Fig. 2: Demonstrator of a programmable switch using two magnets (in blue) for generating a static magnetic field. The Pt electrode is shown in gray, AlOx in yellow and the Co dot is at the center of the stack
Spintec researchers, in collaboration with the Catalan Institute of Nanotechnology and the Autonomous University of Barcelona have developed a new technique to write information in a magnetic memory cell in a more stable manner and at a lower energy cost. This technique opens up very promising concepts in terms of performance memories and offers new features to develop logic functions using magnetic memory cells.
MRAMs (Magnetic Random Access Memories) are a new type of memory that combines speed, low power, high-density non-volatile information (persistent even in the absence of power) and immunity to ionizing radiation. A set of properties that no other type of memory possesses. MRAMs consist of magnetic tunnel junctions (MTJ), stacks of two ferromagnetic layers (FM) and a tunnel barrier oxide. Their resistance varies with the relative orientation of the magnetization of FM layers. To write information in a cell, the magnetization direction of these layers must be changed with a magnetic field, or a spin-polarized current must be injected perpendicular to the layer plane (STT writing). Each method has its drawbacks: difficulty of increasing the integration density for the first, loss of long-term reliability due to the passage of the polarized current for the second. In these structures, a compromise must always be struck between stability and power consumption for writing: an FM layer that is "hard" (with higher anisotropy) retains information better because it is less sensitive to thermal fluctuations but it requires more energy to be written.
Fig. 1: The magnetization of the Co layer is perpendicular to the layer plane, and according to its direction, it can encode a 1 or a logic 0. The asymmetry of interface between the electrodes of Pt and AlOx of the device creates an electric field E parallel to the magnetization direction. Then, by applying a current I in the plane of layers, an effective magnetic field Heff parallel to Ux is created due to the Rashba effect, but also a torque directed along the same axis. Applying a weak magnetic field Happ parallel to the current direction simultaneously with the injection current then results in the bipolar reversal of the magnetization.
Writing with an in-plane current
Our new method is to reverse the magnetization of a magnetic nanostructure with perpendicular magnetization by applying a current in the plane of the layers. How? The nanostructure - cobalt in our sample - is inserted between two different electrodes of platinum-and aluminum oxide (Fig. 1). The asymmetry of the interfaces creates an electric field which, combined with the injected current, generates an effective magnetic field that can flip the magnetization of the Co layer. Our device measures 200 x 200 nm, and the current pulse duration is less than 10 ns with an amplitude of the order of 2 mA. These values should improve significantly in the future with technological advances.
What did we win? By being able to flip the magnetization of the storage layer of an MTJ with a write current in-plane, the current flow through the tunnel barrier is avoided (in STT writing) thus avoiding deterioration. Reading is done as usual by measuring the resistance of the MTJ, thus the paths of reading and writing are separate. This makes it possible to use higher resistance values that improve readability and avoid the risk of accidental writing.
Balancing stability and low power
This method has other advantages. First, this property seems to be stronger in hard layers compared with soft layers. This means that unlike existing structures, high stability would come with low power consumption for writing! Second, it is possible to control the direction of the reversal due to the presence of a weak constant magnetic field applied parallel to the current direction. Thus, the same current can lead to a parallel or antiparallel state depending on the direction of the field. This makes it possible for example to change the functionality of a circuit or part of it by changing the direction of the static field locally (Fig. 2). Finally, the separation of reading and writing current paths allows for three-terminal devices (ex. transistors), where a control terminal determines the electrical behavior between the other two. This additional degree of freedom in particular, makes it possible to associate an intrinsic memory function and a logical function in the same device.
Although the physical origin of this phenomenon is still poorly understood and will require developing a solid theoretical framework, the actual performance and expected improvements suggest that it will contribute significantly to expand the boundaries of microelectronics.
Further reading: Miron IM et al., Nature 476 (2011) 189-193
Maj : 17/02/2014 (920)