Ching-Ming Lee

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.

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%.

Influence of asymmetry on vortex nucleation and annihilation in submicroscaled permalloy disk array

Magnetic properties of vortex nucleation, annihilation, and switching field distribution (SFD) in NiFe disk arrays, where the elements are with 300nm diameter and different degrees of asymmetry, were investigated through measurements and simulations of hysteresis loop. The nucleation and annihilation of vortex state show strong dependences on the asymmetry. More interestingly, the width of SFD, the crucial factor for high-density storage application, oscillating with the degree of asymmetry is observed. The simulation results agree well with the experimental data.

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.

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.

Interplay between out-of-plane anisotropic L11-type CoPt and in-plane anisotropic NiFe layers in CoPt/NiFe exchange springs

Films of L11-type CoPt/NiFe exchange springs were grown with different NiFe (Permalloy) layer thickness (tNiFe = 0–10 nm). X-ray diffraction analysis reveals that the characteristic peak position of NiFe(111) is not affected by the CoPt-layer—confirming the absence of any inter-diffusion between the CoPt and NiFe layers. Magnetic studies indicate that the magnetization orientation of NiFe layer can be tuned through varying tNiFe and the perpendicular magnetic anisotropy of L11-type CoPt/NiFe films cannot sustain for tNiFe larger than 3.0 nm due to the existence of exchange interaction at the interface of L11-CoPt and NiFe layers. Magnetic force microscopy analysis on the as-grown samples shows the changes in morphology from maze-like domains with good contrast to hazy domains when tNiFe ≥ 3.0 nm. The three-dimensional micro-magnetic simulation results demonstrate that the magnetization orientation in NiFe layer is not uniform, which continuously increases from the interface to the top of NiFe layer. Furth...

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.

THE DEVELOPMENT OF PERPENDICULAR MAGNETIC TUNNELING JUNCTIONS

This paper reviews the progress made over the last few years in understanding the development of perpendicular magnetic tunneling junctions (pMTJs). The material systems for making pMTJs, including rare-earth/transition metal alloys, L10-ordered (Co, Fe)–Pt alloys, Co/(Pd, Pt) multilayers, and CoFeB–MgO crystallized structures, are briefly introduced. The fabrication processes of the MTJ devices are focused on, consisting of open-trench, etch-back and self-aligned techniques. The authors also propose a spin-torque nano-oscillator based on pMTJ, for application in GHz range telecommunications.

Synchronization of spin torque nano-oscillators through dipolar interactions

In an array of spin-torque nano-oscillators (STNOs) that combine a perpendicular polarized fixed layer with strong in-plane anisotropy in the free layers, magnetic dipolar interactions can effectively phase-lock the array, thus further enhancing the power of the output microwave signals. We perform a qualitative analysis of the synchronization of an array based on the Landau-Lifshitz-Gilbert equation, with a spin-transfer torque that assumes strong in-plane anisotropy. Finally, we present the numerical results for four coupled STNOs to provide further evidence for the proposed theory.

Effects of MgO Barrier Thickness on Magnetic Anisotropy Energy Constant in Perpendicular Magnetic Tunnel Junctions of (Co/Pd)

In this study, we report experimental results on perpendicular magnetic tunnel junctions composed of the binary multilayer Co/Pt and Co/Pd with various MgO barrier thicknesses. The coercivities of both Co/Pt and Co/Pd layers varied substantially with the MgO thickness. Very interestingly, the perpendicular anisotropy constant Ku of the Co/Pt multilayer measured by the extraordinary Hall effect oscillated with the MgO thickness, and the period of this oscillation was about 0.3 nm. Furthermore, both the Ku value and the oscillation amplitude decreased after annealing at 150deg C and 200deg C, which suggest that the oscillation may result from the film morphology.