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

Abstract

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.

Sequential magnetic switching in Fe/MgO(001) superlattices

“…The size of the coupling is similar to that found in other studies but in our case it extends through MgO layers which are significantly thicker [8]. The reasons for this are as yet unknown but it has been shown that the size and sign of the coupling is strongly affected by a number of factors such as strain [17], impurities/vacancies in the MgO [13,14,17], and interface roughness [18]. These factors are highly sensitive to the growth conditions which vary somewhat between experiments.…”

“…Although the interaction mechanism is different in a metallic superlattice, similar arguments could apply in the Fe/MgO case. In addition, it is known that the interaction can be either ferromagnetic or antiferromagnetic in the Fe/MgO system, depending on the thickness of the layers, strain, and other factors [17] and recently, a metalliclike oscillatory interaction has even been found in very thin MgO layers [18]. Therefore it is not inconceivable that longer-range interactions could have a different sign to the nearest neighbor interaction.…”

Ruderman–Kittel–Kasuya–Yosida-type interfacial Dzyaloshinskii–Moriya interaction in heavy metal/ferromagnet heterostructures

“…Consequently, there is a possibility that, in our case, the tunneling barrier may be very low, which can lead to a considerable increase in the tunneling depth. Moreover, the charge-neutral gap formed by the vacancies can even enable MgO to behave like a metal 38 , in which case the interlayer RKKY exchange constant demonstrates an oscillating-like behavior similar to that observed by us.…”

“…It should be mentioned that it bears a certain resemblance to the oscillating-like behavior of the interlayer RKKY exchange constant in the case of a conducting spacer. Since the quasi-conducting behavior of MgO spacers has been reported previously 38 , we have developed a simplified theoretical formalism taking advantage of this possibility.…”

Understanding nanoscale effects in oxide/metal heteroepitaxy

“…MgO thin films are of particular interest in this context, since high crystalline quality is obtained when governed by cube-on-cube epitaxy. They are quite successfully used in magnetic tunnel junctions as crystalline barriers which, due to a highly spin-dependent evanescent decay of metallic wave functions, give rise to much higher tunnel performance than amorphous films [22][23][24][25][26][27][28] . On Ag(100), the MgO film grows in the form of islands which progressively cover the surface [29][30][31][32][33] .…”