An interplay among the Mg2+ ion coordination, structural order, oxygen vacancies and magnetism of MgO thin films (original) (raw)
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Surface and Coatings Technology
a¨F raunhofer-Institute fur Elektronenstrahl-und Plasmatechnik (FEP), Germany bÄ bstract MgO films were deposited by a mid frequency dual magnetron reactive sputtering system composed of two identical magnetrons and powered by a sinusoidal generator. Atomic force microscopy, scanning electron microscopy, X-ray diffraction, and a diode discharge device were used to characterize surface morphology, crystalline structure, and secondary electron emission (g) coefficient of the films, respectively. The influence of O flow rate with a constant Ar flow rate of 120 sccm on the structure and 2 properties of the films was systematically studied. As the O flow rate is increased from 5 to 14 sccm, the intensity ratio of the 2 (111) to (200) peak of cubic MgO phase increases significantly, leading to a change of the film texture from a combined and (111) preferred orientation to a highly (111) preferred orientation. The (111) peak center shifts to a larger angle, and the full width at half maximum of the (111) peak decreases greatly, indicating a considerable decrease of defect density in the films. The increases in the relative intensity of the (111) peak and the decrease in the defect density in the films contribute to a remarkable increase in the g coefficient of the films. Furthermore increase of O flow rate results in no significant change in the 2 crystalline structures and the g coefficient of the MgO films. ᮊ
Magnetism of Amorphous and Nano-Crystallized Dc-Sputter-Deposited MgO Thin Films
Nanomaterials, 2013
We report a systematic study of room-temperature ferromagnetism (RTFM) in pristine MgO thin films in their amorphous and nano-crystalline states. The as deposited dc-sputtered films of pristine MgO on Si substrates using a metallic Mg target in an O 2 containing working gas atmosphere of (N 2 + O 2) are found to be X-ray amorphous. All these films obtained with oxygen partial pressure (P O2) ~10% to 80% while maintaining the same total pressure of the working gas are found to be ferromagnetic at room temperature. The room temperature saturation magnetization (M S) value of 2.68 emu/cm 3 obtained for the MgO film deposited in P O2 of 10% increases to 9.62 emu/cm 3 for film deposited at P O2 of 40%. However, the M S values decrease steadily for further increase of oxygen partial pressure during deposition. On thermal annealing at temperatures in the range 600 to 800 °C, the films become nanocrystalline and as the crystallite size grows with longer annealing times and higher temperature, M S decreases. Our study clearly points out that it is possible to tailor the magnetic properties of thin films of MgO. The room temperature ferromagnetism in MgO films is attributed to the presence of Mg cation vacancies.
Room temperature ferromagnetism in pristine MgO thin films
Applied Physics …, 2010
Robust ferromagnetic ordering at, and well above room temperature is observed in pure transparent MgO thin films ͑Ͻ170 nm thick͒ deposited by three different techniques. Careful study of the wide scan x-ray photoelectron spectroscopy rule out the possible presence of any magnetic contaminants. In the magnetron sputtered films, we observe magnetic phase transitions as a function of film thickness. The maximum saturation magnetization of 5.7 emu/ cm 3 is measured on a 170 nm thick film. The films above 500 nm are found to be diamagnetic. Ab initio calculations suggest that the ferromagnetism is mediated by cation vacancies.
Synthesis of high-density MgO films by a novel magnetron sputtering system
Vacuum, 2004
A novel magnetron sputtering system, which included simply designed two grids between target and substrate, was developed in our laboratory for the synthesis of high-density MgO films. In order to investigate the effect of grids assisted magnetron sputtering, MgO films were deposited by conventional magnetron, one grid assisted magnetron and two grids assisted magnetron. The saturated ion current density and Mg ion fraction in MgO discharges generated by grids assisted magnetron were increased in comparison with those obtained in conventional magnetron, which means that grids assisted magnetron led to the enhancement of plasma density. As a result of microstructure analysis, grids assisted magnetron produced a higher density MgO film with smoother surface compared to that obtained in conventional magnetron.
Suppression of Ferromagnetic Ordering in thicker co-sputtered Mn doped MgO Films
MRS Proceedings, 2013
We report on preliminary studies of low (14 at.%) and high (53at.%) concentration Mn doped MgO films deposited by co-sputtering from metallic Mn and Mg targets. The structural, surface morphologies and magnetic properties of the films of different thickness were studied. All the as grown films are found to be amorphous and film surfaces are found to be flawless and homogeneous. We observe at room temperature robust ferromagnetic loops with a saturation magnetization value that is a function of film thickness reaching a maximum of ~38.5 emu/cm 3 in the Mn 0.53 Mg 0.47 O film at a thickness of ~92 nm. In thicker films room-temperature ferromagnetic ordering is suppressed and eventually at a thickness around 120nm the expected diamagnetism of the bulk appears. The origin of ferromagnetism may be attributed to cation defects at the Mg-site.
Aging of magnetic properties in MgO films
Applied Physics Letters, 2010
In this work we report on the magnetic behavior of MgO thin films prepared by sputtering. A severe aging process of the ferromagnetic properties is detected in magnetic samples exposed to ambient atmosphere. However, ferromagnetism can be successively switched on again by annealing samples in vacuum. We suggest this behavior reflects the key role played by defects in stabilizing ferromagnetism in MgO films and is likely to be closely related to the hydrogen-driven instability of V-type centers in this material.
Ferromagnetism and diamagnetism behaviors of MgO synthesized via thermal decomposition method
Journal of Alloys and Compounds, 2017
MgO powders were synthesized by a thermal decomposition method which was determined by differential scanning calorimetry coupled with thermogravimetric analysis (TG/DSC). The starting material, Mg(OH) 2, was calcined at 400°C, 450°C and 500°C for 1 hour each to obtain the MgO powders. Phase composition, morphology and magnetic properties at room temperature of calcined powders were characterized using X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), fourier transform infrared Spectroscopy (FTIR) and vibrating sample magnetometer (VSM), respectively. Both Mg(OH) 2 and MgO phases were found in all calcined powders. The combination of ferromagnetism and diamagnetism at room temperature of all powders were observed. It is believed to be attributed to Mg vacancies and defect in the MgO and Mg(OH) 2 structures.
Ferromagnetism in transparent thin films of MgO
Physical Review B, 2010
We show both theoretical and experimental evidences of the appearance of ferromagnetism in MgO thin films. First-principles calculations allow predicting the possibility of the formation of a local moment in MgO, provided the existence of Mg vacancies which create holes on acceptor levels near the O 2p-dominated valence band. Magnetic measurements evidence of the existence of room-temperature ferromagnetism in MgO thin films. High-resolution transmission electron microscopy demonstrates the existence of cation vacancies in our samples. Finally, by applying the element specificity of the x-ray magnetic circular dichroism technique, we also demonstrate that the magnetic moments of the system arise from the spin polarization of the 2p electrons of oxygen atoms surrounding Mg vacancies.
Structural characterization and magnetic profile of annealed CoFeB∕ MgO multilayers
Magnetic tunnel junctions ͑MTJ͒ have become of strategic importance due to the large tunneling magnetoresistance ratio ͑TMR͒ that they can achieve at room temperature. The largest TMR values observed until now were recorded in MTJs with MgO barriers and CoFeB electrodes after annealing of the junction above the recrystallization temperature of the amorphous CoFeB layers. We have used x-ray reflectivity combined with polarized neutron reflectivity to characterize the structure and the magnetism of ͓Co 60 Fe 20 B 20 / MgO͔ 14 multilayers, where the MgO layers were prepared by different methods and annealed at different temperatures. We have found that the MgO preparation method as well as the annealing temperature play a significant role in the systems. A gradient in thickness together with a variation of the scattering length density along the multilayer stacks induce a process of underoxidation or overoxidation, strictly dependent on the MgO production method.
Study of the surface electronic structure of MgO bulk crystals and thin films
Surface Science, 1996
The electronic structures of the surfaces of MgO single crystals, oxidized Mg polycrystals and oxidized Mg films grown by molecular beam epitaxy on Si(100) surfaces were studied using several techniques. These include metastable impact electron spectroscopy (MIES), ultraviolet photoelectron spectroscopy (UPS (He I)), and X-ray photoelectron spectroscopy (XPS). Spectra of oxidized Mg layers on Si(100) show additional features to those obtained for cleaved MgO crystals. These spectral features are attributed to dissociative adsorption of oxygen at bulk oxygen sites. Weak heating of the oxidized Mg layers removes these features and the electronic spectra for all three studied systems become similar. However, the experimental MIES and UPS spectra, both arising mainly from the ionization of the O 2p orbitals, have different structures. They are interpreted on the basis of ab initio Hartree-Fock and density functional calculations of the electronic structures of the ideal MgO(100) surface. It is shown, that the differences in the spectra can be understood by taking into account that UPS spectra reflect the density of electronic states within several surface layers, whereas MIES probes the surface states which are the most extended into the vacuum.