ThMn12 Type Research Articles

Electrical conductivity of ThMn12- and Th2Zn17-type ternary intermetallic compounds in R–T–Al systems (R=Y, La, Ce, Gd, Tb; T=Mn, Fe)

Electrical resistivity measurements in the temperature range 1.5–300K were performed for ThMn12 and Th2Zn17 structure type RMnxAl12−x and R2MnxAl17−x (R=Y, La, Ce, Gd, Tb; T=Mn, Fe) ternary intermetallic compounds. There is a correlation between the ordering of the crystal structure and the type of the electrical conductivity in R–Mn–Al ternaries. Hopping conductivity in terms of variable range hopping (VRH) occurs in RMnxAl12−x (structure type ThMn12) and R2MnxAl17−x (structure type Th2Zn17) aluminides with disordered or partially ordered crystal structure, both for magnetic and nonmagnetic rare earths. Only for the RMn4Al8 stoichiometry with an ordered distribution of atoms, the resistivity exhibits metallic-type behavior. The presence of magnetic Fe atoms suppresses hopping conductivity in R–Fe–Al ternaries, evidencing that the 3d-element has a predominant influence on electronic transport in R–T–Al intermetallics.

Magnetic properties of NdMn12−xNix alloys

The RMn12−xTx alloys where R denotes rare earth element and T denotes Fe, Co and Ni have been a subject of intense investigation. Here, we present the magnetic properties of the NdMn12−xNix system, which exists as a single phase for 3.6≤x≤6, and crystallizes in the tetragonal ThMn12 type of structure (space group: I4/mmm). The magnetic susceptibility of these alloys has been examined at the temperature T=1.9–300K, in a magnetic field of 5kOe with a SQUID magnetometer. The field dependence of the magnetization has been obtained at T=1.9K up to 50kOe, and at T=4.2K up to 140kOe. The magnetic susceptibility, χ, follows a modified Curie–Weiss law above about 100K and at low temperatures some anomalies are seen in the χ(T) plot. The linear dependence of magnetization versus magnetic field at low temperature excludes a magnetic ordering of ferromagnetic character. Also, the absence of any maximum in the temperature dependence of the magnetic susceptibility suggests that the anomalies do not have intrinsic character and the investigated alloys are nonmagnetic (paramagnetic).

Lattice parameters, magnetic susceptibility and thermal conductivity of ScFe 4Al 8 and YFe 4Al 8

ScFe 4Al 8 and YFe 4Al 8 crystallise in a tetragonal structure of the ThMn 12-type. Although antiferromagnetic ordering in the Fe sublattice exists below about 100 K in these compounds, they can be considered as the reference materials for the RFe 4Al 8 ternaries where R is a magnetic lanthanide, U or Np. The present magnetic measurements in a low magnetic field of 50 Oe have revealed another pronounced anomaly in the temperature dependence of magnetic susceptibility at temperatures lower than the Néel point. Low temperature (LT) X-ray diffraction (XRD) examination has confirmed the stability of the crystal structure in the whole investigated temperature range. Therefore the lattice distortion as a reason for the LT magnetic anomalies has been excluded. Also heat conductivity measurements have not given any explanation for the LT anomalies. Based on a thermal conductivity investigation it was possible to separate the electron and phonon components from the total thermal conductivity. Thus a deviation from stoichiometry, spin-reorientation transition (SRT) and spin-glass (SG) state are discussed as reasons for the LT anomalies.

THEORETICAL INVESTIGATION ON THE PHASE STABILITY AND SITE PREFERENCE OF R(Co,T)12 AND R(Co,T)12Nx (R = Y, Ce, Pr, Nd, Sm, Gd, Tb, Ho, Er, Dy, T = Mo, Mn, Ni)

The phase stability and site preference of some ternary elements of ThMn12 type rare-earth intermetallic compounds R ( Co , T )12 and R ( Co , T )12 N x are evaluated based on ab initio converted interatomic potentials. The calculated results show that adding either Mo or Mn makes the crystal cohesive energy decrease markedly. It proved that these elements can stabilize R ( Co , T )12 with the ThMn12 structure. The systemic energy is the lowest when T atoms occupy symmetrically 8i sites. For T = Mo or Mn, the doped nitrogen atom does not affect the order of site preference of these stabilizing elements. Moreover, the nitrogen atom occupies 2b sites. According to the calculated results, it can be seen that lattice parameters and X-ray patterns are in good agreement with experiment. All the above results indicate that the ab initio converted pair potentials are effective for the calculation of these kinds of anisotropy materials and some related nitrides.

High Pressure Studies Of UFe5Al7 and UFe7Al5 Actinide Compounds

The ternary inter-metallic compounds, UFe5Al7 and UFe7Al5, crystallize in a tetragonal ThMn12 type structure. In the as-cast samples a residual phase of FeAl (~2%wt) was identified in the grain boundaries. The amount of the residual cubic phase of FeAl was determined by Rietveld analysis and reduced by the annealing process.UFe5Al7 and UFe7Al5 maintain the tetragonal symmetry as a function of pressure, while FeA1 keeps the cubic structure as was determined by the Rietveld analysis. The volume-pressure curve calculated from the X-ray analysis is V/V0=0.87 for UFe5Al7 at 26.0GPa and 0.89 at 24.6GPa for UFe7Al5.

Direct evidence for the occurrence of superconductivity in the magnetic compound YFe4Al8

For the first time we present direct evidence for superconductivity in the ternary magnetic compound YFe4Al8 with the ThMn12 type structure, found via point-contact (PC) experiments on contacts between a silver needle and single-crystal YFe4Al8, which reveal a distinct Andreev-reflection current. The spectra measured prove the existence of a normal–superconducting interface and exhibit a triangular-like shape in the vicinity of zero bias voltage, implying an unconventional type of superconductivity. The derived dependences of the order parameter versus temperature Δ(T) and magnetic field Δ(H) are presented. Δ(T) follows BCS theory, whereas Δ(H) does not satisfy any theoretical predictions. In some cases there exists noticeable superconductivity enhancement by a weak magnetic field. The data obtained imply a very inhomogeneous distribution of superconductivity over the sample volume in spite of its single-crystal structure. We assume that the reason is associated with inherent magnetic inhomogeneities of this material. The highest values for the critical temperature Tc, upper critical magnetic field Hc2, and ratio 2Δ(0)/kTc are 7.4 K, 5 T, and 7.2, respectively.

The magnetization processes, spin reorientation transitions and magnetic domain structure in DyFe 10CoTi single crystal

The tetragonal ThMn 12-type, single crystalline DyFe 10CoTi sample has been investigated by torque and magnetization measurements and observation of domain structure at various temperatures between 10–300 K and in magnetic field from B=0 to 0.15 T. These examinations showed that the magnetic structure of DyFe 10CoTi changes from “easy axis” ( c-axis) type to conical at 225 K and to “easy plane” ( ab plane) type at 100 K.

Structural and magnetic properties of Sm 2(Fe 1− xTi x) 17 ( x=0–0.1) alloys prepared by hydrogenation processes and their nitrides

Sm 2(Fe 1 − x Ti x ) 17 alloys ( x=0, 0.02, 0.04, 0.06, 0.08, 0.1) were prepared by arc-melting, subsequently annealing, hydrogenation and nitrogenation processes. The phase components, structures and magnetic properties involved in the processes have been investigated systematically. The Sm 2(Fe 1 − x Ti x ) 17 alloys have different main phases including Sm 2(Fe,Ti) 17 with Th 2Zn 17-type for x=0–0.04, Sm 3(Fe, Ti) 29 with Nd 3(Fe,Ti) 29-type for x=0.06, and Sm(Fe,Ti) 12 with ThMn 12-type for x=0.08–0.1. Curie temperature T c of the main phase of the alloys increases from 125°C for x=0 to 318°C for x=0.1. The corresponding hydride phases similar to the main phase structures of the parent alloys were formed after a hydrogen decrepitation (HD) process at 300°C. The hydrogenation at 800°C mainly shows a hydrogenation–disproportionation–desorption–recombination (HDDR) process. The HD and nitrogenation resulted in the Curie temperature increments of alloys by 30–150°C and by 140–350°C due to lattice expansions, respectively. The magnetic properties achieved for the Ti-containing alloys and their nitrides are lower than those of Sm 2Fe 17N δ prepared by the same processes of HD and HDDR.

The Intermetallic Compounds GdRe2Al10 and TbRe2Al10, Crystallizing with a Stacking Variant of the YbFe2Al10 Type Structure

The new compounds GdRe2Al10 and TbRe2Al10 were obtained in well-crystallized form by reaction of the elemental components with an excess of aluminum after dissolving the matrix in hydrochloric acid. They crystallize with a new structure type which has been determined for TbRe2Al10 from single-crystal X-ray data: Cmcm, a = 932.2(1), b = 1030.4(1 ), c = 1803.2(3)pm, Z = 8, R = 0.031 for 1159 structure factors and 77 variable parameters. Of the two terbium sites, one does not have full occupancy with terbium, however, it may have mixed Tb/Al occupancy. The resulting compositions are Tb0.948(5)Re2Al10 or Tb0.936(3)Re2Al10.064(3), respectively. The terbium atoms are coordinated by 4 Re and 16 Al atoms. The rhenium atoms are situated in distorted icosahedra formed by 2 Tb and 10 Al atoms. The nine different aluminum atoms have between 12 and 14 neighbors (1 or 2 Tb, 2 Re, and between 8 and 11 Al atoms). The structure may be viewed as consisting of two kinds of alternating layers. One of these is puckered, hexagonal close packed, with a mesh content of 4ReAl3; the other is planar and less densely packed. It has the mesh content 2TbAl4. The same kinds of atomic layers have been found in the structures of YbFe2Al10 and LuRe2Al10. Thus, the three structure types may be considered as stacking variants of each other. The tetragonal CaCr2Al10 (ordered ThMn12) type structure also belongs to this structural family, although the CaAl4 layers somewhat differ from the layers TbAl4, YbAl4, and LuAl4.

Magnetic structure and site occupancies in YFe11−xCoxTi (x=1,3,7,9)

The crystal and magnetic structures of the system YFe11−xCoxTi have been investigated by high-resolution neutron powder diffraction for the compositions x=1, 3, 7, and 9. All compounds have the ThMn12 type structure, with space group symmetry I4/mmm. The distribution of the atoms over the independent crystallographic sites was found to be significantly different from random, with a strong preference for Ti to be in the 8i site and Co in the 8j and 8f sites. This behavior can be explained in terms of the enthalpy of formation of binary alloys. The magnetic moments are parallel to the c axis for the compositions x=1, 3, 9, and lie in the a-b plane for x=7. No evidence of additional nuclear or magnetic phase transitions was observed for sample YFe4Co7Ti down to 20 K.

Structure, Magnetic, and Electrical Properties of the LnAg6In6 Intermetallics (Ln=La, Ce, Pr, and Nd)

Tetragonal LnAg6In6 compounds have been obtained for the first time for Ln=La, Pr, and Nd. CeAg6.4In5.6 has slightly different stoichiometry, and a Pr compound also exists with this composition. These materials exhibit ThMn12 type structure (I4/mmm). LaAg6In6 is diamagnetic whereas other compounds are paramagnetic following the Curie–Weiss law above 100 K. For paramagnetic compounds the effective magnetic moments are fairly close to free ion values and Weiss constants are very small. NdAg6In6 exhibits in the χ(T) plot a maximum at 5 K which most probably corresponds to the Néel point. At 1.7 K the magnetization of the paramagnetic samples linearly depends on magnetic field strength up to 5 T. The temperature dependence of the electrical resistvity, ρ(T), suggests a metallic character of all compounds without any indication of superconducting transition. The electrical resistivity of PrAg6.4In5.6 is slightly higher than that for PrAg6In6 and it is most probably related to some disorder in an occupation of the crystallographic positions.

Physical properties of UFe 4Al 8 carbides

Two types of UFe 4Al 8 carbides were identified, one isostructural to UFe 4Al 8 (A) and another, with a different structure related with the ThMn 12-type but with the a- and c- axis doubled (B). Two magnetic transitions at ∼ 143 and 133 K were observed for both compounds, the former one attributed to the magnetic ordering of the Fe atoms. From Mössbauer results ∼ 30% and ∼ 18% of the Fe atoms, for A and B, respectively, were found in the paramagnetic state down to 5 K.

Crystallographic and intrinsic magnetic properties of Nd 1− xDy xFe 10.5Mo 1.5 compounds and their nitrides ( x=0.0−1.0)

The variation of the crystallographic and magnetic properties with Dy concentration has been investigated for Nd 1− x Dy x Fe 10.5Mo 1.5 and Nd 1− x Dy x Fe 10.5Mo 1.5N y compounds. In the Nd 1− x Dy x Fe 10.5Mo 1.5 system, the lattice parameters a, c and the saturation magnetization at 1.5 K decrease with increasing Dy concentration, while the Curie temperature changes only little. At room temperature, all Nd 1− x Dy x Fe 10.5Mo 1.5 samples exhibit weak easy-axis anisotropy with anisotropy fields up to about 1 T with a clear increase with increasing Dy concentration. Upon nitrogenation, the Nd 1− x Dy x Fe 10.5Mo 1.5N y compounds retain the original tetragonal ThMn 12-type of structure but the unit-cell volume increases about 3.1%. At room temperature, the Nd 1− x Dy x Fe 10.5Mo 1.5N y compounds show easy-axis anisotropy with anisotropy fields above 7.2 T. At room temperature, in the Nd 1− x Dy x Fe 10.5Mo 1.5N y system the anisotropy field slightly increases with increasing Dy concentration.

Influence on magnetic properties of substitution of Co for Fe in TbFe 11.3Nb 0.7

The magnetic properties of Tb(Fe 1− x Co x ) 11.3Nb 0.7 compounds with x=0, 0.05, 0.1, 0.15, 0.2 and 0.3 have been investigated. All compounds studied crystallize in the ThMn 12-type of structure. Substitution of Co for Fe leads to a contraction of the unit-cell volume. The Curie temperature clearly increases with increasing Co content from 551 K for x=0 to 831 K for x=0.3. The magnetic moment of the transition-metal sublattice increases with increasing Co content from 22.2 μ B/f.u. for x=0 to 23.1 μ B/f.u. for x=0.3. As the temperature increases, a spin reorientation from easy-plane to easy-cone is found in all compounds investigated. The spin-reorientation temperatures T sr have been derived from the temperature dependence of the magnetization in a low field and decrease monotonously with increasing Co content. The easy magnetization direction at room temperature has been determined by X-ray diffraction on magnetically-aligned powder samples. The influence of the substitution of Co for Fe on the magnetic anisotropy is discussed in terms of crystal-field theory.

Structure and element distribution in the UFe10-x Al x Si2 system

The neutron diffraction studies of powdered alloys withx = 2,2.5, and 3 gave detailed information on crystallographic data of the compounds. The structure refinement at 300 K for allx = 2,2.5, and 3 and at 450 K forx = 2 confirmed the ThMn12 type of structure with uranium located in 2(a) positions, iron in 8(f), 8(i), and 8(j) positions, aluminum atoms in 8(i) and 8(j) positions, and Si atoms in 8(f) and 8(j) positions. Impurity phases, FeAl and UFe2Si2 have been detected in all investigated compounds.

Investigation of spin-reorientation phase transitions in single-crystal DyFe11Ti

In this paper we study the phenomenon of spin reorientation in the compound DyFe11Ti, which has a tetragonal crystal structure of ThMn12 type. We experimentally measured curves for the mechanical torque that acts on singlecrystal samples placed in a magnetic field. Literature data on the spin-reorientation transition temperatures in this compound and on the character of the spin reorientation are quite contradictory. Thus, for example, Hsu et al. observed that a second-order spin-reorientation transition takes place in DyFe11Ti at T15 200 K and a first-order spinreorientation transition at T2558 K, whereas Kudrevatykh et al. 1 reported that the second transition occurs at T25120 K. Only one spin-reorientation transition was observed in Ref. 3. Analysis of the other papers also results in contradictory information, since the measurements ~primarily studies of the temperature dependence of the magnetic susceptibility and magnetization! were most often made on oriented powder samples by methods that do not allow an unambiguous answer to the question about the nature of the spinreorientation transition. This prompted us to investigate the spin-reorientation transition in DyFe11Ti single crystals. The technology for obtaining single crystals and the method for making measurements were described previously in Refs. 7,8. For the magnetic measurements we took samples whose crystallographic axes in single-crystal blocks were misoriented by no more than 3°. Samples cut along the crystallographic planes ~010! and ~110! had a weight of around 30 mg; they had the shape of disks with diameter ;4 mm and thickness ; 0.5 mm. Curves of the mechanical torque L(u), where u is the angle between the crystallographic direction @001# and the field H, were plotted for a DyFe11Ti single crystal on a torque magnetometer in the temperature range 78–300 K for magnetic fields up to 13 kOe. Although a magnetic field H5 13 kOe is insufficient for saturation far from the easy magnetization axis, near this axis the magnetization curves and mechanical torques both saturated in this field. The use of stronger magnetic fields leads to such undesirable effects as disruption of the collinearity of the magnetic moments of the dysprosium and iron sublattices, which hinders the analysis of the experimental data. Figure 1 shows experimental curves for L(u) measured in the ~010! plane at various temperatures in a field H5 13 kOe. At T5 300 K the crystallographic directions @001# (L 50 and ]L/]u, 0! and @100# (L50 and ]L/]u. 0! are easy-magnetization and difficult-magnetization axes respec-

Crystallographic transformations of rapidly quenched Sm10Fe90−xTix and magnetic properties of their nitrides

Sm 10 Fe 90−x Ti x alloys (x=0, 1, 3, 4, 5, 6, 8, 10) were prepared by rapid quenching and sequent annealing. Structural transformations from amorphous, metastable-to-equilibrium phases, and magnetic properties of their nitrides were systematically investigated. A CaCu5-type metastable phase was formed for 0⩽x⩽10 as the crystallization of the amorphous alloys occurred below 700 °C. The CaCu5-type structure is transformed into a TbCu7-type one with increased annealing temperature (above 700 °C). These alloys have different structures including Th2Zn17 type for 0⩽x⩽3, TbCu7 type for 3<x<6, and ThMn12 type for 6⩽x⩽10, at high annealing temperatures (above 950 °C). Nitrogenation was carried out at 460 °C for 6 h. The best hard magnetic properties are obtained after nitriding for x=3. It gives Hc=17.6i kOe, 4πMr=7.7 kG, and (BH)max=6.8 MGOe for the TbCu7-type phase and Hc=8.6i kOe, 4πMr=8.4 kG, and (BH)max=10.2 MGOe for the CaCu5-type one. Although the coercivity of the CaCu5-type phase is lower than that of the TbCu7-type one, enhanced remanence of the former has been observed.

Magnetic properties of rare-earth compounds of the type R(Fe, Ni) 10Si 2

The magnetic properties of rare-earth compounds of the type HoFe 10− x Ni x Si 2 were investigated by means of X-ray diffraction and measurements of the magnetisation and the AC-susceptibility. It is shown that these compounds form a complete series of solid solution in which the tetragonal ThMn 12-type of structures is retained for all concentrations. From magnetic measurements made on HoNi 10Si 2 and GdNi 10Si 2, it is shown that the presence of Fe moments is essential for the occurrence of magnetic ordering above 4.2 K. For sufficiently high Fe concentration a magnetic moment is induced on the Ni atoms.

Magnetic properties of Y(Co1−xFex)10Si2 compounds

The magnetic properties of Y(Co1−xFex)10Si2 (x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0) compounds have been studied. All compounds crystallize in ThMn12 type structures. The lattice parameters a and c increase with increasing iron content, while the Curie temperature has a maximum at about x = 0.6. The easy magnetization direction at room temperature changes from basal plane to the c-axis at about x = 0.2. The anisotropy field obtained by the SPD technique reaches a maximum at x = 1.0, with values of 2.1 T at room temperature and 2.6 T at 77 K respectively.

The crystal structure of YbFe2Al10, a combined substitution and stacking variant of the ThMn12 and CeMn4Al8 type structures

Abstract YbFe2Al10 crystallizes with a new structure type, which was determined from single-crystal X-ray data: Cmcm, a = 896.6(1)pm, b = 1015.3(2)pm, c = 900.3(1) pm, Z = 4, R = 0.024 for 795 structure factors and 41 variable parameters. The structure is closely related to the tetragonal ThMn12 and CeMn4Al8 type structures, from which it can be derived by substitution of some manganese by aluminum atoms, and in addition it has a different stacking of atomic layers. The ytterbium atoms have 20 neighbors (4 Fe + 16 Al), the iron atoms have distorted icosahedral coordination (2 Yb + 10 Al), and all of the five different aluminum sites are coordinated by ytterbium, iron, and aluminum atoms with coordination numbers 12, 13, and 14.