arXiv subjects

Ritwik Mondal

Publications and source records attributed to Ritwik Mondal.

At least 19 recordsLinked to original sources

arXiv2026

The interplay of crystal-field transitions and exchange spin dynamics in a ferrimagnet

Rare-earth iron garnets offer an ideal platform for exploring the interplay of low-energy excitations and the complex temperature-dependent magnetization dynamics. In these systems, exchange coupling between rare-earth and iron sublattices generates high-frequency collective spin excitations. In addition, the robust spin-orbit coupling of localized 4$f$ electrons triggers the crystal-electric-field (CEF) transitions at THz frequencies. Despite extensive research into the garnet spin dynamics, the interplay between CEF excitations and exchange modes has remained largely unmapped. Using temperature-dependent THz time-domain spectroscopy, we demonstrate a hybridization between the Yb-ion CEF excitation and the Yb-Fe exchange mode in Gd$_{3/2}$Yb$_{1/2}$BiFe$_{5}$O$_{12}$. This coupling is characterized by a significant redistribution of spectral and temporal weights as the material approaches its magnetization compensation temperature. Notably, the Yb-Fe exchange mode exhibits an anomalous redshift upon cooling -- a reversal of the conventional blue shift typically driven by increased exchange coupling. We trace this phenomenon to a modification of Yb-Fe exchange anisotropy, driven by the interplay of the Fe exchange field and Yb CEF excitations. These findings highlight the critical role of CEF-mediated exchange coupling in shaping low-energy spin dynamics, positioning rare-earth garnets as a cornerstone for future THz spintronic technologies.

cond-mat.mtrl-sci↗

arXiv2026

Relativistic theory for coupled orbital and spin angular momentum dynamics in magnetic systems

We develop a complete relativistic theory to describe the dynamics of electronic angular momentum including both spin (S) and orbital (L) contributions in magnetic systems. We start with the relativistic Dirac-Kohn-Sham Hamiltonian under the influence of an electromagnetic field and apply a unitary transformation to formulate the extended Pauli Hamiltonian. Using the transformed semirelativistic Hamiltonian, we derive the angular momentum dynamics for the orbital and spin angular momenta. Thereby, we formulate the coupled dynamics of orbital and spin moments consistent with the relativistic Dirac framework. Considering especially the conservation of the total angular momentum, J = S +L, we show first that J is conserved in the absence of a spin-polarized Kohn-Sham exchange field, but is no longer conserved under the application of an electromagnetic field, e.g., laser pulse, THz field, etc. Second, considering magnetic systems with atomic spin and orbital momenta, we derive the coupled equations of motion of angular momenta dynamics whilst making the atomistic Heisenberg approximation for the exchange interaction. Our results suggest that, under these assumptions, the total angular momentum remains conserved, even with electromagnetic field, but atomic spin and orbital angular momenta individually are not conserved.

cond-mat.other↗

arXiv2026

Unquenched orbital angular momentum as the origin of spin inertia

The recent proposal and observation of spin inertia, and the consequent high-frequency spin nutation mode, have raised key questions for our understanding of magnetization dynamics, especially considering its high relevance for magnetic memories and ultrafast switching. Notwithstanding recent progress, a clear identification of spin inertia's physical origin thereby offering predictive power remains to be accomplished. Here, discussing general principles for identifying this physical origin, we examine unquenched orbital angular momentum (OAM) finding it to be a key candidate, despite its typically small value. Treating OAM and spin within a two-sublattice model, we derive the equivalent single-sublattice framework for magnetization dynamics making appropriate approximations. The latter naturally manifests the spin inertia term and parameter, which are otherwise introduced phenomenologically. The inertia parameter evaluated within our model is found to be in good agreement with its experimentally observed value in cobalt. We further delineate key experimental signatures that could verify or rule out the unquenched OAM as the origin of the observed high-frequency mode, and avoid a spurious optical mode in a two-sublattice ferromagnet from being identified as nutation. Our analysis offers a potential link between the recently emerged fields of orbitronics and spin inertia, thereby motivating investigations at their intersection.

cond-mat.mtrl-sci↗

arXiv2026

Spin Inertia as a Source of Topological Magnons: Chiral Edge States from Coupled Precession and Nutation

Spin inertia has been demonstrated to give rise to high-frequency nutational excitations beyond the conventional low-frequency precessional modes. Here, we demonstrate that the hybridization between precessional and nutational magnons may give rise to topological phenomena in the spin-wave spectrum. This hybridization requires the presence of interactions breaking angular-momentum conservation, such as the pseudodipolar interaction. We show on the example of a honeycomb ferromagnet how topological gaps open between the precessional and nutational bands that host chiral edge states in slab geometries. Our work establishes a theoretical foundation for exploring inertial spin dynamics as a new route to engineer topological phases in magnetic materials.

cond-mat.mtrl-sci↗

arXiv2026

Ultrafast switching of antiferromagnetic order by field-derivative torque

Control of magnetic order in antiferromagnets is a central challenge in the development of next-generation spintronic devices. Here, we propose and analyze magnetization switching driven by the field-derivative torque, a torque that originates from the time-derivative of an applied THz pulse acting on the staggered order parameter. Using atomistic spin simulations, we show that the field-derivative torque couples efficiently to the N\'eel vector, enabling deterministic switching without net spin accumulation. Further, we show that using the circularly polarised THz pulse, the FDT-induced magnetization switching reduces the required THz magnetic field by two-fold. To this end, we compute the switching and non-switching areas as a function of THz pulse width, THz magnetic field, and damping of the antiferromagnetic material. We find that the switching and non-switching areas are completely deterministic in antiferromagnets. Moreover, the switching area increases by about 55% when the FDT is considered.

cond-mat.other↗

arXiv2025

Engineering spin-wave spectrum via the magnetization inertia tensor

Magnetic inertial dynamics has recently been predicted and experimentally demonstrated in two-sublattice ferromagnets such as CoFeB and NiFe permalloy. In this work, we investigate the spin-wave spectrum of such systems by incorporating the complete magnetic inertia tensor. By decomposing the tensor into symmetric and antisymmetric components, we identify isotropic, anisotropic, and chiral contributions to magnetic inertia. Within linear spin-wave theory, we find that the spectrum comprises two precessional and two inertial magnon bands. Remarkably, the upper precessional band intersects the lower inertial band within the Brillouin zone. Both cross-sublattice and chiral components of the inertia tensor act as effective control parameters for tuning these magnonic band structures. Furthermore, we show that the inertial spin-wave spectrum becomes nonreciprocal along propagation directions where the Dzyaloshinskii-Moriya interaction is finite. Strikingly, a similar nonreciprocity can also arise purely from chiral inertia, even in the absence of Dzyaloshinskii-Moriya interaction. Our findings establish magnetic inertia as a new pathway to engineer nonreciprocal magnon transport and ultrafast spintronic functionalities.

cond-mat.mes-hall↗

arXiv2025

Field-derivative torque induced magnetization reversal in ferrimagnetic Gd$_{3/2}$Yb$_{1/2}$BiFe$_5$O$_{12}$

Understanding the mechanism of spin switching in ferrimagnets via the excitation of THz pulses holds promise for future-generation magnetic memory devices. Such spin switching can be accomplished by the Zeeman torque exerted by the THz pulses on the magnetic spins. Theoretical and experimental works have established that the field-derivative of a terahertz pulse also exerts a torque, field derivative torque (FDT). Here, we investigate the role of the FDT in the spin switching in ferrimagnetic Gd$_{3/2}$Yb$_{1/2}$BiFe$_5$O$_{12}$ using a computational approach. Our results foresee that the spin switching in the presence of the FDT requires less THz magnetic fields than the spin switching without the FDT. Without the FDT terms, the spin switching in the considered system requires an extremely high magnetic field. Furthermore, we compute the switching and non-switching contour diagrams to show that the FDT tremendously enhances the possibility of spin switching. These results not only shed light on the significance of the FDT in magnetization switching but also suggest materials where the switching effect is pronounced.

cond-mat.mtrl-sci↗

arXiv2024

Role of material-dependent properties in THz field-derivative-torque-induced nonlinear magnetization dynamics

The traditional Landau-Lifshitz-Gilbert (LLG) equation has often delineated the linear and nonlinear magnetization dynamics, even at ultrashort timescales e.g., femtoseconds. In contrast, several other non-relativistic and relativistic spin torques have been reported as an extension of the LLG spin dynamics. Here, we explore the contribution of the relativistic field-derivative torque (FDT) in the nonlinear THz magnetization dynamics response applied to ferrimagnets with high Gilbert damping and exchange magnon frequency. Our findings suggest that the FDT plays a significant role in magnetization dynamics in both linear and nonlinear regimes, bridging the gap between the traditional LLG spin dynamics and experimental observations. We find that the coherent THz magnon excitation amplitude is enhanced with the field-derivative torque. Furthermore, a phase shift in the magnon oscillation is induced by the FDT term. This phase shift is almost 90 for the antiferromagnet, while it is almost zero for the ferrimagnet under our investigation. Analyzing the dual THz excitation and their FDT, we find that the nonlinear signals can not be distinctly observed without the FDT terms. However, the inclusion of the FDT terms produces distinct nonlinear signals which matches extremely well with the previously reported experimental results.

cond-mat.mtrl-sci↗

arXiv2024

Theory of tensorial magnetic inertia in terahertz spin dynamics

Magnetic inertia has emerged as a possible way to manipulate ferromagnetic spins at a higher frequency e.g., THz. Theoretical treatments so far have considered the magnetic inertia as a scalar quantity. Here, we explore the magnetic inertial dynamics with a magnetic inertia tensor as macroscopic derivations predicted it to be a tensor. First, the inertia tensor has been decomposed into three terms: (a) scalar and isotropic inertia, (b) anisotropic and symmetric inertia tensor, (c) chiral and antisymmetric tensor. Further, we employ linear response theory to the inertial Landau-Lifshitz-Gilbert equation with the inertia tensor and calculate the effect of chiral and anisotropic inertia on ferromagnets, antiferromagnets, and ferrimagnets. It is established that the precession and nutation resonance frequencies decrease with scalar magnetic inertia. Our results suggest that the nutation resonance frequencies further reduce due to inertia tensor. However, the effective damping of the nutation resonance increases with the chiral and antisymmetric part of the inertia tensor. We show that the precession resonances remain unaffected, while the nutation resonances are modified with the chiral magnetic inertia.

cond-mat.mes-hall↗

arXiv2024

Evidence of relativistic field-derivative torque in nonlinear THz response of magnetization dynamics

Understanding the complete light-spin interactions in magnetic systems is the key to manipulating the magnetization using optical means at ultrafast timescales. The selective addressing of spins by terahertz (THz) electromagnetic fields via Zeeman torque is one of the most successful ultrafast means of controlling magnetic excitations. Here we show that this traditional Zeeman torque on the spins is not sufficient, rather an additional relativistic field-derivative torque is essential to realize the observed magnetization dynamics. We accomplish this by exploring the ultrafast nonlinear magnetization dynamics of rare-earth, Bi-doped iron garnet when excited by two co-propagating THz pulses. First, by exciting the sample with an intense THz pulse and probing the magnetization dynamics using magneto-optical Faraday effect, we find the collective exchange resonance mode between rare-earth and transition metal sublattices at 0.48 THz. We further explore the magnetization dynamics via the THz time-domain spectroscopic means. We find that the observed nonlinear trace of the magnetic response cannot be mapped to the magnetization precession induced by the Zeeman torque, while the Zeeman torque supplemented by an additional field-derivative torque follows the experimental evidences. This breakthrough enhances our comprehension of ultra-relativistic effects and paves the way towards novel technologies harnessing light-induced control over magnetic systems.

cond-mat.mtrl-sci↗

arXiv2024

Inertial spin waves in spin spirals

Inertial effects in spin dynamics emerge on picosecond time scales, giving rise to nutational excitations at THz frequencies. Here, we describe a general framework for investigating the precessional and nutational excitations in any type of spin structure within linear spin-wave theory. We consider the particular cases of planar and conical spin spirals in detail. We observe a change in the sign of the curvature of the high-frequency nutational spin-wave band as the spiral period is decreased when passing from the ferromagnetic to the antiferromagnetic limit. We identify conditions for the interaction parameters where the curvature changes sign and asymptotical flat bands are formed.

cond-mat.mes-hall↗

arXiv2023

Inertial effects in ultrafast spin dynamics

The dynamics of magnetic moments consist of a precession around the magnetic field direction and a relaxation towards the field to minimize the energy. While the magnetic moment and the angular momentum are conventionally assumed to be parallel to each other, at ultrafast time scales their directions become separated due to inertial effects. The inertial dynamics give rise to additional high-frequency modes in the excitation spectrum of magnetic materials. Here, we review the recent theoretical and experimental advances in this emerging topic and discuss the open challenges and opportunities in the detection and the potential applications of inertial spin dynamics.

cond-mat.other↗

arXiv2023

Magnetization precession after non-collinear dual optical excitation

We investigate the impact of non-collinear dual optical excitation on the magnetization precession in a permalloy thin film using two ultrashort laser pulses. By analyzing the magnetization dynamics using time-resolved magneto-optical methods, we find that the excitation with two ultrashort optical pulses introduces a long-lasting modification of the electron system seen as a sizable decrease of the precession frequency and significant increase (about 25%) of the decay time. Our results reveal that the observed effect strongly depends on the respective polarization of the two excitation pulses and the time delay between two optical pulses. Our findings indicate the occurrence of a new nonlinear opto-spin effect during photoexcitation with two interfering optical pulses which can potentially be observed in various materials and at different photon wavelengths.

cond-mat.mtrl-sci↗

arXiv2022

Theory of inertial spin dynamics in anisotropic ferromagnets

Recent experimental observation of inertial spin dynamics calls upon holistic reevaluation of the theoretical framework of magnetic resonance in ferromagnets. Here, we derive the secular equation of an inertial spin system in analogy to the ubiquitous Smit-Beljers formalism. We find that the frequency of precessional ferromagnetic resonances is decreased as compared to non-inertial case. We also find that the frequency of nutational resonances is generally increased due to the presence of magnetic anisotropy and applied magnetic field. We obtain exact solutions of the secular equation and approximations that employ the terminology of non-inertial theory and thus allow for convenient estimates of the inertial effects.

cond-mat.mes-hall↗

arXiv2022

Inertial spin waves in ferromagnets and antiferromagnets

Inertial effects in spin dynamics are theoretically predicted to emerge at ultrashort time scales, but their experimental signatures are often ambiguous. Here, we calculate the spin-wave spectrum in ferromagnets and two-sublattice antiferromagnets in the presence of inertial effects. It is shown how precession and nutation spin waves hybridize with each other, leading to the renormalization of the frequencies, the group velocities, the effective gyromagnetic ratios and the effective damping parameters. Possible ways of distinguishing between the signatures of inertial dynamics and similar effects explainable within conventional models are discussed.

cond-mat.mes-hall↗

arXiv2021

Spin pumping at terahertz nutation resonances

We investigate spin pumping current injected by the nutation resonances of a ferromagnet or an antiferromagnet into an adjacent metal. Comparing the dc spin pumping current between the normal precession and nutation resonances, we find that the ratio of spin pumping current at the nutation resonance to the precession resonance is more pronounced in antiferromagnets. We further show that the spin pumping current injected by the nutation resonance is opposite in sign as compared to the normal precession mode. This could offer a useful experimental signature for identifying such nutation resonances. Analyzing the nature of the nutational eigenmodes, we show that the sign change in spin current is rooted in a reversal of the precession sense for the nutation mode(s). Furthermore, the nutational modes in antiferromagnets are found to be dominated by precession of one of the two sublattices only.

cond-mat.mes-hall↗

arXiv2021

Theory of magnetic inertial dynamics in two-sublattice ferromagnets

The magnetic inertial dynamics have been investigated for one sublattice ferromagnets. Here, we develop the magnetization dynamics in two-sublattice ferromagnets including the intra- and inter-sublattice inertial dynamics. First, we derive the magnetic susceptibility of such a ferromagnet. Next, by finding the poles of the susceptibility, we calculate the precession and nutation resonance frequencies. Our results suggest that while the resonance frequencies show decreasing behavior with the increasing intra-sublattice relaxation time, the effect of inter-sublattice inertial dynamics is contrasting.

cond-mat.other↗

arXiv2021

Influence of inter-sublattice coupling on the terahertz nutation spin dynamics in antiferromagnets

Spin nutation resonance has been well-explored in one-sublattice ferromagnets. Here, we investigate the spin nutation in two-sublattice antiferromagnets as well as, for comparison, ferrimagnets with inter-and intra-sublattice nutation coupling. In particular, we derive the susceptibility of the two-sublattice magnetic system in response to an applied external magnetic field. To this end, the antiferromagnetic and ferrimagnetic (sub-THz) precession and THz nutation resonance frequencies are calculated. Our results show that the precession resonance frequencies and effective damping decrease with intra-sublattice nutation coupling, while they increase with inter -sublattice nutation in an antiferromagnet. However, we find that the THz nutation resonance frequencies decrease with both the intra-and inter-sublattice nutation couplings. For ferrimagnets, conversely, we calculate two nutation modes with distinct frequencies, unlike antiferromagnets. The exchange-like precession resonance frequency of ferrimagnets decreases with intra-sublattice nutation coupling and increases with inter-sublattice nutation coupling, like antiferromagnets, but the ferromagnetic-like precession frequency of ferrimagnets is practically invariant to the intra and inter-sublattice nutation couplings.

cond-mat.mtrl-sci↗