Metalliclike behavior of the exchange coupling in (001) Fe/MgO/Fe junctions

Exchange magnetic coupling between Fe electrodes through a thin MgO interlayer in epitaxial junctions has been investigated as a function of temperature, MgO thickness, and interface quality. Depending on the MgO thickness, which has been varied from 1.5 to 4 monolayers, two opposite temperature dependences are clearly disentangled. For a thin MgO spacer, the main component decreases with temperature following a metalliclike behavior. On the contrary, for the thickest MgO layers, the main component increases with temperature, following an Arrhenius law. Moreover, the insertion of a monoatomic roughness at the bottom MgO interface, induced by the addition of a fraction of a Fe monolayer, exacerbates the metallic features as an oscillatory behavior from antiferromagnetic to ferromagnetic is observed. These results allow questioning the simple tunneling mechanism usually invoked for MgO coupling, and suggest a crossover behavior of the thin MgO spacer from metallic to insulating with a progressive opening of the gap.

Figure 1

High-resolution transmission microscopy of Fe (50 nm)/Fe ($x$)/MgO (3.1 ML)/Fe (5 nm) where $x=0\mathrm{ML}$ or 0.5 ML. (a,b) correspond to a flat Fe/MgO interface and (c,d) to an interface with Fe islands.

Figure 2

(a) Magnetization loop of a Fe (54.0 nm)/MgO (3 ML)/Fe (4.3 nm) junction measured at 300 K. Inset: minor loop of the thinnest layer where the switching fields are pointed out with dotted lines; an arrow points to the saturation field $H_s$ and plateau field $H_P$; they correspond to the average value of the switching fields from or towards saturation (respectively, plateau) magnetization. (b) Magnetization loop of a Fe (50 nm)/MgO (2.5 ML)/Fe (5 nm)/Co (100 nm) measured at 5 K.

Figure 3

Temperature dependence of the bilinear antiferromagnetic coupling $\text{−}J$ of three Fe (50 nm)/MgO ($x$ nm)/Fe (5 nm) junctions with (a) $x=3.0\mathrm{ML}$; (b) 3.25 ML; (c) 3.5 ML. The straight lines are fits with the model described in the text.

Figure 4

MgO thickness dependence of the parameters deduced from the fit of the temperature dependence of the coupling constant $j\left(T\right)$ for a Fe (50 nm)/MgO ($x$ nm)/Fe (5 nm) junction. (a,b) Exchange coupling; (c,d) thermally activated coupling. The dotted lines are guides for the eyes. The error bars correspond to a confidence level of 95%.

Figure 5

Antiferromagnetic bilinear coupling constant measured at room temperature by Kerr magnetometry as a function of the Fe covering rate of the Fe buffer for three MgO spacer thicknesses: (a) 3.25 ML; (b) 3.4 ML; (c) 3.65 ML. The stack of the sample is Fe (50 nm)/MgO ($t$ ML)/Fe (5 nm).

Figure 6

Temperature dependence of the coupling in soft/hard MgO junctions with different MgO thicknesses. (a) Flat interface; (b,c) atomic rough interface. The lines are fits using Eq. (3). The fitted parameters are (a) $T^0=140\mathrm{K}$, $-J^{\mathrm{EXC}}\left(0\right)=0.21\mathrm{erg}/\mathrm{c}{\mathrm{m}}^2$, $-J^{\mathrm{th}}=0.13\mathrm{erg}/\mathrm{c}{\mathrm{m}}^2$, $E=73\mathrm{K}$; (b) $T^0=134\mathrm{K},-J^{\mathrm{EXC}}\left(0\right)=1.04\mathrm{erg}/\mathrm{c}{\mathrm{m}}^2$, $-J_{}^{\mathrm{th}}=0.185\mathrm{erg}/\mathrm{c}{\mathrm{m}}^2,E=105\pm 20\mathrm{K}$; (c) $T^0=150\pm 30\mathrm{K},-J^{\mathrm{EXC}}\left(0\right)=-0.046\mathrm{erg}/\mathrm{c}{\mathrm{m}}^2$, $-J^{\mathrm{th}}=0.20\mathrm{erg}/\mathrm{c}{\mathrm{m}}^2$, $E=83\mathrm{K}$.

Figure 7

Mean interlayer exchange coupling measured at 5 and 300 K as a function of MgO thickness wedge in the case of a (Fe 50 nm/MgO $x$/Fe 5 nm/Co 100 nm) stack with flat (0 Fe ML) or atomic rough (0.5 Fe ML) interface.

Figure 8

(a) Supercell model used in ab initio calculation. (b) Layer projected density of states for up and down spins in the Fe/MgO/Fe supercell slab.

Figure 9

Sketch of the bilayer illustrating the angles ${\theta }_1,{\theta }_2$, and φ; $K_i$: fourfold anisotropy constant of layer $i$; $M_i$: magnetization of layer $i$, $t_i$: thickness of layer $i$; $J$: bilinear coupling; $J_b$: biquadratic coupling.

Figure 10

Sketch of an experimental minor loop of the thin layer.

Figure 11

Temperature dependence of the biquadratic coupling of three Fe (50 nm)/MgO ($x$ nm)/Fe (5 nm) junctions with $x=3.0\mathrm{ML}$; 3.25 ML; 3.5 ML.

Figure 12

Biquadratic coupling constant measured at room temperature by Kerr magnetometry as a function of the Fe covering rate of the Fe buffer for three MgO spacer thicknesses: (a) 3.25 ML; (b) 3.4 ML; (c) 3.65 ML. The stack of the sample is Fe (50 nm)/MgO ($t$)/Fe (5 nm).

Figure 13

Extended temperature dependence of $\text{−}J$ of two Fe (50 nm)/MgO ($x$ nm)/Fe (5 nm) junctions with (a) $x=3.0\mathrm{ML}$; (b) 3.5 ML. The straight lines are fits with the model described in the text. The red points correspond to measurements performed with increasing temperature, the black one, with decreasing temperature.