A pictorial view of the coupling mechanism between hard and soft ferrimagnetic alloys with perpendicular magnetization. Picture: RUB/Abrudan |
Researchers
at Helmholtz Center in Berlin, Germany, have developed a magnetic valve
that could be an enabling technology for spintronics.
It
is suitable as a kind of magnetic valve for data-storage units of the
most recent generation and makes use of the effects spintronics, with
which, in addition to the charging and recharging process, magnetic
characteristics of the electrons can also be used for
information-processing and storage. The advantage of the new structure:
Data remains intact even after the electric current has been switched
off and the memory can be re-written more or less indefinitely.
Everything began with basic academic curiosity.
“First
of all we just wanted to create a defined anisotropy with two thin,
stacked ferromagnetic layers,” says Florin Radu, physicist at the
Institute for Complex Magnetic Materials 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 behavior 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 ferromagnetic layers in order to diminish this
effect.
What
the scientists saw next was surprising; the system behaved
fundamentally differently as compared to the conventional systems made
of ferromagnetic and anti-ferromagnetic layers. The ferromagnet,
described as magnetically “soft” and consisting of the chemical elements
iron and gadolinium, unexpectedly indicated an alteration in the
hysteresis, while the existing magnetism remained unaltered for the
“hard” ferromagnetic 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 spintronics.
“Know
how, Show how!!”, says Radu. “I would not be surprised to see this
discovery implemented into PC’s, smart phones and tablets in the
future.”
For his invention, the spin-valve, 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 tuning the magnetic properties of the hard ferromagnetic 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.
Citation:
F. Radu, R. Abrudan, I. Radu, D. Schmitz, H. Zabel: Perpendicular exchange bias in ferrimagnetic spin valves. Nature Communications, 2012. DOI: 10.1038/ncomms1728
