摘要:

In this work we report a Mo¨ssbauer study of iron‐copper clusters isolated in a rare gas solid. For the FeCu dimer we were able to determine the electronic ground state. The samples were prepared in a liquid‐helium cryostat evacuated to a pressure better than 10−7torr. The Cu and (90&percent; enriched)57Fe atomic beams were produced in alumina crucibles contained in resistance heated tantalum furnaces and codeposited with a stream of argon gas at 4.2K onto a Be disk. In this study very dilute samples were used (Ar/metal ?500) in order to identify correctly the FeCu dimer. From the Mo¨ssbauer parameters we were able to identify the molecule as2&pgr; with a spin at the iron atom of 1/2. A concentration dependence study of the iron‐copper multimer was carried out. It was observed that by increasing the number of copper nearest neighbors the isomer shift approaches the value obtained for iron in a copper matrix. For very high concentrations the Mo¨ssbauer parameters are similar to those of pure iron multimer. It seems that the presence of a large number of copper nearest neighbors produces a drastic change in the electronic structure of the iron atom.

ISSN:0021-8979    

DOI:10.1063/1.326967

出版商:AIP    

年代:1979

数据来源: AIP

摘要:

Mossbauer spectra of olivines of composition (MgxFe1−x)2SiO4(x?0.3 to 0.9) have been examined in the temperature range 4.2 to 300 K, and found to exhibit superparamagnetic relaxation effects originating from clusters of Fe2+ions. Relaxation times were determined by comparison with theoretical relaxation spectra calculated from the stochastic model of Blume and Tjon, which was modified to include the asymmetry parameter and general orientations of the hyperfine field direction with respect to the crystal axes. From the variation of the relaxation time with temperature, the mean cluster‐size diameters of the Fe‐rich regions were found to be in the range 20 to 40 A˚, and to increase with increasing Fe content. The distribution of cluster diameters was asymmetric, with higher probabilities for small diameters. Potential applications with regard to the determination of the thermal histories of rocks containing olivine are discussed.

ISSN:0021-8979    

DOI:10.1063/1.326968

出版商:AIP    

年代:1979

数据来源: AIP

摘要:

In order to understand the role of shock waves in modifying existent remanent magnetization and in inducing magnetization via 1st and 2nd order shock transitions in iron and iron‐nickel alloys a model system has been adopted. Dilute iron in copper (1.5 wt &percent; Fe solutions can be annealed arbitrarily to produce a near ideal dispersion of spherical fcc precipitates having diameters ≲2000 A˚. The size range is arbitrarily specified by the anneal. These spherical iron (antiferromagnetic‐fcc) particles are then transformed to the ferromagnetic state (bcc) during a shock pulse in controlled external fields (≲1 Oe). The axial vector always takes on the sign of the external field indicating that on the microsecond time scale of the transformation the particles record the field sense. The amount of fcc iron transformed depends on both precipitate size and the peak input pressure. The observed deformation induced anistropy is dependent on particle size and shock level, but the 400 A˚–600 A˚ particles show the maximum degree of anisotropy at all shock levels. Different sizes of precipitates support different degrees of magnetic hardening (i.e., response to alternating field demagnetization). Anisotropy in magnetic hysteresis, using hysteresis ratios ‐ RI(ratio of saturation remanence to saturation magnetization) and RH(ratio of remanent coercive force to coercive force) indicate a shock induced weak field remanence anisotropy. Measurement of the angular variation of coercive force (HC) and remanent coercive force (HR) and the computed RHclearly identifies the direction of the shock, RHbeing a minimum and HCa maximum parallel to P. Unique properties observed in hysteresis behavior are related to the narrow iron particle size distribution, orientation, and mean particle spacing. These unique properties include discrete switching fields for magnetization reversal, and separation of region of hysteresis from region of shape response.

ISSN:0021-8979    

DOI:10.1063/1.326969

出版商:AIP    

年代:1979

数据来源: AIP

摘要:

&ggr;‐Fe2O3powder of acicular shape was modified by reaction with Co2+and Fe2+ion containing solution or Co(OH)2to obtain a cobalt ferrite layer of various cobalt content on the surface of each particle. It was found that this epitaxial Co‐doping raised the coercivity of &ggr;‐Fe2O3from 420 (Oe) to 770 (Oe). The coercivity of such epitaxial Co‐doped &ggr;‐Fe2O3powders depends on the packing density in the same manner as the acicularly shaped &ggr;‐Fe2O3powder. The dependence of the coercivity of the powders on temperature, the chemical composition and preparation technique were also studied. The effective uniaxial anisotropy constants were measured. The shapes of the powders were observed by electron microscopy. On the basis of these studies, we are of the opinion that the development of a surface anisotropy of the epitaxial layer is the most probable cause of the increasing coercivity.

ISSN:0021-8979    

DOI:10.1063/1.326970

出版商:AIP    

年代:1979

数据来源: AIP

摘要:

The correlation between the magnetic properties and microstructures of sputtered pure Co and Co‐Re alloy thin films have been investigated. Magnetic properties were investigated via hysteresis loop measurements while microstructure was determined using TEM. (AIP)

ISSN:0021-8979    

DOI:10.1063/1.326971

出版商:AIP    

年代:1979

数据来源: AIP

摘要:

Detectable magnetic fields are associated with electrical activity in the brain that may be evoked by sensory stimuli. Neuromagnetic fields as weak as 20 femtotesla can be studied with modern SQUID detectors without recourse to magnetic shielding. Studies in our laboratory and elsewhere reveal that the field patterns may be sharply localized near the appropriate area of the cortex for visual, somatosensory, or auditory stimuli. The temporal features of the magnetic response to visual stimuli reveal aspects of the organization of the visual system of the brain.

ISSN:0021-8979    

DOI:10.1063/1.326972

出版商:AIP    

年代:1979

数据来源: AIP

摘要:

The conversion of coal to a clean fuel through liquefaction requires the physical separation of undissolved sulfur‐rich mineral matter. The hydrogenation‐liquefaction reaction produces residual solids in which the original coal impurity pyrite, FeS2, has largely been converted to pyrrhotite, Fe1−xS, which is a complex non‐stoichiometric sulfide. Using thermomagnetic analysis and Mo¨ssbauer spectroscopy we show that the iron sulfide in coal liquefaction residues corresponds to an intermediate pyrrhotite. Striking transformations between various magnetic and non‐magnetic states are induced by thermal treatments in inert or sulfidation atmospheres. The kinetics of the transformations are strongly influenced by prior thermal and atmosphere history. Transformation to a high magnetization state has practical application wherein magnetic separation techniques become attractive for the liquid‐solids separation step in coal liquefaction.

ISSN:0021-8979    

DOI:10.1063/1.326973

出版商:AIP    

年代:1979

数据来源: AIP

摘要:

Eutectic Bi/MnBi (97.8 a/o Bi) samples have been plane‐front directionally solidified. The resultant microstructures consist of elongated, aligned particles of MnBi dispersed in a Bi‐matrix. Magnetization as a function of temperature (4.2 to 300 K) and applied field (up to 220 kG) has been used to evaluate solidification parameters and magnetic properties. At room temperature, in addition to the diamagnetic contribution of Bi, one finds a superposition of the ferromagnetic, low temperature (LTP) MnBi phase and paramagnetic phases. At cryogenic temperatures, one of the room temperature paramagnetic phases is ferromagnetic with an intrinsic coercivity of 120 kOe while the other remains paramagnetic for low fields and orders ferromagnetically at high fields in a complicated way. Annealing of as‐grown samples was found to produce significant changes in magnetic properties. The origins of the paramagnetic phases and their relation to the mechanisms which control the coercive field of the hard magnetic LTP MnBi phase are discussed.

ISSN:0021-8979    

DOI:10.1063/1.326974

出版商:AIP    

年代:1979

数据来源: AIP

摘要:

A Class I lodestone is a massive iron ore which owes its magnetic hardness to oxidation. Oxidation of the Fe3O4results in a fine intergrowth of &ggr; Fe2O3in host Fe3O4, a requisite for all lodestones studied. The natural remanence (NRM) to saturation isothermal remanence (ISR) values are ≳0.5 compared to values <0.05 for most other naturally magnetized materials. The behavior of the lodestone magnetization has been studied from 4 K to 825 K, and x‐ray, optical and transmission electron microscopy data have been used to confirm the &ggr;‐FeO3‐Fe3O4epitaxy. On heating to 250 °C coercive force (HC), remanent coercive force (HR), and ISRdecrease, while about 70&percent; of the NRM is lost. On returning to ambient temperature an increase in saturation magnetization (IS) is noted which can be interpreted as an ordering of the &ggr; Fe2O3. Further heating to 425 °C results in destruction of the &ggr; Fe2O3, a complete destruction of the lodestone remanence, while Hc, HR, and ISRincrease. Beyond 425 °C the material behaves normally. On returning to ambient temperatre Hcand HRrecover in smooth and continuous fashion while the thermoremanence (TRM) acquired in the geomagnetic field is less than 5&percent; of the NRM value. Initial and final HCand HRvalues are the same. However as the thickness of the &ggr; Fe2O3lamellae increase the final HCand HRvalues are reduced. ISis reduced by an amount dependent on the &ggr; Fe2O3?&agr;Fe2O3conversion. The intense lodestone remanence was apparently acquired during the oxidation of Fe3O4, though the lodestone could be charged by fields associated with a lightening discharge.

ISSN:0021-8979    

DOI:10.1063/1.326975

出版商:AIP    

年代:1979

数据来源: AIP

摘要:

Conventional pulse‐echo techniques and spin‐phonon spectroscopy have been employed to examine magnetoelastic excitations in films of Fe, Ni, Gd, Er, and crystals of Gd. With perpendicular rf and dc fields, transverse magnetoelastic excitations are observed with intensities appropriate to a coupled mode model. Recent experiments on Er, with parallel rf and dc fields applied in the film plane, demonstrate the excitation of longitudinal phonons with signal intensities which increase monotonically with an applied field, varying from 0–24 kOe at 10K. Experiments on Gd, Ni, and Fe films with the same field configuration also yield longitudinal phonons at applied fields sufficiently large to magnetize the specimens. Experiments on Gd crystals give similar results. In all cases, the phonon signal is at the rf frequency and the signal intensity varies linearly with the rf field intensity. The data support the conclusion that a first order transition is responsible for the signals; however, with this so‐called ’’parallel‐pumping’’ configuration of the rf and dc magnetic fields, first order transitions are forbidden. An addition to the conventional Hamiltonian, which may reconcile these difficulties, is discussed.

ISSN:0021-8979    

DOI:10.1063/1.326976

出版商:AIP    

年代:1979

数据来源: AIP