Data storage

It was at the end of the 1950s that the American physicist and later Nobel laureate Richard Feynman prophesied that all the knowledge in the world would one day be stored in a memory the size of a grain of sand. Today we can fit terabyte capacity into the volume of a lunch box, while the first computers with just a few kilobytes of storage filled entire halls 70 years ago. Yet the possibilities today are not infinite – the system can no longer be reduced in size or made faster at will. That is why new technologies are needed to accommodate the increasingly complex tasks.

The system investigated by the team led by the experimental physicists Prof. Heiko Wende, Prof. Wolfgang Kleemann and Dr. Carolin Schmitz-Antoniak comprises a layer of barium titanate in which tiny pillars of cobalt ferrite just a few nanometres in size are embedded. The components of this composite differ in two significant ways: The pillars are ferrimagnetic, i. e. they align themselves like miniature compass needles to a magnetic field. They are also deformed by a magnetic field. The surrounding layer, also known as the “matrix”, is ferroelectric and generates an electric voltage when under a mechanical load. This “piezoelectric effect”, as it is called, is used in flintless fire lighters: When pressure is applied to the button, a spring forces a small hammer onto the piezo crystal and produces a large electrical voltage. This in turn causes a spark on the adjacent metal contacts, which ignites the gas to light the flame.

The aim is to deform the nanopillars with a magnetic field and exert mechanical pressure on the matrix. This is also referred to as magnetoelectric coupling of a multiferroic composite system. Magnetoelectric coupling is actually based on the minute movements of the atoms in the composite: If a magnetic field is applied along the long axis of the pillars, they contract in this direction. At the same time, their surface area increases to keep the pressure constant, causing them to press against the surrounding matrix on all sides. Under this pressure, the matrix generates an electric voltage.

If the magnetic field is perpendicular to the pillars, they contract in the direction of the field while expanding perpendicular to it. This means that the matrix is only compressed perpendicular to the magnetic field and produces an asymmetrical electric polarisation never before observed in this system.

The system is interesting for digital data storage because the electric polarisation remains, even after the magnetic field has been switched off. The researchers have already developed a strategy by which specific pillars are compressed in a lengthwise and perpendicular direction with electric pulses to write bits of information.
The principle should also work in the opposite direction: The direction of magnetisation can only be switched and a bit written when there is an electric voltage but no current flowing. The information could then be read out in the same way as to date through the magnetic structure. This would also mean that no heat is generated, which is important since heat would be very damaging to the extremely densely packed storage elements. Unlike some other high-tech visions, magnetoelectrical storage devices also work at room temperature without additional cooling and offer extreme stability for the data stored on them.