Morin Transition Research Articles

Morin transition in Hematite nanoparticles analyzed by neutron diffraction

Hematite (α-Fe2O3) undergoes a first order spin reorientation transition called the Morin transition: upon cooling, the moments align antiferromagnetically along the rhombohedral axis, and the net magnetic moment goes to zero. Morin transition temperature is around to 260K in bulk materials and depends on the mean particle size. In this work, the Morin transition has been studied by neutron diffraction as function of temperature and applied magnetic field in 47 nm nanoparticles. The Rietveld analysis of the diffraction spectra around the Morin transition shows a similar behavior to that found in bulk samples. On the other side, the magnetic field induced phase transformation has been analyzed.

Laser-induced shift of the Morin point in antiferromagnetic DyFeO_3

Imaging domain structure of antiferromagnetic DyFeO(3) reveals that intense laser excitation can control the temperature of the Morin transition from collinear to non-collinear spin state. Excitation of the antiferromagnet with femtosecond laser pulses with the central wavelength of 800 nm leads to a shift of the transition temperature over 1 K to higher values as if the light effectively cools the irradiated area down. It is suggested that the optical control of the Morin point can be a result of photo-ionization of Dy(3+) ions.

Fabrication and magnetism of α-Fe2O3 nanotubes via a multistep ac electrodeposition

Hematite Fe2O3 nanotubes were successfully synthesized by a multistep ac electro-deposition on anodic aluminum oxide templates remaining the barrier layers. X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), high-resolution transmission electron microscopy (HRTEM), and magnetic measurements were used to characterize the morphology, structure, and magnetic properties of α-Fe2O3 nanotubes. Based on the characterization results, a self-assemble mechanism for electro-deposition α-Fe2O3 nanotubes was proposed. The magnetic property investigation showed that hematite Fe2O3 nanotubes exhibit ferromagnetic behavior with the Morin transition.

Detection of an Anomalous Magnetic Transition in Hematite by Means of Derivative Microwave Absorption

We have investigated the derivative microwave power absorption (dP/dH) at X-band (8.8–9.8 GHz) in the hematite (a-Fe2O3) powders, for the 180-280 K temperature range. Two microwave absorption techniques are mainly used: the magnetically modulated microwave absorption spectroscopy (MAMMAS) and the extended field microwave absorption (EFMA). MAMMAS response showed distinctive features associated with the Morin transition at T Mo = 260 K, and below T Mo, a weak ferromagnetism is also observed. Additionally, EFMA technique is used to give further knowledge on this material, where this technique shows a change in microwave absorption dynamics in the vicinity of 200 K and it is associated with an anomalous magnetic transition, different to the Morin transition, and which is also confirmed by means of magnetic measurements and the low-field microwave absorption (LFMA).

Coexistence of blocked, metamagnetic and canted ferrimagntic phases at high temperature in Co–Nd ferrite nanorods

One-dimensional Co–Nd ferrite nanostructures were prepared by controlled co-precipitation method. Co–Nd ferrite nanorods exhibits different magnetic phases: blocking phase with the corresponding blocking temperature (TB) of about 350K, metamagnetic phase with the corresponding Morin transition temperature (TM) of about 550K and canted ferrimagntic phase with the corresponding Neel temperature (TN) of about 800K. These novel magnetic one-dimensional structures can potentially be used in nanoelectronic devices, magnetic sensors, and flexible magnets.

Strong shape-dependence of Morin transition in α-Fe2O3 single-crystalline nanostructures

Single-crystalline hematite (α-Fe2O3) nanorings (short nanotubes) and nanotubes were synthesized by a hydrothermal method. High-resolution transmission electron microscopy and selected-area electron diffraction confirm that the axial directions of both the nanorings and nanotubes are parallel to the crystalline c-axis. Intriguingly, the Morin transition occurs at about 210 K in the short nanotubes with a mean tube length of about 115 nm and a mean outer diameter of about 169 nm. However, it does not occur in the nanotubes with a mean tube length of about 317 nm and a mean outer diameter of about 148 nm. Detailed analysis of magnetization data, X-ray diffraction patterns, and room-temperature Mossbauer spectra demonstrates that this very strong shape-dependence of Morin transition is intrinsic to hematite. We explain this intriguing shape-dependence quantitatively, in terms of the opposite signs of the surface magnetic anisotropy constants of the surface planes parallel and perpendicular to the c-axis.

Morin transition temperature in (0001)-oriented α-Fe2O3 thin film and effect of Ir doping

The structural properties and Morin transition in c-plane-oriented α-Fe2O3 and Ir-doped α-Fe2O3 thin films have been investigated. The enhancement of the Morin transition temperature (TM) in α-Fe2O3 film by Ir doping has been demonstrated. The TM in the c-plane-oriented α-Fe2O3 thin film was determined from the temperature-dependent in-plane magnetization and change of coercivity (Hc); this TM value was found close to that of bulk α-Fe2O3. The spin directions of non-doped and Ir-doped α-Fe2O3 at room temperature were also estimated from conversion electron Mössbauer spectroscopy measurements. We confirmed that Ir doping dramatically enhances the TM of α-Fe2O3 thin film.

Effect of magnetic spins flipping process on the dielectric properties of α-Fe1.6Ga0.4O3 system

The α-Fe1.6Ga0.4O3 (Ga doped α-Fe2O3) sample has been stabilized in rhombohedral structure. The sample is a canted ferromagnet at 300 K and above. The spins structure starts flipping from in-plane direction to out of plane direction of the rhombohedral structure to exhibit an antiferromagnetic order below a typical temperature ∼ 215 K, known as Morin transition. The magnetic and dielectric properties of α-Fe1.6Ga0.4O3 system have been discussed in the temperature range 123 K to 350 K to examine the effect of magnetic spins flipping process on dielectric properties. The dielectric constant has shown an anomalous peak at ∼ 310 K, followed by a rapidly decrease of dielectric constant with temperature and becomes weakly temperature dependent below Morin transition. The temperature dependent dielectric constant is accompanied with the changes in electrical conductivity, dielectric loss and phase shift of the current with respect to applied ac voltage across the material. The magnetization and dielectric constant showed a linear relation over a wide range of temperature across the Morin transition. The dielectric constant at room temperature decreases under magnetic field, which indicates magneto-dielectric effect in the system. The signature of magneto-dielectric effect reveals a coupling between spins degrees of freedom (magnetic order) and charge degrees of freedom (electric polarization) in corundum structured non-traditional ferroelectric systems.

Mesoporous urchin-like α-Fe2O3 superstructures with high coercivity and excellent gas sensing properties

This paper reports the synthesis, morphology, magnetic properties and gas sensing characteristics of mesoporous urchin-like α-Fe2O3 superstructures prepared by one-step thermal decomposition of iron nitrate aqueous solution. The advantages of this fabrication technique include simplicity, convenience, efficiency, capability of scale-up fabrication, and good control of product structure and morphology. The urchin-like α-Fe2O3 superstructures are made of oriented aggregations of nanorods (diameter ∼ 10–130 nm; length ∼ 150–700 nm) with an overall size in the range of 0.5–5 μm. The nanobuilding blocks of the urchin-like superstructures are either single-crystal nanorods or a mixture of single-crystal knotted or branched sub-nanorods and elongated particles. Unusually high intrinsic coercivity, iHc, of up to 5.3 kOe and reduced Morin transition temperatures were observed for the urchin-like α-Fe2O3 superstructures. The urchin-like α-Fe2O3 samples showed excellent gas sensing performance for acetone, ammonia, and ethanol.

Spin orientation, structure, morphology, and magnetic properties of hematite nanoparticles

Monodisperse hematite (α-Fe2O3) nanoparticles were synthesized by forced hydrolysis of acidic Fe3+ solution. Rietveld analysis was applied to the X-ray powder diffraction data to refine the lattice constants and atomic positions. The lattice constants for a hexagonal unit cell were determined to be a ∼ 0.50327 and c ∼ 1.37521 nm. High resolution transmission electron microscopy was employed to study the morphology of the particles. Atomic scale micrographs and diffraction patterns from several zone axes were obtained. These reveal the high degree of crystallinity of the particles. A series of observations made on the particles by tilting them through a range of ±45° revealed the particles to be micaceous with stacking of platelets with well defined crystallographic orientations. The Morin transition in these nanoparticles was found to occur at 210 K, which is lower temperature than 263 K of bulk hematite. It was ascertained from the previous Mössbauer studies that the spin orientation for nano-sized hematite particle flips from 90° to 28° with respect to the c-axis of the hexagonal structure during the Morin transition, which is in contrast to that observed in bulk hematite where spin orientation flips from 90° to 0°.

Synthesis and microstructural properties of mixed iron–gallium oxides

Various mixed Fe–Ga oxides were prepared by the calcination of mixed Fe–Ga oxyhydroxide precursors (α-Fe1−xGaxOOH solid solutions, 0⩽x⩽1) at 500 or 1000°C. Isostructural corundum-type oxides α-Fe2O3 and α-Ga2O3, as well as their solid solutions α-Fe2−xGaxO3 in the entire concentration range (x=0, 0.4, 1, 1.6 and 2) were prepared by calcination at 500°C. An increase in the Ga content in α-Fe2−xGaxO3 resulted in a reduction of the unit cell size, a weakened hyperfine magnetic field, a disappearance of the Morin transition, an increase of magnetization at low temperatures, a shift in the position of IR bands to higher wavenumbers, and a lower intensity of absorption bands in the UV–Vis-NIR spectra. The calcination of Fe–Ga oxyhydroxides at 1000°C resulted in the formation of a rhombohedral α-Fe2−xGaxO3 phase for x=0 and 0.4. An orthorhombic GaFeO3 phase with Fe3+ cations located in 3 different octahedral sites was obtained by the calcination at 1000°C of mixed Fe–Ga oxyhydroxide with the Fe:Ga ratio of 1:1. A monoclinic phase β-Fe2−xGaxO3 (x=1.6 and 2) was obtained by the calcination at 1000°C of Fe–Ga oxyhydroxides with 80 and 100mol% Ga. The monoclinic β-Ga1.6Fe0.4O3 showed two strong luminescence peaks in the blue region of the PL spectrum.

Effect of Surfactant on Magnetic and Optical Properties of α-Fe2O_3Nanoparticles

Hematite (α-Fe2O3) nanoparticles and hematite nanoparticles coated with polyvinylpyrrolidone (PVP) are synthesized chemically by co-precipitation. Prolysis and microemulsion methods respectively. An average size of nanoparticles (both coated and uncoated) was found by the broadening of the X-ray di raction peaks using Scherrer's formula. It was found that coating reduced particle sizes. The attachment of the polymer on the surface of particles was con rmed by the Fourier transform infrared spectroscopy and thermogravimetric analysis. Optical ultraviolet/visible re ectance test also indicated the presence of PVP in coated nanohematites. In magnetic studies, it was observed that coating does not change main magnetic character of the α-Fe2O3 nanoparticles however it reduces DC magnetization (as a function of applied eld and temperature) to a considerable amount. Moreover it was observed that after coating Morin transition temperature TM and its width ∆TM shifts to lower values, which is another indication of size reduction.

Fabrication of α-Fe2O3 hexagonal disc/SnO2 nanoparticle semiconductor nanoheterostructures and its properties

Semiconductor nanoheterostructures with different weight ratios were successfully fabricated by hydrothermal method. X-ray diffraction analysis indicates that the samples were composed of α-Fe2O3 and SnO2. HRTEM images confirmed the immobilization of SnO2 nanoparticles on the surface of α-Fe2O3 hexagonal discs. Optical studies reveal that the band gaps of the samples enlarge from 1.862(2)eV to 1.9931(7)eV with increase in SnO2 weight ratio. Hysteresis behavior clearly displayed a shift of loops toward field axis for nanoheterostructures. From the ZFC curve an increase of Morin transition from 230K to 240K was noticed when the SnO2 weight ratio increases.

Dual Morin transitions during the oriented attachment process

In this work, two Morin transitions were observed in hematite product at the intermediate stage during the oriented attachment process, which is universal and plays an important role in biogeochemical processes that shape Earth's near-surface environments. Seemingly, these Morin transitions originate from the existence of two sizes of hematite particles. While in-depth study reveals that they were also interdependent on the defreezing behaviors of two different spin configurations at their interfaces. Such pioneering study on the magnetic performance during the oriented attachment process provides a fundamental and new physical basis for the materials' potential applications.

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.

Fluorescent nanonetworks: a novel bioalley for collagen scaffolds and tissue engineering.

Native collagen is arranged in bundles of aligned fibrils to withstand in vivo mechanical loads. Reproducing such a process under in vitro conditions has not met with major success. Our approach has been to induce nanolinks, during the self-assembly process, leading to delayed rather than inhibited fibrillogenesis. For this, a designed synthesis of nanoparticles - using starch as a template and a reflux process, which would provide a highly anisotropic (star shaped) nanoparticle, with large surface area was adopted. Anisotropy associated decrease in Morin temperature and superparamagnetic behavior was observed. Polysaccharide on the nanoparticle surface provided aqueous stability and low cytotoxicity. Starch coated nanoparticles was utilized to build polysaccharide - collagen crosslinks, which supplemented natural crosslinks in collagen, without disturbing the conformation of collagen. The resulting fibrillar lamellae showed a striking resemblance to native lamellae, but had a melting and denaturation temperature higher than native collagen. The biocompatibility and superparamagnetism of the nanoparticles also come handy in the development of stable collagen constructs for various biomedical applications, including that of MRI contrast agents.

Glycine assisted hydrothermal synthesis of α-Fe2O3 nanoparticles and its size dependent properties

α-Fe2O3 with different sizes and morphologies have been successfully synthesized via a biomolecule assisted hydrothermal method. The samples are characterized by X-ray diffraction, Fourier transform infrared spectroscopy, transmission electron microscopy, UV–visible diffuse reflectance spectroscopy and vibrating sample magnetometer. A clear blue shift is observed in the optical absorption spectra, indicating that band gap increases with size reduction. Magnetic measurements reveal that coercivity and Morin transition temperature decreases as the size decreases.

Morphological and magnetic properties of the hydrothermally prepared α-Fe2O3 nanorods

α-Fe2O3 nanorods were synthesized via hydrothermal method. X-ray powder diffraction revealed the formation of rhombohedral α-Fe2O3 single crystal phase with fiber texture. Scanning and transmission electron micrographs analyses showed that the rhombohedral α-Fe2O3 has nanorods in shape with diameters of 40–85 nm and lengths of 150–45,000 nm. Isothermal magnetization vs. applied magnetic field curves measured at room and liquid nitrogen temperatures displayed a variation on magnetic ordering: from weak ferromagnetism at room temperature to not hysteretic behavior at liquid nitrogen temperature that is well described by a Langevin function. Moreover, the zero field cooling-field cooling curves under applied magnetic field of 100 Oe confirms the decreasing of Morin temperature transition due to nanometric size of the samples.

Terahertz spectroscopic observation of spin reorientation induced antiferromagnetic mode softening in DyFeO3 ceramics

Dysprosium orthoferrite has been found to undergo the Γ4(Gx,Fz)→Γ1(Gy) spin reorientation transition (Morin transition) at about 37K. In this study, the Morin transition in DyFeO3 ceramic is investigated using a temperature-tunable terahertz spectroscopy system. It is experimentally demonstrated that the DyFeO3 ceramics exhibit obvious terahertz absorption and emission at the antiferromagnetic resonant frequency. It is also shown that the lifetime of magnetic dipole emission remarkably shortens and the quasi-antiferromagnetic mode softens below 198K, which can be ascribed to the decreased terahertz absorption and reduced internal magnetic field induced by the spin reorientation, respectively. This contributes to the research of rare earth orthoferrites׳ terahertz electromagnetic properties.

Local vibrational dynamics of hematite (α-Fe₂O₃) studied by extended x-ray absorption fine structure and molecular dynamics.

The local vibrational dynamics of hematite (α-Fe2O3) has been investigated by temperature-dependent extended x-ray absorption fine structure spectroscopy and molecular dynamics simulations. The local dynamics of both the short and long nearest-neighbor Fe-O distances has been singled out, i.e., their local thermal expansion and the parallel and perpendicular mean-square relative atomic displacements have been determined, obtaining a partial agreement with molecular dynamics. No evidence of the Morin transition has been observed. More importantly, the strong anisotropy of relative thermal vibrations found for the short Fe-O distance has been related to its negative thermal expansion. The differences between the local dynamics of short and long Fe-O distances are discussed in terms of projection and correlation of atomic motion. As a result, we can conclude that the short Fe-O bond is stiffer to stretching and softer to bending than the long Fe-O bond.