Magnetic Tunnel Junction Switching via Thermal Spin Torque
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Solution Overview
Problem
Current magnetic random access memory (MRAM) technologies face challenges in reducing the high current densities required for switching magnetic tunnel junctions, which hinders the viability of spin-transfer-torque-based memory devices, and the demonstration of thermally induced spin currents for MRAM has been limited by the difficulty in creating sufficient temperature gradients across ultra-thin tunnel barriers.
Innovation Solution
A method is developed to create a temperature gradient across the tunnel barrier of a magnetic tunnel junction, inducing thermal spin currents that reduce the magnitude of magnetic and electrical spin currents needed to switch the free layer, utilizing an asymmetric tunneling conductance configuration to enhance thermal spin torque effects.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If spin polarized charge currents are passed directly through the MTJ to induce switching via spin transfer torque, then switching of the magnetic tunnel junction is achieved, but the current densities required are too large to make it a viable technology
Solution Approach 1:
The patent introduces a temperature gradient as an intermediary mechanism to generate spin currents indirectly. Instead of applying high current densities directly through the MTJ, a temperature gradient is applied across the tunnel barrier, which generates spin currents via the spin-Seebeck effect. This intermediary thermal field acts as a mediator that produces the required spin angular momentum transfer without demanding excessive charge current, thereby resolving the contradiction between achieving reliable switching and reducing energy consumption.
2Use of energy by moving object
If temperature gradients are used to generate spin currents through the tunnel barrier, then current density requirements are reduced, but sufficiently large temperature gradients across ultra-thin tunnel barriers are difficult to create
Solution Approach 1:
The patent applies local quality by concentrating the temperature gradient generation at specific locations within the device structure. Rather than attempting to create a uniform temperature gradient across the entire device, the invention focuses thermal excitation locally at the tunnel barrier region where it is most needed. This localized approach to heat application makes it feasible to generate sufficient temperature gradients across ultra-thin barriers without requiring complex device-wide thermal management systems.
3Productivity
If the tunnel barrier thickness is reduced to enable useful applications, then device performance is improved, but creating sufficiently large temperature gradients across the barrier becomes more difficult
Solution Approach 1:
The patent employs parameter changes by optimizing multiple device parameters simultaneously to compensate for the reduced barrier thickness. Specifically, the invention adjusts the magnetic layer compositions, anisotropy fields, and tunnel barrier materials to enhance the spin-Seebeck coefficient and thermal conductivity ratios. By changing these material and structural parameters, the device maintains high temperature gradient generation efficiency even with ultra-thin barriers, thereby preserving both improved device performance and effective thermal spin current generation.
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 achieves significant thermal spin torque effects, comparable to or exceeding those from high current densities, allowing for efficient switching of magnetic tunnel junctions with reduced current requirements, potentially enabling more viable MRAM technology.
Implementation Method 1
a temperature gradient of 1 K/nm across a 0.9 nm thick MgO tunnel barrier in an MTJ induces modest charge currents (corresponding to current densities on the order of 1×10³ A/cm²), in addition to large spin currents that induce significant TST
Implementation Method 2
Heat currents can also create spin-currents in magnetic materials; the transfer of spin angular momentum through this process has been called thermal-spin-torque (TST)
Data Source
AI summary
Thermal-spin-torque (TST) in a magnetic tunnel junction (MTJ) is demonstrated by generating large temperature gradients across ultrathin MgO tunnel barriers, with this TST being significant enough to considerably affect the magnitude of the switching field of the MTJ. The origin of the TST is attributed to an asymmetry of the tunneling conductance across the zero-bias voltage of the MTJ. Through magneto-Seebeck voltage measurements, it is estimated that the charge-current that would be generated due to the temperature gradient would give rise to spin-transfer-torque (STT) that is a thousand times too small to account for the observed changes in switching fields, indicating the presence of large TST.