STT-MRAM (Spin Transfer Torque Magnetic Random Access Memory) is the second generation of MRAM (magnetic random access memory). Its core advantage lies in the use of spin current technology to achieve efficient information writing. The core component of its storage unit is a finely designed MTJ (magnetic tunnel junction), which consists of two ferromagnetic layers of different thicknesses sandwiched by a non-magnetic isolation layer that is only a few nanometers thick. This unique design makes STT-MRAM an advanced non-volatile memory that performs data writing operations by precisely controlling the spin current.
Working Principle of STT-MRAM
The storage unit is composed of a transistor, a magnetic tunnel junction (MTJ) and a connecting line. The MTJ is stacked by a fixed layer (strong magnetism), a non-magnetic isolation layer and a free layer (weak magnetism), in which the magnetic moment of the free layer is easily reversed. The transistor is used as a site selection switch, and its drain is connected to the MTJ fixed layer. When the transistor is activated by the gate, the source, drain, MTJ and bit line form a closed circuit.
When writing information, the bit line and the additional write information line generate semi-select write magnetic fields respectively, and the two magnetic fields are orthogonal. Only when the storage cell is selected and the additional write information line is powered on, the magnetic moment of the free layer is reversed by the double magnetic field, parallel or anti-parallel to the magnetic moment of the fixed layer, resulting in a change in the MTJ resistance, thus realizing information storage.
When reading information, a small current passes through the selected memory cell, generating a potential difference across the MTJ, reflecting its resistance state. By measuring the potential difference, the relative direction of the magnetic moments of the free layer and the fixed layer can be determined non-destructively, thereby reading the stored information.
（MRAM process flow after MTJ mode）
STT-MRAM characteristics
- In cases of unexpected power failures, STT-MRAM’s inherent non-volatile memory feature allows customers to safeguard their data without relying on a backup battery.
- The fast read/write performance of STT-MRAM significantly reduces system read/write latency, making in-place execution of applications more efficient.STT-MRAM
- STT-MRAM has the characteristics of high bandwidth read-write and non-volatile, and can play the dual functions of memory and running memory; No write delay; Can realize the system power off automatic protection.
- STT-MRAM memory is a domestic independent product, the first batch shipment.
- Data security is significantly strengthened, making it more convenient to develop and deploy applications that are resistant to tampering.
- SRAM+FLASH+EEPROM can be replaced simultaneously.
The comparison of STT-MRAM with common memory is as follows:
Table 1:
| ITEM | STT-MRAM | FRAM | NVSRAM | TOGGLE-MRAM |
| Memory type | Non-Voltile | Non-Voltile | Non-Voltile | Non-Voltile |
| Write method | OverWrite | OverWrite | OverWrite | OverWrite |
| Write cycle | 25ns | 150ns | 25ns | 35ns |
| Number of reads/writes | 1E+13 | 1E+14 | 1E+7 | 1E+13 |
| The density of Data | High | Low | Low | Middle |
| Retention time | >20yrs | 10yrs | 20yrs | >20yrs |
Table 2:
| ITEM | STT-MRAM | EEPROM | FLASH | SRAM | FRAM |
| Memory Type | Non-Voltile | Non-Voltile | Non-Voltile | Voltile | Non-Voltile |
| Write Method | Overwrite | Erase+Write | Erase+Write | Overwrite | Overwrite |
| Write Cycle Time | 25ns | 10μs | 10μs | 5ns | 150ns |
| Read/Write Cycle | 1E+13 | 1E+6 | 1E+5 | unlimited | 1E+14 |
| Booster Circuit | No | Yes | Yes | No | No |
| Data Backup Battery | No | No | No | Yes | No |
Application
As a disruptive technology, STT-MRAM is gradually reshaping the product performance landscape in a wide range of fields, from consumer electronics and personal computers to automotive, medical, military and aerospace. Which predicting an new chapter of semiconductor industry ,moreover energizing the product innovation poential .
In the automotive industry , STT-MRAM, with its superior read speed, ultra-low power consumption and high density, far exceeds eFlash and eSRAM, and has become a key force in promoting automotive intelligence and high performance. For portable devices and the mobile phone market, STT-MRAM simplifies designs by unifying memory subsystems through the elimination of multi-chip packages (MCP), significantly reducing system power consumption and greatly extending battery life.
In the personal computer domain, STT-MRAM demonstrates strong substitution capabilities. Whether serving as high-speed cache for SRAM, replacing flash memory as non-volatile cache, or even substituting for PSRAM and DRAM in high-speed program execution, STT-MRAM exhibits performance advantages. Moreover, in numerous embedded applications, STT-MRAM is gradually replacing NOR flash and SRAM, emerging as a new choice for enhancing system performance and reliability.
