Tunable magnetoben-optic modulation based on magnetically responsive nanostructured magnetic fluid

Magnetic fluid is a kind of functional composite material with nanosized structure and unique optical properties. The tunable magneto-optic modulation of magnetic fluid under external magnetic field, achieved by adjusting the polarization direction of incident light, is investigated theoretically and experimentally in this work. The corresponding modulation depth and response time are obtained. The accompanying mechanisms are clarified by using the theory of dichroism of magnetic fluid and the aggregation/disintegration processes of magnetic particles within magnetic fluid when the external magnetic field turns on/off.     

Novel optical devices based on the tunable refractive index of magnetic fluid and their characteristics

As a new type of functional material, magnetic fluid (MF) is a stable colloid of magnetic nanoparticles, dressed with surfactant and dispersed in the carrier liquid uniformly. The MF has many unique optical properties, and the most important one is its tunable refractive index property. This paper summarizes the properties of the MF refractive index and the related optical devices. The refractive index can be easily controlled by external magnetic field, temperature, and so on. But the tunable refractive index of MF has a relaxation effect. As a result, the response time is more than milliseconds and the MF is only suitable for low speed environment. Compared with the traditional optical devices, the magnetic fluid based optical devices have the tuning ability. Compared with the tunable optical devices (the electro-optic devices (LiNbO₃) of more than 10 GHz modulation speed, acoustic-optic devices (Ge) of more than 20 MHz modulation speed), the speed of the magnetic fluid based optical devices is low. Now there are many applications of magnetic fluid based on the refractive index in the field of optical information communication and sensing technology, such as tunable beam splitter, optical-fiber modulator, tunable optical gratings, tunable optical filter, optical logic device, tunable interferometer, and electromagnetic sensor. With the development of the research and application of magnetic fluid,a new method, structure and material to improve the response time can be found, which will play an important role in the fields of optical information communication and sensing technology.     

The modulation of coupling in the relaxation behavior of light transmitted through binary ferrofluids

With an external magnetic field, a relaxation process is observed when the light transmit through binary ferrofluids composed of ferrimagnetic CoFe₂O₄ and paramagnetic p-NiFe₂O₄ nanoparticles similar to ferrofluids consisting only of CoFe₂O₄. Since only the ferrimagnetic nanoparticles are able to form field-induced chainlike structures for such binary ferrofluids by magnetic interaction between the particles, so the relaxation behavior of the transmitted light is caused mainly by the ferrimagnetic system. In the binary ferrofluids, the paramagnetic nanoparticles, regarded as magnetically polarized gas molecules, are restrained to occupy the space between the ferrimagnetic chains and distribute following the CoFe₂O₄ particle chains covering and diverging, producing a modulation effect on the relaxation behavior of the transmitted light. The modulation effect can be characterized by range and time parameters that describe the relationship of the relaxation behavior of the transmitted light to the properties of the binary ferrofluids and single CoFe₂O₄ ferrofluids.     

Polarization in D-shaped fiber modulated by magneto-optical dichroism of magnetic fluid

The polarization of a D-shaped fiber is modulated after immersing it in magnetic fluid(MF)and applying a magnetic field.Theoretical analysis predicts that magneto-optical dichroism of MF plays a key role in light polarization modulation.During light polarization modulation,the evanescent wave polarized parallel to the magnetic field has greater loss than its orthogonal component.Light polarization of a D-shaped fiber with a wide polished surface can be modulated easily.High concentration MF and a large magnetic field all have great ability to modulate light polarization.     

Influence of switchable magnetic field on the modulation property of nanostructured magnetic fluids

Magnetic fluid is a kind of colloidal material with tunable microstructure and unique optical properties. The tunable magneto-optical modulation property of magnetic fluid under externally switchable magnetic field with various modulation periods is investigated theoretically and experimentally. The transitional modulation period (lower limit of the working frequency) between the square-like and oscillation-like modulation is achieved and found to be magnetic-field- and sample-concentration-dependent. The modulation mechanism is analyzed and ascribed to the dynamic microstructure of magnetic fluid under different modulation periods of external magnetic fields. The result of this work may be helpful for the pragmatic applications of magnetic fluid based on the square-like modulation.     

Optical fiber modulator derivates from hollow optical fiber with suspended core

A fiber optic integrated modulation-depth-tunable modulator based on a type of hollow optical fiber with suspended core is proposed and investigated. We synthesized magnetic fluid containing superparamagnetic Fe(3)O(4) nanoparticles and encapsulated it in the hollow optical fiber as the cladding layer of the suspended core by fusing the hollow optical fiber with the multimode optical fibers. The light with a wavelength of 632.8 nm is coupled in and out of the modulating element by a tapering technique. Experimental results show that the light attenuation in the system can be greatly influenced by only 2.0×10(-2) μL of the magnetic fluid under different magnetic field strengths. The saturated modulation depth is 43% when the magnetic field strength is 489 Oe. The response time of the system is <120 ms. Significantly, this work presents information for the development of all-fiber modulators, including other integrated electro-optic devices, such as optical switch, optical fiber filter, and magnetic sensors utilizing the special structure of this hollow optical fiber with suspended core and superparamagnetic magnetic fluid.     

Longitudinal field-induced polarized light transmittance of magnetic fluids

The complete optical transmittance for a polarized light passing through the magnetic fluids is investigated theoretically and experimentally, when the externally magnetic field is applied along the propagation direction of the incident light. Hybrid effects due to the geometric shadowing and Faraday rotation are considered simultaneously. The Langevin-like functions are employed to describe the magnetic-field-dependent volume concentration of the particle-aggregation (φ′) and the approximate number of magnetic nanoparticles in the particle-aggregation (βN₀). Based on the experiments on the geometric shadowing effect of our magnetic fluid sample, the analytical expression for the total transmitted power with externally magnetic field after an analyzer is derived. Theoretical simulations disclose the influence of certain critical parameters of the magnetic fluids on the field-dependent optical transmittance. For the entire polarized light transmittance, qualitative agreement between the calculations and the experiments is achieved. Applications of magnetic fluids to several polarized devices operating in longitudinal field arrangement are proposed and discussed. The results presented in this work may be useful for designing the corresponding magnetic-fluid-based optical devices.     

Light scattering by aggregates of magnetite nanoparticles in a magnetic field

The effect of variation of the scattered light intensity in a magnetic fluid with aggregates of magnetite particles several microns in size under the action of a magnetic field is studied. The effect relaxation times are determined when the magnetic field is turned on and off. This effect is found to be caused by the orientation of anisotropically magnetized aggregates. Experimental data are used to calculate the average anisotropy of the magnetic susceptibility of the aggregate and estimate its magnetic permeability.     

Fiber magnetic-field sensor based on nanoparticle magnetic fluid and Fresnel reflection

A simple fiber sensor for magnetic field measurement based on nanoparticle Fe(3)O(4) magnetic fluid and relative Fresnel reflection is presented. The sensor includes only a light source, three couplers, two photodetectors, and two fiber sensing ends. Magnetic fields at different concentrations of magnetic fluid are measured. Magnetic fluid with high concentration can be used for the measurement of weak magnetic fields, while low concentration fluid is used for the measurement of strong magnetic fields. The temperature dependence of the sensor is also addressed.     

Relaxation behavior measuring of transmitted light through ferrofluids film

In this paper, relaxation behavior of transmitted light through thin ferrofluid film under an applied magnetic field is measured. The results show that the intensity of transmitted light through a ferrofluid film increases quickly as soon as an external magnetic field is applied then weakens with time. If uniformity of the field is poor, the transmission of light continuously decreases in a measured duration. Otherwise, the transmission of light will tend increasingly towards a stable value after it decreases to a minimum value while the gradient of the field is low. The relaxation time would increase to an order of some hundreds seconds magnitude and is dependent on the strength of magnetic field and viscosity of the ferrofluids. The field-induced relaxation behaviors of transmitted light through ferrofluids correspond to anisotropic microstructure of the ferrofluids under applied magnetic field. PACS 75.50.Mm; 78.20.Ls     

Low frequency magnetic response in antiferromagnetically coupled Fe/Cr multilayers

The magnetic field and temperature dependence of the low frequency magnetic response of antiferromagnetically coupled Fe/Cr(100) multilayers has been studied between +/-500 Oe, from 2 to 300 K. At T = 2 K the losses exhibit an unusually strong frequency dependence which can be described within a single relaxation time scheme. This relaxation time proves to be strongly field dependent. These phenomena are specific for epitaxial multilayers with large magnetoresistance. The behavior of the relaxation time at low temperatures might be related to some quantum tunneling processes.     

The structuralization phenomena in magnetic fluid composites and their influence on transmissivity of light

A magnetic field induced agglomeration of magnetic and μm-sized copper non-magnetic particles in magnetic fluid with several volume concentrations of magnetite and copper particles was studied by means of optical microscope equipped with video camera. Transmission of light through two crossed polarizers and thin magnetic fluid and magnetic fluid composites film as a function of magnetic field was investigated. The experimental data showed that the presence of copper particles influences the aggregation processes of magnetic particles in magnetic fluids and transmissivity of light. Dedicated to Dr. Anton Zentko on the occasion of his 60th birthday. This work was supported by the Slovak Academy of Sciences within the framework of Project GAV No. 4001.     

NdFeB永磁体下YBaCuO块材在交流磁扰动中的悬浮力磁滞特性

研究了交流磁扰动对高温超导块材和永磁体之间悬浮力的影响。实验分析了在零场冷(ZFC)条件下,不同频率的交流磁场扰动下的悬浮力曲线;另外还研究了交流磁扰动在永磁体下降和上升过程中对悬浮力的影响,发现上升过程中悬浮力受交流磁场影响较大,而在下降过程中影响则较为不明显,同时随着交流磁场幅值的增大,悬浮力的滞回曲线逐渐加宽。说明交流磁场导致了超导块磁滞损耗的增加,对超导磁浮特性的研究具有理论意义。     

Spectral energy transfer and dissipation of magnetic energy from fluid to kinetic scales

We investigate the magnetic energy transfer from the fluid to kinetic scales and dissipation processes using three-dimensional fully kinetic particle-in-cell plasma simulations. The nonlinear evolution of a sheet pinch is studied where we show that it exhibits both fluid scale global relaxation and kinetic scale collisionless reconnection at multiple resonant surfaces. The interactions among collisionless tearing modes destroy the original flux surfaces and produce stochastic fields, along with generating sheets and filaments of intensified currents. In addition, the magnetic energy is transferred from the original shear length scale both to the large scales due to the global relaxation and to the smaller, kinetic scales for dissipation. The dissipation is dominated by the thermal or pressure effect in the generalized Ohm's law, and electrons are preferentially accelerated.     

A magnetic field-dependent modulation effect tends to stabilize light transmission through binary ferrofluids

In binary ferrofluids composed of ferromagnetic γ?Fe₂O₃/Ni₂O₃ composite nanoparticles (A particles) and noncrystalline Fe₂O₃ nanoparticles (B particles), the A particles alone will form chain-like aggregates upon application of a magnetic field. Due to both the long-range ‘magnetic convergent force’ (F_C) and the short-range ‘magnetic divergent force’ (F_D), the A-particle chains immersed in the B-particle ‘sea’ will move in a manner similar to the process of vibrational damping. The apparent damping of the ferrofluids will vary from weak to overdamping according to the motion of the chains, so that the intensity of light transmitted through a ferrofluid film along the direction of the field would tend to stabilize after a period of rapid decrements and increments. In binary ferrofluids, the B-particle system can produce a modulation effect on both the damping and the driving force, further stabilizing the behavior of the transmitted light. At low fields (e.g., 500 Gs, 900 Gs) only the modulation of the viscosity drag force (F_v) is considerable, so that overdamping increases linearly with B-particle volume fraction (Ф_B), and the variation in the transmitted light is much slower during the process tending towards stability as Ф_B increases. However, at high fields (e.g., 1300 Gs) the polarization of the B-particle ‘sea’ is enhanced, so that F_D is modulated as well as F_v (i.e., both the practical damping and driving forces are modulated simultaneously). Thus, the apparent overdamping of the binary ferrofluids system will vary non-linearly as Ф_B increases, and the transmitted light will tend to stabilize faster for ferrofluids with high Φ_B than for those with low Ф_B at an applied magnetic field of 1300 Gs.     

Competition between domain walls and the reverse magnetization in the magnetic relaxation of a Pt/Co/Ir/Co/Pt spin switcher

A multilayer Pt/Co/Ir/Co/Pt/GaAs heterostructures demonstrates a long term (to several hours) magnetic relaxation between two stable states of the magnetization of the system. The magnetization reversal of the heterostructure layers occurs both due to the formation of nuclei of the reverse magnetization domains and as a result of their further growth by means of motion of domain walls. The competition between two these processes provides a nonexponential character of the magnetic relaxation. At 300 K, the contributions of these processes to the relaxation are commensurable, while, at temperatures lower than 200 K, the contribution of the nucleation is suppressed and the magnetic relaxation occurs as a result of motion of the domain walls.     

Study on magnetic fluid optical fiber devices for optical logic operations by characteristics of superparamagnetic nanoparticles and magnetic fluids

We propose two optical fiber-based schemes using two magnetic fluid optical fiber modulators in series or in parallel for optical logic signal processing and operation. Here, each magnetic fluid optical fiber modulator consists of a bare multimode fiber surrounded by magnetic fluid in which the refractive index is adjustable by applying external magnetic fields amplifying the input electrical signal to vary the transmission intensity of the optical fiber-based scheme. The physical mechanisms for the performances of the magnetic fluid optical fiber devices, such as the transmission loss related to Boolean number of the logic operation as well as the dynamic response, are studied by the characteristics of superparamagnetic nanoparticles and magnetic fluids. For example, in the dynamic response composed of the retarding and response sub-procedures except the response times of the actuation coil, the theoretical evaluation of the retarding time variation with cladding magnetic fluids length has good agreement with the experimental results.     

Magneto-optically induced retardation and relaxation study in a mixed system of magnetic fluid and cationic micelles

Stable colloidal systems composed of a double surfactant water-based magnetic fluid mixed with a cationic isotropic micellar solution of cetyletrymethyleammonium bromide (CTABr) are prepared for different concentrations of magnetic fluid. The structural integrity between nanomagnetic particles of magnetic fluids and soft micelles is investigated using the magneto-optical techniques like magnetic birefringence, transmission and relaxation measurements. Magneto-optically induced retardation and relaxation measurement indicates the existence of magnetically induced interactions in this mixed system. Similar behaviour is also observed in small angle neutron scattering experiments.     

Rotational Brownian dynamics simulations of non-interacting magnetized ellipsoidal particles in d.c. and a.c. magnetic fields

The rotational Brownian motion of magnetized tri-axial ellipsoidal particles (orthotropic particles) suspended in a Newtonian fluid, in the dilute suspension limit, under applied d.c. and a.c. magnetic fields was studied using rotational Brownian dynamics simulations. The algorithm describing the change in the suspension magnetization was obtained from the stochastic angular momentum equation using the fluctuation-dissipation theorem and a quaternion formulation of orientation space. Simulation results are in agreement with the Langevin function for equilibrium magnetization and with single-exponential relaxation from equilibrium at small fields using Perrin's effective relaxation time. Dynamic susceptibilities for ellipsoidal particles of different aspect ratios were obtained from the response to oscillating magnetic fields of different frequencies and described by Debye's model for the complex susceptibility using Perrin's effective relaxation time. Simulations at high equilibrium and probe fields indicate that Perrin's effective relaxation time continues to describe relaxation from equilibrium and response to oscillating fields even beyond the small field limit.     

Coordinated chain motion resulting in intensity variation of light transmitted through ferrofluid film

While a suitable magnetic field is applied to a ferrofluids film, magnetic nanoparticles in the film would form chain-like structure. Because of the action of magnetic convergent force (MCF) and magnetic divergent force (MDF), the chains will move coordinately towards to the axis of the field, then do apart from the center. From geometric shadowing effect, variation in the intensity of light transmitted through ferrofluids film is in relation to the coordinate motion of the chains. And a radiate synchromotion of the chain groups is constructed equivalently for describing the relation between transmitted light's intensity varying and chains moving. From the motion equation of one chain group, the relation is illustrated qualitatively by computer simulation. The experimental results show that the field-induced variation of light transmitted through ferrofluids film is a nonlinear relaxation process with intrinsic noise, and are in agreement with the behavior simulated by using the model of coordinated chains motion (MCCM).