Tunneling magnetoresistance in ferromagnetic planar hetero-nanojunctions (original) (raw)
Related papers
Tunnel magnetoresistance in magnetic tunnel junctions with embedded nanoparticles
2015 IEEE Magnetics Conference (INTERMAG), 2015
In the present work we attempt the theoretical modeling of the magnetic tunnel junctions (MTJs) with embedded magnetic and non-magnetic nanoparticles. A few abnormal tunnel magnetoresistance (TMR) effects, observed in related experiments, can be easily simulated within our model: we found, that suppressed TMR magnitudes and TMR sign reversing effect at small voltages are related to the electron momentum states of the nanoparticle located inside the insulating layer. All these TMR behaviors can be explained within the tunneling model, where nanoparticle is simulated as a quantum well (QW). The coherent double barrier tunneling is dominating over the single barrier one. The origin of the TMR suppression is the quantized angle transparency for spin polarized electrons being in one of the lowest QW states. The phenomenon was classified as coherent quantum blockade regime.
Spin-polarized tunneling, magnetoresistance and interfacial effects in ferromagnetic junctions
The pioneering studies of spin-polarized tunnelling by Meservey and Tedrow in the early 1970s showed that the conduction electrons in ferromagnetic (FM) metals are spin polarized and that the spin is conserved in the tunnelling process. Only recently (1995) improved material fabrication techniques have permitted realization of the Jullie Á re quantitative model, showing that tunnelling in ferromagnet/insulator/ferromagnet (FM/I/FM) junctions should lead to a large junction magnetoresistance (JMR); JMR values greater than 30% have been achieved at room temperature. This recent success has led to several fundamental questions regarding the phenomenon of spin tunnelling and also the development of JMR devices. In this paper, experimental results, such as the dependence on bias, temperature and barrier characteristics of FM/I/FM tunnelling are reviewed brie¯y. The in¯uence of inelastic tunnelling processes, metal at the interface and material properties on the JMR is discussed. The future direction from both the physics and the applications viewpoints, is also covered. } 1. INTRODUCTION Spin-polarized tunnelling (SPT), discovered by Meservey et al. (1970) and Merservey and Tedrow (1971, 1994), laid the foundation to a new ® eld of research. Meservey and Tedrow measured the conduction-electron spin polarization P in magnetic metals and compounds using the Zeeman split quasiparticle density of states in a superconductor as the spin detector. Tunnelling from a ferromagnetic (FM) ® lm, with its uneven spin distribution at the Fermi level E F , into such a spinsplit superconducting Al ® lm re¯ects the spin polarization of the tunnelling electrons coming from the ferromagnet. Values, of P recently measured are higher owing to improved junction preparation conditions including samples grown by molecularbeam epitaxy. Highly polarized tunnelling electrons can also be obtained through a phenomenon called the spin-® lter e ect using magnetic semiconductors such as EuS and EuSe as tunnel barriers (Moodera et al. 1988, 1990, 1993). Jullie Á re (1975) made the ® rst reported magnetoresistance measurement on a ferromagnet/insulator/ferromagnet (FM/I/FM) trilayer junction and interpreted it by stating that the tunnelling current should depend on the relative orientation of the magnetizations of the electrodes. The tunnel junction magnetoresistance (JMR) is
Physical Review B, 1998
The tunneling conductance ͑TC͒ and magnetoresistance ͑TMR͒ are investigated for magnetic junctions consisting of two ferromagnetic electrodes separated by a ferromagnetic insulator ͑semiconductor͒. The investigations are based on the nearly-free-electron approximation. The results show that TC and TMR strongly depend on the magnetization configuration of the junction such that TC and TMR reach their maximums when the magnetic moments of the two electrodes are parallel to each other but antiparallel to that of the barrier, whereas the minimums appear when the magnetic moments of both electrodes and the barrier are parallel. The tunneling probability ͑TP͒ relates to the spin orientation of incident electrons and the magnetization configuration of the junction. The variation of TC and TMR with the relative orientation of magnetization of both electrodes and the barrier can be explained by two effects: the ferromagnet-ferromagnet tunneling and spinfiltering effect. Our results agree well with experiment.
Assisted tunneling in ferromagnetic junctions and half-metallic oxides
Applied Physics Letters, 1998
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 tunnel magnetoresistance (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. Onemagnon 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 percent. Examples of half-metallic systems are CrO2/TiO2 and CrO2/RuO2. 73.40.Gk, 73.40.Rw, 75.70.Pa, 85.70.Kh Tunnel magnetoresistance (TMR) in ferromagnetic junctions, first observed more than a decade ago, 1,2 is of fundamental interest and potentially applicable to magnetic sensors and memory devices. 3 This became particularly relevant after it was found that the TMR for 3d magnetic electrodes reached large values at room temperature 4 , and junctions demonstrated a non-volatile memory effect.
Scientific Reports, 2015
The tunnel magnetoresistance (TMR) in the magnetic tunnel junction (MTJ) with embedded nanoparticles (NPs) was calculated in range of the quantum-ballistic model. The simulation was performed for electron tunneling through the insulating layer with embedded magnetic and nonmagnetic NPs within the approach of the double barrier subsystem connected in parallel to the single barrier one. This model can be applied for both MTJs with in-plane magnetization and perpendicular one. We also calculated the in-plane component of the spin transfer torque (STT) versus the applied voltage in MTJs with magnetic NPs and determined that its value can be much larger than in single barrier system (SBS) for the same tunneling thickness. The reported simulation reproduces experimental data of the TMR suppression and peak-like TMR anomalies at low voltages available in leterature.
Electron tunneling and noise studies in ferromagnetic junctions
Thin Solid Films, 2000
Transport properties of exchange-biased magnetic tunnel junction structures are reported as a function of tunnel barrier Ž . thickness. The temperature dependence of the tunneling conductance and magnetoresistance MR is consistent with recent theories relating it to the magnon excitations at the electrode᎐barrier interface or the temperature-dependent surface magnetization. We have also measured the bias voltage dependence of the MR. For CoFe magnetic electrodes, the reduction in MR is approximately 50% at biases of ; 250 mV. At low temperatures, we observe a cusp-like dip in the tunneling conductance at zero-bias. The conductance increases as the square-root of the bias voltage, indicating that electron᎐electron interactions as in disordered media may be important. Coating a magnetic electrode or the oxide barrier with an alternate, thin magnetic layer of higher spin polarization is shown to generally increase the MR by several percent but at the expense of higher 1rf noise. ᮊ
SPIN, 2015
Temperature dependence of the tunnel magnetoresistance (TMR) was calculated in range of the quantum-ballistic model in the magnetic tunnel junction (MTJ) with embedded nanoparticles (NPs). The electron tunnel transport through NP was simulated in range of double barrier approach, which was integrated into the model of the magnetic point-like contact. The resonant TMR conditions and temperature impact were explored in detail. Moreover, the possible reasons of the temperature induced resonant conditions were discussed in the range of the lead–tunneling cell (TC)–lead model near Kondo temperature. We also found that redistribution of the voltage drop becomes crucial in this model. Furthermore, the direct tunneling plays the dominant role and cannot be omitted in the quantum systems with the total tunneling thickness up to 5–6[Formula: see text]nm. Hence, Coulomb blockade model cannot explain Kondo-induced TMR anomalies in nanometer-sized tunnel junctions.
Spatially Modulated Tunnel Magnetoresistance on the Nanoscale
Physical Review Letters, 2011
We investigate the local tunnel magnetoresistance (TMR) effect within a single Co nanoisland using spinpolarized scanning tunneling microscopy. We observe a clear spatial modulation of the TMR ratio with an amplitude of 2020% and a spacing of 201:3 nm between maxima and minima around the Fermi level. This result can be ascribed to a spatially modulated spin polarization within the Co island due to spin-dependent quantum interference. Our combined experimental and theoretical study reveals that spin-dependent electron confinement affects all transport properties such as differential conductance, conductance, and TMR. We demonstrate that the TMR within a nanostructured magnetic tunnel junction can be controlled on a length scale of 1 nm through spin-dependent quantum interference.
Physica B: Condensed Matter, 1998
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.
Nano Letters, 2009
Magnetic tunnel junctions (MTJs), composed of two ferromagnetic electrodes separated by a thin insulating barrier layer, are currently used in spintronic devices, such as magnetic sensors and magnetic random access memories. Recently, driven by demonstrations of ferroelectricity at the nanoscale, thin-film ferroelectric barriers were proposed to extend the functionality of MTJs. Due to the sensitivity of conductance to the magnetization