X-ray magnetic circular dichroism study of the induced spin polarization of Cu in Co/Cu and Fe/Cu multilayers

We present an x-ray magnetic circular dichroism (XMCD) study of Co/Cu and Fe/Cu multilayers, finding that the Cu atoms in these structures exhibit an induced magnetic moment in the d shell. The average Cu spin moment is shown to fall-off inversely with the thickness of the Cu layer. Further, for comparable Cu layer thicknesses, the Cu moments in Fe/Cu and Co/Cu multilayers are found to be nearly equal, despite the fact that the Cu layers in the Co/Cu multilayers are shown to be fee while those in the Fe/Cu structures are bcc. These observations suggest that the induced moment is primarily situated at the Co/Cu and Fe/Cu interfaces and is resultant from short range chemical hybridization between the ferromagnetic and Cu atoms. Results from a local spin density functional theory are presented and found to be in excellent agreement with experimental observations. These results indicate that the Cu d electrons play a central role in mediating the exchange coupling between successive ferromagnetic layers.     

Surface dependence of the magnetic configurations of the ordered B2 FeCr alloy

Tight-binding linear muffin tin orbitals calculations with generalized gradient approximation were carried out for the magnetic configurations at the surface of the ferromagnetic ordered B2 FeCr alloys. For both (001) and (111) crystallographic phases, non ferromagnetic configurations are shown to be more stable than the ferromagnetic configuration of the bulk alloy. For (001) surface we display a c ground state for either Cr or Fe at the surface. For Cr top layer the magnetic moments are larger than in the bulk B2 FeCr while they are slightly enhanced for Fe top layer. For (111) surface an antiferromagnetic coupling between surface and subsurface is always obtained i.e. for either Fe or Cr at the surface. This change of coupling between Fe and Cr (from ferromagnetic to antiferromagnetic) is expected to be fundamental to any explanation of the experimental results obtained for the interface alloying at the Fe/Cr interfaces. Received 23 March 1998     

Influence of the thermal power of a Fe atomic flux on the formation of Cu/Fe nanofilms on a Si(001) substrate

The growth of a Fe sublayer 1.5–14.0 monolayers (MLs) thick and a Cu film (about 5 MLs) on this sublayer is studied at a reduced temperature (1240°C) and an elevated temperature (1400°C) of a Fe source and at a reduced temperature (900°C) of a Cu source. The films are examined by Auger electron spectroscopy, low-energy electron diffraction, and atomic force microscopy. As metal sources, thin Fe and Cu strips on a Ta foil are used. It is shown that a nonequilibrium 2D phase forms in the Fe-on-Si(001) film up to a thickness of 4–5 MLs. This phase appears as closely packed atomically smooth nanoislands. When the thickness of the film exceeds 4–5 MLs, the nonequilibrium Fe phase changes to the bulk (3D) phase of Fe and its silicide Fe _x Si. At Fe source temperatures of 1240 and 1400°C, the nonequilibrium phase consists of Fe with Si segregated on the Fe surface, and a Fe-Si mixture. Copper on the nonequilibrium Fe and Fe-Si phases grows, respectively, as a smooth layer Cu with Si segregated on the top and in the form of Cu-Fe and Cu-Si mixtures. Cu islands growing on the bulk Fe and Fe _x Si phases have smaller and larger sizes, respectively.     

Frozen low-spin interface in ultrathin Fe films on Cu(111)

In ultrathin film systems, it is a major challenge to understand how a thickness-driven phase transition proceeds along the cross-sectional direction of the films. We use ultrathin Fe films on Cu(111) as a prototype system to demonstrate how to obtain such information using an in situ scanning tunneling microscope and the surface magneto-optical Kerr effect. The magnetization depth profile of a thickness-driven low-spin to high-spin magnetic phase transition is deduced from the experimental data, which leads us to conclude that a low-spin Fe layer at the Fe/Cu interface stays live upon the phase transition. The magnetically live low-spin phase is believed to be induced by a frozen fcc Fe layer that survives a thickness-driven fcc-->bcc structural transition.     

Local atomic structure of ultra-thin Fe films grown on Cu(100)

Ultra-thin epitaxial Fe films grown by thermal deposition on Cu(100) are analyzed by scanning tunneling microscopy. Evidence is presented that the morphological characteristics and magnetic properties are a direct consequence of FCC-to-BCC transitions reminiscent of those occurring in bulk Fe. In contrast to the assumption of a ferromagnetic FCC phase in previous models of the Fe/Cu(100) system, we observe a tightly twinned and strained BCC-like phase termed nanomartensite in films below 5 ML thickness, which encompasses almost the entire film volume of 3 ML films. In addition, the surface of 7–8 ML films reconstructs by forming non-close-packed structures with BCC-like bond angles. The formation of these BCC-like phases is the reason for the expansion of the interlayer spacing observed in these films and correlates perfectly with their ferromagnetic ordering. PACS 68.55.-a; 64.70.-p; 81.30.-t     

Fe/[NiFe/Cu]30多层膜线列阵的铁磁共振研究

用铁磁共振(FMR)技术研究Fe/［NiFe/Cu］₃₀多层膜线列阵的特性,线列阵采用激光全息光刻和离子束刻蚀技术加工,线密度为250线/mm和500线/mm.线列阵样品的FMR谱在稍高于主峰（声学峰）的共振场下有光学峰出现,相当层间交换耦合为反铁磁型.光学峰与声学峰的峰位间距沿易轴小于沿难轴.又,来自非磁Cu夹层的顺磁共振峰的幅度远大于原始的连续膜,说明线列阵加工致使层结构退化. 关键词：     

Search for ferromagnetism in ultrathin epitaxial films: Cr/Au(100), Cr/Cu(100), and Fe/Cu(100)

The surface magneto-optic Kerr effect (SMOKE) technique was used to search for ferromagnetism in monolayer-range films of Cr and Fe grown on Au(100) and Cu(100). The growth modes were characterized using low energy electron diffraction (LEED) and Auger electron spectroscopy. The fcc structure of Cr could not be stabilized on Cu(100). Ferromagnetism was not observed for the Cr/Au(100) films at temperature above 100 K. Ferromagnetism also was not observed for fcc Fe/Cu(100) grown at room temperature; but for growth at >150°C, a ferromagnetic, metastable state was observed for the top layer of the Fe film, in the absence of bulk ferromagnetism. The ferromagnetic Fe/Au(100) system was used to establish the sensitivity of the approach.     

Magnetic phase transition in Co/Cu/Ni/Cu(100) and Co/Fe/Ni/Cu(100)

Magnetic phase transitions in coupled magnetic sandwiches of Cu/Co/Cu/Ni/Cu(100) and Cu/Co/Fe/Ni/Cu(100) are investigated by photoemission electron microscopy. Element-specific magnetic domains are taken at room temperature to reveal the critical thickness at which the magnetic phase transition occurs. The results show that a coupled magnetic sandwich undergoes three types of magnetic phase transitions depending on the two ferromagnetic films' thickness. A phase diagram is constructed and explained in the process of constructing Monte Carlo simulations, which corroborate the experimental results.     

一种非连续纳米氧化层自旋阀的显微结构研究

利用高分辨电子显微学方法(HREM)研究了纳米氧化层镜面反射自旋阀多层结构Ta(35nm)Ni80Fe20(2nm)Ir17Mn83(6nm)Co90Fe10(15nm)NOL1Co90Fe10(2nm)Cu(22nm)Co90Fe10(15nm)NOL2Ta(3nm).该自旋阀的巨磁电阻(GMR)效应高达15%,较无此镜面反射纳米氧化层(NOL)的自旋阀提高近1倍,同时交换偏置场亦有所增强.高分辨显微结构分析表明,介于钉扎层与被钉扎层之间的氧化层(NOL1)并未完全氧化,即除氧化过程生成的CoFe氧化物 关键词： 自旋阀 纳米氧化层 高分辨电子显微学 巨磁电阻效应     

Coupled perpendicular magnetization in Fe/Cu/Fe trilayers

Ultrathin epitaxial Fe films on Cu(1 0 0) with perpendicular magnetization have been used as templates for the preparation of FCC Fe/Cu/Fe trilayers. The magnetic anisotropy and the coupling of these films have been studied by in-situ magneto optical Kerr effect measurements and Kerr microscopy. The magnetic coupling of both Fe layers is found to be dominated by magnetostatic interaction. Adsorbate-induced spin reorientation in the top layer also causes spin reorientation in the bottom layer. The governing role of the Fe-vacuum interface for the magnetism of the whole trilayer is demonstrated.     

Magnetoresistance and magnetization studies through a Cu interlayer in spin valves of NiFe/Cu/NiFe/FeMn

The influence of the Cu layer thickness on the magnetic and magnetotransport properties has been investigated in Ta/NiFe/Cu/NiFe/FeMn spin valves. The magnetization and magnetoresistance measurements were carried out for magnetic field applied along the easy-axis direction. A phenomenological model, which assumes formation of a planar domain wall at the anti-ferromagnetic side of the interfaces as well as bilinear coupling between the ferromagnetic layers, was used to derive the anisotropy characteristics and orientation of each NiFe layer magnetization. The anisotropy and spin valve magnetoresistance were simulated numerically and compared with the experiment. It was found that the anisotropy magnetoresistance is negligible and that there is a poor agreement for the spin-valve one, which was attributed to the model (valid for ferromagnetic layers in single-domain state only) used for its calculation. It was found that the increase of the Cu layer thickness provokes a decrease of the interdiffusion between the NiFe and FeMn layers, and, as consequence, changes of the uniaxial anisotropy of the pinned NiFe layer, of the exchange interaction between the pinned NiFe layer and the FeMn ones, as well as of the exchange-bias field of the pinned NiFe layer.     

Fen/Crn超晶格磁性和电子结构的第一性原理研究

采用基于密度泛函原理的全势线性缀加平面波方法(FLAPW),计算了超晶格Fen/Crn(n=1,3,5)的电子结构和磁性,结果表明铁磁耦合状态是基态,铁层的磁矩由于铬层的加入而有一些变化,铁层的磁矩随着n的增大而逐渐增强.铬层的磁矩的方向是正负相间变化的,相邻的铁层和铬层之间是反铁磁性耦合的,铁原子的d轨道和铬原子的d轨道在费米能附近有中等程度的杂化.     

Magnetic superlattices with variable interlayer exchange coupling: a new approach for the investigation of low-dimensional magnetism

We have investigated the magnetic order in an [Fe(2)/(VHx)(13)] x 200 superlattice as a function of temperature and hydrogen content in the vanadium layers. A J(radially)-T magnetic phase diagram was established where J(radially) denotes the interlayer exchange coupling between adjacent Fe planes. We propose that Fe/V superlattices, in which the ratio of interlayer to intralayer coupling can be tuned continuously and reversibly via hydrogen in the nonmagnetic vanadium, offer a new approach for the study of low-dimensional magnetism and crossover effects near the transition from ferromagnetic to antiferromagnetic order.     

Spacer induced magnetism and its effect on interlayer exchange coupling in Fe/(Pd,Cu, Au,Ag) multilayered nanowires

We have investigated the one dimensional Fe/Pd, Fe/Cu, Fe/Ag, and Fe/Au multilayered nanowire systems by using first principles density functional theory. Our study reveals a gain in the binding energies of these heterostructures regardless of nature of the spacer. We have identified the electronic structure dependent enhancement of magnetic properties, and a switching behavior of the interlayer exchange coupling, with respect to the nature and dimension of the nonmagnetic spacer layer. We observe a down-spin (minority) d-charge depletion at the Fe site and a up-spin d-charge gain at Pd site in Fe/Pd nanowire which accounts for the enhanced magnetic moment of the Fe atoms and the ferromagnetic behavior of the Pd, in contrast to the paramagnetism appearing in their bulk state. We find the interlayer exchange coupling, I _ex , in Fe/Pd nanowire to be very strong, and it shows a change of sign and a decrease in magnitude with increase in Pd spacer width. On the other hand, I _ex shows an unusual increasing trend with increase in the Cu spacer layer thickness.     

Mössbauer-effect studies of multilayers and interfaces

The usefulness of Mössbauer spectroscopy for the investigation of magnetic multilayer systems is described. By applying ⁵⁷Fe Mössbauer spectroscopy, the behavior of ultrathin magnetic layers, such as FCC-like Fe films on Cu(0 0 1), is studied. Position-specified (depth-selective) information is available by preparing samples in which monatomic ⁵⁷Fe probe layers are placed at specific vertical positions, e.g. at interfaces or at the surface. As demonstrated for epitaxial chemically ordered Fe₅₀Pt₅₀ alloy films and polycrystalline nanostructured Tb/Fe multilayers, the Fe-spin structure can be determined directly, and a site-selective Fe-specific magnetic hysteresis loop can be traced in very-high-coercivity materials. For the studies of non-magnetic layers, on the other hand, hyperfine field observations by ¹⁹⁷Au and ¹¹⁹Sn probes are worthwhile. Spin polarizations in Au layers penetrating from neighboring ferromagnetic 3D layers are estimated ¹⁹⁷Au from Mössbauer spectra and are also studied by inserted ¹¹⁹Sn probes in Au/3D multilayers. In the Sn spectra for Cr/Sn multilayers, it was found that remarkably large spin polarization is penetrating into Sn layers from a contacting Cr layer, which suggests that Cr atoms in the surface layer have a ferromagnetic alignment.     

Surface relaxation and thermal expansion for the (001) face of α-Fe and Cu

The surface relaxation and the near-surface enhancement of thermal expansion have been calculated for the (001) face of a bcc crystal, α-Fe, and an fcc crystal, Cu. The calculations make use of the anharmonic perturbation formalism of Dobrzynski and Maradudin; the results for certain equal-time vibrational correlation functions which arise in this formalism are also presented. The crystal potential is described in terms of several kinds of short-range empirical interatomic potentials, such as have been used in studies of defects in bulk; in the near-surface region, the effects of surface redistribution of the electron distribution are modelled by the addition of a simple surface Madelung (SSM) force. The effect of the SSM force is to limit severely the usual outward relaxation driven by short-range interatomic potentials. For Fe(001), the five and one-half percent outward static relaxation driven by the short-range potentials acting alone is changed to a one percent inward static relaxation when the SSM force is incorporated; for Cu(001), the comparable change is from a one percent outward relaxation to a one-half percent outward relaxation. On the other hand, the SSM force makes only a small effect on the surface-enhanced thermal expansion coefficients (STEC) for interplanar spacings. The STEC for the outermost spacing is between 2.5. and 3.0 times of that for the bulk at the Debye temperature for both Fe(001) and Cu(001); for the second interplanar spacing, the STEC is smaller than 1.5 times of that of the bulk at the Debye temperature. The ratios of the near-surface mean-square amplitudes (MSA) to those of the bulk at high temperatures are, for Fe(001), about 1.75 for z-components (normal to surface) and 1.55 for x-components (parallel to surface) in the surface layer; for Cu(001), about 1.95 for z-components and 1.30 for x-components. The interplanar correlation functions, while smaller than the MSA on an absolute scale, do show considerable surface-enhancement, particularly for the zz-compoments. For example, the zz-correlation between an atom in the outermost layer and its nearest neighbor in the next layer is nearly twice the comparable bulk correlation above the Debye temperature for both Fe(001) and Cu(001).     

Ferromagnetic stability in Fe nanodot assemblies on Cu(111) induced by indirect coupling through the substrate

We report collective ferromagnetic behavior with high Curie temperatures (T(c)) in Fe dot assemblies supported by the Cu(111) surface. Our ability to tune the average size and spacing of the individual dots allows us to conclude that enhanced magnetic anisotropy cannot account for this high-T(c) ferromagnetic order. Because our Monte Carlo simulations have ruled out the dipolar interaction as the dominant factor in this system, we attribute the origin of the ferromagnetic order to indirect exchange coupling via the Cu(111) substrate.     

Superconducting spin valve effect of a v layer coupled to an antiferromagnetic [Fe/V] superlattice

We studied superconducting V layers deposited on an antiferromagnetically coupled [Fe(2)V(11)](20) superlattice. The parallel upper critical magnetic field exhibits an anomalous T dependence up to the ferromagnetic saturation field of the superlattice, indicating that the superconducting transition temperature T(S) decreases when rotating the relative sublattice magnetization directions of the superlattice from antiparallel to parallel. This proves that the pair breaking effect of a Fe2 layer is reduced if at a distance of 1.5 nm a second Fe2 layer with antiparallel spin orientation exists.     

Structure and magnetic properties of Fe4/Cun (n=2,4) superlattices from first-principles study

The structures and magnetic properties of Fe4/Cun (n=2, 4) superlattices have been investigated by the first-principles pseudopotential plane-wave method based on spin density approximation. Compared with the ideal fcc-Cu bulk structure, for the optimized Fe4/Cu2 model, obvious contraction of interlayer distances occurs on the interior Fe layers, whereas the interlayer distances of Fe layers in Fe4/Cu4 are expanded. The anti-parallel alignment magnetic moment and negative polarization of the interior Fe layer have been found in the Fe4/Cu2 model. This can be explained in terms of the magnetic-volume effect, and the moment of anti-parallel alignment attributes to the contracted interlayer distances between the interior Fe layers. The MR ratio has also been evaluated by means of the two-current model. The MR ratio of the Fe4/Cu2 model (4.89%) is much small than that of the Fe4/Cu4 one (23.65%).     

Effect of Ta buffer layer and TaO x barrier thickness on the evolution of the structural and magnetic properties of the Fe/TaO x /Co trilayers

Fe/TaO _x /Co trilayers were grown on Si(100)/SiO₂ substrates and on tantalum buffer layers by a high vacuum magnetron sputtering system. The effects of both Ta buffer layer and tantalum-oxide barrier layer thickness on the structural and magnetic properties and the coupling of the ferromagnetic layers have been studied. It was observed that Ta improves the structural properties of the Fe layer resulting in an increased coercive field. For a barrier thickness of 4 nm a weak decoupling starts to appear between the ferromagnetic layers and a clear step formation is observed with increasing thickness. The minor hysteresis loops predict an interlayer coupling for thin barriers. The annealing of trilayers up to 250°C shows an increased coercivity for only the Fe layer. Annealing further at 400°C has the opposite effect of decreasing the coercivity, indicating intermixing at the interfaces of the Fe. The refractive index of the insulator barrier shows that the barrier layer is not totally in the form of tantalum-pentoxide.