摘要:

Complete understanding of magnetic impurity modes can be obtained from their energies and wavefunctions. Energies are obtained from absorption, Raman, and fluorescent measurements but wavefunction information fromgfactors is most useful when the impurity and hostgfactor are very dissimilar. To this end, impurity modes in MnF2:Co2+have been studied by far ir absorption and Raman scattering, respectively. Thegfactor for thes0mode is measured to be 3.21 and differs substantially from 4.24, the value obtained by EPR in the diamagnetic isomorph MgF2. Thes0energy andgfactor can be explained consistently by exchange mixing of the ground state with excited orbital states of Co2+. Thegfactor for the pair mode at 170.3 cm−1was found to be 1.61, significantly larger thang(s0) ‐g(Mn2+) = 1.2. The Ising pair mode energies andgfactors have been renormalized by including transverse exchange to second order. The correct energies for the impurity modes are obtained but thegfactors of the pair modes do not agree with experiment. Higher‐order spin interactions produced by the orbital motion on the cobalt ion do not significantly alter this result.

ISSN:0021-8979    

DOI:10.1063/1.1659004

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

Recent work has shown that it is essential to include magnon‐magnon interaction effects in order to calculate the two‐magnon line shape for optical processes in antiferromagnets. These experiments are very sensitive to any structure in the density of states, and so it is necessary to include bothJ1andJ2(obtained from inelastic neutron scattering measurements) even thoughJ1is much smaller thanJ2. We have calculated line shapes for Raman scattering and infrared absorption in MnF2and obtained much improved agreement with the experimental results. We show that the sharp structure in the density of states and magnon‐magnon interactions are equally important in determining the line shape. Simple arguments based on the Ising model to get the shift in peak position that are valid for RbMnF3, where there is a single sharp peak in the density of states, are no longer correct in MnF2. We conclude that a smallJ3exchange and a possible extended range of interaction between light and the magnetic system can only have a small effect on the optical spectra.

ISSN:0021-8979    

DOI:10.1063/1.1659005

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

Raman scattering of 5145‐Å argon laser radiation by two magnons in the perovskite antiferromagnet KNiF3has been observed. At low temperature, the spectrum of the scattered light is in excellent agreement with a Green's function theory for a perovskite withS= 1 and nearest‐neighbor exchange constantJ= (71.0±0.8) cm−1. If the effects of anisotropy (obtained from far‐infrared antiferromagnetic resonance measurements) are included, the value ofJis changed only slightly, decreasing ≈0.5 cm−1. The line shape and position were also observed as functions of temperature. A comparison is made with similar scattering observed inRbMnF3 (S=52)and in related magnetic Ni2+fluorides.

ISSN:0021-8979    

DOI:10.1063/1.1659006

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

The inelastic scattering of neutrons by two‐magnon processes has been studied in MnF2at 4.2°K. The two‐magnon scattering was seen most clearly at the reciprocal lattice point (2&pgr;/a, 0, 0) where it took the form of a shoulder on the high‐frequency side of the one‐magnon peak with a well‐defined cut‐off in energy. Similar results were obtained at other wavevectors around this point but sharp peaks due to magnetovibrational scattering were then superposed on the broad two‐magnon scattering distribution. The line shape, the cut‐off in energy, and the intensity calculated from the cross‐section formulas are consistent with experiment.

ISSN:0021-8979    

DOI:10.1063/1.1659007

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

The unusual magnetic and electrical properties of the semiconductor SmB6have been closely investigated recently. Primarily from magnetic susceptibility measurements, it has been hypothesized that the Sm valence changes from 3 + (4f5,6H5/2) at high temperatures to 2 + (4f6,7F0) at low temperatures. We have used the Mo¨ssbauer effect in149Sm to make a direct determination of the valence state by means of the ``isomer shift,'' which measures thes‐electron density at the Sm nucleus. This density changes with the 4fconfiguration because of coulombic shielding effects. Unfortunately, the change between 4f5and 4f6is too small to resolve resonance lines from the two charge states. The observed isomer shift of −0.4 mm/sec lies approximately halfway between the values (∼−0.9 and ∼0. mm/sec−1respectively) obtained for well characterized ionic di‐ and trivalent Sm compounds, and is not observed to vary between room temperature and 1.1°K. A new model is presented to explain both the magnetic susceptibility data and the Mo¨ssbauer results.

ISSN:0021-8979    

DOI:10.1063/1.1659008

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

The Pr3+4f23H4manifold is split by an octahedral crystal field into four levels &Ggr;1, &Ggr;3, &Ggr;4, and &Ggr;5. Transitions between these levels in PrBi have been measured using neutron time‐of‐flight spectrometry. Peaks in the observed spectra corresponding to crystal‐field transitions are identified both by their temperature dependence and by comparison with corresponding data from (nonmagnetic) LaBi. Well resolved crystal‐field transitions are observed at 11.7 and 5.8±0.6 meV at 90°K. Additional scattering observed at room temperature is consistent with unresolved transitions at 7.6 and 4.1 meV. As it is known from magnetic measurements that the &Ggr;1singlet lies lowest, these four transitions may be identified as &Ggr;5to &Ggr;4, &Ggr;4to &Ggr;1, &Ggr;5to &Ggr;3and &Ggr;3to &Ggr;4respectively, thus accounting for all transitions allowed by magnetic‐dipole selection rules. These data give the parametersA40⟨r4⟩=6.89±0.30 meVandA60⟨r6⟩=0.24±0.10 meV.

ISSN:0021-8979    

DOI:10.1063/1.1659009

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

There is no satisfactory fit to the ferromagnetic Curie points of the rare‐earth intermetallic compounds having a Laves‐phase structure using the Ruderman‐Kittel‐Kasuya‐Yosida exchange interaction. By including a pair interaction of the form LA·SBwe are able to improve the fit to the Curie points. This term enters when we include the effects of spin‐orbit coupling of the conduction electrons on the indirect exchange interaction.

ISSN:0021-8979    

DOI:10.1063/1.1659010

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

SmB6behaves as a nonmagnetic semiconductor at low temperatures. Gd‐ and Eu doped samples have been prepared. The paramagnetic behavior of the doped materials shows that Eu remains divalent and consequently it does not alter the low‐temperature electrical properties appreciably. Gd, on the other hand, increases the electrical conductivity and the paramagnetic behavior corresponds to the trivalent Gd ion. The paramagnetic susceptibility flattens out below 0.1°K indicating a magnetic ordering of the Eu samples in contrast to the pure SmB6.

ISSN:0021-8979    

DOI:10.1063/1.1659011

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

Uranium monopnictides (Uv) and monochalcogenides (Uvi) and their solid solutions have a NaCl‐type structure and are good conductors of electricity. Measurements of the magnetic properties of these compounds show that the magnetic interactions between the localized 5f‐electrons are of long range. Such long‐range interactions are responsible for the type‐I antiferromagnetic structure of Uv compounds and the ferromagnetic structure of Uvi compounds, although these magnetic structures could be explained by molecular field theory. A neutron diffraction study of the UP‐US solid solutions has revealed two new long‐range magnetic structures at intermediate compositions, the type‐IA (2+, 2−) antiferromagnetic structure (found also in UAs at low temperatures) and the antiphase (5+, 4−) ferrimagnetic structure. These structures are due to long‐range RKKY‐type interactions via the conduction electrons. Analogous structures are observed in the lanthanides (e.g., thulium) and their metallic compounds. As the weight of the anions present increases, the importance of superexchange interactions increases and affects the type of magnetic ordering. This effect is more pronounced in lanthanide monopnictides and europium monochalcogenides than in the Uv and Uvi compounds.

ISSN:0021-8979    

DOI:10.1063/1.1659012

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

Low‐temperature specific heats were measured in the systems of borides Co2B&sngbnd;Ni2B and FeB&sngbnd;CoB, especially nearcF, the critical composition for ferromagnetism. In the critical concentration range the linear term &ggr;Tis largely increased and the specific heat curves reveal anomalies at low temperature. Except for thiscFrange, the &ggr; values, in conformity with the magnetic moments, describe the filling up of adband in a rigid band model.

ISSN:0021-8979    

DOI:10.1063/1.1659013

出版商:AIP    

年代:1970

数据来源: AIP