Effects of superparamagnetism in MgO based magnetic tunnel junctions

Abstract

We have investigated the magnetic and transport behavior of MgO based magnetic tunnel junctions incorporating Co 40 Fe 40 B 20 free layers with thicknesses in the vicinity of 15 Å. The magnetic response of the free layer changes rapidly as its thickness decreases. Linear and hysteresis-free switching is obtained when the CoFeB free layer thickness is thinner than a critical thickness of ϳ15 Å. The tunneling magnetoresistance and hysteresis properties of the free layer change abruptly around the critical thickness. We have analyzed the transfer curves of junctions below the critical thickness and show that they agree well with the Langevin equation describing superparamagnetism. At even lower thicknesses, the total magnetic moment of the magnetic clusters decreases rapidly, possibly due to the reduction in the magnetic ordering temperature to below room temperature.

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

“…Using the values of CoFeB for the anisotropy constant, 37 K = 19 900 erg cm −3 , density 37 = 7.8 g cm −3 and saturation magnetization 38 M S = 860 emu cm −3 , we obtain an average diameter ͗D͘ Ӎ 46 nm for the CoFeB grains. Such value is in good agreement with recent results published by Shen et al 39 on MgObased MTJs with similar CoFeB free layers thicknesses. The grain dimension obtained from the ZFC fit imply a pancakelike shape for the CoFeB particles, since it is much higher than the nominal thickness of the FM free layer ͑1.55 nm͒.…”

Linearization strategies for high sensitivity magnetoresistive sensors

“…This transition is also illustrated in the inset of Figure 20a with the abrupt drop in coercive field at t critical ∼ 1.45 nm which is in accordance with other reported values [79][80][81][92][93][94]. Tsai et al estimated ∼23 nm as the average lateral size of the ferromagnetic particles at the sensing layer [94] while Shen et al obtained 40-120 nm [95] which for the given t critical implies that the clusters have a pancake-like shape. For t ≤ t critical a linear response is always present, independent of junction area (Fig.…”

“…20b) [92]. The dramatic decrease in TMR is attributed to weakening of ferromagnetic order in the sensing layer, as the barrier conductance is proportional to the magnetization component along the applied field, being the magnetic moment of the clusters and the layer thickness highly correlated [95].…”

Optimization and Integration of Magnetoresistive Sensors

“…29 The use of thin free layers (typically below 1.6 nm) has been explored as an alternative by several groups. 8,9,30 Figure 7 shows transfer curves measured for several free layer thicknesses, showing a transition between square-like to a high sensitivity linear output curve when 1.5-nm thick CoFeB films are used. The increased sensitivity (around 7%/Oe) is associated to a reduction of coercivity for CoFeB around 1.5 nm.…”