Effect of the structural quality of the buffer on the magnetoresistance and the exchange coupling in sputtered Co/Cu sandwiches

The effect of the structural quality of the buffer stack on the structural properties, giant magnetoresistance (GMR) and the quality of the antiferromagnetic coupling has been investigated for Co/Cu/Co sandwiches prepared by DC-magnetron sputtering. Three kinds of buffers were employed: type A: Cr(6 nm)/Co(0.8 nm)/Cu(10 nm), type B: Fe(6 nm)/Co(0.8 nm)/Cu(10 nm) and type C: Cr(4 nm)/Fe(3 nm)/Co(0.8 nm)/Cu(10 nm). For B and C type buffers, the antiferromagnetic alignment is very interesting at zero field with a coupling strength larger than 0.4 erg/cm² and a GMR signal reaching 5% at room temperature. However, for the A type buffer the antiferromagnetic coupling completely disappears, while the GMR drops to about 0.8%. X-ray diffraction, atomic force microscopy and transmission electron microscopy have been performed in order to understand the origin of the observed difference in the magnetic properties. The results show a strong difference in the average surface roughness, 1.15 nm and 0.35 nm, respectively for the A and C types buffers, and demonstrate that the quality of the surface of the buffer is the key to optimize both the GMR and the indirect exchange coupling. Received 11 July 2000     

In situ resistivity and magnetoresistance studies of Co/Au(111) single layers and bilayers

We report here on resistance and magnetoresistance (MR) studies of ultrathin Co/Au(111) single sandwiches and bilayers with perpendicular magnetization. Resistance of the films was measured in situ in ultrahigh vacuum, during depositions and as a function of a perpendicular applied magnetic field. A large MR variation with the thickness of Au coverage was observed and compared to calculations. The coercive field of the Co films shows a drastic variation with the Au coverage thickness, which reflects the theoretical anisotropy variation. It was measured as a function of temperature. For the first time, the effect of interlayer interaction on the resistivity of a Co bilayer during the growth of Co top layer, is evidenced and compared to calculations. Finally, hysteresis loops of strongly antiferromagnetically coupled bilayers are investigated. Received 3 November 1998 and Received in final form 18 January 1999     

Magnetic properties of (Co Pd ) superstructures on Pd(100) and Pd(111)

The magnetic properties of (Co_nPd_m)_r superstructures on Pd(100) and Pd(111) are evaluated using the fully-relativistic spin-polarized screened Korringa-Kohn-Rostoker method. It is found that only in the case of a Pd(111) substrate such superstructures exhibit perpendicular magnetism, while on a Pd(100) substrate the magnetization is oriented in-plane. Also investigated is the effect of interdiffusion in repeated superstructures. By using the inhomogeneous coherent potential approximation (CPA) for layered systems the effect of ordering into (repeated) superstructures can be described in an ab-initio-like manner. It is found that already small amounts of interdiffusion can be decisive for the actual value of the magnetic anisotropy energy. Received 3 November 1999 and Received in final form 18 January 2000     

Interfacial roughness and angle dependence of giant magnetoresistance in magnetic superlattices

Using the two-point conductivity formula, we numerically evaluate the giant magnetoresistance (GMR) in magnetic superlattices with currents in the plane of the layers (CIP), from which the effect of the interfacial roughness and magnetization configuration on the GMR is studied. With increasing interfacial roughness, the maximal GMR ratio is found to first increase and then decrease, exhibiting a peak at an optimum strength of interfacial roughness. For systems composed of relatively thick layers, the GMR is approximately proportional to ,where is the angle between the magnetizations in two successive ferromagnetic layers, but noticeable departures from this dependence are found when the layers become sufficiently thin. Received 21 September 1998 and Received in final form 22 December 1998     

Interlayer exchange coupling: Preasymptotic corrections

In the asymptotic limit, the interlayer exchange coupling decays as D^-2, where D is the spacer thickness. A systematic procedure for calculating the preasymptotic corrections, i.e., the terms of order D^-n with ,is presented. The temperature dependence of the preasymptotic corrections is calculated. The results are used to discuss the preasymptotic corrections for the Co/Cu/Co(001) system. Received 7 January 1999     

The transverse ferromagnet spin-1 Ising model of alternating magnetic superlattice

Within the framework of the effective field theory with a probability distribution technique that accounts for the single-site spin correlations, we examine the critical behavior of the transverse ferromagnetic spin-1 Ising model of an alternating magnetic superlattice. The critical temperature of the alternating magnetic superlattice has been studied as a function of the interlayer and intralayer exchange interactions and the strength of the transverse field and the thickness of the finite superlattice. Received 12 January 2000 and Received in final form 14 September 2000     

Non-equilibrium spin accumulation in ferromagnetic single-electron transistors

We study transport in ferromagnetic single-electron transistors. The non-equilibrium spin accumulation on the island caused by a finite current through the system is described by a generalized theory of the Coulomb blockade. It enhances the tunnel magnetoresistance and has a drastic effect on the time-dependent transport properties. A transient decay of the spin accumulation may reverse the electric current on time scales of the order of the spin-flip relaxation time. This can be used as an experimental signature of the non-equilibrium spin accumulation. Received 6 May 1998     

The temperature dependence of the perpendicular giant magnetoresistance in Co/Cu multilayered nanowires

A theory, based on earlier work by Valet and Fert, is first presented to describe the influence of temperature on the perpendicular giant magnetoresistance (GMR) in multilayers. Then we present GMR measurements performed at T=77 K and at room temperature on Co/Cu multilayered nanowires with layer thicknesses ranging from a few nm to 1 μm. We use our model to obtain a good quantitative fit to the experimental results in both the short spin diffusion length limit and out of this limit. We discuss the temperature dependence of the bulk parameters, the scattering spin asymmetry coefficient and spin diffusion length in the Co layers. Received: 25 January 1998 / Accepted: 6 May 1998     

Effect on alloying at the Fe/Ni(001) interfaces on the interlayer exchange coupling

We investigate the stability of various ordered FeNi alloys at the interfaces of Fe/Ni superlattices by using ab initio density functional calculation. We consider an Fe_0.5Ni_0.5 ordered alloy of one or two monolayers thick at different positions beyond the interface and the possibility of an interdiffusion of a complete monolayer of Ni(Fe) in Fe(Ni) slab. An interfacial atomic layer of Fe_0.5Ni_0.5 exchanged with its adjacent Ni monolayers, leading to a buffer zone of Ni₃Fe composition is found to be the most stable structural configuration. For this atomic arrangement we investigate the magnetic profile and the resulting interlayer exchange coupling between the Ni slabs for Fe spacer thickness of 0 to 4 monolayers.     

Shape anisotropy and magnetic domain structures in striped monolayers

We study the effect of dipolar interactions on a magnetic striped monolayer with a microscopic unit cell of square symmetry, and of size spins. Even if the aspect ratio r=N _x /N _y is very large, an in-plane shape anisotropy is always negligible, except if N_y is fairly small (N _y <40). In-plane domains are not possible, except for values of the dipolar coupling larger than the domain wall energy. Received: 11 July 1997 / Revised: 24 September 1997 / Accepted: 24 October 1997     

High-frequency dynamics of Co/Fe multilayers with stripe domains

Within the framework of two-dimensional (2D) numerical micromagnetic simulations, the equilibrium magnetization configuration and the high-frequency (0.1–30 GHz) linear response of Co/Fe multilayers have been investigated in detail. Due to the perpendicular anisotropy of Co layers, a stripe domain pattern can develop through the whole multilayer, the characteristics of which depend on the magnitude of the perpendicular anisotropy, the respective thicknesses of Co and Fe layers and the number of Co/Fe bilayers in the stack. One of the most striking features associated with the layering effect is the ripening aspect of the static magnetization configuration across the multilayers which induces complicated dynamic susceptibility spectra including surface modes and volume modes strongly confined within the inner Fe layers. The effect of the cubic magnetocrystalline anisotropy of Fe layers and the influence of a nonuniform perpendicular magnetic anisotropy within the Co layers on the static and dynamic magnetic properties of Co/Fe multilayers are then analyzed quantitatively.     

Low frequency and weakly nonlinear spin transport in ferromagnet/insulator single and double junctions

In this paper, we apply Büttiker's gauge invariant, charge conservation, nonlinear transport theory to explore the spin-polarized tunneling of ferromagnet/insulator (semiconductor) single and double junctions. The Green function of spin-polarized tunneling is calculated by the tight-binding approximation method. The energy and the angle (between the molecular field and the vertical axis) dependences of the weakly nonlinear dc transport coefficient and the linear low frequency ac transport coefficient are investigated. The ac tunneling magnetoresistance is also discussed. Received 1st September 2000 and Received in final form 5 December 2000     

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.     

Evidence of domain wall scattering in thin films of granular CoFe-AgCu

Thin films of the giant magnetoresistive granular CoFe-AgCu system prepared by rf sputtering displayed a great variety of domain-like microstructures with a net out-of-plane component of the magnetization for ferromagnetic volume concentrations above about 0.25. Therefore, magnetic percolation takes place at ferromagnetic concentrations much lower than the physical percolation threshold. The out-of-plane structure of the as-deposited samples in magnetic virgin state consisted of a distribution of both quasi-circular domains and short stripes depending on the ferromagnetic content. Furthermore, these samples present high metastability and a variety of remanent in-plane and out-of-plane microstructures can be achieved as a function of the magnetic history. Besides, the evolution of the magnetic microstructure yields strong training effects on magnetotransport properties, due to the extra contribution of the electron scattering at the domain walls. All in all, the observed behavior is the result of a subtle correlation between perpendicular anisotropy produced by residual stresses, exchange interparticle interactions due to CoFe alloyed in the matrix, and dipolar interactions. Thus, as high structural evolution occurs through annealing, the features of randomly distributed ferromagnetic particles are recovered and, the out-of-plane domain structures and the training effects disappear. Received 30 September 1999     

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.     

Phase transitions of a spin-one alternating magnetic superlattice

We examine the critical behavior of a magnetic superlattice which made up of two magnetic materials, A and B. Using the effective field theory with a probability distribution technique that accounts for the single-site spin correlation, we derive the analytical equation for the Curie temperature of the superlattice which alternates as ABAB...AB. The dependence of the Curie temperature on the interface coupling strength J_ab and the layer number of the finite superlattice was calculated. The effects of the surface modification are also studied. Received 2 March 2001     

Dipolar coupling effect in magnetic bilayer system

Equilibrium micromagnetic structures in a bilayer system composed of two thin cobalt films separated by a non magnetic spacer are systematically analysed. These 2D magnetization distributions are obtained by numerical computations according to different set of magnetic and geometric parameters. The coupling effect due to the dipolar long range interaction (or stray-field effect) between the two Co layers is studied through the evolution of the magnetic pattern in the stack with or without an applied field and compared to a continuous film of same thickness. Special attention is paid to the hysteresis process in a bilayer. Even though the general aspect of the magnetization distribution looks like a Landau-Lifshitz structure, the absence of any core in the vortex of the magnetic structure is analysed in relation to a possible disappearence of (topological) hysteresis. Received 12 December 2001     

Evidence of bcc Mn epitaxial growth in Mn/M<Subscript>x</Subscript>V<Subscript>1–x</Subscript>(001) (M = Fe,Nb) superlattices

This study is dedicated to the growth of bcc Mn by molecular beam epitaxy, in order to look at the magnetic properties of bcc Mn near room temperature. For this purpose, Mn is deposited on bcc M_xV_1-x(001) alloy lattices (M = Fe or Nb) for which the lattice spacing is tunable by varying the concentration x. We first show that the parameter of the M_xV_1-x alloy's buffer layers can be adjusted from 2.95 ? to 3.3 ? depending on x and M. Three different structures in Mn films grown on these buffer layers are observed depending on the in-plane spacing of the initial M_xV_1-x lattice. Thick Mn films are always found to grow epitaxially in the Mnstructure. For moderate thicknesses larger than 4 atomic planes, Mn grows in an unidentified structure. Finally, up to four deposited atomic planes, Mn is found to grow in a tetragonal structure close to a bcc one on Fe(001), Fe_xV_1-x(001) and Nb_xV_1-x(001) for . This tetragonal structure is shown to be a distorsion of a Mn bcc structure with . Except for ultra-thin Mn films deposited on Fe(001), no macroscopic magnetization is detected in our strained bcc Mn samples. These results are compared to theoretical predictions. Received 21 June 1999     

Enhancement of magnetoresistance in cobaltites by manganese substitution: the oxide La 0.8 Sr 0.2 Co 1-x Mn x O 3

The substitution of manganese for cobalt in the perovskite La _0.8 Sr _0.2 CoO ₃ has been studied. A significant increase of the magnetoresistance (MR) is obtained, reaching 60% at 5 K under 7 T for . This behavior originates from a spectacular increase of the resistivity correlated to a significant decrease of ferromagnetism by Mn doping. This enhancement of magnetoresistance can be interpreted by the growth of ferromagnetic clusters in the insulating matrix, by applying a magnetic field. Received 7 May 1999