3 Key Things to know about STT-MRAM – #3 How Sensitive Is STT-MRAM To External Magnetic Fields?

Over the last decade, some in the SRAM business have joked that if a technician opens the chassis of an MRAM-based system using an electric screwdriver it will corrupt the contents of the MRAM device!! While an exaggeration, it was effective in creating fear, uncertainty and doubt in the minds of some customers. The very first generation MRAM – Toggle MRAM – used magnetic fields to write data into the memory cells. It was susceptible to bit errors through influence of stray magnetic fields, and had to use magnetic shielding in packages to provide some immunity against these fields. Since then, MRAM has evolved into a much more robust product. Because STT-MRAM uses spin polarized current to write to memory cells, it is much more immune to external magnetic fields, to the extent that magnetic shielding is not required for industrial or even aerospace (high reliability) applications.

There are multiple standards for maximum allowed magnetic fields in Electrical and Electronic Systems. Both CISPR 16 and IEC 61000-4 define requirements for Electromagnetic Compatibility (EMC). For example, IEC 61000-4-8: Electromagnetic compatibility (EMC) – Part 4-8: Power frequency magnetic field immunity test relates to the immunity requirements of equipment to magnetic disturbances at power frequency. Per this standard, the maximum specification for continuous fields is 100 A/m, and for pulsed field 1000 A/m. Factory Automation gear such as PLCs, and Smart Grid equipment such as Smart Meters are rated per IEC61000-4 for electromagnetic compatibility.

For reference, Annex D of EN/IEC 61000-4-8[1] gives some useful information on the power frequency magnetic field strengths likely to occur in various locations. Household appliances were measured to generate fields between 0.03A/m and 21 A/m when the field is measured 300mm from the appliance. At ground level directly below 400kV overhead power transmission lines, the fields have been measured to be 16A/m per kA of line current. Magnetic fields of up to 15kA/m have been measured in some industrial electrolytic processes (a field strength which exceeds health and safety guidelines).

Avalanche Technology’s Persistent SRAM (STT-MRAM) devices are rated at 24000A/m for maximum allowable magnetic field strength during read or write operations (these parameters are provided in the product datasheets). This rating is orders of magnitude beyond what is routinely observed in different industrial settings. The maximum external magnetic field that can be tolerated by an Avalanche Technology P-SRAM when it is in the OFF state is even higher. With a design this robust the only way an electrical screwdriver is going to be able to corrupt the contents of the P-SRAM is if you were to use it to drive a screw into the P-SRAM package itself!

Learn more about Avalanche Technology’s P-SRAM products HERE.


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