Giant Magnetoresistance In Magnetic Multilayers Research Articles

Unconventional Giant "Magnetoresistance" in Bosonic Semiconducting Diamond Nanorings.

The emergence of superconductivity in doped insulators such as cuprates and pnictides coincides with their doping-driven insulator-metal transitions. Above the critical doping threshold, a metallic state sets in at high temperatures, while superconductivity sets in at low temperatures. An unanswered question is whether the formation of Cooper pairsin a well-established metal will inevitably transform the host material into a superconductor, as manifested by a resistance drop. Here, this question is addressed by investigating the electrical transport in nanoscale rings (full loops) and half loops manufactured from heavily boron-doped diamond. It is shown that in contrast to the diamond half-loops (DHLs) exhibiting a metal-superconductor transition, the diamond nanorings (DNRs) demonstrate a sharp resistance increase up to 430% and a giant negative "magnetoresistance" below the superconducting transition temperature of the starting material. The finding of the unconventional giant negative "magnetoresistance", as distinct from existing categories of magnetoresistance, that is, the conventional giant magnetoresistancein magnetic multilayers, the colossal magnetoresistancein perovskites, and the geometric magnetoresistance in semiconductor-metal hybrids, reveals the transformation of the DNRs from metals to bosonic semiconductors upon the formation of Cooper pairs. DNRs like these could be used to manipulate Cooper pairs in superconducting quantum devices.

Spin accumulation dynamics in spin valves in the terahertz regime

The notion of spin accumulation and spin relaxation in the diffusive regime was introduced by Valet and Fert in 1993 to describe the current-perpendicular-to-plane (CPP) diffusive transport in metallic magnetic multilayers. This theory has been quite successful in explaining the giant magnetoresistance of magnetic multilayers in CPP geometry in the frequency range from DC to a few gigahertz. In this paper, we investigate the dynamic aspect of spin accumulation from the theoretical point of view when reaching the terahertz (THz) frequency range. The characteristic relaxation time of electron elastic scattering is typically in the femtosecond range. However, since spin accumulation results from a diffusion process involving a very large number of individual scattering events, the characteristic time of spin accumulation variation when the current and/or the magnetic configuration are varied can be significantly longer than that of the input signal, eventually reaching the picosecond range. In spintronic devices operating in the THz range such as those based on ferrimagnetic or antiferromagnetic materials, the spin accumulation amplitude and, correlatively, the device magnetoresistance can therefore depend on the actual device operating frequency. We investigate this question by extending the Valet and Fert theory in the time domain.

Pressure-enhanced giant magnetoresistance in Fe/Cr magnetic multilayers

The magnetoresistance of Fe/Cr magnetic multilayers (MMLs) on the 1st and the 2nd peaks has been measured at high pressure up to 3 GPa at low temperature in order to make clear the characteristics of the giant magnetoresistance (GMR) under high pressure. The GMRs of the MMLs on the 2nd peak are found to be enhanced strongly by applying pressure whereas those of 1st peak MMLs decrease with increasing pressure. The magnitude of GMR of the 2nd peak MMLs becomes about twice or more at 2 GPa compared with the value at ambient pressure. The large difference in the pressure dependences is explained by assuming a pressure-induced crossover in the interlayer exchange coupling from the biquadratic to the bilinear spin arrangements.

Numerical simulation of giant magnetoresistance in magnetic multilayers and granular films

We perform numerical simulations of conductance and conductivity for magnetic multilayers and magnetic granular films which show giant magnetoresistance (GMR). The system size to which the recursive Green's function method is applied is sufficiently large to eliminate incidental effects caused by the system boundary reported previously. It is shown that the resistivity change in the magnetic granular films is proportional to the inverse of the grain size.

GMR effect across the transition from multilayer to granular structure

We have studied the evolution of giant magnetoresistance in magnetic multilayers obtained by alternately growing thin cobalt and copper layers having proportional thickness. Co nominal layers thickness was progressively reduced from 12 Å down to 2 Å. This decrease produces a fragmentation of the Co layers and the evolution of samples magnetic properties indicates a transition from multilayer to granular behavior. Magnetic and magnetoresistive data of the granular phase have been analyzed as a function of Co thickness in order to determine particle size distribution.

Non-conformal interface roughness effects on the giant magnetoresistance in magnetic multilayers

We study interface roughness effects on the giant magnetoresistance in magnetic multilayers with non-conformal correlated interfaces. The roughness of each interface is characterized by the roughness exponent H (0⩽ H<1), in-plane correlation length ξ, and rms roughness amplitude Δ. It is shown that the magnetoresistance depends on all the roughness parameters: Δ, H and ξ. The case where the roughness parameters of consecutive interfaces are different due to cross-correlations is also studied.

Quantum Theory for Interfacial Roughness and Angle Dependence of Giant Magnetoresistance in Magnetic Multilayers**The project supported by National Natural Science Foundation of China

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 increases and then decreases with increasing interfacial roughness, exhibiting a peak at an optimum value of interfacial roughness. An approximately linear dependence of GMR on is obtained, where 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.

Role of the density of states in the giant magnetoresistance of magnetic multilayers

The role of the density of states in the giant magnetoresistance (GMR) of ferromagnet 1 (FM1)/nonmagnetic (NM=Cu)/ferromagnet 1 (FM1) sandwiches has been investigated by inserting a thin layer of the second ferromagnetic material (FM2=Co) at the FM1/Cu interfaces. As the thickness of the inserted layer increases, the GMR increases to a value seen for purely Co/Cu/Co sandwiches. By using differing FM1 materials [e.g., Py (permalloy (Ni80Fe20), Ni, Fe] we observe the exponential increase in GMR as more Co is inserted to be dependent on the FM1 material behind it. This is explained by realizing that although the GMR is dominated by the scattering caused by defects like those at the FM2 material/Cu interface, the nature of the scattering depends on the density of states of the FM1 material. This result is discussed in terms of advanced theories on the origin of the GMR in magnetic multilayers.

Ab initio calculations of giant magnetoresistance

We present ab initio calculations of the giant magnetoresistance in magnetic multilayers. The electronic structure of the multilayers is calculated by spin density functional theory using a screened Korringa–Kohn–Rostoker method. The scattering of nanostructural defects in the multilayers is described by means of a Green’s function method. The scattering potentials are calculated self-consistently. The transport properties are treated quasiclassically solving the Boltzmann equation including the electronic structure of the layered system and the anisotropic scattering. The solution of the Boltzmann equation is performed iteratively taking into account both scattering out and scattering in terms (vertex corrections). Since we consider ferromagnetic systems a two current model is applied. Trends of residual resistivities and giant magnetoresistance ratios are discussed for Co/Cu multilayers with 3d transition metal defects.

Correlated Roughness Effects in the Giant Magnetoresistance of Magnetic Multilayers

Correlated Roughness Effects in the Giant Magnetoresistance of Magnetic Multilayers

Effect of Pressure on the Giant Magnetoresistance of Magnetic Multilayers.

The magnetoresistance (MR) of magnetic multilayers. Fe/Cr and Co/Cu, was measured at high pressure. It was found that the effect of pressure on the MR depends on the specimen: the MR of Fe/Cr decreases much more than that of Co/Cu. The large effect of pressure on the MR in Fe/Cr is shown to be due to the suppression of spin-dependent scattering at high pressure. The relation between the effect of pressure on GMR and the interfacial roughness is discussed briefly.

Influence of spin-dependent scattering at rough interface on the giant magnetoresistance in magnetic multilayers

Spin-dependent scattering in Co/Cu and Co/Ru multilayers with interface roughness is estimated. The electronic band structures of Co/Cu and Co/Ru multilayers with smooth interfaces for parallel and antiparallel spin configurations are calculated self-consistently using linear muffin-tin orbital atomic sphere approximation (LMTO-ASA) band structure calculations based on density functional theory. The spin-dependent scattering caused by interface roughness is calculated from the transition probabilities obtained from self-consistent wave functions and potential differences found from these band structure calculations. Our results indicate that spin-dependent scattering caused by interface roughness plays an important role in the giant magnetoresistance (GMR) effect for magnetic multilayers, as predicted by calculations using a simple model. Furthermore, it is concluded that the scattering depends on differences in the band structure for different spin configurations, as well as for interface roughness. The GMR difference between Co/Cu and Co/Ru can be explained qualitatively by difference in spin-dependent scattering due to interface roughness, which is caused by the band structure with different spin configuration.

Magnetization configuration and potential dependence of giantmagnetoresistance in magnetic multilayers

The angular and potential dependence of giant magnetoresistance in magnetic multilayers is discussed. The functional dependence of the giant magnetoresistance on magnetic configuration variation is obtained by considering both interface and bulk magnetization contributions. The results show that the giant magnetoresistance in magnetic multilayers varies with magnetization angle and differs for different layer thicknesses. We also clarify the interface and bulk contribution of the giant magnetoresistance in magnetic multilayers for various exchange potentials and impurity scattering potentials.

Magnetic properties of impurities and impurity pairs in magnetic multilayers

Abstract Spin-dependent scattering is considered to be the origin of giant magnetoresistance in magnetic multilayers, although the role of bulk or interface defects is still under discussion. We present calculations of spin-dependent impurity scattering potentials as a function of the impurity position in a Co/Cu-multilayer. The calculations are performed within spin density functional theory using a Green function method (the tight binding Korringa-Kohn-Rostoker method). It is shown that the impurity potential depends strongly on the impurity position, which is reflected in local densities of states and local impurity moments. Furthermore, calculations of magnetic properties of impurity pairs at the interface are presented and compared with results for single impurities.

Validity and Limitation of Semi-Classical Approaches to Giant Magnetoresistance in Magnetic Multilayers**The project supported by National Natural Science Foundation of China under grant No. 19374013

The validity and limitation of semi-classical theories on giant magnetoresistance in magnetic multilayers are discussed on the basis of fundamental quantum statistical mechanics.

Interface roughness effects in the giant magnetoresistance in magnetic multilayers

In-plane electronic transport in thin layered magnetic structures composed of two ferromagnetic films separated by a nonmagnetic spacer is analyzed theoretically in the Born approximation. Particular attention is paid to the role of interface roughness in the giant magnetoresistance (GMR) effect. The analysis applies to self-affine interfaces described by the k-correlation model. Our results show that GMR is sensitive to the roughness exponent H (0⩽H⩽1) in a manner that depends on spin asymmetries for bulk and interfacial scattering. The limit of low electron concentration is also considered.

The role of the magnetic structure and interface scattering in the GMR of magnetic multilayers

Room temperature giant magnetoresistance (GMR) has been investigated for Co/Cu(1 1 1) multilayers grown by DC magnetron sputtering. The effect of annealing on the magnetic structure of multilayers with a nominal copper thickness of 0.9 nm has been studied, a strong linear correlation between the GMR and the volume fraction of ferromagnetic regions is observed. Such behaviour has been used to elucidate the scattering mechanism responsible for the GMR of this system, and suggests that s-d scattering into the split d-band of cobalt is the dominant mechanism responsible for the GMR of Co/Cu multilayers. This is in contrast to systems like Fe/Cr where experimental studies on the effect of interfacial mixing indicate that interfacial scattering resulting from spin-dependent interface potentials contributes strongly to the observed GMR.

Angular dependence of giant magnetoresistance in magnetic multilayers

Angular dependence of giant magnetoresistance in magnetic multilayers

Correlation between the signs of the magnetoresistance and of the interlayer coupling in MnGa/(Mn,Ga,As)/MnGa trilayers

A correlation is found between the sign of the magnetoresistance ΔR=(RP−RAP) and the sign of the interlayer coupling in MnGa/(Mn,Ga,As)/MnGa trilayers (subscript P, AP=parallel, antiparallel magnetic alignment of both MnGa layers). Ferromagnetic coupling results in ΔR&amp;lt;0, antiferromagnetic coupling results in ΔR&amp;gt;0. This correlation has not been observed before and can not be explained by the usual model for giant magnetoresistance in magnetic multilayers.

Microfabrication in the study of giant magnetoresistance: Recent results

Microstructuring can give access to new experimental geometries for studying the giant magnetoresistance in magnetic multilayers, such as the one in which the current is perpendicular to the plane of the multilayer (the so-called CPP-geometry). We discuss several experimental methods that give access to this geometry and discuss in more detail a method in which a multilayer is evaporated at an angle onto nano-grooved substrates. Using the latter technique, we have determined the spin-dependent bulk and interface scattering parameters in Co/Cu multilayers as a function of the temperature.