An insulating compound (MgO) used as a barrier layer in the nanosandwich structure.Magnesium oxide is a white, crystalline solid formed when magnesium burns in oxygen or when magnesium salts are heated. Chemically it is an ionic compound: each magnesium atom gives up two electrons to an oxygen atom, producing Mg2+ and O2- ions packed into a regular lattice. Because its electrons are tightly bound in that lattice, MgO does not conduct electricity — it is an insulator, with a wide band gap of roughly 8 electron-volts. That combination of chemical stability, high melting point (about 2850 °C), and electrical insulation is what makes it useful as a thin barrier layer in layered nanoscale devices, where it separates two conducting or magnetic regions and controls how electrons or spins pass between them.
Magnesium oxide has been known since at least the eighteenth century as magnesia, and its insulating and refractory properties were studied long before nanoscale thin films existed. Its use as a crystalline tunnel barrier in magnetic junctions emerged in the 1990s and 2000s, when researchers learned to grow it epitaxially on metal electrodes.
In a nanosandwich, a layer of MgO only a few atomic layers thick is grown between two metallic films. Electrons cannot simply flow through the oxide, but they can tunnel quantum-mechanically across it: the thinner the barrier, the higher the probability that an electron passes from one metal to the other. The tunneling current depends strongly on the barrier thickness and on the electronic states on either side, so the MgO layer acts as a sensitive valve rather than a simple wall. In magnetic tunnel junctions, the MgO also filters which electron spins can tunnel, which is why the same material underpins read heads in hard drives and magnetic memory cells.
Thin MgO films are used as tunnel barriers in magnetic tunnel junctions, which serve as the read sensors in hard-disk drives and as the storage elements in magnetic random-access memory (MRAM).
Magnesium oxide is the same material as the white powder used in some antacids and as a refractory lining in high-temperature furnaces, so the same compound appears both in medicine and in nanoscale electronics.
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