Surface Spin Disorder Research Articles

Anisotropic fluoride nanocrystals modulated by facet-specific passivation and their disordered surfaces

Rutile-type fluorides have been proven to be active components in the context of emerging antiferr-omagnetic devices. However, controlled synthesis of low-dimensional, in particular two-dimensional (2D), fluorides in a predictable and deterministic manner remains unrealized because of a lack of efficient anisotropic control, which impedes their further development in reduced dimensions. We report here that altered passivation of {110} growing facets can direct the synthesis of rutile-type fluoride nanocrystals into well-defined zero-dimensional (0D) particulates, one-dimensional (1D) rods and 2D sheets in a colloidal approach. The obtained nanocrystals show positive exchange bias and enhanced magnetic transition temperature from the coexistence of long-range antiferromagnetic order and disordered surface spins, making them strong alternatives for flexible magnetic devices and sensors.

Ce3+-doped nanocrystalline cobalt–zinc spinel ferrite: microstructural, magnetic, and optical characterizations

A series of Ce3+ ions-doped cobalt–zinc spinel cubic ferrite nanoparticles with the general formula Co0.5Zn0.5CexFe2–xO4 (x = 0.00, 0.03, 0.06 and 0.09) were synthesized using soft chemical co-precipitation route. The synthesized nanoferrites were in single-phase as ensured by the X-ray diffractograms. Average crystallite sizes were 16 nm, 12 nm, 10 nm, and 07 nm with increasing Ce content as determined using Scherrer’s formula. A significant enhancement in coercivity (HC) and decrement in saturation magnetization (MS) was observed in field-cooled (9 T) low-temperature (3 K) hysteresis loops with increasing Ce content. A correlation between the mean crystallite size of the synthesized nanoparticles and disordered surface spins has been established. The increment in Ce3+ ions substitution also favored the onset of superparamagnetic behavior as confirmed by the room temperature hysteresis loops. The blocking temperature was reduced with the increase of Ce3+ ions content which makes the nanoparticles a suitable candidate for various biomedical applications. The indirect optical bandgaps were observed to increase gradually with the increasing Ce content due to the nanosize effect.

Investigation of electrochemical performance, optical and magnetic properties of NiFe2O4 nanoparticles prepared by a green chemistry method

Green chemistry methods for synthesising metal oxides nanoparticles (NPs) have been extensively used due to their tremendous and remarkable advantages such as easy preparation, reliable results, non toxicity, low cost and eco-friendly. Based on these advantages we have applied a green chemistry method for synthesising Nickel Ferrite (NiFe 2 O 4 ) NPs mediated plant extract from Persa Americano seeds . XRD pattern confirmed the spinel phase structure of the synthesised NiFe 2 O 4 NPs. Fourier Transform Infrared (FTIR) spectroscopy showed the chemical bonds of the sample are consistent with the bonds assigned to NiFe 2 O 4 NPs. Morphological studies revealed the cubic shapes for some of the particles with average sizes range from 15nm to 20nm and irregular shapes for the others. The optical properties were studied using UV–Vis spectrophotometer and the obtained results indicated that NiFe 2 O 4 NPs exhibit a strong absorption behaviour in the UV range with a maximum peak at 250nm. The optical band gap calculated based on UV–Vis result was 4.25 eV. Photoluminescence (PL) spectrum of NiFe 2 O 4 NPs revealed two emission peaks, one in the UV region centred at 379nm and another a broad peak in nearly visible region (Blue emission) centred at 450nm. Magnetic studies showed a decrease in saturation magnetization Ms of NiFe 2 O 4 NPs compared to their bulk system which is attributed to surface spin disorder. Finally, NiFe 2 O 4 NPs exhibited excellent electrochemical properties which can be ascribed to: this compound possess two electrochemical active components and / or the nanocube structure of the particles which can further improve the electrochemistry through the feasible oxidation states and the synergetic effect. • A green chemistry method was successfully used to synthesis NiFe 2 O 4 NPs. • Morphological studies showed nanocubes shape of the particles. • Low value of Ms for NiFe 2 O 4 NPs was observed. • NiFe 2 O 4 NPs exhibited excellent electrochemical performance.

Artificially Engineered Cubic Iron Oxide Nanoparticle as a High-Performance Magnetic Particle Imaging Tracer for Stem Cell Tracking.

Stem cell therapies are increasingly recognized as the future direction of regenerative medicine, but the biological fate of the administrated stem cells remains a major concern for clinical translation, which calls for an approach to efficiently monitoring the stem cell behaviors in vivo. Magnetic particle imaging (MPI) is an emerging technology for cell tracking; however, its utility has been largely restricted due to the lack of optimal magnetic nanoparticle tracers. Herein, by controlled engineering of the size and shape of magnetic nanoparticles tailored to MPI physics theory, a specialized MPI tracer, based on cubic iron oxide nanoparticles with an edge length of 22 nm (CIONs-22), is developed. Due to the inherent lower proportion of disordered surface spins, CIONs-22 exhibit significantly larger saturation magnetization than that of spherical ones, while they possess similar saturation magnetization but smaller coercivity compared to larger-sized CIONs. These magnetic properties of CIONs-22 warrant high sensitivity and resolution of MPI. With their efficient cellular uptake, CIONs-22 exhibit superior MPI performance for stem cell labeling and tracking compared to the commercialized tracer Vivotrax. By virtue of these advantages, CIONs-22 enable real-time and prolonged monitoring of the spatiotemporal trajectory of stem cells transplanted to hindlimb ischemia mice, which demonstrates the great potential of CIONs-22 as MPI tracers to advance stem cell therapies.

Size variation in nanocrystalline Zn0.2Ni0.8Gd0.05Fe1.95O4 ferrites: Exchange bias effect and its correlation with disordered surface spins

The article reports the influence of particle size variation on microstructural, optical and magnetic properties of nanocrystalline Gd doped Ni-Zn ferrites with a generic formula Zn0.2Ni0.8Gd0.05Fe1.95O4. The nanoparticles were synthesized using chemical co-precipitation technique and the average crystallite sizes were found in the range of 4 nm–29 nm as estimated from Scherrer’s relation. Raman spectra confirmed the lowering of symmetry of the octahedral sites of the cubic spinel structure with rare earth doping. A blue shift was observed in the absorption profile of the nanoparticles due to the size effect. The 7 T field cooled M-H plots recorded at 5 K exhibited exchange bias for the sample with the smallest particle size which was attributed to the high anisotropy of the canted surface spins compared to the core spins.

Magnetic properties and thermal stability of polyvinylidene fluoride—Fe2O3 nanocomposites

Abstract

One-step synthesis of polyethyleneimine-coated magnetite nanoparticles and their structural, magnetic and power absorption study.

Magnetic nanoparticles (NPs) are especially interesting for several biomedical applications due to their chemical surface, especially for targeted cancer imaging and therapeutics. In order to realize these applications, it is important to know their magnetic properties among other complementary properties that help to improve the understanding of the synthesis process. In this work, we report the magnetic properties of polyethyleneimine-coated magnetite (PEI-Fe3O4) NPs synthesized by a one-step method via the co-precipitation method and using PEI as a stabilizer. Transmission electron microscopy (TEM) images revealed agglomerated magnetic nanoparticles with an average size of ∼10 nm; meanwhile, the X-ray diffraction (DRX) analysis confirmed a pure magnetite phase. The study of magnetic properties shows a superparamagnetic system with coexistence of non-interacting single NPs with a low blocking temperature (∼35 K) and interacting NPs in the aggregates with a higher blocking temperature (>150 K), in which the interparticle interactions of magnetic cores dominate over surface spin disorder. The interaction between the surface spin-disorder layer and NP core was found to be weak, related to a weak exchange bias effect. A maximum specific loss power (SLP) value of 70 W g−1 was obtained (f = 571 kHz and H = 23.87 kA m−1) indicating that the magnetic response plays a crucial role in determining the heating efficiency for future applications.

Study of structural, magnetic properties and bandgap of spinel Co1−xFe2+xO4 ferrite

Co1−xFe2+xO4 (x = 0.0–0.8) spinel nano ferrite dry gel powders were synthesized by sol gel auto-combustion method. Synthesized samples were investigated by using X-ray diffraction (XRD), magnetic measurements, Ultraviolet–visible (Uv–Vis); Fourier transform infrared (FTIR) spectroscopy. XRD validates the formation of spinel phase, and incorporation of Co, Fe in spinel lattice. Results reveal that compositional variation leads to: i) cationic re-distribution; ii) allowing fine-tuning of bandgap with strong correlation among Bandgap – Fe3+ ions on B-site; iii) changes in degree of inversion; iv) variation of disorder; v) shows better soft magnetic behavior; vi) surface spin disorder leads to reduction of saturation magnetization; vii) observed changes in Debye temperature suggest modification of lattice vibrations.

High coercivity in α-Fe2O3 nanostructures synthesized by surfactant-free microwave-assisted solvothermal method

In this report, the cube-like shaped α-Fe2O3 nanostructures were prepared by the simple microwave-assisted solvothermal method without using any surfactants. The as-prepared samples were characterized by X-ray diffraction, Raman spectroscopy, Fourier transform infrared spectroscopy and field emission scanning electron microscopy. The well dispersed and size-controlled cubic-like shaped α-Fe2O3 nanostructures were obtained by systematic variation of solvents, reaction temperature and time. The magnetic studies manifest that the magnetic properties of α-Fe2O3 samples are strongly dependent on the shape and size of the nanostructures. The maximum coercivity (Hc) ∼5.6 kOe is observed for Fe-160-30 sample, which is originating from the varying synthesis conditions, oriented sub-particle structures, surface spin disorder, surface/interface anisotropy and interactions of the nanoparticles at the surface/interface of the nanostructures. Represented synthesis approach facilitates the preparation of nanostructured materials with controlled morphology and properties.

Interface driven electrical and magneto-transport properties of (100 – x)% La0.7Sr0.3MnO3– x% Paraffin wax (0 ≤ x ≤ 1) hybrid nanocomposites

We report electrical and magneto-transport properties of La0.7Sr0.3MnO3(LSMO)/Paraffin wax nanocomposites prepared through low temperature chemical pyrophoric reaction process. A core shell structure is expected to be formed having a ferromagnetic LSMO core and a spin disordered shell of paraffin wax. Enhanced charging energy due to coulomb blockade contribution with increasing paraffin wax is observed for the composites attributing enhanced intergranular distance between LSMO grains. Effective spin-polarized tunneling of conduction electrons resulting in sharp low field magnetoresistance (MR) of the composites is due to reordering of spin disorder at paraffin wax shell with magnetic field. Moreover, observation of enhanced magnetoresistive hysteresis with decrease in temperature has been attributed to increased blocked states at LSMO grain surface. Calculated surface spin susceptibility (χb) has been found to follow similar behavior of MR with temperature indicating surface magnetization dependent MR. Fitting of magnetoconductance versus field curves reveal increase of Para to ferro transition temperature, TC and χb indicate higher antiferromagnetic coupling of shell spins for composite samples than pristine LSMO above 50 K. χb also indicate systematic decrease in surface spin disorder of the composite samples with respect to pure LSMO which is expected to be due to the probable covalent bond formation between Paraffin wax molecules and surface atoms of LSMO nanoparticles.

Disordered surface spins induced large exchange anisotropy in single-phase Sm3+ ions substituted nickel ferrite nanoparticles

Abstract This article reports the composition dependent microstructural, optical and magnetic behavior of rare earth Sm3+ ions substituted nanocrystalline nickel ferrite. The formation of pure cubic spinel structure was confirmed by X-ray diffraction patterns. The average crystallite size was found to decrease with increasing Sm content because of hindered crystal growth. The field cooled (7 T) hysteresis loops at 5 K displayed large exchange bias of 2 kOe along with considerable coercive field for the smallest particles. A strong correlation between particle size, canted surface spins, vertical shift and exchange bias was observed at low temperature. The reduction in coercivity and saturation magnetization observed in room temperature hysteresis loops was attributed to nonmagnetic behavior of substituted Sm3+ ions. The existence of interparticle interaction below blocking temperature and high surface anisotropy due to disordered surface spins was confirmed by M-T protocols. The indirect allowed optical band gap was found to increase with high Sm content due to finite size effect.

Electrical conduction mechanism for the investigation of charge ordering in Pr0.5Ca0.5MnO3 manganite system

Abstract In the present work, a systematic investigation of electrical transport mechanism has been used as a tool to investigate the charge-order suppression and its crossover in Pr 0.5 Ca 0.5 MnO 3 (PCMO) manganite system with varying particle size and applied magnetic field. The samples with different particle sizes were synthesized by adopting sol-gel method and sintering at different temperatures. The prepared samples were thoroughly characterized by various physicochemical techniques. The activation energy and density of states at Fermi level obtained from the temperature-dependent electrical resistivity data clearly show the charge order crossover signatures and its suppression in the samples below 70 nm particle size and applied magnetic fields above 4 T. The significant change in various electrical transport parameters with the particle size around 70 nm could be attributed to the melting of long-range charge ordering behavior due to surface spin disorder and induced lattice-strain effects in PCMO manganite system.

Superspinglass behavior of maghemite nanoparticles dispersed in mesoporous silica

Abstract γ-Fe2O3 nanoparticles were dispersed in a mesoporous silica (SBA15) matrix using the co-precipitation method. While structural properties were investigated by X-ray diffraction, X-ray photoelectron spectroscopy, micro-Raman and transmission electron microscopy, magnetic studies were performed with DC and AC magnetometry. The γ-Fe2O3 nanoparticles have the regular cubic spinel structure. A strong reduction in the saturation magnetization (∼30 emu/gFe, 300 K) was measured and can be mainly attributed to the large fraction (60%) of surface spin disorder. This phenomenon was studied using the modified Kneller’s law and a modified Bloch spin wave model with a surface disorder term. Values of spin wave stiffness constant D of 391 meVA2 and exchange integral Jex equal to 1.12 meV were obtained, suggesting the presence of a block spinel ferrimagnetic state. The Vogel-Fulcher relation was applied to study the nanoparticles magnetic interactions. The values of zv = 5.3 (5), Tf = 184 K and τ 0 = 10 - 12 s suggest a superspinglass behavior. Thermoremanent experiments have also shown that magnetic relaxation occurs even below Tf (180 K) with an exponential decay with scale parameters in the range from 0.56 to 0.6, in agreement with the slowing down law. These facts indicate that the superspinglass effect is mainly caused by the strong surface spin disorder and frustrated collective state related to an interacting system with a broad particle size distribution.

Synthesis and temperature dependent magnetic properties of nanocrystalline Ni0.5Zn0.5Fe2O4 ferrites

nanoparticles were successfully synthesized by modified citrate gel method. X-ray diffraction (XRD) analysis confirmed the development of single-phase cubic spinel structure with Fd m space group. The cation distribution was estimated by Rietveld refinement of XRD pattern and found that the prepared sample has a mixed spinel structure. The high field regions of magnetic hysteresis loops were modeled using the Law of Approach to Saturation to determine the saturation magnetization () and magnetic anisotropy constant () for the prepared nanoparticles. It is found that the values of and increases significantly with decreasing temperature. However, temperature dependence of saturation magnetization () and coercivity () were found to have considerable deviations from the Bloch’s and Kneller’s laws, respectively. This behavior is elucidated in terms of finite size effects, disordered surface spins and interparticle interactions in the prepared nanoparticles. Furthermore, the effects of interparticle interactions were investigated by using Vogel–Fulcher law for frequency dependent AC susceptibility measurements.

Irreversible Meta‐Magnetic Transition in Charge Ordered Nd0.5Ca0.5MnO3 Manganite

The investigation of charge ordering (CO) suppression and consequence effects in manganites are of interest to the scientific community as they help them to understand the interactions among the spin, charge and orbital degrees of freedom of conduction electrons. In this work, the suppression of CO in small bandwidth half‐doped manganite, Nd0.5Ca0.5MnO3 (NCMO) has been demonstrated by reducing the particle size. Bulk and nano forms of NCMO materials are prepared by sol‐gel technique and characterized by various physicochemical techniques. The magnetic and magnetotransport properties of bulk samples show the exciting features, viz., CO, training and irreversible metamagnetic effects, whereas these features are significantly suppressed in the nanoscale sample. The observed behavior in the present investigation is explained in terms of magnetic phase separation and spin memory effects. Our investigation establishes a combined effect of surface spin disorder and lattice strains for the suppression of CO behavior in NCMO manganite system.

Fabrication and temperature dependent magnetic properties of Co-Ni nanotube arrays

Parallel arrays of Co-Ni nanotubes with external diameter of 150 nm, wall thickness of 20 nm, and length of about 0.8 μm were successfully fabricated in ion-track etched polycarbonate (PC) templates by electrochemical deposition. The morphology and composition of the nanotubes were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDS). Temperature dependent magnetic analyses were performed with superconducting quantum interference device (SQUID) magnetometer which indicate that saturation magnetization of the nanotubes follows modified Bloch's law in the range 80–300 K. An abrupt deviation of the experimental curve (increase in magnetization) was observed from the modified Bloch's law below 80 K, which can be attributed to the disordered surface spins in the nanotubes due to the finite size effects. Finally, it was found that coercivity measured at different temperatures follows Kneller's law within the premises of Stoner–Wohlfarth model for ferromagnetic nanostructures.

Spin canting and surface spin disorder in Ni substituted Co-Cd ferrite nanoparticles synthesized by fuel deficient combustion method

The nanocrystalline Cd0.5-XNiXCo0.5Fe2O4 (x = 0.0 to 0.5) ferrites were synthesized by auto combustion technique using hexamine as fuel. The substitution of non magnetic Cd2+ ions in the tetrahedral site causes canting on the octahedral site. The saturation magnetization increases with Ni substitution up to X = 0.3 and then decreases with further increase in Ni substitution at room temperature. This behaviour is consistent with Yafet–Kittel magnetic ordering. A three sub-lattices model (A, B1 and B2) has been used to describe the magnetic behaviour of the ferrites. Low temperature unsaturated magnetization behaviour indicates the core shell structure of the nanoparticles i:e ordered core spins and randomly oriented surface spins. Reduced remanent magnetization values (Mr/Ms) suggest that the samples have uniaxial anisotropy at room temperature. The variation of magnetizations of the samples with temperature was studied by ZFC–FC technique at three different applied fields of 250 Oe, 500 Oe and 1000 Oe. The nature of the curves showed a change with increase in Ni concentration. The blocking temperature and anisotropy increased with Ni concentration. The average crystallite size was found to be in the range of 6–13 nm. The lattice constant decreased with Ni concentration. The Dielectric property has been explained on the basis of Maxwell-Wagner's two layer model while A.C. susceptibility studies indicate single domain-superparamagnetic domain structure. The Curie temperature shifts to higher temperature with increase in Ni concentration.

Effect of bimetallic (Ca, Mg) substitution on magneto-optical properties of NiFe2O4 nanoparticles

Nanoparticles (NPs) of calcium and magnesium co-substituted nickel ferrite with the chemical composition CaxMgxNi1–2×Fe2O4 (0.00 ≤ x ≤ 0.05) NPs were prepared hydrothermally. The (Ca, Mg) co-substitution effect on morphological, structural, optical and magnetic properties of NiFe2O4 NPs was analyzed via FT-IR, XRD, TEM, SEM, DRS and VSM techniques. The magnetization of CaxMgxNi1–2×Fe2O4 (0.00 ≤ x ≤ 0.05) NPs were examined and analyzed at room temperature (300 K; RT) and low temperature (10 K). The specific magnetic parameters such as saturation magnetization Ms, remanence Mr, coercivity Hc, squareness ratio (SQR=Mr/Ms) and magnetic moment nB of Ni1–2×MgxCaxFe2O4 (0.00 ≤ x ≤ 0.05) NPs were determined and evaluated. The M(H) curves exhibit superparamagnetic behavior at RT and ferrimagnetic nature at 10 K. The substitution significantly changes the magnetizations and coercivity data. An increase in the Ms, Mr, Hc and nB values was observed in substituted products compared to pristine one. The highest values were obtained for x = 0.05 product. The SQR values are found to be below than 0.5, which can be attributed to surface spin disorder effects. The obtained magnetic results are primarily derived from the substitution with Mg and Ca that will strengthen the super exchange interactions.

Reduced surface spin disorder in ZrO2 coated γ-Fe2O3 nanoparticles

Surface spin disorder in microwave plasma synthesized zirconium dioxide (ZrO2) coated maghemite (γ-Fe2O3) nanoparticles have been studied by using AC and DC magnetic measurements. The inverse spinel structure of γ-Fe2O3 was confirmed by X-ray diffraction. The calculated average crystallite size of γ-Fe2O3 and ZrO2 phase was about 13 and 6 nm, respectively. Zero field cooled/field cooled measurements revealed average blocking temperature at 65 K. The fitted value of Keff deduced from simulation was higher than that of bulk γ-Fe2O3 magneto-crystalline anisotropy which is mainly due to surface spin disorder. However, it was lower than the reported value for uncoated γ-Fe2O3 nanoparticles, which is due to reduction in surface effects and interparticle interactions in ZrO2 coated nanoparticles. Below 25 K, a sharp increase in saturation magnetization was observed which is due to extra contribution of frozen surface spins to magnetism at low temperatures. The coercivity also showed a sharp increase below 25 K, which is due to presence of strong core-surface interactions at low temperatures. For AC susceptibility, Arrhenius law fit revealed weak interactions among the nanoparticles which were not strong enough to create a spin-glass state. In summary, ZrO2 coated γ-Fe2O3 nanoparticles showed reduced surface spin disorder and weak interparticle interactions which is due to non-magnetic ZrO2 coating.

Surface effects and spin glass state in Co3O4 coated MnFe2O4 nanoparticles

Surface effects and spin glass state have been studied in Co3O4 coated manganese ferrite (MnFe2O4) nanoparticles by using AC and DC magnetic measurements. Average crystallite size of Co3O4 coated MnFe2O4 nanoparticles was 7 nm as calculated by Debye–Scherrer’s formula. Simulated ZFC/FC revealed higher value of effective anisotropy (Keff) = 5 × 106 erg cm−3 of these nanoparticles as compared to bulk MnFe2O4 due to enhanced surface effects. Temperature dependent saturation magnetization followed the Bloch’s law. Temperature dependent coercivity showed sharp increase below 25 K due to strong surface anisotropy and exchange coupling at interface between ferrimagnetic core and antiferromagnetic surface. For frequency dependent AC-susceptibility, dynamic scaling law fit confirmed the spin glass behavior in Co3O4 coated MnFe2O4 nanoparticles. DC field dependent AC-susceptibility showed the suppression of energy barrier, reduced activation energy and decreased strength of interparticle interactions with increasing DC field. Slow spin relaxation in ZFC protocol further confirmed the presence of spin-glass behavior. All this analysis confirmed the existence of spin-glass behavior as attributed to disordered surface spins and interparticle interactions in these nanoparticles, which gets suppressed after the application of moderate DC field.