H.R. Rechenberg

Magnetic properties of nanostructured CuFe2O4

The structural evolution and magnetic properties of nanostructured copper ferrite, CuFe2O4, have been investigated by x-ray diffraction, Mossbauer spectroscopy, and magnetization measurements. Nanometre-sized CuFe2O4 particles with a partially inverted spinel structure were synthesized by high-energy ball milling in an open container with grain sizes ranging from 9 to 61 nm. Superparamagnetic relaxation effects have been observed in milled samples at room temperature by Mossbauer and magnetization measurements. At 15 K, the average hyperfine field of CuFe2O4 decreases with decreasing average grain size while the coercive force, shift of the hysteresis loop, magnetic hardness, and saturation magnetization at 4.2 K increase with decreasing average grain size. At 295 K the coercive-field dependence on the average grain size is described, with particles showing superparamagnetic relaxation effects. At 4.2 K the relationship between the coercive field and average grain size can be attributed to the change of the effective anisotropy constant of the particles. The interface anisotropy of nanostructured CuFe2O4 is found to be about 1.8(1) × 105 erg cm-3. Although spin canting was present, approximately 20% enhancement of the saturation magnetization in CuFe2O4 nanoparticles was observed, which could be explained by a cation redistribution induced by milling. The high-field magnetization irreversibility and shift of the hysteresis loop detected in our samples have been assigned to a spin-disordered phase, which has a spin-freezing temperature of approximately 50 K.

Structural and magnetic properties of ball milled copper ferrite

The structural and magnetic evolution in copper ferrite (CuFe2O4) caused by high-energy ball milling are investigated by x-ray diffraction, Mossbauer spectroscopy, and magnetization measurements. Initially, the milling process reduces the average grain size of CuFe2O4 to about 6 nm and induces cation redistribution between A and B sites. These nanometer-sized particles show superparamagnetic relaxation effects at room temperature. It is found that the magnetization is not saturated even with an applied field of 9 T, possibly as the result of spin canting in the partially inverted CuFe2O4. The canted spin configuration is also suggested by the observed reduction in magnetization of particles in the blocked state. Upon increasing the milling time, nanometer-sized CuFe2O4 particles decompose, forming α-Fe2O3 and other phases, causing a further decrease of magnetization. After a milling time of 98 h, α-Fe2O3 is reduced to Fe3O4, and magnetization increases accordingly to the higher saturation magnetization va...

Ionic disorder and Néel temperature in ZnFe2O4 nanoparticles

Abstract Magnetic properties of ultrafine ZnFe 2 O 4 particles obtained from mechanosynthesis of the precursor oxides are presented. The ordering temperature of as-milled sample is T N ≈ 115(10) K, and extends over a Δ T ≈ 60 K range. This behavior can be explained by oxygen vacancies and local disorder at Fe sites resulting from the milling process. Magnetic coupling between ordered and disordered phases is discussed.

Magnetic Hyperthermia With Fe

We have studied the magnetic and power absorption properties of a series of magnetic nanoparticles (MNPs) of Fe3O4 with average sizes langdrang ranging from 3 to 26 nm. Heating experiments as a function of particle size revealed a strong increase in the specific power absorption (SPA) values for particles with langdrang = 25-30 nm. On the other side saturation magnetization MS values of these MNPs remain essentially constant for particles with langdrang above 10 nm, suggesting that the absorption mechanism is not determined by MS. The largest SPA value obtained was 130 W/g, corresponding to a bimodal particle distribution with average size values of 17 and 26 nm.

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We present X-ray diffraction (XRD), Mossbauer spectroscopy (MS) and d.c. magnetization measurements performed on ball-milled CuFe2O4 samples. The average particle size was found to decrease to the nanometer range after t=15 min of milling. Room temperature Mossbauer data showed that the fraction of particles above the blocking temperature TB increases with milling time, and almost complete superparamagnetic samples are obtained for = 7(2) nm. Magnetization measurements below TB suggest spin canting in milled samples. The values of saturation moment μS reveal that site populations are slightly affected by milling. Mossbauer resonant intensities are accounted for on the basis of local disorder of Fe3+ environments, and the development of sample inhomogeneities of CuxFe3−xO4 composition.

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A new stable compound in the Nd–Fe system has been identified as Nd5Fe17. The crystal structure belongs to the hexagonal space group P63/mcm, with a=2.0214(8) nm and c=1.2329(8) nm, and with 12 formula units per unit cell. This compound is a ferromagnet with Tc = 503 K and magnetization 162 emu/g at 4.2 K. Mossbauer spectra have been measured and fitted with five Fe sites, yielding an average Beff=26.5 T at room temperature, from which an average Fe moment of 1.83 Bohr magnetons is deduced. X‐ray powder diagrams from magnetically aligned samples suggest basal anisotropy.

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Magnetization and in-field Mossbauer measurements were performed on copper ferrite nanoparticles with average sizes ranging from 3.5 to 10.4nm. Our results show that the nanoparticles are well-crystallized single domains with a magnetically disordered surface shell. A sharp increase in the saturation magnetization at low temperatures, in addition to the usual modified Bloch behavior, was observed for the smallest particles. This jump in magnetization curves seems to be related to the freezing of the surface spins below a temperature of about 45K.

Nanoparticles: The Influence of Particle Size on Energy Absorption

The role of agglomeration and magnetic interparticle interactions in heat generation of magnetic ferrofluids in an ac magnetic field is still unclear, with apparent discrepancy between the results presented in the literature. In this work, we measured the heat generating capability of agglomerated ferrite nanoparticles in a non-invasive ac magnetic field with f = 100 kHz and H0 = 13 kA m −1 . The nanoparticles were morphologically and magnetically characterized, and the specific absorption rate (SAR) for our ac magnetic field presents a clear dependence on the diameter of the nanoparticles, with a maximum SAR = 48 Wg −1 for 15 nm. Our agglomerated nanoparticles have large hydrodynamic diameters, thus the mechanical relaxation can be neglected as a heat generation mechanism. Therefore, we present a model that simulates the SAR dependence of the agglomerated samples on the diameter of the nanoparticles based on the hysteresis losses that is valid for the non-linear region (with H0 comparable to the anisotropy field). Our model takes into account the magnetic interactions among the nanoparticles in the agglomerate. For comparison, we also measured the SAR of non-agglomerated nanoparticles in a similar diameter range, in which N´ eel and Brown relaxations dominate the heat generation. (Some figures may appear in colour only in the online journal)

Superparamagnetic transition and local disorder in CuFe2O4 nanoparticles

We have systematically studied the magnetic properties of ferrite nanoparticles with 3, 7, and 11 nm of diameter with very narrow grain size distributions. Samples were prepared by the thermal decomposition of Fe(acac)3 in the presence of surfactants giving nanoparticles covered by oleic acid. High resolution transmission electron microscopy (HRTEM) images and XRD diffraction patterns confirms that all samples are composed by crystalline nanoparticles with the spinel structure expected for the iron ferrite. ac and dc magnetization measurements, as well in-field Mossbauer spectroscopy, indicate that the magnetic properties of nanoparticles with 11 and 7 nm are close to those expected for a monodomain, presenting large MS (close to the magnetite bulk). Despite the crystalline structure observed in HRTEM images, the nanoparticles with 3 nm are composed by a magnetically ordered region (core) and a surface region that presents a different magnetic order and it contains about 66% of Fe atoms. The high saturati...

Magnetic and structural characterization of Nd5Fe17

Melt‐spun FeCuNbSiB ribbons were annealed at 540–550 °C for various times (≤1 h). The development of a nanocrystalline structure was investigated by means of Mossbauer spectroscopy. From measured hyperfine fields and intensities the crystalline phase was inferred to be pure Fe1−xSix, with x=0.18 after 1 h annealing. The residual amorphous volume fraction was determined to be ≂50%. With help of these results it has been possible to evaluate the amorphous contribution to magnetostriction in the nanocrystalline state. The development of a nanocrystalline structure was found to play a role in the main mechanisms of magnetic disaccommodation.