Modifying the magnetic response of magnetotactic bacteria: incorporation of Gd and Tb ions into the magnetosome structure.

Magnetotactic bacteria Magnetospirillum gryphiswaldense MSR-1 biosynthesise chains of cube-octahedral magnetosomes, which are 40 nm magnetite high quality (Fe3O4) nanoparticles. The magnetic properties of these crystalline magnetite nanoparticles, which can be modified by the addition of other elements into the magnetosome structure (doping), are of prime interest in a plethora of applications, those related to cancer therapy being some of the most promising ones. Although previous studies have focused on transition metal elements, rare earth (RE) elements are very interesting as doping agents, both from a fundamental point of view (e.g. significant differences in ionic sizes) and for the potential applications, especially in biomedicine (e.g. magnetic resonance imaging and luminescence). In this work, we have investigated the impact of Gd and Tb on the magnetic properties of magnetosomes by using different complementary techniques. X-ray diffraction, transmission electron microscopy, and X-ray absorption near edge spectroscopy analyses have revealed that a small amount of RE ions, ∼3-4%, incorporate into the Fe3O4 structure as Gd3+ and Tb3+ ions. The experimental magnetic characterisation has shown a clear Verwey transition for the RE-doped bacteria, located at T ∼ 100 K, which is slightly below the one corresponding to the undoped ones (106 K). However, we report a decrease in the coercivity and remanence of the RE-doped bacteria. Simulations based on the Stoner-Wohlfarth model have allowed us to associate these changes in the magnetic response with a reduction of the magnetocrystalline (K C) and, especially, the uniaxial (K uni) anisotropies below the Verwey transition. In this way, K uni reaches a value of 23 and 26 kJ m-3 for the Gd- and Tb-doped bacteria, respectively, whilst a value of 37 kJ m-3 is obtained for the undoped bacteria.

Correction: Modifying the magnetic response of magnetotactic bacteria: incorporation of Gd and Tb ions into the magnetosome structure.

[This corrects the article DOI: 10.1039/D2NA00094F.].

Influence of W doping on the structure, magnetism and exchange bias in Ni47Mn40Sn13-xWxHeusler alloys.

The influence of the W-doping on the martensitic transformation, magnetic properties and exchange bias (EB) effect in the Ni47Mn40Sn13-xWx(x= 0, 0.5, 1, 1.25 at.%) magnetic shape memory alloys has been investigated. It is found that the W-doping causes a simultaneous reduction of both the ferromagnetic (FM) exchange coupling and enhancement of the magnetic anisotropy, leading to a decrease of the magnetic moment of the low-temperature phase and to a higher attainable EB. The magnetic memory measurements reveal the presence of a glassy magnetic ground state, which can significantly impact the reduction of magnetization and enhancement of EB in the studied bulk alloys. It is argued that the glassy magnetic ground state originates from the partial magnetic disorder resulting from the correlation between the antiferromagnetic and FM states. The results demonstrate that the doping by W instead of Sn is an efficient tool to tailor the EB effect in the Ni-Mn-Sn-based Heusler alloys, whereby they are promising for spintronic applications.

Modelling of magnetoimpedance response of thin film sensitive element in the presence of ferrogel: Next step toward development of biosensor for in-tissue embedded magnetic nanoparticles detection.

In-tissue embedded magnetic nanoparticle (MNPs) detection is one of the most interesting cases for cancer research. In order to understand the origin, the limits and the way of improvement of magnetic biosensor sensitivity for the detection of 3D mezoscopic distributions of MNPs, we have developed a magnetoimpedance biosensor prototype with a [Cu (3 nm)/FeNi(100 nm)]5/Cu(500 nm)/[FeNi(100 nm)/Cu(3 nm)]5 rectangular sensitive element. Magnetoimpedance (MI) responses were measured with and without polyacrylamide ferrogel layer mimicking natural tissue in order to evaluate stray fields of embedded MNPs of γ-Fe2O3 iron oxide. A model for MI response based on a solution of Maxwell equations with Landau-Lifshitz equation was developed in order to understand the origin of the prototype sensitivity which reached 1.3% of ΔZ/Z per 1% of MNPs concentration by weight. To make this promising technique useful for magnetically labeled tissue detection, a synthesis of composite gels with MNPs agglomerates compactly located inside pure gel and their MI testing are still necessary.

Configuration of the magnetosome chain: a natural magnetic nanoarchitecture.

Magnetospirillum gryphiswaldense is a microorganism with the ability to biomineralize magnetite nanoparticles, called magnetosomes, and arrange them into a chain that behaves like a magnetic compass. Rather than straight lines, magnetosome chains are slightly bent, as evidenced by electron cryotomography. Our experimental and theoretical results suggest that due to the competition between the magnetocrystalline and shape anisotropies, the effective magnetic moment of individual magnetosomes is tilted out of the [111] crystallographic easy axis of magnetite. This tilt does not affect the direction of the chain net magnetic moment, which remains along the [111] axis, but explains the arrangement of magnetosomes in helical-like shaped chains. Indeed, we demonstrate that the chain shape can be reproduced by considering an interplay between the magnetic dipolar interactions between magnetosomes, ruled by the orientation of the magnetosome magnetic moment, and a lipid/protein-based mechanism, modeled as an elastic recovery force exerted on the magnetosomes.

Magnetization reversal in circular vortex dots of small radius.

We present a detailed study of the magnetic behavior of Permalloy (Ni80Fe20 alloy) circular nanodots with small radii (30 nm and 70 nm) and different thicknesses (30 nm or 50 nm). Despite the small size of the dots, the measured hysteresis loops manifestly display the features of classical vortex behavior with zero remanence and lobes at high magnetic fields. This is remarkable because the size of the magnetic vortex core is comparable to the dot diameter, as revealed by magnetic force microscopy and micromagnetic simulations. The dot ground states are close to the border of the vortex stability and, depending on the dot size, the magnetization distribution combines attributes of the typical vortex, single domain states or even presents features resembling magnetic skyrmions. An analytical model of the dot magnetization reversal, accounting for the large vortex core size, is developed to explain the observed behavior, providing a rather good agreement with the experimental results. The study extends the understanding of magnetic nanodots beyond the classical vortex concept (where the vortex core spins have a negligible influence on the magnetic behavior) and can therefore be useful for improving emerging spintronic applications, such as spin-torque nano-oscillators. It also delimits the feasibility of producing a well-defined vortex configuration in sub-100 nm dots, enabling the intracellular magneto-mechanical actuation for biomedical applications.

Influence of the bacterial growth phase on the magnetic properties of magnetosomes synthesized by Magnetospirillum gryphiswaldense.

BACKGROUND: The magnetosome biosynthesis is a genetically controlled process but the physical properties of the magnetosomes can be slightly tuned by modifying the bacterial growth conditions. METHODS: We designed two time-resolved experiments in which iron-starved bacteria at the mid-logarithmic phase are transferred to Fe-supplemented medium to induce the magnetosomes biogenesis along the exponential growth or at the stationary phase. We used flow cytometry to determine the cell concentration, transmission electron microscopy to image the magnetosomes, DC and AC magnetometry methods for the magnetic characterization, and X-ray absorption spectroscopy to analyze the magnetosome structure. RESULTS: When the magnetosomes synthesis occurs during the exponential growth phase, they reach larger sizes and higher monodispersity, displaying a stoichiometric magnetite structure, as fingerprinted by the well defined Verwey temperature. On the contrary, the magnetosomes synthesized at the stationary phase reach smaller sizes and display a smeared Verwey transition, that suggests that these magnetosomes may deviate slightly from the perfect stoichiometry. CONCLUSIONS: Magnetosomes magnetically closer to stoichiometric magnetite are obtained when bacteria start synthesizing them at the exponential growth phase rather than at the stationary phase. GENERAL SIGNIFICANCE: The growth conditions influence the final properties of the biosynthesized magnetosomes. This article is part of a Special Issue entitled "Recent Advances in Bionanomaterials" Guest Editors: Dr. Marie-Louise Saboungi and Dr. Samuel D. Bader.

Studying nanoparticles' 3D shape by aspect maps: Determination of the morphology of bacterial magnetic nanoparticles.

Magnetic nanoparticles (MNPs) are widely investigated due to their potential use in various applications, ranging from electronics to biomedical devices. The magnetic properties of MNPs are strongly dependent on their size and shape (i.e., morphology), thus appropriate tools to investigate their morphology are fundamental to understand the physics of these systems. Recently a new approach to study nanoparticle morphology by Transmission Electron Microscopy (TEM) analysis has been proposed, introducing the so-called Aspect Maps (AMs). In this paper, a further evolution of the AM method is presented, allowing determination of the nanoparticles' 3D shape by TEM image. As a case study, this paper will focus on magnetite nanoparticles (Fe3O4), with a mean size of ∼45 nm extracted from Magnetospirillum gryphiswaldense magnetostatic bacteria (MTB). The proposed approach gives a complete description of the nanoparticles' morphology, allowing estimation of an average geometrical size and shape. In addition, preliminary investigation of the magnetic properties of MTB nanoparticles was performed, giving some insight into interparticle interactions and on the reversal mechanism of the magnetization.

Do extraversion and neuroticism moderate the association between bullying victimization and internalizing symptoms? A three-wave longitudinal study.

The current study examined the moderating roles of neuroticism and extraversion in victims of bullying. According to a stress-diathesis model, we hypothesized that adolescents with high levels of neuroticism and low levels of extraversion would react to victimization with increased symptoms of depression and social anxiety. A sample of 1440 adolescents (648 girls and 792 boys; ages between 13- and 17-years-old) completed measures of extraversion and neuroticism at time 1, as well as measures of bullying victimization, depressive symptoms and social anxiety symptoms at time 1, time 2, and time 3 (in intervals of six months). The results of multilevel analyses for longitudinal data indicated that there was a weak association between bullying victimization and social anxiety symptoms for the adolescents who scored high on extraversion. In addition, the adolescents with high levels of extraversion presented a greater reduction in depressive symptoms over time than adolescents with low levels. Although neuroticism predicted both depression and social anxiety, no significant interactions were evident between neuroticism and bullying victimization. Regarding gender differences, the association between bullying victimization and social anxiety was stronger for boys than for girls, whereas the association between neuroticism and depression was stronger for girls.

On the mineral core of ferritin-like proteins: structural and magnetic characterization.

It is generally accepted that the mineral core synthesized by ferritin-like proteins consists of a ferric oxy-hydroxide mineral similar to ferrihydrite in the case of horse spleen ferritin (HoSF) and an oxy-hydroxide-phosphate phase in plant and prokaryotic ferritins. The structure reflects a dynamic process of deposition and dissolution, influenced by different biological, chemical and physical variables. In this work we shed light on this matter by combining a structural (High Resolution Transmission Electron Microscopy (HRTEM) and Fe K-edge X-ray Absorption Spectroscopy (XAS)) and a magnetic study of the mineral core biomineralized by horse spleen ferritin (HoSF) and three prokaryotic ferritin-like proteins: bacterial ferritin (FtnA) and bacterioferritin (Bfr) from Escherichia coli and archaeal ferritin (PfFtn) from Pyrococcus furiosus. The prokaryotic ferritin-like proteins have been studied under native conditions and inside the cells for the sake of preserving their natural attributes. They share with HoSF a nanocrystalline structure rather than an amorphous one as has been frequently reported. However, the presence of phosphorus changes drastically the short-range order and magnetic response of the prokaryotic cores with respect to HoSF. The superparamagnetism observed in HoSF is absent in the prokaryotic proteins, which show a pure atomic-like paramagnetic behaviour attributed to phosphorus breaking the Fe-Fe exchange interaction.

Breakdown of magnetism in sub-nanometric Ni clusters embedded in Ag.

Downsizing to the nanoscale has opened up a spectrum of new magnetic phenomena yet to be discovered. In this context, we investigate the magnetic properties of Ni clusters embedded in a metallic Ag matrix. Unlike in Ni free-standing clusters, where the magnetic moment increases towards the atomic value when decreasing the cluster size, we show, by tuning the Ni cluster size down to the sub-nanoscale, that there is a size limit below which the clusters become non-magnetic when embedded in Ag. To this end, we have fabricated by DC-sputtering a system composed of sub-nanometer sized and non interacting Ni clusters embedded into a Ag matrix. A thorough experimental characterization by means of structural techniques (x-ray diffraction, x-ray absorption spectroscopy) and DC-magnetization confirms that the cluster size is in the sub-nanometric range and shows that the magnetization of the system is dramatically reduced, reaching only 38% of the bulk value. The experimental system has been reproduced by density functional theory calculations on Ni m clusters (m = 1-6, 10 and 13) embedded in Ag. The combination of the experimental and theoretical analysis points out that there is a breakdown of magnetism occurring below a cluster size of six atoms. According to our results, the loss of magnetic moment is not due to Ag-Ni hybridization but to charge transfer between the Ni sp and d orbitals, and the reduced magnetization observed experimentally is explained on the basis of the presence of a narrow cluster size-distribution where magnetic and non-magnetic clusters coexist.

Effect of Carbon Coating on the Physicochemical and Electrochemical Properties of Fe2O3 Nanoparticles for Anode Application in High Performance Lithium Ion Batteries.

Nanoparticulate Fe2O3 and Fe2O3/C composites with different carbon proportions have been prepared for anode application in lithium ion batteries (LIBs). Morphological studies revealed that particles of Fe2O3 in the composites were well-dispersed in the matrix of amorphous carbon. The properties of the γ-Fe2O3 nanoparticles and the correlation with the particle size and connectivity were studied by electron paramagnetic resonance, magnetic, and Mössbauer measurements. The electrochemical study revealed that composites with carbon have promising electrochemical performances. These samples yielded specific discharge capacities of 1200 mAh/g after operating for 100 cycles at 1C. These excellent results could be explained by the homogeneity of particle size and structure as well as the uniform distribution of γ-Fe2O3 nanoparticles in the in situ generated amorphous carbon matrix.

Brief report: the adolescent Child-to-Parent Aggression Questionnaire: an examination of aggressions against parents in Spanish adolescents.

The objective of this study was to develop a questionnaire to assess child-to-parent aggression in adolescents and to document the extent of the problem. The questionnaire developed in this study, the Child-to-Parent Aggression Questionnaire (CPAQ), includes forms of physical and psychological aggression directed at both the mother and the father. It also includes open questions about the reasons for the aggressive acts. The CPAQ was completed by a sample of 2719 adolescents (age range: 13-18 years old, 51.4% girls). Confirmatory factor analysis supported a four-factor correlated structure (physical aggression against mother, physical aggression against father, psychological aggression against mother, and psychological aggression against father). Psychological and physical aggression against the mother was more frequent than against the father. However, there were no differences with regard to severe forms of aggression. Girls scored significantly higher on all indicators of psychological aggression, including severe psychological aggression. Nevertheless, except for the prevalence of physical aggression against mothers, which was higher in females, there were no significant differences in physical aggression against parents. Finally, the reasons provided by the adolescents for the aggression included both instrumental (e.g., to obtain permission to get home late and to access their computers) and reactive reasons (e.g., anger and self-defense). These findings highlight the complexity of child-to-parent aggression in adolescence.

Influence of the interactions on the magnetotransport properties of Fe-Ag granular thin films.

Fe(x)Ag(100-x) granular thin films, 25 < or = x < or = 40, have been prepared by DC-Magnetron sputtering deposition. These samples are composed of small Fe nanoparticles (2.5-3 nm) embedded in an Ag matrix, and their size remains nearly constant with increasing Fe content. A crossover in the collective magnetic behaviour of the samples near x = 35, had been previously reported for these samples. In this article, we show that the transport and magnetotransport properties are clearly correlated with these magnetic behaviours, and that the resistivity, the magnetoresistance and the Hall resistivity depend on the nature of the most relevant interactions, dipolar or direct exchange, as a function of the thermal evolution and the Fe content.

Influence of temperature on structure and magnetic properties of exchange coupled TbCo/FeNi bilayers.

Among amorphous films of rare earth-transition metal (RE-TM) alloys as exchange-biasing layers in magnetoresistive heads and spin-valve sensors, the amorphous Tb-Co films have most high practical potential. In the present work the influence of the temperature and the heat treatment parameters on the structure and magnetic properties was studied for exchange bias FeNi/Tb35Co65 bilayers annealed in vacuum or a nitrogen flow. A simple explanation of the dependence of the magnetic properties on the temperature and the heat treatment parameters connected with structural changes in each one of the layers was proposed.

Interfacial magnetic coupling between Fe nanoparticles in Fe­Ag granular alloys.

The role of the interface in mediating interparticle magnetic interactions has been analysed in Fe50Ag50 and Fe55Ag45 granular thin films deposited by the pulsed laser deposition technique (PLD). These samples are composed of crystalline bcc Fe (2­4 nm) nanoparticles and fcc Ag (10­12 nm) nanoparticles, separated by an amorphous Fe50Ag50 interface, occupying around 20% of the sample volume, as determined by x-ray diffraction (XRD), x-ray absorption spectroscopy (XAS), and high resolution transmission electron microscopy (HRTEM). Interfacial magnetic coupling between Fe nanoparticles is studied by dc magnetization and x-ray magnetic circular dichroism (XMCD) measurements at the Fe K and Ag L2,3 edges. This paper reveals that these thin films present two magnetic transitions, at low and high temperatures, which are strongly related to the magnetic state of the amorphous interface, which acts as a barrier for interparticle magnetic coupling.

[Panic disorder and transcranial magnetic stimulation]. / Trastorno de angustia y estimulación magnética transcraneal.

Transcranial magnetic stimulation (TMS) has been tried in some Anxiety Disorders (Obsessive-compulsive disorder and Posttraumatic Stress Disorder) with different results. We present a pilot study including three Panic Disorder patients. The subjects who were enrolled had a history of the disease for at least 1 year and they had unsuccessfully followed psychotherapy and pharmacological treatment. The patients received 10 sessions during two weeks; each session lasted 30 trains of 60 seconds at a frequency of 1 Hz, on the right dorsolateral prefrontal cortex, at 110% of the motor threshold. All three patients experienced a modest and partial symptom improvement that did not seemed to be clinically relevant. Two patients accepted to participate in a TMS second phase, where the previous stimulation parameters were alternated with an application of 30 trains of 20 Hz during 2 seconds on the left prefrontal cortex. This alternate application of high and low frequency TMS in each session was also well tolerated, but failed to produce additional improvement. In addition to presenting these three cases, we emphasize some features concerning the neurobiological basis of the anxiety disorders and we connect them to the previously described TMS neurophysiological actions in order to justify further investigation.

X-ray magnetic circular dichroism in FeZrB amorphous alloys: the influence of the tensile stress.

We present X-ray Magnetic Circular Dichroism experiments (XMCD) on the Fe K edge of FeZrB metallic glasses performed under tensile stress. In these compounds the application of tensile stresses produces a large increase of the Curie Temperature. The XMCD signal presents the features expected for a weak ferromagnet but a gradual enhancement of the ferromagnetism is observed as boron and zirconium concentrations increase. The main effect of the tensile stress is to increase the density of states at the Fermi level as deduced from the increment of the amplitude of the XMCD signal with the stress.