In-jun Hwang

Suppression of bias voltage dependence in double-barrier magnetic tunnel junctions comprised of freelayers with an amorphous layer insertion

A double-barrier magnetic tunnel junction (DMTJ) comprising an amorphous freelayer was investigated. The typical sample structure consists of Ta42∕CoZrNb9.2∕IrMn15.5∕CoFe3.3∕AlOx2∕freelayer∕AlOx2∕CoFe7.5∕IrMn15.5∕Ru60(nm). The freelayer includes CoFe2.3∕CoZrNb3.4∕CoFe3. A CoFe 5nm single freelayer was also prepared for reference purpose. Although TMR ratio and RA product appear similar, less attenuation in the TMR ratio was observed at 0.4V for the junction with CoZrNb layer. Coercivity and interlayer coupling were reduced while squareness was improved. The top portion of the DMTJ including top barrier became more uniform after amorphous layer insertion.

Influence of freelayer in magnetic tunnel junction on switching of submicrometer magnetoresistive random access memory arrays

As magnetic tunnel junction (MTJ) cells for magnetoresistive random access memory (MRAM) are reduced in size, the presence of a magnetization vortex seriously interferes with switching selectivity. We prepared 0.3 /spl mu/m/spl times/0.8 /spl mu/m, nearly rectangular shape MTJs consisted of PtMn/CoFe/Ru/CoFe/AlOx/NiFe t (t=3,4.5, and 6 nm). Both at-field and remanent state measurements at 0.4 V were conducted to distinguish kinks originating from vortex and domain wall pinning. In addition, we measured samples at various temperatures (from room temperature to 500 K), and with various hard axis fields (from 0 to 90 Oe). As temperature increased, average switching fields decreased, more rapidly for t=6 nm junctions, but kinks were not eliminated completely. When the hard axis field reached about 40-60 Oe, nearly kink-free switching was possible for t=6 nm MTJs.

Technological issues for high-density MRAM

Key attributes of MRAM (magnetoresistive random access memory) technology are known as non-volatility with high speed and density, radiation hardness, and unlimited endurance. A lot of results have been announced for commercial market. It is anticipated that MRAM would play an important role in future memory market through its unique, functional advantages. For high density MRAM as a standalone memory, several technological issues related with MRAM core cells should be preferentially solved, we demonstrated 1 Kbit MRAM array fabricated by combination of hybrid technology of standard deep sub-micron CMOS and MTJ process. The main issues in the array, which can be fundamental limitation of MRAM technology, are considered. The topic covers basic issues of deep sub-micron MRAM core cell and consider the work related to the MRAM issues, such as cell stability and switching process.

Activation barriers of submicron magnetoresistive random access memory cells with single and synthetic antiferromagnet free layers

Thermally activated magnetization reversals of submicron sized magnetic tunnel junctions with NiFe single and NiFe∕Ru∕NiFe synthetic antiferromagnet (SAF) free layers are investigated by varying the pulse durations and current amplitudes in switching pulse measurements. The measured data show good agreement with the switching probability predicted by the Arrhenius-Neel theory and switching behaviors with a single activation energy barrier. Estimated activation barriers for magnetization reversals are higher for magnetic tunnel junctions (MTJs) with SAF free layers than MTJs with single free layers. It is believed that the high activation barrier is achieved by the larger magnetic volume of SAF layers.

Switching characteristics in magnetic tunnel junctions with a synthetic antiferromagentic free layer

The properties of sub-micrometer scaled MTJs using various synthetic antiferromagnetic (SAF) free layer were studied using remanent-state measurement. The magnetic tunnel junction device was a structure consisted of TiN/PtMn/CoFe/Ru/CoFe/AlO/NiFe/Ru/NiFe/Ta. It was found that junctions using single free layer showed complex switching behaviors with larger switching field variation while the junction using a specific SAF free layer exhibited kink-free R-H curves with less switching field variation. MR ratio and magnetization moment of the MTJ were also measured

Improved selectivity of SAF free layer in high density MRAM array

In this paper, switching properties of the patterned sub-micron MTJ cells incorporating NiFe single and SAF as the free layer of MTJ were investigated. MTJs are composed of PtMn 15/CoFe 1.5/Ru 0.8/CoFe 1.5/Al-O/NiFe t/sub 1/(/Ru 0.8/NiFe t/sub 2/) (nm) with the different thickness (t/sub 1/ and t/sub 2/) of NiFe and fabricated with the standard photo-lithography process to the size of 0.3 /spl times/ 1.0 /spl mu/m/sup 2/. Remanence measurements were performed to ensure vortex states. The magnetoresistance of the MTJs were also studied.