Index of content:
Volume 35, Issue 3, 01 March 1964
- RARE‐EARTH METALS AND ALLOYS
35(1964); http://dx.doi.org/10.1063/1.1713364View Description Hide Description
Gd5Pd2 is ferromagnetic below 335°K. The isostructural Tb5.10Pd1.90, Dy5.07Pd1.93, and Ho5.04Pd1.96 have paramagneticmetamagnetic transitions at 62°, 41°, and 27°K, respectively, and become ferromagnetic at lower temperatures with extremely high hysteresis. Energy products (B×H)max = 20 to 26×106 GOe at 4.2°K have been observed.
35(1964); http://dx.doi.org/10.1063/1.1713365View Description Hide Description
Additional data on the magnetic properties of the rare‐earth metal Eu have been obtained from neutron diffraction data utilizing an improved powder sample consisting of metal filings. Previous diffraction data on Eu, obtained from a rolled metal foil sample, indicated that the metal became antiferromagnetic at a Néel temperature of 87°K. Information regarding the magnetic moment could not be obtained from the metal foil since the diffraction peak intensities indicated the sample had a preferred orientation; also, the magnetic structure determination was uncertain.
The present data verify the transition to an antiferromagnetic state below 91°K which consists of a helical spin structure with the magnetic moments lying parallel to a cube face and with the screw axis directed perpendicular to the moments or along the  direction. This structure is based upon the facts that the diffraction data show a single pair of magnetic satellite reflections equally spaced in reciprocal lattice space about each nuclear reflection and, in particular, the [000±] reflection from the magnetic cell is observed. Information regarding the period of the helical structure was obtained from the separation of the satellite reflections. The separation was observed to change very little with temperature and corresponds to a period of 3.5a at T/TN = 1 and increases to 3.6a at T/TN = 0.05, where a is the lattice spacing.
The intense [000±] magnetic reflection occurring at a scattering angle of 4.8° permits a calculation of the ordered magnetic moment in Eu. If one takes the magnetic form factor equal to 1 at the above small scattering angle, the experimental saturated ordered moment is 5.9±0.4 μ B . Assuming that the magnetic constituent of metallic Eu is a Eu++ ion and is, therefore, characterized by an 8 S 7/2 state having spin only, the theoretical ordered moment should be g J = gS = 7 μ B . The smaller value of the measured ordered moment as compared to the maximum theoretical value is also characteristic of other rare earth metals.
Data on the product of magnetic moment times magnetic form factor (μf) can be obtained from the magnetic satellite diffraction peaks occurring at various scattering angles. These values from Eu are in reasonable agreement with similar values of μf obtained from the compound EuO in earlier work.
The intensity of the magnetic diffraction peaks as a function of temperature deviates from the usual Brillouin pattern. Instead, the magnetic intensity variation with temperature is proportional to (TN−T)½ over a considerable region below the Néel temperature; this same temperature dependence was also observed in the rolled foil sample. Data taken on the intensity of the nuclear peaks at four different sample temperatures above the Néel temperature over the range from 100° to 293°K indicate an apparent Debye temperature variation from about 70° to 122°K, respectively.
We expect to publish the above results giving more details in another journal in the near future.
35(1964); http://dx.doi.org/10.1063/1.1713366View Description Hide Description
The magnetic properties of intermetallic compounds of stoichiometry Fe3MnR, where R is Y, Tm, Er, Dy, or Ho, have been measured in fields up to 14 kOe at temperatures ranging from 1.4° to 300°K and in fields up to 80 kOe at 4.2°K. The moments of the Mn and R atoms are in opposition to those of the Fe atoms so that a compensation point is observed for Fe3MnTm. When Fe3MnTm is cooled in a weak field from 300°K through its compensation point at 60° to 4.2°K the magnetization reverses and a field of 25 kOe is required to reduce it to zero.
35(1964); http://dx.doi.org/10.1063/1.1713367View Description Hide Description
The bcc structure occurs as a defectstructure in 2:3 compounds of rare‐earth elements with S, Se, and Te, or as an inverted structure in 4:3 compounds with group VA elements. The structural, electric, and magnetic properties of the metallic 4:3 compounds and the 2:3 rare‐earth semiconductors have been investigated with specific reference to gadolinium, for which the S ground state gives the minimum crystal field effect.
Gd4Bi3 and Gd4Sb3 are both ferromagnetic with a saturation magnetization obeying the spin‐wave law up to 0.8 of the Curie temperature. The Curie temperatureTc varies in the solid solution system Gd4Sb3– Gd4Bi3 with composition from 260° to 340°K. The semiconducting compound Gd2Se3(ρ RT =3 Ω cm) has been found to be antiferromagnetic below TN =6°K. Solution of Gd in the holes of the defect Th3P4structure decreases the electrical resistivity without a measurable variation of lattice constant (a 0=8.718 Å). With increasing conductivity the material changes from antiferromagnetic to ferromagnetic. At the composition Gd2.1Se2.9 the Curie temperature is Tc =80°K with ρ RT =1.4×10−3 Ω cm. The relation between Curie temperature and electrical resistivity has been examined by introducing the same spin concentration into Gd2Se3 by Eu2+ doping. The semiconductor Eu0.5Gd1.6Se2.9 is paramagnetic. However, Y0.5Gd1.6Se2.9 has low resistivity and is ferromagnetic below Tc =47°K.
35(1964); http://dx.doi.org/10.1063/1.1713368View Description Hide Description
Magnetic moment and electrical resistivitymeasurements of the Curie (TC ) and Néel (TN ) points of Gd‐Dy alloys show that the region of spiral structure between T C and TN narrows as Gd is added to Dy and disappears at about 50 at. % Gd. Apparently no spiral structure exists in Gd or Gd‐rich alloys. The magnetic moments of Dy‐rich alloys show the saturation expected when the magnetization is confined to the hexagonal plane (Jgπ/4), but the moments gradually approach the theoretical Jg as the Gd content increases toward 100%. The spin disorder resistivities are not linear in (g‐1)2 J(J+1); this suggests a variation in the electronic structure of these alloys.
35(1964); http://dx.doi.org/10.1063/1.1713369View Description Hide Description
Neutron diffraction measurements have been made on a single crystal of neodymium at temperatures between 1.6° and 20°K. The onset of magnetic ordering occurs at 19°K and the magnetic structure changes at 7.5°K. The crystal structure is hexagonal with a four‐layer stacking sequence of type ABAC. Thus, alternate layers have different nearest‐neighbor environments, either hexagonal or face‐centered cubic. The main features of the diffraction pattern can be interpreted on the basis of an approximate model in which the hexagonal sites order at 19°K with an antiferromagnetic arrangement between alternate hexagonal layers and with a sinusoidal modulation within each layer. The propagation vector for this modulation and the direction of the magnetic moments are both in the basal plane along a b 1 reciprocal lattice vector. The magnitude of the propagation vector changes from 0.13 b 1 at 18°K to 0.11 b 1 at 7.5°K. The cubic sites order at 7.5°K with a similar structure, except that the propagation vector is 0.15 b 1 and the moments are in the basal plane at an angle of 30° from b 1. The amplitude of the modulated moments is 2.3±0.2 Bohr magnetons for the hexagonal sites and 1.8±0.2 Bohr magnetons for the cubic sites. This model fails to account for all the observed intensities but may be improved by adding a small modulated moment on the cubic sites in the upper temperature region which is shifted in phase by ½π from the modulation in the hexagonal layers, and by allowing the hexagonal moments to deviate slightly from the b 1 direction.
35(1964); http://dx.doi.org/10.1063/1.1713370View Description Hide Description
The thermal conductivity (λ) of a polycrystallinegadolinium sample, having an electrical resistivity of 2.41 μΩ cm at 4.18°K, has been studied as a function of temperature (T) between 5° and 310°K. The thermal conductivity has a maximum value of 0.333 W cm−1 °K−1 at 16.5°K. Changes in the slope of the λ vs T curve occur at 230° and 270°K. The former is believed due to the rapidly varying easy cone of magnetization. The second anomaly is associated with the ferromagnetic‐paramagnetic transformation. The Lorenz function, from measured thermal conductivity and electrical resistivity data on the same sample, has been calculated as a function of temperature. This function clearly indicates that in addition to the electronic thermal conductivity there is considerable phonon and possibly some magnonheat transport.
35(1964); http://dx.doi.org/10.1063/1.1713371View Description Hide Description
A single‐crystal disk of Gd cut parallel to (00.1) has been studied in zero magnetic field. A careful search along and perpendicular to the c* axis in the neighborhood of the (00.2) reflection has failed to reveal any satellites in the temperature range 77° to 290°K. Thus unlike the rare‐earth elements from Tb to Tm, gadolinium is a normal ferromagnet. Above 248°K, the moment is aligned along the c axis. Below this temperature, the moments make an angle with the c axis which reaches a maximum of 75° at 195°K and approaches 30° at 4°K. The coherent nuclear scattering amplitude has been estimated by scaling the nuclear scattering to the magnetic scattering in the (00.2) reflection. The result is |b|=1.5±0.2×10−12 cm.
35(1964); http://dx.doi.org/10.1063/1.1713372View Description Hide Description
Measurements are reported on the behavior of magnetization with field of single‐crystal dysprosium,erbium, and holmium in pulsed magnetic fields up to 165 kOe. These elements go progressively on decreasing temperature from a paramagnetic to antiferromagnetic to ferromagnetic state. Sufficiently high external fields applied along spin axes in single‐crystal samples can cause spin realignment by overcoming the internal exchange and anisotropy fields. The strengths of the internal fields can be determined in this manner. Measurements with an external fieldH applied in the basal plane of Dy and of Ho, and with H parallel to the c axis in Er, confirm previous static field measurements of Hc for the AF/F transition. No change in spin alignment is seen in Dy with H parallel to c up to 165 kOe. In Ho below 80°K fields near 100 kOe produce strong changes in magnetization with H parallel to the c axis. Below 20°K fields up to 17 kOe in the basal plane in Er produce a sharp change in magnetization indicative of spin realignment. The magnetization of Er with H parallel to a is measured at 4.2°K in static fields up to 56 kOe. The magnetization increases sharply between 15 and 20 kOe, reaching 50 emu/g at 20 kOe. Little change is found above 40 kOe where M is 120 emu/g. These results are discussed with regard to the internal fields in Dy, Ho, and Er.
35(1964); http://dx.doi.org/10.1063/1.1713373View Description Hide Description
Magnetic moments of the series of rare‐earth copper (RCu2) intermetallic compounds have been measured at temperatures from 1.4° to 300°K. These compounds have an orthorhombic crystal structure and are isostructural with CeCu2. The susceptibility at high temperatures of most of the compounds followed the Curie—Weiss law giving effective moments close to those of the trivalent rare‐earth ions. Eu and Yb appeared to be divalent. At low fields antiferromagnetic behavior was indicated in some of these compounds by peaks in the curves of moment vs temperature, and by a lack of remanence.Metamagnetic behavior was observed when measurements were made at 4.2°K and with fields up to 80 000 Oe. For example, the moment of TbCu2 increased abruptly at a critical field, and at 80 000 Oe, although not yet saturated, had a moment of 6.2 Bohr magnetons. The moments of EuCu2 and GdCu2 increased gradually, and at 80 000 Oe reached moments of 5.7 and 6.0 Bohr magnetons, respectively. These results indicate a relatively weak antiferromagnetic exchange interaction in these compounds.
35(1964); http://dx.doi.org/10.1063/1.1713374View Description Hide Description
TbMn2crystallizes with the cubic Laves‐phase structure (C15‐type) which may be regarded as the spinel structure without the anions. The ``A'' sites are occupied by Tb ions and the ``B'' sites by Mn ions. Neutron diffractionpowder patterns run at 4.2°K exhibit a number of extra reflections characteristic of a modulated magnetic structure. These additional reflections can be indexed on the basis of a spiral modulation propagated along  with a wavelength of 8.0 Å. The spiral axis is along . The ``B'' sites are ordered in a manner which is entirely analogous to the low‐temperature ordering of Fe3O4; the two sublattice spirals being 180° out of phase. The A sublattice is in phase with a particular one of the two B sublattices. The combination of the internally compensated B sites and the ferromagnetically coupled A sites leads to a ferrimagnetic spiral.
35(1964); http://dx.doi.org/10.1063/1.1713375View Description Hide Description
Uranium monosulfide, a high‐melting‐point compound (2740°K) with a NaCl structure has been found to be ferromagnetic with a Curie point at 180°K and a saturationmagnetic moment of 1.02 Bohr magnetons per atom at 0°K. Resistivity,magnetoresistance, and Hall effect measurements were made in the temperature range, 4.2°–360°K, so that these properties could be measured in the ferromagnetic and the paramagnetic solids. Sintered polycrystalline samples were used.
The electrical resistivity was found to be about a hundred times larger in US than in the common ferromagnetic metals but the temperature dependence was similar showing an abnormal decrease below the Curie point. The magnetoresistance was found to be negative with a sharp minimum at the Curie point. Study of the Hall effect revealed that two Hall coefficients could be identified as in the transition metals. The extraordinary Hall coefficient related to the magnetization was found to vary as the 2.1 power of the resistivity in reasonable agreement with previous theories of this effect. The ordinary Hall coefficient related to the applied field was found to be positive and temperature‐dependent with a maximum at the Curie point, whereas in the ferromagnetic metals, the Hall coefficient is found to be temperature‐independent with the exception of an anomalous behavior near the Curie point. A similar anomaly is observed in US which occurs over a much wider temperature range than in the common ferromagnetic metals. The effective carrier concentration evaluated from the ordinary Hall coefficient is 0.45 holes per U atom at absolute zero.
These results can be correlated with a band picture based on overlapping 7s and 5f bands, where the states in the broad 7s band contribute the major conductivity whereas the magnetic behavior is due to the unpaired spins of electrons in the narrow 5f band. Equilibrium between the bands requires a redistribution of electrons between them as the temperature varies giving rise to the observed temperature dependence of the galvanomagnetic properties. Similar measurements on isostructural ThS and UP and their solutions with US will provide a critical test of this band model.