic order in the absence of additional carriers. When Mn doped ZnO is co-doped with C atom at O sites ferromagnetic state gets stabilized. The C doping creates holes which leads to stabilization of ferromagnetic state via hole mediated double exchange mechanism.     

Interface double-exchange ferromagnetism in the Mn-Zn-O system: new class of biphase magnetism

In this Letter, we experimentally show that the room temperature ferromagnetism in the Mn-Zn-O system recently observed is associated with the coexistence of Mn(3+) and Mn(4+) via a double-exchange mechanism. The presence of the ZnO around MnO(2) modifies the kinetics of MnO(2)-->Mn(2)O(3) reduction and favors the coexistence of both Mn oxidation states. The ferromagnetic phase is associated with the interface formed at the Zn diffusion front into Mn oxide, corroborated by preparing thin film multilayers that exhibit saturation magnetization 2 orders of magnitude higher than bulk samples.     

Control of ferromagnetism via electron doping in In2O3:Cr

Carrier-induced ferromagnetism in wide-gap transparent conductive oxides has been widely discussed and debated, leading to confusion and skepticism regarding whether dilute magnetic oxides exist at all. We show from density-functional calculations within a band-gap corrected approach that ferromagnetic Cr-Cr coupling can be switched on and off via electron doping in the wide-gap transparent n-type conductive oxide In2O3. We show that (i) Cr does not produce in In2O3 any free electrons and renders the system an insulating paramagnet. (ii) Extrinsic n-type doping of In2O3:Cr via Sn produces free electrons, whose concentration is controllable via the oxygen partial pressure. Such additional carriers stabilize a strong long-range Cr-Cr ferromagnetic coupling.     

Enhanced ferromagnetism in nano-sized Zn0.95Mn0.05O grains

The present work reports ferromagnetism by doping magnetic Mn atoms in the diamagnetic ZnO matrix and the ferromagnetism has been extended up to 640 K in nano-grained Zn_0.95Mn_0.05O samples. The bulk and nano-grained samples were stabilized in hexagonal crystal structure with space group p63mc. The grain size and lattice strain of the samples were estimated from room temperature XRD spectrum. Surface morphology of the samples was examined at room temperature using SEM picture and EDX spectrum. The ferromagnetism of the bulk material shows enhancement in nano-grained samples, which was mainly due to the solution of Mn atoms into the lattice sites of ZnO by mechanical milling. The enhancement of magnetic moment and ferromagnetic ordering temperature with reduction in grain size has been understood in terms of the core-shell structure and existing theoretical models. The present work also demonstrated the role of surface spin disorder on the enhancement of ferromagnetism in Zn_0.95Mn_0.05O nanograins.     

Effect of impurity ions configurations on the magnetic properties of Mn-doped ZnO

The magnetic and electronic properties of Mn-doped ZnO are studied by first-principles calculations. It is found that the exchange interaction between Mn ions depends on the Mn-Mn distribution configuration and distance. We also found that the ferromagnetism can be existed when the Mn-Mn distance is large and Mn ions are distributed uniformly, and the long ranged ferromagnetism is explained by the interaction between Mn and O atoms. Thus, it is possible to tune ferromagnetism in Mn-doped ZnO semiconductors by controlling the doping position in ZnO lattice.     

Coupling of magnetic and ferroelectric hysteresis by a multicomponent magnetic structure in Mn2GeO4

The olivine compound Mn(2)GeO(4) is shown to feature both a ferroelectric polarization and a ferromagnetic magnetization that are directly coupled and point along the same direction. We show that a spin spiral generates ferroelectricity, and a canted commensurate order leads to weak ferromagnetism. Symmetry suggests that the direct coupling between the ferromagnetism and ferroelectricity is mediated by Dzyaloshinskii-Moriya interactions that exist only in the ferroelectric phase, controlling both the sense of the spiral rotation and the canting of the commensurate structure. Our study demonstrates how multicomponent magnetic structures found in magnetically frustrated materials like Mn(2)GeO(4) provide a new route towards functional materials that exhibit coupled ferromagnetism and ferroelectricity.     

二氧化铈基稀磁氧化物的第一性原理研究

基于第一性原理的计算方法研究了纯CeO_2、Co掺杂CeO_2和同时引入氧空位Vo和Co掺杂的CeO_2稀磁半导体体系.通过计算体系的能带结构和态密度,探讨了该体系磁性产生的机制.计算发现,纯CeO_2体系不具有磁性;没有氧空位Vo的Co掺杂CeO_2体系中,Co离子之间通过O原子发生超交换反铁磁耦合,体系无铁磁性;当氧空位Vo和Co离子同时存在于CeO_2体系中时,Co离子之间通过氧空位Vo发生铁磁耦合,该体系表现出铁磁性能.另外,由氧空位Vo诱导的Co离子之间的铁磁耦合不仅发生在紧邻的两个Co离子,而且可以扩展到几个原子距离的长度.计算结果证明了氧空位Vo诱导铁磁性耦合机制.本文工作将为CeO_2基稀磁半导体体系制备与磁学性质的研究提供支持.     

Spin-fluctuation mediated high-temperature ferromagnetism in Si:Mn dilute magnetic semiconductors

We discuss a possible route to explain high-temperature ferromagnetism in Si:Mn dilute magnetic semiconductors. We argue that most Mn atoms are segregated within nanometer-sized regions of magnetic precipitate and form the alloy, or compound, MnSi_{2 -z} with z ≈ (0.25?\div0.30), whereas a small minority of Mn atoms forms ?ngstr?m-sized magnetic defects embedded in the host. Assuming that MnSi_{2 -z} is a weak itinerant ferromagnet which supports sizable spin fluctuations (paramagnons) far above the intrinsic Curie temperature, we show that the Stoner enhancement of the exchange interaction between the local magnetic moments of the defects occurs. As a result, a significant increase of the temperature of global ferromagnetic order in the system is achieved. We develop a phenomenological approach, to qualitatively describe this effect.     

Nature of the well screened state in hard X-ray Mn 2p core-level photoemission measurements of La1-xSrxMnO3 films

Using hard x-ray (HX; hnu=5.95 keV) synchrotron photoemission spectroscopy (PES), we study the intrinsic electronic structure of La(1-x)Sr(x)MnO(3) (LSMO) thin films. Comparison of Mn 2p core-levels with soft x-ray (SX; hnu approximately 1000 eV) PES shows a clear additional well-screened feature only in HX PES. Takeoff-angle dependent data indicate its bulk (> or =20 A) character. The doping and temperature dependence track the ferromagnetism and metallicity of the LSMO series. Cluster model calculations including charge transfer from doping-induced states show good agreement, confirming this picture of bulk properties reflected in Mn 2p core-levels using HX PES.     

Ab-initio calculation of magnetic properties of Mn-doped ZnGeN2

We have performed a density functional theory within a generalized gradient approximation study of Mn-doped zinc germanium dinitride (ZnGeN₂) semiconductor. Our results show Mn-doped ZnGeN₂ to be antiferromagnetic for Mn_Zn (Mn substitutes Zn site) and ferromagnetic for Mn_Ge (Mn substitutes Ge site). Ferromagnetic state is also preferred if Mn atoms substitute both Zn and Ge sites. Formation of half-metallic ferromagnetism is possible in this type of material.     

Giant magnetic moments of nitrogen-doped Mn clusters and their relevance to ferromagnetism in Mn-doped GaN

Calculations based on density-functional theory show that the stability and magnetic properties of small Mn clusters can be fundamentally altered by the presence of nitrogen. Not only are their binding energies substantially enhanced, but also the coupling between the magnetic moments at Mn sites remains ferromagnetic irrespective of their size or shape. In addition, these nitrogen-doped Mn clusters carry giant magnetic moments ranging from 4mu(B) in MnN to 22mu(B) in Mn5N. It is suggested that the giant magnetic moments of MnxN clusters may play a key role in the ferromagnetism of Mn-doped GaN which exhibit a wide range (10-940 K) of Curie temperatures.