Browsing by Author "Saha, Renata"
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Item Magnetic field detection using spin-torque nano-oscillator combined with magnetic flux concentrator(2023) Tonini, Denis; Wu, Kai (TTU); Saha, Renata; Wang, Jian PingSpin-torque nano-oscillators (STNO) are studied in terms of the Landau-Lifshitz-Gilbert (LLG) equation. The effect on the limit of detectivity of an STNO concerning externally applied magnetic fields is studied with micromagnetic models by placing adjacent magnetic flux concentrators (MFCs) at different distances from the nanopillar to analyze the effect on the induced auto-oscillations and magnetization dynamics. Perpendicular STNO structures allow for different detectivities with respect to externally applied magnetic fields depending on the distance from the MFCs to the nanopillar. The optimal design of an STNO combined with MFCs is proposed to improve the limit of detectivity, where the STNO consists of two out-of-plane (OP) ferromagnetic (FM) layers separated by a MgO insulating nonmagnetic (NM) thin film, and the MFCs positioned in the vicinity of the STNO are made of permalloy. The time evolution of the free-layer magnetization is governed by the Landau-Lifshitz-Gilbert (LLG) equation. The auto-oscillations induced within the free-layer averaged magnetization are provoked by externally applied magnetic fields. In addition, the DC current-driven auto-oscillations in the STNO structure are studied as a function of the externally applied magnetic field strength, with and without MFCs. The suppression of the DC current-driven auto-oscillations is observed due to the damping effect generated by the MFCs positioned at varying distances with respect to the STNO. By placing MFCs adjacent to the STNO, the lowest detectable magnetic field strength is enhanced from 10 (μT) to 10 (nT). Therefore, it is concluded that MFCs improve the sensitivity of STNO to externally applied magnetic fields thanks to the damped magnetization dynamics. The results presented in this work could inspire the optimal design of STNO and MFC-based ultra-low magnetic field sensors based on nanoscale oscillators and spintronic diodes.Item Nanomaterial-Based Biosensors for SARS-CoV-2 and Future Epidemics(2023) Yari, Parsa (TTU); Liang, Shuang; Chugh, Vinit Kumar; Razaei, Bahareh (TTU); Mostufa, Shahriar (TTU); Krishna, Venkatramana Divana; Saha, Renata; Cheeran, Maxim C. J. (TTU); Wang, Jian-Ping (TTU); Gómez-Pastora, Jenifer (TTU); Wu, Kai (TTU)The outbreak of the coronavirus disease 2019 (COVID-19) pandemic has put enormous pressure on global healthcare and economic systems. The cause of this pandemic, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), is a positive-sense single-stranded RNA (+ssRNA) virus belonging to the beta family of coronaviruses. This highly contagious virus mainly spread through droplet transmission and contact transmission. People who are infected can release droplets and aerosol particles that contain the SARS-CoV-2 virus in the air when they exhale, cough, or sneeze. The droplets or aerosol particles of different sizes (from visible to microscopic) will continue to spread in the air or land on subjects. Healthy people may catch the virus via inhalation of aerosol particles from the air, or through the contact with contaminated surfaces before touching their nose, eyes, or mouth. Besides the direct droplet and contact transmissions, other viral transmission routes are also reported, such as spatter (e.g., blood spatter, spatter during intubation, etc.), fecal-eye transmission, nasal-eye transmission, mouth-eye transmission (through contaminated hands or objects) and the transmission of eye secretions and tears. In addition to its strong transmission power, SARS-CoV-2 can cause acute respiratory distress syndrome (ARDS), septic shock, coagulation dysfunction, intestinal dysfunction, and other clinical symptoms post-infection. Many people infected with COVID-19 display light symptoms like the common cold and influenza, and in some cases they are asymptomatic. Nevertheless, these asymptomatic carriers can still transmit SARS-CoV- 2, making the prevention of COVID-19 infection a major challenge in the world. In view of this, implementing fast, low-cost, accurate, easy-to-access, and integrated diagnostic devices available at the point of care (POC) is paramount to contain not only COVID-19, but also to cope with future epidemics.Item Static and dynamic magnetization models of magnetic nanoparticles: an appraisal(2023) Yari, Parsa (TTU); Chugh, Vinit Kumar; Saha, Renata; Tonini, Denis; Rezaei, Bahareh (TTU); Mostufa, Shahriar (TTU); Xu, Kanglin (TTU); Wang, Jian-Ping; Wu, Kai (TTU)Nowadays, magnetic nanoparticles (MNPs) have been extensively used in biomedical fields such as labels for magnetic biosensors, contrast agents in magnetic imaging, carriers for drug/gene delivery, and heating sources for hyperthermia, among others. They are also utilized in various industries, including data and energy storage and heterogeneous catalysis. Each application exploits one or more physicochemical properties of MNPs, including magnetic moments, magnetophoretic forces, nonlinear dynamic magnetic responses, magnetic hysteresis loops, and others. It is generally accepted that the static and dynamic magnetizations of MNPs can vary due to factors such as material composition, crystal structure, defects, size, shape of the MNP, as well as external conditions like the applied magnetic fields, temperature, carrier fluid, and inter-particle interactions (i.e., MNP concentrations). A subtle change in any of these factors leads to different magnetization responses. In order to optimize the MNP design and external conditions for the best performance in different applications, researchers have been striving to model the macroscopic properties of individual MNPs and MNP ensembles. In this review, we summarize several popular mathematical models that have been used to describe, explain, and predict the static and dynamic magnetization responses of MNPs. These models encompass both individual MNPs and MNP ensembles and include the Stoner-Wohlfarth model, Langevin model, zero/non-zero field Brownian and Néel relaxation models, Debye model, empirical Brownian and Néel relaxation models under AC fields, the Landau–Lifshitz–Gilbert (LLG) equation, and the stochastic Langevin equation for coupled Brownian and Néel relaxations, as well as the Fokker–Planck equations for coupled/decoupled Brownian and Néel relaxations. In addition, we provide our peers with the advantages, disadvantages, as well as suitable conditions for each model introduced in this review. The shrinking size of magnetic materials brings about a significant surface spin canting effect, resulting in higher anisotropy and lower magnetization in MNPs compared to bulk materials. Accurate prediction of static and dynamic magnetizations in MNPs Requires both precise data on their magnetic properties and an accurate mathematical model. Hence, we introduced the spin canting effect and models to estimate anisotropy and saturation magnetization in MNPs.Item Static and Dynamic Magnetization Responses of Self-Assembled Magnetic Nanoparticle Chains(2023) Chugh, Vinit Kumar; Liang, Shuang; Tonini, Denis; Saha, Renata; Liu, Jinming; Yari, Parsa (TTU); Krishna, Venkatramana D.; Cheeran, Maxim C-J; Wu, Kai (TTU); Wang, Jian-PingThe dynamic magnetization responses of magnetic nanoparticles (MNPs) subjected to alternating magnetic fields have been exploited for many biomedical applications, such as hyperthermia therapy, magnetic biosensing, and imaging. This dynamic process is governed by the combined Brownian and Néel relaxations via various energy terms. Both extrinsic factors, such as external alternating fields, dipolar fields, and the properties of the MNP medium, and intrinsic factors, such as the shape, size, and the magnetic properties of the MNPs, can affect their dynamic magnetization responses. However, due to the complex energy terms and interparticle interactions involved, it can be challenging to characterize how each factor influences the dynamic magnetization responses. In this study, we systematically examined the static and dynamic magnetization responses of an ensemble of MNPs. By solidifying the MNP suspension under a fixation field, the immobilized MNPs form long chains, and their easy axes are artificially tuned. In this simplified model, factors such as relative orientations of MNPs’ easy axes to the external field and the dipolar interactions of MNPs are studied. Using a magnetic particle spectroscopy (MPS) platform, the time domain dynamic magnetization responses, dynamic hysteresis loops, high harmonics (which are of interest for MPS and magnetic particle imaging applications), and phase lag of MNPs’ magnetizations to external fields were recorded. A strong correlation between the phase lag of MNPs and the nonlinearity in AC magnetization loops was established.