Y.K. Zheng

Low switching current flux-closed magnetoresistive random access memory

A spin valve magnetoresistive random access memory with a flux-closed structure is presented. The flux-closed structure prevents the disruption to magnetization of the recording layer and increases the thermal stability. Simulation results show that the switching field increases under the uniform external field, however, the switching field has no change under the bit current field. Only the bit current that flows mostly at the center is used to switch the cell. Experimental results show that the bit current of 10 mA is sufficient to switch the 1 μm×4 μm cell.

Multistate per-cell magnetoresistive random-access memory written at Curie point

A multistate per-cell magnetoresistive random-access memory (MRAM) that writes data by a thermally assisted technique and reads data using the angular-dependent magnetoresistance is proposed. A hard magnetic layer or pinned ferromagnetic layer (CoFe-IrMn) is used as the recording layer. The free layer serves as the read layer. Before reading, the free layers magnetization is set to the initial state. For the N states per-cell MRAM, the magnetization angle of the ith state (i=0 to N-1) between the free layer and recording layer can be set to be acos(1-2*i/(N-1)). For example, in the four-state per-cell MRAM, the magnetization angle can be set to be acos(1), acos(1/3), acos(-1/3), and acos(-1), which represent the four states, respectively. More states can be obtained if the signal-to-noise ratio is sufficient. At near Curie point, a small external field can be used to record the signal. In order to verify the idea, a spin-valve giant-magnetoresistance memory cell was fabricated using e-beam lithography and ultrahigh voltage sputtering. A 25-mA heating current and a small external field are enough to assist the writing process. A four-state per-cell memory is realized by this method.

Asymmetrically kinked hysteresis loops in exchange biased NiFe/IrMn rings

Magnetic properties of a series of NiFe/IrMn rings with inner diameters from 0.35 to 1.50 μm, while keeping the outer diameter of 2.0 μm, have been studied by magnetic force microscopy and hysteresis loop measurements at various temperatures. The enhancements in exchange bias and coercivity have been observed in the patterned ring structures. Asymmetrical, kinked hysteresis loops are present in the NiFe/IrMn rings with inner diameters less than 1.50 μm. The kinks in hysteresis loops in the increasing field branch correspond to the presence of a flux closure state. The presence of the flux closure state has been attributed to the formation of antiferromagnetic domain walls in the magnetization reversal which acts as an energy barrier and traps the flux closure state.

Field dependence of high-frequency magnetic noise in tunneling magnetoresistive heads

Thermal magnetization fluctuations in tunneling magnetoresistive heads with track width of ∼100nm have been studied through high-frequency magnetic noise measurement as a function of both transverse and longitudinal fields. The study suggests that in the dynamic region of the heads, the related power spectral density is determined by the magnetization fluctuations of the free layer. At high fields which drive the sensor to the magnetization saturation, an “abnormal” resonance with substantially high amplitude in comparison with that of the free layer is observed when the magnetization of the free layer is nearly antiparallel to that of the reference layer. This abnormal resonance is excluded from the spin torque effect. The detailed analysis suggests that this resonance is due to the magnetization fluctuations in the reference/pinned layers.

Spin-flop switching of the guided synthetic anti-ferromagnet MRAM

In this work, a new MRAM structure with guided SAF layers is presented. This new structure addresses the issues regarding additional read operation, reduction of saturation field, write operation margin, and unintentional switching.

Flux-enhanced giant magnetoresistance head design and simulation

The possibility of introducing a back flux-guide to increase the efficiency of a shielded giant magnetoresistive (GMR) head was studied. The flux-guide instead of the air effectively increases flux at the top of the GMR element, which leads to an increase of GMR heads sensitivity. The flux distribution was calculated by a two-dimensional (2-D) finite element method. In the simulation, the shield to shield spacing, free layer thickness, and GMR element height are assumed to be 90 nm, 4 nm, and 100 nm, respectively. Parameters such as permeability, thickness and height of flux-guides and spacing between the GMR element and flux-guides have been taken into account. The detailed magnetization distribution is simulated using a three-dimensional (3-D) micromagnetic modeling. A back flux-guide with permeability of more than 100, thickness of more than 10 nm, as well as short spacing between the element and flux-guide enhances the sensors sensitivity. After optimization, the sensitivity of flux-enhanced magnetic tunnel junctions (MTJ) GMR head can be 40% higher than that of the conventional MTJ GMR head.

Side-shielded TGMR reader with track width reduction scheme

As the recording density of the hard disk increases, the smaller reader track width is required. The tunneling GMR reader which has been used in the hard disk drive has the potential for lTb/in2 due to its large MR ratio. To maintain the SNR and match the impedance of the preamplifier, the low RA less than 0.5 Omegamum2 is required. However, the low RA less than 1 Omegamum2 is difficult to achieve due to both the thickness and barrier height limitation. On the other hand, as the track width reduces, the side reading effect will increase the magnetic track width. In this paper, a track width reduction scheme is presented in order to reduce the resistance.

Spin valves with canted pinning field

Spin valves with a canted pinning field were studied. Magnetoresistance (MR) and planar Hall effect (PHE) in the spin valves were simultaneously measured in fields applied in the film plane at different angles (α) with respect to the easy axis of the free layer. The Boltzmann transport equation was used to simulate MR and PHE curves, and to determine the magnetization orientation of the free layer at different α. Results showed that in the magnetization configuration in which the pinning field was perpendicular to the easy axis of the free layer, there existed two distinct magnetization reversal modes when the applied field angle α was near 90°. The magnetization reversal was an incoherent rotation when α 90°. However, in the configuration in which the angle between the pinning field and the easy axis of the free layer was about 45°, there was only one magnetization reversal mode. The interlayer coupling kept the free layer in a...

Switch-free read operation design and measurement of magnetic tunnel junction magnetic random access memory arrays

A switch-free read operation design and cell measurement method for magnetic tunnel junction magnetic random access memory arrays with a peripheral circuitry is presented. The design effectively reduces the shunting effect and maintains the signal at the original level even for the cases when the lead resistance cannot be neglected. It also allowed us to measure the magnetoresistance curve of each cell independently without the need of a transistor or a diode. Both simulation and experiments showed that this method is useful in the readout operation of the tunnel junction magnetic random access memory as well as in the characterization of its individual cells.

Sub-100-nm current-perpendicular-to-plane sensor fabrication

Two sets of processes for sub-100-nm current-perpendicular-to-plane (CPP) spin-valves (SVs) were developed. In the first method, a CPP SV was fabricated by using via-hole instead of small cell patterning. This method makes it easy to characterize the CPP spin valve films. The shunting effect has been analyzed in this method. In the second method, a resist layer for planarization, ion-milling, and wet etching for self-opening via-hole in a self-aligned fashion by making use of the height difference between the cell and surrounding regions was introduced. This method is suitable for the actual recording heads with small dimension even without an ion-beam-deposition system. CPP sensors with size from 0.05 /spl mu/m/spl middot/0.05 /spl mu/m to 0.4 /spl mu/m/spl middot/ 0.5 /spl mu/m with different free- layer structures have been investigated by means of the first method.