Magnetoresistance In Magnetic Multilayers Research Articles

Theoretical characterization of the magnetoresistance in magnetic multilayers with antiferromagnetic coupling

Spin-valves (SVs) constitute a significant class of magnetic metal structures with a large range of technological applications. This class of structure offers interesting magnetic properties due to their alternation with the non-magnetic layers. The SVs fortunately have good magnetic properties at room temperature (RT). This study intends to use Boltzmann Approach–BA, to treat the magetotransport properties of Co-based SV. Outer-surface scattering effects on magnetoresistance (MR) performance encountered in these structures are investigated by varying the interfacial chemical composition. Analysis of the results reveals the existence of a significant improvement of magnetic phenomenon for a scattering that is more and more specular of the conduction electrons through outer-surface. A giant MR (GMR) of amplitude 136.9 % is obtained for an interfacial geometric roughness of magnitude 2 Å. The GMR ratio is the result of large asymmetry in the spin-dependent interfacial scattering and a completely specular outer-surface scattering.

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.

Current-perpendicular-to-plane spin transport properties of CoFe alloys: Spin diffusion length and scattering asymmetry

From measurements of the current-perpendicular-to-plane magnetoresistance of magnetic multilayers and exchange-biased spin-valves combining the ferromagnetic (F) alloys Co50Fe50 and Co70Fe30 with the nonmagnetic metal Cu, we extract values of the bulk scattering asymmetry, βF, the spin-diffusion length, lsfF, the interface scattering asymmetry, γF/Cu, and twice the enhanced specific resistance, 2ARF/Cu∗. The parameters of the two alloys lie within mutual uncertainties of each other. The values of βF∼0.88 are unusually large, those of lsfF∼10 nm are moderate and consistent with values for other F-alloys, and those of 2ARF/Cu∗∼0.6 fΩ m2 and γF/Cu∼0.6 are both less than those for Co/Cu. We do not need to include any spin-flipping at the F/Cu interfaces to fit our data.

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.

Hall Effect and Magnetoresistance in Magnetic Multilayers with Alternating In-Plane and Out-of-Plane Anisotropies

A direct comparison between the Hall effects and giant magnetoresistance of ferromagnetic multilayers of similar composition (Ni80Fe20/Au/Co/Au)N with alternating in-plane and out-of-plane magnetization direction of Co layers is presented. The characteristic features at magnetic field-dependence of giant magnetoresistance were correlated with the creation and annihilation of the stripe domains in Co layer, with perpendicular anisotropy. The nucleation field values were investigated as an Au layer thickness function. Furthermore, the in situ conductance measurement results characterised the island growth mode of the ferromagnetic layers. The percolation thicknesses were also indicated.

Magnetoresistance of magnetic multilayers: a phenomenological approach

A phenomenological approach is presented for the giant magnetoresistance (GMR) ofmagnetic multilayers oriented in the CPP mode (current perpendicular to the plane of thelayers). New results are found for the dependence of the GMR on the number of repeats ofthe multilayer.

Direct evidence for mean-free-path effects in the magnetoresistance of magnetic multilayers with the current perpendicular to the planes

Experimental evidence is presented showing that for magnetic multilayers measured in the CPP mode (current perpendicular to the planes), the magnetic-field dependence of the magnetoresistance MR(H) is dominated by scattering processes in which the electron is scattered by the potential due to pairs of neighboring magnetic layers. It is demonstrated experimentally that curtailing such scattering processes leads to a significant decrease in MR(H). These results are confirmed by a calculation.

Magnetoresistance of magnetic multilayers containing three types of magnetic layers

The magnetic-field dependence of the magnetoresistance $\mathrm{MR}(H)$ has been measured in the CPP mode (current perpendicular to the plane) for multilayers containing three different types of magnetic layers: permalloy $\mathrm{Py}(80\mathrm{\AA{}}),\mathrm{Co}(10\mathrm{\AA{}}),$ and $\mathrm{Co}(70\mathrm{\AA{}}).$ These data clarify the role of the electron mean free path in interpreting the $\mathrm{MR}(H)$ curves. A critical discussion is given of the view that denies the role of the electron mean free path for determining the $\mathrm{MR}(H)$ curves.

Absence of mean-free-path effects in the current-perpendicular-to-plane magnetoresistance of magnetic multilayers

The series resistor and Valet-Fert models widely used to describe the current-perpendicular-to-plane (CPP) magnetoresistances of ferromagnetic/nonmagnetic (F/N) metal multilayers were recently claimed to be valid only for mean-free paths shorter than layer thicknesses; otherwise the mean-free path was claimed to be an important length scale in the CPP magnetoresistance (MR). This claim was based on observations of differences in the CPP MR's, after the samples were taken to above their saturation magnetic fields, of two different kinds of multilayers involving Co and Cu: interleaved $[\mathrm{Co}(6)/\mathrm{Cu}(20)/\mathrm{Co}(1)/\mathrm{Cu}(20){]}_{N}$ and separated $[\mathrm{Co}(6)/\mathrm{Cu}(20){]}_{N}[\mathrm{Co}(1)/\mathrm{Cu}(20){]}_{N},$ with N repeats and thicknesses in nm. The maximum CPP MR's of separated samples were only about half as large as those for interleaved ones. In two short papers, we provided experimental evidence that mean-free paths are not important length scales in the CPP MR by showing that the differences in CPP MR's upon which the above claim was made did not change when the mean-free paths in the N and F layers were reduced from well above to well below their layer thicknesses. We ascribed part of the behaviors of interest to finite spin-memory loss (spin flipping) in the F and N metals, and proposed that the rest might be due to spin flips at F/N interfaces. In the present paper we (a) present further experimental evidence against mean-free-path effects, (b) provide details of the calculations we use to analyze the data, and (c) use measurements of magnetization and polarized neutron reflectivity to show that the differences in CPP MR are not due to spurious differences in magnetic structure between interleaved and separated multilayers, but only to the differences in the relative magnetic alignment of adjacent layers. Additional evidence for this last point is our observation that the CPP MR's of separated samples in their as-prepared states are as large as those of the equivalent interleaved samples after they are taken to above their saturation fields. We show that similar differences between interleaved and separated data appear also in the current-in-plane (CIP) MR's and when the Cu is replaced by Ag.

Time dependence of spin accumulation and magnetoresistance in magnetic multilayers

We derive and solve the equations that determine the time dependence of the spin accumulation attendant to driving a current across a magnetic multilayer. The time scales with the spin-flip relaxation time for the multilayer, which is of the order of picoseconds. As this is much shorter than the time it takes for current-induced switching of magnetic layers, the lack of spin accumulation does not seem to be a factor for the long switching times recently observed.

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.

Magnetoresistance of magnetic multilayers: understanding Ohm's law

The magnetic-field dependence of the magnetoresistance MR( H) of magnetic multilayers in the CPP mode (current perpendicular to the layers) was recently measured for two samples that differ only in the ordering of the layers: [Fe/Cu/Co/Cu] N and [Fe/Cu] N [Co/Cu] N , where N denotes the number of repeats. Ohm's law states that MR( H) should be the same for these two samples because in a series circuit, the resistance is independent of the ordering of the resistors. However, the measured MR( H) curves were found to be completely different for the two samples. We show that this unexpected result can be explained as follows. The mean free path is sufficiently long that MR( H) is due to the scattering of electrons by pairs of neighbouring magnetic layers, which differ for the two samples. To confirm these ideas, we carried out calculations of MR( H) and obtained quantitative agreement with experiment.

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.

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.

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.

The role of the bulk spin-dependent scattering on the origin of the enhanced magnetoresistance in magnetic multilayers

A detailed analysis of the current density distribution along the perpendicular direction of a spin-valve-like structure is presented. It is shown that, even when only bulk spin-dependent scattering is considered, the carriers responsible for the enhanced magnetoresistance in these systems are flowing in a narrow strip of the ferromagnetic layer close to the ferromagnetic/nonmagnetic interface. A modification of the bulk transport properties of this region affects drastically the giant magnetoresistance (GMR) of the overall structure.

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.