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Magnets, how do they work?
Enlarge / Magnets, how do they work?

Magnetic media, in the form of disk and tape drives, have been the dominant way of storing bits. But the speed and low power of flash memory have displaced it from consumer systems, and several forms of long-term memory are under development that are even faster. But a new paper suggests that magnetic media could still be competitive — you just have to stop reading and writing with magnets.

Using a specific shape of grenade and some ultra-fast laser pulses, a Dutch-Polish team of researchers performed what they believe is the fastest read/write speed of magnetic media ever. And, for the record, the process was extremely energy efficient.

Heat is actually a problem for both hard drives and flash. While not a problem in most consumer systems, dealing with excess heat is a major concern in data centers. The problem, according to the authors of the new paper, is one of scale. While we can calculate the minimum energy required to flip a magnetic bit, we use much more than that to ensure that each bit is written as intended. Eight orders of magnitude more even. Most of that excess energy eventually dissipates into the environment, where it ends up as heat.

The authors decided to find a way around the electromagnetic switching of bits. Instead, bits are read and written by shining light onto a largely transparent medium. Presumably, a few well-designed mirrors can dissipate all the waste energy.

The magnetic medium that makes all this work is a yttrium iron garnet crystal doped with cobalt. The garnet itself is largely transparent to light, but the cobalt atoms give it interesting properties. They are ultimately oriented by the crystal structure, which places the orbitals of their electrons in a specific geometry. However, with the correct wavelength of polarized light, the orientation can be shifted to one of three others. Photons that make this shift possible are absorbed and any photons that are not transmitted simply pass through the transparent garnet crystal.

Changing the cobalt’s magnetic orientation has a dramatic effect on the propagation of light at a different wavelength. The authors showed they could change patches of the garnet from fully reflective to fully absorbing, based on the precise wavelength and polarization of a laser pulse. Thus, the laser’s initial pulse can act to write a bit that can then be read by a second pulse. The authors showed that bits, once written, remain stable for days, but the entire substrate can be erased with an external magnetic field.

(Note that this differs from the technology in magneto-optical drives, which would use lasers to heat part of a disc to make it susceptible to change from an external magnetic field. An external magnet is never involved here , and the lasers don’t do heating.)

What is striking is that both the read and write laser functions were performed with femtosecond lasers: “for the recording and readout, we used only two femtosecond laser pulses; to our knowledge, this experiment is the fastest ever write-read magnetic recording event.”

The authors estimate that the energy lost in writing a bit 20 nanometers square would dissipate only 22 attojoules (that’s 10-18 joules). In contrast, in flash memory, each bit written spreads 10 nanojoules into the hardware – that’s nine orders of magnitude higher.

In principle, this is all fantastically good. But it omits something important: that femtosecond laser. Even the most compact femtosecond is typically the size of a laptop hard drive. And they can only fire pulses at about 100 MHz, which means we have to divide each pulse and/or fire a number of lasers in parallel to really get a good bandwidth. And finally, they are not particularly energy efficient. Thus, the energy benefit you get from reading and writing with light can be completely absorbed by the light generation process.

Still, it’s pretty neat technology, and maybe one day someone will find a way around some of these issues. And if there’s any indication that this goes beyond scientific curiosity, rest assured that the authors of this article are trying them out.

Nature2016. DOI: 10.1038/nature20807 (About DOIs).

By akfire1

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