Resetting the future of MRAM

Mar 07, 2012
This is a pictorial view of the coupling mechanism between hard and soft ferrimagnetic alloys with perpendicular magnetization. Credit: Picture: RUB/Abrudan

In close collaboration with colleagues from Bochum, Germany, and the Netherlands, researchers from the Helmholtz-Zentrum Berlin, Germany, have developed a novel, extremely thin structure made of various magnetic materials.

It is suitable as a kind of magnetic valve for units of the most recent generation and makes use of effects in the context of so-called spintronics, with which, in addition to the (re-)charging process, magnetic characteristics of the can also be used for and -storage. The advantage of the new structure: data remain intact even after the has been switched off and the memory can be re-written more or less indefinitely.

The scientists published their results in Nature Communications.

Everything began with basic academic curiosity. "First of all we just wanted to create a defined with two thin, stacked ferrimagnetic layers", says Florin Radu, physicist at the Institute for Complex of the Helmholtz-Zentrum Berlin (HZB) and principal author of the research paper. In other words, the researchers just wanted to create a structure in which a magnetic characteristic within the material changes in a well defined way. Experts in this field define this as magnetic hysteresis. It describes the behaviour of magnetic substances vis-à-vis an externally-applied magnetic field. However, the task proved to be much more difficult; the magnetic energies at the interfaces turned out to be so powerful that the magnetization of the films reverses together. It was necessary to place an additional, non-magnetic layer made of tantalum between the ferrimagnetic layers in order to diminish this effect.

What the scientists saw next was truly astounding; the system behaved fundamentally differently as compared to the conventional systems made of ferromagnetic and anti-ferromagnetic layers. The ferrimagnet described as magnetically "soft", which consists of the chemical elements iron and gadolinium, unexpectedly indicated an alteration in the hysteresis, while the existing magnetism remained unaltered for the "hard" ferrimagnetic film that consists of the chemical elements dysprosium and cobalt.

This discovery paves the way for an even more vigorous research in the field of .. "Know how, Show how!!", thus proclaims the research maxim of Radu. "I would not be surprised to see this discovery implemented into PC's, smart phones and tablets in the future", he predicts. For his invention the so-called spin-valve the HZB filed a patent application this week.

Nowadays, the data storage units are either volatile or non-volatile. For the former, the information is lost as soon as the device is switched off, and for the latter the information remain intact for many years. Due to thermal effects, they are also practically unusable after about ten years. In particular, when the bits are only a few nanometres in size, they lose stability. Once lost, the magnetization direction of the hard magnetic layer cannot easily be set again in the original direction.. This leads irretrievably loss of data.

This stability issue can now be addressed with the new spin-valve concept. By tunning the magnetic properties of the hard ferrimagnetic layer, the so-called RAM memory building-blocks (RAM stands for random access memory) can be manufactured with controlled life-time of the stored information of weeks, months or years.. Thereafter, the magnetic orientation can be resettedin the original state, which increases considerably the overall life expectancy of the information as compared to the existing non-volatile MRAM (Magnetoresistive Random Access Memory). These memory building-blocks are now certainly highly sought-after in the field of micro-electronics, but have not been able, to date, to be established in the markets due to high costs and technical problems.

With the spin-valve concept by Radu and his colleagues, electronic devices can now be developed that, similar to the MRAM technology, are operable immediately after being switched on and allow their data storage units to be re-written more or less indefinitely.

Explore further: Thermoelectric power plants could offer economically competitive renewable energy

More information: F. Radu, R. Abrudan, I. Radu, D. Schmitz, H. Zabel: Perpendicular exchange bias in ferrimagnetic spin valves. Nature Communications, 2012. DOI: 10.1038/ncomms1728

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User comments : 6

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Eikka
not rated yet Mar 07, 2012
Despite reading it over many times, I couldn't find a reasonable explaination amidst the typographical errors to what is actually being described here.

Some "tunning" happens to something, which is called a spin valve, and then hard drives retain data for longer, apparently. Does this "tunning" happen continuously or is it a one-time process? Is it reversible, can it change? How does one "tunne" the valve?
Cluebat from Exodar
Mar 07, 2012
This comment has been removed by a moderator.
Eikka
not rated yet Mar 07, 2012
Taking a guess from the picture provided, it seems that adjusting the thickness of the tantalum layer between the ferromagnetic materials affects the hysteresis curve of the top layer.

Ferromagnetic materials align the spins of their electrons to an external magnetic field, and this alignment is also what gives rise to a magnetic field for a permanent magnet. In other words, applying a field to a ferromagnetic material will permanently magnetize it to some degree. The extents of the hysteresis curve basically tell you how much of that magnetic field the material will retain, and the area within the curve tells you how much energy is needed to change the field. Tuning this property makes it possible to choose how difficult it is to flip a bit intentionally or randomly, without changing the materials involved.

This I don't understand though:
the magnetic orientation can be resettedin the original state, which increases considerably the overall life expectancy of the information
Fiorino
not rated yet Mar 07, 2012
Hi Eikka,

"
This I don't understand though:

the magnetic orientation can be resettedin the original state, which increases considerably the overall life expectancy of the information
"
I think that if one replaces the "information" by "device", then it may sound meaningful:

" the magnetic orientation can be reset in the original state, which increases considerably the overall life expectancy of the device"

I believe that the trick here is to design the stability of the device by tuning the coercive field of the ferrimagnetically hard layer. The higher the coercive field, the higher the stability. Since one would like to reset the orientation of the hard layer by some moderately low fields, then the stability of the written information may not be exceptionally high. But the advantage may be that it can be reset again and again for many many years. That's why this concept may be considered, perhaps, as a semi-volatile MRAM.

Eikka
not rated yet Mar 07, 2012
I suspect that the bottom magnet provides a sort of natural zero state for the top magnet, which makes it always tend to align with the bottom magnet's orientation as the thermal effects try to randomize it. It other words, I think it slowly drifts towards a known state instead of pointing every which way.

The randomized magnetic domains present a problem for writing new information, because adjacent domains may affect each other in unpredictable ways.

Although, anything is guesswork because the article is so poorly written that it's difficult to tell what they're on about.
Fiorino
not rated yet Mar 07, 2012
Hi Eikka,

"The randomized magnetic domains present a problem for writing new information, because adjacent domains may affect each other in unpredictable ways."

Indeed, this may be the case when using ferromagnets. Due to the stray fields, the interaction between neighbouring bits is stronger, causing randomization. However, when using ferrimagnets, the stray fields are much lower because the net magnetization of these ferrimagnets is much reduced. Then, the influence of the stray fields on the next bits is lower for ferrimagnets as compared to ferromagnets.
hb_
4 / 5 (1) Mar 08, 2012
I would guess that the distance to the lower magnetic layer will determine how "hard" the top layer is. Nothing surprising really.

The energy states of the top layer - i.e. the work required to change the magnetic orientation - will of course be affected by how strongly coupled it is to the lower (hard) magnetic layer.

So, instead of having a really "soft" magnet (a magnet that is easily demagnetized by heat) on top, you can tune how "hard" it is.

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