Tunneling Magnetoresistance Effect Research Articles

Supramolecular spin valves

Magnetic molecules are potential building blocks for the design of spintronic devices. Moreover, molecular materials enable the combination of bottom-up processing techniques, for example with conventional top-down nanofabrication. The development of solid-state spintronic devices based on the giant magnetoresistance, tunnel magnetoresistance and spin-valve effects has revolutionized magnetic memory applications. Recently, a significant improvement of the spin-relaxation time has been observed in organic semiconductor tunnel junctions, single non-magnetic molecules coupled to magnetic electrodes have shown giant magnetoresistance and hybrid devices exploiting the quantum tunnelling properties of single-molecule magnets have been proposed. Herein, we present an original spin-valve device in which a non-magnetic molecular quantum dot, made of a single-walled carbon nanotube contacted with non-magnetic electrodes, is laterally coupled through supramolecular interactions to TbPc(2) single-molecule magnets (Pc=phthalocyanine). Their localized magnetic moments lead to a magnetic field dependence of the electrical transport through the single-walled carbon nanotube, resulting in magnetoresistance ratios up to 300% at temperatures less than 1 K. We thus demonstrate the functionality of a supramolecular spin valve without magnetic leads. Our results open up prospects of new spintronic devices with quantum properties.

Finite tunnel magnetoresistance at the compensation point of Sm1−xGdxAl2, a ferromagnetic electrode with zero magnetization

A tunnel magnetoresistance effect is observed in magnetic tunnel junctions with an original zero magnetization ferromagnetic electrode composed of Sm1−xGdxAl2. A finite tunnel magnetoresistance is measured when the Sm1−xGdxAl2 electrode is in a magnetic compensated state proving that Sm1−xGdxAl2 with zero magnetization is able to give rise to a spin-polarized current. The temperature dependence of tunnel magnetoresistance reveals that this effect is related to the relative orientation of spin contributions to magnetic moment in both electrodes and that the tunnel polarization of the Sm1−xGdxAl2 electrode is negative.

Observation of a tunneling magnetoresistance effect in magnetic tunneling junctions with a high resistance ferromagnetic oxide Fe 2⋅5Mn 0⋅5O 4 electrode

A high resistance ferromagnetic oxide Fe 2⋅5Mn 0⋅5O 4 (FMO) property as a novel spin injector was investigated with a structure of a magnetic tunneling junction (MTJ) composed of FMO/Al–O/Ni 80Fe 20, in order to reduce an impedance mismatch problem on molecular spintronics. A tunneling magnetoresistance (TMR) effect in the MTJs was observed. The maximum TMR ratio observed in the MTJs was approximately 0.85% at room temperature (RT), and the spin-polarization of FMO was estimated to be at least 0.94% at RT.

Effect of cotunneling and spin polarization on the large tunneling magnetoresistance effect in granularC60-Co films

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Giant Tunneling Magnetoresistance Effect in Ferromagnet/Spin-triplet Superconductor Junctions Caused by Andreev Reflection

We study theoretically the tunneling magnetoresistance effect in ferromagnet/chiral triplet superconductor junctions using the generalized Blonder-Tinkham-Klapwijk theory. This effect rely entirely on the dependence of Andreev reflection on the relative orientation between the magnetic moment in ferromagnet and the d-vector in spin-triplet superconductor, The tunneling differential conductance and magnetoresistance are calculated for three types of chiral p-wave gap functions, and huge tunneling magnetoresistance effect is obtained in the strong spin-polarization and low-temperature regime. This effect might provide a valuable new source of device application.

Role of Coulomb blockade and spin-flip scattering in tunneling magnetoresistance of FeCo-Si-O nanogranular films

In this work, we report the effect of FeCo atomic fraction (0.33 &amp;lt; x &amp;lt; 0.54) and temperature on the electrical, magnetic, and tunneling magnetoresistance (TMR) properties of FeCo-Si-O granular films prepared by atom beam sputtering technique. Glancing angle x-ray diffraction and TEM studies reveal that films are amorphous in nature. The dipole–dipole interactions (particle–matrix mixing) is evident from zero-field cooled and field-cooled magnetic susceptibility measurements and the presence of oxides (mainly Fe-related) is observed by x-ray photoelectron spectroscopy analysis. The presence of Fe-oxides is responsible for the observed reduction of saturation magnetization and rapid increase in coercivity below 50 K. TMR has been observed in a wide temperature range, and a maximum TMR of −4.25% at 300 K is observed for x = 0.39 at a maximum applied field of 60 kOe. The fast decay of maximum TMR at high temperatures and lower TMR values at 300 K when compared to PFeCo2/(1+PFeCo2), where PFeCo is the spin polarization of FeCo are in accordance with a theoretical model that includes spin-flip scattering processes. The temperature dependent study of TMR effect reveals a remarkably enhanced TMR at low temperatures. The TMR value varies from −2.1% at 300 K to −14.5% at 5 K for x = 0.54 and a large MR value of −18.5% at 5 K for x = 0.39 is explained on the basis of theoretical models involving Coulomb blockade effects. Qualitatively particle–matrix mixing and the presence of Fe-oxides seems to be the source of spin-flip scattering, responsible for fast decay of TMR at high temperatures. A combination of higher order tunneling (in Coulomb blockade regime) and spin-flip scattering (high temperature regime) explains the temperature dependent TMR of these films.

A spin-valve-like magnetoresistance of an antiferromagnet-based tunnel junction

A spin valve is a microelectronic device in which high- and low-resistance states are realized by using both the charge and spin of carriers. Spin-valve structures used in modern hard-drive read heads and magnetic random access memoriescomprise two ferromagnetic electrodes whose relative magnetization orientations can be switched between parallel and antiparallel configurations, yielding the desired giant or tunnelling magnetoresistance effect. Here we demonstrate more than 100% spin-valve-like signal in a NiFe/IrMn/MgO/Pt stack with an antiferromagnet on one side and a non-magnetic metal on the other side of the tunnel barrier. Ferromagneticmoments in NiFe are reversed by external fields of approximately 50 mT or less, and the exchange-spring effect of NiFe on IrMn induces rotation of antiferromagnetic moments in IrMn, which is detected by the measured tunnelling anisotropic magnetoresistance. Our work demonstrates a spintronic element whose transport characteristics are governed by an antiferromagnet. It demonstrates that sensitivity to low magnetic fields can be combined with large, spin-orbit-coupling-induced magnetotransport anisotropy using a single magnetic electrode. The antiferromagnetic tunnelling anisotropic magnetoresistance provides a means to study magnetic characteristics of antiferromagnetic films by an electronic-transport measurement.

Bias-voltage-dependence of magnetoresistance for epitaxial Fe/MgO/Co2MnSn tunnel junctions

Tunnel magnetoresistance (TMR) was investigated for fully epitaxial Fe/MgO/Co2MnSn magnetic tunnel junctions (MTJs) fabricated on MgO (001) substrates. The bias-voltage-dependence of the TMR ratio showed that the sign of TMR ratio at 300 K becomes negative for the bias voltage between −800 mV and +200 mV, which may be caused by the impurity state or other imperfections at the interface. The sign change in TMR effect was also observed with changing the temperature at low fixed bias voltage, implying that the effect is not simply due to a bias-dependent change of the density of state at the Fermi level.

The effect of inserting thin Co2MnAl layer into the Co2MnSi/MgO interface on tunnel magnetoresistance effect

We fabricated an epitaxially grown B2-ordered Co2MnAl Heusler alloy film by optimizing fabrication conditions, such as composition of the films and annealing temperature. Tunnel magnetoresistance (TMR) effect was investigated in Co2MnSi/Co2MnAl(0-1.0 nm)/MgO/CoFe magnetic tunnel junctions (MTJs). TMR ratio was enhanced by inserting a thin Co2MnAl layer at the Co2MnSi/MgO interface. The MTJ with Co2MnAl thickness of 0.5 nm exhibited the highest TMR ratio at 310 K and 10 K.

Magnetoresistance characteristics of [formula omitted] heterostructures grown on Si(111) by molecular beam epitaxy

Fe3Si/CaF2/Fe3Si magnetic tunnel junctions (MTJs) have been investigated to demonstrate the tunnel magnetoresistance effects. We fabricated Fe3Si(20 nm)/CaF2(2 nm)/Fe3Si(15 nm) heterostructures epitaxially on a Si(111) substrate by molecular beam epitaxy. The current-voltage characteristics for the MTJs measured at room temperature (RT) were well fitted to Simmons’ equation. The fitting yields the barrier height φ=2.5 eV and the barrier thickness d=1.26 nm. The magnetoresistance ratio for the MTJs were approximately 0.28% under a bias voltage of 20 mV at RT.

Interfacial interaction in Fe/MgO/Fe magnetic tunneling junctions

We present a study on interfacial structures and tunneling magnetoresistance (TMR) in Fe/MgO/Fe junctions using a MgO(111) film with {100} facets. It is shown using X-ray photoelectron spectroscopy that a FeO layer occurs at MgO/Fe rather than Fe/MgO interface, which could be used to tune the TMR effect. At the Fe/MgO interface, such a change in electronic structure is attributed to the band bending associated with a change in thickness of Fe films. The present study provides a new understanding on the Fe/MgO/Fe interfacial behavior and metal/oxide barriers involving electron transport.

Temperature and bias voltage dependence of Co/Pd multilayer-based magnetic tunnel junctions with perpendicular magnetic anisotropy

Temperature- and bias voltage-dependent transport measurements of magnetic tunnel junctions (MTJs) with perpendicularly magnetized Co/Pd electrodes are presented. Magnetization measurements of the Co/Pd multilayers are performed to characterize the electrodes. The effects of the Co layer thickness in the Co/Pd bilayers, the annealing temperature, the Co thickness at the MgO barrier interface, and the number of bilayers on the tunneling magneto resistance (TMR) effect are investigated. TMR-ratios of about 11% at room temperature and 18.5% at 13 K are measured and two well-defined switching fields are observed. The results are compared to measurements of MTJs with Co–Fe–B electrodes and in-plane anisotropy.

Superparamagnetism in MgO-based magnetic tunnel junctions with a thin pinned ferromagnetic electrode

MgO-based magnetic tunnel junctions have been fabricated with a thin ${\text{Co}}_{40}{\text{Fe}}_{40}{\text{B}}_{20}$ (CoFeB) layer in the pinned synthetic antiferromagnetic CoFe/Ru/CoFeB stack. An inverted tunneling magnetoresistance is observed due to the unbalanced synthetic antiferromagnet. Superparamagnetic nanoparticles form when the CoFeB layer is thinner than 1.5 nm, and an abnormal temperature dependence of the junction resistance is associated with superparamagnetism when the thermal fluctuation energy exceeds the magnetic anisotropy energy. This explanation accounts for the temperature dependence of the tunneling magnetoresistance effect.

Perpendicular magnetic tunnel junctions using Co-based multilayers

CoFeB/MgO/CoFeB magnetic tunnel junctions (MTJs) with perpendicular magnetic anisotropy (PMA) free and reference layers composed of Co/M (where M=Pd or Ni) multilayers have been optimized for high PMA and high tunneling magnetoresistance (TMR). The effects of Co thickness, Pd thickness, and the number of Co/Pd bilayers on the anisotropy and coercivity of the [Co/Pd]n multilayer films have been studied for both free and reference layers. The damping parameter α of CoFeB capped multilayers was determined using broadband ferromagnetic resonance. The transport properties of the patterned MTJ stacks were measured from 10 to 400 K. A maximum TMR of 10% at 10 K (5%–10% at 300 K) was obtained for these perpendicular MTJs, regardless of whether or not they were magnetically annealed for MgO–CoFeB crystallization. This indicates that the fcc-bcc-fcc transitions from the fcc multilayers to the bcc CoFeB/MgO/CoFeB do not promote the “giant MgO TMR effect” caused by symmetry filtering.

Inverse and normal tunneling magnetoresistance effects in FeCoGd/FeCo/AlO/FeCo multilayers

FeCoGd/FeCo(t)/AlO/FeCo magnetic tunnel junctions were fabricated with a very thin FeCo layer inserted between the FeCoGd layer and the AlO barrier. At room temperature, inverse tunneling magnetoresistance (TMR) effect and normal TMR effect were observed simultaneously in the low and high magnetic field regions, respectively. The absolute values of both inverse and normal TMR ratios, the saturation magnetic field corresponding to the normal TMR effect have similar dependence on t. Transmission electron microscopy studies indicate that a thin granular film is likely formed between the FeCoGd layer and the AlO barrier, which may result in the normal TMR effect at high magnetic fields.

Local magnetism of Co2MnSn Heusler alloy films and magnetoresistance of Co2MnSn-based magnetic tunnel junctions

Co2MnSn Heusler alloy films were prepared by atomically controlled alternate deposition on MgO(001) substrates and the local magnetism was investigated using 119Sn Mössbauer spectroscopy. It turned out that the magnetic environments around the Sn sites in the films prepared by this method are uniform in comparison with those in bulk alloys prepared by arc melting. The local uniformity around the Sn sites depends on the growth temperature. The magnetoresistance of the magnetic tunnel junctions with a Co2MnSn/MgO/ Co2MnSn/Co structure, where the Co2MnSn layers were prepared by the same method, was also examined. Tunneling magnetoresistance effect has been observed for the samples where the local magnetism is relatively uniform, although the observed ratio is not so large at the present stage.

Simple model for detection of spin accumulation in a ferromagnetic double tunnel junction

We study the effect of spin accumulation in ferromagnetic double tunnel junctions based on a simple model. The analytical results for typical cases show that spin accumulation enhances the tunnel magnetoresistance (TMR) effect and furthermore affects electric potential of the center electrode. These effects are possibly used for the detection of spin accumulation in ferromagnetic double tunnel junctions.

Temperature and bias voltage dependence of tunnel magnetoresistance effect in magnetic tunnel junctions with Co2Fe(Al,Si) electrodes fabricated by MBE

The temperature and bias voltage dependence of tunnel magnetoresistance ratio of two types of magnetic tunnel junctions were investigated. These two types of junctions have almost the same ordered structured Co2FeAl0.5Si0.5 Heusler bottom electrodes and different barrier layers of Al oxide or MgO, and top electrodes. A similar temperature dependence of tunnel magnetoresistance ratio measured at low bias voltage was found. A larger asymmetric bias voltage dependence of the tunnel magnetoresistance was obtained for the junction with Al oxide barrier compared with the junction with MgO barrier, and negative tunnel magnetoresistance ratio was also observed for both junctions.

Effect of Buffer Layer Texture on the Crystallization of CoFeB and on the Tunnel Magnetoresistance in MgO Based Magnetic Tunnel Junctions

Two different buffer layers (Ta/Ru/Ta and thick Ta) were tested for MgO MTJs. The influence of buffer layer texture on the crystallization of CoFeB bottom and top electrodes and on the tunnel magnetoresistance effect was investigated. X-ray results suggest that, after anneal, the CoFeB layer above MgO is well (200) textured and it does not depend on the buffer layer since MgO (100) barrier supplies a good template for CoFeB (200) orientation, while the crystallization of bottom CoFeB layer relies on the buffer layer texture. Different from Ta (110) found in Ta/Ru/Ta buffer layer, a thick Ta buffer layer has beta-(200) texture, which induces (001) oriented grains in MnPt layer. Because of the epitaxy relationship between MnPt and FeCo with MnPt(001)[100]//FeCo(200)[110], MnPt (001) oriented grains lead to the crystallization of bottom CoFeB layer with (200) orientation. As a result, higher TMR ratio up to 290% was achieved in the EB-MTJs with thick Ta buffer layer.

Detection of Continuous Spinwaves Along Magnetic Nanostripes Using Magnetoresistive Devices

In this study, we have theoretically investigated the scheme to detect continuous spinwaves using magnetoresistive devices. First, the spinwaves along a magnetic nanostripe related to this detection scheme have been characterized using a linearization approach on the Landau-Lifshitz-Gilbert equation. Then, the frequency (wavelength) responses of the magnetoresistive devices have been derived from the giant- or tunneling-magnetoresistive effect. The frequency (wavelength) responses have been compared with those of the conventional detection methods using microstrip lines or coplanar waveguides. Finally, we have qualitatively analyzed the impact of noise in spinwave detection, from which this detection scheme can be improved.