Antiferromagnetism for spintronics

In ferromagnetic materials, the resistance variation of interest in nanodevices is due to the so-called giant magneto-resistance effect which requires reversal of the magnetization over 180°. In antiferromagnetic materials, the system remains unchanged after 180° rotation. The exploited resistance variation must be associated to a 90° rotation. It is due to an effect called anisotropic magnetoresistance. It is intrinsically linked to the so-called spin-orbit effect, itself characteristic of magnetic materials containing heavy elements such as platinum or gold.

Born with the discovery of Giant Magnetoresistance by Fert and Grünberg in 1988 (Nobel prize in 2007), spintronics is a branch of nanoelectronics which exploits the fact that the electron has a spin. The electric resistance of spintronic devices depends on their magnetic state. Today, this so-called « magneto-resistive » effect is essentially exploited in nano-sized high-sensitivity sensors, such as the read heads of computer hard disk drives. In the future, it is expected that a new generation of magnetic memories will develop, combining the functions of random access memories (RAM) (the active part of today’s computers) and those of the hard disks. This will lead to a fundamental transformation in the computer’s architecture, permitting much faster operation and dramatic energy saving. Such memories will incorporate new types of magnetic materials able to keep the recorded information at extremely small sizes. Ferromagnetic materials, most often considered, are very sensitive to the effect of parasitic stray fields, whose detrimental effects tend to become critical as the size of the objects is progressively reduced. For this reason, a recent interest is being put on antiferromagnetic materials.

In ferromagnetic materials, the resistance variation of interest in nanodevices is due to the so-called giant magneto-resistance effect which requires reversal of the magnetization over 180°. In antiferromagnetic materials, the system remains unchanged after 180° rotation. The exploited resistance variation must be associated to a 90° rotation. It is due to an effect called anisotropic magnetoresistance. It is intrinsically linked to the so-called spin-orbit effect, itself characteristic of magnetic materials containing heavy elements such as platinum or gold. Scientists from Institut Néel, together with colleagues from Instituto de Fisica of the Federal University of Rio de Janeiro and Laboratoire National des Champs Magnétiques intenses in Grenoble, have recently demonstrated that Mn2Au is the most promising material identified so-far for « antiferromagnetic spintronics ». Combined NMR and a neutron studies have revealed that the manganese magnetic moment in this material reaches 4 µB (in pure Fe metal, the Fe moment reaches 2.2 µB), the arrangement of the magnetic moments derived from the neutron study is shown in the figure. The extrapolated Néel temperature (at which the antiferromagnetic order disappears under the effect of thermal activation) is above1400 K, higher than the Curie temperature of cobalt (1388 K) the (ferro)magnetic material with the highest known ordering temperature. Finally two equivalent privileged directions exist for the moment, at 90° one from the other. Note that it is actually not easy to distinguish an antiferromagnetic material which orders at high temperature (i.e . in which the magnetic interactions are high) and a non-magnetic one, and, until the present study, Mn2Au was thought to be non-magnetic.

Figure :Magnetic structure of Mn2Au derived from the neutron diffraction study. The Mn moments form ferromagnetic sheets perpendicular to c. Their orientation alternates from one sheet to the next. The moments are confined in the plane perpendicular to the uniaxial axis, c. The neutron analysis does not allow the determination of the direction of the moments within the plane.

For the promise of this material to become reality, the controlled growth of films is needed, as well as the development of processes permitting a controlled rotation of the moments. This constitutes subjects of our present research.

Contributors

  • V. Barthem Instituto de Fisica, Universidade Federal do Rio de Janeiro, Brazil.
  • D. Givord, C.V. Colin Institut NEEL, Grenoble, France
  • H. Mayaffre, M.-H. Julien, Laboratoire National des Champs Magnétiques Intenses, Grenoble, France

Our related publication :

Corresponding author  :

D. Givord

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  • - Antiferromagnetism for spintronics

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