Effect of hydrogen on magnetic properties in MgO studied by first-principles calculations and experiments

Introduction

Magnesium oxide (MgO) has shown attractive functionality in many applications, including as a material for refractory protective layers of AC plasma displays and a magnetic shielding/insulator layer in tunneling magneto-resistance (TMR) sensors for the hard disk drive (HDD) industry. MgO is a great insulator with a band gap of 7.8 eV1,2. It can also be synthesized in many forms by several methods3-5 and has been intensively studied as a barrier layer for magnetic tunnel junctions6-10. MgO is an important material for developing spintronic devices and has been widely utilized as a miniaturized magnetic sensor. However, there are still some features of MgO that are not understood. For example, MgO has been reported to exhibit ferromagnetism at room temperature11,12, while it is known to be a diamagnetic material. This makes it unsuitable for spintronic applications since the barrier is supposed to be an electrical insulator that does not interfere with the spins of other layers.

The d0 magnetism13 is one of the phenomena that cause ferromagnetism in diamagnetic materials such as MgO, HfO2, ZnO, and CaO. The ferromagnetism arises from metal atom vacancies in the structure and the free electrons in oxygen atoms surrounding the vacancies couple together, resulting in the unbalanced magnetic moment that generates a ferromagnetic signal in the material12,14. Many studies involving both calculations and experiments indicate that the strength of magnetization results from d0 ferromagnetism, which is related to the roles of defects and impurities in the system15-20. Many experiments have shown that the magnetism in MgO is induced by intrinsically charged defects, such as neutral oxygen vacancies and singly and doubly charged magnesium vacancies11-13,17,18. Kuang et al. used the PBEsol functional with density functional theory to simulate the electronic structure of MgO with vacancies and related it to the magnetic properties of the material12. Additionally, there have been other calculation methods leading to similar results21-24. This research confirmed that Mg vacancies can induce ferromagnetism in the structure. While several theoretical approaches have been used to study intrinsic cation and anion vacancies in MgO, only a few studies have been focused on ubiquitous impurities such as hydrogen. Thus, the characterization of magnetism resulting from defect complexes with different charge states formed by combination of hydrogen impurity atoms and MgO intrinsic defects is still limited. Experiments have revealed that the magnetic moment and ferromagnetism of MgO can be introduced by defect states involving cation vacancies and hydrogen. For example, the formation of oxygen vacancies upon absorption of hydrogen impurities can induce ferromagnetism in MgO nanocrystallites at room temperature17. Balcells et al. reported that the reduction of ferromagnetic properties in MgO thin films prepared by radio frequency (RF) magnetron sputtering was related to the hydrogen-driven instability of vacancy centers in the material24. By using X-ray absorption spectroscopy (XAS) and Fourier transform infrared spectroscopy (FTIR), our previous work showed that both the ferromagnetism and diamagnetism of MgO at room temperature can be attributed to defects involving magnesium (Mg) vacancies and oxygen-hydrogen (O-H) bonding20. Moreover, it was found that suppression of the ferromagnetic properties may be due to formation of O-H bonds via chemical bonding between hydrogen impurities and oxygen in the MgO structure.

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Here, we perform first-principles density-functional theory calculations and experiments, such as X-ray diffraction (XRD), vibrating sample magnetometry (VSM) and Fourier transform infrared spectroscopy (FTIR), to gain a deeper understanding of the roles of hydrogen and intrinsic defects in MgO ferromagnetism based on formation of defects studied with first-principles calculation and provide a comparison between these predictions and experiments.

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