Double tunnel junctions for magnetic random access memory devices

Abstract

Optimum junction resistance and minimum tunnel magnetoresistance (TMR) ratio required for high density and high performance magnetoresistive random access memory (MRAM) devices with a TMR cell plus field effect transistor (FET) switch architecture are discussed by taking into account the variation of FET resistance causing noise. This implies that a TMR ratio over 25% at a 400 mV bias voltage and junction resistance of several tens of kilo-ohms for TMR cells are required with a signal voltage of 30 mV and a sense current of 10 μA, which leads to about 10 ns read time. This large magnetoresistance ratio at the elevated bias voltage requires low bias voltage dependence of TMR for the MRAM devices. In order to try to meet this requirement, double tunnel junctions were fabricated which possess the central ferromagnetic layer consisting of a thin discontinuous layer of hard ferromagnetic Co80Pt20 nanoparticles and insulating Al2O3 prepared by alternate sputtering of Co80Pt20 and Al2O3 targets. The maximum TMR obtained was 20.5% at room temperature for FeCo top and bottom electrodes without annealing. Bias voltage dependence of the (NiFe/CoFe)/1.5 nm Al2O3/discontinuous CoPt/2.6 nm Al2O3/(CoFe/NiFe) double tunnel junctions were revealed to be small compared to that of single junctions, the barrier of which was also fabricated by sputtering of an Al2O3 target.

Molecular dynamics simulations on the direct sputtering of Al2O3 insulating film in a magnetic tunneling junction

“…The insulating layer can be also generated by directly depositing Al 2 O 3 film using Al 2 O 3 ceramic target. This technique has been applied on the fabrications of double tunnel junctions for MRAM devices [13,17], tunnel giant magnetoresistance [18], and organic thin film transistors [19], although less observed for conventional FM/ insulating/FM MTJs. Depending on the final applications, this method may yield a very thin Al 2 O 3 film, e.g.…”

Microstructure investigation on barrier shapes of double barrier magnetic tunnel junctions

“…As we know that the MTJs based on the single crystal barrier MgO͑001͒ with over 200% TMR ratio at room temperature ͑RT͒ can be obtained, 4 this ratio decreases with increasing bias voltage, which is one of the drawbacks for applications of MTJs in devices. [5][6][7][8] To overcome this difficulty, researchers have focused on the fabrication of double barrier MTJs ͑DBMTJs͒ using molecular beam epitaxy 9 and magnetron sputtering. 6,7 By using these two techniques, the DBMTJs with relatively high TMR and high V 1/2 have been obtained successfully.…”

Evidence for hot electron magnetocurrent in a double barrier tunnel junction device

“…Recently, double barrier magnetic tunnel junctions ͑DBMTJ͒ have been considered for use in MRAM technology. The TMR exhibits a reduced bias dependence in two terminal measurements 4 and hot electron transport may provide asymmetric I-V characteristics 5 when a contact is made to the middle electrode.…”