In-field Mossbauer Spectroscopy Research Articles

Local Structure of Core–Shell MnFe2O4+δ-Based Nanocrystals: Cation Distribution and Valence States of Manganese Ions

We investigate the local structure of nanoparticles based on a manganese ferrite core surrounded or not by a maghemite layer obtained after hydrothermal surface treatment. Results of X-ray powder diffraction (XRD) and neutron powder diffraction (NPD) measurements are crossed with those of in-field Mossbauer spectroscopy and X-ray absorption spectroscopy (XANES/EXAFS) to study the valence state of Mn ions and the cation distribution at interstitial sites of the core–shell nanoparticle structure. Linear combination fitting of XANES data clearly indicates the existence of mixed valence states of Mn cations in the Mn ferrite phase. As a direct consequence, it induces nonequilibrium cation distributions in the nanoparticle core with the presence of a large amount of Mn cations at octahedral sites. The quantitative results of the inversion degree given by NPD, Mossbauer spectroscopy measurements, and EXAFS are in good accordance. It is also shown that both the proportions of each oxidation degree of Mn ions and...

Size and surface effects in the magnetic order of CoFe2O4 nanoparticles

Abstract In this work, we have focused on the size dependence of the magnetic properties and the surface effects of CoFe2O4 nanoparticles synthesized by high-temperature chemical method with diameter d~2, 4.5, and 7 nm, with narrow size distribution. transmission electron microscopy (TEM) images and X-ray diffraction (XRD) profiles indicates that samples with 7 and 4.5 nm present a high crystallinity while the 2 nm sample has a poor one. We have investigated by magnetization measurements and in-field Mossbauer spectroscopy the influence of the surface in the internal magnetic order of the particles. Particles with d=7 nm have almost single domain behavior and the monodomain occupies approximately the whole particle. In the sample with d=4.5 nm the surface anisotropy is large enough to alter the ferrimagnetic order in the particle shell. Then, a surface/volume ratio of ~60% is the crossover between a single domain nanoparticle and a frustrated order in a magnetic core–shell structure, due to the competition between surface anisotropy and exchange interaction+crystalline anisotropy in cobalt ferrite. In the d=2 nm sample the poor crystallinity and the large surface/volume ratio avoid the ferrimagnetic order in the particle down to T=5 K.

Ferrofluids from Magnetic−Chitosan Hybrids

Magnetic nanoparticles coated with chitosan or quaternized chitosan were synthesized by a new route and characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), temperature dependent and in-field Mossbauer spectroscopy, FTIR, thermal analysis (TG/DTA), zeta potential, and magnetic measurements. The prepared ferrofluids were stable for a relatively long time. Mossbauer spectra recorded in an external magnetic field of 5 T establish the formation of “nonstoichiometric magnetite” exhibiting the perfect ferrimagnetic ordering with a low degree of spin canting and frustration. Magnetic measurements confirm the desired properties of the magnetic chitosan for biomedical applications including the superparamagnetic character at room temperature and a high saturation magnetization achievable at low applied fields. The successful and full capping of particles is indicated mainly from magnetic and DTA data through the suppressed interparticle interactions and a significant shift in the trans...

Nanocrystalline Nickel Ferrite, NiFe2O4: Mechanosynthesis, Nonequilibrium Cation Distribution, Canted Spin Arrangement, and Magnetic Behavior

Nickel ferrite (NiFe2O4) nanoparticles with an average crystallite size of about 8.6 nm were prepared by mechanochemical synthesis (mechanosynthesis). In-field Mossbauer spectroscopy and high-resolution TEM studies revealed a nonuniform structure of mechanosynthesized NiFe2O4 nanoparticles consisting of an ordered core surrounded by a disordered grain boundary (surface) region. The inner core of a NiFe2O4 nanoparticle is considered to possess a fully inverse spinel structure with a Neel-type collinear spin alignment, whereas the surface shell is found to be structurally and magnetically disordered due to the nearly random distribution of cations and the canted spin arrangement. As a consequence of frustrated superexchange interactions in the surface shell, the mechanosynthesized NiFe2O4 exhibits a reduced nonsaturating magnetization, an enhanced coercivity, and a shifted hysteresis loop. The study also demonstrates that one can tailor the magnetic properties of mechanosynthesized NiFe2O4 particles by suit...

Size Dependence of Magnetic Properties of Co-Ferrite Nanoparticles

Different-sized Co-ferrite nanoparticles synthesized from nanophase cobalt-iron hydroxide carbonate precursors in an inverse microemulsion system have been studied by using X-ray diffractometry, transmission electron microscopy, and zeroand in-field Mossbauer spectroscopy. The lattice constant a0 tends to decrease with decreasing particle size. At liquid-helium temperature, the magnetic hyperfine field at each site decreases with decreasing particle size. The particle size dependence of the site distribution and the superparamagnetic relaxation frequency is also examined.

Zero-field and in-field Mössbauer spectroscopy as a tool for structural and magnetic characterization of maghemite (γ-Fe2O3) nanoparticles

Nowadays, nanoparticles of maghemite (γ-Fe2O3) represent one of the most useful materials in modern advanced nanotechnological applications due to their superior magnetic properties. For their characterization,57Fe zero-field and in-field Mossbauer spectroscopy have proved themselves to be very powerful and effective tools which are crucial for an investigation of the local surrounding of iron atoms and observation of dynamic effects. The structural and magnetic characteristics of maghemite and its nanoparticles are thus discussed with regard to their zero-field and in-field Mossbauer spectra recorded at various temperatures and applied external magnetic fields. In addition, a special attention is also devoted to remarkable physical phenomena (superparamagnetism, spin canting) occurring largely in maghemite nanosized particles.

Sign determination of the57Fe electric field gradient in Al-based crystals and icosahedral quasicrystals

The icosahedral quasicrystals i-AlMn isomorphically suostituted by 28 at.% Fe or by (CrFe) are studied for the first time by in-field Mossbauer spectroscopy (MS) in order to determine the sign and asymmetry parameter of the dominant EFG term. In addition, the orthorhombic o-Al(MnFe) and cubic α- and hexagonal β-Al(MnFe)Si phases are studied. We show that previous MS results are inadequate to determine if there are two sites in the i-AlMn structure in the ratio of the golden number. Our results for i-Al(MnFe) show that the dominant EFG is negative, with asymmetry parameter of about 0.6. For i-Al(CrFe), essentially no deviations are found from the model of Czjzek or the Gaussian isotropic model. The β phase is found to have a very similar quadrupole effect as that in i-Al(MnFe).

Sign determination of the 57 Fe electric-field gradient in Al-based crystals and icosahedral quasicrystals

The icosahedral quasicrystals i-AlMn isomorphically suostituted by 28 at.% Fe or by (CrFe) are studied for the first time by in-field Mossbauer spectroscopy (MS) in order to determine the sign and asymmetry parameter of the dominant EFG term. In addition, the orthorhombic o-Al(MnFe) and cubic α- and hexagonal β-Al(MnFe)Si phases are studied. We show that previous MS results are inadequate to determine if there are two sites in the i-AlMn structure in the ratio of the golden number. Our results for i-Al(MnFe) show that the dominant EFG is negative, with asymmetry parameter of about 0.6. For i-Al(CrFe), essentially no deviations are found from the model of Czjzek or the Gaussian isotropic model. The β phase is found to have a very similar quadrupole effect as that in i-Al(MnFe).

Electric field gradients at57Fe impurities in crystalline NiB phases

The electric field gradient (EFG) parameters are investigated by in-field Mossbauer spectroscopy at57Fe doping the crystalline compounds Ni3B, Ni2B, Ni4B3 and NiB. A preferential substitution of Ni by Fe is found in Ni3B and Ni4B3. The knowledge of qzz andη at both the 3d metal and boron sites /1/ permits to develop a representation of EFG in terms of empirical screened point charges.