They say small is beautiful. In the present context of data storage in personal computers, small hard disks have brought a huge revolution by allowing us to save enormous amount of data in just a few inches of space. We use this space in the hard disk of our laptop which is like a personal library for us to store our e-books, games,software, photos, videos, documents, spreadsheets and many other stuffs. The best part about this library is its compact size which allows storage of terabytes of data in just a few inches. However, this portability wasn’t always there some years back! Hard drives in the 1950’s occupied a whole room. Thankfully, hard disks have undergone a massive reduction in size and increase in storage capabilities over the years making laptops and computers lighter in weight. While we all enjoy more storage capacity of data storage devices, have you ever wondered how the huge amount of data get stored in these hard drives?
Well, the hard disk uses magnetism to store large amounts of data.The spinning disc-like plate in hard disk is magnetic in nature and is divided into billions of small areas. As the computers only understand the binary language of 0s and 1s, every letter, picture or program is coded in these binary codes in hard disks. The small areas in the disc are either magnetized which correspond to 1 or demagnetized which correspond to 0. The different magnetization, i.e. 0 and 1 at a particular area is sensed by a pointed readout head scanner in the hard disk.
With a single letter composed of 8 bits (0s and 1s), one can imagine the huge number of magnetizations required to be present on a magnetic disc. While compacting the size of the hard drive, the major challenge was to ensure that the readout scanner is sensitive to reading the closely spaced different magnetizations on the platter. This problem was solved by the discovery of giant magnetoresistance (GMR), where thin films composed of alternating magnetic and non-magnetic metallic layers, which when expose with a magnetic field show a huge drop of electrical resistance. This large degree drop can be easily identified and differentiated as a different state (0 or 1) by the readout head scanner.
The phenomenon of giant magnetoresistance opened up various avenues for application and therefore, the discoverers, Albert Fert (French) and Peter Grünberg (German) were conferred with the distinguished Nobel Prize in Physics in the year 2007. While giant magnetoresistance is phenomenal, scientists across the world are now looking at utilizing the phenomenon of “colossal magnetoresistance” which is more powerful than giant magnetoresistance and can be the next leap in data storage technology. Toward this, scientists are on a look for materials which show large magnetoresistance – a material that shows low resistance to the flow of current at a smaller intensity of the magnetic field.
“We know the electron has spin and is orbiting around the nucleus in the atoms. The motion of the electron along with spin around the nucleus gives rise to the magnetic moment, which contributes to the atomic magnetic moment. The magnetic moment of each atom in a magnetic material is aligned along a particular direction. However, magnetic materials contain grains, grain-boundaries, and structural distortions. Hence, the magnetic moment of all atoms is not perfectly oriented in a single direction. After the application of the external magnetic field, magnetic moments of each atom get oriented along the direction of the applied field, reduces the scattering of conduction electrons, which leads to the decrease of the resistance of the materials,” says Dr. Prahallad Padhan, who is an Associate Professor at IIT Madras, while explaining the atomic-level details of the phenomenon of magnetoresistance in which the magnetic field and magnetism of material is utilized for controlling the intensity of the current.
Prof. Prahallad Padhan and his team at IIT Madras are working in the field of magnetoresistance and are experimenting with materials, which require low magnetic fields to creategiant magnetoresistance. In their latest research, published in the prestigious international journal Nanoscale Advances, the team reports 99.8 % magnetoresistance at seven Tesla magnetic field in manganites.
“The magnetoresistance of the conventional magnetic materials such as iron, cobalt, or nickel, is around 5 %. Significant enhancement of the magnetoresistance is observed in the artificial material prepared using iron and chromium, and the associated quantum mechanical phenomenon is popularly known as giant magnetoresistance. The artificial material is a thin film multilayer system with the alternate stacking of thin layers of iron and chromium. More recently, the magnetoresistance of the manganites thin-films is found to be orders of magnitude larger than the typically found magnetoresistance of GMR multilayer films. The magnetoresistance of these manganite thin-films is called colossal magnetoresistance (CMR). The maximum magnetoresistance is observed at the ferromagnetic-to-paramagnetic phase transition temperature of the CMR materials,” adds Prof. Padhan while explaining the choice of using manganites for the study.
For this study, the team used 30 % strontium doped lanthanum manganite thin films which were stabilized over lanthanum aluminate substrates by adopting a unique pulsed plasma deposition growth method. Although the variation in magnetoresistance of a series of films with different thickness has been reported earlier, the team observed 96 % magnetoresistance under a Tesla magnetic field at 40 Kelvin temperature range which has not been observed earlier. Further, they found that magnetoresistance increased to 99 % on increasing the magnetic field to three Tesla. As per the team, this study is the first to report the achievement of 99.8 % magnetoresistance at seven Tesla of magnetic field in Colossal magnetoresistance manganites even if the film shows lower magnetic moments as compared to the expected value.
“In the study published recently by Prof. Padhan and coworkers, they have mainly focused on the structure of interfacial spins and its consequence for magneto transport, especially at low (or weak) magnetic fields. Through several careful thickness-dependent measurements of several physical properties, they have elucidated the origin of the high low field magnetoresistance and attributed the same to non-collinear interfacial spins of Manganese. While the scientific results are quite interesting, applications could emerge only when such an effect will be realized at room temperature by further interfacial material engineering,” comments Prof. Satishchandra B. Ogale, Emeritus Professor at IISER Pune and Director at Kolkata based Research Institute for Sustainable Energy (RISE), who is an expert in the same field.
After achieving this feat, the team is all geared to work towards this large magnetoresistance of 99% at room temperature which will open up avenues for its practical applications. The team led by Prof. Padhan at IIT Madras includes Umesh Kumar Sinha and Bibekananda Das.
Article by Aditi Jain
Link to the article : https://pubs.rsc.org/fi/content/articlepdf/2020/na/d0na00287a