摘要:

Cerium hexaboride is an antiferromagnetic metal with a Ne´el temperature of 3°K. The high‐temperature inverse susceptibility of this material is observed to decrease with decreasing temperature more rapidly than would be expected from a Curie‐Weiss law. Since the temperature at which this deviation appears is much higher than the eventual ordering temperature, short‐range ordering effects should be negligible. Also, it can be shown that crystal‐field effects alone cannot produce such a decrease in &khgr;−1. In this paper we offer a possible mechanism which explains this behavior. Basically, we assume that the Ce&sngbnd;Ce exchange arises through an indirect exchange involving the conduction electrons. We visualize the conduction electrons as moving in the spaces between the boron complexes so that the magnitude of thes‐fexchange will be different for the different crystal‐field states of cerium. In a cubic field the2F5/2state of Ce3+splits into a &Ggr;7and a &Ggr;8. Low‐temperature measurements of the saturation moment indicate that the &Ggr;7state lies lowest. Therefore, as the temperature decreases, the &Ggr;8state, which protrudes into the space between the borons and, hence, contributes strongly to thes‐fexchange, is frozen out, leaving only the weakly coupled &Ggr;7state.

ISSN:0021-8979    

DOI:10.1063/1.1657507

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

A new technique which has been useful in growing single crystals of a number of the chalcogenide spinels is described. CrystalsACr2X4, whereA= Zn or Cd,X= S or Se, are grown by reacting compoundAXand CrCl3in a sealed and evacuated silica vial. The compounds, which are separated to suppress an interface reaction, are heated to 800°–1000°C maintaining about a 50°C gradient along the vial. Spinel crystals form at the low‐temperature end of the vial embedded in a liquid phase which consists principally ofACl2. Commonly a number of crystals grow, usually separated by the liquid phase, but sometimes as a cluster of crystals. This technique yields larger crystals than have been obtained by previous methods (for CdCr2Se4and CdCr2S4) in which the maximum size was reported to be 2–3 mm. The antiferromagnetic ZnCr2Se4and ZnCr2S4crystals have, to our knowledge, been grown for the first time. The growth mode is principally octahedral with well‐developed (111) faces, although the larger crystals tend to be flat, truncated along [100] and [110]. Microscopic examination shows the crystals to be sound, containing few internal voids. The complex growth mechanism is discussed in general terms involving an initial vapor transport of the system components and the resultant liquid‐vapor evironment required for crystal formation. The effect of some substitutional (A‐site) doping experiments in CdCr2S4crystals is reviewed.

ISSN:0021-8979    

DOI:10.1063/1.1657508

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

The ferromagnetic semiconductors CdCr2S4and CdCr2Se4both show large (∼0.2 eV) shifts in their absorption edges as the temperature decreases below their Curie temperatures. In CdCr2S4, the absorption edge increases with decreasing temperature; while in CdCrSe4, it decreases with decreasing temperature. It has been proposed that these shifts arise from a lattice expansion or contraction which is, in turn, produced by exchangestriction. Using x‐ray diffraction, we have measured the lattice parameters of these two materials from room‐temperature down to 4.2°K. CdCr2S4is found to expand with decreasing temperature below its Curie point. The total fractional change is of the order of 10−3. CdCrSe4contracts, but its fractional change is even smaller than that of the sulfide. Therefore, if this small expansivity is to explain the absorption‐edge shift, it requires a deformation potential of the order of 100 eV, which is at least an order of magnitude larger than what one would reasonably expect. Therefore, it appears that exchangestriction is not responsible for the large absorption‐edge shifts in these materials.

ISSN:0021-8979    

DOI:10.1063/1.1657509

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

The shift in the Curie temperatureTchas been measured as a function of hydrostatic pressure up to 10 kbars for the chromium chalcogenide spinels CdCr2Se4, HgCr2Se4, CdCr2S4, and HgCr2S4, by measuring the mutual inductance of an astatic coil assembly containing the sample. Also the compressibility of these compounds has been measured for the same pressure range at three temperatures. The measured values ofTcanddTc/dPfor CdCr2Se4, HgCr2Se4, CdCr2S4, and HgCr2S4are 128°, 105.5°, 84°, and 36.1°K and −0.82, −0.95, −0.58, and +0.14°K/Kbar, respectively. Combining the compressibility measurements withdTc/dPthe values ofdTc/da=+66, +99, +46, and −10°K/Å are obtained for CdCr2Se4, HgCr2Se4, CdCr2S4, HgCr2S4, respectively. These values ofdTc/dafor CdCr2Se4and CdCr2S4are in qualitative agreement with the dilution studies for these compounds. However, the value ofdTc/dafor HgCr2S4is of opposite sign from that predicted from the solid‐solution studies for this compound. High‐pressure measurements seem to provide a better picture of variation of magnetic exchange interaction than dilution studies, because for the latter, the covalency effects become different due to changes in the nature of atomic environments. Conclusions are also made about the volume dependence of exchange interactions in these materials, and again it is quite different for HgCr2S4as compared to the volume dependence of exchange interactions for the other three compounds. This seems to be consistent with the recent experimental and theoretical studies for HgCr2S4.

ISSN:0021-8979    

DOI:10.1063/1.1657510

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

Electrical resistivities &rgr;, normal Hall coefficientsR0, and Seebeck coefficientsQ, are reported for a series of samples of the semiconducting ferromagnetic compound CdCr2Se4, doped with varying amounts of indium. The behavior of &rgr; and ofR0indicates that from room temperature down to 150°K the electrons deionize onto the donor centers. From 150°K down to about 55°K, the carrier concentration increases by approximately four orders of magnitude, accompanied by a proportional drop in &rgr;; this can be attributed to the known decrease in bandgap with lowering ofT, if the donor level is assumed to remain constant with reference to the valence‐band edge. The rise of &rgr; below 55°K is ascribed to impurity conduction. The mobilities reach a maximum nearTc, and fall off on both sides. Strong dependence ofR0on the magnetic fieldHis found above 110°K. The magnetoresistance is negative, and peaks sharply at 130°K; it obeys different power laws on the two sides of its peak. The Seebeck coefficientQshows unique dependence onT, Hand on In content: it displays local extrema at 150°K and at 125°K. Between these two temperatures,Hhas a strong effect onQ, depending on the In concentration. No simple relation between the behavior ofQon the one hand, and of &rgr; andR0on the other, has been deduced. The model suggested by the data requires two bands, with charge carriers whose concentrations and mobilities are very different and have strong dependencies onT.

ISSN:0021-8979    

DOI:10.1063/1.1657511

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

The spinel HgCr2S4has a spiral antiferromagnetic spin structure which can be ferromagnetically aligned by application of a moderate magnetic field.1The presence of the nonmagnetic Hg and S ions affords an excellent opportunity to corroborate and investigate the details of the magnetic structure by study of the hyperfine interaction of these nuclei. We have observed the NMR of the199Hg,201Hg, and53Cr nuclei in polycrystalline samples at 1.4°K. The Hg nuclei, which are situated in layers between aligned sheets of Cr spins, exhibit a magnetic anisotropy which disappears with ferromagnetic alignment. The spectral position and shape depend strongly on magnetic field. In zero magnetic field, the center of the Hg spectra is at 507 kOe, and that of the Cr spectrum is at 190 kOe. The intensities of the spectra are field dependent, and are observed to initially increase up to 5 kOe and then decrease in higher magnetic fields. An analysis of the Hg spectra has been made in which the magnetic anisotropy is shown to be a consequence of the spiral spin structure. The strong magnetic‐field dependence of the spectra is found to be a result of the changing spin structure. A fuller account of this work will be presented elsewhere.

ISSN:0021-8979    

DOI:10.1063/1.1657512

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

One possible explanation for the rapid development of antiferromagnetism in the system MnCr2S4&sngbnd;MnInCrS4is the existence of antiferromagnetic interactions among theA‐site Mn. The normal spinel MnSc2S4was prepared and studied to determine the actual magnitude ofA‐Ainteractions in sulfide spinels. The cubic lattice parameter isa=10.615 Å withu=0.381. Magnetic susceptibility measurements showed this material to be an antiferromagnet with a Ne´el temperature below 4.2°K and with a &thgr;‐parameter of about −21°K. The magnitude of the Mn&sngbnd;Mn exchange interaction was determined by fitting the susceptibility data from 15° to 295°K to the [3, 3] Pade´ approximant to the Rushbrooke‐ Wood series expansion. The value of the Mn&sngbnd;Mn exchange constant was found to beJ/k=−0.80°K. The use of Pade´ approximants rather than the truncated series itself greatly improves the applicability of the power series expansions at temperatures comparable to or below the value of the &thgr;‐parameter.

ISSN:0021-8979    

DOI:10.1063/1.1657514

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

We have studied the transitions induced by fields of arbitrary direction between the antiferromagnetic, the spin‐flop, and the paramagnetic phase of a two‐sublattice uniaxial antiferromagnet at zero temperature in molecular‐field approximation. We find that the phase boundary between the antiferromagnetic and the spin‐flop phase in theHplane extends only to a maximum angle with respect to the easy axis of order (anisotropy energy/exchange energy), where it ends in critical points. This behavior explains the sensitive angular dependence of ultrasonic absorption and magnetic resonance experiments.

ISSN:0021-8979    

DOI:10.1063/1.1657515

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

A crystal‐field calculation of the low‐lying levels of a Fe2+ion in FeCl2shows that the lowest lying levels, a doublet and a singlet, are well separated from the remaining levels. The relative isolation of these levels suggest taking the effective spin as unity. The Hamiltonian incorporates a Zeeman term, an exchange term between spinsS=1, and an anisotropy term. An RPA Green's function theory is derived, and, because of the highly anisotropic exchange, is reduced to an expansion about the molecular‐field theory, in powers of the non‐Ising exchange factors. It is found that even in MFT the metamagnetic‐transition field has temperature dependence which follows well that of the zero‐field sublattice magnetization.

ISSN:0021-8979    

DOI:10.1063/1.1657516

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

Recent quantitative measurements of frequency and temperature dependence of the ultrasonic attenuation in the vicinity of the magnetic phase transition for a number of materials (Gd, Tb, Dy, Ho, RbMnF3, MnFr2, Cr) enable us to compare these results critically with a number of recent theories. For each of these materials we know the spin‐phonon coupling mechanism and the coupling strength fairly well. The comparison shows that the anisotropic case (materials with a rather large uniaxial anisotropy) is better understood than the isotropic one. The only case where the spin‐phonon coupling is of the single ion magnetostrictive type, shear wave in Dy, the critical attenuation can be semiquantitatively accounted for by considering higher‐order spin components in the magnetoelastic Hamiltonian.

ISSN:0021-8979    

DOI:10.1063/1.1657517

出版商:AIP    

年代:1969

数据来源: AIP