Current Perpendicular To The Plane Research Articles

Microstructure of magnetic metallic superlattices grown by electrodeposition in membrane nanopores

Magnetic metallic multilayers grown by electrodeposition in nanopores are good candidates for the study and application of giant magnetoresistance (GMR) in the `current perpendicular to the planes (CPP)' geometry. We show that, in spite of the relative simplicity of growth by electrolysis, these multilayers are indeed superlattices, with single-crystal grains including several tens of layers. We analyse their microstructure in detail, using X-ray diffraction and analytical transmission electron microscopy.

Quantum effects in the giant magnetoresistance (GMR) of magnetic multilayers

We present a quantum-statistical theory using the Kubo formalism to describe both CIP (current in plane) and CPP (current perpendicular to the plane) giant magnetoresistance (GMR) of thin magnetic sandwiches. It is shown that GMR for both geometries oscillates with the thicknesses of the sandwich's layers due to the confinement of electrons in quantum wells. The deviation of the GMR dependence on the angle γ between the magnetizations of successive layers from linear cos γ dependence is predicted. It is shown that for CIP geometry this deviation is large only for a large value of the parameter( k F ↑ − k F ↓)/( k F ↑ + k F ↓), where k F ↑(↓) is Fermi momentum of spin-up (down) electron, but for CPP GMR for noncollinear configuration of the magnetizations a new mechanism (interference between spin-up and spin-down electron waves) is predicted, and the deviation from linear cos γ dependence in this case is large even for small values of the parameter ( k F ↑ − k F ↓)/( k F ↑ + k F ↓), but not for very large values of the parameter ( k F ↑ − k F ↓)/(1/ l ↑ + 1/ l ↓), where l ↑(↓) is the mean free path for spin-up(down) electron.

Local field and quantum effects for current perpendicular to planes in multilayers

The calculation of giant magnetoresistance and in general, of electron transport for multilayers in the case of current perpendicular to the planes (CPP) requires both the two-point conductivity and the solution to the local field problem. In this paper we present a solution to the local field problem at an interface using two approaches. In the first approach we find the semiclassical solution for the local field when there is a band mismatch between two sides of an interface, and examine the deviation of the total resistance from the result of “self-averaging,” in the lowest order of the value of the potential step. In the second approach, we solve for the quantum correction to the local field through a numerical iterative scheme. The oscillations due to the quantum correction are surprisingly large, but their correction to the total resistance is remarkably small. Our results imply that the “self-averaging” of the resistance, which is usually assumed in analysis of CPP, is only approximate.

Spin dependent transport perpendicular to CoPt/S/Co (S=Si, Ge) trilayer

In this article, we report spin dependent electron transport perpendicular to a ferromagnet/semiconductor/ferromagnet trilayer with the current perpendicular to the plane (CPP) geometry and fabricated by a photolithographic technique. The CPP current was defined with a contact hole of 20 μm diameter produced in a SiO2 spacing layer. A CoPt/S/Co trilayer was used in the present study. Various film deposition methods were studied for optimum performance. A superior hard magnetic property was obtained in rf sputtered CoPt, which results in a well defined selective magnetization switching between CoPt and Co layers. The magnetoresistance (MR) effects have been confirmed by the measurement of MR hysteresis for various semiconductor layers. The values of MR change ΔR at room temperature observed in sputtered Ge (42 nm), evaporated Si (30 nm), and sputtered Si (16 nm) were 0.40 mΩ (R=1.3 Ω), 0.30 mΩ (R=2.4 Ω), and 5.5 mΩ (R=19 Ω), respectively. The magnetron sputtering was found to be a most promising method for producing a pinhole free semiconductor layer of order 20 nm thickness.

Towards a Unified Picture of Spin Dependent Transport in and Perpendicular Giant Magnetoresistance and Bulk Alloys

From data on $({\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{V}}_{x}/\mathrm{Cu}/\mathrm{Co}/\mathrm{Cu}{)}_{N}$ multilayers, we show that Fe doped with V gains a negative spin asymmetry for bulk scattering $(\ensuremath{\beta}<0)$, which, combined with the positive asymmetry of Co, accounts for the inverse current perpendicular to the plane (CPP) giant magnetoresistance (GMR) we observe. More precisely, the competition between positive and negative asymmetries for interface and bulk scatterings in FeV leads to inverse (normal) GMR for layers thicker (thinner) than a compensation thickness. The negative $\ensuremath{\beta}$ of FeV is consistent with theoretical predictions and bulk alloy data. The current in the plane (CIP) GMR is not reversed, which illustrates the role of channeling in CIP.

Perpendicular giant magnetoresistance of NiFe/Cu and Co/Cu multilayered nanowires

We report a comparative study of the giant magnetoresistance (GMR) in CPP (current perpendicular to the planes) geometry in two different types of multilayered nanowire systems: Co/Cu and Ni 80Fe 20/Cu. The nanowires were produced in the cylindrical pores of a polymer membrane using a pulsed electrodeposition technique. A GMR ratio as large as 78% was obtained for Ni 80Fe 20/Cu multilayered nanowires. Using the results of the Valet-Fert model for perpendicular transport, we deduce the bulk and interface spin asymmetry coefficients and comparison is made between the two systems.

Development of the spin-valve transistor

As the easiest experimental approach, GMR (giant magnetoresistance) is usually measured using the current in plane (CIP)-GMR. The spin-valve transistor has previously been presented as a spectroscopic tool to measure current perpendicular to the planes (CPP)-GMR. Hot electrons cross the magnetic multilayer base quasi-ballistically and the number reaching the collector depends exponentially on the perpendicular hot electron mean free path. Collector current changes of 390% at 77 K have already been measured. Apart from the substantial fundamental value, such properties may be useful for sensor applications. The electron energy range fills the gap between the Fermi surface transport in resistance measurements and other hot electron techniques such as spin polarised electron energy loss spectroscopy (SPEELS). The preparation problem of the spin-valve transistor and metal base transistor structures in general, the deposition of a device quality semiconductor on top of a metal, has now been tackled by bonding of two semiconductor substrates during vacuum deposition of a metal: an excellent bond is achieved at room temperature. TEM photos show a continuous buried metal film. Apart from preparation of various metal base transistor like structures, many other fields may benefit form this new technique.

Perpendicular Giant Magnetoresistance of Co/Cu Multilayers with Fluctuating Co Layer Thicknesses

ABSTRACTJ. Mathon has predicted that introducing pseudorandom fluctuations (PRF) in the Co layer thickness could greatly enhance the Current Perpendicular to the Plane (CPP) magnetoresistance (MR) of a Co/Cu superlattice with dimensions comparable to the electron mean-free-path. We have searched for CPP-MR enhancement in sputtered Co/Cu multilayers with Cu layer thicknesses near both the first and second antiferromagnetically coupled peaks in the oscillatory region of the CPP-MR. In both cases, inserting PRF only decreased the CPP-MR.

Perpendicular magnetoresistance in Co/Cu multilayered nanowires

We have carried out magnetoresistance measurements in the CPP (current perpendicular to the planes) geometry of electrodeposited Co/Cu multilayered nanowires with Cu and Co layer thicknesses varying over wide ranges. The data are compared with the results of the Valet-Fert model for perpendicular transport. The observed behaviour agrees with the corresponding predictions in various limits of the layers thicknesses. 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.

Perpendicular current giant magnetoresistance in 0.4 μm diameter Co-Cu multilayer sensors

We have fabricated a novel giant magnetoresistive (GMR) multilayer (ML) flux sensor that is designed to operate in the current perpendicular to the plane (CPP) mode. The CPP-GMR sensor is a 0.4 /spl mu/m diameter, 0.09 /spl mu/m tall Co-Cu ML pedestal. The sensors are patterned using electron beam lithography. The Al/sub 2/O/sub 3/-TiC substrate is coated with a sputter deposited Al/sub 2/O/sub 3/ film that is polished to <0.2 nm root mean square (RMS) roughness. Contact to the bottom of the CPP-GMR sensor is made by depositing and Co-Cu multilayers onto a smooth 0.45 /spl mu/m thick Mo-Si ML stack. The top contact is self-aligned to the CPP-GMR sensor. This is accomplished, in part, by chemical mechanical polishing (CMP). The top and bottom contacts are electrically isolated by a plasma enhanced chemical vapor deposited (PECVD) Si/sub 3/N/sub 4/ film. The configuration of the contacts allows four point probe resistance measurements. The CPP-GMR coefficient for these 0.4 /spl mu/m diameter sensors is 13%.

Recent results on the giant magnetoresistance in magnetic multilayers (anisotropy, thermal variation and CCP-GMR)

We present some recent results obtained on the electrical transport properties in magnetic multilayers. Three points are addressed. The first one is an experimental demonstration of the existence of an intrinsic anisotropy of the giant magnetoresistance (GMR). The experiments have been carried out on spin-valve samples for which there is no contribution of the usual anisotropic magnetoresistance to the observed magnetoresistance. The GMR amplitude is found to be larger (lower) in the direction perpendicular (parallel) to the sensing current. The second point concerns a quantitative analysis of the thermal variation of the CIP (current-in-plane) GMR in magnetic multilayers. This analysis is based on a semi-classical theory including the spin-intermixing due to spin-flip scattering by magnons. This approach allows quantitatively evaluation of the respective weights of the various contributions to the thermal decrease in GMR: (i) scattering by magnons in the bulk of the ferromagnetic layers; (ii) phonon scattering in the non-magnetic spacer layer; and (iii) interfacial scattering by paramagnetic interfacial layers which may form as the temperature is increased. The third point is a theoretical investigation of the CPP (current perpendicular to the plane) electrical transport through an interface between two semi-infinite metallic materials. It is shown that when a potential step U exists at such an interface, this step gives rise to an interfacial resistance proportional to U 2. It also leads to the existence of large oscillations in the electric fields on both sides of the interface.

超微細加工技術の磁性薄膜への応用 金属人工格子膜の膜面垂直ＭＲ特性

Giant magnetoresistance (GMR) in Co/Cu metallic multilayers was measured with a current perpendicular to the plane (CPP) at room temperature. Micro-pillar structures with a height of 1.0 a m and widths ranging from 15 to 25 u m, fabricated by using optical lithogrphy and Ar ion etching techniques, were used for the measurement. The CPP MR in Co (3 nm)/Cu (2 nm) multilayers was found to be 2.5 times larger than the MR with a current in the film plane (CIP).

Ｃｏ／Ｃｕ金属人工格子膜の垂直磁気抵抗

Giant magnetoresistance (GMR)in Co/Cu metallic multilayers was measured with a current perpendicular to the plane (CPP) at room temperature. Micro-pillar structures with a height of 1.0μm and widths ranging from 15 to 25 μm, fabricated by using optical lithography and Ar ion etching techniques, were used for the measurement. The CPP MR in Co (3 nm)/Cu (2 nm) multilayers was found to be 2.5 times larger than the MR with a current in the film plane (CIP).

人工格子MR膜

MR (Magnetoresistance) charateristics of metallic multilayers such as [M/X(M'/X)] and spinvalve multilayers such as [{Fe-Mn/M/X/M}/X] were studied, where M=Ni-Fe-Co, Co-Fe, … and X=Cu. [Ni-Fe-Co/Cu(/Co/Cu)] showed considerably large MR ratios and small saturation fields, and are suitable for MR sensors. [{Fe-Mn/Ni-Fe-Co/Cu/Ni-Fe-Co}/Cu] showed sharp and very small switching fields and are suitable for low field operations. Thermal stabilities of these multilayers were studied and the films having Co/ Cu interfaces were found to be more stable than those having Ni/Cu interface. MR characteristics of MBE grown epitaxial multilayers and CPP (Current Perpendicular to the Plane) MR were also discussed.

The influence of potential barriers on magnetoresistance of magnetic multilayers (abstract)

There is an emerging interest in the CPP (current perpendicular to the plane of layers) geometry for transport measurements1 and we report on new developments on the CPP-MR (magnetoresistance). Specifically we show that the effect of spin-dependent potentials in magnetic multilayers is far more pronounced for CPP-MR than for CIP-MR (current in the plane of layers). This adds to the difference between CIP-MR and CPP-MR that is due to the spin-dependent scattering in the multilayers.2 The potentials seen by the spin up and the spin down conduction electrons are different, and they depend on the orientation of the magnetic layers relative to one another, i.e., they are different for ferro and antiferromagnetically aligned layers. We have determined the Bloch functions appropriate to these spin-dependent potentials,3 and have used the Kubo formalism to calculate the resistivity and magnetoresistance for both CIP and CPP geometries. In general these potentials do affect the CIP-MR; however, for uniform scattering, we find no effect of the potentials on the CIP-MR. On the contrary, these potentials affect the CPP-MR in all circumstances, even for uniform scattering of the conduction electrons. Due to spatial and spin inhomogeneities in magnetic multilayers, the current varies from one layer to another for a uniform electric field parallel to the layers, CIP. However, for current perpendicular to the plane of the layers, CPP, it is the electric field that varies from one layer to another. We describe the distributions in both cases, and we compare our results on the CPP-MR with recent data on Co/Ag and Co/Cu.4