Zero-field Cooled Magnetization Curves Research Articles

Temperature dependent magnetic and electrical transport properties of lanthanum and samarium substituted nanocrystalline nickel ferrite and their hyperthermia applications

Structural, optical, temperature dependent magnetic and electrical properties have been investigated for the nanocrystalline NiFe1.97RE0.03O4 (RE = La3+ and Sm3+) compound. Without any signs of a secondary REFeO3 phase, the Rietveld refinement reveals a single-phase spinel structure possessing cubic symmetry for current samples. The decrease in lattice constant has been observed as a result of RE ions substitution. The TEM micrographs reveal nanosized particles with an average size of 35 ± 3 nm and 32 ± 3 nm for La3+ and Sm3+ substituted samples. EDX spectra of both samples show good compositional homogeneity. HRTEM micrographs of both samples show well resolved lattice fringes and their inter-planar spacing matches with that obtained from the Rietveld analysis of the XRD patterns. The concentration of Fe2+ and Fe3+ ions has been estimated from the XPS spectra analysis and it is found to be ∼ 22% and 78% for NiFe1.97La0.03O4 and, 20% and 80% for NiFe1.97Sm0.03O4 compound. The optical energy band gap for rare earth substituted samples is found to be more than that of pure nickel ferrite. The value of saturation magnetization (Ms) and magneto-crystalline anisotropy constant (K1) estimated from the “Law of Approach to Saturation” equation for rare earth substituted samples are found to be less than that of pure nickel ferrite. However, an enhanced value of coercivity has been observed with RE substitution. ZFC (zero field cooling) and FC (field cooling) DC magnetization curves measured at 100 Oe in the temperature range of 60–400 K reveal a combination of weak and intermediate forms of magnetic interaction between the particles for both samples. Temperature dependent analysis of saturation magnetization and coercivity employing modified Bloch’s law and Kneller’s relation supports the nanomagnetic behavior of both samples. Under an AC magnetic field of 14.92 kA/m and frequency 337 kHz, the SAR and ILP values have been found to be ∼ 331 W/g and 4.22 nHm2/kg for NiFe1.97La0.03O4 and, ∼ 241 W/g and 3.21 nHm2/kg for NiFe1.97Sm0.03O4. The dielectric relaxation data have been analyzed at various temperatures ranging from 40 to 300 0C over the frequency range 100 Hz-1 MHz using electrical impedance spectroscopy. The dielectric constant for rare earth substituted samples is found to be more and their AC conductivity is observed to be less than that of pure nickel ferrite. The analysis of temperature dependent AC conductivity employing Jonscher’s power law suggests that the charge carrier’s conduction mechanism of both samples follows the small polaron hopping mechanism below 200 0C, thereafter; it follows the correlated barrier hopping mechanism. The activation energy estimated by imaginary impedance spectra is found to be 0.411 eV and 0.398 eV for NiFe1.97La0.03O4 and NiFe1.97Sm0.03O4 which is more than that of pure nickel ferrite. The Cole-Cole plots at different temperatures suggest the presence of non-Debye-type relaxation. The modeling of Cole-Cole plots with equivalent circuits for both samples confirms the contribution of both grain and grain boundary in electrical conduction. The estimated stretching exponential factor by fitting the modified KWW (Kohlrausch-Williams-Watts) equation to the imaginary modulus curve reveals relatively more dipole-dipole interaction in NiFe1.97Sm0.03O4 than that of the La3+ substituted sample.

Non-equilibrium magnetic response in concentrated spin-glass Au0.89Fe0.11 alloy

We report a detailed study of dc magnetization and ac susceptibility performed on the zero field cooled (ZFC) and field cooled (FC) state of polycrystalline Au0.89Fe0.11 alloy. The temperature variation of ZFC and FC dc magnetization at low fields shows a distinct peak around Tf = 33 K, which indicates the cooperative freezing of the finite size spin clusters. A weak thermomagnetic irreversibility between ZFC and FC magnetization appears at a temperature Tir, which is slightly below Tf. Further down the temperature at Tsh<Tir, the ZFC and FC magnetization curves exhibit strong thermomagnetic irreversibility. The ac susceptibility recorded after cooling in presence of field shows significant dispersion below Tf, and also, the FC state of Au0.89Fe0.11 alloy exhibits a pronounced memory effect in the dc magnetization which clearly underline that the FC state is not an equilibrium state. In contrast to the general perception obtained through the mean-field theories of thermodynamic phase transition in spin-glass envisaging the FC state to be an equilibrium state, the present experimental results clearly indicate that the energy landscape of the FC state of Au0.89Fe0.11 alloy is a nontrivial one.

Grain size influence upon magnetic behavior at nanoscale. A computational approach

In this work, a study of magnetic properties of nanogranular films constructed with different grain sizes is presented. The total energy of the system is proposed by mean a Hamiltonian with four contributions, which was solved using the Monte Carlo Method. The proposed terms include: (i) The exchange interactions, which consider the stress at grain boundaries. (ii) Three anisotropy contributions: cubic magnetocrystallinity in the grain cores, the Neel’s approximation in the film surface and the cubic magnetoelastic distortion at the grain boundaries. (iii) The dipolar interactions with a cutoff radius of five replicas and (iv) the energy contribution from the external magnetic field. Hysteresis loops and the temperature dependence of the magnetization under the zero-field-cooled (ZFC) and field-cooled (FC) conditions were computed. The used parameters, such as grain size, grain polydispersity, exchange values among others were chosen considering typical values reported from experimental works. The ZFC magnetization curve shows a broader maximum of which determines the blocking temperature. This maximum moves towards higher temperatures as the grain volume increases, fact related to greater stability of the ferromagnetic (FM) coupling on those grains with larger sizes, being a direct consequence of the decrease in the number of atoms located at the boundaries and of the reduction in the surface disorder. The hysteresis loops features, as well as the trend of some parameters (e.g., coercive field and remanence magnetization) as function of grain size, are in acceptable agreement with the experimental data tendencies. Local anisotropy in boundaries observed around coercive field explains grain size dependence of coercive field at nanoscale.

Synthesis and characterizations of magnesium and titanium doped M-type barium calcium hexaferrites by a solid state reaction method

Magnesium and titanium co-substituted M-type barium calcium hexaferrites Ba0.5Ca0.5Fe12-2xMgxTixO19 (0.0 ≤ x ≤ 0.5) have been synthesized by a solid state reaction method and characterized by XRD, XPS, FTIR, SEM, SQUID. XRD patterns confirm single phase M-type hexaferrite and various parameters such as lattice constants, cell volume, average grain size, X-ray density and profile parameters have been calculated from XRD data. XPS survey and FTIR spectra confirm that hexagonal crystal structure is completely formed, which are correlate closely with the results of Reitveld refined XRD patterns. SEM images of the magnetic powders confirm the hexagonal platelet-like shape. The apparent irreversibility between the zero-field-cooling (ZFC) and field-cooling (FC) curves is observed below 390 K, and the ZFC magnetization curves display a broad peak at a temperature TM. The results are discussed within the framework of random particle assembly model and associated with the magnetic domain wall motion. The saturation magnetization Ms slightly increases with increasing x up to 0.2 at 300 K, and then decreases for x > 0.2. The first anisotropy constant (K1), magnetic anisotropy field (Ha) and coercivity (Hc) decrease with increasing x. These results are discussed in light of the cationic distributions based on the results of the structural refinements.

Magnetic properties of hematite (α − Fe2O3) nanoparticles synthesized by sol-gel synthesis method: The influence of particle size and particle size distribution

Abstract Using the sol-gel method we synthesized hematite (α − Fe2O3) nanoparticles in a silica matrix with 60 wt % of hematite. X-ray diffraction (XRD) patterns and Fourier transform infrared (FTIR) spectra of the sample demonstrate the formation of the α − Fe2O3 phase and amorphous silica. A transmission electron microscopy (TEM) measurements show that the sample consists of two particle size distributions of the hematite nanoparticles with average sizes around 10 nm and 20 nm, respectively. Magnetic properties of hematite nanoparticles were measured using a superconducting quantum interference device (SQUID). Investigation of the magnetic properties of hematite nanoparticles showed a divergence between field-cooled (FC) and zero-field-cooled (ZFC) magnetization curves and two maxima. The ZFC magnetization curves displayed a maximum at around T B = 50 K (blocking temperature) and at T M = 83 K (the Morin transition). The hysteresis loop measured at 5 K was symmetric around the origin, with the values of coercivity, remanent and mass saturation magnetization H C10K ≈ 646 A/cm, (810 Oe), M r10K = 1.34 emu/g and M S10K = 6.1 emu/g respectively. The absence of both coercivity (HC300K = 0) and remanent magnetization (Mr300K = 0) in M(H) curve at 300 K reveals super-paramagnetic behavior, which is desirable for application in biomedicine. The bimodal particle size distributions were used to describe observed magnetic properties of hematite nanoparticles. The size distribution directly influences the magnetic properties of the sample.

Dielectric, magnetic and magneto-dielectric properties of (La, Co) co-doped BiFeO3

BLFCO [Bi1−xLaxFe1−xCoxO3 (x = 0.00, 0.01, 0.03, 0.05, 0.10)] ceramics were synthesized using sol-gel technique. The effect of Lanthanum (La) and Cobalt (Co) co-doping in BiFeO3 (BFO) on structural, electrical, magnetic and magneto-dielectric (MD) properties are systematically investigated. The phase purity and crystal structures were studied using x-ray diffraction technique. The dielectric properties [dielectric constant (ε) and tangent loss (tanδ)] were studied as a function of frequency and temperature for pure and co-doped BFO samples. The magnetization study reveals that the ferromagnetic (FM) properties in La and Co co-doped samples enhance with an increasing dopant concentration. The coercive field is found to increase with the doping concentration suggest a competition between the antiferromagnetic and FM ordering. The zero-field-cooled (ZFC) and field-cooled (FC) magnetization measurements of these samples also confirm the change in the magnetic behaviour of BLFCO as doping concentration increases. ZFC and FC magnetization curves split at low temperature with sharp cusps suggest a spin-glass like behaviour of BLFCO. The pure and co-doped samples exhibit a MD coupling effect. Furthermore, the anomaly near the magnetic phase transition (around the Neel temperature of BFO) implies the MD coupling in BLFCO samples.

Tracking of the electronic re-ordering in Fe3O4/OA nanoparticles using magnetometry

Abstract We performed structural and magnetic investigation of Fe3O4/OA nanoparticles prepared by solvothermal method. Based on XRD, TEM and FTIR examination, investigated sample contained monodisperse, 5 nm-sized spherical magnetite nanoparticles coated with oleic acid. Magnetic measurement of the hysteretic curves at 5 K in zero-field cooled (ZFC) and field-cooled (FC) regime confirmed presence of exchange bias effect. ZFC/FC measurements were performed in applied magnetic fields: 10 Oe, 100 Oe, 200 Oe, 500 Oe and 1000 Oe. Zero-field cooled magnetization curves recorded in 10 Oe and 100 Oe exhibited behavior characteristic for low coercivity Nanomaterials. Same sample exposed to the ZFC/FC measurement protocol in 200 Oe, 500 Oe and 1000 Oe showed increase in ZFC magnetization in the certain temperature range. The observed feature is attributed to the local changes in the magnetite electronic structure, occurred through the spin reorientation process.

Ferromagnetic ordering and electromagnons in microwave synthesized BiFeO3 thin films

Abstract Bismuth iron oxide sols are synthesized using microwave radiations with variation in microwave power in the range of 180 W–1000 W. Magnetic analysis of bismuth iron oxide sols indicate ferromagnetic behavior at microwave powers of 540–720 W. Paramagnetic behavior is observed at microwave powers of 180 W and 270 W. Mixed ferromagnetic and antiferromagnetic behavior of sols is obtained at microwave power of 360 W, 450 W and 810 W–1000 W. Sols synthesized using microwaves are spin coated onto copper substrates and annealed at 300 °C, in the presence of magnetic field, to obtain bismuth iron oxide thin films. X-ray diffraction patterns of thin films indicate amorphous behavior at low microwave power of 180 W and 270 W. Mixed BiFeO3 and Bi25FeO40 phases are obtained at microwave powers of 360 W and 450 W. Transition to phase pure BiFeO3 takes place at microwave power of 540–720 W. Mixed BiFeO3 and Bi2Fe4O9 phases are obtained at high microwave powers of 810 W–1000 W. Energy dispersive X-ray spectrum also confirms formation of BiFeO3 phase with 66.21%, 17.63% and 15.20% for Bi, Fe and O, respectively. Strong ferromagnetic behavior with low magnetocrystalline anisotropy is observed for films prepared using microwave power of 540–720 W. High saturation magnetization of 26.38 emu/cm3 is obtained at microwave power of 720 W. Field cooled and zero-field cooled magnetization curves show spin orientation transition at ∼80 K. Low temperature Raman analysis indicates presence of electromagnons with anomaly observed at 80 K.

Spin glass freezing, magnetocapacitance and dielectric anomalies in 0.3NiFe2O4-0.7BiFeO3 nanocomposite

Abstract We report on magnetocapacitance, spin dynamics through AC succeptability measurements and dielectric anomalies in 0.3NiFe2O4 – 0.7BiFeO3 nanocomposite. Structural studies (purity and phase) and surface morphology of the 0.3NiFe2O4 – 0.7BiFeO3 nanocomposite were examined by X-ray diffraction (XRD), Raman Spectra, transmission electron microscopy (TEM) and scanning electron microscopy (SEM). TEM and SEM images indicate the uniform distribution of nanoparticles in parent pahase as well as in the composite. Energy dispersive X-ray (EDX) analysis indicates the presence of Ni, Fe, O and Bi elements in 0.3NiFe2O4 – 0.7BiFeO3 nanocomposite. Magnetization measurements were performed at 300 K, 50 K and at 2 K and the Field Cooled (FC) and Zero Field Cooled (ZFC) measurements were carried out from 350 K to 2 K. ZFC and FC magnetization curves exhibit a hump around 30 K indicating the existence of competing magnetic interactions and possible spin galss behaviour. AC succeptibility measurements at various frequencies confirm the spin glass freezing behaviour of 0.3NiFe2O4 – 0.7BiFeO3 nanocomposite around 25 K with the relaxation time and activation energy in the order of 10−7 s and 0.938 meV respectively. The dielectric constant and dielectric loss at various frequencies and temperatures indicate the dielectric relaxation behaviour of 0.3NiFe2O4 – 0.7BiFeO3 nanocomposite. A rapid increase in the dielectric constant near the antiferromagnetic transition temperature (TN ∼ 643 K) of BiFeO3 indicates the existence of magneto-electric coupling in this composite. Magnetocapacitance (MC) of 0.3NiFe2O4 – 0.7BiFeO3 composite was measured at room temperature and the MC values of 13.5% and 7.8% (at 5000 Oe) are observed when the pellet is perpendicular and parallel to the magneticfield direction respectively. The presented magnetic and dielectric studies reveal the existence of magnetoelectric coupling and spin glass frezing of 0.3NiFe2O4 – 0.7BiFeO3 nanocomposite.

Effect of annealing on the magnetic properties of zinc ferrite thin films

We report on the magnetic properties of zinc ferrite thin film deposited on SrTiO3 single crystal using pulsed laser deposition. X-ray diffraction result indicates the highly oriented single phase growth of the film along with the presence of strain. In comparison to the bulk antiferromagnetic order, the as-deposited film has been found to exhibit ferrimagnetic ordering with a coercive field of 1140Oe at 5K. A broad maximum, at ≈105K, observed in zero-field cooled magnetization curve indicates the wide grain size distribution for the as-deposited film. Reduction in magnetization and blocking temperature has been observed after annealing in both argon as well as oxygen atmospheres, where the variation was found to be dependent on the annealing temperature.

Effect of Disorder and Dipolar Interactions in Two-Dimensional Assemblies of Iron-Oxide Magnetic Nanoparticles

We examine the magnetic properties of two-dimensional monolayers of iron-oxide magnetic nanoparticles arranged on a hexagonal lattice and interacting through magnetic dipole forces. Monte Carlo simulations are used to reconstruct the zero-field cooled magnetization curve, as well as other relevant quantities (Zeeman and dipolar energies). The blocking temperature is found to increase with shorter interparticle distance. The effect of structural disorder is studied by randomly removing some nanoparticles from their lattice sites. Surprisingly, it is found that disorder has little impact on the global magnetic structure of the sample. The numerical results are compared to existing experimental measurements.

Magnetic properties of NiO (nickel oxide) nanoparticles: Blocking temperature and Neel temperature

Crystalline nickel oxide (NiO) nanoparticles dispersed in an amorphous silica matrix have been prepared by a sol–gel combustion synthesis method. The sample was characterized using X-ray powder diffraction, energy-dispersive X-ray spectroscopy, scanning electron microscopy, transmission electron microscopy and superconducting quantum interference device magnetometry. The nanoparticles were spherical with an average size of about 5 nm and narrow particle size distribution. The magnetization measurements revealed two maxima in the zero-field cooled magnetization curve at 5 K and 56 K. An analysis of DC and AC magnetic susceptibility measurements revealed that the higher temperature maximum corresponded to the onset of antiferromagnetic ordering within the particles. The Néel temperature of TN = 56 K is much lower than in bulk NiO. The 5 K maximum corresponded to the particle blocking temperature TB. The frequency dependence of TB was successfully fitted using the Vogel–Fulcher law, indicating the presence of weak inter-particle interactions (C1 = 0.08 and C2 = 0.76). A Curie–Weiss law fit to the magnetic susceptibility above TN revealed antiferromagnetic exchange interactions with a Curie–Weiss temperature of θ = 24 K and an effective magnetic moment of μeff = 2.87 μB per NiO unit. We conclude that these magnetic properties can be attributed to the smallness of the NiO nanoparticles and to crystal lattice defects.

Solution-combustion synthesis and magnetodielectric properties of nanostructured rare earth ferrites

Rare earth ferrites exhibit remarkable magnetodielectric properties that are sensitive to the crystallite size. There is a major challenge to produce these materials in nanoscale due to particles conglomeration during the ferrite nucleation and synthesis. In this paper we report the fabrication of nanostructured particles of rare earth ferrites in the Me-Fe-O system (Me = Y, La, Ce, and Sm) by Solution-Combustion Synthesis (SCS). The yttrium, lanthanum, cerium, samarium and iron nitrates were used as metal precursors and glycine as a fuel. Thermodynamic calculations of Y(NO3)3-2Fe(NO3)3−nC2H5NO2 systems producing Y3Fe5O12 predicted an adiabatic temperature of 2250 K with generating carbon dioxide, nitrogen and water vapor. The considerable gas evolution helps to produce the synthesized powders friable and loosely agglomerated. Adjusting the glycine/metal nitrates ratio can selectively control the crystallite size and magnetodielectric properties of the ferrites. Increasing the glycine content increased the reaction temperature during the SCS and consequently the particle size. Magnetization of zero-field-cooled (ZFC) and field-cooled (FC) ferrites in the temperature range of 1.9–300 K showed different patterns when the fraction of glycine was increased. Analysis of ZFC and FC magnetization curves of annealed samples confirmed that nanoparticles exhibit superparamagnetic behavior. The increasing concentration of glycine leads to escalation of blocking temperature. Reduction of dielectric permittivity (ɛr) toward frequency indicates the relaxation processes in the composites, and the values of ɛr are shifted upward along the operating temperature.

Effects of aluminum substitution on the crystal structure and magnetic properties in Zn2Y-type hexaferrites

Crystal structure and magnetic properties of multiferroic Y-type hexaferrites Ba0.5Sr1.5Zn2(Fe1−xAlx)12O22 (x = 0, 0.04, 0.08, and 0.12) were investigated. The Z- and M-type impurity phases decrease with increasing Al content, and the pure phase samples can be obtained by modulating Al-doping. Lattice distortion exists in Al-doped samples due to the different radius of Al ion (0.535 Å) and Fe ion (0.645 Å). The microstructural morphologies show that the hexagonal shape grains can be observed in all the samples, and grain size decreases with increasing Al content. As for magnetic properties of Ba0.5Sr1.5Zn2(Fe1−xAlx)12O22, there exist rich thermal- and field-driven magnetic phase transitions. Temperature dependence of zero-field cooling magnetization curves from 5 K to 800 K exhibit three magnetic phase transitions involving conical spin phase, proper-screw spin phase, ferromagnetic phase, and paramagnetic phase, which can be found in all the samples. Furthermore, the phase-transition temperatures can be modulated by varying Al content. In addition, four kinds of typical hysteresis loops are observed in pure phase sample at different temperatures, which reveal different magnetization processes of above-motioned magnetic spin structures. Typically, triple hysteresis loops in low magnetic field range from 0 to 0.5 T can be observed at 5 K, which suggests low-field driven magnetic phase transitions from conical spin order to proper-screw spin order and further to ferrimagnetic spin order occur. Furthermore, the coercive field (HC) and the saturation magnetization (MS) enhance with increasing Al content from x = 0 to 0.08, and drop rapidly at x = 0.12, which could be attribute to that in initial Al-doped process the pitch of spin helix increases and therefore magnetization enhances, but conical spin phase eventually collapses in higher-concentration Al-doping.

Magnetic properties of hematite (α-Fe2O3) nanoparticles prepared by hydrothermal synthesis method

Hematite (α-Fe2O3) nanoparticles are successfully synthesized by using the hydrothermal synthesis method. An X-ray powder diffraction (XRPD) of the sample shows formation of the nanocrystalline α-Fe2O3 phase. A transmission electron microscopy (TEM) measurements show spherical morphology of the hematite nanoparticles and narrow size distribution. An average hematite nanoparticle size is estimated to be about 8nm by TEM and XRD. Magnetic properties were measured using a superconducting quantum interference device (SQUID) magnetometry. Investigation of the magnetic properties of hematite nanoparticles showed a divergence between field-cooled (FC) and zero-field-cooled (ZFC) magnetization curves below Tirr=103K (irreversibility temperature). The ZFC magnetization curve showed maximum at TB=52K (blocking temperature). The sample did not exhibit the Morin transition. The M(H) (magnetization versus magnetic field) dependence at 300K showed properties of superparamagnetic iron oxide nanoparticles (SPION). The M(H) data were successfully fitted by the Langevin function and magnetic moment μp=657μB and diameter d=8.1nm were determined. Furthermore, magnetic measurements showed high magnetization at room temperature (MS=3.98emu/g), which is desirable for application in spintronics and biomedicine. Core–shell structure of the nanoparticles was used to describe high magnetization of the hematite nanoparticles.

Investigation of cation distribution, electrical, magnetic properties and their correlation in Mn2-xCo2xNi1-xO4 films

X-ray photoelectron spectroscopy was performed on Mn2-xCo2xNi1-xO4 (x = 0.2, 0.4, 0.5, 0.6, and 0.8) series spinel films to determine their cation distributions. The results show that both Ni and Co ions exhibit bivalence, while Mn ions exhibit coexisting multivalence. The hopping frequency ν0 of each sample was obtained by combining cation distribution results and electrical measurement data. Analysis of the divergence at Neel temperature between field cooling and zero-field cooling magnetization curves revealed that the ferromagnetic coupling between octahedral site ions first leads to spontaneous magnetization at TC, after which a second spontaneous magnetization arises from the antiferromagnetic couple between tetrahedral and octahedral sites at a lower temperature TN. The correlation between hopping conductivity and magnetic transition temperature is discussed. We propose that the strength of the ferromagnetic couple between octahedral sites is subject to the product term of Nc(1 − c)ν0, which determines the ferromagnetic transition temperature (TC) of the system. Our results provide an avenue to understand the origin of magnetic interaction in small-polaron hopping semiconductors.

Investigation On Magnetic Behaviour Of BiFeO3: SPIN Glass View Point

Field cooled (FC) and zero field cooled (ZFC) magnetization measurements of Bismuth Ferrite (BiFeO3) multiferroic obtained by microwave-assisted gel combustion method are reported. The structural investigation of sythesized and calcined sample is done by XRD and the obtained data is well fitted with Rietveld refinement using full-pro software suite. M-H hysterisis shows that BFO nanoparticles exhibit ferromagnetic properties at room temperature, which is unusally observed in BFO. The M -T plot at H = 1 and 2 kOe shows that the FC and ZFC magnetization curve start to differ at below 331K and 236 K respectively with sharp cusp around 124 K revealing spin glass behaviour of BiFeO3. Both ferromagnetic properties and spin-glass-like behavior are observed in BFO nanoparticles. Ferromagnetic behavior is attributed to a partial destruction in nanoparticles of the long-wavelength cycloid spin structure expected in bulk BFO. Spin-glass-like behavior is assigned to diffusion of domain walls, with possible contributions from pinning of the cycloid spin structure at the nanoparticle surface.

Synthesis, structural and magnetic properties of self-organized single-crystalline nanobricks of chalcopyrite CuFeS2

A thermal pyrolysis method has been developed to synthesize the tetragonal phase of chalcopyrite magnetic semiconductor CuFeS2 nanoparticles. All nanoparticles have the same anisotropic brick-like shape, and the “bricks” are self-organized in a certain orientation, creating well-ordered nanocomposites. High-resolution transmission electron microscopy and electron diffraction data show that every nanobrick is a single crystal with a layered atomic structure and characteristic dimensions of about 5nm×20nm in plane. Magnetic measurements support the antiferromagnetic spin structure and reveal the appearance of a small ferromagnetic component below 60K. Magnetic anomalies observed in the zero-field cooled magnetization curves at low temperatures may be related to an appearance of magnetic moment at the Cu ion site. The Mössbauer spectra show that only about 50% of Fe atoms are in the magnetically ordered α-phase of chalcopyrite. The remaining Fe is non-magnetic and may be located either in the γ-phase of chalcopyrite or in isocubanite.

Magnetic structure of La2O3FeMnSe2: neutron diffraction and physical property measurements

We report on the characterization of the mixed layered lanthanum iron manganese oxyselenide La2O3FeMnSe2, where Fe and Mn share the same crystallographic position. The susceptibility data show a magnetic transition temperature of 76 K and a strong difference between field cooled and zero field cooled (ZFC) data at low fields. While the ZFC magnetization curve exhibits negative values below about 45 K, hysteresis measurement reveals, after an initial negative magnetic moment, a hysteresis loop typical for ferromagnetic material, pointing to competing ferromagnetic and antiferromagnetic interactions. Resistivity and dielectric permittivity measurements indicate that La2O3FeMnSe2 is a semiconductor. We performed x-ray diffraction at 295 K and neutron diffraction at 90 and 1.7 K. The nuclear and magnetic structure was refined in the space group I4/mmm with a = 4.11031 (3) Å and c = 18.7613 (2) Å at 295 K. We did not detect a structural distortion and the Fe and Mn atoms were randomly distributed. The magnetic order was found to be antiferromagnetic, with a propagation vector q = (0,0,0) and magnetic moments of 3.44 (5) μB per Fe/Mn atom aligned within the a–b plane. This magnetic order is different with respect to the pure Fe or Mn compositions reported in other studies.

Magnetic nanoparticles formed in glasses co-doped with iron and larger radius elements

A new type of nanoparticle-containing glasses based on borate glasses co-doped with low contents of iron and larger radius elements, Dy, Tb, Gd, Ho, Er, Y, and Bi, is studied. Heat treatment of these glasses results in formation of magnetic nanoparticles, radically changing their physical properties. Transmission electron microscopy and synchrotron radiation-based techniques: x-ray diffraction, extended x-ray absorption fine structure, x-ray absorption near-edge structure, and small-angle x-ray scattering, show a broad distribution of nanoparticle sizes with characteristics depending on the treatment regime; a crystalline structure of these nanoparticles is detected in heat treated samples. Magnetic circular dichroism (MCD) studies of samples subjected to heat treatment as well as of maghemite, magnetite, and iron garnet allow to unambiguously assign the nanoparticle structure to maghemite, independently of co-dopant nature and of heat treatment regime used. Different features observed in the MCD spectra are related to different electron transitions in Fe3+ ions gathered in the nanoparticles. The static magnetization in heat treated samples has non-linear dependence on the magnetizing field with hysteresis. Zero-field cooled magnetization curves show that at higher temperatures the nanoparticles occur in superparamagnetic state with blocking temperatures above 100 K. Below ca. 20 K, a considerable contribution to both zero field-cooled and field-cooled magnetizations occurs from diluted paramagnetic ions. Variable-temperature electron magnetic resonance (EMR) studies unambiguously show that in as-prepared glasses paramagnetic ions are in diluted state and confirm the formation of magnetic nanoparticles already at earlier stages of heat treatment. Computer simulations of the EMR spectra corroborate the broad distribution of nanoparticle sizes found by “direct” techniques as well as superparamagnetic nanoparticle behaviour demonstrated in the magnetization studies.