Magnetic and hyperfine interaction in RFe4Al8 (R = Ce,Sc) compounds

Magnetic properties of ScFe4Al8 and CeFe4Al8 compounds have been studied by magnetization and Mössbauer effect measurements. Magnetic transition temperatures estimated from Mössbauer spectra (B = 0) 170 K for CeFe4Al8 and 225 K for ScFe4Al8 are not confirmed by magnetization measurements. Contrary, the pronounced maxima at Tmax = 130 and 125 K in DC magnetization curves (B = 1 kOe) were found for ScFe4Al8 and CeFe4Al8, respectively. Thermomagnetic, the so-called zero field (ZFC) and field cooling (FC) experiments show temperature-dependent irreversibilities below the "freezing" temperature, Tf, which diminishes with application of external magnetic field. The Mössbauer studies show the coexistence of magnetically (sextet) and non-magnetically (quadrupole doublet) split patterns in the wide temperature range far away from Tmax. All these observations indicate that the systems studied are either a spin-glass or are the mixture of AF and spin-glass state. © 2001 Elsevier Science B.V

Magnetic and hyperfine interaction in RFe4Al8 (R = Ce,Sc) compounds

Magnetic properties of ScFe4Al8 and CeFe4Al8 compounds have been studied by magnetization and Mössbauer effect measurements. Magnetic transition temperatures estimated from Mössbauer spectra (B = 0) 170 K for CeFe4Al8 and 225 K for ScFe4Al8 are not confirmed by magnetization measurements. Contrary, the pronounced maxima at Tmax = 130 and 125 K in DC magnetization curves (B = 1 kOe) were found for ScFe4Al8 and CeFe4Al8, respectively. Thermomagnetic, the so-called zero field (ZFC) and field cooling (FC) experiments show temperature-dependent irreversibilities below the "freezing" temperature, Tf, which diminishes with application of external magnetic field. The Mössbauer studies show the coexistence of magnetically (sextet) and non-magnetically (quadrupole doublet) split patterns in the wide temperature range far away from Tmax. All these observations indicate that the systems studied are either a spin-glass or are the mixture of AF and spin-glass state. © 2001 Elsevier Science B.V

Magnetic and hyperfine interaction in RFe4Al8 (R = Ce,Sc) compounds

Magnetic properties of ScFe4Al8 and CeFe4Al8 compounds have been studied by magnetization and Mössbauer effect measurements. Magnetic transition temperatures estimated from Mössbauer spectra (B = 0) 170 K for CeFe4Al8 and 225 K for ScFe4Al8 are not confirmed by magnetization measurements. Contrary, the pronounced maxima at Tmax = 130 and 125 K in DC magnetization curves (B = 1 kOe) were found for ScFe4Al8 and CeFe4Al8, respectively. Thermomagnetic, the so-called zero field (ZFC) and field cooling (FC) experiments show temperature-dependent irreversibilities below the "freezing" temperature, Tf, which diminishes with application of external magnetic field. The Mössbauer studies show the coexistence of magnetically (sextet) and non-magnetically (quadrupole doublet) split patterns in the wide temperature range far away from Tmax. All these observations indicate that the systems studied are either a spin-glass or are the mixture of AF and spin-glass state. © 2001 Elsevier Science B.V

Magnetic and hyperfine interaction in RFe4Al8 (R = Ce,Sc) compounds

Magnetic properties of ScFe4Al8 and CeFe4Al8 compounds have been studied by magnetization and Mössbauer effect measurements. Magnetic transition temperatures estimated from Mössbauer spectra (B = 0) 170 K for CeFe4Al8 and 225 K for ScFe4Al8 are not confirmed by magnetization measurements. Contrary, the pronounced maxima at Tmax = 130 and 125 K in DC magnetization curves (B = 1 kOe) were found for ScFe4Al8 and CeFe4Al8, respectively. Thermomagnetic, the so-called zero field (ZFC) and field cooling (FC) experiments show temperature-dependent irreversibilities below the "freezing" temperature, Tf, which diminishes with application of external magnetic field. The Mössbauer studies show the coexistence of magnetically (sextet) and non-magnetically (quadrupole doublet) split patterns in the wide temperature range far away from Tmax. All these observations indicate that the systems studied are either a spin-glass or are the mixture of AF and spin-glass state. © 2001 Elsevier Science B.V

The influence of hydrogenation on superconducting properties of MgB₂

In the paper we present the results of the ac susceptibility measurements of two newly discovered superconducting diboride - MgB₂

Influence of lattice volume on magnetic states of Ce2Fe16MnDy compounds y 0,1,2.3

The substitution of Mn for Fe in Ce2Fe17 suppresses its ferromagnetic ground state completely for Mn contents x amp; 8805; 0.5 Mn f.u. Ce2Fe16Mn has only an antiferromagnetic phase incommensurate helix along c axis below TN 198 K. In this paper we present and discuss the effects of deuterium insertion that can be considered as application of negative chemical pressure on the magneto structural properties of Ce2Fe16Mn. Application of positive high pressures up to 10 kbar on Ce2Fe16MnDy deuterides y 1, 2.3 allowed us to estimate the role of lattice volume and to divide it from the role of modified electronic band structure that both determines the magnetic states of the deuterated compounds. The results show that a ferromagnetic phase is stabilized by the insertion of D into the antiferromagnetic Ce2Fe16Mn. The Curie temperature TC of Ce2Fe16MnDy deuterides increases with increasing D content reaching TC 258 K for y 2.3 and remarkably decreases with pressure dTC dP 5.4 K kbar and dTC dP 3.6 K kbar for Ce2Fe16MnD1 and Ce2Fe16MnD2.3, respectively. Significant difference between the magnetization of Ce2Fe16MnD1 under pressure and the one of the parent compound at ambient pressure indicates that changes of the volume alone cannot determine the magnetic states upon the initial deuteration. However, the volume expansion becomes dominant when increasing the deuterium content up to 2.3 D f.

Forced-ferromagnetic state in a Tm2Fe17H5 single crystal

International audienceWe report the attainment of the ferromagnetic state in an interstitially modified heavy rare-earth-iron intermetallic compound in an external magnetic field. The starting composition is RE2Fe17, which is the RE-Fe binary richest in iron. We concentrate on the Tm-Fe compound, which is the most sensitive to magnetic field. The maximum possible amount of hydrogen (5 at. H/f.u.) is inserted into a Tm2Fe17 single crystal. We demonstrate that in a magnetic field of 57 T Tm2Fe17H5 reaches the ferromagnetic state with an enviably high polarization of 2.25 T

Magnetic properties and crystal structure of RENiA1 and UniA1 hydrides.

RENiAl (RE = rare-earth metal) and UNiAl compounds crystallizing in the hexagonal ZrNiAl-type structure (space group P{bar 6}2m) can absorb up to 2 and 3 hydrogen (deuterium) atoms per formula unit, respectively. Hydrogenation leads to a notable lattice expansion and modification of magnetic properties. However, the impact of hydrogenation on magnetism is the opposite for 4f- and 5f-materials: TN(T{sub c})is lowered in the case of rare-earth hydrides, while for UNiAlH(D){sub x} it increases by an order of magnitude. Here we present results of magnetic and structure studies performed of these compounds, focusing on the correlation between magnetic and structural variations and discussing possible reasons of the striking difference in effect of hydrogenation on rare-earth and actinide intermetallics

F)twpPnqcP-qyy3q MAGNETIC PROPERTIES AND CRYSTAL STRUCTURE OF lU?NiAl AND UNiAl

Introduction RENiAl (RI3 = rare-earth metal) and UNiAl compounds crysta.llising in the hexagonal ZrNiA1-type structure (space group P~2nz ) can absorb up to 2 and 3 hydrogen (deuterium) atoms per formula unit, respectively. Hydrogenation leads to a notable lattice expansion and modification of magnetic properties. However, the impact of hydrogenation on magnetism is the opposite for 4f-and 5f-materials: TN(TC)is lowered in the case of rare-earth hydrides, while for UNiAIH(D)x it increases by an order of magnitude [1][2][3][4]. Here we present results of magnetic and structure studies performed of these compounds, focusing on the correlation between magnetic and structural variations and discussing possible reasons of the strilcing difference in effect of hydrogenation on rare-earth and actinide intermetallics. Experiment. Hydrides (deuterides) have been synthesised using a two-stage process. First the parent intermetallics were arc-melted from the constituent elements under Ar atmosphere, and he phase purity was veritled by means of X-rays diffraction. Secondly the material was crushed and hydrogenated, by means of activating the specimen in a vacuum of 10<Torr at 350"C for 1 hour, followed by exposure to H2(Dz) at 20 atm pressure for HoNiAl and 55atm pressure for UNiA1. The synthesis of HoNiA.lH2.0was initiated by raising the temperature in the reaction chamber to 50 'C. The amount of absorbed D(H) was determined by monitoring the decrease of pressure in a calibrated volume. In order to avoid sample decomposition material was stored in sealed quartz or glass ampulas. Magnetic measurements were performed using an Oxford Instruments Faraday balance and SQUID magnetometer from Quantum Design. DISCLAIMER Results and discussion. HoNiAl has the highest absorption capability among the whole 12ENiAl series. It forms , towards lower temperatures at higher fields demonstrates that HoNiAIHzo orders antiferromagnetically at TN= 6 K, while &e critical temperature of the parent compound is 13 K [5]. These two materials also display different magnetic phase diagrams: the hydride undergoes only one magnetic phase transition within the experimentally achievable temperature range (i.e. down to 1.8 K), while HoNiAl experiences spin re-orientation ffom amplitude-modulated ferromagnetic phase to a canted ferromagnetic structure at TN = 4.9 K [5,6]. The absence of the latter transition in the hydride maybe either due to its shift below 1.8 K or due to the modtilcation of the magnetic structure by the incorporation of hydrogen. For T> 11 K, the temperature dependence of magnetic susceptibility, AT), of HoNiAlH2.0 follows the Curie-Weiss law with~~~= 10.9 p~f.u. and@= -11& compared to 10.7 p~f.u. and 7.2 K for HoNiA1. We can conclude that the effective moment remains approximately unaffected by the hydrogenation, and the paramagnetic Curie temperature ,changes its sign, retaining the same order of magnitude. Field dependencies of magnetisation M(H) are quite different for HoNiAlH2.0 and HoNiAl: magnetisation of hydride reaches lower value (5.9 p.tif.u.) at the maximum field of 5 T, than that of pure compound (-7.6 ptif.u. [5]), the latter also has a remnant magnetisation, whereas M(H) of HoNiAlH2.0 shows zero remanence. Qualitatively new feature, the inflection point at 0.5 T, appears on the M vs. H dependence of hydride, while for HoNiAl it has not been observed up to~= 40 T [5]. UNiAl hydride was first obtained by Drulis et a2. [2], and the first magnetic characterisation was done by Zogal et al. [1]. Larger number of anomalies on XT) reported in [1] may be attributed to the presence of impurity phase, but the ordering temperature above 100 K looks realistic. We have studied both hydride and deutefide, uNiA1.Hz.sand " UNiAlD2.J, indicating antiferromagnetic order at 99 K and 94, respectively Non-uniform expansion in the basal plane, accompanying hydrogenation, leads to the orthorhombic distortion of the HoNiAl lattice (Table l.). Besides that the unit cell is contracted along the c-axis but due to higher multiplicity of the a-axis the total-volume is increased: AV7V= 5.8 %. The latter value is lower then the maximum volume increase in the lWNiAIHXseries observed for SmNiAIH1.2:8.7 % [4]. In spite of notably lower deuteriurn content in HoNiAID0.g7the unit cell volume remains appro~ately the same AVIV= 6.0 %, but the orthorhombic distortion is weaker and b/a ratio approaches +3, typical for hexagons ymmetry. Hydrogenation of UNiAl leads to similar non-uniform deformation of the unit cell but its symmetry remains unchanged, and the volume increase is twice higher than in for HoNiAlH2.00 Due to higher coherent scattering cross-section of deuterium neutron diffraction studies have been performed on HoNiAIDo.g7 Collusions. Hydrogenation has noticeable effect on magnetic properties and crystal structure of RENiAl compounds and UNiA1. It leads to the change of the ordering temperatures and expansion of the unit cell in both cases. While the crystal lattice modification has some common features in 4j-and 5~-compounds, i.e. expansion in the basal plane, contraction along the c-axis, positive value of AWV, magnetic uro~erties are chamzed in the omosite 6 manner. The ordering temperature of UNiAlH2.3is higher by ahnost 100 K compared to its parent compound,~shifts to more negative values, indicating stronger antiferromagnetic interaction. However, we do not have any reliable estimate for the magnitude of ordered moments~u. All these changes can be attributed to the lattice expansion, which leads to decrease of the 5~-ligand and 5~-5~hybridisation. On the contrast to UNiAl, HoNiAlH2.0has lower ordering temperature than HoNi.Al, it also has different type of magnetic ordering: AFM vs. FM in parent compound. & changes its sign after hydrogenation, but retains the same order of magnitude, indicating rather modification of the type of magnetic exchange than its strength. The mechanism leading to . these effects in HoNiAlH2.0should be different from that in UNiAlH2.3. As in the rest of RENiAl series [3,4] it can be ascribed to the weakening of the RKKY exchang~interaction, responsible for the magnetic ordering in REIW41's due to the decrease of the conduction electron density. Geometrical effect plays a secondary role since 4~-states responsible for. magnetism are located quite far from EF. The minor importance of the lattice expansion is reflected, for example, in the absence of the direct correlation between AV7Vand the decre~e of the ordering temperature. For instance ATC= -46 K and AV7V= 5.9% for GdNiASH1.ss [3,4], and ahnost the same increase of volume in HoNiAlH2.0 leads to-the reduction of the ordering temperature by 7 K only. The wedcening of the exchange interaction cannot be attributed either to the symmetry change under hydrogenation because GdNiAIH1.35and GdN~l.Ob having orthorhombic and hexagonal unit cell, respectively, show ahnost identical TN. The role of the symmetry changes maybe quite crucial for the magnetic structures for the incorporation on hydrogen affects local symmetry, thus, altering possibilities for certain arrangement of magnetic moments. &apos