Perpendicular giant magnetoresistance of magnetic multilayers

The theoretical and experimental studies of the giant magnetoresistance effect in metallic magnetic multilayers with measuring current perpendicular to the interface planes are reviewed. Theoretical formalisms of electronic transport in the inhomogeneous electron gas are critically compared with emphasis on the perpendicular magnetoresistance in multilayers. The effects of interface roughness, potential steps at the interfaces, and realistic band structures are addressed. The experimental determination of the perpendicular resistance of metallic multilayers requires either low-resistance measurement techniques or microfabricated samples with enhanced resistances. The experimental methods known at present are discussed and, where possible, compared with each other.     

Perpendicular giant magnetoresistance in magnetic multilayered nanowires

We present giant magnetoresistance (GMR) measurements performed on electrodeposited Co/Cu multilayered nanowires. The variation of the GMR with the thicknesses of the Cu and Co layers over wide ranges is discussed in the framework of the Valet-Fert model for perpendicular GMR. The interface and bulk spin-dependent scattering parameters as well as the spin diffusion lengths in the nonmagnetic and ferromagnetic layers are extracted from this analysis.     

Pressure-induced enhancement of giant magnetoresistance due to crossover of interlayer exchange coupling in Fe/Cr multilayers

We report the first observation of a large pressure-induced enhancement of giant magnetoresistance (GMR) in magnetic multilayers (MML). In Fe/Cr MMLs with the Cr layer thickness of approximately 30 A, a crossover from biquadratic to bilinear interlayer exchange coupling (IEC) was observed by applying pressure, and simultaneously the GMR under high pressure (>2 GPa) was enhanced to be twice as large as that at ambient pressure. The enhanced GMR is attributed to the suppression of the biquadratic IEC by applying pressure, and the electrical resistivity in parallel alignment of magnetization also showed a crossover behavior, suggesting an electronic origin for the observed pressure effects.     

Fe/Cr磁性多层膜的制备及其磁电阻测定

利用磁控溅射制备了不同非磁性层厚度的Fe/Cr多层磁性薄膜系统,利用四探针法测定了该多层膜系统在不同磁场下磁电阻效应,用饱和场法给予佐证,溅射制备的多层膜系统饱和场明显地随Cr层厚度增加而衰减振荡,得出了Fe/Cr多层膜的铁磁反铁磁耦合的交换耦合强度随非磁性层厚度变化的物理规律.     

Role of interface scattering on the giant magnetoresistance of magnetic multilayers

A discussion is presented of the effect of interface scattering on the giant magnetoresistance of magnetic multilayers. It has recently been found experimentally that increased interface scattering causes the magnetoresistance to increase for Fe/Cr multilayers, but to decrease for Co/Cu multilayers. An explanation is proposed for this striking difference in behaviour for these two systems.     

Interactive effect of impurities on giant magnetoresistance of Co–Fe/Cu multilayers

Giant magnetoresistance of Co–Fe–B/Cu multilayers fabricated in the sputtering atmosphere, where the amount of oxygen impurity is varied, is discussed in connection with their interfacial roughness. The magnetoresistance (MR) ratio of Co–Fe/Cu multilayers is enhanced by up to 33% when the oxygen content is varied between 10 and 100 ppm of processing Ar gas. The enhancement of the MR ratio was due to the flattening effect of impurity oxygen on the multilayer interfaces: the root mean square roughness of the multilayer was decreased from 7.5 to 5 Å. With increasing boron content in Co–Fe layers, however, the enhancing effect of MR ratio by oxygen diminished and nearly vanished for 12 at%-B–(Co–Fe) case. The strong affinity of boron for oxygen is suggested as a probable mechanism.     

Low frequency magnetic response in antiferromagnetically coupled Fe/Cr multilayers

The magnetic field and temperature dependence of the low frequency magnetic response of antiferromagnetically coupled Fe/Cr(100) multilayers has been studied between +/-500 Oe, from 2 to 300 K. At T = 2 K the losses exhibit an unusually strong frequency dependence which can be described within a single relaxation time scheme. This relaxation time proves to be strongly field dependent. These phenomena are specific for epitaxial multilayers with large magnetoresistance. The behavior of the relaxation time at low temperatures might be related to some quantum tunneling processes.     

Thermopower of amorphous metals from 4.2 to 300 K

Thermopowers of Metglas 2204, 2605, 2605A, 2826 2826A and 2826MB have been measured. Generally the results do not follow the linear temperature dependence found in 2204 by Nagel. The absence of structure in the thermopower is explained using the Nordheim-Görter relation.     

An estimation of the magnetic disorder at Fe/Cr interfaces in coupled and non-coupled Fe/Cr multilayers

The antiferromagnetic coupling at the Fe/Cr interfaces, inferred from the orientation of the Cr magnetic moments, is used to estimate the magnetic disorder resulting from the interfacial roughness in Fe/Cr multilayers. A crossover from in-plane to out-of-plane orientation of Cr moments depends on the energy cost in either case: (i) to break the interfacial Fe–Cr antiferromagnetic coupling or (ii) having sites with frustrated Cr–Cr antiferromagnetic coupling in the Cr interlayers. A quantitative model of the magnetic frustration due to interfacial disorder in Fe/Cr multilayer systems is described. The step edge density, or terrace size, required to break the interfacial Fe–Cr coupling and destroy the Fe–Fe interlayer exchange coupling is estimated.     

Radiofrequency magnetoresistance of Fe/Cr superlattices

The rf magnetoresistance of Fe/Cr superlattices is studied for two orientations of the current: parallel and across the superlattice layers. A mutually single-valued correspondence is established between the relative magnetoresistance measured at dc current and the change in the transmission coefficient of electromagnetic waves in the magnetic field. When rf currents flow across the layers, the relative change in the signal amplitude is proportional to twice the change in the electrical resistance of the superlattice and is of opposite sign. It is shown that the rf losses are determined by the surface resistance which is proportional to the superlattice thickness and inversely proportional to its conductivity. An equation is derived for the rf electric field distribution in the superlattice. It is established that when the thickness of the superlattice is small compared with the skin layer depth, field and current components which penetrate through the entire superlattice exist.     

Magnetic property and magnetoresistance in Fe/ITO multilayers

Giant magnetoresistance was found in DC magnetron sputtering Fe/ITO multilayers. The magnetic properties, electrical properties and magnetoresistance were investigated. A critical temperature is found around 50 K where the temperature dependence of resistivity and magnetoresistance ratio exhibit an abruptly change. The temperature dependence of resistance is found to obey Mott's 1/4 law for low temperature. The max magnetoresistance ratio of 2.0% and 6.7% is found at room temperature and 12.5 K, respectively. The increase of magnetoresistance ratio at low temperature is due to the decrease of spin-mixing effect.     

Low frequency magnetic noise in epitaxial antiferromagnetically coupled Fe/Cr multilayers

For antiferromagnetically coupled epitaxial [Fe/Cr(001)]₁₀ multilayers we detected a strong enhancement of the magnetism-related electrical noise in the vicinity of the orientation transition between the easy and hard axes. Our measurements are performed at different temperatures and we also identified the noise caused by depinning of domain walls (DWs). We are able to detect and follow in real time the motion of rather extended (of the order of 100 μm) DWs by comparing the magnetic noise in the presence and absence of a DC transport current, respectively. The presence of large and small (<1 μm) DWs is confirmed by magnetic force microscopy images obtained at room temperature.     

Percolation—related magnetic coupling and magnetoresistance properties in Fe／Si1—xAgx multilayers

In this paper, we present a study of the magnetic coupling and magnetoresistance (MR) properties in Fe/Si_1-xAg_x multilayers with a granular Si_1-xAg_x spacer layer. We have found that, with increasing silver content (x) in a silicon matrix, the magnetic state of multilayers changes from a nonmagnetic coupling state to weak antiferromagnetic around the percolation point of the ～2.4 nm thick spacer Si_1-xAg_x. The MR measurements also reveal an abrupt increase of MR near the same percolation point. These changes are ascribed to the formation of the percolation path in the granular spacer.     

Structure of sputter-deposited Pt/Fe and Cr/Fe multilayers

Conversion electron Mössbauer spectroscopy (CEMS) and X-ray diffraction (XRD) have been used to investigate the structure of Pt/Fe and Cr/Fe multilayers deposited by magnetron sputtering. The Cr/Fe samples consisted of four samples prepared under Ar sputtering pressures of 1.3, 3.0, 5.0, and 10.0 mT, all with the same multilayer structure of 3.5 nm Cr/2.5 nm Fe, repeated 35 times onto c-Si wafer substrates. The quality of the interfaces between Cr and Fe is clearly degraded with increasing sputter pressure, as seen by changes in the relative intensities of four magnetic subspectra in the CEMS and the gradual appearance of a single-line resonance similar to Fe in solution in Cr. The low-angle XRD superlattice peaks also disappear with increasing sputter pressure, while the high-angle XRD shows a tendency for loss of the preferred (110) texture. Two films of Pt/Fe were deposited epitaxially onto MgO single crystals with bilayer periods of 1.3 nm and 2.6 nm and total thickness of 300 nm each. A transition from fcc-PtFe with near-perpendicular magnetic anisotropy to a bcc-Fe/fcc-PtFe mixture with in-plane magnetic texture is observed by CEMS for the factor of two increase in bilayer period.