C. Y. Kuo

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

Angular Dependence of Magnetoresistance During Magnetization Reversal on Magnetic Tunnel Junction Ring

Microstructured magnetic tunnel junction ring with outer/inner diameter of 2/1 mum has been fabricated to investigate the angular dependence of magnetoresistance during magnetization reversal process. The minor loop of magnetoresistive curve reveals four distinct resistance levels associated with four magnetization configurations within the free layer throughout the magnetization reversal. The magnetoresistance decreases with increasing angle between applied external field and biasing direction, which is resulted from the relative alignment of total magnetization of pinned and free layer. An extra feature appeared in the minor loop when the external field is transverse to the biasing direction; this can be attributed to a vortex-pair formation/annihilation in the free layer. Furthermore, a series of schemes of magnetic configurations of pinned and free layers are illustrated to explain the routes of minor loops

Temperature Dependence of Electrical Transport and Magnetization Reversal in Magnetic Tunnel Junction

A series of tunneling magnetoresistance (TMR) has been measured at various temperatures ranging from 4 K to 360 K for characterizing the electrical transport and magnetization reversal of nanostructured magnetic tunnel junctions (MTJs) with thin effective MgO barrier of 1 nm thickness and resistance-area (RA) product of 10 ¿¿m2. MTJs with 150 250 nm2 elliptical shape were fabricated by using electron beam lithography in combination with ion beam milling. Typical TMR curves were observed at temperature above 70 K, below which there was no significant anti-parallel (AP) state revealed. A linear relationship is found between resistance and temperature in both parallel (P) and AP states, having linear coefficients of -4.15 ×10-4 and -8.07 × 10-4 (¿/K), respectively. The TMR ratio was found to be proportional to 1-BT3/2. The negative temperature coefficients and TMR tendency with temperature indicated that electrical transport is dominated mainly by tunneling mechanism. In addition, the biasing field of pinned CoFeB layer due to RKKY coupling increased with decreasing temperature until a maximum biasing field reached at 200 K, after which the biasing field decreased with decreasing temperature.

Fabrication and magnetic properties of 100-nm-scaled permalloy nanotube arrays

Fabrication and magnetization characteristics of permalloy nanotube arrays that are in 100-nm-scale have been demonstrated. Permalloy nanotube array is made by using a standard electron-beam lithography combining with an ion-beam milling process. Geometric parameters of individual permalloy nanotube are fixed to be 300/20 nm for outer diameter/wall thickness, and the heights are varied from 180 to 600 nm. Height dependent magnetization reversal behaviors are investigated by using longitudinal magneto-optical Kerr effect with the external field applied perpendicular to the tubular axis. Micromagnetic simulations are performed to scrutinize the micromagnetization configurations. Up to two pairs of head-to-head and tail-to-tail domain walls on both tubular ends and vortex structure motion on the sidewall are identified during the magnetization reversal.

Current-induced switching of exchange bias in nano-scaled magnetic tunnel junctions with a synthetic antiferromagnetic pinned layer

This report investigates the current-induced switching of exchange bias without an external magnetic field in nano-scaled magnetic tunnel junction (MTJ) cells. An MTJ stack film was patterned into an ellipse with dimensions of 120 nm × 270 nm by using standard electron beam lithography in combination with ion beam etching. A spin-polarized current pulse with a duration of 100 ns was used to switch the exchange bias direction of the synthetic antiferromagnetic (SAF) pinned layer. It is worth noting that the MTJ cell was initialized in a high resistance state before applying the current pulse. For the application of both positive and negative current pulses, the resistance can be switched from the high (antiparallel) state to the low (parallel) one at 2.95 and −2.80 mA, respectively. After the current-induced switching, it was found that the magnetoresistance curve is reversed relative to the one before the current-induced switching. Predominantly, this behavior is independent of the polarity of the current...

Investigation on the Exchange Coupling Properties of Ring-Shaped MnIr/CoFe Bilayers

In this study, the exchange coupling properties of submicron patterned Mnlr/CoFe bilayers with strong exchange coupling strength were investigated. The large area of 2.5 mm × 2.5 mm of submicron sized bilayer rings were fabricated by electron beam lithography and ion-milling processes. The clear variations of normalized magnetic hysteresis loops of patterned films are the degradation of pinned CoFe at the Mnlr/CoFe interfaces and the decrease of exchange bias field, comparing with sheet film. After post-field-annealing treatment, only the exchange field was improved. A magnetic tunnel junction ring with outer diameter/linewidth of 2/0.5 μm was constructed as well to further certify the speculations. The magnetoresistance loops of as-fabricated and post-field annealed MTJ ring revealed that the slightly improved magnetoresistance ratio and exchange field. These results were attributed to the rearrangement of distorted spins at the sample edges. However, in comparison with sheet film, the great degradation of magnetoresistance ratio of patterned device, without respect to as-fabricated or post-field annealed, was not retrieved, revealing that the futility of post-field-annealing treatment to the pinning portion of CoFe layer.

Controllable Remanent States on Microstructured Magnetic Tunnel Junction Rings

Controllable remanent states have been studied on the microstructured magnetic tunnel junction (MTJ) rings through magnetoresistance measurements. These rings were designed accordingly with an outer/inner diameter of 2/1 and 1/0.5 mum to reveal two and one metastable states, respectively, during the magnetization reversal process on the free layer. The distinct magnetoresistance levels based on the tunneling magnetoresistance effect are associated with the relative alignment of magnetization of free layer and pinned layer. As a result, four and three controllable remanent magnetic states on the free layer were manipulated by ramping external magnetic fields, applied in the biasing direction, with various field ranges, giving rise to four and three stable magnetoresistance values at zero field. These results may provide a great potential in magnetic multibit memory applications using ring-shaped cells

Probing the magnetization reversal process of permalloy nano-rings with high wall height-to-thickness ratios

We report a study of magnetization reversal process on a tall magnetic nano-ring with a wall height-to-thickness ratio of up to 21. Samples in a hexagonal lattice pattern of ring array with an outer/inner diameter of 300/260 nm and a ring height of 420 nm are fabricated using electron beam lithography in conjunction with an ion beam etching technique. A longitudinal magneto-optical Kerr effect (MOKE) measurement reveals that meta-stable states exist during the magnetization reversal process. In particular, the exact magnetization configurations at the top end of the nano-ring are illustrated using magnetic force microscopy (MFM) that is facilitated with a photoresist etch-back technique. Most interestingly, MFM images of two sets of head-to-head (H2H)/tail-to-tail (T2T) domain walls are captured during the magnetization reversal process. It is believed that the domain walls formation and evolution processes at the top end of the nano-ring are mediated by the anti-symmetrical distribution of the vortex dom...

Study of inter-dot coupling in nano-patterned permalloy dots array

We present a series of studies on interdot coupling in the nanometer-scaled permalloy dots array. A standard electron beam lithography in conjunction with lift-off process was employed for patterning 30 nm thick of millions of permalloy dots array with diameter (D) of 500 nm and the aspect ratio S/D of spacing (S) to diameter ranging from 0.2 to 1.25. The magnetization reversal processes were identified to evolve through the vortex nucleation, movement, and annihilation based on magnetic force microscopy (MFM) imaging in the presence of external magnetic fields. The nucleation field, annihilation field, and moving rate of vortex core were analyzed using M-H loops measured by the alternating gradient magnetometer (AGM). These behaviors are associated with the dipole-dipole interaction in dots array with various interdot spacings.

Coupling strength with off-axial external field in magnetic tunnel junction cells

A series of hysteresis loops with off-axial external magnetic fields have been measured to identify magnetization reversal properties as well as coupling effects in magnetic tunnel junctions (MTJs). MTJ films, consisting of a synthetic artificial antiferromagnetic pinned layer of CoFeB/Ru/CoFe/IrMn, were patterned into an elliptical cell array with sizes of 200 × 300 nm and 500 × 750 nm. The hysteresis loops were measured using a vibrating sample magnetometer with various directions of the external magnetic field; the angle θ between the external magnetic field (H→E) and the biasing field of the pinned layer (H→B) was varied from 0° to 90°. The two significant coupling effects, ascribed to exchange bias between CoFe/IrMn and Ruderman–Kittel–Kasuya–Yosida–like coupling in CoFeB/Ru/CoFe, can be observed in these hysteresis loops. The angular dependence of the hysteresis loops is employed to characterize the variation of these coupling effects in extended and patterned MTJ films.