ABSTRACT This paper focuses on the study of the magnetic properties of 9 nm magnetite nanocrystal... more ABSTRACT This paper focuses on the study of the magnetic properties of 9 nm magnetite nanocrystals. XRD and TEM measurements indicate the presence of crystalline particles, with a fraction of them only partially crystallized or highly defective. The analysis of the temperature dependence of the zero-field-cooled/field-cooled magnetization and of the thermoremanent magnetization provides evidence of the existence of three magnetic regimes: a high temperature regime (300–100 K), an intermediate regime (100–20 K), and a low temperature regime (below 20 K). The characteristics of such regimes are discussed.
ABSTRACT This paper focuses on the magnetic properties of CoFe2O4 nanoparticles, discussing the i... more ABSTRACT This paper focuses on the magnetic properties of CoFe2O4 nanoparticles, discussing the influence of nanoparticles arrangements obtained by different synthesis methods. Using high thermal decomposition (HTD) and direct micellar (DM) routes, three samples of CoFe2O4 nanoparticles with equal primary particle size (5 nm) were prepared. The HTD method allows one to obtain highly crystalline primary nanoparticles coated by oleic acid organized in a self-assembling arrangement (ACoFeHTD). The DM method results to be appropriate to prepare either irregular arrangements (IACoFeDM) or spherical iso-oriented nanoporous assemblies (SACoFeDM) of primary CoFe2O4 nanocrystals. Despite the same particle size, magnetization measurements of the HTD sample show a tendency toward cubic anisotropy (Mr/Ms ≈ 0.7), while in DM samples, a uniaxial anisotropy (Mr/Ms ≈ 0.4) is observed. The comparison between IACoFeDM and SACoFeDM samples indicates that the ordering of nanocrystals at the mesoscopic scale induces an increase of the coercive field (μ0Hc ≈ 1.17 T → μ0Hc ≈ 1.45 T) and of the reduced remanent magnetization (Mr/Ms ≈ 0.4 → Mr/Ms ≈ 0.5). The reason for these differences is discussed. In particular, a detailed study on interparticle interactions is carried out, highlighting the influence of the molecular coating and the formation of spherical iso-oriented assemblies.
The possibility to finely control nanostructured cubic ferrites (M(II)Fe2O4) paves the way to des... more The possibility to finely control nanostructured cubic ferrites (M(II)Fe2O4) paves the way to design materials with the desired magnetic properties for specific applications. However, the strict and complex interrelation among the chemical composition, size, polydispersity, shape and surface coating renders their correlation with the magnetic properties not trivial to predict. In this context, this work aims to discuss the magnetic properties and the heating abilities of Zn-substituted cobalt ferrite nanoparticles with different zinc contents (ZnxCo1-xFe2O4 with 0 < x < 0.6), specifically prepared with similar particle sizes (∼7 nm) and size distributions having the crystallite size (∼6 nm) and capping agent amount of 15%. All samples have high saturation magnetisation (Ms) values at 5 K (>100 emu g(-1)). The increase in the zinc content up to x = 0.46 in the structure has resulted in an increase of the saturation magnetisation (Ms) at 5 K. High Ms values have also been revealed at room temperature (∼90 emu g(-1)) for both CoFe2O4 and Zn0.30Co0.70Fe2O4 samples and their heating ability has been tested. Despite a similar saturation magnetisation, the specific absorption rate value for the cobalt ferrite is three times higher than the Zn-substituted one. DC magnetometry results were not sufficient to justify these data, the experimental conditions of SAR and static measurements being quite different. The synergic combination of DC with AC magnetometry and (57)Fe Mössbauer spectroscopy represents a powerful tool to get new insights into the design of suitable heat mediators for magnetic fluid hyperthermia.
The effect of the deposition temperature (Tdep) on the crystallographic orientation of pulsed las... more The effect of the deposition temperature (Tdep) on the crystallographic orientation of pulsed laser-deposited FeCo/MgO(100) thin film was determined by means of X-ray reflectivity and high resolution trasmission electron microscopy analysis and was correlated with the magnetic anisotropy properties measured by angle dependent hysteresis loops. Highly textured films with a bcc structure and very smooth surface were obtained even at room temperature, the film being [100] and [110] oriented, at Tdep = 25 °C and 150 °C, respectively. The cubic symmetry is reflected in the angular dependence of remanent magnetization, showing a 4-fold character, whose in-plane distribution is consistent with the different crystallographic orientations of the films. The high structural quality, even at room temperature, is reflected in a high value of the saturation magnetization and low coercivity, matching the requirements for technological applications.
Chromium nanoparticles are formed using superfluid helium droplets as the nanoreactors, which are... more Chromium nanoparticles are formed using superfluid helium droplets as the nanoreactors, which are strongly ferromagnetic. The transition from antiferromagentism to ferromagnetism is attributed to atomic-scale disorder in chromium nanoparticles, leading to abundant unbalanced surface spins. Theoretical modeling confirms a frustrated aggregation process in superfluid helium due to the antiferromagnetic nature of chromium.
ABSTRACT In this work we studied the dynamic and static magnetic properties of ZnO-core/CoFe2O4-s... more ABSTRACT In this work we studied the dynamic and static magnetic properties of ZnO-core/CoFe2O4-shell and CoO-core/CoFe2O4-shell nanoparticles. Both systems are formed by a core of ~ 4 nm of diameter encapsulated in a shell of ~ 2 nm of thickness. The mean blocking temperature changes from 106(7) K to 276(5) K when the core is diamagnetic or antiferromagnetic, respectively. Magnetic remanence studies revealed the presence of weak dipolar inter-particle interactions, where Hint is approximately - 0.1 kOe for ZnO/CoFe2O4 and - 0.9 kOe for CoO/CoFe2O4, playing a minor role in the magnetic behavior of the materials. Relaxation experiments provided evidence that the magnetization reversal process of CoFe2O4 is strongly dependent on the magnetic order of the core. At 10 K, activation volumes of ~ 46(6) and ~ 69(5) nm3 were found for CoO/CoFe2O4 and ZnO/CoFe2O4 nanoparticles, respectively, corresponding to one-third and one-fifth of the total shell volume. While the magnetic behavior of ZnO/CoFe2O4 nanoparticles is strongly affected by the surface disorder, the exchange coupling at the CoO/CoFe2O4 interface rules the magnetization reversal and the nanoparticles’ thermal stability by inducing a larger energy barrier and promoting smaller switching volume.
ABSTRACT In this work we present an Extended X-ray Absorption Fine Structure (EXAFS) characteriza... more ABSTRACT In this work we present an Extended X-ray Absorption Fine Structure (EXAFS) characterization of ternary FePtCu alloys with different Cu content. The EXAFS measurements have been carried out at the Cu-K and Pt-LIII edges in order to describe the local environment around these elements in the FePtCu samples and to compare the structural evolution as a function of the Cu content. The EXAFS study, based on a substitutional model where the Cu atoms occupy Fe sites in the tetragonal structure, has been performed by using linear dichroism to enhance the sensitivity to differently oriented bonds and to gain a detailed description of the atomic environment. The study allowed to distinguish experimentally the effects on the chemical order and lattice distortion induced by the Cu atoms. We correlate the determined positions of the Cu atoms in the chemically L10 ordered fct lattice with the magnetic properties of FePtCu ternary alloys. In particular, the main effect of Cu atoms in the alloy is a linear reduction of the c/a ratio, while the non-monotonic behaviour of the chemical order is consistent with the variation of the magnetocrystalline anisotropy.
Small (Weinheim an der Bergstrasse, Germany), Jan 30, 2015
Manganese oxide nanoparticles (MONPs) are capable of time-dependent magnetic resonance imaging co... more Manganese oxide nanoparticles (MONPs) are capable of time-dependent magnetic resonance imaging contrast switching as well as releasing a surface-bound drug. MONPs give T2/T2* contrast, but dissolve and release T1-active Mn(2+) and L-3,4-dihydroxyphenylalanine. Complementary images are acquired with a single contrast agent, and applications toward Parkinson's disease are suggested.
The out-of-equilibrium dynamical properties of two very different superspin glass systems are dis... more The out-of-equilibrium dynamical properties of two very different superspin glass systems are discussed: 1) MnFe 2 O 4 nanoparticles, in powder form, with very strong dipolar interactions and 2) a system consisting of a film of Co particles dispersed (5%-10% volume filling fraction) in an Mn matrix, where interparticle interactions are mainly mediated by the matrix. In the latter system, the influence of interface exchange coupling between the ferromagnetic particles and the antiferromagnetic matrix on the out-of-equilibrium dynamics is discussed.
The structure of 2 nm diameter (340 atoms) Fe nanoparticles embedded in a Cr matrix was determine... more The structure of 2 nm diameter (340 atoms) Fe nanoparticles embedded in a Cr matrix was determined using X-ray Absorption Fine Structure (EXAFS) and the magnetic properties studied by Superconducting Quantum Interference Device (SQUID) magnetometry. The thin films were produced by the co-deposition of pre-formed gas-phase Fe clusters synthesised by a gas aggregation source with an atomic vapour of Cr produced by an MBE source. The behaviour was studied as a function of Fe nanoparticle volume fraction in the range 5-20% and was compared to previous results on ferromagnetic nanoparticles in antiferromagnetic matrices. EXAFS showed that the atomic structure in the Cr-embedded Fe nanoparticles is the same as the bulk bcc structure. Whereas alloying between the nanoparticles and matrix material has previously been shown to be very pronounced for Co nanoparticles in antiferromagnetic Mn, it was found that any alloying between Fe nanoparticles and Cr matrix material is limited. For dilute samples of Fe nanoparticles in Cr the measured saturation magnetisation (M S ) was 1m B /Fe atom, which is significantly less than the bulk M S value of 2.22m B /Fe atom indicating that the surface of Fe nanoparticles is either antiferromagnetic or non-magnetic. An increase in the volume fraction produces an increase in the value of M S and at a volume fraction of 20%, M S exceeds the value of bulk Fe showing that some Cr spins provide a ferromagnetic contribution. After field cooling below 30 K, all films show Exchange Bias (EB) and an increase of coercivity, which are both much larger for the most concentrated sample. The Cr spins at the surface of the Fe particles play a key role in determining the overall magnetic behaviour.
Magnetic nano-sensors based on niobium dc SQUID (Superconducting Quantum Interference Device) for... more Magnetic nano-sensors based on niobium dc SQUID (Superconducting Quantum Interference Device) for nanoparticle characterization are presented. The SQUIDs consists of two Dayem bridges of 90nm x 250nm and loop area of 4, 1 and 0.55 μm2. The SQUIDs were designed to have a hysteretic current-voltage characteristic in order to work as a magnetic flux-current transducer. Current-voltage characteristics, critical current as a function of the external magnetic field and switching current distributions were performed ...
Nanoparticles of iron and iron oxide are widely explored in several biomedical and technological ... more Nanoparticles of iron and iron oxide are widely explored in several biomedical and technological applications. We report on the magnetic properties of amorphous Fe/Fe-O core/shell nanoparticles compared to those of a reference system with crystalline Fe-O nanoparticles. These nanoparticles are prepared by thermal decomposition of iron precursor, where the amorphous and crystalline nature of core and shell is determined by the choice and concentration of the ligand. The crystalline system exhibits a blocking temperature higher than 300 K and negligible exchange bias effect. In contrast, the amorphous systems display large exchange bias, and collective magnetic behavior at low temperatures, with features of magnetic frustration and disorder reminiscent of those observed in spin glass and superspin glass systems. We discuss the origin of the dynamical magnetic behavior of the amorphous particles and study the dependence of the exchange bias field on the cooling field.
ABSTRACT This paper focuses on the study of the magnetic properties of 9 nm magnetite nanocrystal... more ABSTRACT This paper focuses on the study of the magnetic properties of 9 nm magnetite nanocrystals. XRD and TEM measurements indicate the presence of crystalline particles, with a fraction of them only partially crystallized or highly defective. The analysis of the temperature dependence of the zero-field-cooled/field-cooled magnetization and of the thermoremanent magnetization provides evidence of the existence of three magnetic regimes: a high temperature regime (300–100 K), an intermediate regime (100–20 K), and a low temperature regime (below 20 K). The characteristics of such regimes are discussed.
ABSTRACT This paper focuses on the magnetic properties of CoFe2O4 nanoparticles, discussing the i... more ABSTRACT This paper focuses on the magnetic properties of CoFe2O4 nanoparticles, discussing the influence of nanoparticles arrangements obtained by different synthesis methods. Using high thermal decomposition (HTD) and direct micellar (DM) routes, three samples of CoFe2O4 nanoparticles with equal primary particle size (5 nm) were prepared. The HTD method allows one to obtain highly crystalline primary nanoparticles coated by oleic acid organized in a self-assembling arrangement (ACoFeHTD). The DM method results to be appropriate to prepare either irregular arrangements (IACoFeDM) or spherical iso-oriented nanoporous assemblies (SACoFeDM) of primary CoFe2O4 nanocrystals. Despite the same particle size, magnetization measurements of the HTD sample show a tendency toward cubic anisotropy (Mr/Ms ≈ 0.7), while in DM samples, a uniaxial anisotropy (Mr/Ms ≈ 0.4) is observed. The comparison between IACoFeDM and SACoFeDM samples indicates that the ordering of nanocrystals at the mesoscopic scale induces an increase of the coercive field (μ0Hc ≈ 1.17 T → μ0Hc ≈ 1.45 T) and of the reduced remanent magnetization (Mr/Ms ≈ 0.4 → Mr/Ms ≈ 0.5). The reason for these differences is discussed. In particular, a detailed study on interparticle interactions is carried out, highlighting the influence of the molecular coating and the formation of spherical iso-oriented assemblies.
The possibility to finely control nanostructured cubic ferrites (M(II)Fe2O4) paves the way to des... more The possibility to finely control nanostructured cubic ferrites (M(II)Fe2O4) paves the way to design materials with the desired magnetic properties for specific applications. However, the strict and complex interrelation among the chemical composition, size, polydispersity, shape and surface coating renders their correlation with the magnetic properties not trivial to predict. In this context, this work aims to discuss the magnetic properties and the heating abilities of Zn-substituted cobalt ferrite nanoparticles with different zinc contents (ZnxCo1-xFe2O4 with 0 < x < 0.6), specifically prepared with similar particle sizes (∼7 nm) and size distributions having the crystallite size (∼6 nm) and capping agent amount of 15%. All samples have high saturation magnetisation (Ms) values at 5 K (>100 emu g(-1)). The increase in the zinc content up to x = 0.46 in the structure has resulted in an increase of the saturation magnetisation (Ms) at 5 K. High Ms values have also been revealed at room temperature (∼90 emu g(-1)) for both CoFe2O4 and Zn0.30Co0.70Fe2O4 samples and their heating ability has been tested. Despite a similar saturation magnetisation, the specific absorption rate value for the cobalt ferrite is three times higher than the Zn-substituted one. DC magnetometry results were not sufficient to justify these data, the experimental conditions of SAR and static measurements being quite different. The synergic combination of DC with AC magnetometry and (57)Fe Mössbauer spectroscopy represents a powerful tool to get new insights into the design of suitable heat mediators for magnetic fluid hyperthermia.
The effect of the deposition temperature (Tdep) on the crystallographic orientation of pulsed las... more The effect of the deposition temperature (Tdep) on the crystallographic orientation of pulsed laser-deposited FeCo/MgO(100) thin film was determined by means of X-ray reflectivity and high resolution trasmission electron microscopy analysis and was correlated with the magnetic anisotropy properties measured by angle dependent hysteresis loops. Highly textured films with a bcc structure and very smooth surface were obtained even at room temperature, the film being [100] and [110] oriented, at Tdep = 25 °C and 150 °C, respectively. The cubic symmetry is reflected in the angular dependence of remanent magnetization, showing a 4-fold character, whose in-plane distribution is consistent with the different crystallographic orientations of the films. The high structural quality, even at room temperature, is reflected in a high value of the saturation magnetization and low coercivity, matching the requirements for technological applications.
Chromium nanoparticles are formed using superfluid helium droplets as the nanoreactors, which are... more Chromium nanoparticles are formed using superfluid helium droplets as the nanoreactors, which are strongly ferromagnetic. The transition from antiferromagentism to ferromagnetism is attributed to atomic-scale disorder in chromium nanoparticles, leading to abundant unbalanced surface spins. Theoretical modeling confirms a frustrated aggregation process in superfluid helium due to the antiferromagnetic nature of chromium.
ABSTRACT In this work we studied the dynamic and static magnetic properties of ZnO-core/CoFe2O4-s... more ABSTRACT In this work we studied the dynamic and static magnetic properties of ZnO-core/CoFe2O4-shell and CoO-core/CoFe2O4-shell nanoparticles. Both systems are formed by a core of ~ 4 nm of diameter encapsulated in a shell of ~ 2 nm of thickness. The mean blocking temperature changes from 106(7) K to 276(5) K when the core is diamagnetic or antiferromagnetic, respectively. Magnetic remanence studies revealed the presence of weak dipolar inter-particle interactions, where Hint is approximately - 0.1 kOe for ZnO/CoFe2O4 and - 0.9 kOe for CoO/CoFe2O4, playing a minor role in the magnetic behavior of the materials. Relaxation experiments provided evidence that the magnetization reversal process of CoFe2O4 is strongly dependent on the magnetic order of the core. At 10 K, activation volumes of ~ 46(6) and ~ 69(5) nm3 were found for CoO/CoFe2O4 and ZnO/CoFe2O4 nanoparticles, respectively, corresponding to one-third and one-fifth of the total shell volume. While the magnetic behavior of ZnO/CoFe2O4 nanoparticles is strongly affected by the surface disorder, the exchange coupling at the CoO/CoFe2O4 interface rules the magnetization reversal and the nanoparticles’ thermal stability by inducing a larger energy barrier and promoting smaller switching volume.
ABSTRACT In this work we present an Extended X-ray Absorption Fine Structure (EXAFS) characteriza... more ABSTRACT In this work we present an Extended X-ray Absorption Fine Structure (EXAFS) characterization of ternary FePtCu alloys with different Cu content. The EXAFS measurements have been carried out at the Cu-K and Pt-LIII edges in order to describe the local environment around these elements in the FePtCu samples and to compare the structural evolution as a function of the Cu content. The EXAFS study, based on a substitutional model where the Cu atoms occupy Fe sites in the tetragonal structure, has been performed by using linear dichroism to enhance the sensitivity to differently oriented bonds and to gain a detailed description of the atomic environment. The study allowed to distinguish experimentally the effects on the chemical order and lattice distortion induced by the Cu atoms. We correlate the determined positions of the Cu atoms in the chemically L10 ordered fct lattice with the magnetic properties of FePtCu ternary alloys. In particular, the main effect of Cu atoms in the alloy is a linear reduction of the c/a ratio, while the non-monotonic behaviour of the chemical order is consistent with the variation of the magnetocrystalline anisotropy.
Small (Weinheim an der Bergstrasse, Germany), Jan 30, 2015
Manganese oxide nanoparticles (MONPs) are capable of time-dependent magnetic resonance imaging co... more Manganese oxide nanoparticles (MONPs) are capable of time-dependent magnetic resonance imaging contrast switching as well as releasing a surface-bound drug. MONPs give T2/T2* contrast, but dissolve and release T1-active Mn(2+) and L-3,4-dihydroxyphenylalanine. Complementary images are acquired with a single contrast agent, and applications toward Parkinson's disease are suggested.
The out-of-equilibrium dynamical properties of two very different superspin glass systems are dis... more The out-of-equilibrium dynamical properties of two very different superspin glass systems are discussed: 1) MnFe 2 O 4 nanoparticles, in powder form, with very strong dipolar interactions and 2) a system consisting of a film of Co particles dispersed (5%-10% volume filling fraction) in an Mn matrix, where interparticle interactions are mainly mediated by the matrix. In the latter system, the influence of interface exchange coupling between the ferromagnetic particles and the antiferromagnetic matrix on the out-of-equilibrium dynamics is discussed.
The structure of 2 nm diameter (340 atoms) Fe nanoparticles embedded in a Cr matrix was determine... more The structure of 2 nm diameter (340 atoms) Fe nanoparticles embedded in a Cr matrix was determined using X-ray Absorption Fine Structure (EXAFS) and the magnetic properties studied by Superconducting Quantum Interference Device (SQUID) magnetometry. The thin films were produced by the co-deposition of pre-formed gas-phase Fe clusters synthesised by a gas aggregation source with an atomic vapour of Cr produced by an MBE source. The behaviour was studied as a function of Fe nanoparticle volume fraction in the range 5-20% and was compared to previous results on ferromagnetic nanoparticles in antiferromagnetic matrices. EXAFS showed that the atomic structure in the Cr-embedded Fe nanoparticles is the same as the bulk bcc structure. Whereas alloying between the nanoparticles and matrix material has previously been shown to be very pronounced for Co nanoparticles in antiferromagnetic Mn, it was found that any alloying between Fe nanoparticles and Cr matrix material is limited. For dilute samples of Fe nanoparticles in Cr the measured saturation magnetisation (M S ) was 1m B /Fe atom, which is significantly less than the bulk M S value of 2.22m B /Fe atom indicating that the surface of Fe nanoparticles is either antiferromagnetic or non-magnetic. An increase in the volume fraction produces an increase in the value of M S and at a volume fraction of 20%, M S exceeds the value of bulk Fe showing that some Cr spins provide a ferromagnetic contribution. After field cooling below 30 K, all films show Exchange Bias (EB) and an increase of coercivity, which are both much larger for the most concentrated sample. The Cr spins at the surface of the Fe particles play a key role in determining the overall magnetic behaviour.
Magnetic nano-sensors based on niobium dc SQUID (Superconducting Quantum Interference Device) for... more Magnetic nano-sensors based on niobium dc SQUID (Superconducting Quantum Interference Device) for nanoparticle characterization are presented. The SQUIDs consists of two Dayem bridges of 90nm x 250nm and loop area of 4, 1 and 0.55 μm2. The SQUIDs were designed to have a hysteretic current-voltage characteristic in order to work as a magnetic flux-current transducer. Current-voltage characteristics, critical current as a function of the external magnetic field and switching current distributions were performed ...
Nanoparticles of iron and iron oxide are widely explored in several biomedical and technological ... more Nanoparticles of iron and iron oxide are widely explored in several biomedical and technological applications. We report on the magnetic properties of amorphous Fe/Fe-O core/shell nanoparticles compared to those of a reference system with crystalline Fe-O nanoparticles. These nanoparticles are prepared by thermal decomposition of iron precursor, where the amorphous and crystalline nature of core and shell is determined by the choice and concentration of the ligand. The crystalline system exhibits a blocking temperature higher than 300 K and negligible exchange bias effect. In contrast, the amorphous systems display large exchange bias, and collective magnetic behavior at low temperatures, with features of magnetic frustration and disorder reminiscent of those observed in spin glass and superspin glass systems. We discuss the origin of the dynamical magnetic behavior of the amorphous particles and study the dependence of the exchange bias field on the cooling field.
At the end of the course each student should discuss a scientific paper about synthesis and chara... more At the end of the course each student should discuss a scientific paper about synthesis and characterization of nanostructured m agnetic m aterial. The contents of the article will be chosen on the basis of the background and on the scientific interest of each student. A short powerpoint presentation could be required.
Nanomagnetism: Fundamentals and Applications, C. Binns Ed., Elsivier, 2014
Magnetic nanostructures are characterized by a wide variety of geometries, which cover the range ... more Magnetic nanostructures are characterized by a wide variety of geometries, which cover the range from granular bulk materials to low-dimensional magnetic systems, such as nanoparticles, nanowires/nanotubes, granular/continuous thin films and multilayers, dots, etc. The magnetic behaviour of ensembles of magnetic entities depends on their intrinsic magnetic properties and many other factors, including short and long range magnetic interactions. The chapter focus on the effect of interactions in assemblies of single-domain nanoparticles, each of them being considered as a “superspin” with a magnetic moment related to the particle volume. Depending on the type and the strength of the interactions among the magnetic entities, the magnetic behavior of an assembly of super spins evolves from ferromagnetic-like behavior to paramagnetic-like behaviour, through spin glass like behaviour. Due to the enhanced time and magnetization scale with respect to atomic systems, the suffix super has been used to define these magnetic states and the magnetism of nanoparticle assemblies has been often called supermagnetism. The aim of this chapter is to describe how the ability to synthesise suitable nanoparticles systems, makes interactions among magnetic entities a fundamental tool to modify magnetic properties of nanostructured based materials. The last section of the chapter is dedicated to some examples of application, i.e. magnetic recording and permanent magnets, where the interactions among magnetic element plays a fundamental role in determining their performances.
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Papers by Davide Peddis