From the tunnel barrier in MRAM to the thermal path in a power module

At the thin-film scale, MgO stops being a filler and becomes a functional layer. It is the reference tunnel-barrier material in magnetic tunnel junctions — the structure behind today's MRAM and high-density read heads — and at the same time a workhorse insulating filler that moves heat away from power devices.

01The MgO tunnel barrier

A magnetic tunnel junction is two ferromagnetic layers separated by an insulator only a few nanometres thick. Electrons tunnel through it, and the resistance changes with the relative magnetisation of the two layers — the tunnelling magnetoresistance (TMR) effect.

With amorphous alumina barriers the effect was modest. When crystalline MgO(001) replaced it, TMR ratios jumped by an order of magnitude, because the MgO lattice acts as a symmetry filter for the tunnelling electrons. That single materials change is why STT-MRAM and modern TMR read heads are commercially viable.

02Dielectrics, targets and buffer layers

• High-k dielectric. MgO is used as a gate dielectric and in thin-film capacitor stacks where a moderate-k, wide-bandgap insulator is needed.

• Sputtering targets. High-purity MgO targets and evaporation materials for PVD deposition of barrier and dielectric films.

• Buffer layers for HTS tapes. Textured MgO is a standard buffer on the textured metal substrate of second-generation superconducting tapes.

03Thermal management — the other half

MgO has an unusual combination: it is an electrical insulator, yet its thermal conductivity is far higher than that of most oxide fillers. That makes it valuable in the unglamorous but critical places — potting compounds, thermally conductive but electrically insulating plastics, LED substrates, and the insulation layer inside sheathed heating elements.

In all of these, purity is what determines performance. Transition-metal impurities (Fe, Ni, Cu, Mn) degrade dielectric strength and encourage leakage, so electronic grades are specified at 99.9% and above, with tight limits on individual impurity elements and controlled particle-size distribution for reproducible deposition or compounding.

04Recommended grades