摘要:

The antiferromagnetic perovskites have a simple cubic‐magnetic structure with nearest‐neighbor exchange interactions only. The perfect‐crystal Green functions can be easily expressed in terms of tabulated integrals, so that they are particularly suitable for making quantitative comparison between theory and experiment. This is done for exciton‐magnon interactions in pure crystals, and for magnons associated with impurities. Electric‐dipole absorption in the pure crystal can occur when an exciton is created on one magnetic ion and a spin deviation on its nearest neighbor. The resulting magnon sideband reflects the interaction between the exciton and magnon which are in close proximity. If the exchange between a pair, when one is electronically excited isJ′, and the normal exchange isJ, then satisfactory agreement with the experimental lineshape can be obtained forJ′=0.5Jin RbMnF3. The positions of localized magnons are calculated as a function of impurity‐host exchange, and compared with experimental results on Ni2+in RbMnF3. The symmetry of the defect system is also considered, showing that additional terms are allowed. The most important of these is probably electric‐dipole absorption in the far infrared, creating two localized magnons.

ISSN:0021-8979    

DOI:10.1063/1.1657815

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

We have observed a magnetic dipole‐active localized mode in MnF2, doped with 0.04 mol% FeF2at 94.8 cm−1at 1.2°K, using an ir lamellar interferometer. To our knowledge, this is the first direct spectroscopic evidence using far ir techniques for a magnetic dipole‐active impurity mode in an otherwise magnetically ordered system. The line has the following features: the half‐width is 0.4 cm−1and the absorption strength ∫&agr;dv∼1 cm−2. With increasing temperature, the line shifts to lower frequencies and finally disappears at about 25°K. Magnetic fields up to 45 kG have been applied parallel to thecaxis of the sample, yielding a linear splitting of the line. Agfactor of 2.3 is derived from the data. The experimental results at 1.2°K can be understood satisfactorily both with a simple molecular field picture and, also, with the help of a simple spin‐wave calculation. In the molecular field approximation, the line is correlated to a transition of the ground spin state to the first excited state of the orbital ground state of Fe2+. The observed value for the frequency is fitted by a Fe2+&sngbnd;Mn2+exchange coupling constantJ=1.9 cm−1in both models. Preliminary results on a localized mode in MnF2:Co2+are also reported.

ISSN:0021-8979    

DOI:10.1063/1.1657816

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

Measurements have been made of the absorption and fluorescence spectrum of Cr3+in orthorhombic YCrO3, LuCrO3, and GdCrO3. We observe several very sharp emission lines at energies about 100 cm−1lower than the intrinsic4A2to2ECr3+absorption, and attribute the emission to impurity‐perturbed Cr3+ions. Intrinsic emission was not observed. Magnon sideband fluorescence is found at lower energy and results in the simultaneous optical de‐excitation of a perturbed Cr3+ion and the creation of a magnon on a neighboring lattice site. The sideband is spin allowed if the photon and magnon are created on the same sublattice, and has electric dipole polarization. Analysis of the sideband line shape shows the nearest‐neighbor exchange interaction between pairs along thecaxis, and between pairs in the plane perpendicular to thecaxis are not equal. The value ofJaveraged over the six nearest neighbors is 22.1, 17.6, and 25.5 cm−1for the Y, Lu, and Gd chromites, respectively. These results are in good agreement withJvalues computed from magnetic data.

ISSN:0021-8979    

DOI:10.1063/1.1657817

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

A study has been made of spin waves in the antiferro‐, ferri‐, and ferromagnetic phases of FeCl2·2H2O. These levels are found to interact strongly with a field independent level at 31.5 cm−1. A similar interaction is found in CoCl2·2H2O near 29.3 cm−1. This field independent level has been tentatively identified as ak=0, low‐lying optical phonon. This phonon appears to be infrared active only because of its coupling with nearby magnon levels. The experimental data for both materials is well described by a phenomenological magnon‐phonon interaction.

ISSN:0021-8979    

DOI:10.1063/1.1657819

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

The anomalous and ordinary Hall coefficientsRsandR0defined by the relation &rgr;H≡R0B+Rs4&pgr;Mhave been obtained for single‐crystal Tb in both ordered and paramagnetic states. Results for the component of the Hall resistivity &rgr;Hcharacterized byH(andM) in the basal plane 101¯0 easy axis in Tb) show that &rgr;His linear inHaboveTNand exhibits a strong temperature dependence which is almost entirely due to that of the magnetic susceptibility.Rsis found equal to −40.4×10−12&OHgr;·cm/G and temperature independent. BelowTNthe anomalous coefficient found from the saturation Hall resistivity reverses sign, passes through a maximum near 140°K (whereRs=+22.4×10−12&OHgr;·cm/G) and falls to 0 below 30°K. Possible mechanisms for this unique sign reversal inRsare discussed. TheT‐independent nature ofRsaboveTNplus a linear dependence ofRson the derived magnetic resistivity below 120°K indicates that thermal disorder of the 4fspin system is the dominant scattering mechanism in the Hall effect. The normal coefficient is essentially temperature independent above approximately 260°K (R0=−1.0×10−12&OHgr;·cm/G) and below 90°K. At intermediate temperaturesR0rises to a positive maximum of +8.8×10−12&OHgr;·cm/G at 205°K. Hall resistivity data aboveTNwithHalong ⟨0001⟩ directions (second independent component in hcp symmetry) give significantly larger values ofRsandR0reflecting the Fermi surface anisotropy.

ISSN:0021-8979    

DOI:10.1063/1.1657500

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

The magnetic structure properties of several specimens in the gadolinium‐scandium and gadolinium‐yttrium alloy systems have been determined by neutron diffraction using the low‐absorbing isotope160Gd. Neutron patterns were taken from annealed powder samples at temperatures from 4° to 300°K. In both systems, the alloys show ferromagnetism at high gadolinium concentrations and antiferromagnetism at low concentrations. The concentration at which this change from ferro‐ to antiferromagnetism occurs is about 80 at.% for Gd&sngbnd;Y and about 70 at.% for Gd&sngbnd;Sc. The observed antiferromagnetic structures are spiral type with turn angles between the moments in adjacent layers which approach 50 deg. per layer at low Gd concentrations. The ordering temperatures decrease with decreasing Gd concentration more rapidly in the Sc than in the Y alloys. Thus these alloys behave similarly to the other rare earth Y and Sc alloys studied previously. Dilution of Gd by Y and Sc affects the anisotropy of gadolinium differently. An 80 at.% Gd in Sc alloy has a temperature variation of the angle the ferromagnetic moment makes with thecaxis similar to pure Gd in which this angle is 0° atTc, increases to 65° as the temperature is lowered, and then returns to about 32° at very low temperatures. An 80 at.% Gd in Y alloy, on the other hand, has its ferromagnetic moment about 70° from thecaxis at all temperatures belowTc. In both systems, the antiferromagnetic spiral structure has the moments in the base plane perpendicular to thecaxis.

ISSN:0021-8979    

DOI:10.1063/1.1657501

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

The thermal conductivities of gadolinium, terbium, and holmium single crystals have been measured as a function of temperature from 5° to 300°K. The steady‐state heat flow method was used. For each element measurements were taken in the ⟨112¯0⟩ (aaxis) and in the ⟨0001⟩ (caxis) directions of the hcp crystal structure. Electrical resistivity measurements were made on the same samples to obtain Lorenz functions. Anomalies near magnetic transition temperatures are interpreted in terms of magnetic superzones and spin‐disorder scattering. High‐temperature anisotropy is interpreted in terms of Fermi surface anisotropy. Lorenz numbers appreciably larger thanL0suggest that, in addition to electrons, phonons and magnons can carry heat.

ISSN:0021-8979    

DOI:10.1063/1.1657502

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

SmB6has been found to show no magnetic ordering down to a temperature of 0.35°K, in spite of the fact that the other rare earth hexaborides showed ordering in the neighborhood of 10–20°K.1,2We interpret this to mean that the Sm ion is in its nonmagnetic divalent configuration at low temperatures. Direct evidence for the changing electronic configuration of Sm from trivalent to divalent with decreasing temperature has now been found by the observation that the size of the unit cell goes through a minimum near 150°K. The magnetic susceptibility, which has been measured between 1° and 800°K shows evidence for the presence of primarily Sm+3above 300°K and Sm+2below 100°K. Below 40°K the resistivity of SmB6increases asTdecreases with an activation energy of about 0.002 eV. Such semiconducting behavior is to be expected from divalent Sm in the hexaboride lattice.

ISSN:0021-8979    

DOI:10.1063/1.1657504

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

Measurements of the low‐temperature (0.1° to 1.0°K) heat capacities of several cubic Laves intermetallic compounds containing praseodymium indicate the presence of a substantial nuclear hyperfine contribution to the specific heat. It is possible to relate the magnitude of this contribution to the magnetic moment carried by the Pr3+ion in each of the materials studied. The moments deduced in this way are PrNi2, 0.582±0.006; PrCo2, 2.18±0.05; PrAl2, 2.65±0.03 &mgr;B. These are each significantly less than the free Pr3+ionic moment of 3.20 &mgr;B. The results are discussed in terms of a theory of Bleaney and it is concluded that the crystalline electric field is the same in PrCo2and PrNi2, the large moment in the former being due to a larger exchange interaction. In the case of PrAl2, the data indicate an accidental degeneracy in the crystalline field ground state.

ISSN:0021-8979    

DOI:10.1063/1.1657505

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

The two groups of metallic compounds Gd3+X3−(X=N, P, As, Sb, Bi) and Gd3+Z2−(Z=S, Se, Te) have the rocksalt structure and except for ferromagnetic GdN they are antiferromagnetic. Powder neutron diffraction measurements on GdS, GdSe, GdSb, and GdBi show them to have order of the second kind. The exchange interactions are discussed on the basis of the magnetic structure and susceptibility measurements.

ISSN:0021-8979    

DOI:10.1063/1.1657506

出版商:AIP    

年代:1969

数据来源: AIP