Tuned AFM–FM coupling by the formation of vacancy complex in Gd_0.6Ca_0.4MnO₃ thin film lattice

Abstract

The effect of in situ oxygen and vacuum annealings on the low bandwidth manganite Gd1−x Ca x MnO3 (GCMO) thin film with x = 0.4 was investigated. Based on the magnetic measurements, the AFM–FM coupling is suppressed by the vacuum annealing treatment via destroying the double exchange interaction and increasing the unit cell volume by converting the Mn4+ to the Mn3+. Consequently, resistance increases significantly compared to pristine film. The results are explained by a model obtained from the positron annihilation studies, where the vacuum annealing increased the annihilation lifetime in A and B sites due to the formation of vacancy complexes V A,B–V O, which was not the case in the pristine sample. The positron annihilation analysis indicated that most of the open volume defects have been detected in the interface region rather than on the subsurface layer and this result is confirmed by detailed x-ray reflection analysis. On the other hand, the effect of oxygen annealing on the unit cell volume and magnetization was insignificant. This is in agreement with positron annihilation results which demonstrated that the introduction of oxygen does not change the number of cation vacancies significantly. This work demonstrates that the modification of oxygen vacancies and vacancy complexes can tune magnetic and electronic structure of the epitaxial thin films to provide new functionalities in future applications.

Strain-Induced Domain Structure and Its Impact on Magnetic and Transport Properties of Gd0.6Ca0.4MnO3 Thin Films

“…The formation of the second phase can be due to oxygen vacancies in this system. In our previous report, positron annihilation spectroscopy measurements showed that the GCMO can take oxygen from the STO substrate during the deposition to compensate for the oxygen deficiency in the lattice, and thus the defect concentration could be larger near the interface region. The existence of the second phase, which affects the Mn 4+ /Mn 3+ ratio and the number of oxygen vacancies could suppress the DE interaction, decreasing magnetization.…”

“…13,14,23 In our case, the maximum for the GCMO/STO film has lower values compared to that for the GCMO/LSAT and the GCMO/ SLAO films. Previously, 20 we showed that GCMO/STO films contain oxygen vacancies in the lattice and GCMO takes oxygen from STO to compensate the vacancies. It seems that oxygen vacancies and other defects in this film can suppress the double exchange (DE) interaction, resulting in lower ferromagnetic alignment.…”

“…As temperature increases, the magnetic moments of the Mn ions dominate, reaching a maximum at around 50 K. The strength of the maximum depends on the Mn 4+ /Mn 3+ ratio, lattice distortion, and oxygen content in the perovskite lattice. ,, In our case, the maximum for the GCMO/STO film has lower values compared to that for the GCMO/LSAT and the GCMO/SLAO films. Previously, we showed that GCMO/STO films contain oxygen vacancies in the lattice and GCMO takes oxygen from STO to compensate the vacancies. It seems that oxygen vacancies and other defects in this film can suppress the double exchange (DE) interaction, resulting in lower ferromagnetic alignment.…”

“…18 On the other hand, the fabrication of high-quality perovskite thin films by controlling the strain and the lattice mismatch between the bulk and substrate can affect the electrical and magnetic properties of manganites. 10,19 In our previous paper, 20 positron annihilation studies of Gd 0.6 Ca 0.4 MnO 3 (GCMO) thin films grown on SrTiO 3 (STO) showed that most of the oxygen vacancies and open volume defects are in the interface region between the film and the substrate. This is due to the transfer of oxygen from STO to the film bulk to compensate oxygen vacancies in the GCMO lattice.…”

“…In our previous paper, positron annihilation studies of Gd 0.6 Ca 0.4 MnO 3 (GCMO) thin films grown on SrTiO 3 (STO) showed that most of the oxygen vacancies and open volume defects are in the interface region between the film and the substrate. This is due to the transfer of oxygen from STO to the film bulk to compensate oxygen vacancies in the GCMO lattice.…”