ISSN:0021-8979    

DOI:10.1063/1.1709687

出版商:AIP    

年代:1967

数据来源: AIP

摘要:

For some time NMR and ESR have been applied to the study of biological materials. Two somewhat distinct applications have developed: as a means of detecting a given complex and following it through a reaction, and as a source of information on the electronic state and structure of a material. Mo¨ssbauer spectrometry has recently been applied to biological studies, principally in the latter mode. Unfortunately, the method may be used only on atoms with suitable nuclear properties. Work has been reported on hemoglobin, myoglobin, various cytochromes, ferritin, ferridoxin, and xanthine oxidase, all of which contain iron. Hemoglobin has been the most intensively studied. The magnetic compounds (e.g., the azide, fluoride, nitric oxide) are sufficiently dilute that reasonably well‐resolved hyperfine spectra are seen at low temperatures. Many are consistent with ESR data and one (the azide) can be predicted from thegvalues. Recent measurements in applied magnetic fields confirm the original interpretations. The nitric oxide spectrum provides information on the spread of the unpaired NO spin onto the iron ion. Some spectra are not yet understood, with Mo¨ssbauer and ESR spectra indicating different spin states. A number of interesting problems relating to these, to spin‐spin interaction, and to other cooperative iron‐iron effects remain to be investigated.

ISSN:0021-8979    

DOI:10.1063/1.1709688

出版商:AIP    

年代:1967

数据来源: AIP

摘要:

Experimental investigations on the characteristic length over which two magnetic atoms interact in a nonmagnetic metal have been carried out. Two methods are used: (a) measurement of the transition temperature as a function of the magnetic atom concentration in zero field, and (b) study of the magnetization distribution in alloys in the region just below the transition temperature as determined from the magnetic hyperfine field distribution.These experiments have been done using palladium as the nonmagnetic and iron as the magnetic atoms. In the FePd alloys it is found that the transition temperature and the nature of the magnetic transition depend strongly on the method of sample preparation and in particular on the annealing of the samples. Samples quenched from the melt to room temperature show a diffuse transition and a relatively slow rise in magnetization with decreasing temperature. Those which have been annealed show a relatively sharp transition and a narrower magnetization distribution below the transition temperature, indicating that the process of annealing produces a more homogeneous distribution of the iron atoms. Interpretation of this phenomenon and its relation to the range parameter are discussed.Assuming that the magnetic interaction between two iron atoms is due to the overlap of the matrix polarization, the range obtained in this investigation from a simple statistical model is in excellent accord with neutron diffraction measurements.

ISSN:0021-8979    

DOI:10.1063/1.1709689

出版商:AIP    

年代:1967

数据来源: AIP

摘要:

Ferroelectricity is a very general phenomenon. Yet it is possible to divide it into three main groups with essentially three different mechanisms: the order‐disorder of hydrogen bonds, the trigger effect of nonspherical radicals, and the 4&pgr;/3 catastrophe due to high electronic polarizabilities. Recently it has been realized that some of the metallic bronzes crystallizing in the perovskite structure may also be ferroelectric. At present, it is very difficult to determine ferroelectricity in these metals.

ISSN:0021-8979    

DOI:10.1063/1.1709690

出版商:AIP    

年代:1967

数据来源: AIP

摘要:

The relevance of domain studies to both intrinsic and technical magnetic properties and the reasons for the continued interest and activity in the subject are discussed. The particular advantages of certain of the many methods of observing domains are noted, without attempting any comprehensive survey. Reference is made to some recent developments using x‐rays, to the interpretation of powder patterns, and to dynamic studies. There is no general domain theory, i.e., no theory which predicts the domain structure which will exist in a specimen with specified shape and intrinsic properties. However, it is possible to evaluate, with varying degrees of approximation, the energies corresponding to some observed or postulated structures. Domain observations can then be used to verify magnetostatic calculations, and can give measurements of certain intrinsic properties. Also, precise calculations of magnetization curves can be made for postulated structures in the hope that these will elucidate the technical properties of real materials.

ISSN:0021-8979    

DOI:10.1063/1.1709691

出版商:AIP    

年代:1967

数据来源: AIP

摘要:

In general, the symmetry of the spin‐wave spectra is determined by the magnetic space group characteristic of the spin order in the crystal. For many magnetic materials, however, interactions such as dipole‐dipole, antisymmetric exchange, and higher‐order anisotropic exchange are quite small, so that most of their properties are essentially determined by isotropic Heisenberg exchange plus a few crystal field parameters. It was pointed out previously that this fact is reflected in the spin‐wave spectra in terms of additional symmetry not predicted by the magnetic‐space‐group theory and that this symmetry is describable in terms of spin‐space groups which include independent spin and space rotations. In this paper the theory of magnetic space groups and spin‐space groups and the characterization of the spin‐wave spectra in terms of their representations is briefly reviewed. The effect on the spin‐wave spectra of the various types of interactions that have been proposed for the rare‐earth metals is then analyzed in detail and compared with the recent experimental data for Tb. The calculation of selection rules for neutron scattering is also discussed and applied to Tb.

ISSN:0021-8979    

DOI:10.1063/1.1709692

出版商:AIP    

年代:1967

数据来源: AIP

摘要:

The compounds (RE)23+Cr3+Sb5+O7(RE=Y or a rare earth except Ce, Pm, Lu) with pyrochlore structure have asymptotic Curie temperatures between 4° and 16°K; Ho2CrSbO7and Dy2CrSbO7are ferromagnetic belowTc=10°K andTc=16°K, respectively. The moments of the chromium and the rare‐earth ions are oriented parallel in these latter two compounds. The positive Cr3+&sngbnd;Cr3+interaction in the pyrochlore structure is compared to the interaction between chromium ions in other compounds.

ISSN:0021-8979    

DOI:10.1063/1.1709693

出版商:AIP    

年代:1967

数据来源: AIP

摘要:

Electrical conductivity, Hall effect, magnetoresistance and thermoelectric properties have been studied on a number of undoped and doped polycrystalline CdCr2S4and CdCr2Se4samples. Undoped CdCr2S4and CdCr2Se4aren‐type andp‐type, respectively. With suitable doping, the conductivity of CdCr2Se4can be changed by about six orders of magnitude [10−5to 10 (&OHgr;·cm)−1] without changing the magnetic properties. In Ag‐dopedp‐type CdCr2Se4the mobility at room temperature is 32 cm2/V·sec and rises with decreasing temperature. By doping with In, CdCr2Se4can be maden‐type. The mobility of electrons is found to be two orders of magnitude lower than the mobility of holes. The conductivity ofp‐type CdCr2Se4decreases monotonically with decreasing temperature without any anomaly around the Curie temperature (Tc=130°K1). However, the conductivity ofn‐type CdCr2Se4has a minimum at 146°K and rises sharply with further decreasing temperature. Thisn‐type material also exhibits a large negative transverse magnetoresistance effect which has a maximum at the Curie temperature (&Dgr;&rgr;/&rgr;0=−0.8 forH=6 kG). The magnetoresistance of thep‐type material is positive in the paramagnetic range and negative in the ferromagnetic range. Optical transmission experiments on thin single crystals show that the absorption edges of CdCr2S4and CdCr2Se4are at 1.6 and 1.3 eV, respectively, at room temperature.2For the ferromagnetic range an anomalously strong shift of the absorption edge takes place to higher energies in CdCr2S4and to lower energies in CdCr2Se4. The edge is shifted further in both materials by an externally applied magnetic field. This additional field shift exhibits a linear magnetic dichroism and has its maximum at the Curie temperature. The observed anomalous optical properties and also the strong increase in conductivity ofn‐type CdCr2Se4in the ferromagnetic range are closely related to magnetic ordering. Apparently the strong shift of the absorption edge decreases the energy difference between the donor level and conduction band and results therefore in an increase of the carrier density. A complete account of this work will be published elsewhere.

ISSN:0021-8979    

DOI:10.1063/1.1709695

出版商:AIP    

年代:1967

数据来源: AIP

摘要:

Mo¨ssbauer spectra of57Fe in Fe2SiO4, which has iron atoms in two distinct crystallographic sites, have been obtained from 9°K to 300°K, a temperature range in which two magnetic transitions occur as indicated by magnetic susceptibility measurements. Above the first transition temperature, 66°K, the spectra consist of a pure quadrupole split line. Below this temperature Zeeman splitting is observed, resulting in composite spectra attributed to the two different iron sites. Magnetization curves indicate the presence of weak ferromagnetism between 66°K and the second transition temperature, 20°K. Below the second transition temperature Fe2SiO4is an ideal antiferromagnet. The hyperfine spectra were analyzed by numerical solution of the complete interaction Hamiltonian. From this analysis values of the magnetic fields, the electric field gradients, and their relative orientations are obtained. These results are compared with the spin assignments that have been made on the basis of neutron diffraction studies.

ISSN:0021-8979    

DOI:10.1063/1.1709696

出版商:AIP    

年代:1967

数据来源: AIP

摘要:

The ferromagnetic resonance of single crystals of CdCr2S4and CdCrSe4has been investigated as a function of temperature at 9.49 and 24.24 GHz. The CdCr2S4crystals were grown by vapor transport, and the CdCr2Se4crystals were grown from CdCl2flux. The magnetocrystalline anisotropy and linewidth were found to be dependent on sample. TheK1at 4.4°K varied between 3.8×103and 4.4×104erg/cm3for CdCr2S4and 2.2×103and 1.8×104erg/cm3for CdCr2Se4. These values are in good agreement with those based on a calculation using the single‐ion model of anisotropy. The linewidth of the more anisotropic samples was found to be orientation‐dependent and broad compared to the less anisotropic samples. The narrowest linewidth observed at 4.4°K was 37 Oe. This value is higher than expected for these materials and may be caused by crystalline imperfections. It is suggested that the observed variation of properties with sample may be due to small concentrations of divalent chromium.

ISSN:0021-8979    

DOI:10.1063/1.1709697

出版商:AIP    

年代:1967

数据来源: AIP