Direct imaging of the structural change generated by dielectric breakdown in MgO based magnetic tunnel junctions

Abstract

MgO based magnetic tunnel junctions are prepared to investigate the dielectric breakdown of the tunnel barrier. The breakdown is directly visualized by transmission electron microscopy measurements. The broken tunnel junctions are prepared for the microscopy measurements by focussed ion beam out of the junctions characterized by transport investigations. Consequently, a direct comparison of transport behavior and structure of the intact and broken junctions is obtained. Compared to earlier findings in Alumina based junctions, the MgO barrier shows much more microscopic pinholes after breakdown. This can be explained within a simple model assuming a relationship between the current density at the breakdown and the rate of pinhole formation.PACS numbers: 68.37. Lp, 85.30.Mn, In 1975 the tunnel magnetoresistance effect (TMR) in ferromagnet/ insulator/ ferromagnet systems was discovered by Jullière [1]. In 1995 significant TMR was found at room temperature [2,3]. Since then, magnetic tunnel junctions (MTJ) became potential candidates for magnetic random access memory (MRAM) [4] and have already replaced the giant magnetoresistance [5,6] read heads in hard discs.The dielectric breakdown of the junctions, which is of major importance for their reliability, has been investigated for the formerly used Alumina based [7,8,9, 10] as well as the recently introduced MgO based MTJs [11] due to their possible use in electronic devices. These studies were generally done by analyzing the transport properties of the MTJs [7,8,9,11] and/ or indirect imaging of the proposed breakdown mechanism [10], generally single pinholes in the insulating barrier. Direct investigations of intact MTJs were also performed [12,13].Here, we try to directly image the broken barriers of the tunnel junctions. In order to do this, we prepared MTJs and performed standard transport measurements. Then, one half of the junctions was stressed by high voltages to induce the dielectric breakdown. Finally, slices were cut out of the broken and intact junctions by focussed ion beam (FIB) and investigated with transmission electron microscopy (TEM). This allows to directly compare structure and transport properties of the intact and broken samples and to compare the findings with those of alumina based MTJs. A simple model is presented to explain the behavior. The magnetic tunnel junctions are prepared in a magnetron sputter system with a base pressure of 1 × 10 −7 mbar. The layer stack is * Electronic address: athomas@physik.uni-bielefeld.de; URL: http: //www.spinelectronics.de (100) wafer. To activate the exchange biasing and for the crystallization of the MgO barrier, the layer stack is annealed after sputtering at 623 K for 60 minutes in a magnetic field of 6500 Oe. The stack is patterned by optical lithography and ion beam etching. The junction sizes are between 7 × 7 and 22.5 × 22.5 µm 2 . All measurements are done by conventional two probe technique.The transmission electron microscopy samples were prepared by FIB with a FEI NOVA NANOLAB 600, which allow...

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“…Baker et al 14 15 reported a rapid decrease in the damping due to reduced spin pumping with the addition of an ultra-thin MgO barrier layer between Fe and Pt, from which it was concluded that spin current can tunnel through a few monolayers of an insulating oxide barrier. The work was supported by transmission electron microscopy (TEM) imaging, which is limited to sampling very small areas and provides a projection of a thin 3D sample volume that may not show pinhole defects, and any defects present may be difficult to directly image 16 .…”

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“…We swept the voltage from zero to −600 to 600 mV and back to zero. Voltages of more than 600 mV led to a dielectric breakdown of the junctions (Thomas et al, 2008 ; Schaefers et al, 2009 ). All measurements are done with the bottom electrode as the reference potential.…”

“…These values are close to the dielectric breakdown voltage of the devices. Transmission electron microscopy images of MgO junctions before and after the dielectric breakdown are presented in our previous work (Thomas et al, 2008 ; Schaefers et al, 2009 ). Consequently, the investigations utilizing sequences of voltage pulses were limited to a maximum voltage of 500 mV.…”