Research Interest Themes

The behavior of an electron in many-electron system is utterly different from single electron behavior. Emergent phenomena in strongly correlated materials occur due to strong electron-electron repulsion. This strong coulomb repulsion leads to exotic states of matter in materials like cuprates, iridates, etc. Understanding these phenomena is one of the key and important issues in the field of Condensed Matter physics. The rich variety of emergent behavior in these materials is the outcome of the interplay between Coulomb repulsion, spin, orbital, charge degree of freedom, and chemistry of ingredients. The highly non-linear physics of strongly interacting many-body system makes them extremely sensitive to external small perturbations like doping, pressure, strain, magnetic and electric fields.

Magnetic anisotropic interactions and magnetic anisotropy energy are very useful tools in many industrial and technical fields, ranging from information processing, power distribution and generation, and communication devices, to information storage devices. Magnetic anisotropy determines the magnetization stability of a material, whereas the anisotropy energy describes the magnetization tendency of a material along a particular crystallographic direction.

During his Ph.D., Nauman worked with Professor Younjung Jo at Kyungpook National University Daegu South Korea where he studied the electronic transport and magnetic anisotropy in strontium iridate (Sr₂IrO₄) [1]. The exchange interactions between electron spins are completely isotropic in strontium iridate (Sr₂IrO₄), one of the materials belonging to the family of strongly correlated system. Orbital magnetization through spin-orbit interactions links the spin magnetization and the atomic structure and induces the magnetic anisotropy in the material. He performed in-plane magnetic anisotropy measurements of Sr₂IrO₄ single crystals in the low-magnetic field regions. The nature of the alignment of the magnetic moments and the spin-flop transitions under the applied magnetic fields were studied using torque magnetometry. The nature of the arrangement of the magnetic moments were determined by performing angle-dependent torque measurements τ(θ) in the basal plane. This provided a method to study the contribution of in-plane anisotropy to the total magnetic anisotropy of the system.

Magneto-transport and strain-dependent magneto-transport properties remained under his study where he developed a strain device to engineer the strain in single crystal Sr₂IrO₄ and investigated the effect of uniaxial and biaxial strain on the electronic structure and electronic transport of material.

He is also looking for magnetic anisotropy, anisotropic magnetoresistance, magneto-transport, and strain-dependent magneto-transport in other strongly interacting many-body systems.

1. M. Nauman, Y. Hong, T. Hussain, M. Seo, S. Park, N. Lee, Y. Choi, W. Kang, and Y. Jo, "In-plane magnetic anisotropy in strontium iridate Sr₂IrO₄," Physical Review B, vol. 96, p. 155102, 2017.

DOI: 10.1103/PhysRevB.96.155102

Magnetic anisotropy also leads to anisotropic transport in many systems that play a crucial role in device applications such as spintronics and magnonics etc. AFM materials have several advantages over ferromagnetic magnetic materials such as the absence of stray fields, resilience to external magnetic fields, and intrinsically fast dynamics of magnetic moments. The effect of the macroscopic ordering of isospins on transport was measured by applying a magnetic field along different crystallographic axes in Sr₂IrO₄ single crystal [1]. The strong relationship between the magnetoresistance and magnetization data indicates that isospin ordering controls magnetotransport in the system whereas a large and highly anisotropic magnetoresistance (AMR) is obtained by manipulating the AFM isospin domains. First principle calculations performed by collaborators at Yonsei University suggest that electrons whose isospin directions are strongly coupled to the in-plane net magnetic moment encounter an isospin mismatch when moving across the AFM domain boundaries, which generates a high resistance state. By rotating a magnetic field that aligns in-plane net moments and removes domain boundaries, the macroscopically ordered isospins govern dynamic transport through the system, which leads to the extremely angle-sensitive AMR. The work provided a bridge between isospins and magnetotransport in strongly spin–orbit-coupled AFM Sr₂IrO₄, the peculiar AMR effect provides a beneficial foundation for fundamental and applied research on AFM spintronics.

Nauman has a deep interest in exploring new avenues in the field of experimental condensed matter physics. He is not confined to any particular experimental technique and interested to devise his own experimental tools that best fit his research requirements.

1. N. Lee, E. Ko, H. Y. Choi, Y. J. Hong, M. Nauman, W. Kang, H. J. Choi, Y. J. Choi, and Y. Jo, "Antiferromagnet‐Based Spintronic Functionality by Controlling Isospin Domains in a Layered Perovskite Iridate” Advanced Materials, p1805564, 2018.

DOI: 10.1002/adma.201805564

Magnetic Van der Waals (vdW) is an attractive and new class of materials that show two-dimensional attributes both in chemical and magnetic structure. These compounds are stacked in layers pattern and layers are feebly connected to each other perpendicular to the basal plane through van der Waal bonds. The entry of new magnetic vdW material with the name of transition metal phosphorus tri-chalcogenide (TMPX₃) remains ‘talk of the town’ in 2-D magnetism. This material hosts many transition metal (TM) elements like Fe, Co, Mn, and Ni at the transition metal (TM) site. Almost all the TMPX₃ elements show antiferromagnetic (AFM) ordering of one type or the other type with a transition temperature in the range of 80-150K.

Nauman studied magnetic properties and magnetic anisotropy for both in-plane and out-of-plane directions using magnetic properties measurement system (MPMS) and torque magnetic measurements on FePS₃ single crystals [1]. The properties were studied at high magnetic fields (14T and above) and cryogenic temperatures (4K and below).

Magnetic anisotropy in magnetic van der Waals (vdW) TMPS₃ (TM: Fe, Co, Ni), Fe₃GeTe₂ (FGT), and CrI₃ were studied using SQUID and torque magnetometry to unveil and explore the dynamics of magnetic moments under different temperatures and dissimilar magnetic fields.

Apart from TMPS₃, Dr. Nauman is working on other quasi-2D vdW materials including XI₃ (X: V, Cr), FGT, CrGT, etc.

1. Muhammad Nauman^†, Do Hoon Kiem^†, Sungmin Lee, Suhan Son, Je-Geun Park, Woun Kang, Myung Joon Han^*, and Younjung Jo^*, "Complete mapping of magnetic anisotropy for prototype Ising van der Waals FePS₃" Nanoscale Horizons (Submitted)

The multipole description is the sum of terms with progressively finer angular moments that starts with monopole and dipole moments, and continues with higher-order moments. Higher-order multipoles (quadrupoles, octupoles, dodecapoles, etc.) exist and are often neglected in favor of the monopole or dipole description, often for the sake of simplicity or to make calculations more manageable.

The experimental investigation of higher-order multipoles is becoming increasingly more important due to their anticipated role in a variety of materials, such as quantum spin liquids, high-temperature superconductors, and quantum critical metals. The complete understanding of higher-order multipoles is still lacking, mainly due to a large dipolar contribution in traditional magnetic materials. The goal of my research in multipoles is to advance a new technique, named resonant torsion magnetometry, to explicitly detect higher-order multipoles in the Pr-based cage compounds PrT₂Al₂₀ (T= V, Ti). They are a special class of materials for observing multipole physics due to their absence of dipole moments. These compounds become superconducting below T_SC ~ 0.5K and several claims of quadrupolar and octupolar ordering above T_SC have been made, but without definitive proof. The novelty of our approach comes from pairing the perfect material class with a technique that is highly sensitive to moments with a fine angular structure. By exploiting our repurposed technique in other strongly-correlated systems and gaining an overall understanding of the role of higher-order multipoles, our efforts could lead to the resolution of several longstanding mysteries in condensed matter, such as determining the origin of the pseudogap in the superconducting cuprates.

QSL is qualitatively a new class of materials having long-range entanglement but no long-range order. This can be visualized as the liquid of disordered spins and thus known as QSL. These are different from the well-known quantum paramagnet as the latter have short-range entanglement. Theoretical studies predicted that this class of material may host the quasi-particles, such as Spinons and Majorana Fermions, with fractional quantum numbers or quantum statistics which normally happens in long-range entangled phases with topological order (TO). Beyond TO, these may have gapless excitations that may not have a quasi-particle description.

Understanding and investigating this material may lead us to Topological quantum computing and high-temperature superconductivity.