Te-Ho Wu

Effect of cap layer thickness on the perpendicular magnetic anisotropy in top MgO/CoFeB/Ta structures

The perpendicular magnetic anisotropy of a series of top MgO/CoFeB/Ta layers is studied. Similar to the bottom Ta/CoFeB/MgO structure, the critical thickness of CoFeB is limited in a range of 1.1–1.7 nm. However, the cap layer shows much sensitive effect. Not only the type of material is crucial, but the thickness of the cap layer also affects the magnetic anisotropy. The perpendicular anisotropy of a 1.2 nm-thick CoFeB can only exist with the capping Ta thickness less than 2 nm. The magnetic characterizations, including the magnetic remanence and coercivity, also show strong dependence on the Ta thickness. The diffusion of Ta into CoFeB layer is considered to play an important role, which could explain changes in perpendicular anisotropy and related magnetic responses. In addition, the asymmetric role of Ta layer in the top structure and bottom structures is also discussed.

Influence of crystal structure on the perpendicular magnetic anisotropy of an epitaxial CoPt alloy

By molecular beam epitaxy CoPt1.1 alloys were simultaneously prepared on Mo seeding layers on Al2O3(11–20), (1–102), and (1–100) substrates, respectively. Distinct crystal structures and chemical ordering of the CoPt1.1 alloys were observed for substrate temperatures of 300 and 400 °C. Structural and magnetic observations for CoPt1.1 alloys grown on separate sapphire substrates show that the appearance of the ordered L11(111) phase results in an enhancement of the perpendicular magnetic anisotropy and Kerr rotations in the CoPt alloys.

Effect of Annealing and Barrier Thickness on MgO-Based Co/Pt and Co/Pd Multilayered Perpendicular Magnetic Tunnel Junctions

In this study, the effects of annealing conditions on the magnetic characteristics of multilayered perpendicular magnetic tunnel junctions (pMTJ) of the structures SiNx/Pt/(Co/Pd) 10/MgO/(Co/Pt) 5/Pt with various MgO barrier thicknesses were explored. We found that both the fixed and free layers exhibit coercivity growth with increasing annealing temperature. Insertion of a 0.4 nm Mg layer under the MgO barrier layer increases the corecivities further. Magnetoresistance measured by the current-in-plane tunneling (CIPT) method reveals that the insertion of Mg layers on both side of the MgO layer can increase the MR ratio by up to 32%.

Perpendicular magnetic tunnel junctions with synthetic antiferromagnetic pinned layers based on [Co/Pd] multilayers

We fabricated MgO-based perpendicular magnetic tunnel junctions (p-MTJ) with Ta/CoFeB magnetic electrodes. Synthetic antiferromagnetic (SAF) pinned layers with perpendicular magnetic anisotropy (PMA) were included into the p-MTJs by using two Co/Pd multilayers (MLs) separated by a thin Ru spacer layer. The MTJs stack has the structure bottom contact/free layer CoFeB (1.0)/MgO (1)/pinned layer CoFeB (1.0)/Ta spacer layer/SAF/Ru cap layer/top contact (the units in parenthesis are in nanometers). The SAF was optimized by changing the repetition period n in one of the Co/Pd multilayers and the Ru thickness in order to obtain PMA with antiferromagnetic (AFM) coupling. The Ru spacer values were 0.7, 0.75, 0.8, 0.85, and 0.9 nm. The magnetic studies show that all magnetic films, including the Ta/CoFeB layers, are perpendicularly magnetized. The two Co/Pd MLs are AFM coupled for n > 2. Controlling the Ru thickness, the interlayer exchange coupling strength Jiec can be tailored. Jiec vs. Ru thickness exhibits a si...

Effect of Ta thickness on the perpendicular magnetic anisotropy in MgO/CoFeB/Ta/[Co/Pd]n structures

We studied the effect of a thin Ta layer on the perpendicular magnetic anisotropy (PMA) of composite FM1/Ta/FM2 magnetic structures, where FM1 represents the subsystem MgO/CoFeB, and FM2 denotes a [Co/Pd]6 multilayer. The stack without Ta spacer layer shows no PMA. Once a Ta layer is inserted between the thin CoFeB layer and the [Co/Pd]6 multilayer, PMA is observed. The perpendicular magnetization loops show squareness ratios close to unity, indicating the presence of almost complete perpendicular anisotropy. These hysteresis loops also show sharp switching characteristics, indicating that the MgO/CoFeB bilayer and the [Co/Pd]6 multilayer are ferromagnetically coupled together. The coercive field Hc of the composite structure increases as Ta thickness increases. Our results show that Ta layer is essential for integrating MgO/CoFeB and [Co/Pd]6 into a composite magnetic structure with perpendicular anisotropy.

Perpendicular Magnetic Tunneling Junction With Double Barrier Layers for MRAM Application

A double-barrier-layer perpendicular magnetic tunneling junction (DpMTJ) structure consisting of Si substrate/Pt/GdFeCo/AlOx/GdFeCo/FeCo/AlOx/FeCo/TbFeCo/Pt/Ti-cap was prepared by a direct current (dc) and radio frequency (RF) magnetron sputtering method. An elliptical DpMTJ element with 3.5 mumtimes2.5 mum size was fabricated using a top-down technique. A conducting atomic force microscope (CAFM) was used to obtain I-V curves of DpMTJ structures. We obtained the magnetoresistance (MR) ratio value from measured I-V curves by applying two opposite magnetic fields value of plusmn200 Oe perpendicular to the plane of film. The MR ratio was reached as high as 74% at zero applied bias voltage. Furthermore, the MR ratio decreased as bias voltage increased. It could make the DpMTJ structure to be used in the high-density MRAM devices

Magnetic properties of TbFeCo-based perpendicular magnetic tunnel junctions

The perpendicular magnetic tunnel junctions of structure Si/Ti(10 nm)/TbFeCo(22 nm)/CoFeB(2 nm)/MgO(1 nm)/CoFeB(1 nm)/TbFeCo(3 nm)/Ti(10 nm) were deposited on SiNx substrates at room temperature with various Tb contents. The samples were then annealed at different temperatures ranging from 150 to 350 °C. We found that the magnetic tunnel junctions with free-layer Tb content near the compensation point show largest coercivities and best squareness, and more important, best annealing endurance. The maximum magnetoresistance ratio is 4.62% for Tb content 19.7% as deposition. Annealing treatment cannot increase MR further because no crystalline MgO structures were present, even though the samples were annealed up to 350 °C.

Fabrication and Characterization of Microstructured Magnetic Tunnel Junction Rings

Microstructured magnetic tunnel junction rings have been fabricated by a top-down technique combining electron beam lithography and ion milling process. Four-terminal magnetoresistance measurements and magnetic force microscopy were used to successfully explore a four-transition process within the free layer throughout the magnetization reversal. Various magnetization configurations were identified to be the onion state, vortex-pair state, vortex state, vortex-core state, and reverse onion state. In addition, the various durations of each magnetic state observed in the magnetoresistance curve can be utilized for the study of a coupling effect between the pinned layer and the free layer

Micromagnetic Simulation for Spin-Transfer Switching With a Tilted Spin Polarizer

This paper reports the micromagnetic simulations of spin-wave modes excited by spin-transfer torque for an elliptical nanomagnet sized 178 × 133 × 2.1 nm3. Through power spectral analysis we discovered that, for the collinear case, the excited spin-wave mode exhibits the same spatial symmetry as the initial ground state. The additional action of the Oersted field not only changes the mode pattern but also shifts the eigenfrequency from 8.24 GHz to 2.99 GHz due to the degradation of symmetry. For the non-collinear case, without taking into account the Oersted field, a tilted polarizer leads to the excitation of the quasi-uniform mode and reduces the switching time substantially. However, considering the Oersted field, the activated mode is the center mode. The simulated results indicate that the spatial symmetry of the Oersted field and the spin-torque term play crucial roles in determining the activated spin-wave eigenmodes.

Current-in-plane Tunneling Measurement Through Patterned Contacts on Top Surfaces of Magnetic Tunnel Junctions

This study reports an alternative method for measuring the magnetoresistance of unpatterned magnetic tunnel junctions similar to the current-in-plane tunneling (CIPT) method. Instead of using microprobes, a series of point contacts with different spacings are coated on the top surface of the junctions and R-H loops at various spacings are then measured by the usual four-point probe method. The values of magnetoresistance and resistance-area products can be obtained by fitting the measured data to the CIPT theoretical model. The test results of two types of junctions were highly similar to those obtained from standard CIPT tools. The proposed method may help to accelerate the process for evaluating the quality of magnetic tunnel junctions when commercial CIPT tools are not accessible.