Density functional studies on the ferromagnetic properties of (Zn,Cr)Te

We have investigated the electronic and magnetic properties of the room temperature ferromagnetic diluted magnetic semiconductor (DMS) (Zn,Cr)Te with density functional calculations. The electronic and magnetic properties of (Zn,Cr)Te are very similar with those of the typical DMS (Ga,Mn)As. The stronger ionicity of ZnTe plays a key role in the electronic and magnetic properties of (Zn,Cr)Te.     

High temperature ferromagnetism with a giant magnetic moment in transparent co-doped SnO(2-delta)

The occurrence of room temperature ferromagnetism is demonstrated in pulsed laser deposited thin films of Sn(1-x)Co(x)O(2-delta) (x<0.3). Interestingly, films of Sn(0.95)Co(0.05)O(2-delta) grown on R-plane sapphire not only exhibit ferromagnetism with a Curie temperature close to 650 K, but also a giant magnetic moment of 7.5+/-0.5 micro(B)/Co, not yet reported in any diluted magnetic semiconductor system. The films are semiconducting and optically highly transparent.     

稀磁半导体(Ga1-xFex)As的电子结构,磁性及其稳定性的第一性原理研究

采用基于第一性原理的紧束缚近似线性muffin-tin轨道(TB-LMTO-ASA)的方法,在原子球近似的基础上计算了均匀掺杂的稀磁半导体(Ga1-xFex)As在各掺杂浓度下(x=1,1/2,1/4和1/8)的总能量,由能量最低原理得到其在各稳定点的晶格常数,磁性及相应态密度.计算结果表明了(Ga1-xFex)As的晶格常数随掺杂浓度的增大而减小,在各掺杂浓度下(除x=1)样品都是反铁磁态的,Fe 3d和As 4p之间杂化是引起样品电子结构和磁性变化的主要原因.     

Theoretical optoelectronic analysis of intermediate-band photovoltaic material based on ZnY1-xOx （Y = S,Se, Te） semiconductors by first-principles calculations

The structural, energetic, and electronic properties of lattice highly mismatched ZnY1-xOx （Y = S, Se, Te） ternary alloys with dilute O concentrations are calculated from first principles within the density functional theory. We demonstrate the formation of an isolated intermediate electronic band structure through diluted O-substitute in zinc-blende ZnY （Y = S, Se, Te） at octahedral sites in a semiconductor by the calculations of density of states （DOS）, leading to a significant absorption below the band gap of the parent semiconductor and an enhancement of the optical absorption in the whole energy range of the solar spectrum. It is found that the intermediate band states should be described as a result of the coupling between impurity O 2p states with the conduction band states. Moreover, the intermediate bands （IBs） in ZnTeO show high stabilization with the change of O concentration resulting from the largest electronegativity difference between O and Te compared with in the other ZnSO and ZnSeO.     

高组分稀磁半导体Cd1-xMnxTe/CdTe超晶格的荧光谱研究

报道用分子束外延(MBE)技术生长的x=0.4,0.8的高组分稀磁半导体Cd1-xMnxTe/CdTe超晶格低温和室温荧光谱研究结果.基态激子跃迁能级荧光谱实验结果显示高组分超晶格中具有高量子效率和高质量光发射.对激子能级随温度的变化进行了详细研究,给出激子跃迁能量的温度系数.激子能级线型的展宽随温度变化关系可用激子-纵向光学声子耦合模型解释.与光调制反射谱实验结果进行了比较.     

Evolution of magnetic properties in the normal spinel solid solution Mg(1-x)Cu(x)Cr2O4

We examine the evolution of magnetic properties in the normal spinel oxides Mg(1-x)Cu(x)Cr2O4 using magnetization and heat capacity measurements. The end-member compounds of the solid solution series have been studied in some detail because of their very interesting magnetic behavior. MgCr2O4 is a highly frustrated system that undergoes a first-order structural transition at its antiferromagnetic ordering temperature. CuCr2O4 is tetragonal at room temperature as a result of Jahn-Teller active tetrahedral Cu2+ and undergoes a magnetic transition at 135 K. Substitution of magnetic cations for diamagnetic Mg2+ on the tetrahedral A site in the compositional series Mg(1-x)Cu(x)Cr2O4 dramatically affects magnetic behavior. In the composition range 0 ≤ x ≤ ≈0.3, the compounds are antiferromagnetic. A sharp peak observed at 12.5 K in the heat capacity of MgCr2O4 corresponding to a magnetically driven first-order structural transition is suppressed even for small x. Uncompensated magnetism--with open magnetization loops--develops for samples in the x range ≈0.43 ≤ x ≤ 1. Multiple magnetic ordering temperatures and large coercive fields emerge in the intermediate composition range 0.43 ≤ x ≤ 0.47. The Néel temperature increases with increasing x across the series while the value of the Curie-Weiss Θ(CW) decreases. A magnetic temperature-composition phase diagram of the solid solution series is presented.     

室温CdxZn1—xTe／ZnTe多量子阱光学双稳态的研究

研制了室温ＣｄｘＺｎ１－ｘＴｅ／ＺＮＴｅ多量子阱法布里－珀罗腔光双稳器件，并在该器件上观察到皮秒一级的室温激子光双稳。研究结果表明，ＣｄＺｎ１－ｘＴｅ／ＺｎＴｅ多量子阱光双稳器件的光双稳值和对比度分别为３６３ｋＷ／ｃｍ＾２和４：１。根据ＣｆｘＺｎ１－ｘＴｅ／ＺｎＴｅ多量子阱的吸收谱和激子非线性理论，归结了ＣｄｘＺｎ１－ｘＴｅ／ＺｎＴｅ多量子阱光双稳的主要非线性机理为激子的饱和和吸收。     

Oxygen vacancies effects on phase diagram of epitaxial La_(1-x)Sr_xMnO_3 thin films

We investigated the effects of oxygen vacancies on the structural,magnetic,and transport properties of La_(1-x)Sr_xMnO_3(x=0.1,0.2,0.33,0.4,and 0.5)grown around a critical point(without/with oxygen vacancies)under low oxygen pressure(10 Pa)and high oxygen pressure(40 Pa).We found that all films exhibit ferromagnetic behavior below the magnetic critical temperature,and that the films grown under low oxygen pressures have degraded magnetic properties with lower Curie temperatures and smaller magnetic moments.These results show that in epitaxial La_(1-x)Sr_xMnO_3 thin films,the magnetic and transport properties are very sensitive to doping concentration and oxygen vacancies.Phase diagrams of the films based on the doping concentration and oxygen vacancies were plotted and discussed.     

n-Hg_(0.80)Mg_(0.20)Te界面积累层中二维电子气的输运特性研究

通过变磁场霍耳测量研究了MBE生长的Hg0 .80 Mg0 .2 0 Te薄膜在 1 5— 2 5 0K温度范围内的输运特性 .采用迁移率谱 (MS)和多载流子拟合过程 (MCF)相结合的方法对实验数据进行了分析 ,由该方法获得的结果和ShubnikovdeHass(SdH)振荡测量的结果都证明材料中存在二维 (2D)电子和三维 (3D)电子 .其中 2D电子主要来自于Hg1-xMgxTe CdTe的界面积累层或Hg1-xMgxTe与真空界面附近的积累层 .3D电子迁移率随温度的变化关系表明了Hg1-xMgxTe中的电子散射机理与Hg1-xCdxTe中的非常相似 :在低温下电离杂质散射 (考虑了屏蔽效应 )起主导地位 ,而温度在 10 0K以上时 ,晶格散射占主导地位 .     

铬掺杂氧化锌薄膜的超声喷雾法制备及其光学与磁学性质

用注射超声喷雾法将前驱体由针管直接送入超声喷头内,在石英基板上制备Zn_(1-x)Cr_xO(x=0.0,0.01,0.03,0.05)薄膜。采用X射线衍射仪、扫描电子显微镜、荧光光谱仪、紫外-可见分光光度计、振动样品磁强计(VSM)等对薄膜的结构、光学和磁学性质进行测量。实验结果表明,未掺杂的ZnO薄膜为六角纤锌矿结构,沿着c轴(002)择优取向,而Cr掺杂抑制了薄膜的c轴择优取向性;掺杂后的薄膜平均晶粒尺寸均增大,且当x=3%时,晶粒尺寸最大,达31.4nm。扫描电镜(SEM)下薄膜呈球形颗粒状,并且在x=5%下,薄膜出现了长条状的微观形貌。Cr掺杂使样品的光致发光谱(PL)发生了很大的变化:未掺杂的样品的PL谱在378nm处存在一个紫外发射峰,对应于550nm附近还存在一个由于缺陷态引起的绿光发射峰;掺Cr样品只有在350~550nm的很宽的范围内存在一个发射峰,对其进行高斯拟合后,发现掺Cr量为x=1%,3%,5%下样品均存在V_(Zn)(锌空位)、Zn_i(Zn间隙位)、V_(Zn)~-(带一个电子的锌空位)内部缺陷态,且当x=3%时,V_(Zn)最多。Cr的掺杂使得薄膜的带隙增大,并且x=3%时,禁带宽度最大,达到3.374eV。掺Cr的三个样品均具有室温铁磁性,且x=3%样品的磁化强度最大,其与V_(Zn)(锌空位)最大相对应,验证了Cr~(3+)和V_(Zn)的缺陷复合体是ZnO∶Cr样品具有稳定的铁磁有序的最有利条件的理论预测。     

Magnetic properties of Sn1-xCrxTe crystals

Magnetization measurements of magnetic semiconductor Sn_1-xCr_xTe (x <5 at%) crystals with the Curie temperature T_c = 150–300 K were made down to 2 K. The magnetic properties are sensitive to isothermal annealing under Zn vapor. The overall magnetizations of the Zn-annealed crystals have paramagnetic and ferro- or antiferromagnetic contributions.     

The magnetic properties of diluted CoFe₂O₄ nanomaterials

The magnetic properties of (Cox Fe1-x )A(Zn1-xFe1+x) BO4 are studied using mean-field theory and the probability distribution law to obtain the saturation magnetization, the coercive field, the critical temperature, and the exchange interactions with different values of D (nm) and x. High-temperature series expansions (HTSEs) combined with the Pad′e approximant are used to calculate the critical temperature of (Cox Fe1-x) A (Zn1-x Fe1+x) BO4 , and the critical exponent associated with magnetic susceptibility is obtained.     

Neutron powder diffraction investigation on the crystal and magnetic structure of (Ho(0.50+x)Ca(0.50-x))(Mn(1-x)Cr(x))O3

The crystal and magnetic structure of (Ho(0.50+x)Ca(0.50-x))(Mn(1-x)Cr(x))O(3) (x = 0.00, 0.01, 0.02, 0.03) has been investigated between 5 and 300 K by means of neutron powder diffraction followed by Rietveld refinement and dc magnetic measurements. During cooling an orthorhombic to monoclinic phase transition occurs on account of the charge and orbital ordering taking place in the Mn sub-lattice; at low temperature phase separation takes place and the main monoclinic phase coexists with a secondary orthorhombic phase, whose amount slightly increases with the increase of Cr content. Cr(3+) is not involved in orbital ordering or superexchange interactions. The charge and magnetic ordering are decoupled: the Mn moments order according to a CE-type structure in all samples.     

Ferromagnetism in Ga(1-x)Mn(x)P: evidence for inter-Mn exchange mediated by localized holes within a detached impurity band

We report an energy gap for hole photoexcitation in ferromagnetic Ga(1-x)Mn(x)P that is tunable by Mn concentration (x < or = 0.06) and by compensation with Te donors. For x approximately 0.06, electrical transport is dominated by excitation across this gap above the Curie temperature (TC) of 60 K and by thermally activated hopping below TC. Magnetization measurements reveal a moment of 3.9 +/- 0.4 muB per substitutional Mn while the large anomalous Hall signal demonstrates that the ferromagnetism is carrier mediated. In aggregate these data indicate that ferromagnetic exchange is mediated by holes localized in a Mn-derived band that is detached from the valence band.     

Critical phenomena and the quantum critical point of ferromagnetic Zr(1-x)NbxZn2

We present a study of the magnetic properties of Zr(1-x)NbxZn2, using an Arrott plot analysis of the magnetization. The Curie temperature Tc is suppressed to zero temperature for Nb concentration xc = 0.083+/-0.002, while the spontaneous moment vanishes linearly with Tc as predicted by the Stoner theory. The initial susceptibility chi displays critical behavior for x or= xc we find that chi(-1) = chi0(-1) + aT(4/3), where chi0(-1) vanishes as x-->xc. The resulting magnetic phase diagram shows that the quantum critical behavior extends over the widest range of temperatures for x=xc, and demonstrates how a finite transition temperature ferromagnet is transformed into a paramagnet, via a quantum critical point.     

Effects of chemical pressure on diluted magneticsemiconductor(Ba,K)(Zn,Mn)_2As_2

Chemical pressure induced by iso-valent doping has been widely employed to tune physical properties of materials. In this work, we report effects of chemical pressure by substitution of Sb or P into As on a recently discovered diluted magnetic semiconductor(Ba,K)(Zn,Mn)_2 As_2, which has the record of reliable Curie temperature of 230 K due to independent charge and spin doping. Sb and P are substituted into As-site to produce negative and positive chemical pressures, respectively.X-ray diffraction results demonstrate the successful chemical solution of dopants. Magnetic properties of both K-underdoped and K-optimal-doped samples are effectively tuned by Sb-and P-doping. The Hall effect measurements do not show decrease in carrier concentrations upon Sb-and P-doping. Impressively, magnetoresistance is significantly improved from7% to 27% by only 10% P-doping, successfully extending potential application of(Ba,K)(Zn,Mn)_2 As_2.     

Absence of conventional spin-glass transition in the Ising dipolar system LiHo(x)Y(1-x)F(4)

The magnetic properties of single crystals of LiHo(x)Y(1-x)F(4) with x = 16.5% and x = 4.5% were recorded down to 35 mK using a micro-SQUID magnetometer. While this system is considered as the archetypal quantum spin glass, the detailed analysis of our magnetization data indicates the absence of a phase transition, not only in a transverse applied magnetic field, but also without field. A zero-Kelvin phase transition is also unlikely, as the magnetization seems to follow a noncritical exponential dependence on the temperature. Our analysis thus unmasks the true, short-ranged nature of the magnetic properties of the LiHo(x)Y(1-x)F(4) system, validating recent theoretical investigations suggesting the lack of phase transition in this system.     

新型稀磁半导体Fe掺杂LiZnP的光电性质

采用基于密度泛函理论的第一性原理计算法研究了新型稀磁半导体Li_(1±)_y(Zn_(1-)_xFe_x)P (x=0, 0.0625;y=0, 0.0625)的电子结构、磁性及光学性质.结果表明,Fe的掺入使体系产生自旋极化杂质带,Fe的3d态与Li2s态,Zn4s态以及P3p态的态密度峰在费米能级处出现重叠,产生sp-d轨道杂化,此时体系净磁矩最大,材料表现出金属性,导电性增强.当Li空位时,导电性减弱,但杂质带宽度最大,居里温度最高.而Li填隙时,体系形成能最低,材料变为半金属性,表现为100%自旋注入,表明掺杂体系的磁性和电性可以分别通过Fe的掺入和Li的含量进行调控.对比光学性质发现,Li空位时,在介电函数虚部和复折射率函数的低能区出现新峰,扩大了对低频电磁波的吸收范围.能量损失函数表明掺杂体系具有明显的蓝移效应,且Li填隙时有更强的等离子共振频率.     

Electronic Structure and Magnetic Properties of Cr-Doped AlN

To understand the electronic and magnetic properties, we have studied Cr-doped zinc-blende AlN system in detail by applying a first-principle plane wave pseudopotential method based on the density functional theory within the local spin density approximation. The analyses of the band structures, density of states, exchange interactions, and magnetic moments show that Al1-xCrxN alloys may exhibit a half-metallic ferromagnetism character, that Cr in the diluted doping limit forms near-midgap deep levels, and that the total magnetization of the cell is 3μB per Cr atom, which does not change with Cr concentration. Moreover, we have succeeded in predicting that Al1-xCrzN alloys in x = 0.0625 has a very high Curie temperature, and lind that ferromagnetic exchange interaction between magnetic dopants is short-ranged.     

Thermoelectric power in ferromagnetic Pb0.16Sn0.72Mn0.12Te semiconductor

Thermoelectric power was studied in semimagnetic (diluted magnetic) Pb_1−x−ySn_yMn_xTe (x=0.12,y=0.72) semiconductor in the temperature range T=10–100 K covering both ferromagnetic and paramagnetic range of temperatures of this material. In addition to standard diffusion contribution to the thermoelectric power (α_D∝T), we found in PbSnMnTe an additional “magnetic” contribution (α_FM). Employing the effect of carrier-concentration-controlled ferromagnetism in PbSnMnTe, we demonstrate that the temperature dependence of α_FM(T) shows a clear maximum at the ferromagnetic Curie temperature. This experimental finding is discussed in terms of Kasuya model for the thermoelectric properties of diluted magnetic metallic systems with sd-exchange interaction.