Defect-induced magnetism in chemically synthesized nanoscale sheets of MgO

Abstract

Highly defective MgO nanosheets were prepared by a colloidal synthesis and exhibited low-temperature ferromagnetism which was significantly larger that the magnetization potentially obtainable from the low transition-metal impurity concentration. Electron paramagnetic resonance experiments confirmed that the magnetization did not significantly involve impurities and that the nanosheets consisted of strongly interacting spin clusters which disappeared upon high-temperature annealing. These spins were concentrated along extended defects, possibly as unpaired electrons trapped at oxygen vacancies. DIM should be highly sensitive to the various defect or impurity characteristics, posing experimental and theoretical difficulties that demand prudence. Experimentally, Abraham et al.14 and Garcia et al. 15 showed the ease with which a ferromagnetic signal is obtained in such oxides due to impurities. The problem of paramagnetic transition-metal ion impurities as a source of magnetism was recently comprehensively studied by Khalid et al. 16 Much of the previous experimental work claiming to have detected DIM in MgO 9-13 as well as other materials ignored the need to precisely determine the paramagnetic metal impurity concentration in the analysis of results. Preparation of samples by deposition in a stainlesssteel apparatus, 12 using characterization techniques such as x-ray diffraction (XRD) and x-ray photoelectron spectroscopy (XPS) with insufficient sensitivity to low concentrations of impurities, 9,13 or using less-than ultrapure precursors or not addressing precursor purity 10,11 may lead to confusion about the source of the observed ferromagnetism. The theory of DIM is also problematic: In the early 70s large magnetic moments were predicted on neighboring oxygen atoms due to Mg vacancies.17 Elfimov et al. 3 predicted cation vacancy-induced ferromagnetism using density functional theory (DFT) calculations. Also, further theoretical work attributed the formation of magnetic moments and the resulting FM to spin-polarized holes residing on cation p orbitals either at vacancy or impurity sites. 3,18 Most DIM studies, both experimental [10][11][12][13] and theoretical, 12,[19][20][21] suggested that the origin of the observed FM in these oxides is related to cation vacancies or even nitrogen doping in MgO.22-24 Recent theoretical work discussed interdefect coupling mechanisms, 9,25-27 such as high carrier mobility near interfaces. Very recently, Zunger and coworkers 28,29 and Droghetti et al. 2 drew attention to the Achilles' heel of all DFT calculations performed with local approximations of the exchange correlation potentials, such as the local spin density approximation and the generalized gradient approximation. These methods overestimate spin density delocalization, leading to a half-metallic ground state with a relatively strong interdefect ferromagnetic interaction.In this paper we present results obtained on highly defective MgO nanosheets (MgO-NSs) prepared by a colloidal synthesis from the ultrapure precurso...

Resistance switching in transparent magnetic MgO films

“…Moreover, electrical switching that has been observed in MgO films [13] and MgO-based magnetic tunnel junctions [14][15][16] is attributed either to nanostructural rearrangements of metallic ions in the electrode/barrier interfaces or reversible displacement of oxygen vacancies. On the other hand, defect-induced ferromagnetism in MgO has already been observed at room temperature [17][18][19][20][21]. Our previous results demonstrate that the presence of cation vacancies has a strong impact on the electronic properties of MgO.…”

Spin-polarized ground states and ferromagnetic order induced by low-coordinated surface atoms and defects in nanoscale magnesium oxide

“…In the case of the triplet state, however, the <S 2 > values are ~3.02 and ~2.05 before and after annihilation of the first spin contaminant, respectively [29]. This indicates that the calculation for the triplet state is 8 affected by mixing with a higher spin (possibly quintet) state. Figures 3(a)-3(c) …”

Defect and adsorbate induced ferromagnetic spin-order in magnesium oxide nanocrystallites

“…9 It was assumed that vacuum annealing results in creation of oxygen vacancies, which subsequently reduces magnetic moments and, therefore, the observed RTFM spin-order was perceived as induced by magnesium vacancies. Recently, Maoz et al 10,11 noticed that air annealing too reduced magnetic spin-order in MgO nano-sheets and suggested one electron trapped oxygen vacancies (F þ ) to cause the observed ferromagnetic spin-order. Very recently, Wang et al 12 exhibited invariable magnetic moments when annealing in air was performed below 1000 C. Further, Balcells et al 13 observed vacuum annealing of five months air exposed MgO thin films at 850 C to reveal enhanced magnetic moments.…”