Experimental evidence for ferromagnetism at room temperature in MgO thin films

Abstract

Ferromagnetic ordering at room temperature (RTFM) in MgO thin films deposited by RF magnetron sputtering under various atmospheric conditions and temperatures is reported. A saturation magnetization (M(S)) value as high as 1.58 emu g(-1) is (0.046 μB/unit cell) observed for a 170 nm film deposited at RT under an oxygen pressure of 1.3 × 10(-4) mbar. In contrast, films deposited at elevated temperature (under an identical oxygen pressure), or at higher oxygen pressures, as well as under a nitrogen atmosphere at RT show significantly suppressed magnetization. The ferromagnetic order in the MgO matrix is believed to be defect induced.

d° Ferromagnetism of Magnesium Oxide

“…This aspect is in correlation with experimental results obtained from various experimental studies [51][52][53][54][55][56][57][58]. Mg vacancies help to retain local magnetic moments in this material irrespective of the concentration [63][64][65][66][67][68][69].…”

“…The magnetization (M) versus the applied magnetic field (H) curve of this film measured at room temperature is shown in Figure 7b. The behavior of the M-H curve is very similar to that already reported for MgO thin films of similar thickness [40,[50][51][52][53][54][55][56][57][58][59][60][61]. This behavior reveals the onset of d • ferromagnetism in this film.…”

“…These authors also observed thickness dependent transition from ferromagnetic to paramagnetic state in pure MgO thin films [51]. Magnetization of 170 nm film is suppressed at higher oxygen pressure which reflects role of cation vacancies [52]. Cation vacancies induced RTFM are also observed in homogeneously (200) oriented MgO~85 nm thin film deposited on Si substrates by inkjet printing [53] and MgO thin films deposited by pulsed laser deposition [54].…”

“…The values of Mg vacancy formation energy were 2.73 eV and 1.37 eV for MgO bulk and MgO quantum dots [74]. Thus, Mg vacancies occur in quantum dots due to low formation energy and lead to magnetism, as observed by various experimental reports [51][52][53][54][55][56][57][58]. In the case of MgO thin films, Mg vacancy formation energies are 2.45 and 2.74 eV for the surface and the subsurface layers, which are less than that of MgO bulk.…”

“…Though O vacancy destroys the ferromagnetic ground state, experimental studies envisage an important role of oxygen vacancies to control magnetism in this material, as elaborated in previous section [51][52][53]. Thus, the theoretical investigations were carried out in this direction to revive the role of oxygen vacancies [79][80][81][82][83].…”