Dynamics and Manipulation of Nanomagnets

This thesis presents my work on the spin dynamics of nanomagnets and investigates the possibility of manipulating nanomagnets by various means. Most of the work has been published\cite{LC-PRB2010, LC-PRB2012, LC-PRB2013, LC-EPL2014}. Some has been submitted for publication\cite{LC-arxiv2014}. The structure of this thesis is as follows. In Chapter 1, I present the theory of manipulation of a nanomagnet by rotating ac fields whose frequency is time dependent. Theory has been developed that maps the problem onto Landau-Zener problem. For the linear frequency sweep the switching phase diagrams are obtained on the amplitude of the ac field and the frequency sweep rate. Switching conditions have been obtained numerically and analytically. For the nonlinear frequency sweep, the optimal time dependence of the frequency is obtained analytically with account of damping that gives the fastest controllable switching of the magnetization. In Chapter 2, interaction between a nanomagnet and a Josephson junction has been studied. The I-V curve of the Josephson junction in the proximity of a nanomagnet shows Shapiro-like steps due to the ac field generated by the precessing magnetic moment. Possibility of switching of the magnetic moment by a time-linear voltage in the Josephson junction is demonstrated. Realization of the optimal switching is suggested that employs two perpendicular Josephson junctions with time-dependent voltage signals. The result is shown to be robust against voltage noises. Quantum-mechanical coupling between the nanomagnet considered as a two-level system and a Josephson junction has been studied and quantum oscillations of the populations of the spin states have been computed. In Chapter 3, the switching dynamics of a nanomagnet embedded in a torsional oscillator that serves as a conducting wire for a spin current has been investigated. Generalized Slonczewski\u27s equation is derived. The coupling of the nanomagnet, the torsional oscillator and the spin current generates a number of interesting phenomena. The mechanically-assisted magnetization switching is studied, in which the magnetization can be reversed by tilting the torsional oscillator. The effect of the torsional oscillator on the switching of the magnetization in the presence of spin-polarized current is computed. Combined effects of the spin current and a mechanical kick of the torsional oscillator have been studied. In Chapter 4, skyrmion dynamics and interaction of the skyrmion with an electron have been studied. Corrections to the spin texture of the skyrmion due to the crystal lattice have been computed. Due to the lattice effects the skyrmion collapses in clean ferromagnetic and anti-ferromagnetic materials. The lifetime of the skyrmion has been computed numerically and compared with analytical theory. In doped anti-ferromagnetic materials the weak attraction between a skyrmion and an electron may generate a bound state. In Chapter 5, experimental results of the NIST group on magnetic multilayer microcantilevers have been analyzed. Theoretical framework has been suggested that explains the observed strong damping effect of the platinum layer on the mechanical oscillations of Py-Pt bilayer cantilevers. The strong spin-orbit coupling of platinum is shown to impede the motion of the domain wall in permalloy and to dramatically increase the damping of the cantilever motion

Reversal of magnetization of a single-domain magnetic particle by the ac field of time-dependent frequency

We report numerical and analytical studies of the reversal of the magnetic moment of a single-domain magnetic particle by a circularly polarized ac field of time-dependent frequency. For the time-linear frequency sweep, the phase diagrams are computed that illustrate the dependence of the reversal on the frequency sweep rate v, the amplitude of the ac field h, the magnetic anisotropy field d, and the damping parameter alpha. It is shown that the most efficient magnetization reversal requires a non-linear time dependence of the frequency, omega(t), for which an exact analytical formula is derived with account of damping. The necessary condition of the reversal is h > alpha d. Implementation of a small-scale magnetization reversal is proposed in which a nanomagnet is electromagnetically coupled to two weak superconducting links controlled by the voltage. Dynamics of such a system is analyzed with account of the back effect of the magnet on the superconducting links.Comment: 11 pages, 16 figures, 0 table

The effect of refined nursing combined with targeted psychological care on postoperative pulmonary function and self-management ability in male patients with lung cancer

This study aimed to investigate the effects of refined nursing combined with targeted psychological care on postoperative pulmonary function and self-management ability in male patients diagnosed with lung cancer. The clinical data of 80 patients who underwent lung cancer surgery at our institution between January 2022 and January 2024 were retrospectively retrieved, and they were stratified into two groups based on intervention methods: an experimental group (received refined nursing in conjunction with targeted psychological care, n = 40 cases) and a control group (received refined nursing; n = 40 cases). After these interventions, the effects on patients’ postoperative pulmonary function and self-management ability were compared. The results showed that the experimental group had a significantly shorter hospitalization duration, higher Forced Expiratory Volume in one second (FEV1), Forced Vital Capacity (FVC), FEV1/FVC ratio and Maximum Voluntary Ventilation (MVV), and higher scores in self-care concept, sense of self-responsibility, self-care ability, and health knowledge level compared to the control group (p < 0.05). Additionally, the experimental group had significantly lower scores on the Self-Rating Anxiety Scale (SAS) and Self-Rating Depression Scale (SDS) (p < 0.001) and a lower incidence rate of complications (p < 0.05). In conclusion, the combined approach of refined nursing and targeted psychological care showed promising outcomes in improving postoperative pulmonary function, mitigating negative emotions, reducing complication rates, and enhancing self-management ability among male patients with lung cancer, thereby warranting further exploration for potential clinical implementation

Interaction of a Nanomagnet with a Weak Superconducting Link

We study electromagnetic interaction of a nanomagnet with a weak superconducting link. Equations that govern coupled dynamics of the two systems are derived and investigated numerically. We show that the presence of a small magnet in the proximity of a weak link may be detected through Shapiro-like steps caused by the precession of the magnetic moment. Despite very weak magnetic field generated by the weak link, a time-dependent bias voltage applied to the link can initiate a non-linear dynamics of the nanomagnet that leads to the reversal of its magnetic moment. We also consider quantum problem in which a nanomagnet interacting with a weak link is treated as a two-state spin system due to quantum tunneling between spin-up and spin-down states.Comment: 7 pages, 4 figure

Dimethyl acridine-based self-assembled monolayer as a hole transport layer for highly efficient inverted perovskite solar cells

Self-assembled monolayers (SAMs) have recently emerged as excellent hole transport materials in inverted perovskite solar cells (PSCs) owing to their ability to minimize parasitic absorption, regulate energy level alignment, and passivate perovskite defects. Herein, we design and synthesize a novel dimethyl acridine-based SAM, [2-(9,10-dihydro-9,9-dimethylacridine-10-yl)ethyl]phosphonic acid (2PADmA), and employ it as a hole-transporting layer in inverted PSCs. Experimental results show that the 2PADmA SAM can modulate perovskite crystallization, facilitate carrier transport, passivate perovskite defects, and reduce nonradiative recombination. Consequently, the 2PADmA-based device achieves an enhanced power conversion efficiency (PCE) of 24.01% and an improved fill factor (FF) of 83.92% compared to the commonly reported [2-(9H-carbazol-9-yl)ethyl] phosphonic acid (2PACz)-based control device with a PCE of 22.32% and FF of 78.42%, while both devices exhibit comparable open-circuit voltage and short-circuit current density. In addition, 2PADmA-based devices exhibit outstanding dark storage and thermal stabilities, retaining approximately ~98% and 87% of their initial PCEs after 1080 h of dark storage and 400 h of heating at 85 °C, respectively, both considerably superior to the control device

Magneto-mechanical investigation of spin dynamics in magnetic multilayers

The Einstein-de Haas effect is used to study experimentally the interfacial spin transport in a bilayer metallic system. Specifically, mechanical torque on a permalloy film interfaced with a non-magnetic metallic film (platinum or copper), deposited on a microcantilever, is measured. The torque is generated by the transfer of the spin angular momentum from the permalloy film to the mechanical angular momentum of the cantilever. Measurement of the cantilever deflection shows that the presence of a thin non-magnetic metallic layer with strong spin-orbit interaction (platinum) changes the interfacial spin transport and causes a dramatic reduction of the mechanical torque. The observed behavior of the cantilever is attributed to the increased effective damping of the domain wall motion in the permalloy layer

Pneumocystis jirovecii with high probability detected in bronchoalveolar lavage fluid of chemotherapy-related interstitial pneumonia in patients with lymphoma using metagenomic next-generation sequencing technology

Abstract Background Previous studies achieved low microbial detection rates in lymphoma patients with interstitial pneumonia (IP) after chemotherapy. However, the metagenomic next-generation sequencing (mNGS) is a comprehensive approach that is expected to improve the pathogen identification rate. Thus far, reports on the use of mNGS in lymphoma patients with chemotherapy-related IP remain scarce. In this study, we summarized the microbial detection outcomes of lymphoma patients with chemotherapy-related IP through mNGS testing of bronchoalveolar lavage fluid (BALF). Methods Fifteen lymphoma patients with chemotherapy-related IP were tested for traditional laboratory microbiology, along with the mNGS of BALF. Then, the results of mNGS and traditional laboratory microbiology were compared. Results Of the 15 enrolled patients, 11 received rituximab and 8 were administered doxorubicin hydrochloride liposome. The overall microbial yield was 93.3% (14/15) for mNGS versus 13.3% (2/15) for traditional culture methods (P ≤ 0.05). The most frequently detected pathogens were Pneumocystis jirovecii (12/15, 80%), Cytomegalovirus (4/15, 26.7%), and Epstein-Barr virus (3/15, 20%). Mixed infections were detected in 10 cases. Five patients recovered after the treatment with antibiotics alone without glucocorticoids. Conclusion Our findings obtained through mNGS testing of BALF suggested a high microbial detection rate in lymphoma patients with IP after chemotherapy. Notably, there was an especially high detection rate of Pneumocystis jirovecii. The application of mNGS in patients with chemotherapy-related IP was more sensitive