Evidence of spin-polarized direct elastic tunneling and onset of superparamagnetism in MgO magnetic tunnel junctions

Abstract

Magnetic tunnel junctions ͑MTJs͒ with crystalline MgO barriers are currently being used in a variety of applications, namely forefront magnetic sensors and memories. In this work we probed the temperature ͑T͒ dependence of the transport and magnetic properties of MgO-based MTJs with different CoFeB free layer thicknesses ͑t fl = 1.55, 1.65, 1.95, and 3.0 nm͒. All samples have the same insulating MgO barrier with a nominal thickness of 1.35 nm. Our results show that the tunnel magnetoresistance ͑TMR͒ temperature behavior is mainly due to a strong variation of the conductance ͑G͒ of the antiparallel state. Also, we provide evidence that direct elastic tunneling is the dominant mechanism determining the temperature dependence of the tunneling conductance and TMR in the studied MgO MTJs. Intrinsic to this mechanism is the thermal smearing of the electron energies near the Fermi level which then plays a key role in G͑T͒, especially in the parallel state where the overall change in G is very small. Furthermore, we show a clear change in the MTJ properties as the free layer thickness is reduced. Besides the typical decrease of TMR related with the loss of spin polarization, we were able to probe the thickness dependence of the spin wave ␣ parameter. MTJ with the thinner free layer show both absence of hysteresis in the room temperature TMR cycles and interesting freezing effects in the zero and field cool magnetization curves at low temperatures, revealing the discontinuous nature of thin free layers.

Temperature dependence of interlayer coupling in perpendicular magnetic tunnel junctions with GdOX barriers

“…The polarization is assumed to vary with temperature in the same way as the magnetization such that, = 0 (1 − Fitting ΔG data to equation (2) gives P0 and α equal to 0.521 and 6.30 x 10 -5 K -3/2 , respectively. This value of α is in the expected range based on previous MTJs with MgO 43 and AlOX barriers 44 . The inset of Fig.…”

Localized states in advanced dielectrics from the vantage of spin- and symmetry-polarized tunnelling across MgO

“…Compared with the standard MgO MTJ theory 35,36 , this leads to the unexpected relation f P 4f AP at room temperature. As future experiments on appropriate stacks 24,40,65 (and perhaps those of ref. 43 with high TMR) with possible defect control 27 can now explicitly confirm, we expect that a sufficient density of M centres would lower f P to that centre's energy position of B0.4 eV from E F (see Table 1), thereby yielding f P of AP and larger values of TMR.…”

LAO-NCS: Laser Assisted Spin Torque Nano Oscillator-Based Neuromorphic Computing System

“…All these parameters are temperature-dependent. Elevating the temperature will increase G T and reduces P 1 and P 2 (Teixeira et al, 2010). As a result, R P is almost independent of temperature while R AP reduces approximately linearly with temperature.…”

“…Two tunneling mechanisms contribute to the STNO resistance including electron spin-polarized direct elastic tunneling and spin independent tunneling. The total conductance of the STNO can be described as (Teixeira et al, 2010).…”