Electronic structure and magnetism of surfaces,interfaces and modulated structures (superlattices)

Recent developments in the study of surfaces and interfaces of metals and of artificial materials such as bimetallic sandwiches and modulated structures are described. Key questions include the effects on magnetism of reduced dimensionality and the possibility of magnetically dead layers. These developments have stimulated an intensified theoretical effort to investigate and describe the electronic and magnetic structure of surfaces and interfaces. One notable success has been the development of a highly accurate full-potential all-electron method (the FLAPW method) for solving the local spin density equations self-consistently for a single slab geometry. We describe here this advanced state of ab initio calculations in determining the magnetic properties of transition metal surfaces such as those of the ferromagnetic metals Ni(001) and Fe(001) and the Ni/Cu(001) interface. For both clean Fe and Ni(001) we find an enhancement of the magnetic moments in the surface layer. The magnetism of surface and interface Ni layers on Cu(001) (no dead layers are found) is described and compared to the clean Ni(001) results. Finally, the role ofSR experiments in answering some of the questions raised in these studies will be discussed.     

Angle-dependent Ni2+ X-ray magnetic linear dichroism: interfacial coupling revisited

Using x-ray magnetic linear dichroism (XMLD) for magnetometry requires detailed knowledge of its dependence on the relative orientation of polarization, magnetic moments, and crystallographic axes. We show that Ni2+ L(2,3) XMLD in cubic lattices has to be described as a linear combination of two fundamental spectra--not one as previously assumed. The spectra are calculated using atomic multiplet theory and the angular dependence is derived from crystal field symmetry. Applying our results to Co/NiO(001) interfaces, we find perpendicular coupling between Ni and Co moments.     

Spin and Orbital Magnetisms of Ni Fe Compound: Density Functional Theory Study and Monte Carlo Simulation

The self-consistent ab initio calculations based on the density functional theory approach using the full potential linear augmented plane wave method are performed to investigate both the electronic and magnetic properties of the Ni Fe compound. Polarized spin within the framework of the ferromagnetic state between magnetic ions is considered. Also, magnetic moments considered to lie along(001) axes are computed. The Monte Carlo simulation is used to study the magnetic properties of Ni Fe. The transition temperature TC, hysteresis loop,coercive field and remanent magnetization of the Ni Fe compound are obtained using the Monte Carlo simulation.     

Surface and interface studies with ASPIC

Magnetic properties at surfaces and interfaces have been investigated performing Perturbed Angular Correlation (PAC) measurements in the UHV chamber ASPIC (Apparatus for Surface Physics and Interfaces at CERN) using different PAC probes. The results are: (i) determination of magnetic hyperfine fields of Se on Fe, Co, Ni which are explained by a theoretical study on the magnetic hyperfine fields of 4sp-elements in adatom position on Ni and Fe;(ii) static magnetic hyperfine fields in ultrathin Pd on Ni(0 0 1) which indicate an induced magnetic order in Pd;(iii) the observation of induced fluctuating magnetic interactions in Pd when thick Pd is in contact with Ni. Monolayer-resolved measurements of the magnetic hyperfine fields in magnetized Pd are in accordance with theoretical predictions of the layer dependence of the induced magnetic moments in Pd. This revised version was published online in July 2006 with corrections to the Cover Date.     

Fe clusters on Ni and Cu: size and shape dependence of the spin moment

We present ab-initio calculations of the electronic structure of small Fe clusters (1–9 atoms) on Ni(001), Ni(111), Cu(001) and Cu(111) surfaces. Our focus is on the spin moments and their dependence on cluster size and shape. We derive a simple quantitative rule that relates the moment of each Fe atom linearly to its coordination number. Thus, for an arbitrary Fe cluster the spin moment of the cluster and of the individual Fe atoms can be readily found if the positions of the atoms are known. PACS 75.75.+a; 75.70.-i; 73.22.-f     

Magnetic anisotropy of Ni modified by extreme lattice expansion

The induced magnetism of isolated (Cd) adatoms, soft-landed onto ultrathin Ni layers grown pseudo-morphologically on Pd(001) (a_Pd = 3.8907 Å), was studied using the perturbed angular correlation (PAC) technique. The magnitude of the induced magnetic response (magnetic hyperfine field of |5.2| T) was found to be ca. 30% smaller than on bulk Ni (a_Ni = 3.524 Å) surfaces. This result is compared to ab initio calculations. Additionally, the magnetic anisotropy of the induced response was found to be modified as compared to bulk Ni surfaces and the induced hyperfine field was observed to emerge from the plane at an angle of 50(5)^○ to the surface normal. This canted magnetic anisotropy is attributed to the large lattice expansion of the ultrathin Ni film on Pd(001).     

Magnetic surfaces

The magnetic ground state, magnetic anisotropies and spin excitations of surfaces, interfaces and ultra-thin films of ferromagnetic 3d-metals are discussed. Enhanced magnetic ground state moments and altered hyperfine fields as predicted by ab initio band calculations have not been conclusively verified by experiments up to now. Future calculations should take into account dipolar fields and the role of interface roughness. Very large magnetic anisotropies are observed at magnetic surfaces and interfaces. In Ni/Cu multilayered films, the superposition of surface and stress-induced anisotropies was used to switch the easy axis of magnetization from the film plane to a perpendicular orientation by a proper choice of the Ni layer thickness. This could be an attractive possibility to develop new magnetic materials for technical applications. The temperature dependence of the spontaneous magnetization at surfaces and in ultra-thin films deviates from the behaviour of bulk material. Size effects as well as surface effects of spin wave excitations are discussed, comparing theoretical and experimental results. The need for more complete theories including surface exchange, surface anisotropy and realistic surface structures is emphasized.     

Atomic-layer resolved magnetic and electronic structure analysis of ni thin film on a Cu(001) surface by diffraction spectroscopy

Up until now there has been no direct method for detecting the electronic and magnetic structure of each atomic layer at the surface, which is an essential analysis technique for nanotechnology. For this purpose, we have developed a new method, diffraction spectroscopy, based on the photon energy dependence of the angular distribution of Auger electron emission. We have applied this method to analyze the magnetic structure of a Ni ultrathin film on a Cu(001) surface around the spin reorientation transition. Atomic-layer resolved x-ray absorption and magnetic circular dichroism spectra were obtained. Surface and interior core-level shifts and magnetic moments are determined for each atomic layer individually.     

Magnetism of free and supported clusters: a comparative study

The presented work investigates the differences in magnetic properties of free and supported clusters via ab-initio calculations. The electronic structure of the clusters was calculated using a spin polarized relativistic multiple-scattering Green’s function formalism. We focus on Fe clusters of 2–9 atoms, either free or supported by Ni(001), and on Co clusters of 2–7 atoms, either free or supported by Au(111). For the supported clusters, the spin and orbital magnetic moments depend on the cluster size nearly monotonically, while for the free clusters large quasi-oscillations of magnetic moments with the cluster size were observed. Similarly, for supported clusters, the local spin magnetic moments decrease nearly linearly with increasing coordination number, while for free clusters of the same size range the trend is much more complicated. These findings are consistent with the fact that the spectral distribution function contains much sharper features for free clusters than for supported clusters.      

Ab initio study on magnetism at GaN (100) and (101) surfaces

The magnetism of GaN (100) and (101) surfaces containing neutral intrinsic defects has been investigated using ab inito calculations. Ideal Ga-ended GaN (100) surfaces and (101) surfaces are nonmagnetic. After surface relaxation, an N-ended GaN (100) surface transforms to a Ga-end, which presents local magnetic moments being ferromagnetically coupled. Neutral gallium vacancies at the (100) surface bring about large magnetic moments, which are ferromagnetically coupled. The spin-polarization of 2p electrons of nitrogen atoms is responsible for the induced magnetic moments. Neutral nitrogen vacancies at the (101) surface induce a zero magnetic moment. Neutral gallium vacancies at the (101) surface might lead to an antiferromagnetic state.     

Magnetic properties of 3d transition metal monolayers on metal substrates

We report results of systematic calculations for magnetic properties of 3d transition metal monolayers on Pd(001) and Ag(001). We find large similarities to interactions of magnetic 3d impurities in the bulk. Therefore the overlayer results are supplemented with results for 3d dimers in Cu, Ag, and Pd. Differences between the two classes of systems are utilized to reveal the interaction within the overlayers and between overlayers and substrates. In virtually all cases we find both ferromagnetic and antiferromagnetic solutions, showing large magnetic moments and similar densities of states. From the trend of the calculations we conclude that V, Cr, and Mn overlayers favor the antiferromagnetic c(2×2) structure, while Ti, Fe, Co, and Ni prefer the ferromagnetic one.     

Hyperfine fields at surfaces and interfaces

Magnetism and hyperfine fields at transition metal surfaces are discussed using state-of-the-art local spin density methods. Emphasis is placed on recent results obtained for the Fe(001), Ni(001), Cr(001) and Ag/Fe(001) ferromagnetic surfaces, and for the Knight shift in Pt(001).     

Layer-resolved magnetic moments in Ni/Pt multilayers

The magnetic moments in Ni/Pt multilayers are thoroughly studied by combining experimental and ab initio theoretical techniques. SQUID magnetometry probes the samples' magnetizations. X-ray magnetic circular dichroism separates the contribution of Ni and Pt and provides a layer-resolved magnetic moment profile for the whole system. The results are compared to band-structure calculations. Induced Pt magnetic moments localized mostly at the interface are revealed. No magnetically "dead" Ni layers are found. The magnetization per Ni volume is slightly enhanced compared to bulk NiPt alloys.     

The influence of hydrogen on the electronic and magnetic structures of TM(0 0 1) (TM=Fe, Co, Ni, and Cu) surfaces and interfaces: Ab-initio calculations

We present ab-initio investigation of the electronic and magnetic structure of TM(0 0 1) surfaces and TM/Cu(0 0 1) systems (TM=Fe, Co, Ni, Cu) with and without hydrogen adsorbed layer. The adsorption energy of hydrogen atom is found to be energetically more stable above the surface layer of Ni(0 0 1) surface than other TM(0 0 1) surfaces. The adsorption energies of hydrogen on TM/Cu(0 0 1) systems are larger than those on TM(0 0 1) surfaces. The relaxed geometries show that hydrogen has a strong influence on the interlayer distance. Furthermore, a marked reduction of Fe, Co, and Ni surface magnetic moments to 2.54, 1.41 and 0.25 μ_B, respectively, is obtained due to the presence of hydrogen.     

Nonlinear magneto-optical Kerr spectra of thin ferromagnetic iron films calculated with ab initio bandstructure theory

We show that the nonlinear Kerr effect reveals sensitively the geometrical and magnetic structure of surfaces and interfaces. We present calculations of the film-thickness dependent nonlinear magneto-optical Kerr spectra for thin bcc Fe films within a slab geometry using the spin-polarized full-potential linear muffin-tin orbital method. Our results show clearly that the nonlinear Kerr spectra of thin films are characteristicaly different from those at surfaces of bulk materials and moreover reflect more sensitively film features such as magnetic moments and substrate effects than linear spectra which are also calculated. In the case of linear Kerr spectra of Au/Fe(_bcc)/Au(001) films our theoretical results are in good agreement with observed frequency- and thickness-dependent spectra.     

The Influence of Hydrogen Adsorption on Magnetic Properties of Ni/Cu(001) Surface

Ni/Cu(001) is known as a unique system showing the spin-reorientation transition from an in-plane to out-of-plane magnetization direction when the Ni-overlayer thickness is increased. We investigate different relaxed multilayer structures with a hydrogen adlayer using the full-potential linearized augmented plane-wave method. The relaxed geometries, determined by total energy and atomic force calculations, show that H-monolayer strongly influences the interlayer distance between the Ni-surface and sub-surface layers yielding the outward relaxation of Ni-layer at H/Ni interface. Furthermore, large decrease of local magnetic moments at the top surface area is found for the surface covered by H. The magneto-crystalline anisotropy energies are calculated for fully relaxed H/Ni-films. The spin-reorientation transition critical thickness of 4 ML is found in good quantitative agreement with the experiment.     

Magnetic stripe domains in coupled magnetic sandwiches

Magnetic stripe domains in the spin reorientation transition region are investigated in (Fe/Ni)/Cu(001) and Co/Cu/(Fe/Ni)/Cu(001) using photoemission electron microscopy. For (Fe/Ni)/Cu(001), the stripe domain width decreases exponentially as the Fe/Ni film approaches the spin reorientation transition point. For Co/Cu/(Fe/Ni)/Cu(001), the Fe/Ni stripe orientation is aligned with the Co in-plane magnetization, and the stripe domain width decreases exponentially with increasing the interlayer coupling between the Fe/Ni and Co films. By considering magnetic stripes within an in-plane magnetic field, we reveal a universal dependence of the stripe domain width on the magnetic anisotropy and on the interlayer coupling.     

Simultaneous probing of exchange and spin-orbit interaction in spin polarized low energy electron diffraction from magnetic surfaces

An experimental set-up allowing simultaneous probing of exchange and spin-orbit scattering in spin polarized electron diffraction (SPLEED) from magnetic surfaces is described. In a theoretical analysis it is made clear why the measured scattering asymmetries are, in very good approximation, linear superpositions of the exchange and spin-orbit asymmetries. As an illustration SPLEED data for Ni (001) collected at various energies and angles of incidence of the primary beam, are presented and shown to be in good agreement with theoretical results.     

Magnetic properties and electronic structures of hcp Fe,Co and Ni nanowires encapsulated in a zigzag (12,0) BN nanotube

The magnetic properties and electronic structures of ferromagnetic nanowires (FM=Fe, Co and Ni) encapsulated inside a zigzag (12,0) boron nitride nanotube (BNNT) are investigated by first-principle calculations. The relaxed geometry structures of FM/BNNT systems have only slightly changed. Formation energy analysis shows that the combining processes of Co/BNNT and Ni/BNNT systems are exothermic, and therefore the Co and Ni nanowires can be encapsulated into a semiconducting zigzag (12,0) BNNT and form stable hybrid structures. The charges are transferred from ferromagnetic nanowires to more electronegative BNNTs, and the formed FM–N bonds have covalent bond characteristics. The magnetic moments of FM/BNNT systems are smaller than those of the freestanding ferromagnetic nanowires, especially for the atoms on the outermost shell of the nanowires. The stable FM/BNNT systems exhibit higher magnetic moments, which can be useful for a wide variety of next-generation nanoelectronic device components.     

Au-Ni弱铁磁合金的自发磁矩和居里点

x=0.42—0.55的Au_1-xNi_x晶态合金是从液态直接淬火到室温,测量低温下这些合金的磁化曲线(2—70kOe),得到在温度1.5K下的自发磁矩值,并用Arrott-Noakes图线方法确定居里点。首先用Perrier等人提出的最近邻模型计算平均磁矩,和实验结果进行了比较。并提出最及次近邻模型,即考虑到次近邻的影响,假设一个Ni原子的最、次近邻18个原子有11个以上是Ni原子,它就具有和纯Ni一样的磁矩(0.606μ_B),否则磁矩为零,由此模型计算的平均磁矩值和实验结果比较符合。本文还对合金中自旋集团(spincluster)的存在和自旅集团之间的相互作用作了讨论,给出了表示自旋集团之间铁磁相互作用的平均内场μ_BH/k_B的值。 关键词：