A. Kostikas

Effect of oxide layer on the hysteresis behavior of fine Fe particles

The effects of surface oxidation on the structural and magnetic properties of fine Fe particles prepared by the evaporation technique have been studied using transmission electron microscopy, x‐ray photoelectron spectroscopy, superconducting quantum interference device magnetometry, and Mossbauer spectroscopy. By varying the argon pressure, particles were obtained with sizes in the range of 60–350 A. The hysteresis behavior was found to be strongly dependent on the variation in the amount of surface oxidation. The differences in the magnetic behavior due to variation in size and oxidation have been explained by considering a shell/core model for the particle morphology with the shell consisting of Fe oxides surrounding the α‐Fe core.

Magnetic hysteresis and Mössbauer studies in ultrafine iron particles

The magnetic hysteresis behavior of ultrafine Fe particles has been examined by SQUID magnetometry and Mossbauer spectroscopy. Ultrafine Fe particles have been prepared by vapor deposition under argon atmosphere in pressures from 1 to 6 Torr and passivated by exposure to oxygen at low pressure. Particles with size below 10 nm showed a small coercivity (∼100 Oe) at room temperature, which increased drastically upon cooling to cryogenic temperatures (1000–1500 Oe). The larger particles had the highest coercivity at room temperature (∼1000 Oe) which increased by 100% at 10 K. Mossbauer studies revealed that the Fe particles are surrounded by an Fe oxide layer composed of ultrafine Fe3O4 or γ‐Fe2O3 grains. At room temperature the oxide component gives a very broad absorption superimposed on an α‐Fe spectrum. The enhanced coercivity at room temperature may be attributed to anisotropy induced by exchange interaction between the ferromagnetic core and the ferri‐ or antiferromagnetic oxide layer.

Synthesis, thermogravimetric and 57Fe Mössbauer studies of the oxygen deficient perovskite REBaCuFeO5 + x series (RE = Y, Nd, Sm, Gd, Dy, Tm, Lu)

Abstract We present the results of thermogravimetric and 57 Fe Mossbauer studies of the oxygen deficient perovskite REBaCuFeO 5+x series with RE = Y, Nd, Sm, Gd, Dy, Tm, Lu. The presence of the removable content x of oxygen was determined by analysis of TGA measurements in a stream of Ar gas. The amount of removable oxygen was highest for the Nd compound and varied between 0.07 and 0.12 for the other compounds. By means of the Rietveld method we performed the refinement of the structure of the compounds, based on the space group P4mm, after taking into consideration that the Fe ion occupies the lb 1 ( z = 0.27) site and Cu the lb 2 ( z = 0.74) site. From the analysis of the Mossbauer spectra we found that the compounds become magnetically ordered with transition temperatures depending on the oxygen content. The values of the hyperfine parameters in all the samples indicate the ferric iron to be in a high spin state.

Origin of coercivity in (Fe,Co)-based granular films

The magnetic behavior of Co and Fe granular films was studied relative to their host matrix environment (BN, SiO2). The crystal structure of Co and Fe in the as‐deposited samples is α‐Co (hcp) and α‐Fe (bcc) respectively, with particle sizes ranging between 3 and 9 nm. The coercivities measured ranged from a few tens to a few hundreds Oe, with the higher values observed for particles embedded in the oxygen based matrix.

Stability range, structure and magnetic properties of YFe12−xMox (0.5 ⩽ x ⩽ 2.0) alloys

Intermetallic compounds with the general formula YFe12-xMox (0.5 ⩽ x ⩽ 2.0) were prepared by arc melting. Crystal structure determination by X-ray powder diffractometry and Rietveld refinement techniques has shown that a tetragonal ThMn12 is formed with small admixture of α-Fe only for x<1.0. Extrapolation of the linear dependence of the lattice parameters to x = 0 gives a = 8.4727 A, c = 4.7682 A and a unit cell volume V = 342.3 A3. Curie temperatures increase with decreasing x but deviate from linear behavior below x = 1.0 leading to an estimate of Tc = 480 K for the limiting case of x = 0. A more linear dependence on x is observed for the saturation magnetization. The extrapolated value for YFe12 is approximately 26 μB/f.u. Anisotropy fields at 4.2 K increase from 5 T for x = 0.5 to 7.5 T for x = 1.0. Irreversible phenomena in the temperature dependence of magnetization are observed in the x = 2 sample. Hyperfine fields have been determined for the i,j and f sites at room temperature from Mossbauer spectra.

Magnetic anisotropy and magnetic transitions in RFe10Mo2

Compounds of the composition RFe10Mo2 with R = Y, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm and Lu were studied by means of the ac-initial susceptibility (chi) and the singular point detection (SPD) technique in the temperature range from 4.2 to 300 K. Spin reorientation transitions were detected for the compounds RFe10Mo2 with R = Nd and Dy. For the compounds RFe10Mo2 with R = Y, Ho, Er, Tm and Lu the ac-initial susceptibility data indicate a magnetic transition of unidentified nature. For SmFe10Mo2 a first-order magnetization process (FOMP) is observed below 170 K.

Size effects on the magnetic properties of fine Fe-Cr particles

Abstract The magnetic and structural properties of Fe 1003− x Cr x ultrafine particles with x = 5–20 have been studied as a function of particle size. Particles with a size in the range of 80–360 A were prepared by gas evaporation under argon atmosphere. The particles with smaller diameter had a high coercivity at low temperatures and showed a stronger temperature dependence of coercivity. The x = 20 particles with a size 80 A had a coercivity about 2100 Oe at 10 K with a superparamagnetic blocking temperature about 150 K. Mossbauer spectra showed the presence of Fe-Cr, α-Fe and Fe-oxide components in the bigger particles, and Fe-Cr and Fe-oxides in the smaller particles. The coercivity at low temperatures increased with decreasing particle size and this was attributed to the higher percentage of Fe-oxide on the surface of the smaller particles. This interpretation was further supported by the temperature dependence of coercivity of Fe–Cr particles sandwiched between two Ag films.

Structure and magnetic anisotropy of Fe/Pt multilayers

A series of Fe/Pt multilayers prepared by magnetron sputtering have been studied by XRD, conversion electron Mossbauer spectroscopy (CEMS) and SQUID magnetometry. The samples with individual Fe thickness of 3 A show orientation of the magnetic moment close to perpendicular to the film plane and Curie temperatures near 350 K. Interdiffusion at the interface is manifested in a distribution of hyperfine fields.

Mössbauer studies in Fe(SiO2,BN) granular solids

Abstract Mossbauer spectroscopy has been used in Fe ( SiO 2 , BN ) granular solids in order to elucidate the large differences in their coercivity values. The Mossbauer data suggest a core/shell morphology for the Fe SiO 2 films with an Fe(Si) core surrounded by an FeSiO shell whose surface consists of Fe2+SiO particles. In Fe BN the data indicate the presence of amorphous FeB particles in a BN matrix.

Magnetic phase transitions in RFe10Mo2 compounds

A variety of magnetic phase transitions have been observed for all the RFe10Mo2 compounds from AC and DC magnetic measurements in the temperature range between 4.2 and 300 K. The alloys with R = Nd, Tb and Dy exhibit spin reorientation transitions and with R = Ho, Er and Tm some unidentified magnetic phase transitions occur which are accompanied by strong relaxation effects in the 57Fe Mossbauer spectra of TmFe10Mo2. The observed magnetic transitions may broadly interpreted in terms of a mean-field mixed-exchange model, although the detailed behaviour is somewhat different.