CPP Geometry Research Articles

Monte Carlo Computer Simulations of Spin-Transfer Torque.

This article performs computer simulations of the change in magnetization in the ferromagnetic film when polarized electric current passes through it. The model examines multilayer structures from ferromagnetic and nonmagnetic films. A sandwich system comprises two ferromagnetic layers separated by a nonmagnetic gasket. Ferromagnetic films have different magnetic susceptibility. The first ferromagnetic film is magnetically hard and acts as a fixed layer. The second ferromagnetic film is magnetically soft, with a switched direction of magnetization. The current direction is perpendicular to the film plane (CPP geometry). Spin transfer is carried out by electrons that polarize in the first ferromagnetic film and transmit spin to the second ferromagnetic film. We use the Ising model to describe the magnetic properties of the system and the Metropolis algorithm to form the thermodynamic states of the spin system. Simulations are performed at temperatures below the Curie points for both materials. The result of computer simulation is the dependence of magnetization in the magnetically soft film on the current strength in the system. Calculations show that there is a critical value of the current at which the magnetization sign of the controlled film changes. The magnetization versus current plot is stepwise. The change in the magnetization sign is due to an increase in the polarization of the electron gas. The plot of electron gas polarization versus current is also stepwise.

Technology features of creating InSb-based spin-valve

Technological route is reported how to fabricate an InSb spin device for studying spin-transport phenomena in CPP geometry. The device, which consists of ferromagnetic CoFe layer and MgO tunnel barrier deposed on the n-InSb single-crystal substrate, was manufacturing by method of contact photolithography with lift-off technology of drawing transfer in the topological layers. The optimal methods for preparation of InSb substrate surface and photoresist exposure parameters are found by measuring the voltage between InSb and ferromagnetic probe (Hanle effect) that appears due to the spin polarization of electrons injected from ferromagnet into InSb.

Calculation of CPP-and CIP-magnitoresistance in multilayer magnetic structures

A Monte Carlo simulation of magnetic properties for structures Fe/Cr/Fe and Co/Cu/Co, constructed from two ferromagnetic films divided by nonmagnetic film are carried out. The calculation of the magnetoresistance is carried out in case of CPP and CIP geometry for different thicknesses of the ferromagnetic films with the use of the anisotropic Heisenberg model for determination of magnetic properties. It was shown, that the obtained temperature dependence for the magnetoresistance agrees very well with experimental results.

Origin of the size-dependent fluorescence blueshift in [n]cycloparaphenylenes

Quantum chemical electronic structure calculations were employed to investigate the nature of the low-lying excited states of [n]cycloparaphenylenes ([n]CPPs) and the role of static and dynamic geometrical distortions in the bright states. The lowest-energy bright states involve single-electron excitations from S0 ground state to S2 and S3 states, which are at the Franck–Condon geometry the two components of a twofold degenerate 1E state. They couple to a twofold degenerate e vibration which induces Jahn–Teller (JT) deformation of the CPP geometry from circular to oval shape. Non-radiative decay from the S2/S3 states to the ground S0 and first excited, dark S1 states is suppressed due to symmetry rules. The emission spectral features in CPPs with large number of phenylene units n can therefore largely be attributed to the E⊗e JT system associated with S2 and S3. However, absorption and emission energies computed at the respective S0 and S2/S3 minimum energy geometries are found to be nearly identical, independent of the molecular size n in the CPP molecules. In contrast, molecular dynamics simulations performed on the excited state potential surfaces are able to explain the experimentally observed fluorescence blueshift of the strongest emission peaks with increasing molecular size. This unusual feature turns out to be a consequence of large vibrational amplitudes in small [n]CPPs, causing greater Stokes shifts, while large [n]CPPs are more rigid and therefore feature smaller Stokes shifts (“dynamic blueshift”). For the same reasons, symmetry rules are violated to a greater extent in small [n]CPPs, and it is expected that in their case a “static blueshift” due to emission from S1 contributes in the fluorescence spectra.

Lithography-free study of spin torque

We developed a non-lithographic technique to contact sub-100 nm nanowires for spin transfer torque experiments. Co/Cu multilayers were grown by electrodeposition in nanoporous commercial polycarbonate membranes from a Co/Cu bath. A home-made sample holder allows bottom and top electrical contacts to be made to individual nanowires in the CPP geometry. Experimental evidence of the spin transfer torque effect is given for (Co/Cu) n multilayers by recording d V/d I spectra as a function of the DC current amplitude.

Spin-dependent scattering of a domain wall of controlled size

Magnetoresistance measurements in the CPP geometry have been performed on single electrodeposited Co nanowires exchange biased on one side by a sputtered amorphous GdCo layer. This geometry allows the stabilization of a single domain wall in the Co wire, the thickness of which can be controlled by an external magnetic field. Comparing magnetization, resistivity, and magnetoresistance studies of single Co nanowires, of GdCo layers, and of the coupled system, gives evidence for an additional contribution to the magnetoresistance when the domain wall is compressed by a magnetic field. This contribution is interpreted as the spin dependent scattering within the domain wall when the wall thickness becomes smaller than the spin diffusion length.

Transport properties of discontinuous Co/sub 80/Fe/sub 20//Al/sub 2/O/sub 3/ multilayers, prepared by ion beam sputtering

Ion beam sputtered Co/sub 80/Fe/sub 20/(t)/Al/sub 2/O/sub 3/ (30 /spl Aring/) multilayers were obtained. The Co/sub 80/Fe/sub 20/ layers become discontinuous for nominal thickness t/spl les/18 /spl Aring/. Tunnel magnetoresistance was measured in CIP and CPP geometries, reaching up to 6.5% at room temperature and 11% at 15 K, for as-deposited films in CIP geometry. The temperature dependence of MR was found quite different for the two geometries: fairly strong in the CIP case and almost absent in the CPP geometry. A model is proposed to explain these large differences in behavior.

Microwave Current-Perpendicular-to-Plane Giant Magnetoresistance of Fe/Cr Superlattices

The microwave measurement of GMR effect in the CPP geometry is realized and investigated. It is shown that the magnetic field dependence of microwave absorption undoubtedly correlate with the CIP DC magnetoresistance of superlattices.

Effect of sputtering pressure on the structure and current-perpendicular-to-the-plane magnetotransport of Co/Ag multilayered films

Co/Ag multilayers have been sputter-deposited at pressures ranging from 0.86 to 10 mTorr, and magnetotransport measurements have been performed with the current both perpendicular to the plane (CPP) and in the plane (CIP) of the samples. X-ray-scattering studies indicate that the roughness of the interfaces is influenced by the presence of the Nb layers used to make electrical contact for the CPP measurements, as well as by the sputtering pressure. For samples deposited at higher sputtering pressures, the Nb contact layers also influence the low-temperature coercive field of the multilayers. The CPP-giant magnetoresistance is observed to decrease with increasing sputtering pressure and, in the case of the signal observed after cycling to above the saturation field, this decrease is found to come in roughly equal measure from an increase in the high field-saturated specific resistance and a decrease in the field-dependent specific resistance. Making use of the direct connection between microscopic physical parameters and experimental results available through the CPP geometry, we show that both of these changes arise primarily from increased scattering in both spin channels. We see very little, if any, evidence for a change in the spin-dependent scattering with sputtering pressure.

Perpendicular giant magnetoresistance of Co/Cu multilayers on grooved substrates: Systematic analysis of the temperature dependence of spin-dependent scattering.

The giant magnetoresistance of Co/Cu multilayers deposited at an angle onto grooved substrates is measured with the current perpendicular to the layer plane. The spin-dependent scattering parameters due to magnetic bulk and interface scattering are determined as a function of temperature, which is done by comparing our experiments with the two-channel model. We find that the decrease of the magnetoresistance from 4.2 K to room temperature is mainly due to an increase of the bulk resistivities of the Co and Cu layers, while the temperature dependence of the interface resistance and the spin-asymmetry parameters for electron scattering is small. @S0163-1829~96!03622-3# The discovery of the giant magnetoresistance ~MR! effect in magnetic multilayers, 1 has led to numerous experimental studies on this effect by many groups. Different multilayer systems as well as different experimental geometries have been investigated. Transport experiments in magnetic multilayers can be divided in two types: ~i! The current flows in the plane of the layers ~CIP! and ~ii! the current is directed perpendicularly to the layer plane ~CPP!. A description of the giant magnetoresistance effect is theoretically more complex in the CIP geometry than in the CPP geometry. However, experimentally the CPP MR is much harder to access than the CIP MR due to the low perpendicular resistances involved. Several methods have been developed to measure the magnetoresistance of a multilayer in the CPP geometry. The first CPP MR measurements have been done by using super-conducting contacts and ultrasensitive superconducting-quantum-interference-device- ~SQUID-! based voltage measuring techniques at low temperatures. 2 Subsequently also microfabricated, so-called ‘‘pillar,’’ structures have been employed. 3 This method allows one the investigation of the temperature dependence of the CPP MR effect. Unfortunately, this fabrication method is rather complicated and the contact resistance to the pillars is often a problem. A third technique is the electrolytic growth of multilayers into nanopores made into insulating membranes resulting in columns with a large aspect ratio and resistances in a convenient range. 4 Recently, we introduced an alternative and simple technique for measuring CPP MR, based on the oblique evaporation of multilayers on to grooved substrates. 5 At the same time also perpendicular growth of thick multilayers on grooved subtrates was investigated. 6 From measurements with currents at different angles to the grooves @the so-called current at an angle to layer plane ~CAP! geometry#, 5 it has been possible to derive the CPP MR of the multilayer. In this paper we report on temperature-dependent CPP MR experiments on Co/Cu multilayers fabricated with the oblique evaporation technique. We have chosen for the Co/Cu multilayer system since this is by now a well-known system, making comparison with the literature possible. It is emphasized, however, that this technique can also be applied to other systems. We compare the experimental data with the phenomenological two-channel model introduced by Lee et al. 7 We systematically analyze the relevant spindependent scattering parameters as a function of temperature; this has previously not been possible with the other techniques. We find that the Co/Cu interface resistance keeps its strong spin dependence at all temperatures. The main reason for the decrease of the MR with temperature is the increasing importance of bulk scattering with intrinsically much smaller spin dependence. We compare our lowtemperature data with CPP MR experiments on Co/Cu done at Michigan State University, 8 and find that these are similar.

Electron interaction in the GMR in multilayers in the CPP geometry

We discuss the effect of electron interaction on the GMR in multilayers in the perpendicular geometry.

Perpendicular giant magnetoresistance of Co/Cu multilayers deposited under an angle on grooved substrates

We propose a novel experimental technique for investigating the giant magnetoresistance effect measured with the current perpendicular to the layer plane (the so-called CPP geometry). Using holographic laser interference nanofabrication techniques and anisotropic etching the surface of semi-insulating InP substrates is patterned into V-shaped grooves of 0.2 μm width. Subsequently, a magnetic multilayer can be evaporated under an angle with the substrate normal, naturally resulting in a CPP-like magnetoresistance configuration. The technique is illustrated for Co/Cu multilayers, for which we present magnetization and magnetoresistance experiments.

Perpendicular Resistance of Co/Cu Multilayers Prepared by Molecular Beam Epitaxy

ABSTRACTResults are presented of the magnetoresistance of MBE-grown (111) Co/Cu multilayers measured with the current perpendicular to the plane of the layers (CPP). Although for measurements made with the more common geometry of current in the plane of the layers (CIP) there are large differences between the results on samples made by sputtering and those prepared by MBE, for these new CPP data the results on samples made by the two techniques are very much alike. For copper layers with thicknesses between 0.9nm to 6nm the magnetoresistance shows oscillations with copper thickness that were almost non-existent in the earlier CIP data. At the second peak the magnetoresistance in the CPP geometry is an order of magnitude greater than that in the CIP configuration. Although the interfaces in these samples have been shown to be very sharp, they appear to form a mosaic structure with the antiferromagnetic regions embedded in a ferromagnetic structure. It is argued that for CIP measurements the GMR is greatly reduced by these ferromagnetic correlations over lengthscales long compared to the electron mean free path. For CPP measurements, on the other hand, it is the spin diffusion length that is the determining factor with the mean free path no longer a key parameter and with values of the GMR virtually independent of the growth process.