Toshiba is developing a magnetic head able to write and read data on layers of stacked substrates on the same platter by using microwaves. We can indeed call this technique 3D reading and writing!
The MAMR technology (Microwave Assisted Magnetic Recording) was announced and demonstrated in July 2015 during the 20th International Conference on Magnetism (ICM) in Barcelona. It is reminiscent of the principle of reading / writing multilayer optical discs used for the DVD9 and other Blu-Ray formats, and also some variations of opto-magnetic discs released in recent decades. But any similarity ends here: there is no optics in technology that Toshiba wants to develop. Everything here is only magnetism and electromagnetism.
A technology in progress
We must realize that we are at the level of research and development, not yet on technology that may land in two or three years. In principle, because of 3D technology, it means that at one location on the disk we can record multiple separate data. The number of bits depends of the number of stacked layers. In operation, we have to put in resonance only one layer at a specific time at a specific location with a certain wavelength of the microwave transmitter which is adjacent of the magnetic head.
Activating the right wavelength determines the data layer (1, 2, etc.) and will be in a state wherein we can read or write it. Remember that writing is generally more complicated, especially considering the stack of layers! Normally it takes significant energy input to reverse the polarity of the medium. With the addition of the microwaves frequency that puts the layer in resonance, energy is no greater than if the layer was on the first stacking level, and may even be less. This is the ferromagnetic resonance frequency of each layer which is specific.
Toshiba at the forefront of research
To demonstrate the level at which Toshiba stands in terms of research, the emphasis is now placed on the precision of the microwave resonance layout element so that it can access the disk for writing with increasingly fine bits. With its MAMR technology, Toshiba expects to multiply by 10 the storage density of the disk surface compared to the existing. In addition, Toshiba sees its technology as more interesting than HAMR (Heat Assisted Magnetic Recording) used notably by Seagate (see article) because it does not pose some thermal problems of demagnetization over time.
Diagrams courtesy of Toshiba.