Inherent temperature effects in magnetic tunnel junctions

Abstract

Theoretical studies of the temperature dependence of the tunneling magnetoresistance ratio (TMR) are presented. A successful elastic tunneling model has been extended to handle temperature dependence. It treats Fermi smearing and applies Stoner-like behavior to the exchange split band structure in the electrodes to calculate TMR(T). As expected, the effects of Fermi smearing are small, but small changes in the magnetic band structure produce large changes in TMR. For a Co/I/Co junction produced by LeClair et al. [Phys. Rev. Lett. 84, 2933 (2000)], calculations using bulk magnetization predicted 33% of the experimental loss of TMR from 0 to 300 K with only a 1.5% change in magnetization. A mere 3.2% change in magnetization produced 100% of the observed drop in TMR. These results imply larger than imagined intrinsic temperature dependence for TMR.

Temperature dependence of transport spin polarization inNdNi5from point-contact Andreev reflection

“…Since much of the practical interest is in the value of spin polarization close to room temperature, the bulk spin polarization at elevated temperature is often estimated based on the assumption P͑T͒ ϰ M s ͑T͒, namely, that the spin polarization of the electrons close to the Fermi level is proportional to the spontaneous magnetization ͓M s ͑T͔͒ of the ferromagnet. [14][15][16][17] Though the spin polarization as a function of temperature has been measured using spin-polarized photoemission for a few ferromagnets, 18,19 this simple relation has so far remained experimentally unverified even for relatively simple ferromagnets. 20 The experimental verification of this relation is important for two reasons.…”

Band Structure and Density of States Effects in Co-Based Magnetic Tunnel Junctions

“…By a careful analysis of the conductance-voltage (dI͞dV -V ) and magnetoresistance-voltage characteristics, in combination with 59 Co nuclear magnetic resonance (NMR) to directly investigate local physical structure, we show experimentally that junctions with electrodes of two different crystalline phases show obviously asymmetric transport characteristics. The observed asymmetries can be qualitatively explained by a model developed by us [13][14][15] based on ballistic tunneling between the calculated band structures (and resulting DOS) of fcc-Co and hcp-Co. Furthermore, these experimental and theoretical results may explain several related observations of similar conductance asymmetries [8,[16][17][18].…”

“…Since the differential TMR more strongly decreases at the position of the shoulder and local minimum, one expects that either the minority DOS increases or the majority DOS decreases. As we will show, the dip and shoulder features found in dI͞dV͑V ͒ can be accounted for theoretically by a combination of band structure and DOS arguments, incorporated into a ballistic model of tunneling [13][14][15].…”

Origin of temperature dependence in tunneling magnetoresistance

“…In a different approach, Julliere's model was extended [9] to include both a temperature-dependent surface spin polarization, and a spin-independent current channel, assumed to arise from hopping within the barrier [10][11][12]. Other proposed mechanisms include barrier defect and/or magnetic impurity scattering [13][14][15][16] and band structure effects [17]. Although thermal smearing is known to dominate the temperature dependence of superconducting tunneling [18], the same mechanism has been largely ignored in MTJs, based on the conclusions in Simmons' original theoretical work [19] where thermal effects are found to be very small for elastic normal-state tunneling through an ideal trapezoidal barrier.…”