Magnetic order and hyperfine interactions in RFe6Al6 (R = rare earth)

The systems RFe₆Al₆(R = Y, Eu, Gd, Tb, Dy, Ho, Er, Tm and Yb) crystallize in the tetragonal body centered I4/mmm structure. In striking contrast to the magnetic behaviour of RFe₄Al₈ (weakly coupled R and Fe sublattices, complicated magnetic structure, low T_c ~ 130 K), in the RFe₆Al₆ systems all magnetic sublattices order simultaneously at a relatively high temperature. The magnetization curves start with low values at low temperatures and rise to very high values at T_max ~ 230 K and then drop to 0 at T_c ~ 330 K. All samples show strong hysteresis effects at temperatures just below T_max. Mossbauer studies of ⁵⁷Fe in the (f) and (j) sites and ¹⁵¹Eu, ¹⁵⁵Gd, ¹⁶¹Dy, ¹⁶⁶Er and ¹⁷⁰Yb in the (a) site yield all hyperfine interaction parameters and temperature dependence of the local magnetic moments. All Mossbauer and magnetization experimental results can be explained in a self consistent way with a simple molecular field model. The Fe in the (j) site plays the dominant role in its strong intrasublattice ferromagnetic exchange and its strong antiferromagnetic exchange with the rare earth site. The Fe in the (f) site have an antiferromagnetic intrasublattice exchange, they have a canted strcuture with the ferromagnetic component parallel to the (j) sublattice magnetization.     

Magnetic and structural characteristics of the R3Ni7B2 compounds (R = rare earth)

Magnetic and structural behaviour and phase relationships of materials of composition R₃Ni₇B₂ (R = Nd-Lu) were investigated. Detailed X-ray analysis yields that two hexagonal structures are encountered. For the heavy rare earth (Gd-Lu) the compounds crystallize in the CeNi₃ structure. The space group is P6₃/mmc and each unit cell contains two formula units. The R₃Ni₇B₂ where R = Nd-Sm (including Yb₃Ni₇B₂) crystallize in the CeCo₄B structure. The space group is P6/mmm and each unit cell contains one formula unit. The detailed crystal structures are discussed. The magnetic measurements show that Yb₃Ni₇B₂ and Lu₃Ni₇B₂ are Pauli paramagnetic. Sm₃Ni₇B₂ is ferromagnetically ordered with a huge intrinsic magnetic hardness. The magnetization at the coercive field at low temperatures is extremely time dependent. The R₃Ni₇B₂ which crystallize in CeNi₃ structure are antiferromagnetic at low temperatures. All Mossbauer and magnetization experimental results can be explained assuming an antiferromagnetic exchange interaction in both 2(c) and 4(f) crystallographic sites and a ferromagnetic interaction between these sites.     

Magnetic properties of RAu2Si2 rare earth compounds

The magnetic properties of the R Au₂Si₂ compounds with R = Ce-Er have been investigated. It was found that the compounds for which R = Ce, Sm, Gd, Tb and Dy are antiferromagnetically ordered at temperatures ranging from 5.7 to 15.9°K. PrAu₂Si₂ and NdAu₂Si₂ exhibit paramagnetic behavior for temperatures as low as 4.2°K. The magnetic structure is ferrimagnetic for the compounds in which R = Eu, Ho, and Er. The Eu compound is in the divalent state. The Néel and Curie points for this system do not follow the De-Gemnes function. Curie-Weiss Behavior is exhibited by all the compounds with effective moments in good agreement with that of a free tripositive lanthanide ion. The difference in magnetic properties between R Au₂Si₂ and the isomorphous R Fe₂Si₂ series is discussed.     

Magnetic properties of RAuNi4 rare earth compounds

Magnetization studies of new f.c.c. RAuNi₄ intermetallic compounds were performed. The compounds with R = Gd, Tb, Dy, Ho and Er are ferromagnetically ordered at temperatures ranging from 14 to 38K. Two ferromagnetic transitions were observed in the magnetization curves. TmAuNi₄ and YbAuNi₄ exhibit paramagnetic behaviour for temperatures as low as 4.2 K.     

Ferromagnetic rare-earth and diamagnetic iron sublattices in ternary RFe0.67Ge1.33 type compounds

The magnetic properties of RM_2?xSi_x and RM_2?xGe_x where R = La, Ce, Nd, Sm, Eu and Gd and M = Fe, Co, Ni and Ag have been measured. It has been found that for samples of x = 0.4 both R and M sublattices are paramagnetically above 4.2δ K in the high spin state, whereas, if x = 0.67, R is magnetically ordered whereas M is diamagnetic.     

Magnetic properties of R3Rh2 compounds with R=Gd, Tb,Dy, Ho and Er

Bulk magnetic measurements performed on polycrystalline samples of the tetragonal compounds R₃Rh₂ with R=Gd, Tb, Dy, Ho and Er are presented. All the compounds are ferromagnetic at low temperature. However in Tb₃Rh₂ an antiferromagnetic behaviour is observed between 14 and 24 K. In Gd₃Rh₂, where the magnetocrystalline anisotropy must be negligible, it seems that the magnetic structure is not collinear. In the other compounds the observed properties essentially result from indirect exchange interactions and crystal field effects acting on the rare earth ions which lie in low symmetry sites.     

Magnetic ordering in GdI2

GdI₂ powder samples have been investigated by means of a.c. susceptibility and magnetisation measurements indicating magnetic ordering below T_C = 313 K. The structure consists of strongly coupled ferromagnetic layers of Gd³⁺ moments aligned perpendicular to the hexagonal crystal axis. The inter-layer coupling is much weaker and probably antiferromagnetic suggesting that GdI₂ is to a very good approximation a two-dimensional Heisenberg system.     

Magnetic ordering in CeMnCuSi2

Temperature and field-dependent magnetization measurements on polycrystalline CeMnCuSi₂ reveal that the Mn moments in this compound exhibit ordering with a ferromagnetic (FM) component ordered instead of the previously reported purely antiferromagnetic (AFM) ordering. The FM ordering temperature, T_c, is about 120 K and almost unchanged with external fields up to 50 kOe. Furthermore, an AFM component (such as in a canted spin structure) is observed to be present in this phase, and its orientation is modified rapidly by the external magnetic field. The Ce L₃-edge X-ray absorption result shows that the Ce ions in this compound are nearly trivalent, very similar to that in the heavy fermion system CeCu₂Si₂. Large thermomagnetic irreversibility is observed between the zero-field-cooled (ZFC) and field-cooled (FC) M(T) curves below T_c indicating strong magnetocrystalline anisotropy in the ordered phase. At 5 K, a metamagnetic-type transition is observed to occur at a critical field of about 8 kOe, and this critical field decreases with increasing temperature. The FM ordering of the Mn moments in CeMnCuSi₂ is consistent with the value of the intralayer Mn–Mn distance R^a_Mn–Mn=2.890 Å, which is greater than the critical value 2.865 Å for FM ordering. Finally, a magnetic phase diagram is constructed for CeMnCuSi₂.     

Magnetic ordering in PrNi2

A peak has been observed in the magnetic susceptibility of the singlet ground state system PrNi₂, indicating a magnetic ordering. The transition is believed to be of a mixed nuclear-electronic type.     

Magnetic properties of ternary RMn2Si2 and RMn2Ge2 compounds

Magnetic properties of RMn₂Si₂ and RMn₂Ge₂ compounds, where R is a rare earth metal, have been investigated by magnetometric measurements. RMn₂Ge₂ (where R is a light rare earth) and LaMn₂Si₂ are ferromagnets. Remaining compounds have antiferromagnetic properties. DyMn₂Si₂ and ErMn₂Si₂ show ferromagnetic properties at low temperatures. It was confirmed that the value of Curie (or Néel) temperature for the Mn sublattice decreases with increasing c constant.     

Neutron diffraction studies of the magnetic ordering in the rare earth compounds TbNi2Si2, HoCo2Si2 and TbCo2Si2

Neutron diffraction studies and magnetic measurements on the compounds TbNi₂Si₂ (1), HoCo₂Si₂ (2) and TbCo₂Si₂ (3) revealed a collinear antiferromagnetic order below T_N = 10 ± 1 K (1), T_N = 13 ± 1 K (2) and T_N = 30 ± 2 K (3) with the rare earths moments oriented along the c-axis [m₀ = 8.8 ± 0.2 μ_B (1), m₀ = 8.1 ± 0.2 μ_B (2), m₀ = 8.8 ± 0.2 μ_B (3)] and the corresponding wavevector are $\text{k}\text{= [}\text{1}\text{2}\text{1}\text{2}\text{0] (1)}\text{and}\text{k}\text{= [ 0 0 1] (2) (3)}$. The magnetic structure of the compounds HoCo₂Si₂ and TbCo₂Si₂ consists of ferromagnetic layers perpendicular to the c-axis coupled antiferromagnetically (+?+?) while for TbNi₂Si₂ the ordering within (0 0 1) plane is antiferromagnetic and the planes (0 0 1) are indeed decoupled.     

Magnetic ordering in NdRh2Si2 and ErRH2Si2

Neutron diffraction and magnetization study of polycrystalline NdRh₂Si₂ and ErRh₂Si₂ was performed in the temperature range from 4.2 to 293 K. Both compounds are of ThCr₂Si₂ type crystal structure and exhibit antiferromagnetic ordering below T_N = 53 K and T_N = 12.8 K respectively. The magnetic structure wave vector is τ = [0, 0, 1].     

Magnetic properties of R4Co3 (R = Gd, Y) compounds as observed by NMR

The zero field spin echo nmr spectrum of Gd₄Co₃ taken at 4.2 K is analysed and discussed on the basis of having magnetic moments of Co atoms at different structural lattice sites. For Y₄Co₃ the spin echo nmr spectrum taken as a function of the external field is discussed and explained on the basis of the coexistence of Co atoms carrying localized magnetic moments and paramagnetic Co atoms in this compound at determined structural sites.     

Magnetic properties and structural data of ternary silicides: (RE,Th,U) Os2Si2 (RE= rare earth)

New ternary silicides (RE,Th,U) Os₂Si₂ have been synthesized from the elements. All the compounds (RE= Y,La,Ce,Pr,Nd,Sm,Gd,Tb,Dy,Ho,Er,Tm,Yb, Lu) were found to be isotypic and to crystallize with the ordered BaAl₄-type of structure (ThCr₂Si₂-type). Magnetic properties of these alloys — studied in the temperature range 1.5<T<1100 K — reveal a typical Van Vleck paramagnetism of free RE³⁺ ions at temperatures higher than 300 K. The observed effective paramagnetic moment of CeOs₂Si₂, μ(eff)=0.98 BM, is compatible with a rather low concentration (15%) of Ce³⁺. The effective moment of SmOs₂Si₂, μ(eff)=0.47 BM, is in reasonable agreement with a Hund's rule $\text{J=}\text{5}\text{2}$ ground level for free Sm³⁺. For temperatures above 25 K, the magnetic susceptibility as a function of temperature corresponds to the Van Vleck behavior for free Sm³⁺ (closely spaced multiplet, $\text{J=}\text{5}\text{2}$,$\text{J=}\text{7}\text{2}$. Magnetic ordering temperatures of REOs₂Si₂ silicides are generally below 42 K. (Pr,Nd,Ho,Er,Tm) Os₂Si₂ exhibit ferromagnetic ordering whereas (Sm,Gd, Tb,Dy) Os₂Si₂ show antiferromagnetic behavior. Above 1.8 K none of the samples was found to be superconducting.     

Magnetic properties of RCoSi2 compounds (R=rare earth)

New ternary silicides of composition RCoSi₂ (R=rare earth and Y) have been prepared and found to crystallize in the orthorhombic CeNiSi₂-type structure. Their magnetic properties have been studied by means of susceptibility measurements between 2 and 250 K. The Ce and Y compounds show essentially temperature independent Pauli paramagnetism. The compounds with R=Nd, Sm, Gd, Tb, Dy, Ho, Er and Tm show antiferromagnetic ordering below 20 K. The effective rare earh moments in the paramagnetic state agree well with the free ion values, and, for the heavy rare earths, the Néel temperatures vary with the De Gennes factor. There is no indication for a magnetic contribution from the Co sublattice.     

The ν2, 2ν2, 3ν2, ν4, and ν2 + ν4 bands of 15NH3

The ir absorption of gaseous ¹⁵NH₃ between 510 and 3040 cm^?1 was recorded with a resolution of 0.06 cm^?1. The ν₂, 2ν₂, 3ν₂, ν₄, and ν₂ + ν₄ bands were measured and analyzed on the basis of the vibration-rotation Hamiltonian developed by V. ?pirko, J. M. R. Stone, and D. Papou?ek (J. Mol. Spectrosc.60, 159–178 (1976)). A set of effective molecular parameters for the ν₂ = 1, 2, 3 states was derived, which reproduced the transition frequencies within the accuracy of the experimental measurements. For ν₄ and ν₂ + ν₄ bands the standard deviation of the calculated spectrum is about four times larger than the measurements accuracy: a similar result was found for ν₄ in ¹⁴NH₃ by ?. Urban et al. (J. Mol. Spectrosc.79, 455–495 (1980)). This result suggests that the present treatment takes into account only the most significant part of the rovibration interaction in the doubly degenerate vibrational states of ammonia.     

Mössbauer effect studies of RFe11Ti and RFe11TiH (R=Y,Gd, Er)

The Mössbauer spectra of ferromagnetic RFe₁₁Ti and RFe₁₁TiH_x   (R=Y$R=\mathrm{Y}$, Gd, Er, x=0$x=0$, 1) have been investigated in the temperature range 20–300 K and analysed in terms of a new model which allows a better fit of the spectra using less constraints than previously reported analyses. In addition to the dependence of magnetic hyperfine field and isomer shift on the number of Ti nearest-neighbours (NN) of the Fe atoms, this new model also considers the effect of the average interatomic distances between Fe atoms on the 8i, 8f and 8j crystallographic sites. Magnetic hyperfine fields were found to decrease with the average interatomic distances for Fe atoms on the same sites and with the same number of iron NN. This decrease was however lower than that due to the presence of one additional titanium NN. Furthermore the present model distinguishes the 8f from the 8j sites on the basis of the relative areas of the corresponding sextets. It leads to higher average magnetic hyperfine fields and isomer shifts for the 8f site as compared to the 8j sites which may be understood on the basis of electronic charge transfer between the rare-earth and Fe atoms. The dependence of hyperfine parameters on the number of titanium NN and on hydrogenation is similar to those previously reported.     

Fourier transform spectroscopy on the 3ν2, 2ν2 + ν6 and ν3 + ν5 bands of H2CO

Fourier transform spectra covering the range from 1500 to 5400 cm^?1 with 0.02-cm^?1 resolution have been obtained for formaldehyde. A study of the region above 4000 cm^?1 has yielded rotational constants and other asymmetric rotor parameters for three bands: 3ν₂ (ν₀ = 5177.7611 ± 0.0005 cm^?1)2ν₂ + ν₆ (ν₀ = 4734.193 ± 0.004 cm^?1), and ν₃ + ν₅ (ν₀ = 4335.102 ± 0.001 cm^?1). An analysis of the A-type Coriolis interaction between the 2ν₂ + ν₆ state and the unobserved 2ν₂ + ν₄ state has yielded partially deperturbed rotational constants for the 2ν₂ + ν₆ state. Vibration-rotation interaction constants have been obtained for the ν₂ and ν₆ normal modes by combining the present results with those of previous workers.     

Magnetic ordering in GdH3

In contrast to a previous report, GdH₃ is found to order antiferromagnetically at 1.8 K.