Multi-bit MRAM Cell Using Voltage-Controlled Magnetic Anisotropy
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Solution Overview
Problem
Conventional MRAM devices with single-bit cells face limitations in storage density, power consumption, and manufacturing costs, necessitating an increase in storage capacity while maintaining performance.
Innovation Solution
The implementation of multi-bit MRAM cells with magnetic tunnel junctions and a magnetic anisotropy axis, allowing for three or more logic states per cell using a single pair of magnetic field directions, reduces power consumption and manufacturing costs by leveraging thermal assisted switching and dual-function bit/field lines.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Quantity of substance
If single-bit MRAM cells are used, then device simplicity and manufacturing ease are maintained, but storage density is limited
Solution Approach 1:
The patent changes the magnetization switching mechanism from conventional spin-transfer torque to voltage-controlled magnetic anisotropy switching. By applying voltage pulses that temporarily modify the magnetic anisotropy energy landscape, the storage layer magnetization can be switched between multiple stable states (not just binary). This parameter change enables multi-bit storage in a single cell without increasing physical cell complexity, thereby improving storage density while maintaining device simplicity
Solution Approach 2:
The patent introduces dynamic control of magnetic anisotropy through voltage application. The magnetic anisotropy is not fixed but can be dynamically switched between different states by applying voltage pulses, allowing the same physical cell structure to encode multiple logic states. This dynamic property enables a single cell to function as multiple storage units, increasing storage density without adding more cells
2Quantity of substance
If more logic states per cell are implemented, then storage density increases, but power consumption increases
Solution Approach 1:
The patent utilizes voltage-controlled magnetic anisotropy switching, where voltage pulses temporarily modify the energy barriers between magnetic states. This approach requires significantly less energy than conventional current-based switching methods because it exploits the voltage-induced change in anisotropy rather than requiring large currents to generate magnetic fields or spin-transfer torque. The multi-bit storage is achieved by controlling the switching dynamics through voltage pulse parameters, enabling high storage density with low power consumption
Solution Approach 2:
The patent replaces the conventional current-based magnetic switching mechanism with a voltage-controlled mechanism. Instead of using high-current pulses to switch magnetization, the invention uses voltage pulses to modulate the magnetic anisotropy, which then enables low-current or current-free switching. This substitution dramatically reduces power consumption while enabling multi-state storage capability
3Quantity of substance
If multi-bit cells are implemented, then storage density increases, but manufacturing complexity and costs increase
Solution Approach 1:
The patent segments the magnetic anisotropy control function from the storage function. By using a separate control electrode structure that applies voltage to modulate anisotropy independently from the storage layer magnetization, the manufacturing process can use standard semiconductor fabrication techniques. The control electrode is manufactured as a separate layer that can be patterned and connected without requiring complex multi-layer magnetic structures, thus maintaining ease of manufacture while enabling multi-bit storage
Solution Approach 2:
The patent designs the control electrode structure to serve multiple functions: it acts as both the voltage application terminal for anisotropy control and as part of the readout circuitry. This multi-functional design reduces the number of separate components needed, simplifying the manufacturing process. The same basic cell structure can be used for both single-bit and multi-bit operations by simply changing the voltage pulse programming, without requiring different fabrication processes
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach enhances storage density, reduces power consumption, and lowers manufacturing costs by encoding multiple logic states with reduced complexity, enabling efficient write and read operations.
Implementation Method 1
discovery of magnetic tunnel junctions having a strong magnetoresistance at ambient temperatures
Implementation Method 2
the other ferromagnetic layer, the so-called storage layer, is characterized by a magnetization with a direction that is varied upon writing of the device, such as by applying a magnetic field
Data Source
Figure 1
Figure 2
Figure 3A~3B
AI summary
A magnetic random access memory (MRAM) cell includes a storage layer, a sense layer, and a spacer layer between the storage layer and the sense layer. A field line is magnetically coupled to the MRAM cell to induce a magnetic field along a magnetic field axis, and at least one of the storage layer and the sense layer has a magnetic anisotropy axis that is tilted relative to the magnetic field axis. During a write operation, a storage magnetization direction is switchable between m directions to store data corresponding to one of m logic states, with m > 2, where at least one of the m directions is aligned relative to the magnetic anisotropy axis, and at least another one of the m directions is aligned relative to the magnetic field axis. During a read operation, a sense magnetization direction is varied, relative to the storage magnetization direction, to determine the data stored by the storage layer.