Can MRAM Challenge Flash Dominance?

By Bertrand Chatelus, Technical Sales Manager, Future Electronics Europe

READ THIS TO LEARN ABOUT The trade-offs between memory technologies MRAM architecture and parameters

Memory comes in several forms, but selection always imposes a trade-off between speed, cost, density, power consumption and volatility. It is no surprise then that memory manufacturers are investing heavily to identify a single type of memory that can do it all. Magnetoresistive RAM (MRAM) is a leading contender and is now commercially available.

A new generation of non-volatile RAM devices are emerging in three categories: Magnetoresistive RAM (MRAM), Ferroelectric RAM (FRAM or FeRAM) and Phase Change Memory (PCM), also known as Ovonic Unified Memory (OUM).

While there is still some way to go before these new memory types seriously start to challenge the use of DRAM, SRAM and Flash – the three types of conventional memory dominating the market - Freescale is leading the way with the recent commercial release of its second-generation 4-Mbit MRAM chip.

But what effect can MRAM technology have on a world dominated by SRAM, DRAM and Flash? Is it likely to become a mainstream memory technology or a novel technology consigned to niche applications?

 

Taking MRAM for a Spin

MRAM technology results from research into the field of spin electronics or “spintronics.” Spin, the essential quantum property of an electron, is analogous to a magnet. By aligning or polarizing these tiny magnets into a uniform pattern, they can be used to signify elements of binary code.

MRAM does not store data as electric charge on capacitors or transistors, but instead uses magnetic storage elements. The elements are formed from two thin-film ferromagnetic plates, each of which can be magnetized in specific directions, separated by an ultra-thin, insulating spacer-layer. One of the two plates has its magnetization direction fixed, similar to a permanent magnet, while the magnetization of the other plate can be changed between two opposite directions (see Figure 1). These are known as the “fixed-layer” or “pinnedlayer” and the “free-layer,” respectively.

 

 

The two different magnetized directions of the free-layer are used to represent one or zero. A memory circuit is built from a grid of these cells and data is written to the cells by passing current through write lines above and below the cells, creating magnetic fields that switch the magnetization of the free-layer.

To determine the state of the free-layer, a bias voltage is applied to the cell so that electrons tunnel through the insulating spacer-layer, creating a tunneling current. Since the ferromagnetic materials produce a polarized tunneling current, the tunneling resistance depends on the magnetic direction of the free-layer with respect to the fixed-layer. The resistance is highest when the magnetizations point in opposite directions and the resistance is lowest when they are parallel. The state of the bit is therefore determined by measuring the current using a comparator.

Two types of cells for MRAM have been extensively studied: the Stoner–Wohlfarth cell and Freescale’s toggle cell. Figure 2 illustrates the advantage of Freescale’s toggle-bit cell versus the Stoner– Wohlfarth type. The Stoner-Wohlfarth cell uses a single free-layer (Figure 2a). This switches when the field is sufficient to cross the switching curve shown in Figure 2b. However, when a field is applied in the half-select direction, the activation energy needed to flip the cell decreases dramatically, (see Figure 2c). This increases the probability of unintentional switching.

 

 

In contrast, Freescale’s toggle switching uses two ferromagnetic free-layers separated by a spacer-layer (Figure 2d). The resulting structure switches when the field crosses an L-shaped critical switching curve (Figure 2e). When the field is applied in the half-select direction, the activation energy initially increases, making the bit more stable (Figure 2f).

A single free-layer switches when the vector-sum of the field at the selected cell reaches a threshold value. But in toggle switching, the free-layer is switched by a specific pulse sequence, making the toggle cell more immune to unintentional switching. Figure 3 illustrates how the toggle cell’s two free-layers respond to the applied field pulses to change their magnetizations from one direction to the opposite.

 

 

Memory Technologies Comparison

The technical parameters that really count in memory technology are:

• Read time
• Erase/write time
• Data retention
• Storage capacity
• Maximum number of erase cycles
• Cost
• Density
• Power consumption (particularly in battery-driven applications)

Current memory types trade one attribute against another, as shown in Table 1.

 

 

SRAM is faster than DRAM and is used where speed is the prime requirement, such as in CPU caches and router buffers. Both SRAM and DRAM lose data when the power is turned off, and DRAM will also lose its data unless periodically refreshed. Flash memory does not need power to maintain the information stored in the chip (hence it is non-volatile). In addition, Flash offers fast read-access times and better shock resistance than hard disks. These characteristics explain the popularity of Flash memory for applications such as storage in battery-powered devices.

However, reading from Flash memory is generally slower than either SRAM or DRAM, and erasing and writing Flash memory is much slower. Flash memory also has limited write-endurance (typically around 100,000 cycles) since the storage cells degrade each time they are erased and rewritten.

MRAM does not need its memory cells to be refreshed, and thus boasts low power consumption (MRAM only requires about 1% of the power needed for DRAM). In addition, MRAM is considerably faster than DRAM, largely due to the much lower current needed to store a bit into the cells. And, unlike Flash, MRAM does not degrade during writing, nor is it slower to write than to read.

MRAM, however, is noted for its high write-current. For example, the total current requirement for a word of data could approach 200mA, although it is unlikely for all bits within a given word to be flipped during each write cycle. Since the current-draw takes place over a short time, the energy consumption of MRAM is comparable with that of other memory technologies.

In terms of memory cost, the main determinant is the density of the components used. MRAM is physically similar to DRAM in makeup, consisting of metal plates and insulators. Moreover, as the technology matures, it will be produced in higher densities. In contrast, SRAM and Flash have relatively complex cells and are consequently more expensive. FRAM is more expensive still, as it features a complex cell arrangement and requires additional processing during fabrication.

 

Universal Memory

If MRAM were at a comparable state of maturity to the other memory technologies, it would be a good choice to replace DRAM (which imposes a large overhead for refreshes) and offer a trade-off of increased write power against much lower power for standby and data retention. Depending on the application, this could be an attractive compromise. Unfortunately, MRAM is not yet sufficiently developed to make this a practical proposition for many applications.

However, as the technology matures, MRAM is expected to take market share. Initially, this looks likely to be taken from Flash applications. MRAM protagonists also claim that MRAM will be able to replace hard drives as typical density increases, eliminating the relatively slow mechanical components that lead to performance bottlenecks.

MRAM can therefore offer an alternative to every type of memory currently being used. It has similar speeds to SRAM, similar density but much lower power than DRAM, and is non-volatile yet does not suffer degradation over time like Flash. It is this combination of features that some suggest make it the “universal memory,” able to replace SRAM, DRAM and Flash.

However devices such as Freescale’s 4-Mbit MRAM chip – while being too low in capacity to directly challenge mature memory technologies such as DRAM and Flash – already provide designers with the opportunity to explore its advantages in certain areas. These include data-logging applications, write-cache for larger storage systems, system configuration storage, and as a replacement for battery-backed SRAM.

Future Electronics can offer customer support for Freescale’s MRAM chip, including applications engineers who are able to offer design advice for MRAM implementation.