Giant exchange bias impressed on a Heusler compound by field cooling

Exchange bias is a magnetic phenomenon where the hysteresis loop of a magnetic material can be shifted by anisotropic magnetic coupling. It is widely used in e.g. magnetic recording and developing materials with a large exchange bias is of tremendous technological importance.

Scientists from the Max Planck Institute for Chemical Physics of Solids in Dresden, Germany, have designed and synthesised a new family of so-called Heusler compounds where a giant exchange bias is induced by cooling the material through the magnetic transition in an external magnetic field above 15 T. Even after the field is removed the material retains an internal magnetic field of more than 3 T.

The compound, Mn_2.4Pt_0.6Ga, is a compensated ferrimagnet with virtually zero net magnetisation. The Mn atoms of the two magnetic sublattices are strongly coupled antiferromagnetically and their magnetic moments compensate each other. Experiments performed at HLD showed a non-saturating magnetization up to 60 T reflecting the strength of this inter-sublattice Mn-Mn interaction.

Subsequent experiments performed in static magnetic fields up to 32 T at HFML then made it possible to further manipulate the magnetic properties of the compound. In particular, by field cooling the material in fields up to 25 T, an internal magnetic field of more than 3 T was induced and the mechanism of this giant exchange bias could then be addressed experimentally.

Though these new results are particularly important for the fundamental research dealing with the synthesis of new materials and their basic physical properties, possible applications already appear on the horizon. In the same publication, a Heusler compound Mn-Fe-Ga was presented where the magnetic transition occurs above room temperature. This may enable new magnetic devices with unprecedentedly stable magnetic properties.

Reference
Ajaya K. Nayak, Michael Nicklas, Stanislav Chadov, Panchanana Khuntia, Chandra Shekhar, Adel Kalache, Michael Baenitz, Yurii Skourski, Veerendra K. Guduru, Alessandro Puri, Uli Zeitler, J. M. D. Coey and Claudia Felser; Design of compensated ferrimagnetic Heusler alloys for giant tunable exchange bias; Nature Materials (2015) doi:10.1038/nmat4248.

Figure 1: Hysteresis loops of a compensated Mn_2.4Pt_0.6Ga ferrimagnet after field cooling at B_fc = 15 T through the magnetic transition. Compared to zero-field cooling the hysteresis loop is shifted by a giant exchange bias B_ec shown in inset (b) as a function of B_fc. Inset (a) shows the magnetic order in this compensated Heusler compound: The magnetic moments of the two Mn sublattices with opposite orientation and different magnitude are compensated by replacing part of the Mn-II atoms with non-magnetic Pt resulting into a total magnetisation that is virtually zero. However, when field cooling the compound in an external magnetic field, a large internal field of up to 3.3 T remains.