The intrinsic temperature dependence and the origin of the crossover of the coercivity in perpendicular MgO/CoFeB/Ta structures

A collapse of tunnel magneto resistance (TMR) in the perpendicular magnetic configuration of CoFeB/MgO/CoFeB tunnel junction at high annealing temperatures has been reported recently [Ikeda et al., Nature Mater. 9, 721 (2010)]. This observation indicates that not only the temperature-dependent magnetic characterization is important in a pseudo-spin valve type devices but also implies an asymmetrical dependence on the magnetic behavior between the top and bottom CoFeB layers. In this report, we have measured a series of MgO/CoFeB/Ta with different thicknesses of CoFeB (1.0−1.7 nm) and Ta cap layer (1–5 nm) and found a intrinsic dependence of magnetic coercivity of Hc = Hco[1−(T/TB)1/2], where Hco is Hc at 0 K and TB is the blocking temperature, for all films. A systematic study shows that Hco varies in the range of 2500 Oe−250 Oe with a rough inverse linear dependence on CoFeB layer thickness. The TB for all films except the thinnest one (1.0 nm) is in a smaller range of 280–300 K, but drops to 150 K for t...