Magnetoresistance and spin electronics

Journal of Physics: …, 2003

Journal of Applied Physics, 2010

We propose a general formalism to describe accurately the angular dependence of the magnetoresistance. A parabolic band model is used to determine without approximation the conductance of arbitrary complex heterostructures. Simple analytical expressions are obtained in some limit cases. Particularly, we show that significant deviation from the cosine dependence is expected for ferromagnetic barriers. Numerical computations are used to quantify the deviation from the cosine dependence for normal and ferromagnetic barriers and support the precedent conclusion. Finally, the influence of the applied voltage on the angular dependence of magnetoresistance is discussed.

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.

Applied Physics Letters, 2005

We present spin transfer switching results for MgO based magnetic tunneling junctions (MTJs)with large tunneling magnetoresistance (TMR) ratio of up to 150% and low intrinsic switching current density of 2-3 x 10 MA/cm2. The switching data are compared to those obtained on similar MTJ nanostructures with AlOx barrier. It is observed that the switching current density for MgO based MTJs is 3-4 times smaller than that for AlOx based MTJs, and that can be attributed to higher tunneling spin polarization (TSP) in MgO based MTJs. In addition, we report a qualitative study of TSP for a set of samples, ranging from 0.22 for AlOx to 0.46 for MgO based MTJs, and that shows the TSP (at finite bias) responsible for the current-driven magnetization switching is suppressed as compared to zero-bias tunneling spin polarization determined from TMR.

Applied Physics Letters, 2002

MRS Bulletin, 2006

Spin-polarized currents can be generated by spin-dependent diffusive scattering in magnetic thin-film structures or by spin-dependent tunneling across ultrathin dielectrics sandwiched between magnetic electrodes.By manipulating the magnetic moments of the magnetic components of these spintronic materials, their resistance can be significantly changed, allowing the development of highly sensitive magnetic-field detectors or advanced magnetic memory storage elements.Whereas the magneto-resistance of useful devices based on spin-dependent diffusive scattering has hardly changed since its discovery nearly two decades ago, in the past five years there has been a remarkably rapid development in both the basic understanding of spin-dependent tunneling and the magnitude of useful tunnel magnetoresistance values.In particular, it is now evident that the magnitude of the spin polarization of tunneling currents in magnetic tunnel junctions not only is related to the spin-dependent electronic s...

Journal of Physics D: Applied Physics

Journal of Magnetism and Magnetic Materials, 2007

A theoretical study is presented concerning the relationship between spin transfer torque and absolute current-perpendicular-to-plane magnetoresistance in metallic spin-valves (SV) and magnetic tunnel junctions. In a first step, using Valet and Fert Boltzmann-like theory extended to any metallic magnetic multilayers with non-collinear magnetizations, linear relationships between spin torque and absolute current-perpendicular-to-plane giant magnetoresistance are derived numerically, when varying the parameters of the structure one by one. The obtained results are compared with an extension of J.C. Slonczewski's circuit theory. The latter model gives an analytical expression of this linear dependence. In a second step, using an out-of-equilibrium perturbation formalism (Keldysh technique), we study the tunnel magnetoresistance (TMR) dependence of the spin torque amplitude in magnetic tunnel junctions when varying the parameters one by one in the junction. Once again, linear dependences are obtained, with different characteristics when the TMR vanishes. It is shown that these two equivalent behaviours are associated with different origins of the spin torque. r

Journal of the Physical Society of Japan, 2008

A magnetic tunnel junction (MTJ), which consists of a thin insulating layer (a tunnel barrier) sandwiched between two ferromagnetic electrode layers, exhibits tunneling magnetoresistance (TMR) due to spin-dependent electron tunneling. Since the discovery of room-temperature (RT) TMR effect in 1995, MTJs with an amorphous aluminum oxide (Al-O) tunnel barrier have been studied extensively. The Al-O-based MTJs exhibit magnetoresistance (MR) ratios up to about 70% at RT and are currently used in the read heads of hard disk drives and magnetoresistive random access memory (MRAM). MTJs with MR ratios significantly higher than 70% at RT, however, are needed for next-generation spintronic devices. In 2001, first-principle theories predicted that the MR ratios of epitaxial Fe/MgO/Fe MTJs with a crystalline MgO(001) barrier would be over 1000% because of the coherent tunneling of fully spinpolarized Á 1 electrons. In 2004, MR ratios of about 200% were obtained at RT in MTJs with a singlecrystal MgO(001) barrier or a textured MgO(001) barrier. CoFeB/MgO/CoFeB MTJs for practical applications were also developed and found to have MR ratios up to 500% at RT. MgO-based MTJs are of great importance not only for device applications but also for clarifying the physics of spin-dependent tunneling.

IEEE Transactions on Magnetics, 2000

The dependence of magnetotransport on field orientation is an important issue in spintronics-related devices where the applied field is not necessarily in the ideal field-in-plane (FIP) geometry. In this study, we perform tunneling magnetoresistance (TMR) measurements on CoAl 2 O 3-CoFe-NiFe spin-dependent tunnel (SDT) junctions prepared at different conditions with varying field orientation ranging from FIP to field-perpendicular-to-plane (FPP). The TMR ratio decreases drastically, whereas the switching field of Co increases when the field direction is set close to FPP. Furthermore, in a situation near FPP, a peculiar TMR looping behavior is observed for one set of samples. Interface effect is thought to be related. Index Terms-Field-in-plane (FIP), field-perpendicular-to-plane (FPP), spin-dependent tunnel (SDT) junction, tunneling magnetoresistance (TMR).

M agnetically engineered magnetic tunnel junctions (MTJs) show promise as non-volatile storage cells in high-performance solid-state magnetic random access memories (MRAM) 1 . Th e performance of these devices is currently limited by the modest (<~70%) room-temperature tunnelling magnetoresistance (TMR) of technologically relevant MTJs. Much higher TMR values have been theoretically predicted for perfectly ordered (100) oriented single-crystalline Fe/MgO/Fe MTJs. Here we show that sputter-deposited polycrystalline MTJs grown on an amorphous underlayer, but with highly oriented (100) MgO tunnel barriers and CoFe electrodes, exhibit TMR values of up to ~220% at room temperature and ~300% at low temperatures. Consistent with these high TMR values, superconducting tunnelling spectroscopy experiments indicate that the tunnelling current has a very high spin polarization of ~85%, which rivals that previously observed only using half-metallic ferromagnets 2 . Such high values of spin polarization and TMR in readily manufactureable and highly thermally stable devices (up to 400 °C) will accelerate the development of new families of spintronic devices.

Journal of Applied Physics, 2014

An extension of the standard spin diffusion theory is presented by introducing a density-gradient (DG) term that is suitable for describing interface quantum tunneling phenomena. The magnetoresistance (MR) ratio is modified by the DG term through an interface electric field. We have also carried out spin injection and detection measurements using four-terminal Si devices. The local measurement shows that the MR ratio changes depending on the current direction. We show that the change of the MR ratio depending on the current direction comes from the DG term regarding the asymmetry of the two interface electronic structures. 73.43.Qt,85.75.Hh Spin injection and detection between silicon and magnetic material via tunneling barriers constitute one of the most important issues in spintronics, and the key factor that determines the performance of spin devices such as spin transistors . The difference in electronic structure between ferromagnet (FM) and semiconductor (SC) generates the conductance mismatch between the two materials, which has inspired much important research regarding spin transport properties through the interface. With regard to the spin diffusion theory (standard theory) , which has to a great content succeeded in explaining tunneling phenomena of nonmagnet (NM) sandwiched by two FMs, the challenge is to explain new phenomena that appears in SC: Jansen et al. showed the effect of the depletion layer in the SC interface [23] and pointed out the importance of the consideration of band structure at the interface. Tran et al. discussed the relation between the localized states and the enhancement of spin accumulation signal at Co/Al 2 O 3 /GaAs interface. Yu et al.

Applied Physics Letters, 2003

We report on spin-polarized tunneling in fully epitaxial Fe/MgO/Fe/Co tunnel junctions. By increasing the thickness of the insulating layer (t MgO ), we have strongly enhanced the tunnel magnetoresistance. Values up to ϳ100% at 80 K ͑ϳ67% at room temperature͒ have been observed with t MgO ϭ2.5 nm. This tunnel magnetoresistance ratio, which is much larger than the one predicted by the Jullière's model, can be understood in the framework of ab initio calculations.

Applied Physics Letters, 2008

Physical Review B, 2006

A current problem in semiconductor spin-based electronics is the difficulty of experimentally expressing the effect of spin-polarized current in electrical circuit measurements. We present a theoretical solution with the principle of transference of the spin diffusion effects in the semiconductor channel of a system with three magnetic terminals. A notable result of technological consequences is the room temperature amplification of the magneto-resistive effect, integrable with electronics circuits, demonstrated by computation of current dependence on magnetization configuration in such a system with currently achievable parameters.