First-principles studies of the magnetic hyperfine field in Fe multilayers

A detailed theoretical study of the magnetic moments and magnetic hyperfine fields in several Fe multilayers (Fe fcc(001)/5X fcc(001), X=Cu and Ag, and Fe bcc(001)/5X fcc(001), X=Ag and Au) as well as in bulk Fe is presented. The calculations have been performed using the spin-polarized, relativistic linear muffin-tin orbital (SPR-LMTO) method of band structure calculation. Therefore, not only the contribution to the hyperfine fields due to the conventional Fermi contact interaction but also due to the spin dipolar and orbital contributions induced by the crystal field and by spin-orbit coupling are accounted for. To decompose the hyperfine field of non-s-electrons into these contributions it has been assumed that they are proportional to the corresponding so-called magnetic dipole moment and the orbital magnetic moment, respectively. In contrast to previous results for pure metals and alloys not only the orbital but also the spin dipolar hyperfine field was found to be non-negligible. The anisotropy of the hyperfine field determined by calculations for in-plane and perpendicular orientation of the magnetisation was found to be very pronounced and closely connected with the corresponding anisotropy of the magnetic dipole moment and the orbital moment.     

Hyperfine fields in 3d impurities in metals

The dynamic spin polarization of conduction electrons by an impurity spin is shown to provide a significant positive contribution to the impurity hyperfine field, accounting for the fact that the hyperfine field per unit impurity spin is only about half as large for impurities in metals as in insulators. The anomalously small hyperfine fields of Fe impurities in the noble metals are found to result from an orbital contribution. Quantitative analysis of this orbital contribution suggests that a strong dynamic Jahn-Teller effect is present. Similar orbital contributions can yield a strongly anisotropic hyperfine field for Fe or Cr impurities in hexagonal host metals, depending on the type of crystal-field orbital ground state.     

Investigations of “soft-landed” Cd surface atoms via nuclear methods: hyperfine-field sign determination

Using refined preparation techniques, cadmium guest atoms have been positioned at different sites on the surfaces of nickel crystals. The magnetic hyperfine fields and the electric field gradients at the Cd nuclei were measured by time-dependent perturbed angular correlation (TDPAC) spectroscopy of the emitted gamma radiations. By measuring the combined interactions, electric field gradients and magnetic hyperfine fields can be unambiguously attributed to each surface site. The signs of the magnetic hyperfine fields are determined by applying an external magnetic field and choosing the appropriate γ-ray detector configuration. The measured fields correlate with the number of neighbouring host atoms. Band structure calculations confirm this finding and predict magnetic fields for various sp elements from the band structure of the s-like conduction electrons. The quadrupolar interactions are manifestations of the balance in the occupation of the guest p-sublevels. These results provide new information on the structure and formation of electronic configurations of sp elements in different local environments and will contribute to understanding electronic effects on surfaces.     

Size effects on the magnetic hyperfine fields at non-magnetic impurities in iron

An important contribution to the magnetic hyperfine fields at non-magnetic impurities in magnetic metals arises from the conduction-electron polarization (CEP). In the model of Stearns the CEP at the impurity site,Pce, arises from the exchange interaction of the conduction electrons at that site with the localized 3d host electrons. In this modelPce is a negative constant, independent of the impurity, so that an emperical volume-misfit correction has to be introduced to explain the observed positive hyperfine fields. The purpose of this paper is to show thatPce is not constant, but is strongly affected by the distortion of the lattice around the impurity. The correct form ofPce includes a volume term equivalent to that postulated by Stearns. The hyperfine fields for Cu, Zn, Sn, Sb and Au are calculated, taking the distortion effects into account. A fair agreement with the observed hyperfine fields is obtained. Furthermore, the effects of pressure on the CEP are discussed.Supported by the NSF Grant No. DMR 73-07665 AO 3     

Study of Hyperfine Fields in CeIn3 by Electronic Structure Calculations

Electronic band structure calculations for the CeIn₃ compound utilizing the full potential linearized augmented plane waves method were performed with the aim to compute the hyperfine fields acting on Ce and In atoms. The latter are found to be in reasonable agreement with the values measured at low temperatures. The 4f orbital contribution dominates the magnetic hyperfine field at Ce ions while the contact field is negligible due to an almost complete cancellation of valence and core contributions. A non-zero magnetic hyperfine field appears at In sites due to the spin-polarization of the 5p sub-shells through the hybridization with the extended 6s, 6p and 5d Ce states which, in turn, are spin-polarized by the Ce 4f states. No net magnetic moment at In is observed since the sum of its 5p sub-shell spins is zero. The 5p shell of In is responsible for the presence of an electric field gradient at In nuclei. This revised version was published online in September 2006 with corrections to the Cover Date.     

Local magnetic moments and hyperfine magnetic fields in Fe-M (M = Si, Sn) alloys at small metalloid concentrations

The main tendencies in the formation of local magnetic moments and hyperfine magnetic fields at Fe nuclei in Fe-Sn and Fe-Si alloys at low metalloid concentrations are analyzed on the basis of “first-principles” calculations. The results of calculations are compared with experimental data. The main differences between these alloys were proved to be due to the differences in their lattice parameters. It is shown that a significant contribution to the formation of the hyperfine field comes from the orbital magnetic moment and the Ruderman-Kittel-Kasuya-Yosida polarization, which depend on the impurity concentration and the distance to an impurity atom in the crystal lattice.     

基于超精细结构下的激光光泵铯磁力仪的理论研究

激光光泵碱金属磁力仪具有很高的灵敏度,测量范围可以从地球磁场到生物磁场。给出了铯（Cs）光泵磁力仪的理论分析和系统设计以及磁场梯度测量原理,铯原子能级在I—J耦合时形成超精细结构,在外磁场的作用下超精细结构进一步产生塞曼分裂形成塞曼子能级,利用激光泵浦和射频磁场能够使电子在超精细结构中进行能级跃迁,产生光磁双共振的结果,最终通过共振频率就能够达到精确测量外磁场的目的。     

Mössbauer effect in iron and dilute iron based alloys

Fifteen years of Mössbauer Effect (ME) studies have significantly widened the insight into the physical properties of iron and iron based alloys. In this review article the various contributions to the hyperfine interactions as measured with the ME technique, namely the isomer shift, the magnetic hyperfine interaction and the quadrupole interaction are summarized. Further the impurity effects as the Friedel type oscillations in the charge and the RKKY type oscillations in the spin density distribution are discussed. Special attention is paid to the peculiar magnetic properties of metallic iron and its alloys. From a comparison of the magnetic hyperfine fields and bulk magnetizations as a function of the impurity concentration and from the temperature dependence of the magnetic hyperfine fields at the various sites in iron alloys, it is concluded that the 3d magnetic moments in iron are largely localized. Further the exchange interaction is provided by the remaining small percentage of itinerant 3d electrons. Finally, from a comparison of magnetic hyperfine field and isomer shift data in alloys an overall picture of the electronic changes involved in alloying has been developed.     

Orbital magnetic moment of a quantum well and a quantum dot in crossed magnetic and electric fields

A new physical effect, namely, oscillations of the orbital magnetic moment with a change in the electric field strength in two types of nanostructures, has been predicted. Explicit analytical expressions for the orbital magnetic moment of a quantum well and a quantum dot in crossed magnetic and electric fields have been derived. The oscillations of the orbital magnetic moment with a change in the electric and magnetic fields have been studied. The oscillation periods in both the electric and magnetic fields have been found and the limiting cases of the strong magnetic and quantum confinement effects have been considered.     

Hyperfine field of156Gd in TbFe2, TbFe3, TbCo2 and TbCo3 compounds

Time integral γ-γ angular correlation measurements have been made on¹⁵⁶Gd in Tb-3d metal compounds, TbFe₂, TbFe₃, TbCo₂ and TbCo₃, using two Ge(Li) detetctors. From the observation of a linear relation between magnetic hyperfine fields at Gd and Tb nuclei in corresponding compounds, it is inferred that crystalfield induced variations of the Tb moment are negligible. The experimental results are discussed in terms of the conduction electron polarization caused by the magnetic moment of 3d metals.     

Orbital and spin magnetism and dielectric response of electrons in metals

A survey is given of the theory of orbital and spin magnetism of electrons in metals, and of the relation of this to dielectric response theory. In particular, the orbital magnetic behaviour of Bloch electrons in a static but spatially varying magnetic field is discussed. It is shown that the treatment of Hebborn and Sondheimer can be generalized to include this spatial dependence of the applied magnetic field and that the result may be written compactly in terms of the zero field canonical density matrix, or Green function.

It is further shown that the frequency dependence of both the orbital and spin susceptibility can be obtained, again from the density matrix, by calculating the current. The connection with neutron scattering is exhibited and for the case of nickel the way in which such experiments can lead to estimates of the role of the electron interactions is discussed.

The effect of a magnetic field on phonon spectra in metals is considered briefly. Finally it is suggested that it may be of interest to study the effect of the magnetic field on the periodic potential usually employed to calculate energy levels of electrons in magnetic fields. Some modifications of pair forces between ions is to be expected in very high fields.     

微波驱动精细结构能级跃迁引起的电磁诱导负折射效应

本文通过建立Λ形四能级原子系统, 研究了微波驱动精细结构能级跃迁引起的电磁诱导负折射效应. 微波场作用于基态精细结构能级之间, 与不同精细结构能级之间的电偶极矩或磁偶极矩发生耦合, 使系统在某些频率处呈现负折射特性.同时, 两个耦合场各自激励一对基态和激发态之间的光学跃迁. 通过改变两个耦合场的频率失谐量控制负折射区域的频带宽度.结果表明, 耦合场失谐时出现负折射特性的频率范围比耦合场共振时迅速缩小, 而且耦合场负失谐和正失谐时的变化规律不同.     

Optical polarization of nuclei and ODNMR in GaAs/AlGaAs quantum wells

Optical orientation of electrons was used to polarize the crystal lattice nuclei in quantum-size heterostructures and to study the effect of the conduction band spin splitting on the spin states of quasi-two-dimensional (2D) electrons drifting in an external electric field. High (～1%) nuclear polarization was registered using polarized luminescence and ODNMR in single GaAs/AlGaAs quantum wells. Measurement was made of the hyperfine interaction fields created by polarized nuclei on electrons and by electrons on nuclei. The spin-lattice relaxation of nuclei on the non-degenerate 2D electron gas was calculated. A comparison of the theoretical and experimental longitudinal relaxation times permitted the conclusion that the localized charge carriers are responsible for nuclear polarization in quantum wells in the temperature range of 2–77 K. A new effect has been studied, i.e. induction of an effective magnetic field acting on 2D electron spins when electrons drift in an external electric field in the quantum well plane. This effective field B_eff is due to the spin splitting of the conduction band of 2D electrons. The paper discusses possible registration of an ODNMR signal when the field B_eff is modulated by an electric current during optical orientation.     

The electronic structure and the ferromagnetic properties of the organic radical 4-methacryloyloxy-2,2,6,6-tetramethyl-1-piperidinyloxyl (MOTMP)

The ab initio method of the full potential linearized augmented-plane-wave has been used to study the electronic band structure and the ferromagnetic (FM) properties of the organic radical MOTMP. The total and the partial density of states and the atomic spin magnetic moments are calculated. The calculation revealed that MOTMP has a stable ferromagnetic ground state and the spin magnetic moment is 1.0 μ_B per molecule, which is in good agreement with the experimental value. It is found that the unpaired electrons in this compound are localized in a molecular orbital constituted primarily of π*(NO) orbital and the main contribution of the spin magnetic moment comes from the NO free radical. It is also found that there exists ferromagnetic intermolecular interaction in the compound.     

铁磁性高锰硅化物Mn_4Si_7电子特性的第一性原理计算

本文采用基于第一性原理的密度泛函理论超软雁势平面波方法,对铁磁性半导体高锰硅化合物Mn_4Si_7进行了理论计算.结果表明块体Mn_4Si_7是准直接带隙半导体材料,其价带主要是由Mn的3d轨道电子构成,导带主要是由Mn的3d及Si的3p轨道电子构成.相同自旋轨道下,自旋向下态的电子更容易占据较高的能级.而自旋向上态的电子对Mn_4Si_7的禁带宽度起主导作用. Mn_4Si_7的费米能级附近各轨道未被电子占满,且自旋向上态与自旋向下态电子的不对称分布使其具有了磁性.为Mn_4Si_7磁学特性提供主要贡献的是Mn的3d轨道电子,而Si的3p和3s轨道电子提供了一个小的贡献.     

H掺杂α-Fe2O3的第一性原理研究

α-Fe₂O₃是一种重要的磁性半导体材料, 在电子器件中应用广泛, 具有重要的研究意义. 本文基于密度泛函理论, 采用GGA+U方法, 应用第一性原理对间隙H掺杂前后的六方相α-Fe₂O₃的晶格常数、态密度、Bader 电荷分布进行了计算分析. 研究了U值对结果的影响, 发现U=6 eV时, 体相α-Fe₂O₃的晶胞平衡体积、Fe原子磁矩、带隙值与实验值最符合. 在选取合适U值后, 第一性原理计算结果表明, H掺杂后, 间隙H部分被氧化, 其最近邻的Fe 和O部分被还原, H和O有一定程度的成键. 在费米面附近, 出现了新的杂化能级, 杂化能级扩展了价带顶的宽度, 同时导带底下移, 引起带隙减小, 表明H掺杂是一种有效的能带结构调控方法.     

单晶Nd3Co的低温压力效应研究

本文测量了沿Nd3Co单晶b轴在不同压力下电阻率随温度的变化,并对Nd3Co的居里温度和磁性转变场随压力的变化规律进行了研究.结果表明:随着压力的增大,样品剩余电阻率逐渐减小,居里温度平均每GPa 升高2.1 K,磁性转变场平均每GPa增大0.9 T.通过对结果的分析,可以认为压力增大使样品中原子间距变小,晶粒间的连接更加紧密,导致电阻率减小;原子间距变小,4f电子和传导电子间的关联增强,导致样品中Nd离子磁矩的转向变得困难,从而磁性转变场增大.     

Study of combined magnetic and electric hyperfine interactions for cadmium in Dysprosium by time differential perturbed angular correlations

The combined magnetic and electric hyperfine interaction at the site of a¹¹¹Cd impurity in magnetically ordered Dysprosium has been investigated as a function of temperature by time differential perturbed angular correlation measurements. Three different phases have been found in metallic Dy with transition temperatures of 85 and 179 °K in agreement with the results of bulk material measurements. In the paramagnetic phase above 179 °K a pure electric quadrupole interaction has been observed. The various contributions to the electric fieldgradient are analyzed and it is shown, that the dominant contribution comes from the conduction electrons. In the ferromagnetic phase which extends from 0 to 85 °K the magnetic hyperfine field at the site of¹¹¹Cd has the same temperature dependence as the spontaneous magnetization. The value of the hyperfine field at 4.2 °K is ¦H _eff¦=(221 ± 4) kG. At 85 °K a transition to the antiferromagnetic phase of Dy occurs, which shows a hysteresis of the transition temperature. In the antiferromagnetic phase the temperature dependence of the hyperfine field deviates considerably from the magnetization curve. It is suggested that this deviation might be due to a temperature dependence of thes-f exchange interaction.     

Calculations of hyperfine fields

First principle methods for calculation of hyperfine fields in different systems are reviewed. The contributions from energy states close to the Fermi level are emphasized and are responsible for different observed systematic behaviours in the hyperfine field, not directly related to the host magnetic moment. Calculations on Fe, on Fe₂P, on surfaces, on impurity atoms in Fe and on the muon Knight shift in Sb are discussed.     

Magnetic hyperfine fields on119Sn nuclei in uranium compounds

119Sn Mössbauer spectroscopy studies were performed on 12 uranium intermetallic compounds in order to investigate correlations between the formation of the magnetic moment on the U atom and the magnetic hyperfine field transferred to 119Sn nuclei in magnetically ordered materials. The measured hyperfine fields (H_hf) are related to the values of the ordered U magnetic moments (UB) by U_oH_hf/n=A UU. The parameter A varies between 0.73 (UGa2) and 1.55 (UGe2). It seems to correlate with the extent of the hybridization of the 5f states with the conduction electron states.