Research in our group revolves around different facets of magnetism. ur work is largely experimental in nature but takes a cue from condensed matter theory, applied quantum mechanics, classical and quantum optics. In this respect, we draw from rigorous theory, rely on mathematical modeling and simulation, and also design and build scientific instruments or test new measurement techniques.
Magnetism and magnetic materials are the cornerstones of modern civilization. The generation and transmission of electricity, storage of data, and information processing rely on the smart engineering and harnessing of this phenomenon inside solid materials. Despite its familiarity with humankind since three millenniums, it was only in the twentieth century when advances in quantum mechanics enabled a thorough understanding of magnetism.
In its simplest form, the atomic origin of magnetism is the electron’s spin. Each electron is a tiny magnet, called a magnetic moment. Strong interactions between spins, called the exchange interaction, allow spins to cooperate in myriad ways, yielding magnetism of various forms, the strongest of them is called ferromagnetism. In a ferromagnetic material, regions of parallel moments nucleate in the form of magnetic domains. The distribution of domains determines the overall magnetic properties of a piece of solid material. The moments, however, are also affected by the lattice, the crystalline anisotropy, spin-orbit interactions and magnetic fields. These magnetic fields could be endogenous to the material (demagnetization fields) or externally applied fields. The properties that emerge, therefore, are a complex interplay between all of these mechanisms.
One of my research themes is in the understanding of interacting magnetic and dielectric properties, which represents the correlation between spin and charge degrees of freedom. The dielectric analog to ferromagnetism is ferroelectricity where the electric polarization nonlinearly responds to applied electric fields. In our work, we synthesize new material compositions with the aim of enhancing magnetoelectric coupling. This allows the control of the electric permittivity using magnetic fields, or conversely, magnetization using electric fields. Another important class of ferroelectric materials is piezoelectric materials that elicit an electric-field dependent elastic response.
Why is this synergy important? One of the most promising candidates for energy-efficient, fast and compact information storage is the magnetoelectric random access memory (MeRAM). The conventionally used dynamic RAM (DRAM) utilizes electric current to inject data into a capacitor whereas the MeRAM allows ultrafast electric fields to efficiently perform the reading and writing of data, promising orders of magnitude savings in energy.
It is essential that scalable, low-cost synthesis routes of material compositions, interfaces, and structures exist and their complete magnetoelectric characterization is available enabling optimization of properties. My articles numbered [7,11,18,20,22,23] focus on this theme. The overarching principle is simple. We synthesize controllable compositions of piezoelectric, ferroelectric, dilute magnetic semiconducting multiferroics, characterize them and present underlying mechanisms for magnetoelectric coupling, providing the community with a degree of predictive capability for materials design.
The electric permittivity at optical frequencies can also be controlled by a magnetic field, allowing us to write. Extending magnetoelectric interactions to optical frequencies, we enter the realm of magneto-optics. Suppose a suitable magneto-optic crystal is placed inside a magnetic field. Incident light interacts with the crystal and undergoes a change in its polarization state after scattering. We can use the change in polarization to determine the.
Various geometries can be envisaged for this scattering experiment. For example, transmission or reflection with diverse configurations of the field, magnetization, and direction of the momentum vector of light provide different measures of magneto-optic effects. Additionally changing also yields spectroscopic information providing information about electronic energy states and the overlap between states (called oscillator strengths). These effects are not only of purely fundamental interest but also harbor promise for practical applications. Furthermore, magnetic microscopy based on the polarization rotations also allows imaging of domains and nanostructures. All of these techniques (Figs. 2, 3, and 4) are built by my Ph.D. student and are being used for scientific investigations described next.
Magneto-optics is a nonreciprocal effect. This means that changing the direction (momentum) of the light beam does not let the polarization rotation retrace its original bending. Technically, this is called a violation of ‘time-reversal symmetry’. This property is employed to make optical isolators which act like one-way valves for the propagation of light. We have investigated these effects in single crystals under various geometries, references [4,12,17,21,26], and demonstrated that ultra-large polarization manipulations are possible in terbium gallium garnet crystals, which are accentuated at low temperatures.
Furthermore, phenomenological modeling of polarization manipulation in the presence of deleterious effects (light is absorbed and the absorption depends on polarization, the refractive index is different for linear polarization states, etc.) has allowed us to provide benchmarking criteria for assessing the performance of practical devices particularly optical isolators.
Magneto-optics with antiferromagnets
Currently, we are investigating magneto-optic effects in antiferromagnetic insulating crystals as well as metals. Antiferromagnets, with oppositely aligned magnetic sublattices result in zero magnetization rendering magneto-optic effects, at best, elusive. However, it is possible that these effects can be switched on or off by controlling the symmetry of the crystal through, for example, spin-orbit interaction, the staggering of the crystal planes in bilayer structures or applying electric fields. This allows precise tuning of the magneto-optic response.
Furthermore, nonlinear effects (such as the Voigt effect) which depend on the square of the magnetization of sublattices, instead of the linear dependence, can also allow determination of the direction of the antiferromagnet’s Neel vector with possible extensions to Neel vector spatial imaging. At present, these experiments are underway in my group and are currently in the stage of growth of single crystals (using crystal pulling in a homemade Bridgman furnace) or chemical vapor deposition (a low-cost system is being constructed in-house).
Magneto-optics for chemical sensing
One of the practical applications of magnetic films comprising compositions of transition metals and rare-earths (e.g. CoxPt100-x, FexPd100-x) lies in the sensing of hydrogen concentrations. Different routes may be adopted for this purpose such as ferromagnetic resonance. We are in the process of real-time magneto-optic Kerr measurements in time-dependent hydrogen supplies to these materials.
Magneto-optics with quantum light
In our laboratory, we have also demonstrated a tabletop generation of entangled photons using nonlinear crystals. Laser light shines on a pair of mutually twisted b-barium borate crystals. The light emerges in the form of a pair of photon beams, both beams being entangled in time, momentum, and polarization. Quantum entanglement is one of quantum physics’ most counter-intuitive facet. One of the down-converted beams can be used as a trigger, signaling the creation of a single photon in the complementary beam. These streams of tagged photons called heralded photons, constitute what is called ‘quantum light’.
We are in the process of conducting magneto-optic experiments with quantum light in an attempt to observe polarization rotations of a single photon as it interacts with a magnetic film (single photon Faraday and magneto-optic Kerr effects). Possible extensions to this work include quantum imaging of spins, magnetic structures, vortices in superconductors, and generation of spin-photon entanglement.
At the outset, we are accustomed to dealing with three-dimensional materials; but the physical size, geometry, and shape of materials can be reduced. Enter nanotechnology! For example, it is absolutely possible to grow single atom thick layers which may be called two-dimensional (2D) materials. The archetypal example is graphene. Electrons, which carry both charge and spin, possess energy and momentum inside the crystal. The crystal momentums are sectored in the form of bands, analogous to the formation of discrete levels in isolated atoms. The band structure can have interesting symmetry and topological properties. Topological phases of matter, have heralded a new field of investigation with baffling properties popping up almost all the time. An example is a topological insulator, a material in two or three dimensions which is insulating in the bulk but conducting on its surface. The presence of spin-orbit interaction leads to current flow only in one direction.
We are studying the magneto-optic signatures of these two-dimensional materials in the presence of spin-orbit, Rashba interaction, gating, chemical doping and applied magnetic and electric fields. Our articles referenced [1,2] survey magneto-optic signatures of such materials (graphene, silicone, phosphorene, borophene) including the giant Faraday and Kerr rotations, quantized lateral and angular shifts of optical beams, and the photonic spin Hall effect. These effects are prominently featured in THz spectroscopy and can be used for ultrasensitive sensing of magnetic fields, probing of quantized energy levels, and in building practically important polarization-sensitive optical components.
Figure 5 Experimental arrangement for a quantum erasure experiment.
Figure 6 Quantum interference experiment for a single photon.
Opto-magnetics at the ultrafast scale
It is clear from the previous discussion that magneto-optics uses light to extract information about magnetism. The converse phenomenon called opto-magnetics aims to achieve the converse: controlling magnetization and spin (precession, polarization) using light pulses. Recently, my PhD student Ali Akbar has forayed into the atomistic simulation of opto-magnetic effects initiated by a femtosecond ( s) laser pulse. The laser pulse heats the electronic system. The electron-spin interaction affects the magnetization, effectively quenching the magnetization. The exchange interaction within the complex spin system promotes spin precession and possibly the regrowth of magnetization in the opposite direction. This laser-induced magnetization switching is the cornerstone of potential techniques for efficient, low-energy information storage and retrieval in the electronics industry. Our recent works published in IEEE Transactions on Magnetics and Journal of Magnetism and Magnetic Materials [5,8] describe the experimental work and simulations on DyFe nanostructures for ultrafast magnetization switching.
Optical detection of spintronic effects notably the spin Seebeck effect
Spintronic effects, in particular the spin Hall effect and its variants have ushered in a new era of low-dimensional materials and the burgeoning field of spintronics which aims at carrying out information processing tasks reliably, efficiently, and robustly. One of my projects involves the optical probing of spin caloritronic effects notably the spin Seebeck effect. The proposed optical read-out obviates the need for electronic measurement which relies on spin injection across interfaces. The readout is being performed by a scanning Kerr reflection technique while time resolved dynamics of the spin current are probed by a high bandwidth, pulsed laser, yielding vital information about the time dynamics of spin transport, and hence the “benchmarking” of spintronic devices. As part of this work, we are investigating the magneto-optic signature of topological insulators.
Presentation: Optical Detection of Spin 2016
Magnetization dynamics of single-molecule magnets
Singe-molecule magnets (SMM) are organometallic molecular complexes that behave as magnets. They are distinct from single-crystal, bulk and single-domain magnetic nanoparticles in several respects. Their unique properties position them at a special vantage point, straddling the interface of classical nanophysics and quantum physics. I am working on the probing of thin films and solutions of such SMM’s using polarized light that is obtained from a stabilized and modulated laser diode. The measurements are performed inside a variable temperature cryostat and are carried out in transmission or reflection geometries. These studies may become useful for exploring the potential use of SMM’s as magneto-optical switches, giant Faraday rotators, magnetic field sensors and futuristic quantum information processors.
This aspect of my research work summarizes my research program for the next ten years.
Theoretical and experimental advances in condensed matter physics are unfolding new physical phenomena with dazzling properties, one after the other. The field of spintronics harbors the immense promise of utilizing an electron’s spin—in contrast to its charge used in conventional ‘electronics’—for faster computing and exponentially smaller power requirements. This potpourri of spins, magnetism, the angular momentum of spins, and the lattice, and in many cases, elastic strain and thermal effects can now be created, controlled, and detected by optical means. These interactions form the mainstay of modern research in condensed matter physics and a good portion of the entire discipline of physics. They also carve my future areas of scientific endeavor.
My aim is to establish a world-class experimental facility capable of probing the ultrafast characteristics of select materials using Femto-optics. In particular, we aim to develop an ultrafast optical technique comprising a pump and a subsequent probe pulse, derived from a femtosecond laser. The laser is capable of outputting pulses of light, of the order of one hundred femtoseconds (1 femtosecond is one-millionth of a billionth of a second). The first pulse, called the pump, creates some change in the property of the material. A second pulse, called the probe, derived from the same laser source, then negotiates the disturbing material and manifests the change in the physical property. The timing interval between the two pulses is an adjustable parameter, allowing us to traverse the temporal dynamics of spins, carriers (holes and electrons), magnetization, lattice, and other quasi-particles and excitations. The dynamics will be probed on the scale of femtoseconds to nanoseconds, providing a window into ultrafast dynamics of various excitations and their interactions.
The overall goal is the complete optical control and detection of magnonic and spin caloritronic effects.
I also envisage that the facility and the ensuing theoretical investigations will also become a nursery for spin-based photoemission experiments of topological insulators, Weyl semimetals, and other quantum materials harboring topological order. This is also one of the main goals of the Laboratory for Quantum Technologies, a part of the National Centre for Nanoscience and Nanotechnology that is being established by the Higher Education Commission from public sector funding.
I now delineate some of the key scientific questions I like to pursue in the next ten years.
One of the most futuristic areas of intense research is femtomagnonics which promises to employ magnons, quantized spin oscillations. Magnons are wave-like excitations in the spins of a system, akin to phonons that are excitations in lattice oscillations and photons that are excitations of an electromagnetic field. Magnons, depending on the mechanism of the spin interaction (exchange or dipolar) occupy a range of frequencies going up to the THz regime. They can be sustained inside ferromagnetic insulators, allowing virtually dissipation-less transport. These can also propagate over long distances ( cm range) as compared to the small diffusion lengths ( nm range) associated with the transport of spins and with high propagation speeds ( several km per second). These spin-waves naturally integrate with ferromagnetic metals and magnetic alloys notably permalloy, half-metals and more importantly insulators such as yttrium-iron-garnet Y3Fe5O12. Magnons offer the birth of a new discipline, complementing electronics and spintronics, and offers an energy-efficient and extremely fast method for information processing and quantum computing. We like to create and detect magnons, measure their dissipation, time-of-flight, and elucidate their role in spin caloritronic effects (see next).
Spin effects are inextricably linked with thermal gradients. It is well known for almost 200 years that temperature differences give rise to electric fields or voltages. This is called the Seebeck effect and has been used for centuries in making thermocouples for temperature measurements. Inside a magnetic material, these temperature gradients can also give rise to “spin voltages”, generating spin currents. This confluence between spintronics and temperature effects has been aptly called spin caloritronics. The role of magnons in these effects is an open question. Our investigations will allow elucidation of the mechanisms underpinning these phenomena.
Terahertz spectroscopy of quantum materials
Quantum materials are the generic name given to a certain class of materials. There is only a broad consensus on the meaning of this term. Broadly these materials harbor some form of emergent property or an exotic form of long-range order appearing either due to strong correlations between electrons and other quasiparticles such as photons, excitons, phonons, and magnons etc. or due to topological properties of the underlying electronic wavefunction in the reciprocal space. Archetypal quantum materials include topological insulators, high-temperature superconductors, graphene, and other honeycombed buckled structures (silicene, phosphorene), Mott insulators, Dirac and Weyl semimetals, etc. This scientific research goal will be the epitaxial growth of such materials using chemical vapor deposition and single crystal, and their time-resolved optical characterization using linear and nonlinear optical spectroscopy. The energy band structure of these quantum materials can be probed using THz spectroscopy. Our initial theoretical results [1,2] provide us with an impetus for conducting experimental investigations.
This work is in collaboration with Dr. Qasim Mehmood from Information Technology University (ITU) and is represented in our publications [2,7,14]. The idea is to create nanostructures and metamaterials using a combination of e-beam and photolithography. These nanostructures demonstrate novel optical properties and features such as flat meta-axicons, dielectric nanowaveguides for optical vortices creation, and dielectric meta-holograms. These structures are first simulated using finite-time-domain techniques. The structures employ low loss materials and are easy to fabricate using scalable techniques. Along with my Phd student, Ali Akbar, we contribute to the patterning of these structures and perform complete optical measurements in our laboratory.
Instrumentation: creating the tools for research and teaching
My research efforts have been sustained by a somewhat invisible effort in creating several important instruments which have enabled our group members to conduct their scientific investigations. This effort is also linked with my efforts to providing new tools and gadgets to our students to enhance their understanding of physical phenomena in the research and teaching laboratories. For example, recently we developed an FPGA based highly linear, time-do-digital converter with an impressive time resolution of ps for our quantum optical experiments as well for detecting lifetimes and speeds of cosmic ray muons. This system is also at the heart of our homegrown MHz NMR rleaxometer. Besides, we have developed an ambient atomic force microscope, MOKE imager, room temperature vibrating sample magnetometer, surface plasmon resonance based sensor, an apparatus for magneto-dielectric measurements, synthesis of nanofibers using electrospinning [featured in references 25,26], a four probe measurement station and numerous smaller gadgets for measuring physical phenomena. Currently, we are engaged in developing a Bridgman type furnace and chemical vapor deposition system for synthesis of quantum materials.
Low field nuclear magnetic resonance
Polarized, miniaturized and mobilized magnetic resonance can revolutionize the applicability of this widespread technique, extending it to developing countries, on-field inspections and testing, ambulatory medical care in disaster-struck areas, and the chemistry lab benchtop or the fume hood. For example, the high polarizations achieved from para-hydrogen enable us to detect trace amounts of chemical species and image their spatio-temporal profile, circumventing the low sensitivity issue plaguing NMR. In addition, the polarized para-hydrogen can be used as a magnetization storage vector for portable, mobile MRI instrumentation. It is known that the storage time can be extended beyond the conventional T1 times by exploiting the symmetry properties of the quantum singlet state. The goals of this project include:
- Building of a compact hyperpolarizer
- Building of a compact NMR system
- Investigation of (multi-step) chemical reactions in the low-field regime; example reactions are polymerization of para-hydrogenated propene, para-hydrogenation of fluorinated compounds, enabling the automatic transfer of polarization to fluorine and subsequent detection of fluorine signal
- Preservation and spatial transfer of spin order in endohedral hydrogen fullerenes.
Presentation: NmR with a small ‘m’ 2014