摘要:

We report measurements of the specific heatCof two single crystals of EuO over the range of temperatures from 0.37° to 4.4°K and in the presence of external magnetic fields up to 13 kOe. The results for these two samples, which were grown from different batches, are consistent within a few percent. The primary contribution toCabove 0.7°K is described in terms of spin‐wave theory. From an analysis of the data we findJ1/kB=(0.76±0.02) °K andJ2/J1=−0.11±0.02 in excellent agreement with another determination. Below 0.7°K, the dominant contribution toCarises from the hyperfine splitting of the151Eu and153Eu nuclear levels. The magnitude of this contribution corresponds to an effective hyperfine fieldHN=300±5 kOe in agreement with the value deduced from NMR and Mo¨ssbauer experiments.

ISSN:0021-8979    

DOI:10.1063/1.1659037

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

The thermodynamics of impure Heisenberg magnets have been studied previously within the random phase approximation (RPA) which neglects some important correlations between exchange‐coupled spins. To test the accuracy of the RPA and to allow more detailed comparison with experiment we have extended the rigorous Dyson low‐temperature boson formalism to impure systems. Spin‐wave interactions are treated in Hartree‐Fock approximation (equivalent to Dyson's first Born results for the pure system). We find only very small differences from the RPA impurity magnetization results, but there is substantial variation from RPA predictions of the temperature dependence of local mode energies.

ISSN:0021-8979    

DOI:10.1063/1.1659038

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

The relationships between the magnetoelectric (ME) susceptibilities in polycrystalline powders and single crystals are derived for antiferromagnetic magnetic crystal classes possessing orthorhombic or higher symmetry. The powder susceptibilities induced by both parallel and perpendicular ME annealing are calculated in terms of the single‐crystal ME susceptibilities. The results indicate that: (a) Measurements of the ME effect in a powder specimen, together with a knowledge of its crystallographic point group in the ordered state, are sufficient to determine its magnetic crystal class. (b) In many cases, the elements of the single‐crystal ME susceptibility tensor can be determined, at least in principle, from the powder measurements. The application of these results to potentially ME materials is discussed.

ISSN:0021-8979    

DOI:10.1063/1.1659039

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

A first‐order (self‐consistent) renormalized spin‐wave theory has been used to fit the temperature dependence of the magnetizationM(T, 0) in the hexagonal ferromagnet CrBr3. However, this theory appears incapable of simultaneously explainingM(T, O) and new measurements we have made ofM(T0,H). Reexamining the first‐order theory we find that (a) self‐consistency corrections are negligible because of the low magnon density and (b) an equally satisfactory fit toM(T, O) can be obtained from a rectangular model with appropriately chosen intralayerJTand interlayerJLexchange constants and reciprocal‐lattice volume. This is possible becauseJTis so large that there is little thermal excitation beyond thekx2+ky2part of the spectrum. Since largekzmagnons are present due to the smallness ofJL, this is a strongly interacting system for which correlation effects are known to be important. Using the rectangular model we have treated spin‐wave interactions to all orders via thet‐matrix and find deviations of up to 40% from the first‐order renormalization. Since the first‐order renormalization effects onM(T, O) were shown to be large, we may infer from our calculations that the first‐order fitting of one thermodynamic function was fortuitous and was made possible by the multiparameter character of the theory.

ISSN:0021-8979    

DOI:10.1063/1.1659040

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

The magnetic susceptibilities of polycrystalline MnCO3and MnO2have been measured in fields up to 300 kOe in the temperature region 80°–300 K. MnCO3shows an increase in susceptibility at about 190 kOe at 300°K. The low‐ and high‐field phases obey an antiferromagnetic Curie‐Weiss law, and constants &thgr;=61°±9°K, and &thgr;=44°±9°K respectively, MnO2shows a decrease in susceptibility at about 120 kOe and a subsequent increase at 270 kOe. The low‐, intermediate‐, and high‐field phases again follow an antiferromagnetic Curie‐Weiss law with constants &thgr;=266°±57°K, 110°±18°K, and 159°±12°K. The transitions are discussed in terms of a suppression mechanism by which the high fields affect the ligand orbitals, and estimates of the transition fields are made on the basis of this theoretical model.

ISSN:0021-8979    

DOI:10.1063/1.1659041

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

The reduction of the ferromagnetic exchange with lattice dilation has been studied in EuO films. Films deposited at high substrate temperatures (400°–500°C) show bulk‐like properties and films deposited at lower substrate temperatures (below 350°C) show as much as a 1% room‐temperature lattice dilation with a corresponding 14% blue shift (2.03–2.31 eV) in the room‐temperature optical absorption band and 28% reduction (69°–50°K) in the Curie temperature. This lattice dilation causes a reduction in the cubic crystal field splitting of the 5dlevels, and consequently a shift to higher energy of the 4f7→4f65t2gtransition. The Eu2+&sngbnd;Eu2+interatomic ferromagnetic exchange is reduced via the Goodenough model which involves overlap of a half‐filled 4flevel on one cation with an empty 5dlevel on a neighboring cation.

ISSN:0021-8979    

DOI:10.1063/1.1659043

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

CsFeF3appears to be a Ne´el type ferrimagnet whose magnetization forms cones of revolution having a 25‐deg angle with the hexagonalcaxis. The moment along the cone angle increases in an anomalous manner in fields up to 25 kG. In addition, the anisotropy energy at 4.2°K is extremely large withK1≈−7.8×106ergs/cm3andK2≈2.2×107ergs/cm3. At room temperature the optical absorption coefficient is about 1 cm−1for wavelengths 6000–8000 Å. In a field of 20 kG at 84°K the Faraday rotation is 100 deg/cm for a wavelength of 6000 Å.

ISSN:0021-8979    

DOI:10.1063/1.1659044

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

The existence of isolated magnetic charges (monopoles) in nature would require profound rethinking not only in elementary particle physics but also in high‐energy cosmic‐ray physics and astrophysics. Monopoles, whose possible existence was first suggested by Dirac in 1931 and whose properties were later recalculated by Schwinger, would be intensely ionizing particles similar in their effects in passing through matter to relativistic rare‐earth ions (or still heavier nuclei). As Parker has noted, only a very minute abundance of monopoles in the galaxy would be necessary to have removed or at least grossly altered the galactic magnetic field on an astrophysically interesting time scale. Solid‐state nuclear track detectors have led to a new series of intensive monopole searches: One of these takes ferromanganese deposits from the deep oceans (as a material which would have magnetically trapped monopoles) and exposes them to high magnetic fields sufficient to loosen monopoles and accelerate them into a detection system; the other experiment is designed to utilize natural detectors (mica and obsidian) to record over geological times the paths of massive, penetrating monopoles. The negative results obtained set new and highly restrictive limits on the abundance of monopoles. These limits are such that the mysterious, high‐energy cosmic rays (1017−1019eV) cannot be monopoles as has been suggested, nor can the galactic magnetic fields be seriously altered by the maximum permitted abundance—even over the galactic age. Finally, the flux of monopoles onto the earth over the earth's entire age is <½ monopole/cm2. Qualifications on these claims are indicated in the article.

ISSN:0021-8979    

DOI:10.1063/1.1659045

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

Rockmagnetism provides an insight into the origin and stability of Natural Remanent Magnetization (NRM) in rocks, which enables one to decipher the geomagnetic field history, continental drift, and seafloor spreading. NRM in some rocks is due to cooling through the Curie point (TRM) and for others, either due to volume growth from superparamagnetic to single domain size (CRM) or alignment along the field vector in a wet sediment (DRM). However, CRM has sometimes been mistaken for TRM; it is important to distinguish between the two so as to assign the correct age for NRM. NRM may also acquire an antiparallel direction to the ambient geomagnetic field (self‐reversal); thus providing an ancient direction 180° in error. Theoretical models for self‐reversal appeal to exchange anisotropy, compensation point, and chemical phase changes. Self‐reversal in nature, however, has been proved only for a limited number of cases. Although some minerals have large magnetostriction, a direct effect on NRM is questioned. Interestingly, stable NRM has recently been found in nominally diamagnetic minerals with 1% iron content and in antiferromagnets.

ISSN:0021-8979    

DOI:10.1063/1.1659046

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

Liquid crystals are characterized by an orientational order of the rod‐like (or possible disk‐like) molecules of which they are composed. The anisotropic part of the molecular diamagnetic susceptibility gives rise to a coupling between external magnetic fields and the orientational structure. In terms of the Frank elasticity theory we discuss several such effects of a static field, e.g. unwinding of a cholesteric helix, Freedericksz transition. The second part of this review is devoted to a consideration of magnetic resonance as a tool to study both the order parameter and the orientational fluctuation modes in a liquid crystal.

ISSN:0021-8979    

DOI:10.1063/1.1659047

出版商:AIP    

年代:1970

数据来源: AIP