Exchange Interactions and Curie Temperatures in Dilute Magnetic Semiconductors

We discuss the origin of ferromagnetism in dilute magnetic semiconductors based on ab initio calculations for Mn-doped GaN, GaP, GaAs and GaSb. We use the Korringa–Kohn–Rostoker method in connection with the coherent potential approximation to describe the substitutional and moment disorder. Curie temperatures (T _C) are calculated from first-principles by using a mapping on a Heisenberg model in a mean field approximation. It is found that if impurity bands are formed in the gap, as it is the case for (Ga, Mn)N, double exchange dominates leading to a characteristic √c dependence of T _C as a function of the Mn concentration c. On the other hand, if the d-states are localized, as in (Ga, Mn)Sb, Zener's p–d exchange prevails resulting in a linear c-dependence of T _C. In order to have more precise estimations of T _C, effective exchange coupling constants J _ij 's are calculated by using the formula of Liechtenstein et al. It is found that the range of the exchange interaction in (Ga, Mn)N is very short. On the other hand, in (Ga, Mn)As the interaction is weaker but long ranged. Monte Carlo simulations show that the T _C values of (Ga, Mn)N are very low since percolation is difficult to achieve for small concentrations and the mean field approximation strongly overestimates T _C. Even in (Ga, Mn)As the percolation effect is still important.     

稀磁半导体材料居里温度的极值点

以Zener模型为基础,考虑反铁磁性交换作用对DMS材料居里温度的影响,理论计算得到了居里温度关于掺杂浓度和反铁磁性交换作用的二元函数,对GaAs ∶ TM(Ga,TM)As (TM=Sc,Ti,V,Cr,Mn,Fe,Co,Ni)的居里温度做了详细分析得到:n型半导体居里温度有一个极大值,而p型掺杂是单调的递增。     

利用RL交流电桥测量磁性材料的居里温度

利用RL交流电桥测量了磁性材料的居里温度,并讨论了工作频率和工作电压的变化对测量结果的影响.     

Electronic and magnetic structures of chain structured iron selenide compounds

Electronic and magnetic structures of iron selenide compounds Ce2O2FeSe2 （2212＊） and BaFe2Se3 （123＊） are studied by the first-principles calculations. We find that while all these compounds are composed of one-dimensional （1D） Fe chain （or ladder） structures, their electronic structures are not close to be quasi-lD. The magnetic exchange couplings between two nearest-neighbor （NN） chains in 2212＊ and between two NN two-leg-ladders in 123＊ are both antiferromagnetic （AFM）, which is consistent with the presence of significant third NN AFM coupling, a common feature shared in other iron-chalcogenides, FeTe （11＊） and KyFe2-xSe2 （122＊）. In magnetic ground states, each Fe chain of 2212＊ is ferromagnetic and each two-leg ladder of 123＊ form a block-AFM structure. We suggest that all magnetic structures in iron-selenide compounds can be unified into an extended J1-J2-J3 model. Spin-wave excitations of the model are calculated and can be tested by future experiments on these two systems.     

Calculation of Half-Metal,Debye and Curie Temperatures of Co_2 VAl Compound: First Principles Study

By FP-LAPW calculations, the structural, elastic, Debye and Curie temperatures, electronic and magnetic properties of Co2 VAl are investigated. The results indicate that Ferromagnetic(FM) phase is more stable than AntiFerromagnetic(AFM) and Non-magnetic(NM) ones. In addition, C11–C12 0, C44 0, and B 0 so Co2 VAl is an elastically stable material with high Debye temperature. Also, the B/G ratio exhibits a ductility behavior. The relatively high Curie temperature provides it as a favorable material for spintronic application. It's electronic and magnetic properties are studied by GGA+U approach leading to a 100% spin polarization at Fermi level.     

Janus monolayer TaNF: A new ferrovalley material with large valley splitting and tunable magnetic properties

Materials with large intrinsic valley splitting and high Curie temperature are a huge advantage for studying valleytronics and practical applications. In this work, using first-principles calculations, a new Janus TaNF monolayer is predicted to exhibit excellent piezoelectric properties and intrinsic valley splitting, resulting from the spontaneous spin polarization, the spatial inversion symmetry breaking and strong spin−orbit coupling (SOC). TaNF is also a potential two-dimensional (2D) magnetic material due to its high Curie temperature and large magnetic anisotropy energy. The effective control of the band gap of TaNF can be achieved by biaxial strain, which can transform TaNF monolayer from semiconductor to semi-metal. The magnitude of valley splitting at the CBM can be effectively tuned by biaxial strain due to the changes of orbital composition at the valleys. The magnetic anisotropy energy (MAE) can be manipulated by changing the energy and occupation (unoccupation) states of d orbital compositions through biaxial strain. In addition, Curie temperature reaches 373 K under only −3% biaxial strain, indicating that Janus TaNF monolayer can be used at high temperatures for spintronic and valleytronic devices.     

钙钛矿La0.8Ca0.2MnO3多晶样品的制备及低温磁学特性

我们利用固相反应法制备了粉末状La0.8Ca0.2MnO3样品,其空间群为pnma,样品中存在超顺磁性粒子,其截止态和超顺磁态的转变温度为185K,从dσ/dT的负峰得到铁磁-顺磁相变居里温度为210K,在顺磁领域线性拟合而得到顺磁居里温度为216.4K,分子式有效磁矩为6.25μB,低温领域磁化曲线服从布洛赫T32定律,其自旋波劲度系数为49.2meV2.     

Exchange Interactions at Surfaces of Fe, Co, and Gd

Magnetic exchange interactions at low-index surfaces of bcc iron, hcp cobalt, and hcp gadolinium are studied using ab initio electronic structure calculations. Interlayer exchange couplings derived from total-energy differences are enhanced at the surfaces over their bulk counterparts. This trend is in contrast to a surface reduction of on-site exchange parameters formulated within a classical Heisenberg model. A particular attention is paid to the sensitivity of exchange interactions at a Gd(0001) surface to relaxation of interlayer distances. The calculated results do not provide support for recently observed surface enhancement of the Curie temperature of the Gd metal.     

Electric field dependence of the Curie temperature and microwave absorption in displacive ferroelectrics with impurities II

Expressions for the complex dielectric constant, microwave absorption and the Curie temperature in doped displacive ferroelectrics, subjected to an external electric field are discussed, using the approach made in our previous study. A cross-term of defect parameters with electric field and anharmonic parameters is obtained. The intrinsic parametersB andC are modified by impurity terms. The qualitativeE ² dependence of tan δ is discussed.     

High-temperature nodal ring semimetal in two-dimensional honeycomb-kagome Mn2N3 lattice

The search for two-dimensional (2D) nodal ring semimetallic materials is a current research hotspot in spintronics, and designing a 2D nodal ring (NR) material with high Curie temperature ($T_{mathrm{C}})$ and strong robustness to spin-orbit coupling (SOC) is an even greater challenge. Here, based on the first-principles calculations and symmetry analysis, we predict that 2D Mn$_{2}$N$_{3}$ is a nodal ring semimetal (NRSM) with three energy bands near the Fermi energy level consisting of electrons in the same spin channel. An electron-like energy band and two hole-like energy bands near the Fermi plane cross to form two NRs centered at the point $varGamma $. Symmetry analysis shows that the spin-polarized NR semimetal is robust to SOC due to the conservation of horizontal mirror symmetry. Monte-Carlo simulations further demonstrate that the $T_{mathrm{C}}$ of the 2D Mn$_{2}$N$_{3}$ reaches 530 K, well above the room temperature. Notably, the 2D Mn$_{2}$N$_{3}$ remains an NRSM on h-BN substrate. Our results not only reveal a general framework for designing 2D NR materials, but also promote further research in the direction of multifunctional quantum devices for spintronics.     

Structure and Curie temperature of Y2Fe17-xCrx

The structures of Y2Fe17-xCrx are simulated by the ab initio potentials. The site preference of Cr atom in Y2Fe17 is evaluated and the order is determined as 4f, 12j, which is close to the experimental result. Based on the site preference behavior, the calculated parameters and the atom sites of Y-Fe-Cr system are studied. The result corresponds well to observed data. Further, the DOS of the relaxed structures are calculated and the variation in Curie temperature is explained qualitatively by the spin-fluctuation theory.     

Magnetic Excitations and the Curie Temperature of Decamethylchromocenium Tetracyanoethylenide

We describe the molecular ferromagnet decamethylchromocenium tetracyanoethanide [C_rC_p2^*] [TCNE] by a quasi-one-dimensional alternating-spin model with ferromagnetic exchange interaction. The spin-wave theory is used to discuss the magnetic excitation of the molecular ferromagnet. We obtain the magnetizations corresponding to different external magnetic fields without interchain exchange interaction which is consistent with the experimental results. The formula of the Curie temperature T_c is obtained by using the thermodynamic Green's function method. It is found to be related to the spin of donors, to the intrachain exchange interaction and to the spatial anisotropic parameter. The result obtained by this T_c formula is consistent with the experimental result T_c = 3.6 K for [C_rC_p2^*][TCNE].     

厚度与应变效应对La0.67Ca0.33MnO3薄膜电输运与居里温度的影响

在厚度为25—400nm范围内，系统地研究了 (001)SrTiO_3(STO), (001)LaAlO_3(LAO)衬底上La_0.67Ca_0.33Mn_O.3 (LCMO)薄膜的电输运与居里温度T_C随薄膜厚度及衬底的变化. 结果表明，随薄膜变薄，电阻率ρ增加，T_C降低. 对于同一薄膜厚度，LCMO/STO薄膜的ρ大于LCMO/LAO基上的薄膜的ρ. T_C衬底的依赖关系则与ρ相反. 分析表明，LCMO薄膜的低温区电阻温度(ρ-T) 符合关系式ρ=ρ_0+Bω_s/sin h^2(ω_s/2/k_BT)+CT^n， 其中ρ_0为剩余电阻；等号右端第二项反映软光学模声子对电子散射的贡献；第三项包括其余可能散射机 理在电输运过程中所起的作用；B，ωs（软光学模声子的平均频率）与C都为拟合系数. 高温区的电输运则由小极化子跃迁模型ρ=DT×exp（E_a/k_BT）描述（E_a为极化子激 发能）. 根据ρ_0，ωs，E_a以及T_Ｃ变化，初步讨论了薄膜中的厚度与应变效应. 进一步 研究发现ωs，E_a的变化与T_C相关，从而说明极化子效应为影响T_C变化的主要因素. 关键词： 锰氧化物薄膜 电输运 居里温度 极化子     

Comparative Theoretical Study of Hyperfine Interactions of Muonium in A- and B-Form DNA

Muon Spin Relaxation (μSR) experiments in A- and B-form DNA have shown evidence for an enhanced electron mobility in the more closely-packed A-form. Besides dynamic effects (electronic diffusion) that could cause the observed difference in muon spin relaxation, one should also carefully examine the difference in the strengths of the hyperfine interactions of the muon (μ ⁺) with the moving electron in the two forms of DNA, since this could contribute to the observed difference in the muon spin relaxation rates as well. We have therefore investigated the (static) trapping properties of muon and muonium (μ ⁺ e ⁻) in A-form and B-form DNA from first-principles with the aim to understand how the different structural geometries of A- and B-form DNA can influence the hyperfine interaction of trapped muonium.     

Magnetic dynamics of two-dimensional itinerant ferromagnet Fe_3GeTe_2

Among the layered two-dimensional ferromagnetic materials(2 D FMs),due to a relatively high T_C,the van der Waals(vdW) Fe_3 GeTe_2(FGT) crystal is of great importance for investigating its distinct magnetic properties.Here,we have carried out static and dynamic magnetization measurements of the FGT crystal with a Curie temperature TC ≈ 204 K.The M-H hysteresis loops with in-plane and out-of-plane orientations show that FGT has a strong perpendicular magnetic anisotropy with the easy axis along its c-axis.Moreover,we have calculated the uniaxial magnetic anisotropy constant(K_1)from the SQUID measurements.The dynamic magnetic properties of FGT have been probed by utilizing the high sensitivity electron-spin-resonance(ESR) spectrometer at cryogenic temperatures.Based on an approximation of single magnetic domain mode,the K_1 and the effective damping constant(α_(eff)) have also been determined from the out-of-plane angular dependence of ferromagnetic resonance(FMR) spectra obtained at the temperature range of 185 K to T_C.We have found large magnetic damping with the effective damping constant α_(eff) ～ 0.58 along with a broad linewidth(ΔH_(pp) 1000 Oe at 9.48 GHz,H ‖ c-axis).Our results provide useful dynamics information for the development of FGT-based spintronic devices.     

Correlation between Curie temperature and system dimension

Curie temperature T_C of spin arrangement with arbitrary dimension was considered. We assumed that interaction of a spin with all other spins vary with a power-law decay rate in exchange integral on Heisenberg model. As a result, we found that T_C, which was obtained from T_C=λC (λ: mean-field coefficient and C: Curie constant), significantly depends on fractal dimension of spin arrangements D, the exchange integral and the decay constant. This semi-quantitatively explains how T_C depends on D (1≤D≤3) in a universal way and also the finite size effect on T_C in low-dimensional spin systems.     

Residual stress and Curie temperature of Fe-N thin films prepared by dc magnetron sputtering at elevated temperature

Fe-N thin films were prepared by dc magnetron sputtering at elevated temperature of 80 °C. The residual stress of the thin film was characterized by means of grazing incidence X-ray diffraction method. The effect of magnetron sputtering parameter on residual stress was investigated. The results indicate that the nitrogen content in working gas has great effects on the residual stress in the Fe-N thin film, and the residual stress increases firstly and then decreases with the increasing of nitrogen content in working gas. Curie temperature measurement shows that tensile residual stress enhances the ferromagnetic-paramagnetic transition temperature of Fe-N thin films under the condition of same phase composition.     

Effects of disorder on the Curie temperature of GaMnN, GaCrN, InCrN, and InMnN diluted magnetic semiconductors

The critical Curie temperatures of ordered and disordered diluted magnetic semiconductors based on GaN, InN, CrN, and MnN compounds are investigated using the classical Heisenberg model within the mean field approximation. The equilibrium structural lattice parameters of all the structures investigated are obtained from first principles. We show that the Curie T_c temperatures of disordered GaN and InN doped with small concentrations of Mn and Cr depends, to a great extent, on the Mn and Cr concentrations. Our calculations on these systems show that a T_c above room-temperature can be observed in these systems and it is affected greatly by the degree of disorder of Mn and Cr randomly distributed on the Ga and In sites. In addition, the ferromagnetic stability in these diluted magnetic semiconductors is studied systematically. Our results indicate that 3d Mn and Cr impurity states in GaN and InN favor the ferromagnetic state rather than the spin-glass phase.     

Magnetic entropy change and magnetic properties of LaFe_(11.5)Si_(1.5) after controlling the Curie temperature by partial substitution of Mn and hydrogenation

Magnetic properties and magnetic entropy changes of La(Fe_(1-x)Mn_x)_(11.5)Si_(1.5)H_y compounds are investigated. Their Curie temperatures are adjusted to room temperature by partial Mn substitution for Fe and hydrogen absorption in 1-atm(1 atm = 1.01325×10~5Pa) hydrogen gas. Under a field change from 0 T to 2 T, the maximum magnetic entropy change for La(Fe_(0.99)Mn_(0.01))_(11.5)Si_(1.5)H_(1.61)is-11.5 J/kg. The suitable Curie temperature and large value of ?S_m make it an attractive potential candidate for the room temperature magnetic refrigeration application.