S. G. Sankar

Magnetic properties of hydrides of the rare earths and rare earth intermetallics

Rare earth elements (R) react with hydrogen at elevated temperatures to form hydrides with compositions approaching RH3. Intermetallic compounds involving the rare earths in chemical union with Mn, Fe, Co or Ni absorb copious quantities of hydrogen rapidly and dissociatively at room temperature. In all the hydrides RH3 and in many of the hydrogenated intermetallics the proton density exceeds that in condensed elemental hydrogen. The presence of such large amounts of hydrogen significantly modifies various physical properties of the host metal. In the elemental rare earths hydrogenation produces a loss in metallic conduction and greatly weakens exchange. Magnetic ordering is suppressed by as much as 250 degrees. In contrast hydrogenation of intermetallics may either strengthen or weaken exchange, the latter predominating numerically in the systems studied to date. Contrasting behavior is exhibited by the isostructural pair of compounds Y6Mn23 and Th6Mn23. Th6Mn23 exhibits Pauli paramagnetism whereas Y6Mn23...

Thermal, magnetic and electrical characteristics of PrNi5

Abstract Results of measurements of the heat capacity, magnetic susceptibility and electrical resistivity of PrNi5 are presented for temperatures ranging from 2 to 300°K. Magnetization of PrNi5 goes through a maximum at about 16°K. Heat capacity measurements fail to show a λ-type thermal anomaly peaking at 16°K, indicating that this is not a Neel point. The difference between the heat capacity of PrNi5 and non-magnetic LaNi5 is Schottky-type with two maxima, at 16° and 58°K. The maxima in Cp and magnetization are consequences of the crystal field interaction, which leads to Van Vleck paramagnetism in this material at low temperatures. A reasonably successful analysis of the heat capacity and susceptibility results was made using the Hamiltonian, ℋ = g μ B J ⋅ H + W 246 { ( 1 − | y | ) O 2 F 2 + y [ x O 4 F 4 + ( 1 − | x | ) O 6 F 6 ] } . Resistivity measurements on NdNi5 show a sharp drop at its Curie point, which is ascribed to the loss of its spin-disorder resistivity. No such effect is observed for PrNi5. The analysis indicates that the decline in resistivity for the latter material is so small and so gradual as to be experimentally imperceptible, this being a consequence of the development of Van Vleck paramagnetism rather than magnetic ordering in PrNi5 at reduced temperatures. The observed magnetic entropy of PrNi5 is consistent with other observations which indicate that the ground state for Pr+3 in PrNi5 is a singlet.

Low temperature heat capacities and thermal properties of DyAl2, ErAl2 and LuAl2

Abstract The heat capacities of the compounds DyAl 2 , ErAl 2 and LuAl 2 were measured in an adiabatic calorimeter from approximately 5 to 300 K. The compounds DyAl 2 and ErAl 2 show C P anomalies at 58.0 and 10.2 K, respectively, which are attributed to the destruction of magnetic order. In order to separate the crystal field and magnetic contributions from the measured heat capacities, it was necessary to evaluate the lattice heat capacity. The lattice term, C L was obtained from the C P data of LuAl 2 by a method of interpolation which gave values of C L for an arbitrary R Al 2 compound. Using this “interpolated lattice blank”, excess entropies associated with the crystal field and magnetic terms were computed throughout the series. These values are quite close to R In (2 J + 1). The results also indicate that, for the compounds studied, the degeneracy of the lowest ground state is completely lifted. In addition, the magnetic contribution to the heat capacity of the magnetically ordered R A1 2 phases was found to exhibit an exponential dependence below the temperature corresponding to the spin wave energy gap and a T 3 2 dependence above this temperature. Detailed calculations were performed to characterize the influence of cubic crystal field in ErAl 2 on the 4 I 15 2 ground state multiplet of the Er 3+ ion. It is concluded that the magnetic ordering in ErAl 2 takes place within the Γ 8 3 quartet state. Smoothed values of heat capacity, entropy and related thermodynamic functions are tabulated.

Magnetic properties of pseudo‐ternaries of the composition Nd(Mn1−xCrx)2Si2

Structural and magnetic properties of a number of pseudo‐ternaries of the composition Nd(Mn1−xCrx)2Si2 have been examined. In the composition range 0<x<0.6 they crystallize in tetragonal BaAl4‐type structure. Magnetic properties were examined between 4.2 K and 400 K in applied fields up to 20 kOe. NdMn2Si2 exhibits anomalies in the magnetization versus temperature plots at about 40 K and 380 K—the former due to the ordering of the rare earth moments and the latter due to the ordering of the transition metal sublattice. Addition of chromium to this compound increases the crystal cell volume significantly and modifies the magnetic behavior. The transition metal sublattice orders antiferromagnetically at 380 K, 260 K and 200 K for compositions of X=0, 0.2 and 0.3, respectively. Compounds of the composition X=0.2, 0.3 and 0.4 exhibit unusual thermomagnetic hystereses. The thermomagnetic curves for these compositions in low external fields (<10 kOe) exhibit an increase in the magnetization upon warming the sam...

Effect of substitution of nickel on the magnetic properties of Sm2Co17

Structural, metallographic and magnetic properties of Y2Co17−xNix and Sm2Co17−xNix have been established to ascertain the effect of nickel on the permanent magnet properties of Sm2Co17. The results indicate that the 2 : 17 compositions exist in the concentration limits 0<x<8 in Y2Co17−xNix and 0<x<6 in Sm2Co17−xNix. Metallographic examination of these materials confirms the absence of secondary phases. Magnetic properties were examined in the temperature range 4.2–1200 K in external fields up to 21 kOe. The ordering temperatures of Sm2Co17−xNix and Y2Co17−xNix decrease with increasing concentration of nickel. Examination of saturation magnetization results indicates a smooth variation with the addition of nickel. The easy direction of magnetization at room temperature was established for the samples by examining the x‐ray diffraction patterns of aligned powders. The results show that the axial anisotropy exhibited by Sm2Co17 is retained with the substitution of nickel. For a few nickel‐rich compositions o...

Temperature compensated magnetic materials of the type SmxR1−xCo5 (R=Tb,Dy,Er)

Permanent magnets made from SmCo5 exhibit negative reversible change in magnetization with increasing temperature, typically of the order of 0.04% per deg. C. between −100 and +200 C. It is desirable to improve this property by suitable substitutions. In the present study, a systematic examination of the preparation, structural and magnetic characterization of SmxR1−xCo5 (R=Tb, Dy and Er) has been undertaken. The ternaries were prepared by induction melting of the metals of proper stoichiometries. Magnetic studies were performed between 4.2 and 1000 K in applied fields up to 20 kOe. The results indicate that the best temperature compensation is achieved in the ternary SmxDy1−xCo5. Saturation magnetization and Curie temperatures are also reported.Permanent magnets made from SmCo5 exhibit negative reversible change in magnetization with increasing temperature, typically of the order of 0.04% per deg. C. between −100 and +200 C. It is desirable to improve this property by suitable substitutions. In the present study, a systematic examination of the preparation, structural and magnetic characterization of SmxR1−xCo5 (R=Tb, Dy and Er) has been undertaken. The ternaries were prepared by induction melting of the metals of proper stoichiometries. Magnetic studies were performed between 4.2 and 1000 K in applied fields up to 20 kOe. The results indicate that the best temperature compensation is achieved in the ternary SmxDy1−xCo5. Saturation magnetization and Curie temperatures are also reported.

Heat capacity studies of RFe2 intermetallic compounds over the temperature region 1.5–10 K (R=Gd, Tb, Dy, Ho, Er, Tm and Lu)

The heat capacities of the RFe2 intermetallic compounds (R=Gd, Tb, Dy, Ho, Er, Tm, and Lu) have been measured over the temperature region 1.5–10 K. Values for the apparent electronic heat capacity coefficients have been determined and exhibit a cusp‐like behavior as the R atom is changed across the rare earth series going from Gd to Lu. The maximum is found to occur for HoFe2.

Magnetic properties of ScFe2

ScFe2 crystallizes in the hexagonal C‐14 Laves phase. Magnetic measurements performed in the temperature range 4‐600 K reveal that the sample orders ferromagnetically with a Curie temperature of 545 K. Saturation magnetization measurements performed at 4.2 K yield a value of 2.9 μB per formula unit of ScFe2. The easy axis of magnetization was determined by examining the changes in the intensities of x‐ray pattern on aligning a powder at room temperature in a field of 20 kOe. This was found to be along the c‐axis. Mossbauer pattern at room temperature exhibits a magnetic hyperfine interaction with a complex spectrum.

Crystal field effects on the magnetocrystalline anisotropy in HoAl2

Abstract Recent work on the heat capacity of HoAl 2 revealed an anomaly at 20°K, the origin of which was not understood. It is now suggested that this anomaly originates from a change in the easy direction of magnetization brought about by a combined effect of crystalline electric field and exchange field. Calculations by the equivalent operator method have been performed employing reasonable combinations of crystal field parameters. The exchange field acting on Ho 3+ is determined in a self-consistent manner. Results are presented to illustrate the expected occurrence of a change in the easy direction of magnetization for a range of crystal field parameters which are compatible with those found for other R Al 2 compounds. The magnetic moment on Ho 3+ ion calculated on the basis of this model is in agreement with experiment.

Heat capacities and related thermal properties of DyNi5, HoNi5 and ErNi5 between 5 and 300 K☆

Abstract Heat capacity data and calculated thermodynamic functions are presented for DyNi 5 , HoNi 5 and ErNi 5 . λ-type thermal anomalies are noted at 12.0 K (DyNi 5 ), 4.1 K (HoNi 5 ) and 8.0 K (ErNi 5 ). Schottky-type anomalies are observed at higher temperatures. The λ and Schottky anomalies are ascribed to the destruction of ferromagnetic order and to crystal field excitation, respectively. A deficiency of magnetic entropy, compared to R ln(2 J + 1), is noted corresponding roughly to R ln2. This suggests that the ground state in the ordered materials is a doublet. ErNi 5 is analyzed using a Hamiltonian containing terms representing the crystal field and magnetic interactions. The analysis shows that a doublet ground state can result with reasonable values of the crystal field parameters. The parameters are shown to be consistent with the heat capacity behavior of ErNi 5 . Ordering temperatures are not proportional to the de Gennes function.