Electronics Weekly
The device is extremely fast in turning on – 20ps has been measured when hit by a femtosecond laser pulse. [Ed: an earlier version of this article incorrectly stated ’20ns’]
It has a stack of two Co22Fe58B20 magnetic layers separated by a magnesium oxide insulator.
To pin its magnetic field in place (with a vertical orientation), the bottom magnetic layer has a thin coating of pure iron just below the oxide.
The top magnetic layer is conversely ‘free’, in that its magnetic direction is free to be influenced by external factors including exposure to light and an applied magnetic bias field.
When both layers have the same magnetic direction, the resistance through the device from top to bottom is ~550Ω, while if the direction of magnetisation in the top layer is reversed or skewed sideways, resistance climbs to around 1kΩ.
Various combinations of magnetic field and light (both 800 and 450nm have been tried) realign the magnetic field in the top section sufficiently to adjust the resistance of the device – allowing the external magnetic field to be used to tune its optical sensitivity.
To give an idea of sensitivity, it was triggered with a simple laser pointer at a public demonstration this week.
While turn-on is fast, turn off is slower – for example a handful of nanoseconds in the proof-of-concept device. Electronics Weekly asked TDK about this and, while details cannot be revealed at this time, TDK replied that likely causes of slow turn-off have been identified and don’t look to be show-stoppers – a second-generation research device is already being studied.
Once made, the light sensor is stable up to 400°C, allowing it to be further processed on an IC surface.
“Because the spin photo detector has a completely different operating principle and uses an electron heating phenomenon, it can operate at ultra-high speeds even when the wavelength is shortened,” according to TDK, which worked with Nihon University on the device. “It has been confirmed that it can operate from visible light to near-infrared light.”
Analogue responses are possible, and arrays of the sensor can be made. A “fast visible light image sensor can be realised. [The] required element size for fast detection is much smaller for spin photo diode” compared to a regular photo diode, the company told Electronics Weekly.
The sensor concept came out of the company’s knowledge of designing and manufacturing magnetic tunnelling read heads for disc drives.
Like other magnetic tunnelling junctions, it is expected to have strong cosmic-ray resistance, possibly opening up aerospace applications.
The Journal of Physics D: Applied Physics has published a detailed article: ‘Spin photo detector by using a CoFeB magnetic tunnel junction‘, which can be read in full without payment. This includes detailed amplitude and spectral sensitivity information.
