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We report on the synthesis and characterization of single phase Sm doped Co-ferrites nanoparticles, CoFe(2−x)SmxO4 with x = {0.0, 0.05, 0.10, 0.15, 0.20}. The samples were characterized by X-ray diffraction, Mossbauer spectroscopy at 12 K and 300 K, transmission electron microscopy and vibrating sample magnetometry. The particle size, saturation magnetization and coercivity field are found to decrease with the increase of Sm3+ concentration. The particles had sizes from 5.6 nm to 9.5 nm. The cation distribution obtained by X-ray diffraction and mossbauer spectroscopy corresponds to a partially inverted ferrite. The magnetocrystalline anisotropy energy was determined from hysteresis curves. Since the samples are composed of superparamagnetic and thermally blocked nanoparticles, we used a combination of Langevin and hyperbolic tangent functions to fit the magnetization curves, the results were in close agreement with the experimental results.

We report a theoretical study of the impact of the dipolar interaction on the magnetic phases of Fe@Py rectangular cylindrical core-shells. Our results show that the Fe@Py core-shell geometrical parameters may be chosen to allow significant changes in magnetic phases of the isolated Fe core and Py shell. For instance, an 18 nm height rectangular (81 nm × 117 nm) and 21 nm thick Py shell inhibits the formation of the intrinsic magnetic vortex of an isolated 18 nm height (33 nm × 33 nm) square Fe core. We show that the geometric parameters of flat Fe@Py core-shell cylinders can be chosen in such a way to control the nucleation of domain walls in the Py shell. Furthermore, it allows fine-tuning the domain wall position and the domain wall width on the Py shell.

We report a theoretical study of the impact of dipolar interactions on the magnetic phases and thermal hysteresis of superparamagnetic Gd nanoparticle clusters. We consider Gd nanoparticles with size ranging from 5.5 to 10 nm, arranged with uniform density in a few-hundred-nanometer-sized ellipsoidal clusters. We show that in densely packed systems the dipolar interaction may grant thermal stabilization of the individual nanoparticle moments at high-temperature values, leading to large thermal loop widths, tunable by external magnetic fields of moderate strength. We have found, for instance, that with an external magnetic field strength of 85 Oe one may reach thermal loop widths ranging from 88 K for 5.5-nm-diam Gd nanoparticle clusters to 660 K for 8-nm-diam Gd nanoparticle clusters.

We report a theoretical investigation of new magnetic phases of thin terbium (Tb) films in the temperature interval between 220 K and 280 K, and under external magnetic field strengths ranging from 50 Oe to 1 kOe. We show that surface effects may produce relevant changes in the helimagnetic phase, such as the nucleation of alternating-helix phases. Changes in the exchange energy balance near the surface, favor the alignment with the external field and the nucleation of the alternating-helix phase.We report a theoretical investigation of new magnetic phases of thin terbium (Tb) films in the temperature interval between 220 K and 280 K, and under external magnetic field strengths ranging from 50 Oe to 1 kOe. We show that surface effects may produce relevant changes in the helimagnetic phase, such as the nucleation of alternating-helix phases. Changes in the exchange energy balance near the surface, favor the alignment with the external field and the nucleation of the alternating-helix phase.

Purpose:To evaluate the effect of hydration with Ringer's solution submitted to magnetic field in experimental model of hepatic ischemia/reperfusion in rats. Methods:Wistar rats weighing 389.58±46.88g were randomly allocated into 2 groups: control group (n = 6) -Hepatic ischemia/reperfusion and hydration with Ringer's solution; and test group (n = 6) -Hepatic ischemia/reperfusion and hydration with Ringer submitted to magnetic field of 20 mT during 2 hours. After anesthesia with ketamine 70 mg/kg and xylazine 7mg/kg i.p., the animals were weighed and operated by aseptic technique. The ischemia of the median and left liver lobes was induced for 30 minutes using a vascular clip. Then, i.v. hydration with 3 ml of Ringer in the control group, and 3 ml of magnetizedRinger’s solution in the test group. After 24 hours, blood sample and fragment of the left liver lobe were collected for the determination of AST, ALT, ALP and albumin levels, and histopathological examination. Results:There was no significant differencein comparison of the rats weight loss, biochemical measurements and histopathological scores between the groups (p>0.05). Conclusion:The hydration with Ringer's solution submitted to magnetic field after hepatic ischemia/reperfusion in rats did not alterliver unction tests neither histopathological scores, comparing with the control group. Effect of hydration with Ringer's solution submitted to magnetic field in an experimental model of hepatic ischemia-reperfusion injury in ratsFreire JM, et alJ Surg Cl Res –Vol. 9(2) 2018:44-5345function tests neither histopathological scores, comparing with the control group.Keywords:Ischemia. Reperfusion. Liver. Magnetics. Fluid therapy.RESUMOObjetivo:Avaliar o efeito da hidratação com solução de Ringer submetido à ação de campo magnético em modelo experimental de isquemia e reperfusão hepática em ratos. Métodos:Ratos Wistar pesando 389,58±46,88g foram distribuídos em 2 grupos: grupo controle (n=6) -Isquemia/reperfusão hepática e hidratação com solução de Ringer; e grupo teste (n=6) -Isquemia/reperfusão hepática e hidratação com Ringer submetido a campo magnético de 20 mT por 2 horas. Após anestesia com cetamina 70 mg/kg de peso e xilazina 7 mg/Kg i.p., os animais foram pesados e operados com técnica asséptica. A isquemia dos lobos mediano e esquerdo do fígado foi realizada por 30 minutos, usando clipe vascular. Em seguida, hidratação i.v.com 3 ml de Ringer no grupo controle, e 3 ml de Ringer submetido a campo magnético no grupo teste. Após 24 horas, sangue total e biópsia do lobo esquerdo do fígado foram colhidos para dosagem de AST, ALT, fosfatase alcalina e albumina, e exame histopatológico respectivamente. Resultados: Não houve diferença significante entre os grupos na comparação da perda de peso, das dosagens bioquímicas e dos escores histopatológicos (p>0,05). Conclusão: A hidratação com solução de Ringer submetida à ação de campo magnético após isquemia/reperfusão hepática em ratos não alterou as provas de função hepática nem os escores histopatológicos, comparando-se com o grupo controle.

Purpose: To evaluate the effect of hydration with simple Ringer's solution submitted to magnetic field in rats under renal ischemia and reperfusion (I/R) model, using biochemical and histopathological evaluation. Methods: Wistar rats were allocated into 2 equal groups of 6 animals each: Renal ischemia/reperfusion group + hydration with magnetic Ringer (mag Ringer). The left kidney underwent ischemia for 30 minutes. The right kidney was not manipulated. Thereafter, i.v. hydration was performed with 3 ml mag Ringer; Group renal I/R + hidration with simple Ringer. The simple Ringer solution was submitted to a magnetic field of 20mT intensity for 2 hours immediately prior to the surgical procedure. After 24 hours, under anesthesia blood was collected for dosing serum urea and creatinine. Histopathological analysis was performed on the left kidney (HE staining). Results: There was no significant difference between the creatinine (p=0.764) and urea (p = 0.926) values ​​between the control and test groups (non-parametric Mann-Whitney test). Comparing the histopathological scores, there was a significant difference between the groups, with higher levels of lesions in the mag Ringer treated group (p=0.025) - Mann-Whitney test. Conclusion: The use of Ringer's solution submitted to magnetic field for intravenous hydration in a renal ischemia/reperfusion model did not alter the renal function and had a negative influence on the histopathological findings on the kidney, when compared to control. Objetivo: Avaliar o efeito da hidratação com solução de Ringer simples submetida à ação de campo magnético, em ratos sob modelo de isquemia e reperfusão (I/R) renal, através de métodos bioquímicos e histopatológicos. Métodos: Ratos Wistar foram divididos por sorteio em 2 grupos iguais de 6 animais cada: Grupo Isquemia/reperfusão renal + hidratação com Ringer tratado com magnético (Ringer mag) – Provocada isquemia no rim esquerdo durante 30 minutos. O rim direito não foi manipulado. Em seguida, realizou-se a hidratação de infusão i.v. com 3 ml de Ringer mag; Grupo I/R renal + reposição volêmica com Ringer simples. A solução de ringer simples foi submetida a campo magnético de intensidade 20mT. A solução foi exposta ao CM durante 2 horas imediatamente antes da realização do procedimento cirúrgico. Após 24 horas, sob anestesia foi colhido sangue para dosagem de Ureia e Creatinina. Análise histopatológica foi realizada no rim esquerdo (coloração HE). Resultados: não houve diferença significativa entre os valores de creatinina (p=0,764) e ureia (p=0,926) entre os grupos controle e tratado, usando o teste não paramétrico de Mann-Whitney. Comparando os escores histopatológicos, houve diferença estatisticamente significativa entres os grupos, observando-se maiores níveis de lesões no grupo tratado p-valor= 0,025 (teste de Mann-Whitney). Conclusão: O uso se solução de Ringer submetida a campo magnético para hidratação intravenosa em modelo de isquemia/reperfusão renal não alterou as provas de função renal e influiu negativamente nos achados histopatológicos do rim, quando comparado com o controle.

We report a theoretical discussion of the impact the composition on the maximum energy product ((BH)max) of core@shell FePt@CoFe2 and FePt@Fe nanocylinders. We have found that the best composition is determined by the competing trends imposed by a strong ferromagnetic core@shell interface exchange energy, and the core@shell dipolar interaction energy. The dipolar interaction has a negative impact on the nanocylinder (BH)max value, for shell thickness above a shell material dependent threshold value. We have also found that Fe is the best shell material owing to its much larger exchange stiffness.We report a theoretical discussion of the impact the composition on the maximum energy product ((BH)max) of core@shell FePt@CoFe2 and FePt@Fe nanocylinders. We have found that the best composition is determined by the competing trends imposed by a strong ferromagnetic core@shell interface exchange energy, and the core@shell dipolar interaction energy. The dipolar interaction has a negative impact on the nanocylinder (BH)max value, for shell thickness above a shell material dependent threshold value. We have also found that Fe is the best shell material owing to its much larger exchange stiffness.

We report a theoretical study of core-shell dipolar interaction effects on the energy product of hard/soft ${\mathrm{SmCo}}_{5}@\mathrm{Fe}$ core-shell spherical nanoparticles. We show that the impact of the core-shell dipolar interaction on the shell reversal process is tunable by the values of the core diameter and shell thickness. Maximum energy product optimization requires small core diameters, and the energy product enhancement may reach more than $370%$ for small core diameters ${\mathrm{SmCo}}_{5}@\mathrm{Fe}$ core-shell nanoparticles.

We report a study of the structural and magnetic properties, as well as of the heat release, of an iron/wustite composite, prepared from iron powder and water by high energy mechanical milling. We identify that the produced sample consists of α-Fe nanoparticles embedded in a wustite matrix and has high stability in time. Moreover, we observe that it presents noticeable features, as exchange bias effect at low temperatures and, when an alternating magnetic field is applied, its temperature reaches ∼46 °C in ∼600 s. Thus, the results and the possibility of tuning the magnetic properties of α-Fe nanoparticles embedded in a wustite matrix, through interface interactions, place this system as a very attractive candidate for biomedical applications such as magnetic hyperthermia agent for cancer therapy.

Purpose: To evaluate the effect of oxacillin bonded to magnetic nanoparticles in local infection model in rat. Methods: Twelve Wistar rats weighing 290±18g were randomly divided into four groups (n=6, each) and all rats had a magnet ring sutured on their right thighs. In the biodistribution group rats 0.1mL of 99mTc-magnetite (0.66 MBq) was injected i.v and after 30 minutes, biodistribution of 99mTc-magnetite was evaluated in right and left thighs. The other groups were inoculated with MRSA in each thigh muscles. Group 1 rats were injected i.v. with magnetite, group 2 with Magnetite + Oxacillin, group 3 with saline twice a day. After 24 hours samples of muscle secretion were harvested for microbiological analysis; muscle, lungs and kidneys for histology. Results: 99mTc-magnetite uptake was three-fold higher in right thigh muscles (with external magnet) than in the left. In magnetite and oxacillin-magnetite groups, bacterial/CFU was significantly lower in thigh muscles than in saline-controls. The inflammatory reaction in muscles and lungs was significantly lower in oxacillin-magnetite group-rats than in other groups (p

The treatment of peptic ulcers induced by H. pylori remains challenging due to the deep mucous layer location of bacteria preventing antimicrobial drug access. The present work aimed to design and evaluate in vitro dual responsive (both pH and magnetic field-sensitive) polymeric magnetic particles loaded with amoxicillin as a smart drug carrier for deep mucous layer penetration and in situ drug release. Magnetite particles were produced by the co-precipitation method and subsequently coated with the Eudragit®S100 and amoxicillin by using the spray-drying technique. The physicochemical characterization of the obtained particles was carried out by optical and scanning electron microscopy, X-ray powder diffraction, Fourier transform infrared spectroscopy, nitrogen adsorption/desorption isotherms, and vibrating sample magnetometry. Additionally, drug release tests and antibacterial activity tests were evaluated in vitro. Microparticles presented 17.2 ± 0.4 μm in size and their final composition was 4.3 ± 1.5% of amoxicillin, 87.0 ± 2.3% of Eudragit, and 9.0 ± 0.3% of magnetite. They were both pH and magnetic field responsive while presenting antimicrobial activity. On one side, magnetic field responsiveness of particles is expected to prompt them to reach bacterium niche in deep mucous layer by means of magnetic forces. On the other side, pH responsiveness is expected to enable drug release in the neutral pH of the deep mucous layer, preventing undesired delivery in the acidic gastric lumen. Smart microparticles were designed presenting both pH and magnetic field responsiveness as well as antimicrobial activity. These may be promising assets for peptic ulcer treatment.

We report the synthesis of Ca2Fe2O5 nanoparticles by high-energy ball milling and thermal annealing from α-Fe2O3 and CaCO3. Magnetization measurements, Mossbauer and X-ray spectra reveal that annealing at high temperatures leads to better quality samples. Our results indicate nanoparticles produced by 10 h high-energy ball milling and thermal annealing for 2 h at 1100 °C achieve improved stoichiometry and the full weak ferromagnetic signal of Ca2Fe2O5. Samples annealed at lower temperatures show departure from stoichiometry, with a higher occupancy of Fe3+ in octahedral sites, and a reduced magnetization. Thermal relaxation for temperatures in the 700–1100 °C range is well represented by a Neel model, assuming a random orientation of the weak ferromagnetic moment of the Ca2Fe2O5 nanoparticles.

We report a theoretical study of the impact of dipolar interactions on the room temperature magnetic phases of superparamagnetic nanoparticles confined in spherical and ellipsoidal clusters. We consider Fe3O4 nanoparticles with size ranging from 9 nm to 12 nm, arranged with uniform density in hundred nanometer-sized clusters. We show that one may have a large enhancement of the initial susceptibility for ellipsoidal clusters of high eccentricity, as required for most biomedical applications. Spherical clusters display a reduction of the initial susceptibility, due to the early nucleation of new magnetic phases. In densely packed systems, the dipolar interaction may lead to thermal stabilization of the individual nanoparticle moments, while keeping the cluster superparamagnetic, with a vanishingly small magnetic moment in the absence of an external field. The theoretical model is used to discuss recent findings on quasi-one-dimensional arrays of superparamagnetic Fe and Co nanoparticles, and on spherical clusters of superparamagnetic Fe3O4 nanoparticles.We report a theoretical study of the impact of dipolar interactions on the room temperature magnetic phases of superparamagnetic nanoparticles confined in spherical and ellipsoidal clusters. We consider Fe3O4 nanoparticles with size ranging from 9 nm to 12 nm, arranged with uniform density in hundred nanometer-sized clusters. We show that one may have a large enhancement of the initial susceptibility for ellipsoidal clusters of high eccentricity, as required for most biomedical applications. Spherical clusters display a reduction of the initial susceptibility, due to the early nucleation of new magnetic phases. In densely packed systems, the dipolar interaction may lead to thermal stabilization of the individual nanoparticle moments, while keeping the cluster superparamagnetic, with a vanishingly small magnetic moment in the absence of an external field. The theoretical model is used to discuss recent findings on quasi-one-dimensional arrays of superparamagnetic Fe and Co nanoparticles, and on spherical c...

Aqueous suspensions containing small magnetic particles have been increasingly used in biosciences and biotechnology. Magnetic particles develop magnetic polarization and magnetophoretic mobility, and because of such unique properties, these carriers may be eligible candidates for delivering drugs to specific sites within the body. Their special properties also allow other uses, such as those in embolization, radioisotope delivery, magnetic cell tracking for monitoring cell therapy, magnetofection, and hyperthermia. This review focuses on a discussion about magnetic particles, the properties and fate of magnetic carriers, the methods used to produce and characterize them, and their other uses in biotechnology.

We report a theoretical investigation of the equilibrium pattern and the spectra of head-to-head and Neel domain walls of flat Fe and Py stripes, exchange coupled with a vicinal antiferromagnetic substrate. We show that the domain wall excitation spectrum is tunable by the strength of the interface field. Furthermore, strong interface coupling favors localized wall excitations. We report a theoretical investigation of the equilibrium pattern and the spectra of head-to-head and Neel domain walls of flat Fe and Py stripes, exchange coupled with a vicinal antiferromagnetic substrate. We show that the domain wall excitation spectrum is tunable by the strength of the interface field. Furthermore, strong interface coupling favors localized wall excitations.

We report a theoretical investigation of new magnetic phases of thin dysprosium films. At the surfaces, the balance between exchange and anisotropy energies favors the alignment of spins along the basal plane easy axis directions. Near the Curie temperature, the helimagnetic phase is strongly modified, allowing surface spins to lock into the basal plane anisotropy easy directions. As the temperature raises, we find transitions to other surface lockin patterns, which are tunable by external field strengths of a few hundred milli-Oersteds.

We report a study to develop a magnetic system for local delivery of amoxicillin. Magnetite microparticles produced by coprecipitation were coated with a solution of amoxicillin and Eudragit ® S100 by spray drying. Scanning electron microscopy, optical microscopy, X-ray powder diffraction and vibrating sample magnetometry revealed that the particles were superparamagnetic, with an average diameter of 17.2 μm, and an initial susceptibility controllable by the magnetite content in the suspension feeding the sprayer. Our results suggest a possible way to treat Helicobacter pylori infections, using an oral drug delivery system, and open prospects to coat magnetic microparticles by spray drying for biomedical applications.

Aqueous suspensions containing magnetic microparticles have been increasingly used in biosciences and biotechnology. This work describes an experimental procedure to produce superparamagnetic microparticles. The particles were prepared based on the coprecipitation of iron salts in alkaline medium. Afterwards, characterization was performed. Characterization data demonstrated that magnetite was the dominant phase in the analyzed sample, and 50% of them were in the size range of 0.5-5 microm. The results suggest that the experimental protocol provided a simple synthesis route to produce superparamagnetic microparticles. Such properties may be very useful for biotechnological purposes.

This work evaluates an experimental set-up to coat superparamagnetic particles in order to protect them from gastric dissolution. First, magnetic particles were produced by coprecipitation of iron salts in alkaline medium. Afterwards, an emulsification/cross-linking reaction was carried out in order to produce magnetic polymeric particles. The sample characterization was performed by X-ray powder diffraction, laser scattering particle size analysis, optical microscopy, thermogravimetric analysis and vibrating sample magnetometry. In vitro dissolution tests at gastric pH were evaluated for both magnetic particles and magnetic polymeric particles. The characterization data have demonstrated the feasibility of the presented method to coat, and protect magnetite particles from gastric dissolution. Such systems may be very promising for oral administration.

Suboptimal disposition behavior of drugs requires innovative delivery approaches. Magnetic drug targeting seems to be a promising one. Magnetic particles develop magnetic polarization and magnetophoretic mobility, and because of such unique properties, these carriers may be eligible candidates for delivering drugs to specific locations within the body. Their special properties also allow other uses, such as those in magnetic separation, hyperthermia, and magnetic resonance imaging. This review focuses on a brief discussion of magnetic drug targeting, the properties and fate of magnetic carriers, the methods used to produce and characterize them, and their other uses in biotechnology.

We show that confinement in small volumes affects the interplay of exchange and dipolar interactions and the magnetic phases of hard and soft spherical core-shell nanoparticles. Large variations in the magnetization of thin shells may occur due to the core dipolar field gradient within the shell. The reversal field is tunable by the trends imposed by the dipolar and core-shell interface exchange energies. We show, for instance, that the reversal field of a ${\mathrm{CoFe}}_{2}{\mathrm{O}}_{4}$(30 nm)@${\mathrm{MnFe}}_{2}{\mathrm{O}}_{4}$(6 nm) particle ranges from 15.5 kOe for antiferromagnetic coupling down to 2.5 kOe for ferromagnetic coupling.

CoFe2O4 nanoparticles were synthesized using gelatin as a polymerizing agent. Structural, morphological and magnetic properties of samples treated at different temperatures were investigated by X-ray diffraction, scanning electron microscopy, Mossbauer spectroscopy and magnetization measurements. Our results revealed that the samples annealed at 623 K and temperatures above 973 K have a cation distributions given by (Co0.19Fe0.81)[Co0.81Fe1.19]O4 and (Co0.06Fe0.94)[Co0.94Fe1.06]O4, respectively. The particle sizes varied from 73 to 296 nm and the magnetocrystalline anisotropy, K1, has values ranging from 2.60x10^6 to 2.71x10^6 J/m3, as determined from the law of approach to saturation applied to the MxH data at high field. At 5 K, the saturation magnetization, coercive field and (BH)max varied from 76 to 95 Am2/kg, 479.9 to 278.5 kA/m and 9.7 to 20.9 kJ/m3, respectively. The reported values are in good agreement with near-stoichiometric cobalt ferrite samples., Comment: To be published in Ceramics International: 10.1016/j.ceramint.2015.05.148

We report a theoretical study of the magnetocaloric effect (MCE) of Dy thin films, and show that the confinement of spins in the direction perpendicular to the surfaces leads to a considerable enhancement of the MCE. The adiabatic temperature change, Δ T , for external field strength change, Δ H , of the order of a few kOe, is much larger than what is found in bulk Dy, reaching Δ T Δ H = 6 K/T for ultra-thin films.

The recent development of superconducting magnets has resulted in a huge increase in human exposure to very large static magnetic fields of up to several teslas (T). Considering the rapid advances in applications and the great increases in the strength of magnetic fields used, especially in magnetic resonance imaging, safety concerns about magnetic field exposure have become a key issue. This paper points out some of these safety concerns and gives an overview of the findings about this theme, focusing mainly on mechanisms of magnetic field interaction with living organisms and the consequent effects.

We calculate the dynamical response of a group of non-interacting magnetic grains dispersed in a medium that imposes on them an uniaxial anisotropy. Numerical results are obtained for a collection of grains that has its volume distribution given by a Gaussian function centered in the average volume V0, and has a width σV0. We assume that the only effect of the medium on the particles is the anisotropy imposed on the grains to study the influence of the volume distribution (σ and V0) on the effective permeability. The results are analyzed through the calculation of numerical values for the components of the magnetic permeability and also by the analysis of the combined effect of the external dc magnetic field and the width of the volume distribution on the skin depth.

We study reorientation of magnetization of a ferromagnetic film grown on a compensated antiferromagnetic substrate and propose a method to determine a lower bound of the interface exchange coupling strength. For uniaxial ferromagnetic films, we show that the frustration induced by the interface exchange leads to a reorientation of the magnetization only if the strength of the interface field is beyond a threshold value. For ferromagnetic films with fourfold crystalline anisotropy the reorientation of the magnetization occurs for any value of the interface exchange field strength. We use a self-consistent two spin mean field model and apply the theoretical model to Fe / FeF 2 and Fe / MnF 2 bilayers. For these systems the reorientation of the magnetization produces minor changes in the antiferromagnetic arrangement of the spins near the interface, allowing a simple analytical formula which relates the threshold value of the interface exchange coupling to the magnetic parameters of the antiferromagnetic substrate and to the anisotropy and thickness of the ferromagnetic film.

We study the nucleation of a domain wall induced by an interface step defect in a bilayer consisting of a four-fold crystalline anisotropy ferromagnetic film exchange coupled with a two-sublattice uniaxial antiferromagnetic (AF) substrate. For thin ferromagnetic (F) films we find a Neel wall, and the surface magnetic profile is a replica of the interface pattern. For thick films the magnetic pattern at the free surface may deviate significantly from the interface pattern. The relaxation is due to the local character of the interface exchange coupling. We find that above a threshold value of the F film thickness the surface is uniformly magnetized in the direction perpendicular to the easy axis of the AF substrate. We discuss how the relaxation affects the hysteresis curves.

We study the field induced instability of the ground state of ferrimagnetic multilayers consisting of a stacking alternating two different uniaxial ferromagnetic layers. For multilayers with even number of layers N, we obtain analytical expressions for the critical fields in terms of the magnetic parameters (anisotropies, and interlayer exchange coupling), for any value of N. The critical fields are calculated from the energy fluctuations for small variations in the equilibrium magnetic profile. The form of the hysteresis curves is discussed, using the expressions of the critical fields.

We study the behavior of surface and bulk polaritons in thin antiferromagnetic films when a dc magnetic field is applied perpendicular to the easy axis. Dispersion relations are obtained for magnetostatic modes, as well as for retarded polaritons. It is shown that, the dispersion relations of localized modes exhibit reciprocity, i.e. Ω( k → )=Ω(− k → ), but they are non equivalent since they are localized in different regions. The non reciprocal character of surface modes in a semi infinite sample is regained for very thick films.

We report a theoretical study of the vortex profile of in-plane magnetized PyTM nano-cylinders subjected to the stray field of perpendicular anisotropy Co nano-cylinders. We consider 6 nm thick PyTM cylinders dipolar coupled to 60 nm thick Co cylinders, at distances from 1.5 nm to 30 nm, with diameters (D) ranging from 45 nm to 105 nm. We find considerable reduction of critical diameter for stable PyTM magnetic vortices and spiral-vortex phases, as well as vortex core diameters twice as large as the bulk value.

We report a theoretical investigation of vortex nucleation along the hysteresis curves of flat iron and permalloy nanoelements exchange coupled to an antiferromagnetic substrate. Contrary to common intuition, the interface exchange field favors vortex nucleation. For lateral dimensions in the sub-100 nm range, vortex nucleation in iron nanoelements require smaller values of the interface field strength. For both materials, strong values of the interface field strength leads to double loop curves, with the magnetization driven by vortex motion in most part of both branches of the hysteresis curve.

We consider a thin rare-earth film with the c axis perpendicular to the surface, to study its nonuniform magnetic structure induced by the lower coordination near the surfaces. The influence of a dc magnetic field, applied perpendicular to the c axis, on the equilibrium configurations is also investigated. The spin-wave frequencies, in the q=0 limit, are calculated as a function of the strength of the external dc field. We use the bulk parameters of dysprosium and holmium to obtain the external field dependence of the equilibrium con- figuration, magnetization, and the lowest spin-wave modes of 30-monolayer-thick films. We consider a thin rare-earth film with the c axis perpendicular to the surface, to study its nonuniform magnetic structure induced by the lower coordination near the surfaces. The influence of a dc magnetic field, applied perpendicular to the c axis, on the equilibrium configurations is also investigated. The spin-wave frequencies, in the q=0 limit, are calculated as a function of the strength of the external dc field. We use the bulk parameters of dysprosium and holmium to obtain the external field dependence of the equilibrium con- figuration, magnetization, and the lowest spin-wave modes of 30-monolayer-thick films.

This paper presents a theoretical study of thermal hysteresis of thin Dy films. Large differences between the isofield magnetization curves during heating or cooling are found. The width of the thermal hysteresis may cover the whole interval between the Curie (TC) and the Neel (TN) temperatures and it is controlled by external field. The thermal hysteresis is controllable by external magnetic fields and thickness, it is modest for ultra thin films and large above a critical film thickness.

We report a theoretical investigation of thermal hysteresis of a pair of interface biased elliptical iron nanoelements, separated by an ultrathin layer of nonmagnetic material. The thermal hysteresis originates in the strong dipolar interaction, and is tunable by the nature of the low temperature state and the eccentricity of the nanoelements. The width of the thermal hysteresis varies from 500 K to 100 K for lateral dimensions of 125 nm × 65 nm and 145 nm × 65 nm.

We report on the preparation of magnetite nanoparticles, with size ranging from 12 nm to 20 nm, by high energy ball milling. The synthesis is made using stoichiometric amounts of distilled water and metallic iron powder. The milled powder samples were analyzed by Mossbauer spectroscopy (MS), X-ray diffraction (XRD) and vibrating sample magnetometry (VSM). Our results indicate that the milling time is a key parameter of the synthesis. By increasing the milling time one achieves high purity magnetite samples. Also, the particle size decreases with the milling time. The sample milled during 10 h contained a fraction of 56 nm metallic Fe particles and 20 nm magnetite particles. By increasing the milling time to 96 h we have obtained a sample that is mainly composed of 12 nm magnetite particles. MS performed at room temperature showed a spectrum consisting of two sextets with hyperfine parameters related to iron ions occupying octahedral (A) and tetrahedral (B) sites. We have used a self-consistent method to investigate the impact of the dipolar interaction to drive the system to a magnetically blocked regime.

We report a theoretical study of the ferromagnetic resonance (FMR) frequency Ω(H) of Fe/FeF2(110) and Fe/MnF2(110) uniaxial anisotropy compensated bilayers. We show that under external field perpendicular to the anisotropy axis, the uniform mode of the Fe-film becomes soft at an external field strength (H*=HAF−Hint⊥) smaller than the Fe anisotropy field. For strong interface exchange coupling, there is a gap in the FMR spectrum. In this case, Ω(H) is a monotonically increasing function of the external field strength and for any value of the external field strength Ω(H)>Ω(0). The value of the effective interface field downshifts (Hint⊥) and the value of Ω(0) may be used to estimate the interface exchange energy.

We present a study of vortex formation in interface biased nano-sized disk and square nanoelements of Fe and PyTM. For small lateral dimensions, the circular nanoelements have smaller vortex core diameter than square nanoelements with equal top surface area. For surface area ranging from 1900 nm2 to 6700 nm2, the vortex core diameter of 30 nm thick Fe (PyTM) nanoelements varies from 32 nm (36 nm) to 36 nm (48 nm). Interface effects are stronger for PyTM nanoelements.

A theoretical study of the phase diagram of antiferromagnetic thin films is presented. The theory is based on a numerical self-consistent local-field calculation that allows for size and surface effects. The properties of magnetic structures as functions of the temperature and external field are calculated. Spatially nonuniform canted states are shown to intermediate the transition from the antiferromagnetic to the spin-flop phases. The model is applied to thin films of FeF2 and MnF2 as examples in order to clarify the role of anisotropy. A theoretical study of the phase diagram of antiferromagnetic thin films is presented. The theory is based on a numerical self-consistent local-field calculation that allows for size and surface effects. The properties of magnetic structures as functions of the temperature and external field are calculated. Spatially nonuniform canted states are shown to intermediate the transition from the antiferromagnetic to the spin-flop phases. The model is applied to thin films of FeF2 and MnF2 as examples in order to clarify the role of anisotropy.

Magnetic polymer particles have been widely used in alarge number of biotechnology and biomedicine applications(1–3). In modern pharmacy, basic investigations for themanufacturing and characterization of new dosage forms arefrequently performed using magneticfillers and drug carriers,which can be classified as the artificial sources of magneticfields. These sources are most typically based on magnetite,iron (III)γ-oxide, and barium ferrite (4)

We report a theoretical study of the impact of finite size on the magnetocaloric effect (MCE) of thin Ho films. For strong external field strengths, the adiabatic temperature change ΔTad is comparable to the value found in bulk Ho, reaching about 12 K for an external field strength change ΔH = 50 kOe. For thicknesses below the helix period, there is a large enhancement. In this thickness range, the helical state does not form, leading to a giant MCE reaching ΔT/ΔH = 6.5 K/T for ΔH = 2 kOe.

Astep defect at the interface between a two } sublattice antiferromagnetic substrate and a uniaxial ferromagnetic thin "lm stabilizes a Ne`el wall. Breathing domain wall modes (BDWM) are studied with the external "eld in the plane of the "lm. The restoring force of the domain wall excitations originates from the interface exchange energy within the domain wall. For low anisotropy ferromagnets there is a signi"cant reduction of the domain wall width and the frequency of the domain wall excitations is determined from the interface exchange coupling in a small interface area. For strong interface coupling, the width of the domain wall excitation spectrum scales with the square root of the ratio between the interface e!ective exchange "eld and the anisotropy "eld of the ferromagnetic "lm. 2001 Elsevier Science B.V. All rights reserved.

We report a theoretical investigation of thermal hysteresis of fourfold anisotropy ferromagnetic FM film exchange coupled to a compensated antiferromagnetic substrate. Thermal hysteresis occurs if the temperature interval includes the reorientation transition temperature, below which the frustration of the interface exchange coupling leads to a 90° rotation of the magnetization of the ferromagnetic layer. The temperature width of the thermal hysteresis is tunable by external magnetic fields of modest magnitude, with values of 43 K for an external field of 110 Oe and of 14 K for a field of 210 Oe, for a Fe 12 nm/MnF2 110 bilayer. For a Fe 3 nm/FeF2 110 bilayer the width of the thermal hysteresis is 23 K at 110 Oe and 13 K at 300 Oe. We discuss how the thickness of the iron film affects the field tuning of the thermal hysteresis width, and also how the thermal loops may be used to identify the nature of the interface exchange energy.

We report a theoretical study of the coercivity and bias of iron dots exchange coupled with an antiferromagnetic substrate. We show that flat dots, with height close to the iron exchange length, and lateral dimensions of a few exchange lengths, exhibit large enhancement of coercivity and exchange bias. For small interface field strength the magnetization reversal is nearly a coherent rotation with symmetrical loops. Interface pinning leads to large reduction in coercivity and asymmetrical loops, if the interface field strength is comparable to the value of the iron exchange field. We discuss the impact of geometrical confinement and interface pinning on the magnetization reversal mechanisms. We show that small area dots with height larger than the exchange length display stronger interface effects.

We report a theoretical investigation of the magnetic states and depinning field of a periodic array of head-to-head domain walls of flat Fe rectangular nanowires, exchange coupled with a vicinal two-sublattice uniaxial antiferromagnetic substrate. We show that for strong interface exchange energy, domain walls are pinned at interface steps perpendicular to the antiferromagnetic easy axis, separating terraces with opposite interface exchange field. The array sequence, which alternates head-to-head and tail-to-tail domain walls, may form a structure with alternate chirality or with the same chirality. The domain wall dipolar field affects the chirality sequence, which is tunable by the geometrical constraints and the strength of the interface exchange field. The depinning field of 10 nm thick, 1 μm long wires, with widths of 100 and 200 nm, is of the order of the interface field strength, and the depinning process involves domain wall motion and the transversal displacement of a periodic array of vortices.

The vibrational spectrum of FCC metal vicinal surfaces is obtained from displacement-displacement Green's functions. Results are presented for the Pt(332) surface. Surface spectral densities are calculated, in the neighbourhood of the surface, for selected directions in the Brillouin zone. The coherent surface vibrations contain features that reproduce the available experimental results and predict others that might be observable by He scattering. The dispersion relation of a high frequency surface mode is calculated. The surface mode frequency is shown to depend weakly on the wave-vector along high symmetry lines in the surface Brillouin zone. The polarization and local character of this mode are closely related. Near the Brillouin zone centre and also near the zone boundary it is strongly polarized perpendicularly to the surface and a sudden polarization change is seen when its frequency approaches the surface projection of the bulk spectrum. The vibrational spectrum of FCC metal vicinal surfaces is obtained from displacement-displacement Green's functions. Results are presented for the Pt(332) surface. Surface spectral densities are calculated, in the neighbourhood of the surface, for selected directions in the Brillouin zone. The coherent surface vibrations contain features that reproduce the available experimental results and predict others that might be observable by He scattering. The dispersion relation of a high frequency surface mode is calculated. The surface mode frequency is shown to depend weakly on the wave-vector along high symmetry lines in the surface Brillouin zone. The polarization and local character of this mode are closely related. Near the Brillouin zone centre and also near the zone boundary it is strongly polarized perpendicularly to the surface and a sudden polarization change is seen when its frequency approaches the surface projection of the bulk spectrum.

We have performed a theoretical investigation of thermal hysteresis in the magnetization of ${\mathrm{Fe}}_{m}∕{\mathrm{Dy}}_{n}∕{\mathrm{Fe}}_{m}$ and ${\mathrm{Gd}}_{m}∕{\mathrm{Dy}}_{n}∕{\mathrm{Gd}}_{m}$ trilayers, where the interface coupling is antiferromagnetic and ferromagnetic, respectively. Our results show a large number of stable states can exist in these structures. The thermal hysteresis occurs because the transitions between these states occur at different temperatures for the heating and cooling processes. The thermal hysteresis can be quite large, spanning about $90\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Furthermore the temperature span of the thermal hysteresis can be substantially reduced by applying a modest external magnetic field.

We investigate the long wavelength soft mode of antiferromagnetic (AF) multilayers near the field induced instability of the AF order. We show that in the vicinity of the surface spin flop field sHSSFd the frequency of the lowest mode is V2 /g2=asHSSF−Hd+bsHSSF−Hd2, where a and b are functions of the magnetic parameters of the multilayer and H is the external field strength. For antiferromagnetic films the first order term is zero (due to the absence of demagnetizing effects) and b=1.

We report a theoretical study of the magnetic heat capacity, cV(T), of exchange coupled compensated ferromagnet/antiferromagnetic bilayers. We show that cV(T) may display a large peak at the temperature TR where the interface magnetic frustration induces a 90° reorientation of the F layer. We study Fe/FeF2 bilayers and show that TR and the magnitude of the cV(T) peak depend on the strength of the interface exchange. We suggest how the sign of the interface exchange energy parameter can be investigated using features produced by external fields.