J.M. Greneche

Magnetic properties of nanostructured ferrimagnetic zinc ferrite

Nanostructured ZnFe2O4 ferrites with different grain sizes were prepared by high energy ball milling for various milling times. Both the average grain size and the root mean square strain were estimated from the x-ray diffraction line broadening. The lattice parameter initially decreases slightly with milling and it increases with further milling. The magnetization is found to increase as the grain size decreases and its large value is attributed to the cation inversion associated with grain size reduction. The Fe-57 Mossbauer spectra were recorded at 300 K and 77 K for the samples with grain sizes of 22 and 11 nm. There is no evidence for the presence of the Fe2+ charge state. At 77 K the Mossbauer spectra consist of a magnetically ordered component along with a doublet due to the superparamagnetic behaviour of small crystalline grains with the superparamagnetic component decreasing with grain size reduction. At 4.2 K the sample with 11 nm grain size displays a magnetically blocked state as revealed by the Mossbauer spectrum. The Mossbauer spectrum of this sample recorded at 10 K in an external magnetic field of 6 T applied parallel to the direction of gamma rays clearly shows ferrimagnetic ordering of the sample. Also, the sample exhibits spin canting with a large canting angle, maybe due to a spin-glass-like surface layer or grain boundary anisotropies in the material.

Néel temperature enhancement in nanostructured nickel zinc ferrite

The Neel temperature of Ni0.5Zn0.5Fe2O4 spinel ferrite increases significantly from 538 K in the bulk state to 592 K when the grain size is reduced to 16 nm by milling in a high-energy ball mill. This has been attributed to an increase in the AB superexchange interaction strength due to a possible enhancement in the magnetic ion concentration in the A-site on milling, as is evident from extended x-ray absorption fine structure and in-field Mossbauer measurements. (c) 2005 American Institute of Physics.

Surface anisotropy in ferromagnetic nanoparticles

The effect of surfaceanisotropy on the magnetic ground state of a ferromagnetic nanoparticle is investigated using atomic Monte Carlo simulation for spheres of radius R=6a and R=15a, where a is the interatomic spacing. It is found that the competition between surface and bulk magnetocrystalline anisotropy imposes a “throttled” spin structure where the spins of outer shells tend to orient normal to the surface while the core spins remain parallel to each other. For large values of surfaceanisotropy, the spins in sufficiently small particles become radially oriented either inward or outward in a “hedgehog” configuration with no net magnetization. Implications for FePt nanoparticles are discussed.

Cationic distribution and spin canting in CoFe2O4 nanoparticles

CoFe(2)O(4) nanoparticles (D(NPD) ~6 nm), prepared by a thermal decomposition technique, have been investigated through the combined use of dc magnetization measurements, neutron diffraction, and (57)Fe Mössbauer spectrometry under high applied magnetic field. Despite the small particle size, the value of saturation magnetization at 300 K (M(s) ͠= 70 A m(2) kg(-1)) and at 5 K (M(s) ͠= 100 A m(2) kg(-1)) are rather close to the bulk values, making the samples prepared with this method attractive for biomedical applications. Neutron diffraction measurements indicate the typical ferrimagnetic structure of the ferrites, showing an inversion degree (γ(NPD) = 0.74) that is in very good agreement with cationic distribution established from low temperature (10 K) Mössbauer measurements in high magnetic field (γ(moss) = 0.76). In addition, the in-field Mössbauer spectrum shows the presence of a non-collinear spin structure in both A and B sublattices. The results allow us to explain the high value of saturation magnetization and provide a better insight into the complex interplay between cationic distribution and magnetic disorder in ferrimagnetic nanoparticles.

Electrical and magnetic properties of chemically derived nanocrystalline cobalt ferrite

Nanocrystalline cobalt ferrite particles of 8nm grain size were synthesized by coprecipitation technique and subsequently suitably heat treated to obtain higher grain sizes. The experimentally observed changes in the dc electrical conductivity and Curie temperature with heat treatment have been attributed to the changes in the cation distributions as obtained from the Mossbauer and extended x-ray absorption fine structure (EXAFS) measurements and to the grain size. The activation energies for conduction as determined from the Arrhenius plots suggest that the conductivity is due to hopping of both electrons and holes. The observed decrease in conductivity when the grain size is increased from 8to92nm is clearly due to the predominant effect of migration of some of the Fe3+ ions from octahedral to tetrahedral sites, as is evident from in-field Mossbauer and EXAFS measurements. But the higher conductivity of the 102 and 123nm particles compared to that of the 92nm particles is attributed to the higher grain ...

Annealing dependence of magnetic properties in nanostructured particles of yttrium iron garnet prepared by citrate gel process

Abstract Yttrium iron garnet (YIG) particles were first synthesized using a low-temperature method based on the citrate gel process. The annealing temperature dependence of the magnetic behaviour was investigated by means of transmission 57 Fe Mossbauer spectrometry and DC magnetic measurements. The as-prepared particles behaves as an amorphous-like structure. Annealing treatments cause aggregation and crystallization of particles. For low annealing temperatures, one can suggest a nanostructured behaviour of the YIG particles, which progressively disappears when the annealing temperature increases. Depending on the annealing temperature, YIG nanostructured particles of different size, in the range 50–700 nm were obtained. The values of the saturation magnetization as well as the Curie temperature for the YIG particles are close to those observed for well-crystallized YIG.

Magnetic properties of nanostructured ball-milled Fe and Fe50Co50 alloy

Nanostructured Fe and Fe50Co50 powders were prepared by high-energy ball milling. Microstructural and magnetic properties changes with milling time were followed by x-ray diffraction, differential scanning calorimetry and vibrating sample magnetometry. The nonequilibrium microstructure originates from a grain size reduction to about 12 nm and the introduction of internal strain up to 1.5% (root-mean-square strain). The occurrence of disorder in the ball-milled powders is evidenced by the broad exothermic reaction during the heating of ball-milled samples, the variation of lattice parameters and the increase of the saturation magnetization during the first 3 h of milling for Fe and continuously for the Fe50Co50 powder mixture. According to both the reduction of Fe Curie temperature, Tc, and the increase of the phase transformation , the paramagnetic temperature domain of nanostructured bcc α-Fe is extended by about 50 °C. The Fe50Co50 nanostructured powder behaves as a soft ferromagnet with low values of both the coercive field and the squareness ratio Mr/Ms.

Magnetic interaction evidence in α-Fe2O3 nanoparticles by magnetization and Mössbauer measurements

Abstract Magnetic properties of α-Fe 2 O 3 antiferromagnetic particles of 5 nm mean diameter prepared by sol–gel method were investigated by means of static magnetic measurements and 57 Fe Mossbauer spectrometry. The large magnetic moment as well as the anisotropy energy are attributed to the lack of compensation of the antiferromagnetic arrangement essentially located at the surfaces. Zero-field and in-field Mossbauer experiments reveal the existence of magnetic interaction among antiferromagnetic nanoparticles in the sample with higher particle concentration and established the presence of 2 atomic thick surface magnetic layer estimated from a core-shell model.

New analysis of the Mössbauer spectra of akaganeite

The Mossbauer spectra of akaganeite have always been interpreted considering both the tetragonal structure and the chlorine content. However, very recently it has been suggested that the crystallographic structure is not tetragonal but monoclinic, thus another interpretation for the Mossbauer spectra is required. For this purpose, we have prepared and characterized by several techniques synthetic akaganeite. Our results suggest that the two crystallographic sites required by the monoclinic symmetry are not distinguishable in the paramagnetic state as previously assumed, but they are only discernible in the low temperature magnetic region. At room temperature the spectrum is fitted with two doublets whose origin is related to the chlorine content, i.e. one Fe site assigned to Fe3+ ions located close to chloride ions and the other Fe site to those located close to chloride vacancy sites. The low temperature spectra can be adequately fitted with four sextets, whose hyperfine parameters must be subjected to some constraints. The origin of these components is related to the two different crystallographic sites and to the chlorine content. In-field Mossbauer spectrometry at low temperature suggests that the magnetic structure behaves as a system which consists of two asperimagnetic-like structures antiferromagnetically coupled, and not as a collinear antiferromagnet as usually assumed.

About the interfacial zone in nanocrystalline alloys

Abstract We discuss the concept of an interfacial zone resulting from the symmetry restriction at the periphery of crystalline grains in the nanocrystalline alloys. The different experimental features are essentially based on Mossbauer investigations performed in a wide temperature range. The results obtained are related to the atom probe field ion microscopy, EXAFS and FMR analysis. The experimental evidence for the existence of the interfacial zone (with a thickness of about 2–3 atomic layers) is presented. The structural, chemical and magnetic properties of the interfaces are discussed including their role in the overall magnetic interactions in nanocrystalline structure.