Magnetic Nanosystems Research Articles

Observation of Distinct Bulk and Surface Chemical Environments in a Topological Insulator under Magnetic Doping

The influence of magnetic dopants on the electronic and chemical environments in topological insulators (TIs) is a key factor when considering possible spintronic applications based on topological surface state properties. Here we provide spectroscopic evidence for the presence of distinct chemical and electronic behavior for surface and bulk magnetic doping of Bi2Te3. The inclusion of Mn in the bulk of Bi2Te3 induces a genuine dilute ferromagnetic state, with reduction of the bulk band gap as the Mn content is increased. Deposition of Fe on the Bi2Te3 surface, on the other hand, favors the formation of iron telluride already at coverages as low as 0.07 monolayer, as a consequence of the reactivity of the Te-rich surface. Our results identify the factors that need to be controlled in the realization of magnetic nanosystems and interfaces based on TIs.

Theranostics of Epitaxially Condensed Colloidal Nanocrystal Clusters, through a Soft Biomineralization Route

Clustering of biocompatible magnetic iron oxide nanocrystallites (MIONs) is a synthetic strategy which improves magnetic manipulation, imaging, and sensing for biomedical applications. In this work we describe the synthesis of condensed clustered MIONs obtained through biomineralization and epitaxial aggregation in the presence of alginate at ambient conditions, mimicking the process that so far has been achieved only by nature, in iron-oxidizing bacteria. These condensed-type magnetic nanostructures exhibit higher magnetophoretic responses compared to other types of magnetic colloids and clustered systems. The soft environmental conditions used for the synthesis of the magnetic nanosystems enables the alginate coating material to retain high drug loading ability for the doxorubicin molecule as well as strong binding proclivity for radionuclides. The strong binding of doxorubicin forms the physical basis to obtain magnetic nanocarriers, where the selective release of the drug occurs only under the action ...

Elastic and inelastic neutron scattering for the study of magnetic thin films and heterostructures

Neutron-based techniques have proved for decades to be essential tools in the investigation of magnetic materials. The strong experience gathered in neutron centers together with high performance instruments have also largely contributed to the researches on magnetic nanosystems. We will present here a few examples of the use of elastic and inelastic neutron scattering for the study of Dysprosium-based thin films and heterostructures. Elastic neutron scattering in rare earth-based nanosystems provides crucial information both on the crystal structure and the modulated magnetic order. This has especially permitted to connect the drastic change in Curie temperature observed in Dy epitaxial films to modifications of both magnetoelastic and exchange contributions. In superlattices, elastic neutron scattering has been successfully used to analyze the magnetic coupling phenomena, in particular the long range coherent propagation of the Dy helical order. The example of Dy/Er superlattices highlights this fascinating coupling mechanism through either a paramagnetic or magnetic spacer. Inelastic Neutron Scattering is the technique of choice to investigate magnetic excitations over the entire Brillouin Zone. The very low inelastic scattering cross section can however make any investigation of nanosystems very difficult and very few studies have indeed been reported so far on nanosystems. We will show that relevant information could be obtained from high quality single crystalline Dy films and superlattices. These promising results are paving the way to new spin wave explorations in a wider range of magnetic nanosystems.

Success of a simulation approach for magnetic nanosystems: Power of physical laws

The new quantum simulation model and the self-consistent algorithm (SCA) for magnetic nanosystems, that we proposed 2years ago, were extended to study the magnetic properties of a nanowire consisting of 3d ions which are coupled ferromagnetically. To test the applicability of the algorithm, our simulations in the present work were started from a magnetic structure in which all spins in the whole nanosample were randomly oriented (defined as the random magnetic configuration for later use) as other authors have been doing with Monte Carlo or micromagnetism method, and such calculated results were all reasonable. Especially, the free energies evaluated at the chosen temperatures were found to attenuate spontaneously and quickly, as the program ran, towards the minima according to the principle of lowest free energy as expected. This suggests that the computational algorithm is able to lead the code to converge rapidly to the equilibrium state automatically without the need to minimize the total (free) energy of the system elaborately that must be done if the Monte Carlo or micromagnetism method is used, demonstrating the great power of natural laws.

Magnetic nanosystem for cancer therapy using oncocalyxone a, an antitomour secondary metabolite isolated from a Brazilian plant.

This paper describes the investigation and development of a novel magnetic drug delivery nanosystem (labeled as MO-20) for cancer therapy. The drug employed was oncocalyxone A (onco A), which was isolated from Auxemma oncocalyx, an endemic Brazilian plant. It has a series of pharmacological properties: antioxidant, cytotoxic, analgesic, anti-inflammatory, antitumor and antiplatelet. Onco A was associated with magnetite nanoparticles in order to obtain magnetic properties. The components of MO-20 were characterized by XRD, FTIR, TGA, TEM and Magnetization curves. The MO-20 presented a size of about 30 nm and globular morphology. In addition, drug releasing experiments were performed, where it was observed the presence of the anomalous transport. The results found in this work showed the potential of onco A for future applications of the MO-20 as a new magnetic drug release nanosystem for cancer treatment.

Investigations on nanoconfinement of low-molecular antineoplastic agents into biocompatible magnetic matrices for drug targeting

Magnetic mesoporous silica nanoparticles are employed as biocompatible matrices to host low-molecular antineoplastic drugs. 5-Fluorouracil is a well-known antimetabolite drug used to treat many malignancies: colon, rectal, breast, head and neck, pancreatic, gastric, esophageal, liver and G-U (bladder, penile, vulva, prostate), skin cancers (basal cell and keratosis). Unfortunately severe gastrointestinal, hematological, neural, cardiac and dermatological toxic effects are often registered due to its cytotoxicity. Thus, this work focuses on development of a magnetic silica nanosystem, capable of hosting high amounts of 5-fluorouracil and delivers it in a targeted manner, under the influence of external magnetic field. There are few reports on nanoconfinement of this particular small molecule antimetabolite on mesoporous silica hosts. Therefore we have investigated different ways to confine high amounts of 5-FU within amino-modified and non-modified mesopores of the silica shell, from water and ethanol, under magnetic stirring and ultrasound irradiation. Also, we have studied the adsorption process from water as a function of pH in order to rationalize drug–support interactions. It is shown that nature of the solvent has great influence on diffusion of small molecules into mesopores, which is slower from alcoholic solutions. More importantly, sonication is proven as an excellent alternative to long adsorption tests, since the time necessary to reach equilibrium is drastically reduced to 1h and higher amounts of drug may be immobilized within the mesopores of amino-modified magnetic silica nanoparticles. These results are highly important for optimization of drug immobilization process in order to attain desired release profile.

Magnetic Nanowires Investigated by Polarized SANS

Polarized SANS results obtained on ferromagnetic Co nanowires embedded in alumina (Al2O3) porous matrices are reported. The triangular arrays of aligned nanowires are investigated as a function of the external magnetic field in order to determine experimentally the real space magnetization M' ('r) distribution inside the material during the magnetic hys- teresis cycle. The observation of field-dependent SANS intensities allows us to characterize the influence of stray fields. Micromagnetic calculations are compared to the SANSPOL experimental data. These results show that SANSPOL is a technique able to address real-space magnetization distributions in magnetic nanosystems.

Nonlinear Ferromagnetic Resonance in Nanostructures Having Discrete Spectrum of Spin-Wave Modes

Nonlinear ferromagnetic resonance (NFMR) at the nanoscale differs from the same phenomenon in bulk magnetic samples due to the nanosize-related discreteness of the spin wave spectrum. Because of the spectrum discreteness, the conservation laws for multiwave parametric interaction processes cannot be fulfilled exactly and become nonresonant. The presence of the nonresonant parametric processes in a strongly driven magnetic nanosystem can be detected experimentally as distortions on the NFMR curve caused by the nonresonant excitation of spin wave modes.

Thermally induced synchronization and stochastic resonance between magnetization regimes in spin-transfer nano-oscillators

The thermally induced synchronization of transitions between different magnetization regimes with weak ac-injected spin-polarized current is considered for a spin-valve-like magnetic nano-system, where magnetization dynamics is described by the Landau-Lifshitz-Slonczewski equation. We apply suitable averaging techniques and derive a non-autonomous stochastic differential equation for the free energy. By using this equation, we demonstrate that synchronization of thermal transitions with weak ac excitation is ascribed to a general form of stochastic resonance. Numerical computations confirm the accuracy of the theory.

Nucleation field, reversal mechanism and coercivity paradox in two-phased magnetic nanosystem

The magnetic reversal mechanism has been determined within a micromagnetic model reliably for a two-phased magnetic nanosystem, with the formulae for nucleation fields derived analytically. It is found that the nucleation field H N decreases uniformly as the size of the soft phase L s increases whereas it increases with the size of the hard phase L h . The analysis shows that whilst the effect of L h could be ignored in most cases, where the nucleation field is dominated by the L s and the calculation could be significantly simplified, the overly simple inverse square law between H N and the soft layer thickness L s adopted by the previous researchers is not a good approximation. While nucleation is the beginning of the magnetic reversal, pinning is the dominant coercivity mechanism in both two-phased and single-phased magnetic materials, where the crystalline defects exist. Comparison with the experimental data confirms this conclusion, indicating that Brown’s paradox results from the much lower effective anisotropy in both single-phased and composite materials, as speculated in the literature.

Single crystal EPR study at 95 GHz of a large Fe based molecular nanomagnet: toward the structuring of magnetic nanoparticle properties

A W-band single-crystal EPR study has been performed on a molecular cluster comprising 19 iron(III) ions bridged by oxo- hydroxide ions, Fe(19), in order to investigate magnetic nanosystems with a behavior in between the one of Magnetic NanoParticles (MNP) and that of Single Molecule Magnets (SMM). The Fe(19) has a disk-like shape: a planar Fe(7) core with a brucite (Mg(OH)(2)) structure enclosed in a "shell" of 12 Fe(III) ions. EPR and magnetic measurements revealed an S = 35/2 ground state with an S = 33/2 excited state lying ∼ 8 K above. The presence of other low-lying excited states was also envisaged. Rhombic Zero Field Splitting (ZFS) tensors were determined, the easy axes lying in the Fe(19) plane for both the multiplets. At particular temperatures and orientations, a partially resolved fine structure could be observed which could not be distinguished in powder spectra, due to orientation disorder. The similarities of the EPR behavior of Fe(19) and MNP, together with the accuracy of single crystal analysis, helped to shed light on spectral features observed in MNP spectra, that is a sharp line at g = 2 and a low intensity transition at g = 4. Moreover, a theoretical analysis has been used to estimate the contribution to the total magnetic anisotropy of core and surface; this latter is crucial in determining the easy axis-type anisotropy, alike that of MNP surface.

A promising strategy for overcoming MDR in tumor by magnetic iron oxide nanoparticles co-loaded with daunorubicin and 5-bromotetrandrin

To overcome both the dose-limiting side effects of conventional chemotherapeutic agents and the therapeutic failure resulting from multidrug resistance (MDR) and minimize adverse effects of chemotherapy agents, a novel chemotherapy formulation of magnetic nanoparticles co-loaded with daunorubicin and 5-bromotetrandrin (DNR/BrTet-MNPs) was developed, and its effect on MDR leukemic cells was explored. After the DNR and Br were co-loaded onto a pluronic-stabilized and oleic acid-modified magnetic nanosystem, the physical characteristic and drug-loading capacity were evaluated. The cell toxicity of the self-prepared DNR/BrTet-MNPs formulation was then determined by MTT (3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide) assay; the cellular uptake of drug was demonstrated by fluorescent microscope. Lastly, the transcription of mdr1 and the expression of P-glycoprotein (P-gp) were detected by the reverse transcription reaction and western blotting assay, respectively. The results showed that the self-prepared DNR/BrTet-MNPs formulation possessed a sustained release of drug and displayed a dose-dependent antiproliferative activity on MDR leukemia K562/A02 cells. It also enhanced the accumulation of intracellular DNR in K562/A02 cells and downregulated the transcription of the mdr1 gene and the expression of P-gp. These findings suggest that the remarkable effect of the novel DNR/BrTet-MNPs formulation, acting as a drug depot system for the sustained release of the loaded DNR and BrTet, on multidrug resistance leukemia K562/A02 cells would be a promising strategy for overcoming MDR.

Reduced exchange coupling and hysteresis loops in two-phased magnetic nanosystem

The reduced exchange coupling has been incorporated in our micromagnetic calculations for the hysteresis loops of Nd 2Fe 14B/α-Fe and Sm 40Fe 60/Ni 80Fe 20 multilayers. Analysis shows that nucleation and pinning fields are sensitive to the value of the interface coupling constant J i when the soft layer thickness L s is small. Hysteresis loops have been obtained for a trilayer system with a soft α-Fe sandwiched between two Nd 2Fe 14B layers for different values of L s and J i . As J i decreases, nucleation field decreases while the pinning field increases. In the meantime, the squareness of the loops is deteriorated, which results in smaller energy product. For thick soft layer the coercivity mechanism transforms from pinning to nucleation as the interface coupling decreases, and vice versa. The above calculations have been extended to a Sm 40Fe 60/Ni 80Fe 20 bilayer and compared with available experimental data. The theoretical loop is consistent with the experimental one when the value of J i is taken as 10% of the bulk one, demonstrating that the interface coupling in the experiment is far away from perfect coupling.

Spin torque dynamics with noise in magnetic nanosystems

We investigate the role of equilibrium and nonequilibrium noise in the magnetization dynamics on mono-domain ferromagnets. Starting from a microscopic model we present a detailed derivation of the spin shot noise correlator. We investigate the ramifications of the nonequilibrium noise on the spin torque dynamics, both in the steady state precessional regime and the spin switching regime. In the latter case we apply a generalized Fokker-Planck approach to spin switching, which models the switching by an Arrhenius law with an effective elevated temperature. We calculate the renormalization of the effective temperature due to spin shot noise and show that the nonequilibrium noise leads to the creation of cold and hot spot with respect to the noise intensity.

Classes of Nanomagnets Created from Alkanethiol‐Coated Pt or Pd Nanoparticles and Their Alloys with Co

AbstractAlkanethiol‐coated metal nanoparticles are fascinating materials for designing unconventional electronic and magnetic functions. Novel classes of magnetic nanosystems can be created using Pd and Pt nanoparticles and their alloys with a 3d transition metal using quantum size, interface and exchange enhancement effects. The charge transfer at the metal–organic interface and the quantum size effect render Pt and Pd nanoparticles magnetic with localised moments. The substitution of capping molecules with a TTF derivative having a long alkanethiol substituent decreases the totalspin concentration due to partial oxidation of the TTF units. Pd nanoparticles containing Co atoms as magnetic centres have significantly enhanced magnetic moments because of the exchange enhancement. Interestingly, in Pd nanoparticles of 2 nm in diameter containing only one Co atom per particle, the Co atom magnetises all of the Pd atoms in the particle. Therefore, the nanoparticles behave as a single nanoparticle magnet that shows blocking behaviour.

Temperature dependence of the magnetic susceptibility of magnetic dispersed nanosystems

The temperature dependence of the complex magnetic susceptibility of different magnetic nano-colloids and finely dispersed magnetite powder is studied. The results are explained with allowance for the magnetic moment relaxation in single-domain particles and phase transitions in a system of interacting dipoles.

Proceedings of the PASPS V Conference Held in August 2008 in Foz do Iguaçu, Brazil

In recent years, spin-related effects have emerged as the key ingredient underlying many fundamental spin-dependent phenomena in nanoscale condensed-matter systems. In this context, the 5th International Conference on Physics and Applications of Spin-related Phenomena in Semiconductors (PASPS V) took place in the city of Foz do Iguacu, Brazil, on August 3–6, 2008. PASPS V followed the successful series of conferences held in Japan (Sendai, 2000, 2006), Germany (Wurzburg, 2002), and The United States (Santa Barbara, 2004). Over the years the PASPS series has stimulated the crossfertilization between the fields of semiconductor and magnetic nanosystems and has become the ideal forum for identifying (new) challenges, open questions and opportunities in spintronics. The fifth edition of PASPS has followed this trend thus focusing on fundamental spin-dependent phenomena arising from spin-related effects in semiconductors and magnetic nanostructures. The scientific program of PASPS V has highlighted the latest theoretical and experimental developments in new semiconductor spintronic materials and devices, as well as advances in metallic magnetoelectronics. Many state-of-the-art developments on relevant themes in the field were presented and extensively discussed during the conference. These include: ferromagnetic III–V semiconductors (control of magnetic properties, anisotropy), di-

Electrodynamic model for low loss microwave nanocomposite magnets using autonomous blocks with Floquet channels

The diagonal and off-diagonal components of the effective permeability tensor and the effective permittivity of three-dimensional (3D) periodic magnetic nanosystems, depending on the size and separation of the magnetic nanoparticles, were calculated using the computed values of the propagation constants and the dispersion relations of clockwise and counterclockwise polarized electromagnetic waves (EMWs), as well as for the ordinary and extraordinary modes in the gyromagnetic medium. The propagation constants of EMWs in a 3D periodic array of 150 nm iron nanospheres in a dielectric matrix, depending on the ratio of the period of the array to the radius of the nanospheres, were obtained at a frequency f=30 GHz for longitudinal and transverse orientations of the bias magnetic field. It is shown that the magnetic and dielectric losses of weakly interacting systems are lower than that of yttrium iron garnet (YIG); however, losses increase due to spin wave (SW) excitations upon reduced separation.

Molecular nanomagnets and magnetic nanoparticles: the EMR contribution to a common approach

The current status and future developments of the use of electron magnetic resonance (EMR) for the investigation of magnetic nano-systems is here reviewed. The aim is to stimulate efforts to provide a unified view of the properties of magnetic nanoparticles (MNP) comprising a few hundred magnetic centres, and molecular nanomagnets which contain up to ca. one hundred magnetic centres (MNM). The size of the systems is becoming the same but the approaches to the interpretation of their properties are still different, being bottom up for the latter and top down for the former. We make the point here of the need for a common viewpoint, highlighting the status of the two fields and giving some hints for the future developments. EMR has been a powerful tool for the investigation of magnetic nano-objects and it can provide a tool of fundamental importance for the development of a unified view.

Cyclodextrin-conjugated nanocarrier for magnetically guided delivery of hydrophobic drugs

A magnetic nanosystem that simultaneously implements the cyclodextrin–drug complexation power, bioadhesive property of gum arabic (GA) and inherent magnetic properties of Fe3O4 nanoparticles, has recently been reported. In this study, a magnetic nanocarrier was fabricated by conjugating 2-hydroxypropyl-cyclodextrin (HCD) onto the gum arabic modified magnetic nanoparticles (GAMNP). The analyses of transmission electron microscopy (TEM) and dynamic light scattering (DLS) revealed that the product had a mean diameter of 14.8 nm and a mean hydrodynamic diameter of 29.3 nm. This nanocarrier showed good loading efficiency for ketoprofen. In addition, the in vitro release profile of ketoprofen from HCD-GAMNP was characterized by an initial fast release followed by a delayed release phase. In view of the better biocompatibility and the combined properties like specific targeting, complexation ability with hydrophobic drugs makes the nanosystem an exciting prospect for drug delivery.