Ferromagnetic Junctions Research Articles

Tunneling in Ferromagnetic Junctions and Half-Metallic Systems.

Tunneling in Ferromagnetic Junctions and Half-Metallic Systems.

Coulomb blockade and enhancement of magnetoresistance change in ultrasmall ferromagnetic tunnel junctions

Coulomb blockade and negative magnetoresistance of ultrasmall ferromagnetic tunnel junctions are studied based on the Feynman path integral approach. It is shown that the change in magnetoresistance is considerably enhanced in the Coulomb blockade regime. This is due to the nonlinear current–voltage characteristics caused by the higher-order tunneling processes which set in since negative magnetoresistance tends to make Coulomb blockade unstable. Results obtained are qualitatively in good agreement with recent experimental findings. Coulomb blockade and the magnetoresistance in ultrasmall ferromagnetic double junction are also discussed briefly.

Assisted tunneling in ferromagnetic junctions and half-metallic oxides

Different mechanisms of spin-dependent tunneling are analyzed with respect to their role in tunnel magnetoresistance (TMR). Microscopic calculation within a realistic model shows that direct tunneling in iron group systems leads to about a 30% change in resistance, which is close but lower than experimentally observed values. The larger observed values of the TMR might be a result of tunneling involving surface polarized states. It is found that tunneling via resonant defect states in the barrier radically decreases the TMR by order of magnitude. One-magnon emission is shown to reduce the TMR, whereas phonons increase the effect. The inclusion of both magnons and phonons reasonably explains an unusual bias dependence of the TMR. The model presented here is applied qualitatively to half metallics with 100% spin polarization, where one-magnon processes are suppressed and the change in resistance in the absence of spin mixing on impurities may be arbitrarily large. Even in the case of imperfect magnetic configurations, the resistance change can be a few 1000%. Examples of half-metallic systems are CrO2/TiO2 and CrO2/RuO2.

Interface Magnetism and Spin Wave Scattering in Ferromagnet-Insulator-Ferromagnet Tunnel Junctions

Careful tunneling studies in high quality $\mathrm{Co}/{\mathrm{Al}}_{2}{\mathrm{O}}_{3}/{\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$ junctions show a junction magnetoresistance (JMR) of 20.2% and 27.1% at 295 and 77 K, respectively, where the latter is in agreement with Julliere's model. The temperature dependence of the JMR can be explained by the temperature dependence of surface magnetization. The decrease of the JMR with increasing dc bias is intrinsic to ferromagnetic junctions. The strong disagreement with recent theories in the low dc bias region can be attributed to magnetic excitations in these junctions, as seen in inelastic tunneling measurements.

Spin-polarized magnetic tunnelling magnetoresistive effects in various junctions

Recent progress concerning spin-polarized magnetic tunnelling effects for (i) trilayer standard ferromagnet (F)/insulator (I)/ferromagnet (F) junctions, (ii) spin-valve-type junctions, (iii) trilayer or multilayer ferromagnet/granular/ferromagnet junctions and (iv) F/I/F junction with a `wedge-geometry' insulator is reviewed. Special emphasis is placed on the dependence of the tunnel magnetoresistance ratio on temperature and also the intensity of the applied voltage. It was found that the resistance for the saturation magnetization state, , and the tunnelling magnetoresistance ratio, TMR, of an junction decreased rapidly with increasing temperature, whereas those of a junction were insensitive to temperature. Concerning the bias voltage dependence of and TMR, the same tendency with temperature was observed for and junctions. Spin-valve-type junction exchange biased by a FeMn layer exhibits a relatively large TMR ratio up to about 400 K.

Effect of Coulomb Blockade on Magnetoresistance in Ferromagnetic Tunnel Junctions

We study spin-dependent electron tunneling in ferromagnetic junctions containing small metallic islands. The tunneling matrix elements depend on the relative direction of magnetization of the island and electrodes. The dependence of the matrix elements amplifies the cotunneling in the Coulomb blockade regime. We show that in single-electron ferromagnetic transistors the magnetoresistance is strongly enhanced by the Coulomb blockade. The results provide a theoretical basis for recent experiments on ferromagnetic single-electron transistors, ferromagnetic double tunnel junctions, and ferromagnetic granular materials.

Tunneling Magnetoresistance in Ferromagnetic Junctions: Bias Dependence

By tunneling magnetoresistance (TMR) we mean in this paper a drop ΔR in the junction resistance when the magnetizations of two ferromagnetic electrodes rotate from antiparallel to parallel alignment. The effect was observed first by Julliere [1] in Co/Ge/Fe junctions. Similar observations were reported later also in other systems [2, 3]. Recent experimental data show additionally that TMR significantly decreases with increasing voltage V [4]. Two microscopic approaches have been developed to account for TMR in ferromagnetic junctions. One of them takes into account many-body features of the corresponding electronic wave functions in ferromagnetic electrodes [5]. In the second approach, on the other hand, one-electron wave functions are used to calculate the tunneling current [6]. In both models a complex electronic band structure of a ferromagnet was approximated by the one-band model with free-electron-like spin-polarized parabolic bands. However, there are some additional features of the electronic structure, which can contribute to TMR, and which have not been taken into account in the above models. The most important seems to be the s-d hybridization in the ferromagnetic electrodes [7] as well as at the electrode/barrier interfaces. Apart from this, the barrier height for d electrons can differ remarkably from the barrier height for s electrons [8]. Thus, the tunneling probability for a particular electronic state depends on the rate of s—d hybridization. This, on

Large magnetoresistance in La0.7Sr0.3MnO3/SrTiO3/La0.7Sr0.3MnO3 ramp-edge junctions

We report on the fabrication of ferromagnet–insulator–ferromagnet junction devices using a ramp-edge geometry based on (La0.7Sr0.3)MnO3 ferromagnetic electrodes and a SrTiO3 insulator. The maximum junction magnetoresistance (JMR) as large as 23% is observed below 300 Oe at low temperatures (T<100 K). Our ramp-edge junctions exhibit JMR of 6% at 200 K with a field less than 100 Oe. The device performance at room temperature is believed to be limited by both the nearly equivalent coercive fields in the electrodes and the magnetization process, rather than by the insulating barrier.

Optimum tunnel barrier in ferromagnetic–insulator–ferromagnetic tunneling structures

Al 2 O 3 tunnel barriers I, formed by the oxidization of Al metal of various thicknesses between two ferromagnetic (FM) films were investigated to understand the influence of overlayer metal Al on the junction magnetoresistance (JMR). The optimum thickness of Al was observed to lie in the range of 1–1.6 nm to achieve good JMR in FM–I–FM junctions. Additionally, such junctions can be used to study the magnetic proximity effect in ferromagnet/normal metal bilayer systems.

Mesoscopic charge-density-wave junctions.

We investigate theoretically charge-density waves (CDW's) in mesoscopic heterostructures for the configuration where the one-dimensional chains are oriented normal to the interfaces. Based on Bogoliubov--de Gennes equations and suitable boundary conditions, ground state properties of phase coherent CDW systems are calculated in the mean-field approximation. It is shown that in a charge-density-wave/normal-metal/charge-density-wave junction the CDW condensates couple through the normal metal region by means of Friedel oscillations. Geometrical resonance effects are predicted. We relate the phase-dependent energy of the junction to a momentum current, carried by electron-hole pairs. The effects of pinning potentials at the interfaces are also considered. Finally, we discuss the analogies with superconductor and ferromagnet junctions. \textcopyright{} 1996 The American Physical Society.

Magnetoresistance in 82Ni-Fe/Al-Al2O3/Co Junction-Dependence of the Tunneling Conductance on the Angle between the Magnetizations of Two Ferromagnetic Layers

The magnetoresistance of a ferromagnetic tunneling junction in 82Ni-Fe/ Al-Al <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> O <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> /Co, prepared by electron beam evaporation in a 1×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-6</sup> Torr vacuum, was studied. Tunneling was confirmed to occur by measuring the I-V characteristic and its temperature dependence, which were in good agreement with Stratton's tunneling theory. The hysteresis curve of the magnetoresistance corresponds well to that of the magnetization. The magnetoresistance ratio δR/R was 2.7% and 3.5% at room temperature and 77K respectively. The dependence of the tunneling conductance (G) on the angle (θ) between the magnetizations of the two ferromagnetic layers was also measured. The G values can be expressed as G=G <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</inf> (l+ϵcosθ) (where G <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</inf> and ϵ are constants), which agrees well with the theory proposed by Slonczewski.

８２Ｎｉ‐Ｆｅ／Ａｌ‐Ａｌ２Ｏ３／Ｃｏ接合における磁気抵抗効果 トンネルコンダクタンスの両磁性層の磁化の相対角度依存性

Magnetoresistance was studied for the ferromagnetic tunneling junction in 82Ni-Fe/Al-Al2O3/Co prepared by an electron beam evaporation in a vacuum of 1×10-6 Torr. The tunneling is confirmed by measuring I-V character and its temperature dependence, which are in good agreement with Stratton's tunneling theory. The hysteresis curve of magnetoresistance corresponds well with that of magnetization. The magnetoresistance ratio ΔR/R is 2.7% and 3.5% at R.T. and 77 K, respectively. Dependence of the tunneling conductance (G) on the angle (θ) between the magnetizations of two ferromagnetic layers was also measured. The G-values can be expressed as G = G0(1+e cos θ) (G0, e: constant). which agrees well with the theory proposed by Slonczewski.

Magnetic-field dependent switching effect in ferromagnetic EuS film junctions

Magnetic-field dependent switching effect in ferromagnetic EuS film junctions

Electrical Conduction in Ferromagnetic Metal Oxide Junctions

The electrical conduction from 4.2°K to room temperature in thin film Ni–NiOx-metal junctions was studied. Tunneling is the dominant conduction mechanism at low temperature; at high temperature, Schottky thermionic emission dominates. The effective barrier thickness determines at what temperature the conductivity changes from tunneling to Schottky emission. Atomic penetration into the oxide is important, and there appear to be broad transition regions at the metal-oxide interfaces. Using the conventional models, the barrier heights are approximately 0.2 eV.