Interaction of phase transformation and magneto- and elastocaloric properties of Heusler alloys
- PDF / 42,882,478 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 28 Downloads / 160 Views
Interaction of phase transformation and magneto- and elastocaloric properties of Heusler alloys 1
2,3
2
1
Peter Entel , Vladimir V. Sokolovskiy , Vasiliy D. Buchelnikov , Denis Comtesse , and 1 Mehmet Acet 1
Faculty of Physics and CENIDE, University of Duisburg-Essen, 47048 Duisburg, Germany
2
Chelyabinsk State University 454001 Chelyabinsk, Russia
3
National University of Science and Technology ‘MISiS’, 119049, Moscow, Russia
ABSTRACT The structural, electronic and magnetic properties of functional Ni-Mn-(Ga, In, Sn) and Pt-Ni-(Ga, Sn) alloys are studied by first-principles and Monte Carlo tools. The ab initio calculations give a basic understanding of the underlying physics which is associated with the complex magnetic behavior arising from the competition of ferro- and antiferromagnetic interactions for excess Mn atoms in the unit cell. We show that the resulting complex magnetic ordering is the driving mechanism of structural transformations and multifunctional properties of Heusler alloys associated with magnetic shape-memory, magnetocaloric and elastocaloric effects. The thermodynamic properties can be calculated by using the ab initio magnetic exchange parameters in finite-temperature Monte Carlo simulations. Entropy and specific heat changes associated with the magnetic changes and emergence of microstructure across the magnetostructural transition are pointed out. We show how to optimize the functional properties by tuning the compositional changes, for example, a magnetic shape-memory effect of more than 14% can be achieved in Pt-Ni-Mn-Ga alloys. The theoretical studies are accompanied by experimental investigations. INTRODUCTION The alloy series Ni-Mn-(Al, Ga, In, Sn, Sb) has outstanding properties ranging from magnetic shape memory behavior to exchange bias and caloric properties where elastocaloric, barocaloric and magnetocaloric effects are involved. In particular, the caloric effects are of interest since they may be strong across the magnetostructural phase transition, in which magnetic and structural phase transitions are coupled. They also exhibit the unique feature that for many compositions the structural transformation occurs at lower temperatures in the ferromagnetic state. Moreover, considering the prototype magnetic shape-memory alloy (MSMA) Ni2MnGa (near stoichiometry) which is ferromagnetic below 375 K and undergoes a martensitic transformation around 200 K with a modulated pre-martensitic phase as precursor, to
various modulated martensitic structures upon cooling, we find a sudden change (drop) of the magnetization which is associated with the structural transformation. This behavior is observed in low magnetic fields and disappears slowly but not completely when the magnetic field approaches its saturation value. The remaining drop is more promounced for Ni-Mn-In and NiMn-Sn alloys. Since in this alloy series one finds a crossover from low temperature magnetism to high temperature antiferromagnetism when approaching the binary alloy system Ni-Mn with increasing valence-electr
