Th2Zn17-type Structure Research Articles

Structural and magnetic properties of R2Fe17-xBex compounds with R = Y, Pr, Nd, Sm, Gd, Tb and Dy

The effect of partial Fe substitution by Be on structure and magneticproperties has been studied across the R2Fe17-xBex series, whereR = Y, Pr, Nd, Sm, Gd, Tb and Dy. On Be substitution, the Th2Zn17 typerhombohedral structure is stabilized across the series, even for R = Dy and Tbwhich stabilize in the Th2Ni17 type hexagonal structure without Besubstitution. Depending on the R element, one or more Be atoms (up to five)per formula unit can be retained in the Th2Zn17 type structure. Thelattice parameters, magnetic ordering temperature and saturation magneticmoment decrease upon Be substitution. The easy magnetization direction of thesamples with a higher magnetic ordering temperature is still within the basalc plane. The variation of magnetic properties is discussed in correlationwith volume effects, dilution effects and electron hybridization.

Structure and magnetic properties of SmCo7−xTix with TbCu7-type structure

The SmCo7−xTix, x=0–0.56 bulk samples are prepared by arc melting. X-ray diffraction indicates that samples with 0.2<x<0.4 form a single disordered TbCu7-type structure phase and other minor phases appear for other values of x, which indicates that Ti helps stabilize the 1-7 phase. The lattice parameters ratio (c/a) increases with increasing Ti concentration. Room temperature saturation magnetization and Curie temperature decrease with increasing x. X-ray diffraction and magnetization measurements on aligned samples show that all samples studied have uniaxial anisotropy. The anisotropy field is found to increase with increasing x reaching a maximum of 175 kOe at x=0.28 and then decreases for higher values of x. This anisotropy field is 20% higher than that of the same compound with Th2Zn17-type structure.

Magnetic properties of Sm2Co17−xGax (x=0–7) compounds

The study of the effect of Ga substitution on the magnetic properties of arc-melted Sm2Co17−xGax (x=0–7) compounds has been carried out by means of x-ray diffraction (XRD) and magnetic measurements. XRD patterns show that all samples of Sm2Co17−xGax (x=0–7) crystallize in the rhombohedral Th2Zn17-type structure. The Ga substitution for Co leads to an approximately linear decrease in the Curie temperature and a rapid decrease in the saturation magnetic moment which is faster than that in the case of magnetic dilution. Spin-reorientation transition is observed for Sm2Co11Ga6 compound at 32 K. In order to determine the room-temperature easy magnetization direction (EMD), XRD measurements are performed on magnetically aligned samples with x⩽5. The result shows that the EMD of these compounds corresponds to the c axis. The anisotropy constants K1 and K2 of Sm2Co17−xGax compounds were derived by fitting the hard-direction magnetization recoil curves measured on magnetically aligned powder samples as well as by fitting the magnetization recoil curves measured on polycrystalline bulk samples. The fitting results indicate that the substitution of Ga for Co in Sm2Co17 compound decreases the anisotropy constants, thus indicating the weakening of the easy-axis anisotropy.

Structure and magnetic properties of Sm2Fe17-xMnx compounds

The effect of Mn substitution for Fe in Sm2Fe17-xMnx compounds on the structure and magnetocrystalline anisotropy was studied by x-ray diffraction and magnetization measurements. The compounds prepared have a rhombohedral Th2Zn17-type structure single phase except for a small amount of -Fe for x = 1-4, while x = 5 has Nd3(Fe,Ti)29-type monoclinic structure. The Curie temperature TC is found to increase to 430 K for x = 1, then decrease to 255 K for x = 5. The saturation magnetization at 5 K decreases with increasing Mn concentration. Room temperature anisotropy changes from easy planar type to cone type and to easy c-axis type with increasing Mn concentration for x = 1, 2 and 3 respectively. The anisotropy field for x = 3 is about 1 T at 100 K and decreases to 0.4 T at room temperature.

Structure, exchange interactions and magnetic anisotropy of Ho2Co17−xSix compounds

The structural and magnetic characteristics of Ho2Co17−xSix (x=0, 0.5, 1.0, 1.5, 2.0, 2.5, and 3.0) compounds have been investigated by means of x-ray diffraction and magnetic measurements. Powder x-ray diffractograms show that all samples are single phase and have the hexagonal Th2Ni17-type or the rhombohedral Th2Zn17-type structure. The substitution of Si for Co leads to a monotonous decrease in unit cell volume v and Curie temperature TC. The compensation points are observed for the samples with x=2.5 and 3.0, and the compensation temperatures shift to higher temperatures with increasing Si content. The intersublattice-molecular-field coefficient, nHo–Co, has been determined on the basis of magnetization curves at the compensation temperature. It has been found that the Si substitution for Co has little influence on Ho–Co exchange interaction. The magnetic phase diagram has been determined from the temperature dependence of magnetization and the XRD patterns of magnetically oriented powder samples. The spin-reorientation temperatures of Ho2Co17−xSix compounds are found to decrease with Si content for x<2.5 and then increase for x>2.5, attaining a minimum value of 237 K at x=2.5.

Structure and magnetic properties of Pr2Co17−xMnx compounds

The structure and magnetic properties of Pr2Co17−xMnx (x=0–14) compounds were studied. A single phase with the rhombohedral Th2Zn17-type structure was obtained from x-ray diffraction patterns for x=0–11. The unit-cell volumes were found to increase linearly at a rate of about 5.7 Å3 per Mn atom. Both the Curie temperature and saturation magnetic moment at 5 K decrease monotonously with increasing Mn concentration. The spin reorientation was found in Pr2Co17−xMnx compounds for x=1–5. The magnetic phase diagram of the compounds was given. A transition from ferromagnetic to antiferromagnetic for x=8–9 was observed due to the Mn substitution. At high Mn concentration (x=12,13,14), a rare-earth intermetallic 3:29 phase was found. This kind of 3:29 phase shows a paramagnetic character when the temperature is above 5 K.

Crystal structure and magnetic properties of the intermetallic compounds La2Co17-xMx (M = Nb, Mo, Mn)

Novel ternary intermetallic compounds La2Co17-xMx (M = Nb,Mo,Mn) with Th2Zn17-type structure were synthesized and their homogeneous ranges were determined: 0.3 x0.6 for Nb, 0.6 x1.2 for Mo and 1.0 x4.0 for Mn. The lattice parameters and the unit-cell volume increase with increasing x in all of the systems. The preferential occupancy of 6c crystallographic positions (dumb-bell sites) and the stabilization of these compounds by M atoms are attributed to the atomic-size and enthalpy effects. These compounds exhibit favourable uniaxial anisotropy with M content x0.5. The Curie temperature decreases monotonically with increasing x for all of the systems. The saturation moment µs decreases rapidly with increasing x for M = Mo and Nb. In the La2Co17-xMnx system, µs increases slightly with x for x2.5 and then decreases for x>2.5.

Structure and magnetic properties of Pr2Fe17−xMnx (x=0–9) compounds

The influence of Mn substitution for Fe on the structure and magnetic properties of Pr2Fe17−xMnx compounds with 0⩽x⩽9 was investigated. X-ray diffraction indicates that all samples are single phase with the rhombohedral Th2Zn17-type structure. The unit cell volume V remains nearly constant with increasing manganese content up to x=2, then increases linearly at a rate of about 2.5 Å3 per Mn atom up to x=8 and there is a large increase of cell volume at x=9. The Curie temperature increases a little from 298 K at x=0 to 308 K at x=1, and then decreases to 37 K at x=9 quickly. The saturation magnetization at 5 K was found to decrease monotonously with increasing Mn concentration. The thermal magnetization curves M(T) of the compounds with x=1–9 show a strong decrease below TC with decreasing temperature. Also it presents considerable magnetic hardness at low temperature. This can be attributed to the presence of narrow Bloch walls.

Influence of nitrogenation on structure development and magnetic properties of mechanically alloyed and annealed Sm–Fe powders

Sm–Fe–N compounds were prepared by mechanical alloying, subsequent annealing and nitrogenation. For crystal structure investigations of the non-equilibrium phases Sm2Fe17+yNx, formed at various annealing temperatures TA for 1 h, X-ray diffraction with following Rietveld analysis was used. A volume expansion of 6.2% was observed after nitrogenation. As for the non-nitrided Sm–Fe alloys a modified TbCu7-type structure (space group P6/mmm) and a modified Th2Zn17-type structure (space group R3m) have been observed. However, for nitrogenated Sm–Fe samples the modified Th2Zn17-type structure forms already for annealing at TA=750°C prior to nitrogenation. For samples annealed, prior to nitrogenation, between TA=600 and 700°C the modified TbCu7-type structure was found, in which the nitrogen occupies randomly the 3f position with an occupancy larger than 1/3. The partially ordered, modified Th2Zn17-type structure formed for 750°C <TA<900°C, is derived from the ideal Th2Zn17-type structure by introducing additional Fe(6c) and Sm(3a) positions. The degree of order of the Sm atoms and Fe-dumbbells increases with increasing TA. The nitrogen occupies the octahedral interstitial positions 9e in the latter cases. The nitrogen content is higher in the hexagonal phase than in the rhombohedral phase. Optimum magnetic properties were obtained for TA=750°C. Here we found a coercivity μ0JHc=3.7 T and a good squareness of the demagnetization curve.

Neutron diffraction studies on (Ho1−xErx)2Fe15Ga2Cy compounds

Powder neutron diffraction studies were carried out on (Ho1−xErx)2Fe15Ga2Cy (x=0, 0.5, 1; y=0, 2) with a view to study the effect of substitution of Ga for Fe and the insertion of carbon at the interstitial sites on the structural and magnetic properties. All Ga substituted compounds (y=0) possess hexagonal Th2Ni17 type structure (space group: P63/mmc). However, the carbides adopt rhombohedral Th2Zn17 type structure (Space group: R3̄m). The unit cell volume increases with Ga substitution as well as with interstitial modification by carbon. An increase in the Curie temperature of about 260 K is observed for the compound HoErFe15Ga2C2 vis-a-vis HoErFe17. The increase is attributed to the preferential occupancy of Ga atoms in 18f(12j) and 18h(12k) sites and the presence of interstitial carbon. The magnetization increases with carbon addition. X-ray diffraction studies on magnetically aligned samples and neutron diffraction studies show that the easy magnetization direction lies in the basal plane at room temperature.

Structure and magnetic properties of Zr-doped nanocrystalline Sm2Fe15Ga2Cx ribbons

The phase formation and magnetic properties of melt-spun Sm2Fe15Ga2Cx ribbons with a Zr addition of 1.5 at. % have been investigated. A metastable phase with the disordered TbCu7-type structure is formed in the samples annealed at temperatures between 620 and 740 °C. Upon heating above 750 °C, a structural transformation from the hexagonal TbCu7- to the rhombohedral Th2Zn17-type structure occurs. It was found that the Zr addition improves the thermal stability of the TbCu7 structure. Transmission electron microscopy observations show that the grain sizes in 1:7-type ribbons are much smaller than that in 2:17-type samples. The highest value of the maximum energy product of 88.7 kJ/m3 is obtained in a 1:7-type ribbon annealed at 740 °C for 15 min. The corresponding remanence and coercivity amounts to 0.8 and 0.43 T, respectively. In contrast, coercivities higher than 0.8 T obtained in 2:17-type ribbons are accompanied by remanences lower than 0.66 T. The 1:7-type ribbons with grain sizes in the range of 15–45 nm show significant remanence enhancement which can be attributed to the exchange coupling between adjacent grains.

Structure change in Sm–Fe–B–Ti permanent magnet materials induced by HDDR process

The structures of Sm8.3Fe85.2B2.0Ti4.5 and Sm9.4Fe84.1B2.0Ti4.5 alloys, which show superior properties as isotropic permanent magnets after a hydrogenation–disproportionation–desorption–recombination (HDDR) process followed by a nitridation, have been investigated by transmission electron microscopy, and electron and X-ray diffraction, paying particular attention to the structural changes induced by the HDDR process. It is found that a hexagonal Th2Ni17-type structure, observed in both alloys, with an approximate composition Sm(Fe,Ti)9.5 before the HDDR process, transforms to a rhombohedral Th2Zn17-type structure with a composition Sm(Fe,Ti)8.5 after the HDDR process. The Ti content in the hexagonal Th2Ni17-type structure is higher than that in the rhombohedral structure. The hexagonal Th2Ni17-type and rhombohedral Th2Zn17-type structures in the Sm9.4Fe84.1B2.0Ti4.5 alloy have lattice parameters a=0.85103(3) nm and c=0.83820(5) nm, and a=0.85549(3) nm and c=1.24826(9) nm, respectively, and show the relationship ahex 1/3crhomb.

Magnetic properties of R2Co15Al2 compounds with R=Y, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, and Tm

An investigation of the structure and the magnetic anisotropy of R2Co15Al2 (R=Y, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, and Tm) compounds has been performed by means of x-ray diffraction and magnetization measurements. X-ray diffraction shows the prepared compounds to be single phase, having the hexagonal Th2Ni17-type structure for R=Y, Ho, Er, and Tm and the rhombohedral Th2Zn17-type structure for R=Ce, Pr, Nd, Gd, Tb, and Dy. Except for the Ce compound, the unit-cell volumes of R2Co15Al2 compounds decrease in accordance with the lanthanide contraction. Substitution of Al for Co in R2Co17 leads to a decrease of the saturation magnetization at 1.5 K and Curie temperature. The exchange-coupling constants JCo–Co and JR–Co have been calculated by using the method based on magnetic ordering temperature. It is found that the JR–Co has a small dependence on the R elements and is almost not affected by the Al substitution. The Ce compound is found to exhibit an anomalous lattice parameter and magnetic characteristic, which are relative to the mixed-valence behavior of the Ce ion. X-ray diffraction measurements on magnetically aligned R2Co15Al2 powders show that the compounds with R=Pr, Nd, Gd, Tb, Dy, and Ho have an easy-plane type of magnetic anisotropy, whereas the compounds with R=Y, Ce, Sm, Er, and Tm exhibit an easy-axis type of magnetic anisotropy at room temperature. The compounds R2Co15Al2 with R=Pr and Ho exhibit a spin-reorientation transition and the spin-reorientation temperature is found to be 531 and 431 K, respectively. A strong uniaxial anisotropy is observed in Sm2Co15Al2 compound with a magnetocrystalline anisotropy field of 84 kOe at room temperature.

HARD MAGNETIC PROPERTIES OF Sm-Fe-Cu-Si-C WITH THE 2∶17-TYPE STRUCTURE

The structural and magnetic properties of Sm2Fe15Si2C and Sm2Fe14CuSi2C were analyzed.The Curie temperature TC and saturation magnetization Ms were measured.The magnetocrystalline anisotropy field HA and anisotropy constant K1 and K2 at temperatures ranging from 1.5K to room temperature were obtained by analyzing the magnetic curve.We can conclude that Cu does not come into the Th2Zn17-type structure,while Cu doping modifies the micro-structure and enhances the coercivity of Sm2Fe14CuSi2C ribbons.The maximum intrinsic coercivity(μ0iHc)around 1.0 and 1.2 T at room temperature for Sm2Fe15Si2C and Sm2Fe14CuSi2C ribbons were achieved, respectively. The microstructural parameter αk and the averaged local effective demagnetization factor Neff were derived from the temperature dependence of coercivity.

Anomalous Magnetic and Transport Properties in Ce2Fe17 and Lu2Fe17

Using single phased Ce2Fe17 and Lu2Fe17 with the rhombohedral Th2Zn17-type and Th2Ni17-type structure, respectively, we performed measurements of magnetization and resistivity under various magnetic field up to 5.5T. Ce2Fe17 exhibits a simple helical spin structure with a wave vecter of τ= 0.0372 Å-1 parallel to the c-axis in the temperature range of TC=125K < T<TN = 210 K. For T<TC, it shows a modified helix of τ= 0.0435 Å-1 parallel to the c-axis with a superstructured magnetic cell, in which the magnetic c-parameter is twice as large as the chemical unit cell. Only below TC, a giant magnetoresistance is induced by a metamagnetic transition from the helix to ferromagnetic phase in the field of H < 2T. The value of Δρ/ρ(= (ρAF-ρF )/ρAF) reaches 0.85 at 4.2 K. Such GMR may be atributed to the suppression of a super-zone gap induced by a formation of the superstructured magnetic cell below TC. On the other hand, GMR could not be observed in Lu2Fe17.

Structural studies of YBCO ramp Josephson junctions for rapid single flux quantum circuits

The influence of Mn substitution for Fe on the structure and magnetic properties of Pr2Fe17-xMnx compounds with 0 less than or equal to x less than or equal to 9 was investigated. X-ray diffraction indicates that all samples are single phase with the rhombohedral Th2Zn17-type structure. The unit cell volume V remains nearly constant with increasing manganese content up to x=2, then increases linearly at a rate of about 2.5 Angstrom(3) per Mn atom up to x=8 and there is a large increase of cell volume at x=9. The Curie temperature increases a little from 298 K at x=0 to 308 K at x=1, and then decreases to 37 K at x=9 quickly. The saturation magnetization at 5 K was found to decrease monotonously with increasing Mn concentration. The thermal magnetization curves M(T) of the compounds with x=1-9 show a strong decrease below T-C with decreasing temperature. Also it presents considerable magnetic hardness at low temperature. This can be attributed to the presence of narrow Bloch walls. (C) 1999 American Institute of Physics. [S0021-8979(99)05621-2].

Hydrogenation–disproportionation–desorption–recombination in Sm2Fe16M (M=Al, Ga and Si) and magnetic properties of their carburized powders

The application of the hydrogenation–disproportionation–desorption–recombination (HDDR) process in Sm2Fe16M (M=Al, Ga and Si) was investigated. The hydrogen absorption behaviour was studied by temperature–pressure analysis (TPA). In the temperature range between 500 and 800°C, Sm2Fe16M samples with M=Ga and Si show a weaker hydrogen absorption than Sm2Fe16Al. This was confirmed by X-ray diffraction which showed a complete disproportionation of Sm2Fe16Al into SmHz (1.9<z<2.8) and α-(Fe,Al), while Sm2Fe16Ga and Sm2Fe16Si exhibit a fraction of undecomposed material with the Th2Zn17-type structure after the disproportionation procedure. These results point to a stabilization of Sm2Fe16M against disproportionation by hydrogen for M=Ga and Si. The magnetization processes of carburized HDDR powders were studied by VSM measurements and Kerr microscopy. The demagnetization curve of our HDDR processed Sm2Fe16AlCy is well shaped, whereas those of Sm2Fe16GaCy and Sm2Fe16SiCy show a large decrease of the polarization at low fields. The favourable behaviour of Sm2Fe16AlCy is due to a homogeneous submicron grain structure. In Sm2Fe16GaCy and Sm2Fe16SiCy samples, an additional, magnetically soft phase with larger domains was observed which causes the low coercivity. As a consequence, HDDR was only favourable for the preparation of Sm2Fe16MCy hard magnets with M=Al but not for M=Ga and Si. Optimization of the HDDR process in Sm2Fe16Al and subsequent nitrogenation or carburization led to coercivities of μ0jHc=2.9 and 2.5 T, respectively.

Magnetic properties of ErFeGa compounds with the 2:17-type structure

An investigation of the structure and the magnetic anisotropy of Er2Fe17−xGax compounds has been performed by means of X-ray diffraction and magnetization measurements. X-ray diffraction shows the prepared compounds to be single phase, having the hexagonal Th2Ni17-type structure for x ≤ 3 and the rhombohedral Th2Zn17-type structure for x ≥ 4. Substitution of Ga for Fe in Er2Fe17 leads to an increase of the unit-cell volume which is linear with the Ga content. The saturation magnetization at 1.5 K decreases linearly with increasing Ga concentration. The Curie temperature is found first to increase, then to decrease with increasing Ga concentration, having a maximum value of 526 K at about x = 3. The Er2Fe17−xGax compounds with x = 2, 3 and 4 exhibit a spin-reorientation transition with the spin-reorientation temperature being 136, 140 and 102 K, respectively. X-ray diffraction measurements on magnetically aligned Er2Fe17−xGax powders show that the compounds with x ≤ 7 have easy-plane type of magnetic anisotropy, whereas the compound with x = 8 exhibits easy-axis type of magnetic anisotropy at room temperature. The easy-magnetization direction of the sample of x = 8 is perpendicular to the c-axis at low temperature. Present result demonstrates that in Er2Fe17−xGax the substitution of high concentration of Ga leads to a change in the second-order crystal field parameter A20 from negative to positive.

Magnetocrystalline anisotropy of Pr2(Fe1−xCox)17−yGay compounds with a Th2Zn17-type structure

The structure and magnetic properties of Pr2(Fe1−xCox)17−yGay compounds with 0⩽x⩽1 and 0⩽y⩽3 have been investigated using x-ray diffraction (XRD) and magnetization measurements. XRD patterns demonstrate that all samples were single phase with the rhombohedral Th2Zn17-type structure, except for Pr2Fe16.5Ga0.5 with some impurity phases. The replacement of (Fe, Co) by Ga results in an approximately linear increase in the unit cell volume, and decrease in the Curie temperature and saturation magnetization. The substitution of Co for Fe leads to a nonlinear variation in the unit cell volume, the Curie temperature and the saturation magnetization. The magnetic anisotropy phase diagrams of Pr2(Fe1−xCox)16.5Ga0.5 (0⩽x⩽1) and Pr2(Fe0.4Co0.6)17−yGay (0⩽y⩽3) compounds have been determined from the temperature dependence of magnetization and the XRD patterns of magnetically oriented powder samples. Most of the samples exhibit easy plane anisotropy. A small range exists in the magnetic anisotropy phase diagrams of Pr2(Fe1−xCox)17−yGay compounds at 300 K, in which the c axis is the easy magnetization direction, however, small amounts of Ga close the easy c-axis region at y=1.5. The spin-reorientation temperature was found to increase with Co and Ga content for Pr2(Fe1−xCox)16.5Ga0.5 (0.5⩽x⩽1) and Pr2(Fe0.4Co0.6)17−yGay (0⩽y⩽1.5) compounds, respectively. The relationship of the magnetocrystalline anisotropy with the lattice parameters and other magnetic properties were analyzed.

Magnetic properties of Pr 2 Co 17- x M x ( M = Ga, Si) compounds

The influence of Ga or Si substitution for Co on the structural and magnetic properties of Pr2Co17 compounds is investigated. All samples studied here are single phase and have the rhombohedral Th2Zn17-type structure. The unit-cell volume is found to increase linearly by the substitution of Ga for Co, but reduce by the substitution of Si for Co in Pr2Co17 compounds. In Pr2Co17-xMx, the Curie temperature decreases monotonically with increasing x at an approximate rate of 153K per Ga atom and 175K per Si atom. The saturation magnetic moment of Pr2Co17-xMx (M = Ga, Si) decreases with increasing x. The rates of the decrease are larger than that expected as a simple dilution. For Pr2Co17-xSix, the spin-reorientation transition is observed above room temperature. The spin-reorientation temperature Tsr first decreases with increasing Si content and then increases at higher x values (x > 2). The spin-reorientation behavior is interpreted by the competition between the Pr and Co sublattice anisotropies. The easy magnetization direction in Pr2Co17-xGax compounds is perpendicular to the c-axis, and no spin reorientation transition is observed.