Vincent Sokalski

Optimization of Ta thickness for perpendicular magnetic tunnel junction applications in the MgO-FeCoB-Ta system

The impact of Ta thickness on magnetic anisotropy and interlayer magnetic coupling is evaluated for the Ta-FeCoB-MgO thin film system commonly used in magnetic tunnel junctions. It is shown that there exists a window of Ta thickness where strong magnetic coupling of FeCoB with another magnetic layer is achievable through Ta while still maintaining properties required for use in a perpendicular magnetic tunnel junction. We also expand on existing knowledge about the role of annealing temperature, film composition, and seedlayer sequence on magnetic anisotropy in Ta/FeCoB/MgO tri-layers of varying FeCoB thickness.

Experimental modeling of intergranular exchange coupling for perpendicular thin film media

We present an experimental model system that enables quantitative assessment of intergranular exchange coupling in CoCrPt-oxide perpendicular magnetic recording media. A thin film structure consisting of a high coercivity CoPt unicrystal layer and a lower coercivity CoPt layer separated by a thin oxide interlayer is used to model perpendicularly magnetized grains separated by oxide grain boundaries. Exchange coupling energy between the CoPt layers was obtained for SiOx, TiOx, and CrOx interlayers by measuring field shifts from the lower coercivity layer. Cr segregation in CoCrPt grains to grain boundaries is also modeled experimentally and found to significantly suppress exchange coupling.

Increased Perpendicular TMR in FeCoB/MgO/FeCoB Magnetic Tunnel Junctions by Seedlayer Modifications

By modifying the seedlayer in perpendicular FeCoB/MgO/FeCoB magnetic tunnel junctions (MTJs), we observe an increase in maximum tunneling magnetoresistance (TMR) from 65% up to 138%. Its found that decreasing the Ta deposition rate in Ta/Ru/Ta underlayers allows for greater annealing temperatures (up to 350 ) while still maintaining a perpendicular easy axis. An improvement is also seen at a lower temperature where both seedlayers maintain a perpendicular FeCoB easy axis indicating that the increase in TMR is not solely related to annealing at a higher temperature.

Magnetic anisotropy and stacking faults in Co and Co84Pt16 epitaxially grown thin films

A combined set of experimental and theoretical diffraction studies are performed to evaluate the possible impact of stacking faults on magnetic anisotropy using epitaxially grown Co/Ru and Co84Pt16/Ru thin films on MgO(111) single crystal substrates. A 3rd nearest neighbor interaction is incorporated into Monte Carlo simulations of faulted film growth used to predict (10.L) diffraction profiles. These are compared with experimental profiles to determine stacking fault content. It is found that stacking fault density decreases with increasing temperature concurrent with an increase in magnetic anisotropy and a compression of the crystallographic lattice parameter, c.

Characterization of Oxide Materials for Exchange Decoupling in Perpendicular Thin Film Media

We report on continued measurements using an experimental model system to quantify intergranular exchange coupling in CoCrPt-oxide perpendicular magnetic recording media. A thin film multi-layered structure comprising a high coercivity CoPt unicrystal layer separated from a low coercivity CoPt layer by a thin oxide interlayer is used to model the vertically aligned grains separated by oxide boundaries in CoCrPt-oxide media. Exchange coupling is measured by field shifts of the minor loop from the low coercivity layer. Results on coupling energy as a function of interlayer thickness are presented for several different oxides. Additional measurements are presented to understand the possible role of the added alloying elements, Cr and Mn, to the magnetic grains in terms of intergranular exchange coupling.

Naturally Oxidized FeCo as a Magnetic Coupling Layer for Electrically Isolated Read/Write Paths in mLogic

Recently, a nonvolatile, low power circuit scheme based on current-induced domain wall motion and perpendicular magnetic tunnel junctions known as mLogic has been proposed that requires electrically isolated, magnetically coupled read and write paths. Here, we suggest naturally oxidized FeCo for the magnetic coupling layer. Relevant properties of the FeCo-oxide were evaluated by a preliminary investigation of [FeCo/FeCo-Oxide]N artificial superlattices. It is found that FeCo thin films form an insulating 11 Å passivating oxide layer with a magnetization of 500 emu/cc. Experimental measurements show that FeCo-oxide can couple two perpendicular FeCoB layers with a coupling strength greater than 0.35 ergs/cm2.

Noise Mechanisms in Small Grain Size Perpendicular Thin Film Media

In this paper, we present a set of systematic experimental investigations on possible noise mechanisms for current perpendicular thin film media of small grain sizes. In particular, we focus on intergranular exchange coupling and grain boundary surface anisotropy in the granular layer of the present continuous-granular-composite film structure. Micromagnetic modeling studies are conducted to study the impact of the observed experimental phenomenon. Modeled experiments show that significant intergranular exchange coupling may occur when oxide grain boundary thickness becomes less than 1 nm. If the grain boundary thickness has significant distribution below this critical value, the exponential dependence of the coupling strength on the oxide thickness would yield significant degradation of the medium signal-to-noise ratio. Carefully designed experiments have also been conducted to study possible grain boundary interfacial anisotropy. Co/Cr, CoPt/Cr, Co/SiO2, Co/Cr2O3, and Co/TiO 2 interfaces are investigated and the corresponding interfacial anisotropy strengths are quantitatively measured. Although Co/SiO2 interfacial anisotropy appears to be the weakest among them, the measured interfacial anisotropy energy strengths for all of them are significant fractions of the crystalline perpendicular anisotropy of the grains at present grain sizes. Finally, we investigated the impact of stacking faults in hcp Co-alloy grains. It is found that when the anisotropy strength of a small segment of a grain substantially reduces due to the existence of stacking faults, it will yield a switching field reduction disproportional to the volume ratio of the segment.

Experimental demonstration of four-terminal magnetic logic device with separate read- and write-paths

Magnetic logic has recently become an attractive candidate for future electronics. This paper describes the demonstration of a four-terminal spintronic device with distinct read- and write-paths (“mCell”). The mCell enables a non-volatile circuit technology (mLogic) with gain sufficient to drive fanout independent of CMOS [1]. Measured material properties and prototype device results are presented.

Annealing effect and under/capping layer study on Co/Ni multilayer thin films for domain wall motion

Co/Ni multilayer structure with Perpendicular Magnetic Anisotropy (PMA) is considered to be one of the most promising film structures for current-driven domain wall motion. In this work, the field annealing effect on Co/Ni multilayer films with different underlayers is studied. The annealing temperature ranges from 250 °C to 375 °C. The effect of Pt capping layer is also investigated. It was found that the annealing process influences magnetic properties of Co/Ni multilayers with different underlayers differently. For Co/Ni multilayers with Ni underlayer, no PMA is observed in the as-deposited state, but they become perpendicular after annealing, and the effective perpendicular magnetic anisotropy (Keff) increases linearly with annealing temperature. The origin of the Keff increase is discussed. For Co/Ni multilayers with Pt underlayers, large PMA is observed for as-deposited films; however, Keff decreases after annealing. The effects of interfacial lattice mismatch, roughness, and impurities to surface a...

Thickness and Interface-Dependent Crystallization of CoFeB Alloy Thin Films

We have analyzed the crystallization processes of sputtered thin films of Co20Fe60B20 and Co20Fe60B20-X alloys (X = Ta, Hf) using the increase in saturation magnetization that accompanies the formation of α-FeCo. Constant heating rate analysis shows a decrease in crystallization temperature for MgO/CoFeB/Ta multilayers with decreasing CoFeB thickness (from 350 °C for a 20 nm film to 275 °C for a 2 nm film) that we attribute to interface effects on critical nucleus formation. Alloying CoFeB with Ta or Hf is found to decrease Curie temperature and inhibit crystallization. For very thin (<;2 nm) CoFeB films, such as those found in modern perpendicular devices, interdiffusion of adjacent Ta layers in the absence of a low-energy nucleation surface, such as MgO, can prevent crystallization altogether. CoFeB films with adjacent TaN layers are found to crystallize regardless of film thickness, and we suggest that these alternate capping layers have lower interdiffusivity.