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 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     

Quantum Theory for Interfacial Roughness and Angle Dependence of Giant Magnetoresistance in Magnetic Multilayers

The giant magnetoresistance(GMR)in magnetic multilayers with current in the plane of the layers is studied by using the quantum-statistical Green‘s function approach,in which the effects of the interfacial roughness and magnetization configuration on the GMR are included,It is shown that the maximal GMR first increses and then decreases with increasing interfacial roughness,exhibiting a peak at an optimum value of interfacial roughness.An approximately linear dependence of GMR on sin^2(θ/2)is obtained,where θ is the angle between magnetizations of the two successive ferromagnetic layers,FUrthermore,the maximal GMR is found to increase with increasing the number of bilayers.     

On the orientational dependence of giant magnetoresistance

The functional dependence of the giant magnetoresistance (GMR) with respect to the relative angle between the orientations of the magnetization in the magnetic slabs of a trilayer system is calculated by using the Kubo-Greenwood formula for electrical transport together with the fully-relativistic spin-polarized screened Korringa-Kohn-Rostoker method for semi-infinite systems and the coherent potential approximation. It is found that the functional dependence of the GMR is essentially of the form . Received 30 November 1998     

Current-in-plane giant magnetoresistance: The effect of interface roughness and spin-depolarization due to the proximity of a buffer layer

We report on proximity effects of a Au buffer layer on the current-in-plane giant magnetoresistance effect (CIP-GMR) in high-quality, epitaxial Fe/Cr/Fe(001) trilayers. The lower Fe layer is grown in the shape of a wedge and allows simultaneous preparation of 24 GMR stripe-elements with different lower Fe thicknesses in the range from 13 to 14.5 ML. The layer-by-layer growth mode in combination with the small thickness variation gives rise to: (i) well-controlled roughness changes from stripe to stripe as confirmed by reflection high-energy electron diffraction (RHEED), and (ii) to a varying influence of the underlying Au buffer. The oscillatory roughness variation along the wedge yields an oscillatory GMR behavior as a function of Fe thickness and confirms the previous result that slightly increased interface roughness causes a higher GMR ratio. The proximity of the Au buffer to the GMR trilayer results in an increase of the GMR ratio with increasing Fe thickness. The latter effect is explained by spin-depolarization at the Fe/Au interface and in the bulk of the Au buffer.     

Interfacial polarisation effect on the interlayer couplings in Co/Rh sandwiches

The structural, magnetic and transport properties of Co/Rh sandwiches grown by ultra high vacuum evaporation and sputtering have been studied. High-energy electron diffraction observations during the growth reveal that both Co and Rh layers have been stabilised in the (111) fcc structure for the evaporated sandwiches. X-ray measurements performed on sputtered samples show a predominant fcc polycrystalline structure of the stacks with a preferential (111) texture. Magnetisation and magnetoresistance measurements show a very strong antiferromagnetic exchange coupling for thin Rh layers, reaching for 4.8? Rh, the strongest ever observed in exchange coupled systems. This value is in good agreement with the value of obtained by ab initio calculations for Co/Rh (hcp) superlattices. This is explained by the magnetic nature of the Co/Rh interfaces. Indeed, the variation of the measured saturation magnetisation as a function of the Co layer thickness shows no evidence of Co moment reduction for the Co atoms located at the interfaces, even for the very thin layers. The value of the preserved magnetic moments of the cobalt atoms at the interfaces is confirmed by ab initio calculations for Co/Rh superlattices taking the intermixing into account. Received: 18 February 1998 / Received in final form: 30 April 1998 / Accepted: 29 May 1998     

Well-defined Co clusters embedded in an Ag matrix: A model system for the giant magnetoresistance in granular films

Well-defined granular Co/Ag films have been prepared by the co-deposition of in-beam prepared Co clusters and Ag atoms. In this way we were able to study the giant magnetoresistance (GMR) as a function of mean Co cluster size for a fixed Co cluster volume fraction v_cl as well as a function of v_cl for a fixed . Mean Co cluster size has been varied between and 6.9 nm, Co cluster volume fraction between 5 and 43%. The GMR was measured in-situ at T=4.2 K in magnetic fields 1.2 T. The analysis of the GMR data obtained from these studies clearly shows that spin-dependent scattering at the Co-cluster/Ag-matrix interface is the only relevant scattering mechanism causing the GMR in our well-defined samples. Received: 21 April 1998 / Received in final form: 17 May 1998 / Accepted: 18 May 1998     

Magnetic and transport properties of sputtered Fe/Si multilayers

The structural, magnetic and transport properties of sputtered Fe/Si multilayers were studied. The analyses of the data of the X-ray diffraction, resistance and magnetic measurements show that heavy atomic interdiffusion between Fe and Si occurs, resulting in multilayers of different complicated structures according to different sublayer thicknesses. The nominal Fe layers in the multilayers generally consist of Fe layers doped with Si, ferromagnetic Fe-Si silicide layers and nonmagnetic Fe-Si silicide interface layers, while the nominal Si spacers turn out to be Fe-Si compound layers with additional amorphous Si sublayers only under the condition either for the series or for the series multilayers. A strong antiferromagnetic (AFM) coupling and negative magnetoresistance (MR) effect, about 1%, were observed only in multilayers with iron silicide spacers and disappeared when -Si layers appear in the spacers. The dependences of MR on and on bilayer numbers N resemble the dependence of AFM coupling. The increase of MR ratio with increasing N is mainly attributed to the improvement of AFM coupling for multilayers with N. The dependence of MR ratio is similar to that in metal/metal system with predominant bulk spin dependent scattering and is fitted by a phenomenological formula for GMR. At 77 K both the MR effect and saturation field increase. All these facts suggest that the mechanisms of the AFM coupling and MR effect in sputtered Fe/Si multilayers are similar to those in metal/metal system. Received: 11 February 1998 / Revised: 9 March 1998 / Accepted: 9 March 1998     

Electrodeposition and characterization of Cu/Co multilayers: Effect of individual Co and Cu layers on GMR magnitude and behavior

The present work discusses the successful electrodeposition of Cu/Co multilayers, exhibiting appreciable GMR of 12-14% at room temperature. The effect of individual Cu and Co layers on the magnitude and behavior of GMR has been studied. By varying the thickness of individual layers the field at which saturation in GMR is observed can be controlled. It was observed that for lower thicknesses of Co layer, the saturation fields are reduced below 1 kOe. The Cu layer thickness seems to control the nature of magnetic coupling and the saturation field, with the two showing a correlation.     

Tetragonal structure of thin nickel films on Cu(001)

The crystallographic structure of thin Ni films deposited on Cu(001) has been studied using Surface Extended X-ray Absorption Fine Structure (SEXAFS). Taking advantage of the linear polarization of the synchrotron radiation, we have shown that Ni adopts the Cu lattice parameter parallel to the interface. This lateral expansion induces a longitudinal compression of the unit cell, leading to a face centered tetragonal structure of the Ni films from 3 to 10 monolayers. The temperature dependence of the EXAFS oscillations has allowed to measure strain inside the Ni layers. Received 22 December 1998     

The segregation of poly(styrene-b-isoprene) diblock copolymers to the surface of a polystyrene melt: the effect of the ratio of block lengths

The effect of the ratio of block lengths on the interfacial partitioning of poly(styrene-block-1,4 isoprene) diblock copolymers from their mixtures with polystyrene homopolymer melt is investigated utilizing a series of copolymers with almost constant molecular weight but different compositions. The concentration profile of the copolymer is measured directly using the nuclear reaction analysis technique; a segregation of the diblock is found at both the air/polymer surface, due to the lower surface energy of polyisoprene, and at the substrate/polymer interface. No significant effect of the block length ratio on the free-surface excess was observed. The block molecular weights have apparently led to dangling chain conformations in the non-overlapping mushroom and in the overlapping mushroom regimes whereas the brush regime was not accessible; no indications of a real border between the two former regimes was found. Received: 20 July 1998 / Received in final form and Accepted: 11 September 1998     

Structural origin of magnetic anisotropy in Co-Pt alloy films probed by polarized XAFS

Polarized X-ray absorption fine structure (XAFS) measurements at the Co K and Pt L₃ edges show that the perpendicular magnetic anisotropy found in epitaxial fcc CoPt₃ (111) films stems from the existence of anisotropic local ordering. Such ordering, induced during the codeposition process and dependent on the growth temperature, is characterized by preferential CoCo pairs in the film plane, balanced by preferential CoPt pairs out of the plane, resulting from some Co 2D-segregation. Polarized XAFS at the Pt edge reveals similar anisotropic local ordering in epitaxial hcp Co₃Pt (0001) films exhibiting a larger magnetocrystalline anisotropy compared to that of bulk hcp Co. Besides, a polarization dependence of the Co XANES profile is observed only for the Co₃Pt films exhibiting hcp symmetry. Received: 29 April 1998 / Revised: 27 July 1998 / Accepted: 31 August 1998     

Microcharacterization of the surface oxidation of Py/Cu multilayers by scanning X-ray absorption spectromicroscopy

The local surface oxidation of the permalloy surface layer in Py/Cu GMR multilayers on a micron lateral scale has been analyzed by means of a microspot-X-ray absorption spectromicroscope utilizing synchrotron radiation from the Advanced Light Source bending magnet beamline 6.3.2. Additionally, the GMR multilayer samples prepared by dc magnetron sputtering have been analyzed by cross-sectional transmission electron microscopy, hard X-ray reflection and magnetoresistance measurements. The formation of a passivating iron-oxide layer on the sample surface was identified by X-ray absorption near edge structure spectroscopy (XANES) near the Fe-2p edge while no indication for nickel-oxide formation could be found. Small micron-size pits of reduced iron-oxide concentration could be identified by XANES microscopy while the corresponding nickel distribution appeared to be homogeneous. The results are explained in terms of a local breakdown of the passivating oxide layer. Received:16 October 2000 / Accepted: 4 December 2000 / Published online: 21 March 2001     

Micromagnetic Simulation of Transfer Curve in Giant-Magnetoresistive Head

The transfer curve of the giant-magnetoresistive （GMR） magnetic head represents its most important property in applications, and it is calculated by the micromagnetic modeling of the free layer and the pinned layer in the heart of the GMR head. Affections of the bias hard magnetic layer and the anti-ferromagnetic pinning layer are modeled by effective magnetic fields. The simulated transfer curve agrees with experiment quite well, therefore the values of these effective magnetic fields can be determined by the model. A synthetic antiferromagnetic spin valve structure GMR head is also analyzed for comparison.     

The lamellar-disorder interface: one-dimensional modulated profiles

We study interfacial behavior of a lamellar (stripe) phase coexisting with a disordered phase. Systematic analytical expansions are obtained for the interfacial profile in the vicinity of a tricritical point. They are characterized by a wide interfacial region involving a large number of lamellae. Our analytical results apply to systems with one dimensional symmetry in true thermodynamical equilibrium and are of relevance to metastable interfaces between lamellar and disordered phases in two and three dimensions. In addition, good agreement is found with numerical minimization schemes of the full free energy functional having the same one dimensional symmetry. The interfacial energy for the lamellar to disordered transition is obtained in accord with mean field scaling laws of tricritical points. Received: 28 March 1997 / Revised: 6 February 1998 / Accepted: 16 February 1998     

Adsorbed and free lipid bilayers at the solid-liquid interface

Single and double phosphocholine (DPPC and DSPC) bilayers adsorbed at the silicon-water interface have been prepared and characterised. The second bilayer, called “free bilayer”, is a novel highly hydrated system floating at above the first one. Robust and reproducible preparation has been possible thanks to a combination of Langmuir-Blodgett and Langmuir-Schaeffer techniques. Carefully optimised neutron reflectivity measurements have allowed a precise non-destructive characterisation of the structure, hydration and roughness of the layers. This work opens new possibilities for the investigation of the interaction between membrane lipids and soluble proteins, in particular peptides too small to be visible with other techniques. Received 17 July 1998     

O2 plasma oxidation effect on magnetic properties of spin valves

The use of nano-oxide to improve the performance of spin valves has been extensively studied. But most of the investigations so far have been carried out on different samples. This may make some of the conclusions drawn from the experiments inconsistent because of the fluctuation in deposition conditions and device structures. In this work, the effect of nano-oxide on the properties of spin valves has been investigated through post-growth oxidation of the same sample in oxygen plasma for different rf powers and durations. The sample investigated was a bottom spin valve with the structure Si/SiO₂/Ta/NiFe/IrMn/CoFe/Cu/CoFe/Ta. A relative increase of 20% and 12% was obtained in the giant magnetoresistance (GMR) ratio of as-deposited and annealed samples, respectively. It was found that, at a fixed rf power, there is a peak of the GMR ratio as the oxidation time increases. A higher peak value of the GMR ratio was obtained for lower rf power, although the required oxidation time is longer. This result can be well understood by considering both the enhanced specularity at the insulator/metal interface and the loss of magnetic effective thickness of the free layer by the oxidation. Magnetic parameters such as the interlayer coupling field (H₀) and the coercivity of the free layer (H_cf) were also greatly influenced by the oxidation process. When only the Ta layer was oxidized, H₀ increases very slightly, and H_cf increases with the oxidation time. However, when the CoFe free layer was oxidized, a significant increase was found for H₀, and H_cf changes to decreasing. These results can be explained based on the Néel and RKKY coupling models. Received: 25 October 2001 / Accepted: 21 December 2001 / Published online: 3 June 2002     

Magnetism of alloyed models of Cr(Mn) V and Mn Cr overlayers on V (001) substrates

The magnetism is calculated for substitutional alloyed of Cr_x(Mn_x)V_1-x monolayers (MLs) and Cr_xMn_1-x on V (001) surface with a variety of concentrations (x = 0.25, 0.50 and 0.75). The substitutional surface alloys were treated by an artificial super cell construction. Parallel magnetic ordering is obtained for all the considered structures. The surface net magnetization increases in terms of Cr (Mn) concentration in Cr_x(Mn_x)V_1-x/V (001) system, while no serious variations occur in Cr_xMn_1-x/V (001). Vanadium atoms at the interfacial layers acquire appreciable magnetic moments antiferromagneticlly (AF) coupled with the surface moments. Received 25 February 2002 / Received in final form 13 May 2002 Published online 14 October 2002 RID="a" ID="a"e-mail: jkalifa@sci.ju.edu.jo