Giant Magnetoresistance Response Research Articles

Effect of Ni addition on CPP-GMR response in electrodeposited Co-Ni/Cu multilayered nanocylinders with an ultra-large aspect ratio

Effect of Co–Ni alloy composition on the current perpendicular-to-plane giant magnetoresistance (CPP-GMR) response of electrochemically synthesized Co–Ni/Cu multilayered nanocylinders was studied using anodized aluminum oxide membranes (AAOM) with nanochannel diameter D ∼67 nm and length L ∼70 μm. Co–Ni/Cu multilayered nanocylinders, which have an aspect ratio L/D of ∼1,045, were fabricated in the AAOM nanochannel templates by utilizing a pulse-current electrochemical growth process in an electrolytic bath with Co2+, Ni2+ and Cu2+ ions. Co–Ni/Cu alternating structure with Co84Ni16 alloy layer-thickness of 9.6 nm and Cu layer-thickness of 3.8 nm was clearly observed in a nanocylinder with a diameter of 63 nm. The alternating structure was composed from crystalline layers with preferential orientations in hcp-CoNi (002) and fcc-Cu (111). The Co–Ni/Cu multilayered nanocylinders were easily magnetized in the long axis direction because of the extremely large aspect ratio L/D. In Co84Ni16/Cu multilayered nanocylinders, the coercivity and squareness were ∼0.46 kOe and ∼0.5, respectively. The CPP-GMR value was achieved up to 22.5% (at room temperature) in Co84Ni16/Cu multilayered nanocylinders.

Magneto-Transport in Flexible 3D Networks Made of Interconnected Magnetic Nanowires and Nanotubes.

Electrochemical deposition of interconnected nanowires and nanotubes made of ferromagnetic metals into track-etched polycarbonate templates with crossed nanochannels has been revealed suitable for the fabrication of mechanically stable three-dimensional magnetic nanostructures with large surface area. These 3D networks embedded into flexible polymer membranes are also planar and lightweight. This fabrication technique allows for the control of the geometric characteristics and material composition of interconnected magnetic nanowire or nanotube networks, which can be used to fine-tune their magnetic and magneto-transport properties. The magnetostatic contribution to the magnetic anisotropy of crossed nanowire networks can be easily controlled using the diameter, packing density, or angle distribution characteristics. Furthermore, the fabrication of Co and Co-rich NiCo alloy crossed nanowires with textured hcp phases leads to an additional significant magnetocrystalline contribution to the magnetic anisotropy that can either compete or add to the magnetostatic contribution. The fabrication of an interconnected nanotube network has also been demonstrated, where the hollow core and the control over the tube wall thickness add another degree of freedom to control the magnetic properties and magnetization reversal mechanisms. Finally, three-dimensional networks made of interconnected multilayered nanowire with a succession of ferromagnetic and non-magnetic layers have been successfully fabricated, leading to giant magnetoresistance responses measured in the current-perpendicular-to-plane configuration. These interconnected nanowire networks have high potential as integrated, reliable, and stable magnetic field sensors; magnetic devices for memory and logic operations; or neuromorphic computing.

Charge-Transfer-Induced Interfacial Exchange Coupling at the Co/BiFeO3 Interface

The interplay between ferroelectricity and magnetism in multiferroic materials is of great scientific and technological interest, allowing magnetic control of ferroelectric properties and electric control of magnetic properties through the magnetoelectric coupling from interfacial strain, exchange bias, or charge-transfer process. We report the charge transfer at the Co/BiFeO3 interface from which the interfacial exchange coupling is achieved with the formation of Fe2+. The x-ray linear dichroism and x-ray magnetic circular dichroism results reveal that the strain as well as the exchange coupling J(ex) in BiFeO3 (BFO) with periodic 109 degrees domains are stronger than that in BFO with 71 degrees domains, resulting in the higher coercive field of Co/BFO with 109 degrees domain samples. The possible electric field control of the charge-transfer process at the Co/BFO interface enables an alternative class of electrically controllable magnetization, exchange bias, and giant magnetoresistive response of spintronic devices.

Tunable Spin Characteristic Properties in Spin Valve Devices Based on Hybrid Organic-Inorganic Perovskites.

The hybrid organic-inorganic perovskites (HOIPs) form a new class of semiconductors which show promising optoelectronic device applications. Remarkably, the optoelectronic properties of HOIP are tunable by changing the chemical components of their building blocks. Recently, the HOIP spintronic properties and their applications in spintronic devices have attracted substantial interest. Here the impact of the chemical component diversity in HOIPs on their spintronic properties is studied. Spin valve devices based on HOIPs with different organic cations and halogen atoms are fabricated. The spin diffusion length is obtained in the various HOIPs by measuring the giant magnetoresistance (GMR) response in spin valve devices with different perovskite interlayer thicknesses. In addition spin lifetime is also measured from the Hanle response. It is found that the spintronic properties of HOIPs are mainly determined by the halogen atoms, rather than the organic cations. The study provides a clear avenue for engineering spintronic devices based on HOIPs.

Topological Insulator GMR Straintronics for Low-Power Strain Sensors.

A quantum spin Hall insulator, i.e., topological insulator (TI), is a natural candidate for low-power electronics and spintronics because of its intrinsic dissipationless feature. Recent density functional theory and scanning tunneling spectroscopy experiments show that the mechanical strain allows dynamic, continuous, and reversible modulations of the topological surface states within the topological phase and hence opens prospects for TI straintronics. Here, we combine the mechanical strain and the giant magnetoresistance (GMR) of a ferromagnet-TI (FM-TI) junction to construct a novel TI GMR straintronics device. Such a FM-strained-FM-TI junction permits several energy spectral ranges for 100% GMR and a robust strain-controllable magnetic switch. Beyond the 100% GMR energy range, we observe a strain-modulated oscillating GMR, which is an alternative hallmark of the Fabry-Pérot quantum interference of Dirac surface states. These strain-sensitive GMR responses indicate that FM-strained-FM-TI junctions are very favorable for practical applications for low-power nanoscale strain sensors.

Ultra-low-pressure sputtering to improve exchange bias and tune linear ranges in spin valves

A series of CoFe/IrMn exchange bilayers was grown by DC-sputtering at different ultra-low argon pressures ranging from 0.008 to 0.1Pa. This pressure range was one to two orders lower than the normal sputtering pressure. Results revealed that the exchange bias increased from 140 to 250Oe in CoFe(10nm)/IrMn (15nm) bilayers of fixed thickness because of the improved crystalline structure and morphological uniformity of films. Since ferromagnetic /antiferromagnetic (FM/AF) bilayers are always used in linear magnetic sensors as detection layers, the varying exchange bias can successfully achieve tunable linear range in a crossed pinning spin valve. The linear range could be adjustable from −80Oe – +80Oe to −150Oe – +150Oe on the basis of giant magnetoresistance responses. Therefore, this method provides a simple method to tune the operating range of magnetic field sensors.

Linear response to a heat-driven spin torque

The existence of a heat-driven spin torque is demonstrated using Co/Cu/Co spin valves embedded in metallic nanowires. Heat currents flowing in one direction or its opposite were obtained by heating optically one end or the other of the nanowires. The spin torque associated with the heat-driven spin current pushes the magnetization out of equilibrium, resulting in a change of the magnetoresistance, which is detected using a charge current small enough not to cause heating or induced fields of any significance. The giant magnetoresistance response to this torque peaks with the magnetic susceptibility, whereas the spurious signal coming from the temperature dependence of the resistance produces merely a field independent baseline.

COMPARATIVE STUDY OF SPIN INJECTION AND TRANSPORT IN Alq3 AND Co–PHTHALOCYANINE-BASED ORGANIC SPIN VALVES

Recent experimental reports suggest the formation of a highly spin-polarized interface ("spinterface") between a ferromagnetic (FM) Cobalt ( Co ) electrode and a metal-phthalocyanine (Pc) molecule. Another report shows an almost 60% giant magnetoresistance (GMR) response measured on Co / H 2 Pc -based single molecule spin valves. In this paper, we compare the spin injection and transport properties of organic spin valves with two different organic spacers, namely Tris(8-hydroxyquinolinato) aluminum ( Alq 3) and CoPc sandwiched between half-metallic La 0.7 Sr 0.3 MnO 3 (LSMO) and Co electrodes. Alq 3-based spin valves exhibit clear and reproducible spin valve switching with almost 35% negative GMR at 10 K, in accordance with previous reports. In contrast, cobalt-pthalocyanine ( CoPc )-based spin valves fail to show clear GMR response above noise level despite high expectations based on recent reports. Investigations of electronic, magnetic and magnetotransport properties of electrode/spacer interfaces of LSMO/ CoPc / Co devices offer three plausible explanations for the absence of GMR: (1) CoPc films are strongly chemisorbed on the LSMO surface. This improves the LSMO magnetic properties but also induces local traps at the LSMO interface for spin-polarized charge carriers. (2) At the Co / CoPc interface, diffusion of Co atoms into the organic semiconductor (OS) layer and chemical reactivity between Co and the OS deteriorates the FM properties of Co . This renders the Co / CoPc interface as unsuitable for efficient spin injection. (3) The presence of heavy Co atoms in CoPc leads to large spin–orbit coupling in the spacer. The spin relaxation time in the CoPc layer is therefore considerably smaller compared to Alq 3. Based on these findings, we suggest that the absence of GMR in CoPc -based spin valves is caused by a combined effect of inefficient spin injection from FM contacts and poor spin transport in the CoPc spacer layer.

Giant magnetoresistance in cluster-assembled nanostructures: on the influence of inter-particle interactions

The giant magnetoresistance response of granular systems has since its discovery been described by a simple model based on the geometric orientation of the magnetic moments of adjacent nanoparticles. This model has been proven quite successful in many cases but its being based on decoupled neighboring grains has never been verified as all available studies rely on samples with too high concentration. Here we report on magnetic and magnetotransport measurements of cluster-assembled nanostructures with cobalt clusters around 2.3 nm diameter embedded in copper matrices at different concentrations. The thorough magnetic characterization based on the recently developed “triple fit” method allows the detection of measurable inter-particle interactions and thus assures true superparamagnetic behavior in the most dilute sample. The spintronic response is compared to theory and we show that only at low concentration (0.5 at.% Co) all experiments are consistent and the common theoretical description is appropriate. Increasing the concentration to 2.5 and 5 at.% implies deviations between magnetometry and magnetotransport.

Crafting the magnonic and spintronic response of BiFeO3 films by epitaxial strain

Multiferroics are compounds that show ferroelectricity and magnetism. BiFeO3, by far the most studied, has outstanding ferroelectric properties, a cycloidal magnetic order in the bulk, and many unexpected virtues such as conductive domain walls or a low bandgap of interest for photovoltaics. Although this flurry of properties makes BiFeO3 a paradigmatic multifunctional material, most are related to its ferroelectric character, and its other ferroic property--antiferromagnetism--has not been investigated extensively, especially in thin films. Here we bring insight into the rich spin physics of BiFeO3 in a detailed study of the static and dynamic magnetic response of strain-engineered films. Using Mössbauer and Raman spectroscopies combined with Landau-Ginzburg theory and effective Hamiltonian calculations, we show that the bulk-like cycloidal spin modulation that exists at low compressive strain is driven towards pseudo-collinear antiferromagnetism at high strain, both tensile and compressive. For moderate tensile strain we also predict and observe indications of a new cycloid. Accordingly, we find that the magnonic response is entirely modified, with low-energy magnon modes being suppressed as strain increases. Finally, we reveal that strain progressively drives the average spin angle from in-plane to out-of-plane, a property we use to tune the exchange bias and giant-magnetoresistive response of spin valves.

Decay in spin diffusion length with temperature in organic semiconductors—An insight of possible mechanisms

This article presents a comparison of spin transport mechanism in two π-conjugated organic polymers namely, regiorandom and regioregular poly (3-hexyl thiophenes) with same elemental composition but different regioregularity of the constituent atoms leading to different crystallinity and charge carrier mobility. Spin-valve devices made with both polymers show substantial low temperature giant magnetoresistance (GMR) response. However, the GMR signal decreases drastically at higher temperatures where charge carrier mobility is higher. Our results suggest that in both the polymers spin diffusion length at low temperature is almost similar, but, temperature dependence of spin diffusion length is greater in the disordered polymer compared to the more structured one. Comprehensive analysis of our experimental data suggest that at low temperature, in the VRH hopping regime (5–50K), spin relaxation due to hyperfine interaction and Elliot-Yafet momentum scattering is the dominant spin relaxation mechanism while in the thermally activated regime Dyakonov–Perel mechanism contribution becomes significant. However, mobility dependence of spin scattering rate in both systems differ from traditional Dyakonov–Perel model signifying that there are coexisting contributions from several spin scattering effects present in the system. Proper understanding and careful modification of spin–orbit coupling in organic semiconductors can be very useful for organic based spin devices.

Influence of nanoparticular impurities on the magnetic anisotropy of self-assembled magnetic Co-nanoparticles

A suspension of monodisperse Au-particles of either 3 or 6nm were mixed with a dilution of 6nm Co-particles. The resulting mixture was employed for the formation of granular films and the transport properties of these assemblies were analyzed. An increased granular giant magnetoresistive response was observed for samples with a high content of Au-particles. The experimental data were compared to numeric solutions of the Landau–Lifshitz–Gilbert equation for discrete magnetic moments. The alteration of the magnetic properties can be related to the formation of a nanoparticular structure resulting from the minimization of the particle stray fields.

Study of surface effects on CoCu nanogranular alloys by ferromagnetic resonance

Room temperature ferromagnetic resonance measurements have been performed on Co5Cu95 melt-spun nanogranular alloys. Results on the ratio of the orbital-to-spin magnetic moment have been compared to previous results obtained by x-ray magnetic circular dichroism, explaining the differences in terms of the degree of magnetic saturation of the samples. Finally, the ferromagnetic resonance results have been related to the giant magnetoresistance (GMR) response of the samples at each stage of annealing, which has been interpreted as a sign of the interfacial origin of the GMR phenomenon.

Domain-wall-induced magnetoresistance in pseudo-spin-valve/superconductor hybrid structures

We have studied the interaction between magnetism and superconductivity in a pseudo-spin-valve structure consisting of a Co/Cu/Py/Nb layer sequence. We are able to control the magnetization reversal process and monitor it by means of the giant magnetoresistance effect during transport measurements. By placing the superconducting Nb-film on the top of the permalloy (Py) electrode instead of putting it in between the two ferromagnets, we minimize the influence of spin scattering or spin accumulation onto the transport properties of Nb. Magnetotransport data reveal clear evidence that the stray fields of domain walls (DWs) in the pseudo-spin-valve influence the emerging superconductivity close to the transition temperature by the occurrence of peak-like features in the magneto-resistance characteristic. Direct comparison with magnetometry data shows that the resistance peaks occur exactly at the magnetization reversal fields of the Co and Py layers, where DWs are generated. For temperatures near the superconducting transition the amplitude of the DW-induced magnetoresistance increases with decreasing temperature, reaching values far beyond the size of the giant magnetoresistive response of our structure in the normal state.

The hyperfine interaction role in the spin response of π-conjugated polymer films and spin valve devices

Although the hyperfine interaction (HFI) has been foreseen to play an important role in organic spin response, clear experimental evidence for it has been scarce. We studied and compared spin dynamics in films and organic spin-valve (OSV) devices based on π-conjugated polymers made of protonated, H-, deuterated, D-hydrogen (having a weaker HFI strength, a HFI), and 13C-rich chains (having stronger a HFI). We demonstrate that HFI indeed plays a crucial role in the spin dynamics of all three polymer isotopes. Films based on the D-polymer show substantial narrower optically detected magnetic resonance of spin ½ polarons; whereas due to the longer spin diffusion, OSV devices based on D-polymers show substantially larger magnetoresistance. We also found that the giant magnetoresistance (GMR) steep temperature dependence in OSV devices is isotope independent, showing that is due to the magnetic response of the ferromagnetic electrodes. In addition, we found that the GMR steep voltage dependence is isotope dependent indicating that is due to a spin injection process at the electrodes, rather than spin transport through the organic interlayer. Finally we report GMR response in OSV devices made of C 60 interlayer having very weak HFI. These devices show sharp GMR response, and may therefore be excellent candidates for room temperature operation.

A General-Purpose Phenomenological Micromagnetic <newline/>Model for Modern Giant Magnetoresistive Devices

A phenomenological model for calculating the giant magnetoresistive (GMR) response of magnetic systems and devices is described. The model can easily be incorporated with minor revisions into most micromagnetic models and accounts for dependencies on sample size, shape and internal electrical current and magnetization distributions. The application of the model to the computation of the MR response of a granular thin-film sample is presented.

Giant magnetoresistance response by the π–d interaction in an axially ligated phthalocyanine conductor with two-dimensional π–π stacking structure

Electrochemical oxidation of [MIII(Pc)(CN)2]− (M = Co and Fe, Pc = phthalocyaninato) and peri-xanthenoxanthene (PXX) in acetonitrile yields partially oxidized salts of [PXX]2[M(Pc)(CN)2]·CH3CN. In the crystal, the Pc ring is formally oxidized by 0.5 e and forms a two-dimensional double-sheet layer. These crystals show semiconducting behavior with small activation energy under ambient pressure, but show metallic behavior under high pressures. When M = Co, the metallic behavior is maintained down to 5 K. On the other hand, in the π–d system with M = Fe, the metallic behavior is disrupted by a steep increase in the resistivity at low temperatures. In this high resistivity state, negative differential resistance appears in the measurements of current–voltage characteristics, suggesting the development of charge disproportionation due to the π–d interaction. The magnetic susceptibility for M = Fe reflects anisotropic molecular magnetism of the Fe(Pc)(CN)2 unit, and shows an anomaly due to antiferromagnetic interaction between the Fe d-spins. The increase in the resistivity at low temperature under high pressures is suppressed by the application of a magnetic field. The magnitude of the negative magnetoresistance effect is extremely large; −99.8% was recorded at a field of 15 T at 3 K under 12 kbar.

Fabrication and testing of a CoNiCu/Cu CPP-GMR nanowire-based microfluidic biosensor

Giant magneto resistance (GMR)-based microfluidic biosensors are used in applications involving the detection, analysis, enumeration and characterization of magnetic nano-particles attached to biological mediums such as antibodies and DNA. Here we introduce a novel multilayered CoNiCu/Cu nanowire GMR-based microfluidic biosensor. The current perpendicular to the plane of multilayers (CPP)-nanowires GMR was used as the core sensing material in the biosensor which responds to magnetic fields depending on the concentration and the flow velocity of bio-nano-magnetic fluids. The device was tested with different control solutions such as DI-water, mineral oil, phosphate buffered saline (PBS), ferrofluid, polystyrene superparamagnetic beads (PSB) and Dynabeads sheep anti-rabbit IgG. The nanowire array resistance decreased with an increase in the ferrofluid concentration, and a maximum 15.8% relative GMR was observed for the undiluted ferrofluid. The sensor was also responding differently to various ferrofluid flow rates. The GMR device showed variation in the output signal when the PSB and Dynabeads of different dilutions were pumped through it. When the tests were performed with pulsing potentials (150 mV and 200 mV), an increased GMR response was identified at higher voltages for PSB and Dynabeads sheep anti-rabbit IgG.

Co layer fragmentation effect on magnetoresistive and structural properties of nanogranular Co/Cu multilayers

We deposited nanogranular Co/Cu multilayers made of thin fragmented Co layers separated by thicker Cu layers to study how the structure and the microstructure of magnetic nanogranular samples change as the average particle size is reduced and how these changes affect the giant magnetoresistive response of the samples. Indeed, thanks to the vertical periodicity of the structure, namely, to the fact that Co/Cu interfaces display an ordered stacking and are not randomly distributed within the samples as in conventional granular materials, their self-correlation and cross correlation can be investigated. In this way, the characteristic length scale of the Co/Cu interfacial roughness that is strictly related to the giant magnetoresistive response of the samples and the universality class of the growth mechanism that affects the systems structure can be both accessed. The Co/Cu nanogranular multilayers were characterized using different x–ray techniques, from specular reflectivity, which allows to probe the multilayer development in the vertical direction, to grazing incidence small angle diffuse scattering, which provides information on the self-correlation and cross correlation of the Co/Cu interfaces. Furthermore, diffraction measurements indicate that the degree of structural disorder increases by decreasing the thickness of the Co layers. Magnetoresistive and magnetization measurements are as well presented and discussed with the results of the structural characterization.

Magnetoresistance behavior of elliptical ring nanomagnets in close proximity with magnetic elements

We have investigated the giant magnetoresistance (GMR) responses of the pseudospin valve elliptical rings in close proximity with individual magnetic elements. Significant modifications of the GMR responses were observed due to the effects of magnetostatic coupling between the rings and the magnetic elements. We observed that the vortex state stability is significantly sensitive to the position, orientation of the magnetic elements, and the direction of the applied field. We also investigated the switching field reproducibility of the rings for onion→vortex and vortex→onion transitions and observed that the ring with asymmetrically placed magnetic elements shows the most reproducible switching states.