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28 January 2014

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X. Marti, I. Fina, C. Frontera, Jian Liu, P. Wadley, Q. He, R. J. Paull, J. D. Clarkson, J. Kudrnovský, I. Turek, J. Kuneš, D. Yi, J-H. Chu, C. T. Nelson, L. You, E. Arenholz,
S. Salahuddin, J. Fontcuberta, T. Jungwirth and R. Ramesh. Nature Materials, 26 January 2014



The bistability of ordered spin states in ferromagnets provides the basis for magnetic memory functionality. The latest generation of magnetic random access memories rely on an efficient approach in which magnetic fields are replaced by electrical means for writing and reading the information in ferromagnets. This concept may eventually reduce the sensitivity of ferromagnets to magnetic field perturbations to being a weakness for data retention and the ferromagnetic stray fields to an obstacle for high-density memory integration. Here we report a room-temperature bistable antiferromagnetic (AFM) memory that produces negligible stray fields and is insensitive to strong magnetic fields. We use a resistor made of a FeRh AFM, which orders ferromagnetically roughly 100 K above room temperature, and therefore allows us to set different collective directions for the Fe moments by applied magnetic field. On cooling to room temperature, AFM order sets in with the direction of the AFM moments predetermined by the field and moment direction in the high-temperature ferromagnetic state. For electrical reading, we use an AFM analogue of the anisotropic magnetoresistance. Our microscopic theory modelling confirms that this archetypical spintronic effect, discovered more than 150 years ago in ferromagnets, is also present in AFMs. Our work demonstrates the feasibility of fabricating room-temperature spintronic memories with AFMs, which in turn expands the base of available magnetic materials for devices with properties that cannot be achieved with ferromagnets.


Don’t be afraid of magnets: Antiferromagnetic memories

Nowadays, information in most computers, credit or transport cards, etc. is stored in a sequence of “zeros” an “ones”, which are defined by the orientation of the magnetic moment (a sort of small compass) which is characteristic of the “ferromagnetic” materials that form the memory (cobalt, iron, nickel, etc) (Fig. 1a). Naturally, it is extremely dangerous to approach a magnet to a credit card or to a hard disk, because the magnetic moments of the memory elements will be reoriented and the information will be lost (Fig. b). We do have so many magnetic fields around!

Now, researchers of the Institut de Ciència de Materials de Barcelona (ICMAB-CSIC), in collaboration with researchers from Berkeley and Prague, have demonstrated that it is possible to use another type of magnetic materials, the so-called “antiferromagnetics” to store information. The work has just appeared in Nature Materials (DOI: 10.1038/NMAT3861; 26th January 2014).

The antiferromagnetic materials are formed by a number of small compasses (the magnetic moments) oriented alternatively in opposite directions and directed along well defined directions of the material (Fig. c), which cannot be modified by conventional magnets. For that reason, these materials are rather insensitive to external magnetic fields. For the same reason that precludes modification of their magnetic ordering, information cannot be written by using magnetic fields. This is why antiferromagnets have not been used so far to build magnetic memories.

The discovery that is now reported consists on using some antiferromagnetic materials that change from antiferromagnetic to ferromagnetic upon heating. To be able to select the magnetic moment direction of the antiferromagnet, it is first slightly heated up to bring the material into the ferromagnetic phase. A magnetic field pointing along a certain direction is applied and the materials is allowed to cool down back into the antiferromagnetic region, where the magnetic moment orientations get blocked (Fig. 1c) along a direction determined by the cooling magnetic field. The information has been written and it will be insensitive to external magnetic fields. Information is subsequently read by simply measuring the electric resistivity, which is known (Lord Kelvin discovered about 160 years ago) to depend on the relative angle formed by the measuring current and orientation of the magnetic moments (Fig. 1c and Fig. 1d). Zeros and Ones forever!

These results can open new perspectives for the design of more robust magnetic memories.

Memòries magnètiques que no “obliden”

A l’actualitat, la informació a la majoria d’ordinadors, càmeres fotogràfiques, targetes de crèdit o de transport, etc, es guarda en forma de “zeros” i “uns”, definits per l’orientació del moment magnètic (com una petita brúixola) característica dels materials “ferromagnètics” que formen la memòria (Cobalt, Ferro, Níquel, etc) (Fig.1a). Naturalment, és extremadament perillós apropar un iman a la tarja de memòria, ja que aquell reorientarà el moment magnètic dels elements de memòria i es perdrà la informació emmagatzemada (Fig. 1b). Al nostre entorn hi ha molts camps magnètics !

Investigadors de l’Institut de Ciència de Materials de Barcelona (ICMAB-CSIC) juntament amb investigadors de Berkeley, i Praga, han demostrat que és possible usar un altre tipus de materials magnètics, anomenats “antiferromagnètics” per a emmagatzemar informació. El treball s’acaba de publicar a Nature Materials (DOI: 10.1038/NMAT3861; 26th January 2014).

Els materials antiferromagnètics estan constituïts per moltes petites brúixoles (els moments magnètics) que apunten alternativament en direccions oposades, i dirigides segons direccions ben precises en el material (Fig. 1c) i que no es poden modificar amb imans convencionals. Per això, aquests materials son insensibles a camps magnètics externs. Malauradament, per la mateixa raó que no es poden modificar fàcilment, no s’hi pot escriure  informació amb camps magnètics. Per aquest motiu aquests materials no s’havien usat fins ara com a memòries magnètiques.

El descobriment que han fet aquests investigadors, consisteix en usar uns certs materials antiferromagnètics que amb un lleuger canvi de temperatura, passen a ser   ferromagnètics.

Aprofitant aquest canvi de fase, aplicant un camp magnètic a la fase ferromagnètica d’alta temperatura, i refredant després el material fins la fase antiferromagnètica, l’orientació dels moments magnètics queda seleccionada i fixada (Fig. 1c) en direccions determinades a voluntat. És a dir: s’ha escrit informació i aquesta serà insensible a camps magnètics externs. La lectura de la informació es fa amb una simple mesura de la resistència elèctrica, que és diferent segons quina sigui la direcció relativa entre el corrent elèctric de mesura i l’orientació dels moments magnètics (Fig. 1c). Un “uns” o “zeros” per  sempre !

Aquest resultats poden obrir noves perspectives en el disseny de memòries magnètiques més robustes.

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Room-temperature antiferromagnetic memory resistor