Magnetic memories, particularly magnetic random access memories (MRAMs), have drawn increasing interest due to their potential for high read/write speed, excellent endurance, non-volatility and low power consumption during operation. An MRAM can store information utilizing magnetic materials as an information recording medium. One type of MRAM is a spin transfer torque random access memory (STT-RAM). STT-RAM utilizes magnetic junctions written at least in part by a current driven through the magnetic junction.
For example, FIG. 1 depicts a conventional dual magnetic tunneling junction (MTJ) 10 as it may be used in a conventional STT-RAM. The conventional dual MTJ 10 typically resides on a bottom contact 12, uses conventional seed layer(s) 13 and is under a top contact 16. The conventional dual MTJ 10 includes a conventional antiferromagnetic (AFM) layer 20, a first conventional pinned layer 30, a first conventional tunneling barrier layer 40, a conventional free layer 50, a second conventional tunneling barrier layer 60, a second conventional pinned layer 70, and a second conventional AFM layer 80.
Conventional contacts 12 and 16 are used in driving the current in a current-perpendicular-to-plane (CPP) direction, or along the z-axis as shown in FIG. 1. The conventional tunneling barrier layers 40 and 60 are nonmagnetic, insulating, and thin. For example, a one nanometer thick layer of MgO might be used for either conventional tunneling barrier 40 or 60. The conventional seed layer(s) 13 are typically utilized to aid in the growth of subsequent layers, such as the AFM layer 20, having a desired crystal structure.
The conventional free layer 50 has a changeable magnetic moment 51. Although depicted as a simple layer, the conventional free layer 50 may also include multiple layers. For example, the conventional free layer 50 may be a synthetic antiferromagnetic (SAF) layer including magnetic layers antiferromagnetically or ferromagnetically coupled through thin conductive layers, such as Ru. The free layer 50 is typically on the order of two to four nanometers in thickness.
The conventional pinned layers 30 and 70 shown are SAFs. Thus, the conventional pinned layer 30 includes ferromagnetic layers 32, 34 and 38 separated by nonmagnetic layers 33 and 36. The nonmagnetic layers 33 and 36 are typically Ru and approximately one Angstrom thick. The ferromagnetic layer 38 closer to the conventional free layer 50 is termed a reference layer 38 and is typically formed of 1-2 nanometers of CoFeB. The ferromagnetic layers 32 and 34 may be made of materials such as CoFe and are approximately each one to three nanometers thick. Similarly, the conventional pinned layer 70 includes ferromagnetic layers 72 and 76 separated by nonmagnetic layer 74. The nonmagnetic layer 74 is typically Ru. The ferromagnetic layer 72 closer to the conventional free layer 50 is termed a reference layer 72 and is formed of one to three nanometers of CoFeB. The ferromagnetic layer 76 is typically one to three nanometers of CoFe. Thus, the pinned layer 30 typically has a thickness of approximately five to eleven nanometers. The pinned layer 70 typically has a thickness of three to seven nanometers. In other conventional MTJs, the pinned layers 30 and 70 could be simple ferromagnetic layers. In such conventional MTJs, the nonmagnetic layers 33, 36 and 74, ferromagnetic layer 34, and reference layers 38 and 72 would be omitted.
The conventional AFM layers 20 and 80 are used to fix, or pin, the magnetic moments of the conventional pinned layers 30 and 70, respectively. For example, the conventional AFM layer 20 pins the magnetic moment 31 of the conventional ferromagnetic layer 32 in the direction shown. This pinning may be accomplished via an exchange interaction. Similarly, conventional AFM layer 80 pins the magnetic moment 77 of the conventional ferromagnetic layer 76 in the direction shown. The thickness of the conventional nonmagnetic layers 33 and 36 are set such that the magnetic moments 37, 35, and 31 of the reference layer 38 and ferromagnetic layers 34 and 32 are antiferromagnetically coupled. Similarly, the thickness of the conventional nonmagnetic layer 74 is set such that the magnetic moment 73 of the reference layer 72 is antiferromagnetically coupled to the magnetic moment 77 of the pinned layer 76. In order to sufficiently pin the magnetic moments 31 and 77, the AFM layers 20 and 80 are thick antiferromagnets. For example, the AFM layers 20 and 80 are formed of antiferromagnets such as PtMn and typically have thicknesses approximately fifteen to twenty nanometers.
Spin transfer torque may be used to write to the conventional MTJ dual 10. In particular, spin transfer torque rotates the magnetic moment 51 of the conventional free layer 50 to one of the two directions along its easy axis. When a write current is passed through the conventional MTJ 10 perpendicular to the plane of the layers, electrons may be spin polarized by transmission through or reflection from the conventional pinned layers 30 and 70. The spin transfer torque on the magnetic moment 51 of the conventional free layer 50 may be adequate to switch the conventional free layer 50 if a sufficient current is driven through the conventional MTJ 10. Therefore, the conventional free layer 50 may be written to the desired state. The conventional MTJ 10 may thus be used for data storage in an STT-RAM.
The conventional dual MTJ 10 may be desirable for use in STT-RAM because it has a lower switching current density and reduced asymmetry as compared to single-barrier MTJ. However, the magnetoresistance of the conventional dual MTJ 10 may be reduced. Further, given the thicknesses of the layers 20, 32, 33, 34, 36, 38, 40, 50, 60, 72, 74, 76, and 80, the conventional dual MTJ 10 may have a thickness on the order of fifty-five nanometers or more. As higher density memories are manufactured, the length and width of the conventional dual MTJ 10 are on the order of its thickness. For example, the conventional dual MTJ 10 might be an ellipse having fifty nanometer and one hundred and fifty nanometer long axes. In such an embodiment, the short axis of the conventional dual MTJ 10 is smaller than the thickness. Consequently, fabrication of the conventional dual MTJ 10 may become challenging at higher densities.
Accordingly, what is desired in an improved magnetic junction usable in higher density STT-RAM.