Interface-induced states with an incommensurate spin-density wave in Fe/Cr-type multilayers

A model is proposed for magnetic ordering in Fe/Cr-type multilayers substantially above the Néel temperature of bulk chromium. Redistribution of the charge (and, hence, spin) density near the Fe/Cr interfaces gives rise to the formation of an essentially inhomogeneous spin-density-wave (SDW) state in the chromium spacer. The spatial structure of the antiferromagnetic order parameter in thick spacers is described. The SDW contribution to the effective exchange coupling between the moments in adjacent iron layers is calculated. The data obtained are used in the interpretation of experimental data on the tunneling spectroscopy of trilayers and neutron diffraction from Fe/Cr superlattices.     

Noncollinear interlayer exchange in Fe/Cr/Fe magnetic structures with different interface roughnesses

The interaction of iron layers through a chromium spacer in Fe/Cr/Fe trilayers with different roughnesses of interfaces was studied by the Kerr magnetometry and Mandel’shtam-Brillouin light scattering techniques so as to trace the interlayer exchange coupling of the Fe layers depending on the Cr spacer thickness and the sample temperature. It is established that, in a broad range of these parameters, the interlayer exchange in Fe/Cr/Fe structures with sufficiently smooth interfaces is adequately described using the proximity magnetism and half-angle coupling models taking into account the antiferromagnetic properties of chromium. As the interface roughness increases, the well-known biquadratic exchange model becomes valid. This is evidence for the decisive role of the magnetic stiffness of a Cr spacer and the structure of interfaces on the noncollinear exchange coupling in Fe/Cr/Fe trilayers.     

Interlayer magnetic coupling in multilayer magnetic Fe/Cr/Fe systems

The interlayer magnetic coupling of iron layers as a function of the chromium spacer thickness and temperature has been studied for three-layer epitaxial Fe/Cr/Fe films by the methods of Kerr magnetometry and Mandelstam-Brillouin scattering. The results obtained indicate that the short-period component of the interlayer exchange is related to the spin density wave in the chromium spacer.     

Noncollinear magnetic states in iron-chromium-type multilayers

A model of noncollinear magnetic ordering in Fe/Cr-type multistructures was suggested. The model was based on the idea of charge (and, as a consequence, spin) density redistribution near a metal-metal interface. A peculiar state of the whole structure characterized by strong short-range antiferromagnetic ordering in the interlayer and a pronounced dependence of magnetic characteristics on the properties of the boundary between iron and chromium layers was shown to be formed in a certain temperature range. Inhomogeneous antiferromagnetic structures with a vector order parameter were found, and the effective exchange coupling between neighboring iron layer moments was calculated using the Ginzburg-Landau expansion of the thermodynamic potential. The results were used to discuss the experimental data on Fe/Cr superlattices obtained in neutron scattering and magnetization measurements.     

Short-range magnetic order in a spin density wave in Cr-based multilayers

A mechanism of the formation of the short antiferromagnetic order with a spin density wave (SDW) in the vicinity of the interfaces in the Fe/Cr type multilayers is proposed. The main reason behind the emergence of magnetic ordering with SDWs is the redistribution of charge (and, hence, spin) density in the vicinity of Fe/Cr interfaces, which leads to the paramagnetic phase instability at a temperature considerably higher than the Néel temperature in chromium. The Ginzburg-Landau expansion for the free energy of the system is used for determining the inhomogeneous collinear structures of CDWs and for constructing the phase diagram (the dependence of the transition temperature on the thickness of the antiferromagnetic interlayer). The obtained results are used for discussing the experimental data on neutron scattering and tunnel microscopy.     

Polarized neutron reflectometry of Fe/Cr/Gd superlattices

The magnetic structure of Fe/Cr/Gd superlattices is investigated using complementary methods of SQUID magnetometry and polarized neutron reflectometry. The complex magnetic behavior of the given system is caused by exchange interaction between the 3d (Fe) and 4f (Gd) layers of the ferromagnetic metals through the Cr antiferromagnetic spacer layer. It is found that a nonuniform profile of magnetization forms within the Gd layers under the influence of this interlayer interaction.     

Interlayer exchange coupling in iron/silicon nanostructures

A superexchange mechanism is considered for the interlayer exchange coupling in Fe/Si nanostructures. It is assumed that the most important role in this mechanism is played by iron-silicon interfaces, in whose immediate vicinity the contact-induced ferromagnetic phase of body-centered iron monosilicide forms and spin-polarized interface states arise. The magnetic coupling between iron layers is accomplished by means of the interaction of interface states through the normal-semiconductor spacer layer and involves both intraband and interband processes. The nonmonotonic character of the dependence of the interband exchange coupling on the spacer thickness and composition (the occurrence of a maximu, the change in shape, the possible sign reversal of the bilinear component) is caused by competition between the ferro-and antiferromagnetic super-exchange components and by the complex character of the spacer electronic spectrum (in particular, by the occurrence of several equivalent extrema in the semiconductor bands). This model qualitatively reproduces the main features of the mechanism of interlayer exchange coupling in real Fe/Si nanostructures.     

Control of interlayer exchange coupling in Fe/Cr/Fe trilayers by ion beam irradiation

The manipulation of the antiferromagnetic interlayer coupling in epitaxial Fe/Cr/Fe(001) trilayers by 5 keV He ion beam irradiation has been investigated. It is shown that even for irradiation with low fluences a drastic change in strength of the coupling appears. For thin Cr spacers (below 0.6-0.7 nm) it decreases with fluence, becoming ferromagnetic for fluences above 2x10(14) ions/cm(2). The effect is connected with the creation of magnetic bridges in the layered system due to atomic exchange events caused by the bombardment. For thicker Cr spacers an enhancement of the antiferromagnetic coupling strength is found. A possible explanation of the enhancement effect is given.     

First-order phase transitions in magnetization of Fe/Cr/Fe three-layer films

The possibility of appearing jumps in the magnetization curves of Fe/Cr multilayers has been predicted under the assumption that magnetic ordering in chromium interlayers has the form of a linearly polarized spin-density wave. This possibility has been analyz ed for an Fe/Cr/Fe three4ayer film with a conventional quality of Fe/Cr interfaces, which does not ensure a change in the ferromagnetic and antiferromagnetic orientations of the magnetizations of neighboring iron layers with a variation in the thickness of the chromium interlayer by one atomic layer (short oscillation period). The model used suggests that the wave vector of the spin-density wave is responsible for the experime ntally observed long period of these oscillations. Relationships have been derived for the range of thicknesses of chromium interlayers in which the appearance of the predicted effect can be expected, and the magnitude of the effect has been evaluated.     

Exchange couplings in magnetic films

Recent advances in the study of exchange couplings in magnetic films are introduced.To provide a comprehensive understanding of exchange coupling,we have designed different bilayers,trilayers and multilayers,such as anisotropic hard/soft-magnetic multilayer films,ferromagnetic/antiferromagnetic/ferromagnetic trilayers,[Pt/Co]/NiFe/NiO heterostructures,Co/NiO and Co/NiO/Fe trilayers on an anodic aluminum oxide(AAO) template.The exchange-coupling interaction between soft-and hard-magnetic phases,interlayer and interfacial exchange couplings and magnetic and magnetotransport properties in these magnetic films have been investigated in detail by adjusting the magnetic anisotropy of ferromagnetic layers and by changing the thickness of the spacer layer,ferromagnetic layer,and antiferromagnetic layer.Some particular physical phenomena have been observed and explained.     

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     

Ab initio calculations of magnetic structure and lattice dynamics of Fe/Pt multilayers

The magnetization distribution, its energetic characterization by the interlayer coupling constants and lattice dynamics of (001)-oriented Fe/Pt multilayers are investigated using density functional theory combined with the direct method to determine phonon frequencies. It is found that ferromagnetic order between consecutive Fe layers is favoured, with the enhanced magnetic moments at the interface. The bilinear and biquadratic coupling coefficients between Fe layers are shown to saturate fast with increasing thickness of nonmagnetic Pt layers which separate them. The phonon calculations demonstrate a rather strong dependence of partial iron phonon densities of states on the actual position of Fe monolayer in the multilayer structure.     

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 POLARIZATION OF Pd SPACERS AND OSCILLATORY EXCHANGE COUPLING BETWEEN Fe AND Pd LAYERS IN Fe/Pd MULTILAYERS

Ay oscillatory behavior in specific saturation magnetization of Fe/Pd multilayers is observed. The conversion electron M?ssbauer spectroscopy and other experiments indicate that this behavior is caused by the magnetic polarization of Pd spacers, which alternates between positive and negative polarization with respect to the magnetic moments of Fe layers. The interlayer coupling between two Fe layers always keeps ferromagnetic. Then, the interlayer couplings in ferromagnetic metal/nearly ferromagnetic metal multilayers were calculated By using the model of Barnas. The results of calculation prove that the above phenomenon might appear under some condition. The influence of magnetic polarization potential of spacers and interface chemical potential on the interlayer couplings is also discussed.     

Magnetisation and remagnetisation peculiarities of Fe/Cr multilayer films caused by instability of Cr layers magnetic structure

A model to describe the magnetic properties of Fe/Cr/Fe films with nanometer-thick layers of Fe and Cr is proposed. The model rests on the assumption that there exists a magnetic order of the linearly polarized spin-density wave type in a chromium interlayer with thicknesses even less than its wavelength. This only assumption proved to be enough to describe the following characteristics, experimentally observed, of the magnetic properties of the multilayer Fe/Cr films: the existence of short and long periods, dependences of their magnetic properties on the chromium layer thickness, noncollinear orientation of the magnetizations of neighboring iron layers, the existence of the magnetization curves beyond the hysteresis loops of magnetization.     

Competing exchange interactions in magnetic multilayers

We have studied alloying of the nonmagnetic spacer layer with a magnetic material as a method of tuning the interlayer coupling in magnetic multilayers. We have specifically studied the Fe/V(100) system by alloying the spacer V with various amounts of Fe. For some Fe concentrations in the spacer, it is possible to create a competition between antiferromagnetic Ruderman-Kittel-Kasuya-Yoshida exchange and direct ferromagnetic exchange coupling. The exchange coupling and transport properties for a large span of systems with different spacer concentrations and thicknesses were calculated and measured experimentally and good agreement between observations and theory was observed. A reduction in magnetoresistance of about 50% was observed close to the switchover from antiferromagnetic to ferromagnetic coupling.     

Possibility of Helical Interlayer Coupling in the Magnetic Superlattices and Overlayers

We consider interlayer magnetic coupling in a superlattice and/or overlayer in the case when spacer layer has the tendency towards formation of the helical spin density wave. Within phenomenological Landau-Ginzburg approach we show that the mutual interplay of the SDW and interlayer coupling spin polarization can lead to the helical interlayer coupling. When surface anisotropy is accounted for the effective interaction mimics interlayer biquadratic coupling.     

Interlayer exchange coupling in ferromagnet-semiconductor digital magnetic alloys

The interlayer exchange coupling in ferromagnet-semiconductor digital magnetic alloys in which monolyers (submonolayers) of transition metals are embedded into a semiconductor matrix is studied theoretically. A mechanism of an indirect exchange between ferromagnetic δ layers is proposed; it is based on the confinement of carriers in two-dimensional spin-polarized states inside the energy gap of the semiconductor. These appear due to strong potential and exchange carrier scattering by the δ layers. The interlayer exchange coupling is shown to occur through a nondegenerate semiconductor interlayer because of virtual electron excitations through an energy barrier separating these partly filled two-dimensional spin-polarized states and the edge of the bulk semiconductor band. The interlayer coupling intensity decreases exponentially with increasing distance between neighboring δ layers, and the type of this coupling can change from ferromagnetic into antiferromagnetic or vice versa as the interlayer thickness or the degree of filling the two-dimensional states increases.     

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.     

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.