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Keywords:

  • density functional theory;
  • Heusler materials;
  • Monte Carlo simulations;
  • spin- and magnetic-cluster glasses;
  • strain glasses

Magnetic Ni–Mn based Heusler intermetallics show complex magnetic behavior in connection with martensitic transformations (see, for instance, the phase diagram of Ni–Co–Mn–Sn on the right-hand side). The cubic austenitic phase at high temperature shows long-range ferromagnetic order which can considerably be weakened by the appearance of competing antiferromagnetic interactions which are induced by Mn excess and chemical disorder. With decreasing temperature a martensitic/magnetostructural transformation takes place from cubic to non-modulated/modulated tetragonal/monoclinic or orthorhombic structure, where long-range ferromagnetic order can no longer be maintained, leading to superparamagnetic behavior. At still lower temperatures superparamagnetism changes to superspin glass because of strong competition of ferromagnetic and antiferromagnetic interactions and chemical disorder. In addition, disorder and local structural distortions can lead to strain glass in austenite, as observed for some non-magnetic martensitic systems. The magnetic intermetallics are of technological importance in view of their functional properties involving magnetic shape-memory and exchange-bias effects as well as magnetocaloric effects. The ‘ferroic cooling’ is of particular relevance since it avoids the use of ozone-depleting and greenhouse chemicals compared with conventional fluid-compression technology.

pssb201451059-gra-0001

Experimental phase diagram of inline image inline image inline image inline image for inline image. Here, inline image is the Curie temperature of austenite; at inline image the system transforms to paramagnetic martensite and at inline image to superparamagnetic martensite (SPM) and then to superspin-glass martensite (SSG) at inline image. The possible strain-glass phases (labeled by question marks) are predicted because of kinetic arrest phenomena and local distortions associated with the magnetostructural transition and ergodicity breaking by field-cooling/zero-field-cooling experiments.