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The Mossbauer spectra of Nd6Fe13X, where X is Si, Cu, Ag, and Au, have been measured between 80 and 500 K. A model corresponding to a basal alignment of the magnetic moments leads to excellent, internally consistent, fits for the Cu, Ag, and Au compounds. The resulting temperature dependences of the spectral hyperfine parameters are uniform and reveal for each Fe site the expected correlations between the isomer shift and the Wigner–Seitz cell volume and the hyperfine field and the number of Fe near neighbors. For Nd6Fe13Si, a different model must be used because of the presence of a spin reorientation below 155 K. Above 155 K, because of the axial alignment of the moments, and in agreement with the 295 K powder neutron diffraction results, the spectra can be analyzed with four sextets. Below 155 K, five additional sextets are required to fit the spectra because of a progressive transition towards a basal alignment of the Fe moments. At 80 K the mixed magnetic phase is 75% basal and 25% axial, whereas at ...

We have studied the magnetic properties of several R 2 Co 17− x Ga x compounds with R=Y, Tb, Dy and Ho. X-ray diffraction measurements made on magnetically aligned powders of Y 2 Co 17− x Ga x compounds showed that these materials retain their easy-plane magnetisation at room temperature up to at least x =5. The difference in behaviour with the corresponding Gd compounds and the absence of spin-reorientation phenomena in R 2 Co 17− x Ga x compounds in which the R component has a negative second-order Stevens factor is discussed in terms of preferred site occupation of the Ga atoms. High-field measurements at 5 K were made on free powders of several Tb, Dy and Ho compounds. The intrasublattice constants derived from these data are somewhat smaller than those of the undiluted R 2 Co 17 compounds, which is also discussed in terms of preferred Ga site occupation.

In this report the possibilities to obtain experimental information on various physical properties of rare earth intermetallics by means of 155 Gd Mossbauer spectroscopy are reviewed. Results are discussed of compounds related to permanent magnet materials, hydrogen absorbing materials, superconducting and heavy-electron systems. Particular attention is paid to the electric field gradient V ZZ and its relation to the second order crystal field parameter A 2 0 . Experimental data are compared with results of electronic structure calculations.

The magnetic properties of Tb2Co17−xGax compounds (x=1,3,5 and 7) were studied by means of neutron diffraction and magnetic measurements. The refinement results of the neutron powder diffraction patterns show that initially the Ga atoms occupy almost exclusively the 18h site in the rhombohedral Th2Zn17 structure and presumably are responsible for the sign reversal of the Co sublattice anisotropy. For x≥3 the Ga atoms increasingly occupy the 6c and 18f sites at the cost of the 18h site that shows a decreasing Ga occupancy in this concentration range. The 9d site is completely avoided by the Ga atoms over the whole concentration range. The sign reversal of the Co sublattice anisotropy is attributed to filling of the 18h site with Ga. From magnetic measurements and neutron diffraction it is derived that the Co moment strongly decreases in size with Ga concentration. There is also a strong decrease of the Curie temperature with Ga concentration.

The crystallographic and magnetic properties of the Ce2Ni15Si2 compound was studied by means of neutron diffraction at room temperature and at 2 K. The refinement results of the neutron powder diffraction pattern show that this compound crystallises in the rhombohedral Th2Zn17-type structure. There is a distinct preference for the Si atoms to occupy the 9d site. No magnetic ordering was detected either at room temperature or at 2 K in the neutron diagram. From XAFS experiments it was derived that Ce is in an intermediate valence state in this compound.

We have studied the Fe magnetic ordering in the tetragonal RFe 4 Al 8 (R=La, Ce, Lu, Y) antiferromagnetic compounds by neutron diffraction at temperatures between 1.5 and 240 K. The structure refinement in the paramagnetic state confirms the preference of the Fe atoms for the 8(f) positions. Below T N the four compounds were found to order with four distinct types of spiral structures requiring two sets of wave vectors for their description: (i) q 1 =0, q 2 =(0, 0, q z ) and (ii) q 1 =0, q 3 =( q x , q x , 0). Most of the structures undergo magnetic phase transitions at lower temperatures. In all cases is the Fe moment value at 1.5 K close to 2.0 μ B /Fe atom. The wave vector length(s) is slightly temperature dependent in the high temperature region. The R=La compound orders with a double cone structure with the cone axis parallel to the wave vector q 2 =(0, 0, 0.265(5)). The cone axes of adjacent Fe chains extending along c have opposite signs. At 1.5 K the cone angle(s) of the Fe sublattices are Φ c =60° and Φ c +π. The rotation angle is Φ s =95(1)°. For the RFe 4 Al 8 compounds with R=Ce, Lu, Y the iron site splits into two independent sets (orbits) under the action of the in-plane wave vector q 3 =( q x , q x , 0), where q x has at 1.5 K the values 0.243(1), 0.145(3), 0.135(1) r.l.u., respectively. Between Fe moments of adjacent chains extending along c (different orbits) there is a non-zero phase while along the face diagonal(s) (the same orbit) the moments have the same phase. The moments of the Y and Lu compounds rotate within the plane of the wave vector (0 0 1) and form a cycloid spiral. The latter compound has in addition a small component along c which corresponds to a double cone cycloid spiral. The Ce compound has a simple spiral structure with the moments rotating in a plane perpendicular to the wave vector q 3 =( q x , q x , 0). The existence of small moment component along q 3 cannot be excluded from the present data. This would give rise to a double cone structure.

We have studied the magnetic properties of Ho2Co17-xGax compounds (x less than or equal to 7) by means of magnetic measurements and X-ray diffraction on magnetically aligned powders. The Curie temperature and the spontaneous moment decrease strongly with increasing Ga concentration. The compounds that are magnetically ordered at roam temperature have an easy magnetisation direction at room temperature which is perpendicular to the c-direction. (C) 1998 Elsevier Science S.A.

The structural properties of Nd 2 Co 17− x Ga x compounds ( x =5, 6 and 7) were studied by means of neutron diffraction. The refinement results of the neutron powder diffraction patterns show that the Ga atoms completely avoid the 9d site in the rhombohedral Th 2 Zn 17 structure. Furthermore, the Ga atoms increasingly occupy the 6c and 18f sites at the cost of the 18h site that shows a decreasing Ga occupancy in this concentration range. At 4.2 K, the easy magnetisation direction is perpendicular to the c -axis for x =5 and 6, but parallel to the c -axis for x =7. From magnetic measurements and neutron diffraction it is derived that the Co and Nd moments at 4.2 K strongly decrease with increasing Ga concentration. There is also a strong decrease of the Curie temperature with Ga increasing concentration.

Rare earth-iron interstitial carbides and nitrides are candidate materials for advanced permanent magnets. However, since the thermal stability of the parent compounds is generally limited, it has not been possible to date to synthesize single-phase materials using the usual gas-phase nitriding and carburizing processes. Very recently, it has been shown by Mao, Strom-Olsen, Altounian and Yang (J. Appl. Phys. 79 (1996), p.4619.) that two-phase Sm 2 Fe 17 C X / α-Fe can be produced by ball milling Sm 2 Fe 17 with graphite and a subsequent heat treatment. Here, we perform ball milling experiments on TbFe 9.5 Mo 2.5 , TbFe 9 Mo 3 , Gd 2 Co 17 , Gd 2 Fe 17 and Sm 2 Fe 17 in order to study whether these compounds can be directly carburized by mechanical alloying with one of the following carbon sources: graphite, glass-like carbon, toluene, solid paraffin or iron carbide. At present, we conclude that a heat treatment is always necessary to start the diffusion of carbon into the host lattice, resulting in the partial decomposition of the compounds.

The specific heat of a CeCoAl 4 single crystal has been studied in magnetic fields up to 17.5 T. In zero field, this compound has been found to order antiferromagnetically below the Neel temperature of about 13.5 K. The effect of the magnetic field is different when it is applied along the different crystallographic axes. Along the a -axis, with increasing magnetic field, the Neel temperature slightly decreases and reaches a value of 12.7 K at 17.5 T. Along the b -axis, the Neel temperature decreases more strongly with increasing magnetic field. At 16 T, we found a value of T N of about 6.9 K. At 17.5 T, along the b -axis, we observed a rather broad peak in the specific heat which is associated with a transition from paramagnetism to ferromagnetic order. Along the c -axis, a drastic change in the specific heat occurs at a field between 7–8 T. At 7 T, the value of T N is about 9 K. At fields higher than 8 T, the specific heat is evidently characterised by a transition from paramagnetism to ferromagnetic order. The critical values of the magnetic field above which ferromagnetic order is formed at low temperature turn out to be consistent with the measurements of the magnetisation curves at 4.2 K.

Magnetic and structural properties of sintered NdFeB magnets with addition of various amounts of DyGa as a separate powder are studied and compared with single-powder magnets of the same nominal composition. When the magnets are annealed above 640 °C, the coercivity decreases strongly as compared to magnets annealed below 640 °C. Decomposition of the δ phase (Nd6Fe14−xGax) accompanies this drop. Measurements of coercivity as a function of temperature show that the decomposition of the δ phase does not increase the effective demagnetization factor, but enlarges either the magnetic inhomogeneous region at the grain surface or the exchange coupling between the grains. Line faults, which are present within the Nd2Fe14B grains when the magnet is annealed below 640 °C, do not act as pinning centers. Phase analysis and diffusion profiles of the Dy and Ga concentration within the grains are made using electron probe micro-analysis. This shows that Ga forms Nd3Ga2 and Nd6Fe12.7Ga1.3 as intergranular phases. Its concentration within the Φ phase is significantly lower than in the Φ phase of single-powder magnets. At 1090 °C, a diffusion coefficient for Ga of D=2.5±0.1×10−15 m2/s is found. Dy is shown only to be present in that part of the Nd2Fe14B grain which has precipitated from the liquid during sintering. At the boundary of the Dy-free center and the outer part of the grain, an increased Dy concentration is found.

The magnetic structure of the compound DyNiSi 2 (CeNiSi 2 type of structure) has been studied by neutron diffraction and magnetic measurements. The paramagnetic neutron data confirmed the structure reported earlier for this compound. The analysis of the data collected in the magnetically ordered regime ( T N =24 K) showed the occurence of a two step magnetic ordering described by the presence of three wave vectors. In a very narrow region immediately below T N the ordering is described by a transversal sine wave modulated structure associated with the wave vector q 2 =1/5 b *. Below 22 K a phase transition takes place manifested by the appearance of higher harmonics pertaining to the wave vectors q 1 =(010)=5 q 2 and q 3 =3 q 2 . The I ( q 1 ) intensities increase at the cost of the I ( q 2 ) and I ( q 3 ) ones. The intensities I ( q 1 ) and I ( q 2 ) coexist down to 1.5 K in varying proportions while the only very weak reflection of q 3 disappears down to 1.5 K. The wave vector q 1 =(010) pertains to a collinear moment arrangement of the Dy moments along c with the magnetic space group C P (22′2 1 ′/m′cm)i′ S 57 391 . The refined moment value at 1.5 K is μ Dy =7.5(1) μ B .

The structural and magnetic properties of MnFe1-xCoxGe compounds with x=0,0.1,0.2,0.3,0.4,0.5,0.6,0.7,0.8,0.85,0.9, and 1.0 were investigated by means of X-ray diffraction (XRD) and magnetization measurements. XRD shows that the MnFe1-xCoxGe compounds crystallize in the hexagonal Ni2In-type crystal structure for xles0.8 and in the orthorhombic NiTiSi-type structure for x>0.8. The magnetization measurements show that the MnFe1-xCoxGe compounds exhibit a complex magnetic behavior. The Curie temperature increases with increasing of x. The saturation magnetization of the compounds with Ni2In type structure increase with increasing of x and the saturation of the magnetization in the NiTiSi-type structure also increases with increasing x. We investigated the magnetocaloric effects in these compounds by means of magnetization measurements. The maximum magnetic-entropy change observed in these compounds reaches 9 J/kgK for x=0.8 in a field change from 0 to 5 T at around 289 K

We report on magnetic properties and magnetocaloric effects of Mn 5 Ge 3− x Si x alloys with x = 0.1, 0.3, 0.5, 1.0, 1.5 and 2.0. All Mn 5 Ge 3− x Si x compounds crystallize in the Mn 5 Si 3 –type hexagonal structure with space group P 6 3 / mcm . The lattice parameters and the Curie temperature of Mn 5 Ge 3− x Si x alloys decrease with increasing x . A considerable large magnetic-entropy change has been observed in these alloys near room temperature. Typical values are 2.3 and 7.8 J/kg K at T c = 299 K for magnetic field changes from 0 to 1 T and from 0 to 5 T, respectively. The average Mn magnetic moment decreases with increasing Si content. The substitution of Si in Mn 5 Ge 3 does not result in a change of the crystal structure. But the Si substitution has two kinds of effects on the magnetocaloric effects. One is that the magnetic-entropy change decreases with increasing Si content, the other one is that the magnetocaloric effect peak becomes broadened.

We have investigated the Gd–Fe exchange-coupling strength in GdFe 6 Ga 6 by means of magnetization measurements on single-crystalline powder particles that are free to rotate into their minimum-energy direction in the applied magnetic field. The experiments have been carried out at 4.2 K and at high magnetic fields up to 55 T in order to achieve bending of the Gd- and Fe-sublattice moments out of their initially collinear antiparallel configuration. The bending starts around 40 T and, from the magnetization behavior above this field value, the strength of the Gd–Fe exchange coupling J GdFe / k B has been determined to be −8.5 K. This value is discussed and is compared with earlier results on the RFe 6 Ga 6 compounds with R = Dy, Ho, Er and Tm.

In previous investigations, it was shown that the compound ErMn 6 Sn 6 , that crystallizes in the HfFe 6 Ge 6 -type of structure, orders in a helimagnetic state at T N = 352 K, with the moments rotating within the [0 0 1] plane. In the helimagnetic state, metamagnetic behavior is observed, the critical field increasing when the temperature decreases from T N to 220 K and then decreasing with further decrease of the temperature from 200 to 75 K. At this latter temperature, the compound orders ferrimagnetically with the moments in the [0 0 1] plane. Very peculiarly, the in-plane magnetocrystalline anisotropy was found to decrease with temperature decreasing from 125 to 4.2 K. The magnetization of ErMn 6 Sn 6 single crystals has been measured in magnetic fields up to 51 T. The measurements were performed at 4.2 K in the ferrimagnetic state and at 125 K in the helimagnetic state with the magnetic field applied along the three principal crystallographic directions. The measured magnetic isotherms are discussed in terms of the competition between the Er-Mn exchange interaction and the magnetocrystalline anisotropies of the Er and the Mn magnetic sublattice.

Neutron powder diffraction has been used to investigate the magnetic order in a series of RMn4Al8 (space group I4/mmm) compounds with R = (Nd, Dy, Ho and Er). For compounds with Mn, no magnetic order was detected down to 1.6 K, whilst for the compound ErCr4Al8, two very weak magnetic reflections were observed at 1.6 K. The observed magnetic peaks cannot be indexed on the tetragonal unit cell, indicating a possible incommensurate magnetic structure. For ErCu4Al8, a type 1 antiferromagnetic structure is observed. The magnitude and temperature dependence of the Er3+ magnetic moment can be adequately modelled by a combined isotropic exchange and tetragonal crystal field Hamiltonian within the Mean Field Approximation.

The magnetic properties of the rhombohedral Ce 2 Fe 17 − x Ga x solid solutions, with x equal to 0.3, 0.5, 0.7, 1.0, and 2.0, have been studied by magnetic measurements, neutron diffraction, and Mossbauer spectroscopy. Magnetization studies indicate that, as the unit cell volume expands with Ga addition, the Curie temperature increases from 238 K for Ce 2 Fe 17 to 406 K for Ce 2 Fe 15 Ga 2 . Powder neutron diffraction measurements at 295 and 14 K indicate that the Fe moments are both oriented in the basal plane and increase with increasing Ga content. The Mossbauer spectra, which have been fit with a binomial distribution of the near-neighbor environments, indicate that the maximum hyperfine field, H max , increases with increasing Ga content and is largest for the 6c site. The decremental field, ΔH , per added Ga atom is virtually independent of temperature and Ga content, indicating that the next-nearest-neighbor interactions are very small. The compositional dependence of the isomer shifts for R 2 Fe 17 − x M x , where R is Ce or Tb and M is Al, Si, or Ga, is analyzed in terms of volume and chemical terms, the second of which reveals the importance of the electronic configuration of M in understanding its influence on the magnetic properties of these solid solutions. Finally, the similarities and differences in the changes in crystallographic, magnetic, and Mossbauer spectral properties of R 2 Fe 17 − x M x , at low x values, indicate that electronic rather than structural factors are more important in modifying the magnetic properties and indicate the need for extensive band structure calculations on these solid solutions.

The magnetic ordering of the novel binary compound with two Tb sites ( structure type, space group Cmcm) has been studied by neutron diffraction and magnetic measurements. orders antiferromagnetically in two steps. In a first step the sublattice orders below with a collinear moment arrangement along the shortest axis a which persists down to . Below a reorientation of the moments takes place. Simultaneously the moments order with a collinear arrangement along c. The ordering of the and sites is associated with the wavevector q = (010) and is described by the monoclinic magnetic space group . At 1.4 K the ordered moment values of and are and , respectively. The moment is tilted away from the x-direction by an angle of and by from the y-direction within the (0yz) plane. The resulting structure corresponds to a three-dimensional canting with six sublattices. The presence of diffuse scattering in the 16.5 - 53 K temperature interval is attributed to short-range ordered magnetic domains of the mean correlation length A. Fourier analysis of the data reveals that the first- (up to 4.5 A) and third- (6.5 - 7.5 A) neighbour correlations are mainly ferromagnetic; the second (4.5 - 6.5 A) and fourth (7.5 - 8.5 A) ones are antiferromagnetic. The magnetic ordering of the coexisting novel phase with the structure type (space group P6/mmm) has been also studied. The Tb moments order antiferromagnetically below . This collinear moment arrangement is associated with the wavevector and is described by the magnetic space group . At and is confined to the (001) plane.

Three CeNi x Sn 2 compounds of different nominal Ni concentrations ( x = 0.65, 0.75, 0.85) were studied by high resolution neutron diffraction. The refinement of the nuclear structure shows that these compounds are single phase and that the actual compositions are close to the nominal ones. Low temperature neutron data of the CeNi 0.85 Sn 2 compound [refined to x = 0.864(2)] confirm the mictomagnetic behaviour already observed in a previous sample ( I ) refined to x = 0.840(4): coexistence of domains of two distinct magnetic phases in equal amounts. The relative amount of the two domains depends on x . One of these phases is ferromagnetic, q 1 = 0 with T C = 3 K. The other phase is a modulated antiferromagnet (antiphase domain type with two amplitudes) with T N = 4.0 K . Its magnetic ordering can be described by two propagation vectors q 2 = (010) ( C p magnetic lattice) and q 3 = 1 3 b ∗ . The ordered magnetic moment valueat 1.5 K in both phases is 2.0 μ B /Ce atom and the moments point into the same direction, along c. In sample ( I ) the two phases were present in equal amounts while in the present sample the amount of the ferromagnetic phase is strongly reduced to approximately 16%. The magnetic properties of these compounds and of compounds with higher and lower nominal Ni concentrations have been studied by us in more detail by means of measurements of specific heat, the magnetisation, the AC and DC susceptibility. In several cases two magnetic phase transitions were observed. The nature of these transitions is discussed in the light of the low-temperature neutron diffraction results obtained for the more Ni-rich samples.

The magnetic ordering of the novel binary compounds Dy 3 Ge 4 and DyGe 1.3(1) , has been studied by neutron diffraction and magnetic measurements. Dy 3 Ge 4 with the Er 3 Ge 4 structure type (space group Cmem ) displays a two step antiferromagnetic ordering below T N = 19 K. The two Dy sites (Dy 1 :8(f) and Dy 2 :4(c)) order independently with two distinct order parameters associated with the same wave vector q = (0, 1, 0). Both sublattices have a uniaxial antiferromagnetic moment arrangement but with mutually perpendicular orientations. Dy 1 orders below T N with the moments along the a -axis, Dy 2 orders only below T N = 6 K along the c -axis. At 1.4 K the moment values of Dy 1 and Dy 2 are 7.31(8) μ B and 5.6(3) μ B , respectively. Their arrangement leads to a symmetry reduction described by the monoclinic magnetic space group Cp11 2 1 ′ m 1′(Sh 11 55 ) . The two-dimensional canted antiferromagnetic structure is the result of a strong magnetocrystalline anisotropy and the presence of various types of magnetic interactions. The phase of DyGe 1.3(1) , composition with the AIB 2 structure type (space group P 6/ mmm ) was detected as an impurity in the studied sample. It orders antiferromagnetically below T N = 16 K with collinear moment arrangement associated with a 2 c -cell enlargement ( q = 0,0, 1 2 ) described by the magnetic space group P 2 11 2′ m′ (Sh 11 56 ) . At 1.4 K μ Dy = 7.2(2) μ B and is confined to the (001) plane.

The magnetic properties of PrRu${}_{2}$Si${}_{2}$ have been investigated experimentally by specific heat, single-crystal magnetization, ${}^{141}$Pr M\"ossbauer and muon spectroscopies, neutron powder diffraction, and inelastic neutron scattering, leading to the determination of its zero-field phase diagram and its crystal electric-field energy levels below 40 meV. PrRu${}_{2}$Si${}_{2}$ undergoes a magnetic phase transition at ${T}_{N}$ $\ensuremath{\simeq}$ 16 K to an axial incommensurate sine-wave magnetic structure characterized by a wave vector $\ensuremath{\tau}$ = (0.133, 0.133, 0), followed by a first-order phase transition at ${T}_{C}$ $\ensuremath{\simeq}$ 14.0 K to an axial ferromagnetic structure. The lowest crystal electric-field states are the two singlets $|{\ensuremath{\Gamma}}_{t1}^{(1)}〉$ and $|{\ensuremath{\Gamma}}_{t2}〉$ separated by 2.25 meV. The low-temperature properties are described by a Hamiltonian identical to that of an Ising system with a transverse magnetic field. Since the ratio of the exchange energy to the energy splitting between the singlets is sufficiently large, it exhibits spontaneous magnetization. The nature of the two singlet states explains the giant magnetic anisotropy. The random-phase approximation predicts the value of the high-field magnetization but yields a low-field magnetization too small by $\ensuremath{\sim}$ 15$%$. Possible application of our results to uranium intermetallic compounds is pointed out.

The magnetic coupling in the GdT 2 Si 2 (T=transition metal) antiferromagnetic compounds is studied by means of DC susceptibility and high-field free-powder (HFFP) measurements. In most compounds, the experimental results can be described in the framework of a two-sublattice model without anisotropy. From this analysis, an intrasublattice exchange coupling constant, n 11 , and an intersublattice exchange coupling constant, n 12 , are obtained. The exchange couplings are discussed in terms of the change of the lattice parameters, the volume V and the filling of the transition metal electron band. The apparently reduced value of the moment per Gd atom in the forced ferromagnetic configuration is attributed to a reduction of the on-site 5d-moment and a possible negative polarisation of the other non-4f electrons.

The 145 Nd Nuclear magnetic resonance (NMR) spin-echo spectra of the Nd x Y 2 − x Co 17 and Nd x Y 2 - x Co 14 B compounds at 4.2K are presented. Quadrupole septets are assigned to the Nd sites 4f, 4g in Nd x Y 2 − x Co 14 B and 6c in Nd x Y 2 − x Co 17 . A large change of the quadrupole splitting between x = 0.1 and x = 0.2 and only slight changes of the hyperfine fields is observed in Nd x Y 2 − x Co 14 B. The effect is attributed to a change of the easy magnetisation direction with Nd content from the c-plane to the c-axis. For Nd x Y 2 − x Co 17 slight changes of quadrupole splitting and hyperfine field in the whole concentration region 0.1 x 2 0 at the neodymium sites. The A 2 0 values amount to 192 Ka 0 −2 for the 4g and 57 Ka 0 −2 for the 4f site in Nd x Y 2 − x Co 14 B. The directions of magnetic moments at the Nd sites are also obtained.

The x-ray-absorption near-edge structure (XANES) spectra obtained at the cerium ${L}_{\mathrm{III}}$ edge of the ${\mathrm{Ce}}_{2}{\mathrm{Fe}}_{17\ensuremath{-}x}{\mathrm{Al}}_{x}$ solid solutions and ${\mathrm{Ce}}_{2}{\mathrm{Fe}}_{14}{\mathrm{Si}}_{3}$ show two absorption peaks characteristic of the ${4f}^{1}$ and ${4f}^{0}$ configurations of cerium, peaks which indicate that cerium is in a mixed valent state in these compounds. All the XANES spectra have been consistently and excellently fit with one sigmoidal function and two Gaussian-broadened Lorentzian functions. The cerium spectroscopic valence obtained from the relative areas of the two peaks decreases from 3.64 to 3.43 between $x=0$ and 9 in ${\mathrm{Ce}}_{2}{\mathrm{Fe}}_{17\ensuremath{-}x}{\mathrm{Al}}_{x},$ and correlates linearly with the cerium site volume. This correlation confirms that the cerium valence is strongly dependent upon steric effects. In contrast, the cerium valence obtained from the XANES spectrum of ${\mathrm{Ce}}_{2}{\mathrm{Fe}}_{14}{\mathrm{Si}}_{3}$ is not determined by steric effects and indicates, in agreement with other measurements and calculations, that silicon is more covalently bonded with its near-neighbor cerium atoms than is aluminum. The neodymium ${L}_{\mathrm{III}}$-edge XANES spectra of the ${\mathrm{Nd}}_{2}{\mathrm{Fe}}_{17\ensuremath{-}x}{\mathrm{Al}}_{x}$ solid solutions, where $x$ is 0, 3, and 8, reveal the presence of only trivalent neodymium and an increase of the empty $5d$ state density when aluminum is substituted in place of iron. XANES measurements at the iron $K$ edge of the ${\mathrm{Ce}}_{2}{\mathrm{Fe}}_{17\ensuremath{-}x}{\mathrm{Al}}_{x}$ and ${\mathrm{Nd}}_{2}{\mathrm{Fe}}_{17\ensuremath{-}x}{\mathrm{Al}}_{x}$ solid solutions show changes in the relative intensity of the multiple scattering peaks, changes which are related to the changing composition of the first three neighbor shells with increasing aluminum content.

The magnetic properties of Tb2Co17-xAlx compounds with 0 less than or equal to x less than or equal to 4 were studied by means of magnetic measurements. There is a strong decrease of the saturation moment and the Curie temperature with AI concentration. In a previous investigation, it was shown that increasing AI concentration in R(2)Co(17-x)AI(x) compounds leads to a sign reversal of the magnetocrystalline Co-sublattice anisotropy from easy-plane anisotropy for low AI concentration to easy-axis anisotropy for higher AI concentration. In the Tb(2)Co(17-x)AI(x) compounds, the competition between the planar Tb-and easy-axis Co-sublattice anisotropy does not lead to spin-reorientation transitions because the Tb-sublattice anisotropy dominates at all temperatures. The occurrence of spin reorientations in R(2)Co(17-x)AI(x) compounds is discussed in terms of a simple model description.

High-pressure ${}^{155}$Gd M\"ossbauer measurements on Gd metal, GdCo${}_{5}$, GdRu${}_{2}$Si${}_{2}$, and Gd${}_{2}$Co${}_{17}$N${}_{3}$ were performed at 4.2 K. The maximum pressures reached were about 18 GPa. The resulting volume reductions of 20--30 % were determined using high-pressure x-ray diffraction. The pressure dependence of the electric-field gradients and hyperfine fields obtained for the first three systems was compared with predictions from first-principles band-structure calculations. Significant changes of the hyperfine parameters are observed, especially for elemental Gd metal. With increasing pressure, an increase of the electron density at the Gd nuclei is found in all compounds. The values of the hyperfine field initially increase with pressure. For Gd metal the induced structural phase transitions result in large changes in the electric-field gradient at the nucleus ${(V}_{\mathrm{zz}})$. The intermetallic compounds show no structural phase transitions and relatively small changes in ${V}_{\mathrm{zz}}.$ The combination of experiment and calculations indicates that the transition-metal magnetic moments decrease at high pressure. Although for zero pressure predictions of the electric-field gradient and the hyperfine fields, based on the calculations, are quite accurate, the calculated pressure dependence of the hyperfine parameters for Gd, GdCo${}_{5}$, and GdRu${}_{2}$Si${}_{2}$ does not in all cases lead to a satisfactory agreement with experiment. The application of pressures therefore may give additional stimulus for the improvement of the theoretical description of band structures and hyperfine parameters.

The magnetostriction in intermetallic compounds based on magnetic rare-earth (R) elements is controlled to a large extent by the electrostatic interaction of the aspherical 4f-charge cloud with the crystal field. Sign and magnitude of the magnetostriction are determined by the Stevens coefficients and the expectation values of the Stevens operators. The temperature dependence of the magnetostriction follows the temperature dependence of the latter expectation values. These can be shown to decrease with increasing temperature as a high power of the reduced R-sublattice magnetisation. Therefore, a mechanism is required to keep the latter magnetisation high in order to have high magnetostriction values also at room temperature. The extent to which the R-sublattice magnetisation in R-T (T=transition-metal) compounds can be kept high at room temperature strongly depends on the strength of the 3d-4f interaction. Therefore, to be able to select compounds which exhibit interesting magnetostrictive behaviour at low temperature and which are still suitable for application at room temperature, it is of prime importance to know the values of the 3d-4f interaction in R-T compounds. We present results of an experimental study of this interaction in practically all R-T compounds, which are known to exist, with R a heavy rare earth and T=Fe, Co or Ni. The observed variation of the 3d-4f interaction within and between series of R-T intermetallics will be discussed.

We present an X-ray absorption spectroscopy study performed at the L 3 edge of cerium of two recently reported compounds Ce 2 Ni 17 Si 9 and Ce 2 Co 17 Si 9 . The electronic valence state of cerium has been measured in both compounds and is discussed in the light of the local atomic environment. An intermediate valence state of cerium previously proposed on the basis of neutron diffraction and magnetization measurements has been confirmed. This study has evidenced that the substitution of nickel for cobalt leads to a slight relocalization of the 4f states of cerium. Finally, the peculiar environment of cerium in this structure is compared to that of other intermetallic phases and is discussed in terms of Wigner Seitz cell volumes.

The thermodynamical stability and magnetic-field response of the ferrimagnetic spiral (FS) structure in DyMn 6 Ge 6 is investigated in terms of a simple model based on a competition between various magnetic interactions, including the R-sublattice anisotropy, antiferromagnetic MnMn exchange interactions, and the RMn exchange interaction. It can be shown that the exchange interaction between the Mn moments belonging to different magnetic units acts as an effective Mn anisotropy, which plays a crucial role in the stabilization of the cone structure observed in DyMn 6 Ge 6 at low temperature. The response of the FS structure to low applied magnetic fields consists of a rotation of the Mn and R moments towards the c -axis. At higher fields, transitions towards a (canted) structure of two collinear single sublattices (2SS or SS) are possible. Experimental high-field free-powder magnetisation data obtained on DyMn 6− x Cr x Ge 6 , are interpreted in terms of the theoretical analysis. From the field regions corresponding to the 2SS structure, estimates for the n RMn mean-field intersublattice coupling parameter are obtained. The estimated n DyMn values agree with n R—3d values obtained in other R—3d compounds. In addition, the temperature dependence of the free-powder magnetisation, established by experiment, is satisfactorily interpreted in terms of the theoretical analysis.

Samples with composition R2Fe14M3 (M = Ga and Si; R = Y, Nd, Sm, Gd, Tb, Dy, Ho, Er and Tm) have been prepared by are-melting. Crystal-structure studies have shown that the prepared samples are single phase with the rhombohedral Th2Zn17 and/or the hexagonal Th2Ni17 structure. Substitution of Ga for Fe in R2Fe17 leads to an increase of the unit-cell volume, whereas Si substitution reduces the unit-cell volume. The Curie temperatures of the R2Fe14Ga3 and R2Fe14Si3 compounds are much larger than those of the corresponding R2Fe17 compounds. The Fe moment decreases upon Ga or Si substitution. The compounds R2Fe14Ga3 and R2Fe14Si3 With R = Er and Tm exhibit a spin-reorientation transition at low temperature. (C) 1997 Elsevier Science Ltd.

The magnetic properties of compounds of the type La 1 − x Gd x Fe 12 B 6 have been studied in high magnetic fields at 4.2 K. LaFe 12 B 6 is a compound with Fe moments close to magnetic instability and values not larger than about 0.5 μ B . Substitution of Gd for part of the La and application of high magnetic fields enhance the Fe moments to values in excess of 2 μ B . The high-moment state in LaFe 12 B 6 is reached via a metamagnetic transition. This first-order transition is associated with an extremely large hysteresis of nearly 7 T at 4.2 K.

High-resolution neutron powder diffraction has been used to determine the crystal structure of the hexagonal compounds RCr 6 Ge 6 (P6/mmm). It is shown that most of the compounds investigated (RY, Dy and Er) can be regarded as partly disordered derivatives of the HfFe 6 Ge 6 -type structure where a comparatively small fraction of the R atoms occupy intermediate lattice positions which are shifted from the normal R positions by c /2 along the c axis, The fully ordered HfFe 6 Ge 6 -type structure was observed only for HoCr 6 Ge 6 . The observed atomic disorder is shown to strongly control the RR interactions and the corresponding magnetic ordering temperatures. The compounds DyCr 6 Ge 6 and ErCr 6 Ge 6 order below T c = 3.0 K and T c = 2.5 K, respectively, Both compounds exhibit canted ferromagnetic moment arrangements, where the moments of the ferromagnetic R and Cr sublattices have different orientations. No magnetic order above 1.5 K was observed in YCr 6 Ge 6 and HoCr 6 Ge 6 .

A Mossbauer spectroscopy study of the intermetallics RE 6 Fe 14−t M t , where RENd or Pr, MGa or Al and x =1 or 2 was undertaken. A comparative analysis of the Mossbauer spectra showed that the second Al or Ga atom substitutes preferentially for the Fe atoms located at the 161 sites. The reduction of the hyperfine fields in Nd 6 Fe 12 M 2 , as compared With Nd 6 Fe 13 M. mainly at the 16k, 161 1 and 161 2 and the corresponding decrease of the average Fe moment gives rise to a total magnetic moment of 41 μ B per formula unit in excellent agreement with previous saturation magnetisation measurements.

A structure determination using high resolution neutron powder data (difference Fourier maps) has shown that the hexagonal compound TbCr 6 Ge 6 (P6/ mmm ) is a partly disordered derivative of the HfFe 6 Ge 6 -type structure where a part of the Tb atoms at 1(a) (0,0,0) and a part of the Ge 3 atoms at 2(e) (0,0,0.347) populate intermediate lattice positions which are shifted by c /2. The observed atomic disorder modifies the R-R inteactions as well as the crystal field, leading to an ordered state and easy axis for the Tb moments different from that found for the (fully ordered) isomorphic TbMn 6 Ge 6 compound. As the CrCr interaction is very weak the ordering is dominated by the R-R interaction. TbCr 6 Ge 6 orders below T c = 10.3 K with a canted ferrimagnetic moment arrangement where the moments of the Tb and Cr sublattices have different orientations making an angle of 25(2) deg. and 143(7) deg. with the c -axis respectively. The ordered moment values of the two sublattices at 1.5 K are 8.74(1) μ B /Tb and 0.48(10) μ B /Cr. The net magnetic moment of the structural model proposed is found to be in satisfactory agreement with results of magnetic measurements reported earlier.

The magnetic and crystallographic properties of the compound TbNi 3 Ga 2 have been studied by neutron diffraction and magnetic measurements. From magnetic measurements it is derived that TbNi 3 Ga 2 orders ferromagnetically below about 14 K. Refinement of the neutron diffraction data obtained in the paramagnetic regime showed that TbNi 3 Ga 2 adopts the hexagonal YCo 3 Ga 2 type structure, but with a different distribution of the 3d atoms and Ga atoms over the three non-rare earth sites available in this structure type. Analysis of the neutron data obtained in the magnetically ordered regime shows that the magnetic structure is essentially ferromagnetic, the preferred direction of the Tb moments being perpendicular to the c direction. The Tb moment at the 2c site (3.9 μ B ) is substantially lower than that at the 1a site (8.3 μ B ).

The magnetic ordering of the novel orthorhombic compound Er3Ge4 (Cmcm) with two Er sites has been studied by neutron diffraction and magnetic measurements.Er3Ge4 orders antiferromagnetically below TN = 7.3 K. The magnetic moments of the Er1 atoms (at 8(f)) make an angle of 35.5(3)° with the y-axis while the moments of the Er2 atoms (at 4(a)) are perpendicular to it within the (O, y, z) plane. This triangular antifferomagnetic momentsarrangement of the two sublattices in the plane (O, y, z) relates to the stacking of the rare earth triangular prism slabs along the a axis. Adjacent prisms along b or c are shifted by x = 1 2 and display different chiralities. The 1.5 K ordered magnetic moment values of Er1 and Er2 are 7.32(5) μB/Er and 6.37(6) μB/Er, respectively. The two Er sublattices order simultaneously but display a completely different thermal behaviour indicating the existence of competing interactions. The magnetic space group Cp ( 2 2′2′ m ′ c m ) (Sh57391) has a primitive lattice associated with the wave vector q = (0 1 0).

The crystal structure and the magnetic ordering of the novel orthorhombic compound ErGe 2− x has been studied by neutron powder diffraction and magnetic measurements. The crystal structure belongs to the DyGe 1.85 -type (space group Cmc2 1 ). ErGe 2− x ( x =0.17(2)) orders antiferromagnetically below T N =6 K with a uniaxial magnetic moment arrangement. The magnetic cell has the same size as the chemical unit cell ( q =0). The magnetic space group is Cmc2 1 ( Sh 36 173 ). At T =1.5 K the magnetic moments of the two erbium sites have the same ordered magnetic moment value of 7.63(6) μ B /Er and are coupled antiferromagnetically along the a direction.

The magnetic properties of Ho2Co17 − xAlx and Dy2Co17 − xAlx compounds (x = 2, 3, 4 and 5) have been investigated by magnetic measurements and X-ray diffraction. The compounds with x = 5 do not stabilise in a 2–17 structure. The Ho2Co17 − xAlx compounds with x = 2, 3, and 4 show with increasing temperature, a spin reorientation from the basal plane to the c-axis. The origin of this reorientation is the change in the Co-sublattice anisotropy from planar to easy-axis upon substitution of Al for Co. This has been confirmed by measurements on Y2Co17 − xAlx. The spin reorientation shifts to lower temperatures with increasing Al concentration, indicating that the strength of the easy-axis Co-sublattice anisotropy increases. As the Dy-sublattice anisotropy is significantly larger than the Ho-sublattice anisotropy, the Dy compounds show no spin reorientation. The Curie temperatures of both series decreases linearly but strongly with increasing Al concentration.

Neutron scattering measurements have been made on SmMn2Ge2 characterized by AC susceptibility. The structure of all three magnetic states have been identified and a buckled magnetic structure is observed in the low temperature ferromagnetic and in the anti-ferromagnetic states. Changes in this structure reflect anisotropic changes in the lattice constants and provide an explanation of the giant magnetoresistance.

We report on bulk electronic properties of UPdSi in conjuction with results of X-ray-and neutron-diffraction studies. Both, diffraction experiments clearly show that UPdSi crystallises in the TiNiSi-type (space group Pnma) structure. Magnetic susceptibility, specific heat and electrical resistivity anomalies around 33 and 27 K indicate that UPdSi orders magnetically below 33 K and undergoes an additional magnetic phase transition at 27 K. The antiferromagnetic ground state is decomented by purely magnetic reflections in the neutron-powder-diffraction patterns collected at low temperatures and by metamagnetic transitions at 4 and 7 T observed in the magnetisation curve at 4.2 K. The magnetisation measured on a sample, consisting of powder particles that are free to be oriented by the applied field does not saturate even in fields up to 35 T, where it reaches 1.3μB/f.u. The magnetic structure determined at low temperatures (T = 3.5 K) is collinear antiferromagnetic with the propagation vector q = (0.25,0,0.25). This structure is built-up of eight-layer + + + + − − − − blocks of ferromagnetic planes of U moments (μU = 1.401 (7)μB) piled up along the [1 0 1] 1 direction. Between TN(= 33 K) and 27 K, we observe an incommensurate magnetic structure with the propagation vector q = (0.217, 0, 0.217). It is characterized by a sine-wave profile along [1, 0, 1] of b-axis oriented U-moments with an amplitude of 0.65(18)μB at 31 K.

The enthalpies of formation of the various intermetallic and amorphous phases of the Ti-Cu binary system were measured by solution calorimetry in molten aluminum. The experimental results were compared with the values predicted by Miedema's model.

We report on the electronic properties of various UCu x Al 5 − x compounds (2.9 ≤ x ≤ 3.5). These compounds crystallize in the hexagonal CaCu 5 structure. For all compounds, we find that the low-temperature specific heat diverges logarithmitically, which may be taken as an indication of non-Fermi-liquid scaling in these materials. Also we find a large magnetic anisotropy in all compounds studied, and we show that the magnetic anisotropy should not be neglected in the analysis of other bulk properties. Though for some of UCU x Al 5− x polycrystals non-Fermi-liquid scaling is found also in the magnetic susceptibility, comparison with single-crystal results on UCu 3 Al 2 indicates that any temperature dependence may be due to averaging anisotropic response over all crystallographic directions.

The structural and magnetocaloric properties of Mn/sub 5/Sn/sub 3-x/Ga/sub x/ alloys with x = 0.1, 0.3, 0.5, 1.0 and 1.5 are investigated. Ingots of the alloys were prepared by arc melting stoichiometric amounts of Mn (99.9%), Sn (99.9%) and Ga (99.99%) in an argon atmosphere. The ingots were annealed at 1073 K for one week in evacuated quartz ampoules, followed by quenching into water. The powder X-ray diffraction (XRD) data were collected at room temperature with Cu K/sub /spl alpha// radiation in the range of 2 /spl theta/ from 20/spl deg/ to 80/spl deg/. The results show that all alloys investigated, crystallize in the Ni/sub 2/In-type hexagonal structure with space group P63/mmc. The lattice parameters a and c decrease with increasing x. Magnetization measurements were performed in the temperature range from 5 to 400 K and in fields up to 5 T. The magnetization of the alloys initially increases with increasing temperature, and then becomes flat which is probably related to the magnetic anisotropy of the alloys. The Curie temperature of these alloys monotonously increases from 236 K for x = 0.1 to 281 K for x = 1.5. Ga substitution leads to an increase in the average moment of Mn atoms and therefore, an increase in the magnetocaloric effects, though the effects are relatively small due to the relatively small moment and the second order nature of the transition.

The magnetic and magnetocaloric properties of Mn/sub 1.1/Fe/sub 0.9/p/sub 1-x/Ge/sub x/ compounds with x=0.20,0.22,0.23,0.26, and 0.30 have been investigated. In the composition range x from 0.20 to 0.30, the compounds crystallize in the Fe/sub 2/P-type hexagonal structure. The Curie temperature of the compounds strongly increases with increasing Ge contents. A history dependence of the magnetic phase transition is observed in this compound system. The optimum composition range and the temperature range of the Mn/sub 1.1/ Fe/sub 0.9/P/sub 1-x/Ge/sub x/ compounds for magnetic refrigeration applications have been determined. In the composition range x from 0.20 to 0.23, the compounds exhibit large magnetic-entropy change in a low magnetic field.

The magnetic properties of Tm${\mathrm{Ni}}_{2}$${\mathrm{B}}_{2}$C were measured with $^{169}\mathrm{Tm}$ M\"ossbauer spectroscopy at temperatures from 0.3 to 700 K. A clear transition in the quadrupole splitting (QS) near 1 K is observed, which might be due to magnetic ordering or to quadrupole ordering. Although a possible small magnetic Tm moment of \ensuremath{\sim}0.1(1)${\mathrm{\ensuremath{\mu}}}_{\mathit{B}}$ is found below \ensuremath{\sim}1 K, the change in QS is larger than expected due to magnetic ordering only. Therefore the nature of the transition is unclear at this moment. From the temperature dependence of QS is deduced that the crystal-field ground-state level most likely equals the singlet $a|\ensuremath{-}4〉+b|0〉+a|+4〉$.

The crystal and magnetic structures of tetragonal compounds ErCo10Mo2 and TbCo10Mo2 (space group I4/mmm) have been investigated by time-of-flight neutron powder diffraction at 5 K. The compound ErCo10Mo2 is planar at 5 K whilst TbCo10Mo2 remains axial even at low temperatures. The magnetic moments of the three Co sublattices have been determined, with moments of 1.0(0.2)μB, 0.8(0.2)μB and 0.8(0.2)μB for 8i, 8j and 8f sites respectively. The Er and Tb moments are reduced by approximately 7% from their free-ion values by the action of the tetragonal crystal field.

The magnetic properties of the compound TmFeAl have been studied using57Fe and169Tm Mossbauer spectroscopy, magnetic measurements and neutron depolarization (ND). The compound exhibits a complex magnetic behaviour. At 4 K it contains a Tm moment of 7μB and a distribution of Fe moments with an average of 0.5μB. Between 10 and 60 K a distribution in magnetic ordering temperatures is observed. ND displays the growth of small magnetic domains when the temperature decreases towards 4 K. The structural disorder of the Fe atoms in the lattice combined with the competing exchange interactions between the different types of atoms and the high anisotropy of the Tm moment gives rise to the observed magnetic behaviour. Results of high field magnetization and57Fe Mossbauer spectroscopy are presented also for GdFeAl and YFeAl.

The crystallographic properties of the compound CeCuAl 3 were studied by means of high-resolution neutron diffraction. The refinement results of the neutron powder diffraction patterns show that this compound does not crystallise as a disordered form of the ThCr 2 Si 2 structure but belongs to the more ordered BaNiSn type.