Crystal structure magnetic properties and hyperfine interactions in RFe4Al8 (R = rare earth) systems

X-ray, magnetic susceptibility and ⁵⁷Fe, ¹⁵¹Eu, ¹⁵⁵Gd and ¹⁷⁰Yb Mössbauer studies were performed. Detailed analysis of X-ray intensities yields all ion locations and interatomic distances in the body centered tetragonal structure (space group I4/MMM). The unit cell contains two formula units. The rare earth, iron and aluminum occupy the 2(a), 8(f) and 8(i) and 8(j) crystallographic sites, respectively. The susceptibility and Mössbauer studies indicate the existence of two independent magnetic sublattices. The iron sublattice orders into an antiferromagnetic structure at about 120 K, whereas the rare earth sublattice orders (excluding those with La, Ce, Eu, Y and Lu) antiferromagnetically at about 20 K. The ⁵⁷Fe, ¹⁵¹Eu, ¹⁵⁵Gd and ¹⁷⁰Yb Mössbauer studies yield, in addition to the hyperfine interaction parameters, also the direction along which the moments are aligned. In EuFe₄Al₈ the Eu ion is in a mixed valent state.     

Correlation of57Fe hyperfine interactions with Fe-Fe distance in Laves phase compounds RFe2 (R=rare earth)

The⁵⁷Fe Mössbauer isomer shifts and magnetic hyperfine fields in Laves phase compounds RFe₂ (R=Pr, Nd and Sm) are studied with particular reference to the effect of the Fe–Fe interatomic distance on the hyperfine interactions. It is shown that the charge density at the Fe nuclei scales linearly with fractional volume change up to 20%. The⁵⁷Fe hyperfine field corrected for the influence of rare-earth moment shows a systematic variation with the distance, which can be understood in terms of the Bethe-Slater curve arguments. The similarity of the atomic volume dependence of the⁵⁷Fe hyperfine interactions in Lves phase compounds to those in iron with close-packed structure is emphasized.This paper is based on a paper presented at the 5th Int. Conf. on hyperfine interactions, Berlin, July 21–25, 1980.     

Giant intrinsic magnetic hardness in RFe5−x Ni x (R=rare earth,x=4 to 5)

Giant low temperature intrinsic magnetic hardness is observed in structurally homogeneous CaCu₅ type compounds RFe_5−x Ni _x . In SmFe_5−x Ni _x , this magnetic hardness peaks approximately at a composition SmFe_0.2Ni_4.8, with an extrapolated coercive force of 230 kOe at absolute zero. The transition metal sublattice is not anisotropic. Thus, the rare earth alone creates giant coercivity. Only compounds withc-axis preference exhibit substantial magnetic hardness (Sm, Er, Tm). Partial substitution of a tetravalent rare earth to produce crystal field anisotropy fluctuations apparently increases coercivity somewhat in the axis-preference compound SmFeNi₄, but has no effect in the plane-preferred compound TbFeNi₄.     

From anisotropic dots to smooth RFe 2 (110) single crystal layers (R = rare earth)

Single crystal RFe₂(110) films were grown by molecular beam epitaxy to a total thickness of 1000 ? at different substrate temperatures ranging from 450 ^° C to 660 ^° C. The first stages of growth and the surface morphology of the deposited layers have been studied using Reflection High Energy Electron Diffraction (RHEED) and Atomic Force Microscopy (AFM). The growth is first strained but further deposit induces the formation of three-dimensional fully relaxed islands. Subsequently, the morphology of the RFe₂(110) nanosystems evolves from anisotropic dots to a smooth surface, as a function of the preparation parameters, i.e. nominal thickness and substrate temperature. It also depends on the rare earth involved in the compound. Received 29 June 2000     

The magnetism of rare earth intermetallics RFe4Al8

Hyperfine Interactions - The YFe4Al8 and ErFe4Al8 intermetallics were studied by57Fe Mössbauer spectroscopy between 2 K and room temperature. The spectra are consistent with iron occupying 8f...     

Semiconducting TlSr2RCu2O7 (R=rare earth) and its superconducting derivatives

Semiconducting TlSr₂RCu₂O₇ (R=Pr or Er) with a 1212-type structure has been synthesized in the single-phase form. Partial substitution of Sr²⁺ for R³⁺ converts this semiconductor to a 90 K superconductor TlSr₂(R_1–y Sr _y )Cu₂O₇. A combination substitution, Sr²⁺ for R³⁺ and Pb⁴⁺ for Tl³⁺, leads to the Ca-free 100 K superconductor (Tl, Pb)Sr₂(R, Sr)Cu₂O₇. The results are explained in the framework of the mixed Cu²⁺/Cu³⁺ valence.     

The magnetic spin-reorientation transitions in the RGa (R=rare earth) intermetallic compounds studied by measurements of the hyperfine interactions of the Sn probe atoms

The magnetic and electric hyperfine interactions of the probe nucleus ¹¹⁹Sn on the Ga site of the ferromagnetic rare-earth (R) gallium compounds RGa (R=Pr–Er) have been investigated by Mössbauer spectroscopy technique. For all of the compounds, the directions of the magnetic moments of the R³⁺ ions have been determined as a function of temperature in the range from 5 K to T_C. For NdGa, SmGa, HoGa, and ErGa compounds, the magnetic reorientation transitions due to the competition between the exchange interaction and the interaction with crystal field have been investigated. At high temperatures, when the electric interaction dominates, the orientation of the magnetic moments is unambiguously determined by the sign of the quadrupole moment of 4f shell of the R³⁺ ion. With decreasing temperature, the magnetic moments rotate gradually from the bc-plane toward the crystallographic a-axis. In the temperature range 5 K?T<100 K, the ferromagnetic structure of the GdGa compound is noncollinear. At 5 K the magnetic moments of the Gd³⁺ ions point in two distinct directions with respect to the crystallographic a  -axis (_θ1≈30°${\theta }_1\approx 30°$ and _θ2≈60°${\theta }_2\approx 60°$).     

Magnetic properties of rare earth compounds of the type RFe4Al8

The magnetic properties and the ⁵⁷Fe Mössbauer effect have been studied for the tetragonal compounds RFe₄Al₈. At temperatures around 150 K the Fe moments order antiferromagnetically. This ordering is followed by a mutually ferromagnetic ordering of the R moments at much lower temperatures (T < 35 K).     

Spin arrangements in R2Co14B compounds (R = rare earth)

R₂Co₁₄B compounds (R = Pr, Nd and Tb) have been studied for evidence of spin reorientation in the temperature range 4.2 K − 1100 K with the use of magnetometry technique. It has been established that Pr₂Co₁₄BandTb₂Co₁₄B undergo spin reorientations at temperatures 664 K and 795 K, respectively. These are axis-to-plane reorientations with increasing temperature. Nd₂Co₁₄B undergoes two spin transitions as temperature progresses: one at ∼ 34 K (cone-to-axis) and the second at ∼ 546 K (axis-to-plane). A diagram of spin arrangements observed in all existing R₂Co₁₄B compounds has been constructed.     

Structure and magnetic properties of RAlSi(R=light rare earth)

We prepared the semimetals RAl Si(R = light rare earth), and systematically study their crystal structures and magnetic properties. X-ray diffractions confirm the coexistence of the site-disordered phase with group space of I41/amd and the noncentrosymmetrically ordered phase with space group of I41 md in RAl Si alloy. The ordered phase is the main phase in RAl Si alloy. RAl Si alloys show nonmagnetic character for R = La, low temperature ferromagnetic order for R = Ce, Pr, and paramagnetic character for R = Nd, respectively. Sm Al Si shows metamagnetic transition at 10 K and ferromagnetic order at 143 K, respectively. Sm Al Si follows the van Vleck paramagnetic model in its paramagnetic region. The magnetization curves of RAl Si(R = Ce, Pr, Sm) follow the mixed model of ferromagnetism and paramagnetism, and the fitted saturation moment MSdepends on the moment of trivalent rare earth. The paramagnetic susceptibility χ of RAl Si is going up with increasing the atomic order numbers of rare earth elements. This reveals that the magnetic property of RAl Si originates from the rare earth.     

Correlation between valence electronic structure and magnetic properties in RCo_5(R=rare earth) intermetallic compound

The magnetisms of RCo_5(R = rare earth) intermetallics are systematically studied with the empirical electron theory of solids and molecules(EET).The theoretical moments and Curie temperatures agree well with experimental ones.The calculated results show strong correlations between the valence electronic structure and the magnetic properties in RCo_5 intermetallic compounds.The moments of RCo_5 intermetallics originate mainly from the 3d electrons of Co atoms and 4f electrons of rare earth,and the s electrons also affect the magnetic moments by the hybridization of d and s electrons.It is found that moment of Co atom at 2c site is higher than that at 3g site due to the fact that the bonding effect between R and Co is associated with an electron transformation from 3d electrons into covalence electrons.In the heavy rare-earth-based RCo_5 intermetallics,the contribution to magnetic moment originates from the 3d and 4f electrons.The covalence electrons and lattice electrons also affect the Curie temperature,which is proportional to the average moment along the various bonds.     

Local and itinerant magnetism and superconductivity in R Rh2si2 (R = rare earth)

Magnetization and Mossbauer studies reveal that R Rh₂Si₂ (R = rare earth) have two magnetic phase transitions, one corresponding to the ordering of the rare earth (T^N = 27?130K) and the other to the itinerant electron ordering of the Rh sublattice (T^M= 5?17K). LaRh₂si₂ has also been studied by resistivity, specific heat and a.c. susceptibility measurements. All studies indicate that LaRh₂Si₂ orders magnetically at T^M= 7K and becomes superconducting, type II, at T^c= 3.8±0.2K.     

Anisotropic magnetic exchange in orthorhombic RCu2 compounds (R = rare earth)

The magnetic excitations in the field induced ferromagnetic phase F3 of a NdCu₂ single crystal were investigated by means of inelastic neutron scattering experiments. A mean field model using the random phase approximation in connection with anisotropic magnetic bilinear R-R (R denotes a rare earth) exchange interactions is proposed to account for the observed dispersion. The relevance of this model to the analysis of the magnetic ordering process in other RCu₂ compounds is discussed. Received 21 April 1999     

Magnetism,spin relaxation and hyperfine interactions of rare earths in RM6Al6(M = Cr,Mn, Cu)

The systems RM₆Al₆ (R = rare earth or Y, M = Cr, Mn, Cu, Rh) were studied by magnetization measurements and by Mossbauer spectroscopy of ¹⁵⁵Gd, ¹⁶¹Dy, ¹⁶⁶Er and ¹⁷⁰Yb. The magnetization studies show weak R-R antiferromagnetic exchange interactions in RCu₆Al₆(T_n)Gd) = 21 K, less than 4 K for all other R and strong crystalline field effects. Similar phenomena are observed in RMn₆Al₆ and RCr₆Al₆, however, due to the presence of a Mn or Cr local moment the systems order ferrimagnetically. In RCr₆Al₆the order temperatures are low T_c ～ 25 K, yet T_c(GdCr₆Al₆) = 170 K. The Mossbauer studies observations are consistent with the magnetiza results. In the case where Er and Yb are not ordered at 4.1 K, the spectra still show magnetic hyperfine structure however of paramagnetic nature. The spectra yield the hyperfine interaction spin Hamiltonian parameters and the spin relaxation rates. These turn out to be extremely slow (1O⁸–1O⁹ sec^?1, a very uncommon phenomenon for a concentrated Er or Yb metallic system.     

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.