Perpendicular Multilayers Research Articles

Enhanced spin–orbit torque switching in perpendicular multilayers via interfacial oxygen tunability

Spin–orbit torque (SOT) offers a promising pathway to electrically manipulate magnetization in perpendicular multilayers, but the ultrahigh current density required for SOT switching limits its applications. Here, we report that field-free SOT switching is achieved in perpendicular Ta/CoFeB/MgO multilayers by inserting ultrathin Mg or Hf layers. A critical current density of 1.18 × 107 A/cm2 is obtained in Ta/CoFeB/Mg(0.1 nm)/MgO multilayers for field-free SOT switching, which is 42% lower than that in the Ta/CoFeB/MgO sample. The results demonstrate that the enhanced SOT switching efficiency is determined by a modified Rashba interface induced by interfacial orbital hybridization due to the presence of an ultrathin inserted layer. Furthermore, SOT exhibits a significant dependence on the interfacial structure, especially the interfacial oxygen content. Our findings provide an effective insight into the interfacial manipulation of SOT-based spintronic devices.

Tunable Giant Anomalous Hall Angle in Perpendicular Multilayers by Interfacial Orbital Hybridization

A spintronic device based on the spin-dependent Hall effect has attracted great interest because of its great potential applications in the multivalue storage and logic gate, which isa promising candidate to break the bottleneck of the information industry in the big data period. It is a technological challenge to implant spintronic devices into semiconductor integrated circuits. The anomalous Hall angle (θ), defined as the deviation of the electron flow from the current direction, is the key parameter to evaluate the capacity of Hall device compatibility. However, the bottleneck for the device is low θ (less than 5%) at room temperature (RT), making it difficult to directly complement with the semiconductor circuit which limits its potential application. Here, we report a simple perpendicular multilayered structure with θ up to 5.1% at RT. Wide working temperature (250-350 K) across RT for our samples will accelerate the potential applications in spintronic memory. A giant Hall angle at RT originates from the enhanced side-jump scattering at the atomic-scale-modified interfacial structure. The high θ at RT together with wide working temperature is practically significant and may provide the way for further 3D spintronic devices based on the spin-dependent Hall effect with ultrahigh storage density and ultralow power consumption.

Tunable anomalous hall effect induced by interfacial catalyst in perpendicular multilayers

The interfacial structures, playing a critical role on the transport properties and the perpendicular magnetic anisotropy in thin films and multilayers, can be modified by inserting an ultrathin functional layer at the various interfaces. The anomalous Hall effect (AHE) in the multilayers with core structure of Ta/CoFeB/X/MgO/Ta (X: Hf or Pt) is tuned by interfacial catalytic engineering. The saturation anomalous Hall resistance (RAH) is increased by 16.5% with 0.1 nm Hf insertion compared with the reference sample without insertion. However, the RAH value is decreased by 9.0% with 0.1 nm Pt insertion. The interfacial states were characterized by the X-ray photoelectron spectroscopy (XPS). The XPS results indicate that a strong bonding between Hf and O for Hf insertion, but no bonding between Pt and O for Pt insertion. The bonding between metal and oxygen leads to various oxygen migration behavior at the interfaces. Therefore, the opposite behavior about the RAH originates from the different oxygen behavior due to various interfacial insertion. This work provides a new approach to manipulate spin transport property for the potential applications.

Tuning anomalous Hall effect in perpendicular multilayers with different oxygen environment by interfacial ionic migration

Interfacial oxygen migration and its induced anomalous Hall effect are reported in perpendicular multilayers with different interfacial oxygen-coordinated. Saturation Hall resistance RAH for Pt/Co/MgO/Pt and Pt/Co/Al2O3/Pt multilayers is 3.66Ω and 4.34Ω in as-deposited state, respectively. After annealing at 400°C, RAH value reaches 4.82Ω and 6.67Ω, which is 32% and 54% larger than that in as-deposited samples, respectively. Especially, the increment value ΔRAH in Pt/Co/Al2O3/Pt multilayers is 101% larger than that in Pt/Co/MgO/Pt film. Interfacial structural analysis shows such differentΔRAH in two samples originates from distinct oxygen migration behavior induced different interfacial oxygen-coordinated.

Spin orbit torques and Dzyaloshinskii-Moriya interaction in dual-interfaced Co-Ni multilayers

We study the spin orbit torque (SOT) and Dzyaloshinskii-Moriya interaction (DMI) in the dual-interfaced Co-Ni perpendicular multilayers. Through the combination of top and bottom layer materials (Pt, Ta, MgO and Cu), SOT and DMI are efficiently manipulated due to an enhancement or cancellation of the top and bottom contributions. However, SOT is found to originate mostly from the bulk of a heavy metal (HM), while DMI is more of interfacial origin. In addition, we find that the direction of the domain wall (DW) motion can be either along or against the electron flow depending on the DW tilting angle when there is a large DMI. Such an abnormal DW motion induces a large assist field required for hysteretic magnetization reversal. Our results provide insight into the role of DMI in SOT driven magnetization switching, and demonstrate the feasibility of achieving desirable SOT and DMI for spintronic devices.

Interfacial effects on magnetic interlayer coupling between perpendicular Co/Ni multilayers across Ru spacer

The interlayer exchange coupling properties in synthetic antiferromagnetic (AF) structures based on Co/Ni perpendicular multilayers were investigated by varying the Ru spacer thickness (tRu) and the ferromagnetic (FM) layer material at the FM/Ru interfaces. Samples of two Co layers adjacent to Ru own much stronger AF coupling field (HAF) and show two HAF peaks located at tRu = 0.47 and 1.08 nm. If the interfacial Co layers are substituted by Ni, the HAF at the 1st peak decreases much more considerably, becomes lower than the 2nd one and even disappears due to strong pinhole-induced FM coupling. Such pinhole effect makes HAF of tRu < 0.85 nm remains unchanged or even decrease with decreasing temperatures. By tuning the interfacial Co layer thickness, pinhole effect has been suppressed as a result of greatly enhanced HAF, leading to a normal temperature dependence of Ruderman-Kittel-Kasuya-Yosida-type HAF even at thin tRu cases, i.e., increase in HAF with temperature decreasing.

Tailoring and isolating the interfacial magnetic coupling in curved Co/Pt multilayer film

The multilayer film Co/Pt was deposited onto the curved substrate made of the ordered polystyrene beads. When the perpendicular multilayer film Co/Pt was deposited, the easy axis was tailored along the film deposition direction. VSM measurements showed the magnetic anisotropy axis of the curved multilayer film [Pt(3nm)/Co(0.45nm)]15 was around 45° for PS beads with diameter 200nm. SEM measurements showed the nanocap film formed on the bead, and TEM images showed the film thickness decreased along the curved colloidal surface. HRTEM images showed the sublayer Co and sublayer Pt were alloyed together near the nanocap brim. The interfacial magnetic coupling happened in the unalloyed part of the magnetic nanocap and the perpendicular part was isolated on the top of each polystyrene bead. This work also showed this curved magnetic film was suitable for the high-density recording media.

Controlling the demagnetizing field of an in-plane free layer by exchange coupling with a perpendicular multilayer

The demagnetizing field of a Co50Fe50 free layer in an in-plane micron-sized magnetic tunnel junction (MTJ) can be partially compensated by exchange coupling with a [Co90Fe10/Pt] N multilayer with perpendicular magnetic anisotropy via a Ru interlayer. The perpendicular anisotropy for N = 5 is optimized for nominal CoFe and Pt thickness of 0.4 nm and 1.0 nm, respectively. An increase of tunnel magnetoresistance (TMR) from 2% to 75% is observed in MTJs as the free layer thickness, t varies from 1.0 nm to 3.0 nm. A phenomenological model is developed to interpret the TMR dependence in terms of the free layer magnetization rotation from in-plane to out-of-plane with decreasing t, a consequence of interlayer exchange coupling with the perpendicular multilayer. We suggest that this strategy could significantly reduce the switching current density in such MTJs.

Magnetic behavior of Co/Pt multilayers deposited on nanosphere array

Ordered polystyrene particles were deposited onto the silicon substrate by the self-assembly process and the colloidal surfaces worked as the curved substrate. We fabricated perpendicular multilayer film [Co/Pt] n with the controllable strength and direction of anisotropy axis based on the 2D curved substrate in the magnetron sputtering system. In contrast to the same multilayer film deposited on flat Si substrate, the squareness hysteresis loops remained almost unchanged between 0 ∘ and 45 ∘ for the curved film, which indicated a narrow distribution of the switch field. All the observations were attributed to the variation of the anisotropy axis across the top of the sphere.

X-ray studies of aligned magnetic stripe domains in perpendicular multilayers

We have investigated the stripe domain structure and the magnetic reversal of perpendicular Co/Pt-based multilayers at room temperature using magnetometry, magnetic imaging and magnetic X-ray scattering. In-plane field cycling aligns the stripe domains along the field direction. In magnetic X-ray scattering the parallel stripe domains act as a magnetic grating resulting in observed Bragg reflections up to fifth order. We model the scattering profile to extract and quantify the domain as well as domain wall widths. Applying fields up to ∼1.2 kOe perpendicular to the film reversibly changes the relative width of up versus down domains while maintaining the overall stripe periodicity. Fields above 1.2 kOe introduce irreversible changes into the domain structure by contracting and finally annihilating individual stripe domains. We compare the current results with modeling and previous measurements of films with perpendicular anisotropy.

Superlattice source and its application in Co/Pd multilayers for perpendicular magnetic recording

This article reports a sputtering system, the Intevac Superlattice Source (SLS), designed to meet the needs of making perpendicular multilayers for hard disk manufacturing. The SLS is a two-sided sputtering station in which each side is capable of depositing multiple sequential bilayers of two different film materials. Each side of the source contains a magnetron (sharing the same centerline as the substrate) and a rotating cathode on which are mounted two semicircular targets. The two rotating cathodes (one on each side of the source) are synchronized to ensure matched elements sputtering simultaneously and to prevent cross contamination between target materials. The design easily integrates onto standard hard disk manufacturing systems and can deposit up to 5 bilayers per second. Using this source, a series of Co/Pd multilayers has been made. We investigated the effect of different seedlayers, Pd, Ti, and Ru, on the coercivity and microstructure of Co/Pd multilayers. We found that although the crystalline growth is very different for each of the three seed layers studied, all three lead to higher coercivity and unique squareness of Co/Pd multilayers as compared to multilayers made without any seedlayer. The coercivity achieved for a 300 Å thick seedlayer [Co 4 Å/Pd 10 Å]×15 multilayer is 6253, 6871, and 6920 Oe for Pd, Ti, and Ru seedlayers, respectively.

Micromagnetic study of subnanosecond magnetic switching in perpendicular multilayers

Measurements and micromagnetic modeling results are presented of the subnanosecond magnetization reversal process of perpendicularly magnetized (CoCr/Pt) multilayers. The switching time of the magnetization is primarily dependent on the field strength and field bias angle to the perpendicular direction. The intergranular exchange coupling results in a domino-type magnetization reversal, significant delay in the media response far from the gap edge, and an asymmetry between the maximum upper and lower magnetization levels attained in response to a periodically varying head field. The thermal excitations of the magnetization assist the switching of the magnetization, however, their effect is small and can be neglected, to first order, in studies of high-speed switching of the magnetization in response to a change in the head field polarity. The gyromagnetic switching in the multilayered film appears to be heavily damped.

Real-time observation of sub-nanosecond magnetic switching in perpendicular multilayers

Measurements are presented of sub-ns magnetization reversal processes of perpendicularly magnetized (CoCr/Pt) multilayers, using real-time Kerr microscopy with a 10 ps temporal resolution. The multilayers are placed in contact with a ring-type recording head, which is used as a sub-ns risetime magnetic field source (Δ t=0.8 ns). By scanning the laser beam across the gap the influence of the field angle on the reversal process is studied.