Fyzikální ústav Akademie věd ČR

In ferromagnetic materials, information can be stored in “zeros” and “ones” defined by the orientation of magnetic moments, which can be pictured as small compasses (see Fig. 1a). This technology is behind a range of memory applications from kilobyte magnetic stripe cards to terabyte computer hard disks. It is dangerous to place a parking ticket or a hard disk next to another magnet or device generating strong magnetic fields because the magnetic moments of the memory can be unintentionally reoriented and the information lost (see Fig. 1b). Moreover, being magnetic on outside, a ferromagnetic bit could disturb the neighboring one if the integration of bits in high-density memories was pushed to limits.

Fig. 1

Researchers from the Institute of Physics of the Academy of Sciences of the Czech Republic, in collaboration with researchers from Berkeley and Barcelona, have demonstrated that it is possible to use another type of magnetic materials, the so-called antiferromagnets to store information. The work entitled “Room-temperature antiferromagnetic memory resistor” has been published in Nature Materials (DOI: 10.1038/NMAT3861; 26th January 2014).

Antiferromagnetic materials are magnetic inside, however, their microscopic magnetic moments sitting on individual atoms alternate between two opposite orientations (see Fig. 1c). This antiparallel moment configuration in antiferromagnets, instead of the parallel configuration in ferromagnets, makes the magnetism in antiferromagnets invisible on the outside. It implies that if information was stored in an antiferromagnetic memory it would be insensitive to disturbing external magnetic fields (see Fig. 1d), and an antiferromagnetic bit would also not affect the neighboring antiferromagnetic bit no matter how densely the bits were arranged in the memory. The outstanding question, however, is how to read and write information on an antiferromagnetic bit.

The answer is provided in the above Nature Materials article by using a special antiferromagnet which changes to a ferromagnet upon heating. To be able to select the magnetic moment direction of the antiferromagnet for encoding “zeros” or “ones” (see Fig. 1c), it is heated up to bring the material into the ferromagnetic phase. A magnetic field pointing along one or another direction is then applied and the material is allowed to cool down back into the antiferromagnetic state where the direction of the antiparallel moments “freezes” (Fig. 1c) along the magnetic field direction applied during cooling. Once in the antiferromagnetic state, the information is written and is no more sensitive to external magnetic fields. Information is subsequently read by simply measuring the electrical resistance which depends on the relative angle between the measuring current run through the antiferromagnetic bit and the direction of the antiparallel magnetic moments in the bit (see Fig. 1c). “We have introduced a new class of materials that can be used for constructing more robust magnetic memories”, says Xavi Marti from the Institute of Physics.

For detailed information contact Xavi Marti or Tomáš Jungwirth from the Institute of Physics ASCR, Cukrovarnická 10, 162 53 Praha 6, e-mail: xmartifzu [dot] cz or jungwfzu [dot] cz