The giant magnetoresistance of Co/Cu superlattices grown by MBE

We report the first observation of a giant magnetoresistance (MR) in Co/Cu superlattices grown by MBE. The maximum value of the MR is - 26% and this is found in a specimen for which the copper layers are about 7 Å thick and which includes a thin 10 Å layer of gold in the buffer region between the substrate and the superlattice. From RHEED and X-ray diffraction it is shown that the metallic layers in the specimens grown in this way are extremely flat and that the orientation of the copper is (111).     

Influence of superparamagnetic regions on the giant magnetoresistance of electrodeposited Co–Cu/Cu multilayers

The room-temperature magnetoresistance (MR) of electrodeposited Co–Cu/Cu multilayers was investigated. Samples were prepared on either a polycrystalline Ti foil or on a silicon wafer covered with a Ta buffer and a Cu-seed layer. The field dependence of the magnetoresistance was analyzed by decomposing the GMR into ferromagnetic (FM) and superparamagnetic (SPM) contributions, whereby the field dependence of the latter could be described by a Langevin function. In order to better understand the influence of the deposition conditions on the GMR in electrodeposited multilayers, the evolution of the relative importance of the two GMR contributions is discussed in terms of the Co dissolution process during the Cu deposition pulse.     

Electronic structures and magnetoresistance of Co/Cu/Ni/Cu superlattices with different Orientations

Electronic structures and magnetoresistance (MR) of pseudo-spin-valves structure (PSV) Co₃/Cu₃/Ni₃/Cu₃(1 0 0), Co₃/Cu₃/Ni₃/Cu₃(1 1 0), and Co₃/Cu₃/Ni₃/Cu₃(1 1 1) are studied using the full potential linearized augmented plane wave (FLAPW) method with generalized-gradient approximation (GGA) for the exchange correlation interactions. It has been found that the spin-dependence scatterings at interfaces are insignificant compared with the ones of the interior layers for the three orientation superlattices. Also, we evaluate the MR ratio by means of the two-current model, and find that the MR ratio of Co₃/Cu₃/Ni₃/Cu₃(1 0 0) (73.72%) is larger than those of structures Co₃/Cu₃/Ni₃/Cu₃(1 1 0) (68.38%), and Co₃/Cu₃/Ni₃/Cu₃(1 1 1) (67.09%).     

Enhancement of magnetoresistance sensitivity in Co/Cu/NiFe trilayers by softening the Co hard layer

Co/Cu/NiFe trilayers were prepared by sputtering without magnetic field applied. We have found that the Co(2 nm)Cu(1 nm)NiFe(2 nm) trilayer using Ta as buffer layer exhibits an enhanced magnetoresistance (MR) sensitivity by a factor of more than 6 and a low saturation field of 9.3 Oe. Experimental results have demonstrated that the low saturation field is attributed to the softening of the Co layer by depositing the Co(2 nm)Cu(1 nm)NiFe(2 nm) sandwich on Ta layer. The decrease of the coercivity of the Co layer also plays an important role in the enhancement of MR sensitivity by reducing the effective coercivity of the NiFe layer, which is discussed in terms of the change in interlayer coupling.     

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     

Effects of interfacial roughness on the magnetoresistance of Co/Cu multilayers

Using a semi-classical approach, Hood, Falicov and Penn have studied the effects of interfacial roughness on the magnetoresistance (MR) of iron based trilayers (Fe/Cr/Fe and Fe/Cu/Fe). We extend their theory to magnetic metallic multilayers composed of N bilayers ferromagnetic-normal metal. The in plane MR of Co/Cu multilayers is calculated for correlated quasiperiodic interfaces. The averaged effects due to impurities, interdiffusion, band structure, etc. are included in a simple way using two phenomenological parameters S^↑ and S^↓ for two directions of spin. MR variation with S^↑, S^↓ and relaxation time is reported. We analyse also recent experimental data giving the influence of number of bilayers on the MR of Co/Cu multilayers for different temperatures.     

Giant magnetoresistance and magnetothermopower in Co/Cu multilayers

The thermopower (Seebeck coefficient, S) has been measured on a series of Co/Cu multilayers that exhibit giant negative magnetoresistance (GMR). Negative in zero applied field, S(H) increases in magnitude as the field is increased, approaching that of bulk Co. The change in S is inversely proportional to the resistance of the sample which is, in turn, proportional to the square of the magnetization. A model is presented that yields both the GMR and S(H) from the spin-split density of states of the Co without the need of a spin-dependent scattering potential at the interfaces.     

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     

Anomalous temperature dependence of the magnetoresistance in Co/Cu multilayers

An anomalous temperature dependence of magnetoresistance (MR) of Co/Cu multilayer films witha ～3 Å thick magnetic layer has been established experimentally. The temperature of the MR maximum T _max is shown to coincide with the Néel temperature. The variation of T _max with the Cu layer thickness follows an oscillatory pattern.     

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.     

Improved giant magnetoresistance in magnetic granular Co5 Cu95 alloys by direct-current joule heating

The dc joule heating technique has been used to produce giant magnetoresistance (GMR) Co₅Cu₉₅ granular alloys. At T=10 K, GMR as large as 28.5% has been observed in the as-quenched sample annealed with I=6A in a magnetic field up to 30 kOe. At room temperature, the joule-heated samples show higher GMR in comparison with that annealed by the conventional method.     

Improved giant magnetoresistance in magnetic granular Co5 Cu95 alloys by direct-current joule heating

The dc joule heating technique has been used to produce giant magnetoresistance (GMR) Co₅Cu₉₅ granular alloys. At T=10 K, GMR as large as 28.5% has been observed in the as-quenched sample annealed with I=6A in a magnetic field up to 30 kOe. At room temperature, the joule-heated samples show higher GMR in comparison with that annealed by the conventional method.     

Perpendicular giant magnetoresistance of Co91Fe9/Cu exchange-biased spin-valves: further evidence for a unified picture

Measurements are reported of the increase in specific resistance, AΔR, with increasing Co₉₁Fe₉ layer thickness in current-perpendicular to the plane (CPP) exchange-biased spin-valves (EBSVs) of Co₉₁Fe₉ and Cu. Analysis of these measurements yields a spin anisotropy parameter for Co₉₁Fe₉ of β=0.65±0.05, and a spin diffusion length of l^CoFe_sf=12±1 nm. The value of β agrees reasonably well with those obtained experimentally and theoretically for dilute CoFe alloys, thus providing additional support for a unified picture of spin-polarized transport in CPP-MR and bulk alloys. This value of l^CoFe_sf, and the previously determined l^Py_sf for Permalloy, scale approximately inversely with the residual resistivities of the two alloys.     

Stability of the magnetoresistance for NiO-containing Co/Cu/Co spin valves naturally placed in air

The Co/Cu/Co spin valves with an additional NiO layer at the top (TSV) and under the bottom (BSV) were set in air at room temperature for about four years. Effects of time on the stability, the magnetoresistance (MR), and microstructures were investigated. Results show that the MR of TSV is more stable than that of BSV. The MR of BSV decreases 12.5% in the first 1000 days, while it increases 4% after another 360 days, while the MR of TSV keeps almost unchanged within the range of measurement error. Different time-dependent behavior of the MR value for BSV and TSV was ascribed to the different contribution of the roughness at NiO/Co and Co/Cu interfaces. PACS 68.60.Dv; 61.10.Eg; 68.35.Ct; 73.43.Qt; 75.25.+z     

Application of magnetically modulated microwave absorption to study of giant magnetoresistance effect in the Ni−Fe/Cu multilayer system

Magnetically modulated microwave absorption (MMMA), magnetic hysteresisM(H), giant magnetoresistance (GMR) and ferromagnetic resonance (FMR) have been examined in the antiferro-magnetically coupled Py/Cu (Py=Ni₈₃Fe₁₇, permalloy) multilayer system. The correlation between results obtained by the MMMA technique, the standard GMR measurements, and magnetization reversal studies is shown. Microwave studies of GMR, magnetization reversal, and FMR for different orientations of the magnetic field with respect to the sample surface are presented.     

Influence of sputtering pressure on the giant magnetoresistance and structure in Fe–Cu–Ni granular thin films

Composition, structure and giant magnetoresistance in Fe_xCu_yNi_z films prepared at different sputtering pressures were investigated. X-ray diffraction studies showed only Cu (1 1 1) peak from the Cu grain. The shifts in the d-spacing d₁₁₁ of Cu indicates a progressive substitution of Ni in the Cu lattice. Similar trends in d₁₁₁ of Cu observed for the samples prepared at different sputtering pressures indicate that the structural behaviour of the samples is nearly independent of the sputtering pressure. A significant enhancement of magnetoresistance (MR) for the samples sputtered at 0.001 mbar as compared to that sputtered at 0.02 mbar is attributed to the reduced role of residual gas impurities in the films upon lowering the sputtering pressure. An interesting observation is that the MR did not significantly decrease even with a large substitution of Ni in the Cu grains.     

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.