Ab initio calculation and analysis of the properties of digital magnetic heterostructures and diluted magnetic semiconductors of IV and III-V groups

We present the first-principles calculations of digital magnetic heterostructures Si/M, Ge/M. GaAs/M, GaSb/M, GaN/M and GaN/M (50%) with M=Cr, Mn, Fe, and Co. The interaction between magnetic dopants results in a wide spin-polarized two-dimensional band inside the gap. It is found that beginning occupation of the minority-spin band greatly increases the energy of the ferromagnetic (FM) state and leads, as a rule, to the antiferromagnetic (AFM) spin ordering. This mechanism causes transition to the AFM state, when interaction between magnetic atoms is too strong, and defines the optimum of Curie temperature as a function of transition element concentration in magnetic layers.     

Structural and magnetic properties of MoS2 monolayer zigzag nanoribbon doped by Ti,V, Cr,and Mn

In order to find new functions of monolayer MoS₂ in nanoelectronics or spin electronic devices, using spin-polarized density functional theory (DFT) calculations with on-site coulomb interaction (U), we investigated substitutional doping of Mo atoms of monolayer zigzag MoS₂ nanoribbon (ZZ-MoS₂ NR) by transition metals (TM) (where TM = Ti, V, Cr, Mn) at the Mo-edge, S-edge, and the middle of the NRs. The results of this study indicate the NR widened irrespective of the doped TM position and type, and the Mo-edge was found as the easiest substitutional position. For ZZ-MoS₂ NR doped by Mn, Cr or V atoms, the preferred magnetic coupling state is the edge atoms of S at the S-edge, exhibiting the same spin polarization with TM (named the FM1 state), attributing the NR with metallic magnetism. For Ti-doped monolayer ZZ-MoS₂ NR, in addition to the FM1 state, other preferred magnetic coupling state was observed in which the edge atoms of S at the S-edge exhibit the opposite spin polarization with that of Ti (named the FM2 state). Thus, the NR doped by Ti atom possesses metallic (FM1 state) or half-metallic (FM2 state) magnetism. The total magnetic moments of the ZZ-MoS₂ NR doped by TM follows a linear relationship as a function of the TM dopants (Mn, Cr, V, and Ti). Under $>4\text{\%}$ applied strain, the NR doped by Ti atom only presents the characteristics of half-metallic magnetism as the initial one in the FM2 state, and its total magnetic moment always remained 0 μB, i.e., it was not affected by the width of the NR. This study provides a rational route of tuning the magnetic properties of ZZ-MoS₂ NRs for their promising applications in nanoelectronics and spin electronic devices.     

TM掺杂的Ⅲ-Ⅴ族稀磁半导体电磁性质的第一原理计算

使用基于自旋局域密度泛函理论的第一性原理方法对3d过渡金属(TM=V，Cr，Mn，Fe，Co和Ni)掺杂的Ⅲ-Ⅴ族半导体(GaAs和GaP)的电磁性质进行了计算.结果发现：用V，Cr和Mn掺杂时体系将出现铁磁状态，而Fe掺杂时将出现反铁磁状态，Co和Ni掺杂时，其磁性则不稳定.其中，Cr掺杂的GaAs和GaP将可能是具有较高居里温度的稀磁半导体(DMS).在这些DMS系统中，V离子的磁矩大于理论期待值，Fe，Co和Ni离子的磁矩小于理论期待值，Cr和Mn离子的磁矩与期待值的差距取决于晶体的对称性以及磁性离子的能带分布.此外，使用Si和Mn共同对Ⅲ-Ⅴ族半导体进行掺杂，将有利于DMS表现为铁磁状态，并可以使体系的T_C进一步提高. 关键词： 稀磁半导体 过渡金属 掺杂 共掺杂     

双轴应变调控二维单层C2X2TM（X = O, H; TM=Fe, Cr） 的多铁性研究

基于密度泛函理论的第一性原理计算,系统地研究了过渡金属原子插层的单层氧化/氢化石墨烯的磁学性质和铁电性质.在考虑了电子在位库仑作用和自旋轨道耦合作用下,得到了过渡金属Fe、Cr插层形成的C₂X₂TM二维单层膜的稳定结构以及基态磁性结构,研究了不同应变作用下C₂X₂TM的磁性、能带、铁电极化以及电子结构的变化.结果发现,对于任何应变下的C₂X₂TM其基态磁性都为手性逆时针反铁磁结构.在无应变时体系存在一个较大的离子翻转势垒,通过外加双轴应变,可有效调控体系的势垒高度和能隙,发现25%应变下C₂O₂Cr和30%应变时C₂O₂Fe单层薄膜具有与GeS等二维铁电材料相近的铁电极化和翻转势垒,这些研究结果表明C₂O₂Fe(Cr)单层薄膜是一种新型二维多铁性材料.     

First-principles studies: Thiolated Au2Cr and Au6Cr clusters

The structural, energetic and magnetic properties of thiol-passivating Au₂Cr and Au₆Cr clusters are investigated by performing first-principles calculation based on density functional theory. We find that the adsorption of thiolate is energetically more favorable than thiol and that the thiolates favor “top” site adsorption on Cr atom in one methanethiolate adsorbed Au₂Cr cluster while they favor “bridge” site adsorption on top of the middle point of Cr–Au bonds in three methanethiolates adsorbed Au₂Cr cluster. In thiol-passivating Au₆Cr cluster, the thiol favor “top” site adsorption on top of atom Au while the thiolate favor “bridge” site adsorption on top of the middle point of Au–Au bonds. The energetics of the reactions indicates that these thiol-passivating Au₂Cr or Au₆Cr clusters can be used as hydrogen storage materials. There are large and positive spin populations on atom Cr. The spin populations of atoms Au, S and H are attributed to both the spin delocalization and the spin polarization mechanisms.     

Half-Heusler合金Cu1-xFexMnSb的电子结构和反铁磁-铁磁相变

基于密度泛函理论（DFT）,使用局域密度近似（LDA）研究了Heusler合金Cu1-xFexMnSb的电子结构和反铁磁-铁磁相变。研究发现,两种磁状态下的合金晶格常数随掺杂浓度x变化很好地满足Vegard定理。当x>0.5时,铁磁态合金的总磁矩很好地符合SP规律,然而当x<0.5时,却发生了明显的偏离。由于整个体系存在RKKY和超交换磁耦合的竞争,因而在x=0.25时,我们观察到了独特的反铁磁—铁磁相变。进一步的态密度分析发现,Cu的掺杂浓度可以有效调整铁磁态合金的费米面位置,并且反铁磁态合金由于不同自旋方向的Mn原子的分波态密度相互补偿,总态密度形成了几乎完全对称的自旋向上带和自旋向下带。     

Half-Heusler合金Cu1-xFexMnSb的电子结构和反铁磁-铁磁相变简

基于密度泛函理论(DFT),使用局域密度近似(LDA)研究了Heusler合金Cu1-xFex MnSb的电子结构和反铁磁-铁磁相变.研究发现,两种磁状态下的合金晶格常数随掺杂浓度x变化很好地满足Vegard定理.当x0.5时,铁磁态合金的总磁矩很好地符合SP规律,然而当x0.5时,却发生了明显的偏离.由于整个体系存在RKKY和超交换磁耦合的竞争,因而在x=0.25时,我们观察到了独特的反铁磁—铁磁相变.进一步的态密度分析发现,Cu的掺杂浓度可以有效调整铁磁态合金的费米面位置,并且反铁磁态合金由于不同自旋方向的Mn原子的分波态密度相互补偿,总态密度形成了几乎完全对称的自旋向上带和自旋向下带.     

First-principles prediction of half-metallic ferromagnetism in Cd(TM)O2 (TM=Cr, Mn, Fe, Co, Ni) compounds

We report the electronic structure of Cd(TM)O₂ (TM=Cr, Mn, Fe, Co, Ni) in the chalcopyrite structures. From this study we find that Cd(TM)O₂ is a half-metallic ferromagnetic compound. From the energy consideration we find that Cd(TM)O₂ is more stable in chalcopyrite structure rather than in rock salt structure. A careful analysis of the spin density reveals the ferromagnetic coupling between the p-d states and the cation dangling-bond p states, which is believed to be responsible for the stabilization of the ferromagnetic phase. The calculated heat of formation, bulk modulus and cohesive energy are reported.     

Understanding Cr segregation at the He bubble surface in Fe

Although Cr segregation at the free Fe surface is weak, noticeable segregation of Cr at the He bubble surface in Fe has recently been observed. To understand the driving force for Cr segregation, we have carried out first-principles density functional theory calculations on the energetics of solute Cr atoms at the He bubble surface, which was modeled by a Fe/He interface. We find that both the compressive stress produced by the He bubble and the direct interfacial interaction promote Cr segregation from inside the bulk to the bubble surface, along with reduced spin polarization. Electronic structure analyses show that at the Fe/He interface, Cr is more compressible than Fe due to having more empty e(g) orbitals and, accordingly, the Fe surface gets energetically more favorable for Cr than in the bulk. On the other hand, the segregation of Cr increases the charge density at the bubble surface, and thus hinders assimilation of further He atoms.     

Electronic structure and magnetism of R(Fe,Si)12 (R = Y, Nd)

The newly developed full-potential linearized augmented plane wave (LAPW) and local orbitals (lo) based on standard APW methods are briefly introduced, and the structure and magnetic properties of R(Fe, Si)12 compounds (R = Y, Nd) are calculated using the method. The distribution of Si at different sites is analyzed based on total energy of one crystal unit with structure having been optimized. The characters of magnetic moments, total density of states (TDOS) and partial density of states (PDOS) for different crystal sites Si occupies are obtained and analyzed. The results show that the total magnetic moments of RFe10Si2 (R = Y, Nd) are larger than those of RFe10M2 (M = Ti, V, Cr, Mn, Mo and W) and the hybridization mechanism is seen as follows. Si(8j) reduce the magnetic moments of Fe at three sites, however, Si(8f) mainly reduce the magnetic moments of Fe(8i) and Fe(8j) atoms. The Curie temperature is markedly enhanced by the introduction of Si atoms according to spin fluctuation of DOS at Fermi level.     

Structure, electronic and magnetic properties of Cr-doped (ZnS)12 clusters: A first-principles study

We have studied the structural, electronic, and magnetic properties of (ZnS)₁₂ clusters doped with one (monodoped) and two (bidoped) Cr atoms in terms of a first-principles method. Substitutional, exohedral, and endohedral doping are considered. The substitutional isomer is found to be most favorable in energy for monodoped clusters, while the exohedral isomers are found to be most favorable for bidoped clusters. The magnetic coupling between the Cr atoms is mainly governed by the competition between direct Cr-Cr antiferromagnetic (AFM) interaction and the ferromagnetic (FM) interaction between two Cr atoms via S atom due to strong p-d hybridization. Finally, we show that the exohedral bidoped (ZnS)₁₂ clusters favor the FM state, which has potential applications in nanoscale quantum devices.     

Reentrant spin glass behavior in Cr-doped perovskite manganite

The magnetic and transport properties of the Cr-doped manganites La(0.46)Sr(0.54)Mn(1-y)Cr(y)O3 ( 0 < or = y < or = 0.08) with the A-type antiferromagnetic structure were investigated. Upon cooling, we find multiple magnetic phase transitions, i.e., paramagnetic, ferromagnetic (FM), antiferromagnetic (AFM), and spin glass in the y = 0.02 sample. The low temperature spin glass state is not a conventional spin glass with randomly oriented magnetic moments but has a reentrant character. The reentrant spin glass behavior accompanied by the anomalous multiple magnetic transitions might be due to the competing interactions between the FM phase and the A-type AFM matrix induced by the random Cr impurity.     

First-Principles Study of Magnetic Properties of TM_(13) and TM_(13)@Au_(32) Clusters(TM = Mn,Co)

The structural and magnetic properties of TM_(13 )and TM_(13)@Au_(32 )clusters(TM=Mn,Co)are studied by firstprinciples calculations.We find that the Au_(32 )cluster can tune not only the magnetic moment but also the magnetic coupling properties between the TM atoms of the TM cluster.The Au_(32 )cluster can increase the net magnetic moment of Mn_(13 )clusters while reducing that of Co_(13 )clusters.The interaction between Au and Mn atoms induces more Mn atoms to form spin parallel coupling,resulting in an increase of the total magnetic moment of Mn_(13 )clusters,while for the Co_(13 )clusters,the interaction between Au and Co atoms does not change the magnetic coupling states between the Co atoms,but reduces the magnetic moment of the Co atoms,leading to a decrease of the total magnetic moment of this system.Our findings indicate that the encapsulation of Au_(32 )clusters can not only raise the chemical stability of TM clusters,but also can tune their magnetic coupling properties and magnetic moment,which enables such systems to be widely applied in fields of spintronics and medical science.     

Structural,electronic, and magnetic properties of transition metal doped ReS<Subscript>2</Subscript> monolayer

Magnetic properties of transition-metal (TM) atoms (TM = Co, Cu, Mn, Fe, and Ni) doped ReS₂ monolayer are investigated by ab initio calculations. It is found that magnetism appears in the cases of Co, Fe, and Ni. Among all the samples, the Co-doped system has the largest magnetic moment. Therefore, we further study the interaction in the two-Co-doped system. Our results show that the interaction between two Co atoms is always ferromagnetic (FM), but such FM interaction is obviously depressed by the increasing Co–Co distance, which is well described by a simple Heisenberg model based on the Zener theory. Our results provide useful insight for promising applications of TM-doped ReS₂ monolayer in the future.     

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.     

A theoretical study of Fe adsorption along Bi-nanolines on the H/Si(0 0 1) surface

We have investigated the energetic stability and equilibrium geometry of the adsorption of transition metal Fe atoms near the self-organized Bi lines on hydrogen passivated Si(0 0 1) surface. Our total energy results show that there is an attractive interaction between Fe adatoms along the Bi-nanolines. For the energetically most stable configuration, the Fe adatoms are seven-fold coordinated, occupying the subsurface interstitial sites aside the Bi-nanolines. With increased coverage, Fe atoms are predicted to form two parallel lines, symmetrically on both sides of the Bi line. Within our local spin-density functional calculations, we find that for the most stable geometries the Fe adatoms exhibit an antiferromagnetic coupling.     

Half-metallic exchange bias ferromagnetic/antiferromagnetic interfaces in transition-metal chalcogenides

To investigate half-metallic exchange bias interfaces, magnetic structures at ferromagnetic (FM)/antiferromagnetic (AFM) interfaces in the zinc blende transition-metal chalcogenides, and with compensated and uncompensated AFM interfaces, were determined by the full-potential linearized augmented plane-wave method. With the uncompensated AFM interface, an antiparallel alignment of the Cr and Mn moments induces an excellent half-metallicity. More striking still, in the compensated AFM interface the Cr moments in the FM layer lie perpendicular to the Mn moments in the AFM layer but the Mn moments strongly cant to induce a net moment so as to retain the half-metallicity. These findings may offer a key ingredient for exchange biased spintronic devices with 100% spin polarization, having a unidirectional anisotropy to control and manipulate spins at the nanoscale.     

Instability of one-dimensional dangling-bond wires on H-passivated C(001), Si(001), and Ge(001) surfaces

We investigate the instability of one-dimensional dangling-bond (DB) wires fabricated on the H-terminated C(001), Si(001), and Ge(001) surfaces by using density-functional theory calculations. The three DB wires are found to show drastically different couplings between charge, spin, and lattice degrees of freedom, resulting in an insulating ground state. The C DB wire has an antiferromagnetic spin coupling between unpaired DB electrons, caused by strong electron–electron interactions, whereas the Ge DB wire has a strong charge-lattice coupling, yielding a Peierls-like lattice distortion. For the Si DB wire, the antiferromagnetic spin ordering and the Peierls instability are highly competing with each other. The physical origin of such disparate features in the three DB wires can be traced to the different degree of localization of 2p, 3p, and 4p DB orbitals.     

Structural,electronic,and magnetic properties of transition-metal atom adsorbed two-dimensional GaAs nanosheet

By using first-principles calculations within the framework of density functional theory,the electronic and magnetic properties of 3d transitional metal(TM) atoms(from Sc to Zn) adsorbed monolayer Ga As nanosheets(Ga As NSs) are systematically investigated.Upon TM atom adsorption,Ga As NS,which is a nonmagnetic semiconductor,can be tuned into a magnetic semiconductor(Sc,V,and Fe adsorption),a half-metal(Mn adsorption),or a metal(Co and Cu adsorption).Our calculations show that the strong p–d hybridization between the 3d orbit of TM atoms and the 4p orbit of neighboring As atoms is responsible for the formation of chemical bonds and the origin of magnetism in the Ga As NSs with Sc,V,and Fe adsorption.However,the Mn 3d orbit with more unpaired electrons hybridizes not only with the As 4p orbit but also with the Ga 4p orbit,resulting in a stronger exchange interaction.Our results may be useful for electronic and magnetic applications of Ga As NS-based materials.     

Ab initio study of magnetic properties in the adsorption of transition-metal atoms on arsenene

The magnetic properties of adsorption of different transition-metal (TM) atoms (Co, Cu, Mn, Fe, and Ni) on arsenene are investigated using density functional theory (DFT). Magnetism appears in the cases of Co, Mn, and Fe. Among all the magnetic cases, the TM atom prefers the same adsorption site. Then, we further study the interaction in two-TM-adsorbed system and different magnetic states are observed. Our results show that both nonmagnetic and ferromagnetic states exist in two-Co-adsorbed system and the p-d hybridization mechanism results in its ferromagnetic state. However, for two Mn and two Fe adsorbed systems, an AFM interaction is found, which could be reasonably explained by the superexchange mechanism. Such multiple magnetic properties may suggest promising applications of TM-adsorbed arsenene in the future.