Article

Heavy-Ion-Induced Displacement Damage Effects in Magnetic Tunnel Junctions With Perpendicular Anisotropy

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Abstract

We evaluate the resilience of CoFeB/MgO/CoFeB magnetic tunnel junctions (MTJs) with perpendicular magnetic anisotropy (PMA) to displacement damage induced by heavy-ion irradiation. MTJs were exposed to 3-MeV Ta <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2+</sup> ions at different levels of ion beam fluence spanning five orders of magnitude. The devices remained insensitive to beam fluences up to $10^{11}$ ions/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , beyond which a gradual degradation in the device magnetoresistance, coercive magnetic field, and spin-transfer-torque (STT) switching voltage were observed, ending with a complete loss of magnetoresistance at very high levels of displacement damage (>0.035 displacements per atom). The loss of magnetoresistance is attributed to structural damage at the MgO interfaces, which allows electrons to scatter among the propagating modes within the tunnel barrier and reduces the net spin polarization. Ion-induced damage to the interface also reduces the PMA. This study clarifies the displacement damage thresholds that lead to significant irreversible changes in the characteristics of STT magnetic random access memory (STT-MRAM) and elucidates the physical mechanisms underlying the deterioration in device properties.

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... Various strategies have been explored to tailor the anisotropy of Heusler alloys, such as strain-induced anisotropy change [60], buffer layer effects [61,62], doping-induced disorder effects [63] and irradiation [8,[64][65][66]. Modification of the magnetic properties of various materials via irradiation has been reported for several types of irradiations, such as plasma of various gasses [67,68], ionizing radiation [69,70], high-energy protons and cosmic rays, X-rays [71], neutrons [72] and even several low and swift heavy ion irradiations [73,74]. Among these techniques, ion irradiation is a well-established technique for shaping the magnetic properties of materials due to its high precision and control over the irradiation parameters, such as the ion species, ion energy and ion beam current density [8,[64][65][66]75,76]. ...
... Displacement per atom (DPA) has been increasingly used to evaluate ion-irradiation or implantation-induced damage in materials, where the DPA denotes the average number of times that an atom from the substrate lattice is displaced during ion irradiation and accounts for all the parameters to provide a single value that represents the total damage endured by the sample. Particularly in magnetic materials, where the structural changes by irradiationinduced damage result in the modification of magnetic properties, DPA is a valuable parameter and has been employed by various researchers in the field [73,[95][96][97]. Park et al. [98] reported that 20 keV proton (p + ) and Cr + ion irradiation of CoFeB/MgO/CoFeB-based MTJs caused different displacement damages. ...
... Proton irradiation up to 1 × 10 18 p + ·m −2 caused negligible displacement damages on MTJ layers, whereas Cr + -irradiation-induced displacement damages reduced the magnetization and magnetoresistance. Xiao et al. [73] focused on the heavy ion irradiation of CoFeB/MgO/CoFeB layers using 3 MeV Ta 2+ ions. They reported that the DPA of up to 3.5 × 10 −4 for 1 × 10 11 ions·cm −2 was below the threshold limit for any significant displacement damages, above which the displacement damages caused structural damages at the CoFeB/MgO interface. ...
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... Following 10 12 ions/cm 2 exposure, minor degradation of switching functionality is observed, which would not preclude normal memory functionality. At fluences of 10 13 ions/cm 2 and above, switching functionality is lost due to severe damage in the interface, FL, or PLs, with the exact nature of the degradation still under investigation [90]. ...
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SRIM is a software package concerning the Stopping and Range of Ions in Matter. Since its introduction in 1985, major upgrades are made about every six years. Currently, more than 700 scientific citations are made to SRIM every year. For SRIM-2010, the following major improvements have been made: (1) About 2800 new experimental stopping powers were added to the database, increasing it to over 28,000 stopping values. (2) Improved corrections were made for the stopping of ions in compounds. (3) New heavy ion stopping calculations have led to significant improvements on SRIM stopping accuracy. (4) A self-contained SRIM module has been included to allow SRIM stopping and range values to be controlled and read by other software applications. (5) Individual interatomic potentials have been included for all ion/atom collisions, and these potentials are now included in the SRIM package. A full catalog of stopping power plots can be downloaded at www.SRIM.org. Over 500 plots show the accuracy of the stopping and ranges produced by SRIM along with 27,000 experimental data points. References to the citations which reported the experimental data are included.
Article
The transport properties of Co/Al2O3/Co magnetic tunnel junctions with ion-irradiated tunneling barrier are reported. The irradiation by He+ and Ar+ with energies ranging from 15 to 105 eV takes place in situ after oxidation of the 1.4-nm-thick Al layer. For both ion species the area resistance of the junctions increases strongly with ion energy, simultaneously the tunneling magnetoresistance is reduced. But the energy dependence of both properties is different for He+ and Ar+ irradiations. Additionally the bias voltage dependence of the tunneling magnetoresistance is deteriorated with increasing ion energy especially for Ar+ irradiation. These experimental results are discussed with respect to the energy-dependent penetration depth of He+ and Ar+ and their energy loss in the barrier. (c) 2005 American Institute of Physics.
Article
In the field of spintronics, researchers have manipulated magnetization using spin-polarized currents. Another option is to use a voltage-induced symmetry change in a ferromagnetic material to cause changes in magnetization or in magnetic anisotropy. However, a significant improvement in efficiency is needed before this approach can be used in memory devices with ultralow power consumption. Here, we show that a relatively small electric field (less than 100 mV nm(-1)) can cause a large change (approximately 40%) in the magnetic anisotropy of a bcc Fe(001)/MgO(001) junction. The effect is tentatively attributed to the change in the relative occupation of 3d orbitals of Fe atoms adjacent to the MgO barrier. Simulations confirm that voltage-controlled magnetization switching in magnetic tunnel junctions is possible using the anisotropy change demonstrated here, which could be of use in the development of low-power logic devices and non-volatile memory cells.
Article
When an electron tunnels from a metal into the barrier in a magnetic tunnel junction it has to cross the interface. Deep in the metal the eigenstates for the electron can be labelled by the point symmetry group of the bulk but around the interface this symmetry is reduced and one has to use linear combinations of the bulk states to form the eigenstates labelled by the irreducible representations of the point symmetry group of the interface. In this way there can be states localized at the interface which control tunneling. The conclusions as to which are the dominant tunneling states are different from that conventionally found. Comment: 14 pages, 5 figures, accepted in PRB, v2: reference 3 completed
Radiation effects on magnetic tunnel junctions and novel magnetic thin films
  • N Anuniwat