Uniaxial Nanoparticles Research Articles

Superparamagnetic effects in the linear magnetic response of polydisperse ensembles of nanoparticles suspended in a fluid.

Within a kinetic theory, the linear magnetic response of uniaxial single-domain particles suspended in a fluid is analyzed. The main qualitatively different types of frequency dependence of the longitudinal dynamic magnetic susceptibility of such particles are described. It is shown that superparamagnetic (related to orientation thermal fluctuations of the magnetic moment inside a particle) peculiarities of the response of a particle to a probing magnetic field are not fully determined by the ratio of anisotropy energy to thermal energy when a stationary bias field is applied. For a case where the indicated ratio is much greater than one, a simple approximate expression for the dynamic magnetic susceptibility of a particle is proposed. The developed approach is extended to polydisperse suspensions of noninteracting uniaxial nanoparticles. It is shown that polydispersity does not vanish away specific superparamagnetic features in the dynamic magnetic response of such systems. Quantitative estimates of the corresponding effects are performed in different frequency ranges of the applied field. It is demonstrated that under certain restrictions on the disperse composition of a suspension, the internal diffusion of the magnetic moment can lead to a splitting of the absorption spectrum of the system. The significant role of the bias field is revealed. In particular, it can cause an additional absorption maximum provided the particle-size distribution meets the outlined condition. Also, it enables one to assess how important it is to take into account superparamagnetism of particles: the effect of the biasing is stronger for particles with smaller anisotropy and thereby more pronounced superparamagnetic properties. A qualitative agreement of some of the inferences with the experimental data is briefly discussed.

Molecular Alignment‐Induced Chemically Patchy Uniaxial Nanoparticles and Their Applications in Anti‐Counterfeiting and Self‐Assembled Superstructures

AbstractWe report an unusual strategy for synthesizing patchy nanoparticles (NPs) by controlling the orientation of the molecules that form the NPs. This is realized by synchronous polymerization and crystallization of liquid crystal (LC) monomers during scalable precipitation polymerization. The resulting NPs are cylinders with highly uniform shapes and have only a single LC domain. The patchy properties originate from the discrepancy of surface chemical compositions on flat and side surfaces and can be switched on and off by solvent. Extra colloidal blocks can be grown onto the patches, resulting in highly uniform triblock patchy dumbbells, which have integrated optical properties, and as demonstrated, show triple‐mode optical authentication in anti‐counterfeiting labels or patterns. We also demonstrate that the triblock patchy cylinders are attractive building blocks for long LC rods or porous colloidal materials through polymerization‐induced self‐assembly.

Molecular Alignment-Induced Chemically Patchy Uniaxial Nanoparticles and Their Applications in Anti-Counterfeiting and Self-Assembled Superstructures.

We report an unusual strategy for synthesizing patchy nanoparticles (NPs) by controlling the orientation of the molecules that form the NPs. This is realized by synchronous polymerization and crystallization of liquid crystal (LC) monomers during scalable precipitation polymerization. The resulting NPs are cylinders with highly uniform shapes and have only a single LC domain. The patchy properties originate from the discrepancy of surface chemical compositions on flat and side surfaces and can be switched on and off by solvent. Extra colloidal blocks can be grown onto the patches, resulting in highly uniform triblock patchy dumbbells, which have integrated optical properties, and as demonstrated, show triple-mode optical authentication in anti-counterfeiting labels or patterns. We also demonstrate that the triblock patchy cylinders are attractive building blocks for long LC rods or porous colloidal materials through polymerization-induced self-assembly.

Dynamics of magnetization of a uniaxial nanoparticle in the region of noncollinear ferromagnetic resonance

Resonance dynamics of the magnetic moment of a uniaxial ellipsoidal nanoparticle under its magnetic biasing along the symmetry axis and excitation by a transverse high-frequency field is studied with the parameters (frequency, magnetic bias field and shape parameter), corresponding to the noncollinear orientation of equilibrium magnetization and the external static field. We revealed the frequency regions where precession becomes nonlinear at a weak alternating field and dynamic bistability, as well as complex spatial attractors and chaos are implemented. Keywords: ellipsoidal nanoparticle, ferromagnetic resonance, static and high-frequency field, nonlinearity, effective anisotropy, bistability, regular and chaotic precession.

Improved averaging of hysteresis loops from micromagnetic simulations of non-interacting uniaxial nanoparticles

Micromagnetic simulations allow us to understand the magnetization reversal of magnetic systems, but the computational cost scales up with the size and, in the case of bulk-scale systems, it becomes an impossible task to face unless certain assumptions are made (e.g. uniform and fully saturated magnetization or simplified anisotropy). However, those simplifications do not work for more complex systems with domain walls, shape anisotropy or exchange-bias. Macroscopic ensembles of non-interacting Magnetic Nanoparticles (MNP) can be modelled as an average of a set of isolated single-domain nanoparticles where magnetocrystalline anisotropies force the particle moments in a wide range of directions. To reduce computational time in such systems, we propose an optimized method of hysteresis loop averaging that takes advantage of high rotational symmetry of spherical particles and proves convenient for energy landscapes such as that in magnetocrystalline uniaxial systems. This improved method reduces the number of simulations required to generate macroscopic-like non-interacting and randomly oriented ensembles of magnetic nanoparticles (i.e. a dilute powder), as compared to the usual mean arithmetic averaging of hysteresis loops. To verify the good agreement of the averaging method we have compared our results with the well-known Stoner-Wohlfarth hysteresis loop, thus matching magnetic properties such as coercivity, remanence and energetic product with a relatively low count of simulations.

Dynamics of Pulsed Remagnetization of Magnetically Uniaxial Nanoparticles

The response of the magnetic moment of a magnetically uniaxial nanoparticle and a planar array of such nanoparticles to a short Gaussian magnetic field pulse is studied in the presence and absence of its modulation. The periodic dependence of the response duration and the final orientation of magnetic moments on the pulse duration and its peak value are revealed and analyzed. The effect of the weak bias magnetic field and the pulse field deviation from the transverse orientation on remagnetization processes is studied. It was shown that the effect of the dipole–dipole interaction leads to modulation of the response to the pulse exposure.

Uniform and nonuniform precession of a nanoparticle with finite anisotropy in a liquid: Opportunities and limitations for magnetic fluid hyperthermia

We focus on an in-depth study of the forced dynamics of a ferromagnetic single-domain uniaxial nanoparticle placed in a viscous fluid and driven by an external rotating magnetic field. The process of conversion of magnetic and mechanical energies into heat is a physical basis for magnetic fluid hyperthermia that is very promising for cancer treatment. The dynamical approximation allows us to establish the limits of the heating rate and understand the logic of selection of the system parameters to optimize the therapy. Based on the developed analytical and numerical tools, we analyze from a single viewpoint the synchronous and asynchronous rotation of the nanoparticle or/and its magnetization in the following three cases. For the beginning, we actualize the features of the internal magnetic dynamics, when the nanoparticle body is supposed to be fixed. Then, we study the rotation of the whole nanoparticle, when its magnetization is supposed to be locked to the crystal lattice. And, finally, we realize the analysis of the coupled motion, when the internal magnetic dynamics is performed in the rotated nanoparticle body. In all these cases, we describe analytically the uniform mode, or synchronous rotation along with an external field, while the nonuniform mode, or asynchronous rotation, is investigated numerically.

Rapidly driven nanoparticles: Mean first-passage times and relaxation of the magnetic moment

We present an analytical method of calculating the mean first-passage times (MFPTs) for the magnetic moment of a uniaxial nanoparticle which is driven by a rapidly rotating, circularly polarized magnetic field and interacts with a heat bath. The method is based on the solution of the equation for the MFPT derived from the two-dimensional backward Fokker-Planck equation in the rotating frame. We solve these equations in the high-frequency limit and perform precise, numerical simulations which verify the analytical findings. The results are used for the description of the rates of escape from the metastable domains which in turn determine the magnetic relaxation dynamics. A main finding is that the presence of a rotating field can cause a drastic decrease of the relaxation time and a strong magnetization of the nanoparticle system. The resulting stationary magnetization along the direction of the easy axis is compared with the mean magnetization following from the stationary solution of the Fokker-Planck equation.