Toward Versatile Sr₂FeMoO₆-Based Spintronics by Exploiting Nanoscale Defects

Abstract

To actualize the high spintronic application potential of complex magnetic oxides, it is essential to fabricate these materials as thin films with the best possible magnetic and electrical properties. Sr2FeMoO6 is an outstanding candidate for such applications, but presently no thin film synthesis route, which would preserve the magnetic properties of bulk Sr2FeMoO6, is currently known. In order to address this problem, we present a comprehensive experimental and theoretical study where we link the magnetic and half metallic properties of Sr2FeMoO6 thin films to lattice strain, Fe-Mo antisite disorder and oxygen vacancies. We find the intrinsic effect of strain on the magnetic properties to be very small, but also that an increased strain will significantly stabilize the Sr2FeMoO6 lattice against the formation of antisite disorder and oxygen vacancies. These defects, on the other hand, are recognized to drastically influence the magnetism of Sr2FeMoO6 in a nonlinear manner. On the basis of the findings, we propose strain manipulation and reductive annealing as optimization pathways for improving the spintronic functionality of Sr2FeMoO6.

The Role of the Fe/Mo Cations Ordering Degree and Oxygen Non‐Stoichiometry on the Formation of the Crystalline and Magnetic Structure of Sr2FeMoO6−δ

“…In its turn, the N ( E ) value depends on the oxygen concentration (6– δ ) and ordering of both antisite Mo Fe , Fe Mo defects, and oxygen vacancies in the SFMO structure . However, the functional dependence δ = f([Mo Fe ], [Fe Mo ]) is not unique and is determined by the variation dynamics of both the oxygen nonstoichiometry and antisite defect concentration as a function of the SFMO synthesis regime (temperature T , duration t , and partial oxygen pressure р (О 2 )), because one and the same δ value can be obtained for different [Mo Fe ], [Fe Mo ] values, and vice versa . This circumstance leads to a situation in which several publications report different magnetic and electrical transport properties of SFMO samples prepared by similar methods .…”

Challenges in Sr2FeMoO6−δ Thin Film Deposition

“…Neglecting the small configurational entropy contribution, the enthalpy (internal energy) of oxygen vacancy formation H f ( V O ) yields about 1.88 eV. This value is significantly lower than the formation energies of neutral oxygen vacancies E f ( V O ) calculated from first principles . For different configurations, the enthalpy values range from about 3 to about 6 eV at 0 K and 0 pressure.…”

“…First‐principle calculations reveal that the most abundant point defects in SFMO are oxygen vacancies (Figure b) as well as antisite defect pairs Fe Mo and Mo Fe (Figure c) . They yield a diminution of half‐metallic character by Fe Mo antisite defects and Mo vacancies, that is, in Fe‐rich SFMO films, as well as by Fe Mo + Mo Fe pairs . According to other first‐principle calculations, the half‐metallic characters are maintained for SFMO containing Fe Mo and Fe vacancies (i.e., in Mo‐rich films), oxygen vacancies, or Sr vacancies .…”

“…A supercell of four‐formula units should be a minimum size . When using DFT + U calculations, the strengths of on‐site interactions are usually described by a parameter which is the difference between the average Coulomb energy and the exchange energy. Thereby, d and f electrons in magnetic systems can be properly modeled.…”

“…Defects deteriorating half‐metallicity should be avoided . Additionally, it was suggested to increase the amount of oxygen vacancies so as to improve T C since they do not even deteriorate spin polarization, a key parameter for applications . A larger M s was predicted in the presence of small amounts of strontium vacancies since a deficit of strontium without the formation of oxygen vacancies will increase the valence of Fe and Mo to ensure electrical neutrality of the formula unit .…”