Magnetic Resonance -- Spintronics

Interests of the Moodera Research Group include:

(*) Manipulating electron spin in solids: spin tunneling; spin filtering; interfacial exchange coupling
(*) Molecular spintronics: towards molecular-scale spin memory
(*) Quantum transport in topological insulators and heterostructures: atomic scale interface effects; spatially resolved interface chemical/physical/magnetic studies in national labs; electrical transport under extreme conditions – mK temperature and high magnetic field
(*) Majorana fermion: search Majorana fermion in unconventional superconductors; interactions and entanglements among Majorana quantum particles.

Investigation of quantum properties of new quantum systems: Topological Insulators (TI) -- TIs represent a class of new quantum states that are predicted to display novel quantum coherent behaviors. They exhibit gapless surface states with Dirac-like band dispersion in momentum space, originating from the strong spin-orbit coupling and band inversion. Consequently this leads to insulating bulk but highly conducting surface states. Moreover the special feature of TI surface states is the spin-momentum locking, giving the charge carriers immunity from scattering due to the conservation of time reversal symmetry. TIs behave as ideal spin source where spin-up and spin-down carriers flow in separate channels without spin scattering or energy dissipation, making them highly valuable for future spintronics. These unique band features and surface properties lead to the predication of myriads of phenomena, and importantly, can encompass different fields, e.g. high energy physics, semiconductors, superconductivity, magnetism, strongly correlated electron systems and so on.

TI materials exhibit a combination of properties ranging from Heusler to magnetic to superconductor to antiferromagnetic and in some cases to Kondo insulating behavior as well. Other predictions such as topological magneto-electric effect, axion electrodynamics, Majorana modes are waiting to be discovered. Hence it is a rich playground for novel physics, chemistry and material science.

Research in this field concentrates on hybrid TI thin films and heterostructures. One example was to demonstrate among others the predicted quantum property namely quantum anomalous Hall (QAH) state by breaking time reversal symmetry in TI. QAH effect describes the dissipationless quantized Hall transport in the absence of external magnetic fields. The realization of the QAH effect in realistic materials requires two stringent conditions: ferromagnetic insulating materials and topologically non-trivial electronic band structures. In the absence of an applied magnetic field, we successfully observed the QAH effect in V doped TI thin films accompanied by dissipationless electron transport (~ zero longitudinal resistance, figure below). This was achieved in collaboration with Don Heiman at Northeastern U and Moses Chan, C –X Liu, Jainendra Jain at Penn State U. Our realization of a robust QAH state and chiral edge channels that conduct a spin polarized dissipationless current is a major step towards energy efficient electronics. This observation in a TI is in sharp contrast to the QH state observed in conventional semiconductor heterostructures that requires tens of tesla magnetic fields.

We also investigate an alternate way to drive TI into magnetic state using pure interface effects. A ferromagnetic insulator is coupled to a TI via short range interfacial exchange interactions (as done in the past with superconductors). This approach builds atomically sharp interface between two different materials. What is distinctive about this route is that it generates many tens of tesla internal field at the all-important TI surface without introducing additional dopant and defects. Extensive spatial-resolved interfacial studies using national synchrotron and polarized neutron reflectivity facilities have shown induced ferromagnetism in TI accompanying with exotic magnetic and transport behavior.

Searching Majorana fermions in unconventional superconductors: superconducting gold surface states -- In 1937, Ettore Majorana proposed an exotic fermionic particle called Majorana fermion, which is its own anti-particle. Neutrinos are believed to be Majorana fermions, but this remains unproven. Recently there is strong activity in the possibility of creating actual Majorana fermion quasi-particles, i.e. Majorana bound states (MBSs), in condensed matter systems. Unlike neutrinos, the MBSs are localized in space and can be thought of as half a fermion and are expected to obey non-abelian statistics. This unconventional behavior guarantees the MBSs’ quantum states from de-coherence, and is crucial for them to become basic elements for fault tolerant quantum computations.

Such exotic objects (MBS) have not been seen in nature, and thus will open up many emerging fields. There have been several theoretically guidance of searching MBSs in exotic superconductors, in the vortex cores or at the end of one dimensional wires. Our search for MBS is with close collaboration of Patrick Lee’s group (Physics Dept), who theoretically proposed topological superconductivity in gold surface states by atomically coupling it to a thin film superconductor. The strong Rashba-type spin-orbit coupling of the gold surface states guarantees the robustness of the MBS from material disorders. The nanowire form of the gold/superconductor heterostructure may lead to fundamental platform for achieving fault tolerant quantum computations. We utilize nano-scale quantum tunneling spectroscopy under extreme conditions (mK temperature and high magnetic field) in gold thin film nanowires to reach our goal.

Control and manipulate spins in molecules: Towards Molecular Spintronics -- The use of molecular state as quanta for information storage, sensing and computing is a subject that meets the demands for future generation data storage and communication. Utilizing multifunctional molecular spin states as quantum bits for memory, sensing and logic etc. is called molecular spintronics. Organic molecules have the potential to lead towards this cherished objective: single molecules have a well-defined number of atoms and electronic structure, and thus form the smallest possible electronic units that can be controlled within atomic precision. As a result, a functional spintronic device can be incorporated within a single molecular building block. Furthermore, one can design them with precisely tuned molecular properties, by attaching specific functional groups etc., and thus it gives rise to limitless possibilities for future science and technology.

However, understanding the interaction down to the molecular level at the interfaces has been an ongoing challenge for decades. In particular, interfacial charge and spin transfer resulting from hybridization and magnetic exchange interactions are essential to realizing functional spin based molecular devices. With this in mind we demonstrated, see figure below, (in collaboration with Swadhin Mondal at IISER, Kolkata, India and Markus Munzenberg at Griefswald U, Germany) the possibility to engineer the FM/molecule interface through a phenalenyl based molecule in developing integrated molecular-scale spin quantum devices. We observe an unexpected MR of over 20% even near room temperature involving only one FM electrode resulting from a spin-filter tunneling effect. Furthermore, using phenalenyl chemistry, we show the possibility to create an artificial nanoscale magnetic molecule with a well-defined magnetic hysteresis at the FM surface. Such molecular spintronic devices could be utilized in information storage and for massive parallel computations far more efficiently than a classical binary memory or a multi-level single molecular qubit.

Success of molecular spintronics relies on tailoring the molecule-FM metal interface interactions. The adsorption of molecules on a FM surface is a complex phenomenon, with various possible configuration and related charge transfer which determines the electronic and magnetic structure of the entire system. Understanding this is our goal, although as complex as imaginable this interaction may be, it is an essential step towards future molecular engineering. It nevertheless opens up prospects for multifunctional molecular spintronics development. This is coupled by the enormous potential to design and synthesize molecules of required diversity, functionality and flexibility making this an open field to explore.