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Vacancy kinetics during magnetic phase transitions

Liedke, M. O.; Butterling, M.; Ehrler, J.; Eggert, B.; Griggs, W.; Anwar, M. S.; Bali, R.; Thomson, T.; Hirschmann, E.; Elsherif, A. G. A.; Wagner, A.

Two model magnetic systems, FeAl and FeRh, will be discussed in terms of defect kinetics during magnetic phase transitions. Open volume defects have been investigated with Doppler broadening and positron annihilation lifetime spectroscopy techniques using continuous [1] and pulsed [2] slow positron beams, respectively.
The first system, FeAl, exhibits the so-called disorder induced ferromagnetism, where anti-site disorder promotes ferromagnetic A2 phase over paramagnetic ordered B2 phase. The overall control of the phase transition is given by ion irradiation and annealing [1,3]. The main physical origin correlates strongly with the anti-site disorder [4], however the concentration and size of open volume defects is crucial for kinetics of the reordering processes. It will be shown that Fe and Al mono-vacancies introduced by Ne+ irradiation increase the A2 → B2 ordering rate, whereas triple defects and vacancy clusters are stable during annealing. The ordering is achieved through the diffusion of Al and Fe atoms which is mediated by vacancies, and the splitting of vacancy clusters and triple defects into single vacancies during irradiation allows control of the A2 → B2 re-ordering rates, strongly accelerating thermal diffusion [3]. These results provide insights into thermal reordering processes in binary alloys, and the consequent effect on magnetic behavior.
The second system investigated, FeRh, shows a first-order metamagnetic transition from a low temperature antiferromagnetic to a high temperature ferromagnetic phase at about 370 K. During this transition the local Fe magnetic moments align ferromagnetically while the Rh atoms acquire a moment of approximately 1 μB. Moreover, the lattice volume expands by about 1%. The phase transition can also be induced by ion or laser irradiation which drives a disorder-induced mechanism where so-called static disorder plays a key role. It can occur in the form of mono-vacancies, vacancy clusters, grain boundaries or as anti-site disorder, which lead to the formation of ferromagnetism. It will be demonstrated that ion irradiation damages the film structure introducing open volume defects, where concentration scales with ion fluence. Moreover, defect kinetics during thermal annealing across the antiferromagnetic-ferromagnetic phase transition critical temperature will be discussed in detail.

[1] M.O. Liedke, W. Anwand, R. Bali et al., J. Appl. Phys. 117 (2015) 163908.
[2] A. Wagner, M. Butterling, M.O. Liedke et al., AIP Conf. Proc. 1970 (2018), 040003.
[3] J. Ehrler, M.O. Liedke, J. Čížek et al., Acta Mater. 176 (2019) 167.
[4] R. Bali, S. Wintz, F. Meutzner et al., Nano Lett. 14 (2014) 435.

* The Impulse- and Networking Fund of the Helmholtz-Association (FKZ VH-VI-442 Memriox), and the Helmholtz Energy Materials Characterization Platform (03ET7015) are acknowledged.

Keywords: positron annihilation spectroscopy; positron annihilation lifetime spectroscopy; FeRh; FeAl

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Publ.-Id: 34098