Temperature Dependence Of Tunneling Magnetoresistance Research Articles

Temperature dependence of tunnel magnetoresistance in serial magnetic tunnel junctions

Magnetic tunnel junctions have been widely used in various applications, such as magnetic sensors and magnetic random-access memories. In the practical application of MTJs, they are usually used in series toward high sensitivity and high stability, especially for sensor applications. In this paper, serial MTJs devices on 8 in. wafers were fabricated. The temperature dependence of the tunnel magnetoresistance ratio, resistances in parallel and antiparallel configurations, and dynamic conductance were systematically investigated. The results of serial MTJs devices are consistent with a single MTJ device. This research suggests that serial MTJs can be directly used to investigate the magnetic tunneling properties of MTJ stacks.

Effect of annealing on the temperature dependence of inelastic tunneling contributions vis-à-vis tunneling magnetoresistance and barrier parameters in CoFe/MgO/NiFe magnetic tunnel junctions

The effect of annealing on the changes in the inelastic tunneling contributions in tunneling conductance of ion beam sputtered CoFe/MgO/NiFe magnetic tunnel junctions (MTJs) is investigated. The inelastic contributions are evaluated using hopping conduction model of Glazman and Matveev in the temperature range of 25–300 K. The hopping through number of series of localized states present in the barrier due to structural defects increases from 9 (in as deposited MTJ) to 18 after annealing (at 200 °C/1 h); although no changes in the interface roughness of CoFe-MgO and MgO-NiFe interfaces are observed as revealed by the x-ray reflectance studies on planar MTJs. The bias dependence of tunneling magnetoresistance (TMR) at 25 K is found to get improved after annealing as revealed by the value V1/2 (the bias value at which the TMR reaches to half of its value at nearly zero bias); which is 78 mV (in MTJ annealed at 200 °C/1 h) 2.5 times the value of 33 mV (in as deposited MTJ). At 25 K the inelastic tunneling spectra revealed the presence of zero bias anomaly and magnon excitations in the range of 10–15 mV. While the barrier height exhibited a strong temperature dependence with nearly 100% increase from the value at 300 K to 25 K, the temperature dependence of TMR becomes steep after annealing.

Observation of magnetic moments at the interface region in magnetic tunnel junctions using depth-resolved x-ray magnetic circular dichroism

The depth-resolved x-ray magnetic circular dichroism (XMCD) technique was applied to observe the interfacial and inner magnetic moments in CoFe/MgO and Co${}_{2}$MnSi(CMS)/MgO. The magnetic moments of the real interface region (two monolayers from the interface) and the inner layers were separately analyzed using the XMCD sum rules. The observed interfacial moments of Co at the CMS/MgO interface show a remarkable reduction from that in the inner layers compared with the CoFe/MgO structure, suggesting small exchange stiffness in the CMS at the MgO interface. The weak exchange stiffness of Co is a possible reason for the large temperature dependence of tunnel magnetoresistance in CMS-based magnetic tunnel junctions.

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.

Tunnel magnetoresistance in textured Co2FeAl/MgO/CoFe magnetic tunnel junctions on a Si/SiO2 amorphous substrate

Magnetic tunnel junctions with B2-ordered Co2FeAl full Heusler alloy as a ferromagnetic electrode were fabricated by sputtering on thermally oxidized Si/SiO2 amorphous substrates. A Co2FeAl/MgO/Co50Fe50 structure showed a highly (001)-textured structure and the tunneling magnetoresistance (TMR) ratio of 166% at room temperature and 252% at 48 K were achieved. The temperature dependence of TMR can be fitted with spin wave excitation model, and the bias voltage dependence of differential conductance demonstrated that the high TMR was mainly contributed by coherent tunneling. This work suggests the B2-Co2FeAl is one of the promising candidates for practical spintronic applications.

Structural instability of FeCo ultrathin films grown on MgO(100)

Structures and their stability of FeCo ultrathin films grown on MgO(100) were investigated. Atomic force microscopy observations revealed dramatic structural changes from a continuous ultrathin layer to coarse islands by low temperature annealing. The results suggest that the thin layered structures of Fe based alloys as grown on MgO(100) are not so stable as expected from the relatively small lattice mismatch between Fe based alloys and MgO. This possibly explains annealing temperature dependence of tunnel magnetoresistance observed in a Fe/MgO/Fe(100) junction.

Influence of chemical and magnetic interface properties of Co-Fe-B/MgO/Co-Fe-B tunnel junctions on the annealing temperature dependence of the magnetoresistance

The knowledge of chemical and magnetic conditions at the Co40Fe40B20∕MgO interface is important to interpret the strong annealing temperature dependence of tunnel magnetoresistance of Co-Fe-B/MgO/Co-Fe-B magnetic tunnel junctions, which increases with annealing temperature from 20% after annealing at 200 °C up to a maximum value of 112% after annealing at 350 °C. While the well defined nearest neighbor ordering indicating crystallinity of the MgO barrier does not change by the annealing, a small amount of interfacial Fe-O at the lower Co-Fe-B/MgO interface is found in the as grown samples, which is completely reduced after annealing at 275 °C. This is accompanied by a simultaneous increase of the Fe magnetic moment and the tunnel magnetoresistance. However, the tunneling magnetoresistance of the MgO based junctions increases further for higher annealing temperature which cannot be caused by Fe-O reduction. The occurrence of an x-ray absorption near-edge structure above the Fe and Co L edges after annealing at 350 °C indicates the recrystallization of the Co-Fe-B electrode. This is a prerequisite for coherent tunneling and has been suggested to be responsible for the further increase of the tunneling magnetoresistance above 275 °C. Simultaneously, the B concentration in the Co-Fe-B decreases with increasing annealing temperature, at least some of the B diffuses toward or into the MgO barrier and forms a B2O3 oxide.

Temperature dependence of the resistance of magnetic tunnel junctions with MgO barrier

The temperature dependent tunneling resistance of magnetic tunnel junctions with MgO barriers was characterized. In the junctions prepared by magnetron sputtering, the tunnel magnetoresistance decreases with increasing temperature. Various contributions to the tunnel conductance are discussed using different models. Not only the direct elastic tunneling contributes to the temperature dependence of tunnel magnetoresistance, but also the assisted, spin-independent tunneling plays an important role in determining the temperature dependent behavior in our magnetic tunneling junctions. The process is further investigated assuming magnon and phonon assisted tunneling and compared to junctions with alumina tunnel barrier.

Temperature dependence of tunnel magnetoresistance and magnetization of IrMn based MTJ

The temperature dependence of spin-polarized tunnel magnetoresistance (TMR) is investigated between 30 K and 300 K for annealed junctions with the structure of Ta(5)/Cu(10)/Ta(5)/NiFe(2)/Cu(5)/IrMn(10)/CoFe(2.5)/Al2O3(1.5)/CoFe(2.5)/NiFe(t)/Ta(5), where t = 10 and 100 nm. For the junction (t = 100 nm) annealed in 270 °C we were able to separate electron polarization spin-dependent and spin-independent contributions of TMR temperature dependence. The thermally spin waves excitation constants determined from temperature dependence of magnetization and polarization are comparable. For junction with t = 10 nm annealed in 300 °C electron spin polarization conductance is small in comparison to high conductance via trapped states, which arises from defects and magnetic impurities diffusion. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Analytical result of the effect of spin scattering and impurity-assistance on magnetoresistance in doped magnetic tunneling junctions

An extended tunneling Hamiltonian method is proposed to study the temperature-dependent tunneling magnetoresistance (TMR) in doped magnetic tunnel junctions. It is found that for nonmagnetic dopants (Si), impurity-assisted tunneling is mainly elastic, giving rise to a weak spin polarization, thereby reduces the overall TMR, while for magnetic ions (Ni), the collective excitation of local spins in δ-doped magnetic layer contributes to the severe drop of TMR and the behavior of the variation of TMR with temperature different from that for Si-doping. The theoretical results can reproduce the main characteristic features of experiments.

Tunneling magnetoresistance of Co clusters in MgF2

Samples of Co clusters embedded in a MgF2 matrix have been prepared by the codeposition of well-defined Co clusters (mean diameter L≃4.5 nm) and MgF2 molecules onto a cold substrate. This system shows a temperature-dependent tunneling magnetoresistance (TMR) which has a value of 34.7(2)% at 2.5 K, the largest TMR value reported for a granular Co system. This large TMR value either is due to an antiferromagnetic coupling of those Co clusters between which the tunneling occurs or is caused by an enhanced spin-polarization of the tunneling electrons. The latter may be caused by a cluster–surface/matrix interaction leading to dominated d-electron tunneling due to Co(3d)–F(2p) hybridization.

Temperature dependent resistance of magnetic tunnel junctions as a quality proof of the barrier

Tunnel junctions of Co(10 nm)/AlOx (nominally 2 nm)/Co(20 nm) have been prepared by molecular beam epitaxy applying a shadow mask technique in conjunction with an UV light-assisted oxidation process of the AlOx barrier. The quality of the AlOx barrier has been proven by x-ray photoelectron spectroscopy and temperature dependent tunneling magnetoresistance (TMR) measurements. Optimum-oxidized tunnel junctions show a TMR of 20% at 285 K and up to 36% at 100 K. At 285 K the TMR values as a function of oxidation time are not symmetric about the optimum time. For underoxidized junctions the TMR is reduced more strongly than for overoxidized junctions. The temperature dependence of the junction’s resistance is a clear and reliable indicator whether pinholes (or imperfections) contribute to the conduction across the barrier.

Analyses of intrinsic magnetoelectric properties in spin-valve-type tunnel junctions with high magnetoresistance and low resistance

A series of experimental data was obtained systematically for a spin-valve-type tunnel junction of ${\mathrm{Ta}\mathrm{}(5\mathrm{}\mathrm{n}\mathrm{m})/\mathrm{N}\mathrm{i}}_{79}{\mathrm{Fe}}_{21}$ (3 nm)/Cu ${(20\mathrm{}\mathrm{n}\mathrm{m})/\mathrm{N}\mathrm{i}}_{79}{\mathrm{Fe}}_{21}$ ${(3\mathrm{}\mathrm{n}\mathrm{m})/\mathrm{I}\mathrm{r}}_{22}{\mathrm{Mn}}_{78}{\mathrm{}(10\mathrm{}\mathrm{n}\mathrm{m})/\mathrm{C}\mathrm{o}}_{75}{\mathrm{Fe}}_{25}$ (4 nm)/Al ${(0.8\mathrm{}\mathrm{n}\mathrm{m})\ensuremath{-}\mathrm{o}\mathrm{x}\mathrm{i}\mathrm{d}\mathrm{e}/\mathrm{C}\mathrm{o}}_{75}{\mathrm{Fe}}_{25}$ ${(4\mathrm{}\mathrm{n}\mathrm{m})/\mathrm{N}\mathrm{i}}_{79}{\mathrm{Fe}}_{21}$ (20 nm)/Ta (5 nm). Analyses of (i) temperature dependence of tunnel magnetoresistance (TMR) ratio and resistance from 4.2 K to room temperature, (ii) applied dc bias-voltage dependence of TMR ratio and resistance at 6.0 K and room temperature, and (iii) tunnel current I and dynamic conductance $(dI/dV)$ as functions of dc bias voltage at 6.0 K were carried out. High-TMR ratio of 64.7% at 4.2 K and 44.2% at room temperature were observed for this junction after annealing at 300 \ifmmode^\circ\else\textdegree\fi{}C for an hour. An anisotropic wavelength cutoff energy of spin-wave spectrum in magnetic tunnel junctions, which is essential for self-consistent calculations, was suggested based on a series of inelastic electron tunnel spectra obtained. The main intrinsic magnetoelectric properties in such spin-valve-type tunnel junction with high magnetoresistance and low resistance can be evaluated based on the magnon-assisted inelastic excitation model and theory.

Fabrication of high-magnetoresistance tunnel junctions using Co75Fe25 ferromagnetic electrodes

Temperature dependence of tunnel magnetoresistance (TMR) ratio, resistance, and coercivity from 4.2 K to room temperature and applied voltage dependence of the TMR ratio and resistance at room temperature for a tunnel junction, Ta (5 nm)/Ni79Fe21 (3 nm)/Cu (20 nm)/Ni79Fe21 (3 nm)/Ir22Mn78 (10 nm)/Co75Fe25 (4 nm)/Al (0.8 nm)-oxide/Co75Fe25 (4 nm)/Ni79Fe21 (20 nm)/Ta(5 nm), were investigated. TMR ratio, effective barrier height and width, and breakdown voltage of the junction can be remarkably enhanced after annealing at 300 °C for an hour. High TMR ratio of 49.7% at room temperature and 69.1% at 4.2 K were observed. The value of spin polarization of Co75Fe25, P=50.7%, deduced from the TMR ratio at 4.2 K was corresponding well to the experimental data measured at 0.2 K in a spin polarized tunneling experiment using a superconductor/insulator/ferromagnet tunneling junction.