In 1956, the first IBM hard drive weighed almost a ton and had a diameter of 61 centimeters and a storage capacity of just under 4 MB. These days, five terabytes can be stored on a device measuring just a few centimeters and storage space for a single bit measures just 20 nanometers, a thousand times smaller than a human hair. But increasing digitalization means that the amounts of data to be stored in the future will require a much higher storage density.
In order to fully comprehend this challenge, it is important to understand how files are written onto a hard drive: A bit is saved when the write head, a tiny electromagnet, magnetizes a section on the hard drive. To do this it has to precisely apply a strong magnetic field within billionths of a second without overwriting neighboring bits. At this point a bit is much smaller than the resolution of typical micro sensors, which is why these are barely capable of providing accurate data about the condition of the magnetic fields. “Currently, new write heads can only be developed by using simulations based on the principle of trial and error”, explains Ingmar Jakobi, a doctoral student at the University of Stuttgart’s 3rd Institute of Physics. “The industry is reliant on new sensors if it is to continue miniaturizing its storage devices.”
The 32-year-old researcher is part of Prof. Jörg Wrachtrup’s team, which has managed to measure a write field at the required length scale. To do this, the physicists used a material that is more commonly seen in a jeweler’s window: a diamond. Or, to be more precise, the nitrogen defects, which give these precious stones a pink color when present in high concentrations. These color centers are atomic flaws in the diamond’s crystal lattice, where a carbon atom is replaced by a nitrogen atom and the neighboring position on the lattice is empty. Single electrons get trapped in this structure and their spin, i.e. a quantum mechanical molecular magnet, reacts sensitively to magnetic fields, but are otherwise relatively shielded from the surrounding environment. The spin has an effect on the fluorescence of the color center. This means that a magnetic field on a single color center can be optically identified using a microscope.
Measurable Field Intensities Varying from A Few Microtesla to Hundreds of Millitesla
As part of an experiment, the team of scientists supporting Wrachtrup moved the hard drive write head gradually over the surface of a diamond crystal where the colour centers were located close to the surface. During this process, they were able to use the color center to measure field intensities ranging from just a few microtesla to hundreds of millitesla – both static fields and alternating fields with frequencies in the gigahertz range.
“Our magnetic field sensor is unique because of its ‘size’. The lattice spacing of the atoms in a diamond is just 150 picometers, that is 1000 times smaller than the pole of the write head and 100 times smaller than a bit on the hard drive”, explains Ingmar Jakobi, who published the study's findings in the scientific journal Nature Nanotechnology* as lead author. He also states that the sensor is easy to use: “Because the spin is trapped in the defect we measure the volume of an atom, but can easily control the position thanks to the size of the whole crystal. The resolutions depend upon how accurately we move the diamond and how close the hard drive head is to the surface.”
The project is the perfect example of successful cooperation between the industrial sector and science, say representatives at Seagate Technology, a leading hard drive manufacturer: “write heads are designed to create huge magnetic fields on the smallest possible space to achieve the best possible storage capacity. The ability to measure these on the nanometer scale is an outstanding development for our industry”, says Dr. Fadi El Hallak, Head Developer at Seagate and co-author of the study.
Magnetic resonance imaging on single molecules
For the Institute director Prof. Jörg Wrachtrup, the findings achieved as part of the special research project SFB/TR 21, mark an important step in establishing a further practical application for quantum technology. Wrachtrup also hopes that this will open new perspectives for the field of research that go far beyond micro electronics: “The findings show that it is possible to use write heads for our experiments and that we can utilize the extraordinary properties of the write field. The strong gradient of the field could make it possible, for example, to use nitrogen defects to carry out magnetic resonance imaging on single molecules.” Subsequent projects of this kind could also involve the Interdisciplinary Center for Applied Quantum Technology ZAQuant at the University of Stuttgart, where physics, engineering sciences and industry want to work together to research and develop new quantum sensors and operational prototypes. Andrea Mayer-Grenu