While electrons are known to carry both charge and spin, in current electronic systems, only the electric charge part is used as an information carrier.
The limitations of modern electronics, as well as the eventual end of Moore’s Law, have reignited interest in the development of “spintronic” devices that can harness electron spin. The widespread deployment of spintronic computing systems is predicted to change information technology in the same way that the invention of electronics did.
Finding an effective and sensitive mechanism to electrically detect the electronic spin state is a significant difficulty in spintronics. The discovery of gigantic magnetoresistance (GMR) in the late 1980s, for example, enabled this capabilities.
In GMR, depending on the parallel or antiparallel spin configurations of the ferromagnetic bilayer, a substantial shift in electrical resistance occurs under the magnetic field. GMR’s discovery paved the way for the development of hard-disk drive technology, which is the first mass-produced spintronic device.
Other related phenomena, such as colossal magnetoresistance (CMR), which occurs in the presence of a magnetic field, have expanded our understanding of the interplay between spin and charge degrees of freedom and served as a foundation for emerging spintronic applications since then.
A research team led by Prof. KIM Jun Sung from the Institute for Basic Science (IBS, South Korea) and the Physics Department at Pohang University of Science and Technology (POSTECH, South Korea) discovered a new magnetotransport phenomenon in the magnetic semiconductor Mn3Si2Te6 in the most recent issue of the journal Nature.
Unlike the previous magnetotransport phenomena, a huge change in resistance is induced by only rotating the spin direction without altering their configurations. This unusual effect originates from the unique topologically-protected band structure of this magnetic semiconductor.
Professor KIM Jun Sung
Under a rotating magnetic field, the magnitude of change in resistance can approach a billion-fold, according to the researchers. The term “colossal angular magnetoresistance (CAMR)” was coined to describe this unusual change in resistance as a function of magnetic field angle.
“Unlike the previous magnetotransport phenomena, a huge change in resistance is induced by only rotating the spin direction without altering their configurations. This unusual effect originates from the unique topologically-protected band structure of this magnetic semiconductor,” notes Professor KIM Jun Sung, one of the co-corresponding authors of the study.
In spintronic applications, topological materials, a newly discovered class of materials, have become increasingly relevant. The term “topological material” refers to a material with “twisted” electrical structures.
The twisted electronic structure in topological materials is kept unless the system’s symmetry changes, just as a Mobius strip cannot be unwound without fundamentally altering its form.
Spin information can be hosted and controlled in such topologically secure states. Topological magnets, which combine magnetism and topological electronic states, have been extensively explored in tandem with the current development of topological materials.
Because their electronic structures are topologically protected but changeable by modifying spin configurations or direction, these topological magnets are of tremendous interest with a wide range of possible uses. This new class of materials provides innovative ways to combine spin and charge degrees of freedom, which is beneficial in spin-electronic applications.
The discovery of a ferromagnetic semimetal Fe3GeTe2 by the research team was published in Nature Materials in 2018. This material was identified as a topological magnet after it was discovered to contain nodal-line-shaped band crossing sites.
Degeneracy can be lifted in the nodal-line states depending on spin orientation, which is a unique property of this topological magnet. The research team expanded on the concept by focusing on magnetic semiconductors with topological nodal-line states in the conduction or valence bands.
The nodal-line state’s band degeneracy is sensitive to spin orientation once again, but in magnetic semiconductors, the lifting of band degeneracy, controlled by spin rotation, can transform the system into a semiconductor or a metal.
As a result, spin rotation can be used to turn on or off charge current flow, similar to how an electric field can be used to turn on or off charge current flow in traditional semiconductors.
The initial challenge was to find a candidate material that has both ferro or ferrimagnetism and a topological band degeneracy. Dr. KIM Kyoo of the Korea Atomic Energy Research Institute (KAERI) predicted a nodal-line-type band degeneracy in the ferrimagnet Mn3Si2Te6 using first-principle calculation methods.
In his calculations, he rotated the net magnetic moment of Mn3Si2Te6, which lifted the nodal-line degeneracy, similar to Fe3GeTe2, which is powerful enough to induce bandgap closure.
The IBS and Seoul National University’s HA Hyunsoo and Prof. YANG Bohm-Jung employed symmetry analysis to discover that the nodal-line degeneracy of Mn3Si2Te6 is protected by a particular crystalline symmetry, demonstrating its topological nature.
The computed changes in the nodal-line states, depending on the spin direction, can be captured by the developed Hamiltonian, which takes into consideration both nodal-line states and strong spin-orbit coupling.
Dr. SEO Junho and Dr. De Chandan in Prof. KIM Jun Sung’s research team at the IBS and POTSECH succeeded in synthesizing single crystals of Mn3Si2Te6 and measuring their resistance at low temperatures while spinning its spin moments using external magnetic fields.
They discovered that as the magnetic field rotates, huge resistance, up to gigaohm, is reduced to tens of milliohm. This massive change in resistance as a function of magnetic field angle has never been seen before and is at least 100 thousand times bigger than previously known angular magnetoresistance magnetic materials.
LEE Ji Eun and Prof. KIM Jae Hoon in the Department of Physics at Yonsei University in Seoul, South Korea used terahertz absorption measurements to experimentally confirm that the observed huge change in resistance is indeed due to electronic gap closure and the resulting insulator-to-metal transition, as it was theoretically predicted.
The gigantic angular magnetoresistance is a direct result of spin-polarized nodal-line states and their distinctive spin-charge coupling, according to these theoretical and experimental discoveries by the research teams involved.
The recently discovered enormous angular magnetoresistance is expected to be used in high-angular-sensitivity vector magnetic sensing or efficient electrical readout of the spin state. Furthermore, by taking use of Mn3Si2Te6’s semiconducting characteristics, a novel sort of spintronic device may be created, in which both charge and spin degrees of freedom can be manipulated simultaneously using electric or magnetic fields.
One of the remaining hurdles is extending the gigantic angular magnetoresistance’s operational temperature range to room temperature. Magnetic topological semiconductors with a triangular lattice as a structural motif are thought to exhibit gigantic angular magnetoresistance as a common attribute.
“In nature, there is a vast possibility of candidate magnetic semiconductors, showing similar or even stronger properties at high temperatures, awaiting theoretical investigation and experimental verification,” noted Professor Yang, one of the co-corresponding authors of the study.
