Large Giant Magnetoresistance Research Articles

Large magnetocaloric effect and giant magnetoresistance in rare earth based intermetallic compound ErAl3: construction of magnetic phase diagram

This study explores the magnetic and magnetotransport behavior of polycrystalline ErAl3 compound. The polycrystalline compound adopts HoAl3-type structures with the R-3m space group, No. 166-2 and hR60 configurations. Multiple magnetic orderings and two field-induced metamagnetic transitions are observed. ErAl3 exhibits a significant magnetocaloric effect (MCE), −ΔSM = 15.25 J kg−1 K−1 and high relative cooling power of 383 J kg−1 with applied magnetic field change (ΔH) of 70 kOe near the paramagnetic to ferromagnetic transition, showcasing its potential for magnetic refrigeration technology. The compound also demonstrates metallic behavior, with a notable magnetoresistance of 48.5 % at 2 K due to the suppression of antiferromagnetism. The magnetic phase diagram reveals four distinct phases influenced by temperature and magnetic field, identified through the study of the MCE.

Vacuum barrier induced large spin polarization, giant magnetoresistance, and pure spin photocurrent in ferromagnetic zigzag graphene nanoribbons

Based on first-principle calculations combined with non-equilibrium Green’s function method, we investigate the spin-polarized transport properties of a two-probe device which consists of a vacuum barrier sandwiched between two semi-infinite ferromagnetic zigzag graphene nanoribbons (ZGNRs). It is well known that any vacuum barrier itself is not spin-polarized, and meanwhile, the infinite pristine ferromagnetic ZGNR presents no spin polarization in the transmission function around the Fermi level. However, our computational results indicate that robust large spin polarization is found in the proposed device. More interestingly, giant magnetoresistance up to can be obtained at low bias voltage. Furthermore, the photogalvanic effect-induced fully spin-polarized photocurrent or pure spin photocurrent can be generated in such device by illumination with polarized light, depending on the symmetry of structure. Consequently, our results indicate that vacuum barrier may show interesting spin-polarized transport properties in certain systems and should be taken into consideration in the construction of molecular junctions and spintronic devices.

Current-Perpendicular-to-Plane Giant Magnetoresistance Effect in van der Waals Heterostructures

Spin-dependent transport in a full van der Waals (vdW) giant magnetoresistance (GMR) junctions with the structure of Fe3GeTe2/XTe2/Fe3GeTe2 (X = Pt, Pd) has been investigated by using first-principles calculations. The ballistic conductance, magnetoresistance (MR) and resistance-area product (RA) have been calculated in a current-perpendicular-to-plane (CPP) geometry. A giant magnetoresistance of around 2000% and RA less than 0.3 {\Omega} {\mu}m2 have been found in the proposed vdW CPP GMR. In addition, the spin-orbit coupling effect on transport and anisotropy magnetoresistance (AMR) has also been investigated. The calculated AMR is found to be around 20% in Fe3GeTe2/trilayer-PdTe2/Fe3GeTe2 CPP GMR. Both GMR and AMR in the proposed vdW CPP GMR mainly originate from the bulk electronic structure properties of Fe3GeTe2. This work demonstrates a vdW CPP GMR with superior advantages including perpendicular magnetic anisotropy, large GMR, low RA as well as sizable AMR may stimulate future experimental explorations and should be appealing for their applications in spintronic devices including magnetic sensor and memory.

Enhanced unidirectional spin Hall magnetoresistance in a Pt/Co system with a Cu interlayer

Spin-dependent scattering at the nonmagnet/ferromagnet interface plays a key role in determining the amplitude of unidirectional spin Hall magnetoresistance (USMR), similar to giant magnetoresistance (GMR). We report the enhancement of USMR by inserting a thin Cu interlayer into the Pt/Co interface, where the Cu/Co system is well known to exhibit a large GMR. A measurement of the spin-orbit torque shows that the spin current injection into the Co layer is not modulated by the Cu interlayer. In addition, USMR increases with the Cu thickness for the ultrathin regime as the Cu/Co interface is formed, reaching a peak before decreasing, owing to the shunting effect. Our results suggest an interfacial origin of the enhanced USMR and highlight the close similarities between USMR and GMR.

Spin-polarized transport properties and spin molecular Boolean logic gates in planar four-coordinate Fe complex-based molecular devices with carbon nanotube bridges and electrodes

Based on density functional theory combined with Keldysh non-equilibrium Green’s function formalism, we investigate the spin-polarized transport properties of planar four-coordinate Fe complexes-based molecular devices with carbon nanotube bridges and electrodes. The results show that these systems can exhibit perfect spin filtering, large giant magnetoresistance and low-bias negative differential resistance effects with the external magnetic field modulation. The underlying mechanism is analyzed by spin-resolved transmission spectra, projected density of states, and spatial distributions of corresponding molecular projected self-consistent Hamiltonian orbitals. On the basis of these intriguing transport properties, we further design basic spin molecular Boolean logic gates, such as YES, NOT, AND, NAND, XOR, NXOR, OR and NOR logic gates.

Large negative giant magnetoresistance at room temperature and electrical transport in cobalt ferrite-polyaniline nanocomposites

At room temperature, a large negative out-of-plane magnetoresistance (MR), [R(H)-R(0)]/R(0), with a value of −35.76% at magnetic field of 9 T has been obtained in the cobalt ferrite (CoFe2O4)/polyaniline (PANI) nanocomposites with CoFe2O4 loading of 40.0 wt% prepared by surface initiated polymerization (SIP) method, which is strongly related to the weak localization (WL) model in the weak disordered system. The negative MR at room temperature in the CoFe2O4/PANI nanocomposites exhibits the obvious nanoparticle loading and magnetic field dependent properties. Both thermal activated transport model and Mott variable range hopping (VRH) model are applied to express the electrical transport mechanism for the temperature regimes of 180–290 K and 50–180 K, accordingly. The electrical transport in the CoFe2O4/PANI nanocomposites obeys the 3D VRH transport mechanism at low temperature range of 50–180 K. The estimated activation energy Eg for the CoFe2O4/PANI nanocomposites with different CoFe2O4 nanoparticle loadings of 10.0, 20.0, 40.0, and 60.0 wt% is 61, 63, 65, and 87 meV, respectively. The coating of PANI on the surface of CoFe2O4 nanoparticle reveals the significant effect on both the remanence and coercivity of CoFe2O4 nanoparticle.

Enhancement of giant magnetoresistance and oscillation by wave-vector filtering in Fe/Ag/Fe/InAs/Ag

The performance of a giant magnetoresistance (GMR) heterostructure Fe/Ag/Fe/InAs/Ag(100) in the current-perpendicular-to-plane geometry is presented. Calculations are based on a realistic tight-banding model with full $spd$ bands and the recursive Green's function algorithm. Results show that the system's GMR can reach values above 1000%. This GMR enhancement mainly is a result of the wave-vector filtering effect imposed by the InAs layer, restricting conductance within a small region around the $\mathrm{\ensuremath{\Gamma}}$ point in the 2D Brillouin zone. Calculations also reveal that when the Fermi level sits in the InAs band gap, MR gradually saturates as a function of InAs thickness with a smooth plateau; whereas when the Fermi level is in the InAs conduction band and close to the band bottom, GMR exhibits an oscillatory behavior with a large period. GMR oscillations are also observed with respect to Ag thickness, with oscillation amplitude determined by the Fermi level position relative to the InAs conduction band edge. The oscillation periods in both cases can be well explained by the concept of quantum-well states, and are determined by the spanning vector of the Fermi surface belly of the material whose thickness is varied. The observed GMR oscillations are due to the quantum interference of conduction electrons near the $\mathrm{\ensuremath{\Gamma}}$ point. The GMR and area-resistance (RA) product profiles at a wide range of Fermi energy positions relative to InAs bands are also compared. Near the GMR peak (with GMR above 1000%) in the conduction band, RA product can be as low as 8.$8\mathrm{\ensuremath{\Omega}}\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}{\text{m}}^{2}$. This feature of large GMR but small RA product results from the wave-vector filtering effect of doped InAs, and it makes the structure under study distinct from conventional GMR systems (small GMR, small RA) or magnetic tunnel junctions (large GMR, large RA).

Large spontaneous exchange bias and giant magnetoresistance in Ni50Mn37−xFexIn13(x=2–4) Heusler alloys

Abstract In the present work, we have obtained a large zero-field cooled exchange-bias (spontaneous exchange bias, SEB) in Ni 50 Mn 35 Fe 2 In 13 Heusler alloy. The experimental results indicate that the sample with x =2 exhibits super-spin glass (SSG), super-paramagnetic (SPM), super-ferromagnetic (SFM) and antiferromagnetic (AFM) behaviors in the martensite state at low temperature. Contributing to the complex magnetic interactions, a large SEB effect with the value of 1567 Oe was obtained at 5 K. At the same time, a non-monotonic behavior of spontaneous exchange bias field (spontaneous H EB ) was observed with the variation of temperature, which is resulted from the competition between the volume fraction of SFM clusters and the exchange coupling of the SFM–AFM interface. In addition, during martensitic transformation (MT), extraordinary electrical transport properties of Ni 50 Mn 37− x Fe x In 13 ( x =2–4) alloys have been observed under various external magnetic field. The maximal value of the giant magnetoresistance (GMR) reaches about 57% at 135 K under the external magnetic field change of 50 kOe. The effect of field induced reverse martensitic transformation (FIRMT) on the GMR has been also discussed.

High power all-metal spin torque oscillator using full Heusler Co2(Fe,Mn)Si

We showed the high rf power (Pout) emission from an all-metal spin torque oscillator (STO) with a Co2Fe0.4Mn0.6Si (CFMS)/Ag/CFMS giant magnetoresistance (GMR) stack, which was attributable to the large GMR effect thanks to the highly spin-polarized CFMS. The oscillation spectra were measured by varying the magnetic field direction, and the perpendicular magnetic field was effective to increase Pout and the Q factor. We simultaneously achieved a high output efficiency of 0.013%, a high Q of 1124, and large frequency tunability. CFMS-based all-metal STO is promising for overcoming the difficulties that conventional STOs are confronted with.

Large positive spin polarization and giant inverse tunneling magnetoresistance in Fe/PbTiO3/Fe multiferroic tunnel junction

We perform first-principles electronic structure and spin-dependent transport calculations of a multiferroic tunnel junction (MFTJ) with an epitaxial Fe/PbTiO3/Fe heterostructure. We predict a large positive spin-polarization (SP) and an intriguing giant inverse tunneling magnetoresistance (TMR) ratio in this tunnel junction. We demonstrate that the tunneling properties are determined by ferroelectric (FE) polarization screening and electronic reconstruction at the interface with lower electrostatic potential. The intricate complex band structure of PbTiO3, in particular the lowest decay rates concerning Pb 6pz and Ti 3dz2 states near the Γ¯ point, gives rise to the large positive SP of the tunneling current in the parallel magnetic configuration. However, the giant inverse TMR ratio is attributed to the minority-spin electrons of the interfacial Ti 3dxz+3dyz orbitals which have considerably weight in the extended area around the Γ¯ point at the Fermi energy and causes remarkable contributions to the conductance in the antiparallel magnetic configuration.

High power radio frequency oscillation by spin transfer torque in a Co2MnSi layer: Experiment and macrospin simulation

We experimentally and numerically investigated rf oscillation induced by spin transfer torque in a current-perpendicular-to-plane giant magnetoresistance (GMR) device with full-Heusler Co2MnSi (CMS) layers. High output power (Prf) of 1 nW was experimentally achieved owing to the large GMR effect resulting from the half-metallic feature of the CMS layers. However, the high power rf oscillation was observed only in the narrow dc current (Idc) region. Macrospin simulation suggested that the high spin polarization of CMS layers led to narrowing the optimum Idc region for the rf oscillation.

Influence of Cu deposition potential on the giant magnetoresistance and surface roughness of electrodeposited Ni–Co/Cu multilayers

It has been shown previously for electrodeposited Co/Cu multilayers that the single-bath electrodeposition process can be optimized from an electrochemical point of view in order to avoid unwanted Co dissolution and incorporation of Co in the non-magnetic layer during the Cu deposition pulse. In the present work, electrodeposition of Ni–Co/Cu multilayers has been studied to clarify if the same optimization method is appropriate when two magnetic elements are present and if this potential results in the largest giant magnetoresistance (GMR) for the particular alloy system studied. For this purpose, several Ni–Co/Cu multilayers were prepared by varying the deposition potential of the Cu layer. The composition analysis of the deposits showed that the Ni:Co ratio exhibits a minimum as a function of the Cu deposition potential, which can be explained by considering both the dissolution of Co and the mass transport of the reactants. Both the saturation GMR value and the intensity of the satellite peaks in the X-ray diffractograms were highly correlated with the resulting surface roughness of the deposits which was strongly varying with the Cu deposition potential. Higher GMR values, lower saturation fields and more perfect multilayer structure were observed for sufficiently positive Cu deposition potentials only which enabled a partial Co dissolution resulting in a reduced surface roughness. The results draw attention to the complexity of the optimization procedure of the deposition of multilayers with several alloying components.

Influence of Spin Polarizer on the Magnetoresistance, Switching Property, and Interlayer Interactions in Co/Pd Single Spin Valves

In order to investigate the effect of polarization and saturation magnetization ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">M</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">S</sub> ) in Co/Pd multilayers-based pseudo-spin valve (PSV) structures with perpendicular magnetic anisotropy (PMA), we studied different spin polarizer layers (SPL). The PMA could be achieved by tuning the thickness and M <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">s</sub> of the SPL. Sharp switching of magnetization for both soft layer (SL) and hard layer (HL) were observed for all studied structures with thin SPL, indicating higher PMA in the film structures. However, interesting behavior was observed for thicker SPLs where a tail in hysteresis loop was observed. For further understanding of the interactions in multilayers we have carried out first-order reversal curve (FORC) measurements. The FORC contours pointed out the correlation between exchange interactions with magnetostatic interaction and minor loop shifts in hysteresis curves. Giant magnetoresistance (GMR) was observed for the PSV films with thick Co <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">65</sub> Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">35</sub> (10 Å and 15 Å) SPL. However, GMR value as a function of thickness decreases for all the other films with different <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">M</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">s</sub> . Moreover, we have clarified that GMR is correlated to the fcc (111) orientation of Co/Pd multialyers. The maximum value of GMR with largest <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">M</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">s</sub> shows the minimum value of full width at half maximum (FWHM) for fcc (111) peak, indicating that the large GMR as well as high PMA occurs for the samples deposited in good fcc (111) texture.

Electronic and magnetic structure of CrO2-RuO2interfaces

CrO${}_{2}$ and RuO${}_{2}$ share the same (rutile) crystal structure and have similar lattice constants. We have used density functional theory (DFT) within the generalized gradient approximation (GGA) to calculate the electronic and magnetic structure of CrO${}_{2}$, RuO${}_{2}$, and their interfaces. We also used DFT-GGA to investigate the electronic and magnetic structure of CrO${}_{2}$ and RuO${}_{2}$. Consistent with previous calculations and experiments, we find that the CrO${}_{2}$ Fermi energy lies in a band gap for the minority channel. RuO${}_{2}$, in agreement with experiment, is predicted to be nonmagnetic. We find relatively good matching between the majority of the energy bands of CrO${}_{2}$ and both RuO${}_{2}$ channels in the (100), (110), and (001) directions. For (100) interfaces, we find a small induced Ru moment oriented opposite to that of the Cr moments. We study the change in energy as a function of the angle between the interfacial and bulk Cr magnetic moments. We investigate both sharp and mixed (100)- and (110)-oriented CrO${}_{2}$-RuO${}_{2}$ interfaces with a supercell approach. We investigate the origin of the large negative moment that forms when an Ru ion substitutes for a Cr ion. We speculate that weakened interionic exchange interactions, noncollinear spins, and induced moments at interfaces may be a common problem in oxides that may be a challenge to overcome for achieving large giant magnetoresistance and tunneling magnetoresistance effects.

Giant magnetoresistance in IrMn based antiferromagnetic spin valve

According to the parameter-free first principles calculations, we investigate the spin polarized transport in antiferromagnetic spin valve (AFSV) based on noncollinear IrMn. The giant magnetoresistance (GMR) in Co/IrMn/Cu(111) with L12-type IrMn shows three-fold rotational symmetry, which is easy to be distinguished from the GMR of normal ferromagnetic spin valves. Moreover, GMR based on -phase IrMn with current-in-plane (CIP) structure shows that GMR is 7.7%, around two time larger than that in current-pendicular-to-plane (CPP) structure (3.4%). Our study demonstrates that the AFSV CIP structure possesses a larger GMR effect than the CPP structure, and the GMR effect in AFSV based on noncollinear antiferromagnetic structure is larger than that based on collinear antiferromagnetic structure such as FeMn.

Growth and characterisation on giant magnetoresistance property of metallic multilayers

Metallic multilayers with specific materials configurations have been shown to exhibit giant magnetoresistance effect at room temperature. In this study, microstructural analysis was carried out on magnetron sputtered Co/Cu multilayers using various diffraction and imaging techniques. Dominant characteristic features associated with the multilayers, such as lateral and vertical columnar grain orientations as well as layer undulation and regularity were identified. By deliberately introducing microstructural changes using a buffer layer and via heat treatment, detailed microstructural analysis carried out on the multilayers has provided an insight into the dependence of giant magnetoresistance on the observed microstructures. The present study shows that high giant magnetoresistance effect of the multilayers was associated with highly correlated interfacial profiles, sharp columnar grain boundaries and high degree of lateral coherency in columnar grain growth. While the introduction of iron buffer layer favoured the growth of these structural features that were responsible for large giant magnetoresistance effect, annealing led to deterioration of giant magnetoresistance property of the multilayers.

Giant magnetoresistance in electrodeposited Co-Cu/Cu multilayers: Origin of the absence of oscillatory behavior

A detailed study of the evolution of the magnetoresistance was performed on electrodeposited Co/Cu multilayers with Cu layer thicknesses ranging from 0.5 nm to 4.5 nm. For thin Cu layers (up to 1.5 nm), anisotropic magnetoresistance (AMR) was observed whereas multilayers with thicker Cu layers exhibited clear giant magnetoresistance (GMR) behaviour. The GMR magnitude increased up to about 3.5 to 4 nm Cu layer thickness and slightly decreased afterwards. According to magnetic measurements, all samples exhibited ferromagnetic (FM) behaviour. The relative remanence turned out to be about 0.75 for both AMR and GMR type multilayers. This clearly indicates the absence of an antiferromagnetic (AF) coupling between adjacent magnetic layers for Cu layers even above 1.5 nm where the GMR effect occurs. The AMR behaviour at low spacer thicknesses indicates the presence of strong FM coupling (due to, e.g., pin-holes in the spacer and/or areas of the Cu layer where the layer thickness is very small). With increasing spacer thickness, the pin-hole density reduces and/or the layer thickness uniformity improves which both lead to a weakening of the FM coupling. This improvement in multilayer structure quality results in a better separation of magnetic layers and the weaker coupling (or complete absence of interlayer coupling) enables a more random magnetization orientation of adjacent layers, all this leading to an increase of the GMR. Coercive field and zero-field resistivity measurements as well as the results of a structural study reported earlier on the same multilayers provide independent evidence for the microstructural features established here. The large GMR reported previously on such Co/Cu multilayers at Cu layer thicknesses around 1 nm can be attributed to the presence of a fairly large superparamagnetic (SPM) fraction rather than being due to a strong AF coupling.

High spin polarization in Co2CrAl–Cr superlattice

The electronic structure, magnetic properties and interface effects in Co2CrAl–Cr superstructures have been investigated by the use of first principle calculations. The results show that at the interface, a large magnetic moment and a high spin polarization can be induced by a strong ferromagnetic exchange interaction at the Cr–Co interface. However, at the CrAl–Cr interface, both the magnetic moment and the spin polarization of the Cr atoms are decreased due to a Cr–Cr antiferromagnetic interaction. It can also be found that the interface effect is only a short range effect. So, high spin polarization in Co2CrAl–Cr superlattice can be obtained. Based on this theoretical analysis, a large giant magnetoresistance effect can be expected in the Co2CrAl–Cr superlattices with proper thickness of the Cr spacer layers.

Transmission electron microscopy and X-ray diffraction study of microstructural evolution in magnetoresistive Cu–Fe–Ni ribbons

The evolution of the microstructure of a granular Cu80Fe10Ni10 (at%) melt-spun ribbon is studied by transmission electron microscopy (TEM), energy-filtered transmission electron microscopy (EFTEM) and X-ray diffraction. This system is interesting as large giant magnetoresistance (GMR) values have been measured for this composition. We have shown the presence of two face-centred cubic phases, an (Fe,Ni)-rich phase and a Cu-rich phase. The lattice parameters of these two phases are close and no diffraction or elastic contrast is involved in displaying the two phases in TEM bright-field mode. With EFTEM imaging, we have shown the presence of a fine-scale (Fe,Ni)-rich precipitation inside the Cu-rich fcc matrix. The precipitates are 2–4 nm in the as-spun state and 4–6 nm after annealing for 2 h at 400°C. The lattice parameter of the Cu-rich phase in the as-spun sample is 0.3608 nm and 0.3610 nm for the (Fe,Ni)-rich phase. After a 24-h annealing treatment at 600°C, the mean diameter of the particle is 20 nm and the lattice parameter of the (Fe,Ni)-rich phase has decreased to 0.3600 nm, while that of the Cu-rich phase has increased to 0.3613 nm, which is consistent with a segregation of Fe and Ni in the precipitates. The composition and volume fraction of the two phases measured for this annealed sample are in good agreement with the Thermocalc® predictions.

A novel spin transfer torque effect in Ag2Co granular films

We studied a spin transfer torque (STT) effect in Ag2Co granular films, induced by a high current density 108 - 9 A cm− 2 injected via a point contact. The system consists of single domain Co nanoparticles randomly embedded in Ag matrix, with a mean distance corresponding to the typical quantity of layer thicknesses used for the nonmagnetic space in multilayer nanopillars. A large giant magnetoresistance (GMR) effect of 55% measured at 4.2 K is an indication for high spin scattering anisotropy which is required for STT observations. Supposedly, a certain amount of large-sized Co particles saturated in external magnetic field H acts here as the spin polarizer for the injected current I and small-sized particles unsaturated at 4.2 K and Hmax (90 kOe) even act as the detector for switching. A novel STT effect was observed thereby as I rises across a threshold value Ic, showing a sharp decrease in R (ΔR/R = 130% with two steps), which arises accordingly from further alignment of the small-sized Co granules. The behavior is polar and hysteretic, similar to properties measured for multilayer nanopillars. The two-step behavior could be an effect related to a lognormal particle size distribution. Depending on the spin polarization, Ic is found to be field disproportional, indicating a larger STT efficiency at a higher H.