29. Februar 2012
Such behaviour would have quite curious effects for a light switch: In the case of a lamp dimmer switch, the correlation, which the physicists at the Max Planck Institute of Microstructure Physics in Halle have discovered, would not only cause the light to change brightness, but also to change colour – from green to red, for instance. Although both properties are characteristics of light, they cannot be manipulated with one switch simultaneously. However, the researchers in Halle have now succeeded in doing something similar with the tunnel current that flows between two ferromagnetic electrode layers of a multiferroic material sandwich.
In this case, multiferroic means that the material sandwich includes the two ferromagnetic substances, as well as a ferroelectric substance. In ferroelectric materials, a voltage switches between the two directions of an electric polarisation – depending on its polarity – not unlike when a magnetic field permanently reverses the polarity of a ferromagnet. As ions shift within the material structure during this process, the polarisation is preserved, even after the voltage has been switched off. It is possible, however, to reverse the switch again with a similarly large voltage with reversed polarity.
The Halle-based researchers prepared their multiferroic material sandwich by depositing an extremely accurately structured ferromagnetic lanthanum strontium manganate (LSMO) layer on a base. The thickness of this layer is just under 30 nanometres − a nanometer is one millionth of a millimetre. On top of that, they deposited a layer of ferroelectric lead zirconate titanite (PZT) only three nanometres thick and with a very regular structure; a top layer of ferromagnetic cobalt finished the sandwich.
The physicists then placed the tip of an atomic force microscope over the cobalt cover of the material stack to apply a voltage to the multiferroic sandwich. Although the non-conductive PZT layer prevents current flow in the traditional sense between cobalt and LSMO, some electrons may overcome the barrier in the quantum-physical tunnel process. The researchers in Halle were interested in precisely these properties of tunnel current.
The strength of this tunnel current depends on the polarisation of the ferroelectric PZT. The polarisation impacts the height of the tunnel barrier, meaning that the tunnel resistance is different for both polarisation directions. The physicists also used electrical voltage to switch between the two polarisation directions. To do this, they applied a voltage pulse, which was considerably stronger than the voltage needed for the tunnel current but lasted less than a millisecond.
“Surprisingly, not only the component of the tunnel junction resistance that depends on the direction of the polarisation, but also the component that usually only depends on the direction of the magnetisation of the electrodes, the so-called tunnel magnetoresistance, changes when reversing the polarity of the ferroelectric”, says Dietrich Hesse, who heads the research team together with Marin Alexe. This tunnel magnetoresistance (TMR) always appears when electrons are tunnelling between two different ferromagnets. It is usually smaller for two ferromagnetic electrodes that are magnetised in the same direction than for two electrodes that are magnetised in opposite directions – in this case, physicists talk of normal tunnel magnetoresistance.