Superparamagnetism in MgO-based magnetic tunnel junctions with a thin pinned ferromagnetic electrode

MgO-based magnetic tunnel junctions have been fabricated with a thin ${\text{Co}}_{40}{\text{Fe}}_{40}{\text{B}}_{20}$ (CoFeB) layer in the pinned synthetic antiferromagnetic CoFe/Ru/CoFeB stack. An inverted tunneling magnetoresistance is observed due to the unbalanced synthetic antiferromagnet. Superparamagnetic nanoparticles form when the CoFeB layer is thinner than 1.5 nm, and an abnormal temperature dependence of the junction resistance is associated with superparamagnetism when the thermal fluctuation energy exceeds the magnetic anisotropy energy. This explanation accounts for the temperature dependence of the tunneling magnetoresistance effect.

Figure 1

(Color online) Temperature dependence of the magnetization $\left(M\text{−}T\right)$ for unpatterned MTJ samples with (a) $t=0.5\text{nm}$, (b) 0.75 nm, and (c) 1.0 nm, measured with ZFC and FC modes.

Figure 2

(Color online) [(a)–(c)] The junction resistance $\left(R\right)$ as a function of field at 300 K, 77 K, and 24 K, respectively. Inset in (a) shows the $R\text{−}{\mu }_{0}H$ curve over a large field range at 300 K. The pinned CoFeB thickness is 1.0 nm for this MTJ, annealed at $300°\text{C}$. The dashed line is of ${\mu }_{0}H=0$.

Figure 3

(Color online) (a) The temperature dependence of $R_{low}$ and $R_{high}$ for one MTJ with $t=1.0\text{nm}$. The schematic temperature dependence of $R_P$ and $R_{AP}$ for MTJs without nanoparticles is also shown. (b) The temperature dependence of the inverted TMR for the same MTJ as shown in (a). Inset in (b) is the temperature dependence of the inverted TMR for one MTJ with $t=0.5\text{nm}$. The solid lines (a) and (b) are the fitting results.

Figure 4

(Color online) (a) Main structure of MgO-based MTJs including the nanoparticles. (b) and (c) The magnetic switching of the MgO-based MTJs with the CoFeB nanoparticles at antiparallel and parallel states.

Figure 5

(Color online) The temperature dependence of TMR with strong and weak superparamagnetism for one MTJ with $t=0.75\text{nm}$ at $T_{\text{a}}=350°\text{C}$. Inset shows the TMR difference $\left(\Delta \text{TMR}\right)$ between the strong and weak superparamagnetism.

Figure 6

(Color online) The effective $T_{\text{B}}$ as a function of the thickness $\left(t\right)$ of the pinned layer, obtained from the $M\text{−}T$ curves and the fitted results. The line is guided to eye.