Magnetoresistance Hysteresis Loops Research Articles

Observation of double sign reversal of the hall resistance in Y 0.55Pr 0.45Ba 2Cu 3O 7− δ polycrystalline samples

In order to study Hall effect in Y 0.55Pr 0.45Ba 2Cu 3O 7− δ polycrystalline samples we have measured longitudinal and transverse voltages at low magnetic field using Van der Pauw technique. The results of the Hall resistance R XY as a function of temperature presented double sign reversal which crosses to the positive sign at same temperature in which branching point ( T Cj) of the electrical resistance R XX occurs. Magneto-resistance hysteresis loops demonstrated that the dissipation is dominated by weak coupling between superconducting clusters. The results suggest that the positive Hall voltage at superconducting state is due to quasiparticle current which is in good agreement with the recent discussion about the applying of the resistively shunted junction model to polycrystalline samples.

Effect of the magnetic field on the branching point of the resistive transition in the Y 1− xPr xBa 2Cu 3O 7− δ system

This work presents results about the influence of the applied magnetic field on the branching point of the resistive transition in granular samples of the Y 1− x Pr x Ba 2Cu 3O 7− δ (Y123 + Pr) system. Single phase samples of x=0.0 and 0.35 composition were prepared by solid state reaction method. Using several resistance curves as a function of the temperature measured at different applied currents we defined branching points at each magnetic field which was ranged from 0 up to 9 T. We have observed that a line in the H– T phase diagram separates the weak coupling state from decoupled regime of the superconducting clusters. Measurements of current–voltage curves and magneto-resistance hysteresis loops agree with such discussion.

Anisotropic magnetoresistance in CoO/Co and CoO/Fe bilayers in the biased and unbiased state

Magnetoresistance and magnetic hysteresis loop measurements have been used to investigate the strong interfacial coupling in cobalt oxide/ferromagnet bilayers at low temperatures. For the biased state, an asymmetry in the magnetization reversal processes has been found. It is characterized by domain wall motion as the dominant mechanism for the decreasing field branch and an appreciable amount of magnetization rotation for the increasing field branch. The observed behavior is different from other exchange bias systems.

Transport and magnetic properties of isolated cobalt nanowires

A multiprobe connection setup has been used to study the magnetoresistance (MR) of isolated electrodeposited ferromagnetic cobalt nanowires. The MR hysteresis loops taken along different segments of the same 60-nm-diameter Co wire are found to be completely different, pointing out local inhomogeneity of the magnetic anisotropy along the same wire. These results are corroborated both by magnetic force microscopy (MFM) and transmission electron microscopy (TEM) experiments, showing a mixing of parallel and perpendicular c-axis orientations along the wire.

Field dependent anisotropic magnetoresistance measurements on CoFe/IrMn bilayers

The exchange anisotropy field, along with the qualitative distribution of the local pinning field at the ferromagnetic/antiferromagnetic interface, has been studied in polycrystalline CoFe/IrMn bilayers. The exchange anisotropy was studied using both the anisotropic magnetoresistance (AMR) technique and hysteresis loop measurements. The AMR technique determines a larger value for the exchange anisotropy than the hysteresis loop technique. It is observed that Hea extracted via the AMR technique for the majority of samples shows a quantifiable dependence on the applied measurement field, even at fields much less than the exchange bias measured by hysteresis loop. It is believed that a reversible measurement of the soft dispersive moments takes place at these low fields, and that the field dependence is indicative of a distribution of the local magnetization direction.

Size and relative Co layer thickness dependence of magnetization flop in magnetic tunnel junctions exchange biased by Co/Ru/Co synthetic antiferromagnets

Computer simulation is used to examine the size and relative Co layer thickness dependence of the magnetization flop in magnetic tunnel junctions with the structure NiFe(I)(7.5 nm)/AlOx (0.7 nm)/Co(I)(y nm)/Ru(0.7 nm)/Co(II)(7−y nm)/FeMn(10 nm). The flop angle, which was obtained at a typical sputtering condition from an energy minimization, increases with the increase of y. The unidirectional pinning reduces the magnetization flop and its effect is greater when the Co thickness difference is smaller due to a larger energy contribution from the pinning term. Magnetoresistance hysteresis loops of “as-sputtered” samples, which were subjected to the magnetization flop, are found to vary significantly with the relative Co layer thickness, the cell size, and the magnitude of the unidirectional pinning.

Tailoring anisotropic magnetoresistance and giant magnetoresistance hysteresis loops with spin-polarized current injection

Current pulses were injected into magnetic nanowires. Their effect on the magnetoresistance hysteresis loops was studied for three morphologies: homogeneous Ni wires, copper wires containing five cobalt/copper bilayers, and hybrid structures composed of a homogeneous Ni half wire and a multilayered Co/Cu half wire. The characteristic features of the action of the current on the magnetization are shown and discussed.

Influence of annealing on magnetoresistance of Co/Cu multilayers

A correlation between magnetoresistance and magnetic properties of as-deposited and annealed Cu/Co multilayers, produced by magnetron sputtering, has been discussed. Two models have been proposed to describe the shapes of magnetoresistance and magnetization hysteresis loops: one taking into account the multidomain structure of the layers and another based on dissimilar magnetic properties of the Fe buffer layer and Co layers. A difference between the initial magnetoresistance and the hysteresis peak as a function of the annealing temperature has been explained in terms of magnetization processes. The annealing treatments of the films with 2.2-nm Cu spacer at temperatures between 300 and 340°C increased the magnetoresistance hysteresis peak to 43% compared to 30–35% for the as-deposited samples. This value remained high even after 1-h annealing at 380°C and the GMR at room temperature decreased only by 27% compared to the low temperature one.

In situ measurement of thickness dependence of magnetoresistance and magnetic hysteresis loops of ultrathin Co films on a SiO 2/Si(1 1 1) substrate

Ultrathin Co films ranging from 1 to 19 monolayers (ML) thick were grown by thermal evaporations on native-oxide-covered Si(1 1 1) surfaces in ultrahigh vacuum (UHV). The thickness-dependent magnetoresistance (MR) and thickness-dependent magnetic hysteresis loops were measured in situ by a collinear four-point probe and surface magneto-optical Kerr effect (SMOKE) techniques, respectively. The magnetoresistance of the Co films, measured with the applied magnetic field parallel to the film plane and perpendicular to the current direction, was obtained only when the Co film thickness reached ∼7 ML. The MR increased monotonically from ∼0.01% at ∼7 ML to ∼0.11% at ∼19 ML. The onset of measurable magnetic hysteresis loops from the Co film occurred at ∼3 ML, earlier than the onset thickness ∼7 ML for measurable MR. The coercivity H c of the Co film decreased with the film thickness d in the range of 3–19 ML and followed a power law H c∝ d − n with n=0.33±0.05. The MR change was attributed to scattering from domain walls. The coercivity decrease as a function of thickness was attributed to the decrease of surface domain-wall pinning.

Magnetoresistance of a single domain wall in Co and Ni nanowires

We report on the domain wall magnetoresistance of Co and Ni nanowires of 35 nm on diameter or less. The enhancement of the resistance due to an isolated domain wall is clearly evidenced from magnetoresistance hysteresis loops obtained under parallel and perpendicular fields. The domain walls magnetoresistance effect is by one order of magnitude smaller in Ni than in Co and it was observed only at low temperature for Ni nanowires. The formation and motion of domain walls is demonstrated by MFM experiments.

Magnetoresistance of Ferromagnetic Nanowires

Magnetoresistance of single Ni and Co nanowires, of about 60 nm in diameter and 6000 nm in length, was measured at room temperature. The full magnetoresistive hysteresis loops of single Ni nanowires, including the irreversible jump, are understood qualitatively, and major progress has been made towards their quantitative description, on the basis of anisotropic magnetoresistance. In contrast, the magnetoresistive hysteresis loops of single Co nanowires could not be described quantitatively, due to the presence of nucleation processes of domain walls or vortices.

Comparative experimental study of exchange biased symmetric and antisymmetric dual magnetoresistive read elements

In this work, the advantage of the antisymmetric dual magnetoresistive head (A-DMR) in comparison to the symmetric design (S-DMR), in terms of common mode longitudinal field rejection is demonstrated experimentally. The experiments were conducted at the wafer level. Magnetoresistive hysteresis loops of heads with the top permalloy longitudinally biased by TbCo exchange tabs, defined either by an ion-milling process or by lift-off, are presented. Initialization methods to have the symmetric or the antisymmetric state at low longitudinal applied magnetic fields are described.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Origin of interfacial energy in coupled films of permalloy and cobalt

The mechanism of magnetic coupling in multilayer films was investigated by analyzing the shape of the magnetoresistance hysteresis loops. A magnetic field was applied during the deposition of the layers in a high vacuum. Each film consisted of a soft layer of Ni-Fe and a hard layer of Co separated by intermediate laminations of SiO or Ag. The thickness of the intermediate lamination varied from 100 to about 5000 Å. The magnetoresistance hysteresis loop of the soft layer was displaced along the driving field axis. The displacement reaches a maximum value when the intermediate lamination has a thickness of about 700 Å and seems to be caused by a negative magnetic coupling. It was also found that the material of the intermediate lamination does not materially alter the characteristics of the films. Both phenomena were discussed in terms of magnetostatic interactions between uniaxial single-domain particles. The explanation for the interfacial energy based on long-range interactions points out that short-range interactions between the separated layers can be neglected.

Magnetoresistance below magnetization saturation in plastically deformed nickel down to liquid helium temperatures

A phenomenological analysis is outlined for the magnetoresistance effect in plastically deformed pure nickel below magnetization saturation down to liquid helium temperatures, which is based on the assumption of time dependent hysteresis phenomena. Both preferred orientation and induced uniaxial magnetic anisotropy are expected to be created by the plastic flow, when the deformation of the polycrystalline material is carried out by cold rolling with high reduction in the thickness of the sample. The magnetoresistance hysteresis loops can be ascribed to a magnetization process, which can be discussed from the viewpoint of a domain structure, in which a rotational process of reorientation of spins predominates. The field interval corresponding to non-saturated states of magnetization is higher than expected in ordinary bulk material, where no orientation and size effects are of importance for the magnetization reversal. The dynamic magnetoresistance loops performed at constant drive amplitude of the applied magnetic field, when a triangular waveform and low frequencies are used, are different from the static loops which can be described when taking into consideration the tensor of the electrical resistivity in a cubic ferromagnetic crystal. These deviations become appreciable at liquid helium temperatures, and as suggested in this analysis, they may be related to a mechanism of magnetization reversal having a relatively long switching time.

Effect of Cold Rolling on Magnetization Reversal of High-Purity Nickel Tapes

Dynamic magnetoresistance hysteresis loops were used for investigation of the magnetization reversal in cold-rolled tapes of high-purity nickel. The tapes were prepared by unidirectional rolling as well as by rolling with reversing tape end for end between passes. The loops were cycled in the range ±4500 Oe with a variety of frequencies f from 0.005 to 0.1 cps in the wide interval of temperatures from 297° to 4.2°K. When the magnetization field was transverse to the rolling axis, the coercive force could be expressed by Hc = A (lnf−lnf0)/T½, where A and f0 are in general dependent on the reduction of the tape thickness, and vary slightly with the temperature T. Such a relation may be related to a magnetization reversal occuring through a process of reorientation of spins, which can be described as a temperature-activated domain-wall relaxation. The magnetoresistance loops of the samples made by unidirectional cold rolling were asymmetric when they were performed by applying simultaneously two crossed magnetic fields, one ac and the other dc. A magnetic uniaxial anisotropy induced by this rolling seems to be responsible for the observed asymmetry, a possible mechanism for this being the deformation of crystals by slips.