T. Dimopoulos

Antiferromagnetically coupled CoFeB/Ru/CoFeB trilayers

This work reports on the magnetic interlayer coupling between two amorphous CoFeB layers, separated by a thin Ru spacer. We observe an antiferromagnetic coupling which oscillates as a function of the Ru thickness x, with the second antiferromagnetic maximum found for x=1.0–1.1nm. We have studied the switching of a CoFeB∕Ru∕CoFeB trilayer for a Ru thickness of 1.1nm and found that the coercivity depends on the net magnetic moment, i.e., the thickness difference of the two CoFeB layers. The antiferromagnetic coupling is almost independent of the annealing temperatures up to 300°C while an annealing at 350°C reduces the coupling and increases the coercivity, indicating the onset of crystallization. Used as a soft electrode in a magnetic tunnel junction, a high tunneling magnetoresistance of about 50%, a well-defined plateau and a rectangular switching behavior is achieved.

Thermal annealing of junctions with amorphous and polycrystalline ferromagnetic electrodes

In this work we study Al-oxide-based tunnel junctions with amorphous Co60Fe20B20 and polycrystalline Co90Fe10 ferromagnetic (FM) electrodes. Focus is given on the evolution of the tunnel magnetoresistance and barrier characteristics (resistance-area product, effective thickness, height, and asymmetry) as a function of the annealing temperature up to 400°C. Junctions with two CoFeB electrodes show the largest thermal stability of the tunnel magnetoresistance. Substituting firstly one and then both CoFeB electrodes with CoFe leads to an increasingly faster degradation of the spin-dependent transport upon annealing. The observed differences suggest an improved interface quality between the amorphous FM and the Al oxide.

Interface sharpening in CoFeB magnetic tunnel junctions

We report grazing incidence x-ray scattering evidence for sharpening of the interface between amorphous Co60Fe20B20 and AlOx during in situ annealing below the Co60Fe20B20 crystallization temperature. Enhancement of the interference fringe amplitude in the specular scatter and the absence of changes in the diffuse scatter indicate that the sharpening is not a reduction in topological roughness but a reduction in the width of the chemical composition profile across the interface. The temperature at which the sharpening occurs corresponds to that at which a maximum is found in the tunneling magnetoresistance of magnetic tunnel junctions.

Magnetic tunnel junctions with yttrium oxide barrier

Magnetic tunnel junctions have been studied, with YOx barriers prepared by plasma oxidation of a 1.5 nm Y film. We report their junction area resistance, tunnel magnetoresistance (TMR) and barrier parameters (height and thickness) as a function of the oxidation time. For the optimum oxidation time, TMR values of ∼25% are obtained at room temperature and ∼44% at low temperature (5 K). The barrier height extracted from the current–voltage curves, is close to 1 eV, which is less than half of what is usually reported for AlOx-based junctions. Structural and topographical characterization of the multilayes revealed that the YOx layer is amorphous with well-defined, smooth, and correlated interfaces with the ferromagnetic electrodes.

Large tunnel magnetoresistance with plasma oxidized MgO barrier

This work focuses on magnetic tunnel junctions with a polycrystalline MgO barrier, prepared by plasma oxidation. Combined with Co50Fe50 ferromagnetic electrodes, a large tunnel magnetoresistance (TMR) of 60% is obtained at room temperature. The TMR effect is comparable to state-of-the-art Al oxide barriers with amorphous CoFeB electrodes. It is also found to decrease with the MgO thickness. Two most significant advantages of the MgO junctions are pointed out: (a) The resistance-area product is approximately two orders of magnitude lower than for AlOX− based junctions of the same thickness. (b) MgO presents unsurpassed thermal stability for high annealing temperatures (up to 370 °C) and long annealing periods. In addition, for small, patterned elements, we have tested the switching behavior of the soft electrode grown on the polycrystalline MgO barrier.

Structural and electrical characterization of SiO2/MgO(001) barriers on Si for a magnetic transistor

We report a structural and electrical study of sputter-deposited SiO2/MgO barriers for developing magnetic Si-based transistors. We propose that SiO2/MgO tunneling barriers may utilize spin-filtering by achieving crystalline MgO (001) while reducing spin-scattering due to the Si/SiO2 interface. We find that MgO (<3 nm thick) crystallizes with (001) preferred orientation on thermally oxidized Si(<2 nm). Typical processing temperatures do not cause significant intermixing with SiO2 or ferromagnetic electrode. Conversely, MgO on Si is amorphous up to 2 nm thick. Capacitance-voltage characteristics of MgO capacitors are influenced significantly by the density of interface-states, as high as 5×1013 cm−2 eV−1 while Si/SiO2/MgO structures are electrically beneficial by reducing to 6×1012 cm−2 eV−1.

Diode effect in all-oxide Sr2FeMoO6-based magnetic tunnel junctions

Sr2FeMoO6(70nm)∕SrTiO3(3nm)∕CoFe2O4(30nm) magnetic tunnel junctions are grown by pulsed laser deposition and patterned using optical lithography. The square tunnel elements with lateral size of 50μm exhibit nonlinear zero-field current-voltage (I‐V) variation. More interesting, when the temperature is decreased, the I‐V curves become highly asymmetric and present a diodelike behavior. In order to explain this behavior, ab initio calculations are performed to obtain the electronic structure for each oxide layer. It is shown that our structure behaves like a usual metal∕insulator∕semiconductor diode due to the small gap into the majority-spin band of the CoFe2O4 layer. Finally, we point out that the I‐V characteristic can be magnetically controlled.

Magnetic properties of antiferromagnetically coupled CoFeB/Ru/CoFeB

This work reports on the thermal stability of two amorphous CoFeB layers coupled antiferromagnetically via a thin Ru interlayer. The saturation field of the artificial ferrimagnet which is determined by the coupling, J, is almost independent on the annealing temperature up to more than 300 D C. An annealing at more than 325 D C significantly increases the coercivity, H-c, indicating the onset of crystallization. © 2004 Elsevier B.V. All rights reserved.

Switching of submicron-sized, antiferromagnetically coupled CoFeB∕Ru∕CoFeB trilayers

This work reports on the magnetic reversal of submicron-sized elements consisting of an CoFeB∕Ru∕CoFeB artificial ferrimagnet (AFi). The elements were patterned into ellipses having a width of approximately 250–270 nm and a varying aspect ratio between 1.3 and 8. The coercivity was found to decrease with an increasing imbalance of the magnetic moment of the two antiferromagnetically coupled layers and is therefore strongly affected by an increase of effective anisotropy due to the antiferromagnetic coupling of the two layers. With respect to a single layer of amorphous CoFeB, patterned in comparable elements, the AFi has an increased coercivity. Switching asteroids comparable to single layers were only observed for samples with a high net moment.

Strong temperature dependence of antiferromagnetic coupling in CoFeB/Ru/CoFeB

The temperature dependence of saturation and spin-flop fields for artificial ferrimagnets (AFi) based on antiparallel coupled CoFeB/Ru/CoFeB trilayers has been investigated in a temperature range between 80 K and 600 K. The results presented in this paper are relevant for magnetic devices using this system, e.g. magnetic-random access memory based on spin-flop switching. In good accordance to the theory, the saturation field Hsat behaves like Hsat ∝ H0(T/T0)/sinh(T/T0) with a characteristic temperature of T0 ≈ 150 K. Within this model, the Fermi velocity for the Ru layer is of the order of 105 m/s, therefore, explaining the strong variation of the coupling strength with the temperature in Ru-based AFi. Furthermore, a strong uniaxial anisotropy of Ku = 2 × 103 J/m3 with a small angular distribution of the anisotropy axes is observed for the AFi trilayers based on amorphous CoFeB alloys.