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Terahertz conductivity sense cation disorder in thin films of half-metallic double perovskites

Ekta Yadav¹, Ketan Navale¹, G. L. Prajapati², K. R. Mavani^1,*

¹Department of Physics, Indian Institute of Technology (IIT) Indore, Simrol, Khandwa Road, Indore-453 552, India

²Department of Physics, Indian Institute of Science Education and Research, Bhauri, Bhopal-000000, India

*krushna@iiti.ac.in

Double perovskites with the chemical stoichiometry of A₂BB′O₆ (A= Ca, Sr, and Ba; B = Fe, B′ = Mo), tend to show half-metallic character with two conducting channels (spin-up and spin-down) for the charge transport. This class of materials have gained attention owing to the high Curie temperature (T_C), 100% spin polarization and large magnetoresistance at room temperature, which help in realizing spintronic applications at room temperature and above [1]. In A₂FeMoO₆ compounds, a high T_C of 360 K, 415 K, and 380 K for A= Ca, Sr, and Ba, respectively, makes this class of materials more promising vis-à-vis mixed-valent manganites, with lower T_C and smaller spin polarization in comparison.

Figure 1: Drude model fit to real and imaginary room temperature THz conductivity of (a) Ca₂FeMoO₆ / LaAlO₃ [111] thin film and (b) Ca₂FeMoO₆ / LaAlO₃ [100] thin film.

Despite promising attributes of A₂FeMoO₆ compounds for applications in the area of spintronics, the anti-site disorder which arises due to interchange of ions with similar size at B and B′ sites, creates a large hindrance in realizing the applications [2,3]. This anti-site disorder generally builds up when thin films are deposited. To overcome this problem, various methods are adopted by researchers [2]. In this direction, we have performed a comparative experimental investigations on two sets of films deposited on [100] and [111] oriented single crystal substrates in order to see whether [111] orientation in combination of compressive strain help order cations in this system, as predicted by theoretical studies [4]. For this study, we characterized the films for structural, electrical and magnetic properties, in addition to the temperature-dependent terahertz (THz) time-domain spectroscopy. The spectroscopic data were collected at different constant temperatures in the range of 5K to 300K. We find that terahertz conductivity of double perovskite thin films is highly sensitive to the anti-site disorder. The frequency-dependent THz conductivity of [111]-oriented films fit to Drude model whereas that of [001] oriented film fit to Drude-Smith model, at all the temperatures. Our results, in combination to THz spectroscopy, clearly show that Ca₂FeMoO₆ films deposited on LaAlO₃ [111] substrates have better cation ordering, which further results into enhanced magnetization and half-metallic character.

References
[1] Kobayashi K I, Kimura T, Sawada H, Terakura K, and Tokura Y, “Room temperature magnetoresistance in an oxide material with an ordered double perovskite structure,” Nature 395, 6703 (1998)
[2] Suchaneck G, Kalanda N, Artsiukh E, and Gerlach G, “Challenges in Sr₂FeMoO_6-δ thin film deposition,” Phys. Status Solidi b 257, 1900312 (2020)
[3] Sarma D D, Sampathkumaran E V, Ray S, Nagarajan R, Majumdar S, Kumar A, Nalini G, and Guru Row T N, “Magnetoresistance in ordered and disordered double perovskite oxide Sr₂FeMoO₆,” Solid State Commun. 114, 465 (2000)
[4] Josée E Kleibeuker, Eun Mi Choi, Edward D Jones, Tse Min Yu, Bianca Sala Belinda, A MacLaren, Demie Kepaptsoglou, David Hernandez-Maldonado, Quentin M Ramasse, Lewys Jones, Ju ri Barthel, Ian MacLaren, and Judith L MacManus Driscoll, “Route to achieving perfect B site ordering in double perovskite thin films,” NPG Asia Materials 9, e406 (2017)