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We present the (x, T) phase diagram of the ferromagnetic Y2Co17−xCrx (x = 1.17, 1.76, 2.34, 3) compounds with rhombohedral structure (space group R 3 ¯ m , Th2Zn17-type), determined for the entire magnetically ordered regime from high-resolution neutron-diffraction data. Substitution of small amounts of Cr for Co was found to have drastic effects on the magnetic properties such as a decrease of the Curie (TC) and spin reorientation (TSR) temperatures as well as a decrease of the ordered-moment values. These effects could be brought into connection to the fact that Cr was found to have a preference for the four Co sites in the following order 6c > 18f > 9d > 18h. The number of Cr atoms found occupying the latter two sites, however, is within experimental error negligible. The magnetic phase diagram comprises three distinct regions of magnetic order in addition to the paramagnetic state: (a) the HT range (easy axis), (b) the LT (easy plane) for 0

The effect of nanoscale exchange coupling has been investigated in the CoPt alloy system. The experimental results of XRD and TEM indicate that in the early stage of the disorder-order transformation, the ordered fct regions with a mean size smaller than 8 nm are uniformly distributed and randomly oriented along the a-, b- and c-axes of the cubic structure of the original disordered fcc matrix phase. Both phases are completely coherent. The degree of atomic order in the ordered regions continuously increases, but this process is different for the various ordered regions. Also the degree of atomic order of the matrix phase increases, albeit at a low rate. There is a broad continuous distribution of the degree of order in the initial stage of the ordering transformation. An ordered region with higher degree of order has a higher magnetocrystalline anisotropy. The effect of nanocomposite exchange coupling between the regions with different degree of order is very clear. A remanence ratio of 0.78 is obtained after annealing the sample at 675 degreesC for 20 min. When the annealing time is longer than 60 min at 675 degreesC, the growth of ordered regions leads to a situation in which the disordered regions are swallowed up and the ordered regions meet each other to form antiphase boundaries. Cross-intersected polytwin bands of several hundreds nm length and a few nm width are formed along the {111} and {211} planes to relax the strain energy. It is suggested that the mechanism of magnetization reversal changes from coherent rotation to magnetic-domain wall movement. Since the widths of the polytwin bands are still within the length of exchange coupling, the strong remanence enhancement is preserved. (C) 2003 Elsevier B.V. All rights reserved.

We have studied the heat capacity of a single crystal PrCoAl4. The data indicate that Pr3+ in this compound has a non-magnetic singlet ground state. The compound becomes antiferromagnetically ordered below the Neel temperature of 17 K. The magnetocrystalline anisotropy in PrCoAl4 is relatively strong. Up to 300 K, the entropy contained in the specific heat is 15.2 J/Kmol, which is only 83% of 18.3 J/Kmol expected for the 2J+1-fold degeneracy of the ground multiplet of the Pr3+ ion.

Modern society relies very much on readily available cooling. Magnetic refrigeration based on the magneto-caloric effect (MCE) has become a promising competitive technology for the conventional gas-compression/expansion technique in use today. Recently, there have been two breakthroughs in magnetic-refrigeration research: one is that American scientists demonstrated the world's first room-temperature, permanent-magnet, magnetic refrigerator; the other one is that we discovered a new class of magnetic refrigerant materials for room-temperature applications. The new materials are manganese–iron–phosphorus–arsenic (MnFe(P,As)) compounds. This new material has important advantages over existing magnetic coolants: it exhibits a huge MCE, which is larger than that of Gd metal; and its operating temperature can be tuned from about 150 to about 335 K by adjusting the P/As ratio. Here we report on further improvement of the materials by increasing the Mn content. The large entropy change is attributed to a field-induced first-order phase transition enhancing the effect of the applied magnetic field. Addition of Mn reduces the thermal hysteresis, which is intrinsic to the first-order transition. This implies that already moderate applied magnetic fields of below 2 T may suffice.

We synthesized for the first time the intermetallic compound Fe2MnGe. To avoid preferential evaporation of volatile components we exploited mechanical alloying. Amorphous Fe2MnGe alloy powder was prepared by planetary ball milling elemental starting materials. The amorphous-to-crystalline transition was studied by means of differential scanning calorimetry (DSC) and X-ray diffraction (XRD). A cubic D03 phase is formed at low temperature and transforms to a high-temperature hexagonal D019 phase. The apparent activation energy was determined by means of the Kissinger method.

GdNi 5 is a simple metallic ferrimagnet with well understood magnetic properties. It has been used in the past as a testing ground for validating muon spin relaxation theories. Here we report on the muon diffusion and site localization in its paramagnetic phase, i.e., above 30.85 K. The muon localizes at the interstitial 3f sites over the whole temperature range. Below ∼70 K the interstitial 6m sites become populated as well. These sites form a ring surrounding the (0,0, 1 2 ) site and the muon can occupy all these sites within its lifetime. Above 40 K the muon residing time in these sites is relatively short and it jumps to the 3f site by a quantum diffusion process.

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.

We have studied the first-order magnetic phase transition and related physical properties of MnFeP0.5As0.4Si0.1 by means of x-ray diffraction, magnetization, thermal expansion, and electrical resistance measurements. The compound crystallizes in the hexagonal structure with Fe2P-type. The temperature dependence of magnetization shows a first-order magnetic phase transition around 299 K. There is no crystallographic structural transition associated with the magnetic phase transition. The linear thermal expansion shows that this magnetic phase transition is accompanied by a negative volume change. The electrical resistance shows an anomalous behavior at the transition, which is probably caused by the change in c/a ratio.

The MnCoGe alloy can crystallize in either the hexagonal Ni2In- or the orthorhombic TiNiSi-type of structure. In both phases MnCoGe behaves like a typical ferromagnet with a second-order magnetic phase transition. For MnCoGeBx with B on interstitial positions, we discover a giant magnetocaloric effect associated with a single first-order magnetostructural phase transition, which can be achieved by tuning the magnetic and structural transitions to coincide. The results obtained on the MnCoGe-type alloys may be extensible to other types of magnetic materials undergoing a first-order structural transformation and can open up some possibilities for searching magnetic refrigerants for room-temperature applications

The easy magnetization direction of DyCo10V2 is parallel to the c axis at room temperature. The magnetization hysteresis loops of magnetically aligned samples were measured from 5 to 300 K with fields applied parallel and perpendicular to the alignment direction. For samples field-cooled to below the compensation temperatures, shifts of the loop centers into the range of negative magnetizations were observed. These features and the occurrence of negative remanences are explained in terms of the magnetization reversal when passing the compensation temperature upon cooling. From measurements of the temperature dependence of the magnetization, it was derived that a spin-reorientation transition takes place at about TSR=41 K.

The ordering transformation and magnetic properties of Fe59.75Pt39.5Nb0.75 bulk alloys have been investigated in detail by using different homogenization temperatures and different low-temperature annealing times to obtain samples with different microstructure and different atomically ordered states. The experimental results indicate that the disorder–order transformation temperature for the presently studied alloys is around 950 °C. When using high homogenization temperatures (in the range 1100–1350 °C) and medium quenching rates, one can reach high nucleation rates of the atomically ordered phase, so that a fine microstructure of ordered regions with dimensions of 10–20 nm is obtained. A uniform nanoscale microstructure with a wide distribution in the degree of order can be obtained by properly controlling the quenching rate and the annealing time. In this way, a high remanence ratio (mr=0.74) can be obtained by profiting from the nanocomposite exchange coupling between nearest-neighbor ordered regions with different magnetic anisotropy. We found that, in the present off-stoichiometric magnetic alloy system, the fct phase is magnetically not very hard, like the Nd2Fe14B phase, and that the fcc phase is not truly magnetically soft due to an increase in the degree of atomic order. Therefore, the samples do not possess the features of an exchange spring magnet as the iron-rich Nd–Fe–B magnets, although the exchange coupling still results in a strong remanence enhancement.

The antiferromagnetic body-centred tetragonal compound GdNi2Ge2 orders at 28 K. Successive magnetic phase transitions are observed by specific-heat and magnetisation measurements as a function of temperature in different applied magnetic fields. Plots of M2 vs. B/M (Arrott-plots) show various anomalies. On the basis of the experimental results, a magnetic phase diagram is constructed. The multiple magnetic phase transitions are discussed in terms of competing ordering modes in the Gd sublattice.

The crystal and magnetic structures of hexagonal intermetallic compounds of the type R2Ni17 with R=Y, Tb, Dy, Ho and Er have been investigated by neutron powder diffraction at 20 and 200 K. The crystal structures at 200 K, which is above the magnetic ordering temperature for these compounds, have been refined and show that they crystallize in the hexagonal Th2Ni17 structure (space group P63/mmc). The magnetic structure at 20 K has a magnetic propagation vector k → =0 and consists of a ferrimagnetic alignment of R and Ni moments. At 20 K, compounds with R=Tb, Dy and Ho are collinear ferrimagnets with moments aligned in the basal plane of the hexagonal unit cell, whilst the compound with R=Er is an axial collinear ferrimagnet. The low-temperature magnitudes of the R moments at 2b and 2d sites are markedly different, with an almost free ion value at the 2b site, whilst the 2d site moment is substantially reduced from its free ion value.

YCo 10 Mo 2− x M x compounds (M=Si or V and x =0, 0.5, 1.0, 1.5, 2.0) with the ThMn 12 structure have been prepared by means of arc-melting and subsequent annealing. The magnetic properties have been investigated by X-ray diffraction on magnetically aligned powders, by a.c. susceptibility and by magnetization measurements. The lattice constants of the YCo 10 Mo 2− x M x compounds decrease linearly with increasing x , according to Vegard's law. The magnetic properties of both series are different. Deviations of the spontaneous moment and the Curie temperature from a linear x dependence are observed, occurring in opposite directions in the two series investigated. This behaviour is discussed in terms of preferred Co-site occupation in the materials.

The magnetic properties of Pr2Co17-xAlx compounds with 0 less than or equal to x less than or equal to 4 were studied by means of magnetic measurements. From these measurements it is derived that there is an almost linear decrease of the Curie temperature with Al concentration accompanied by a somewhat less strong decrease of the saturation moment at 5 K. For the compounds with 2 less than or equal to x less than or equal to 4, we have observed spin-reorientation transitions at temperatures that decrease strongly with increasing Al concentration. These spin-reorientations are interpreted as originating from a competition between the Pr- and Co-sublattice anisotropies. (C) 1997 American Institute of Physics.

We report on crystal and magnetic structures of the UTSi compounds (T = transition metal) studied by X-ray and neutron diffraction. They crystallize in the orthorhombic TiNiSi-type structure except for UPtSi (CeCu2-type). We have resolved magnetic structures of URhSi (ferromagnetic with strongly reduced U moments), UPtSi (incommensurate, collinear AF structure) and UPdSi at low temperatures. UIrSi is not ordered down to 1.3 K. Our findings are consistent with results of bulk measurements. The development of 5f-magnetism in the UTSi series can be conceived in the framework of the 5f-ligand hybridization involvement in the formation of U moments.

The magnetic properties of Pr2Co17-x Al-x and Y2Co17-xAlx compounds with 0 less than or equal to x less than or equal to 4 were studied by means of magnetic measurements. These measurements have shown that increasing Al concentration leads to a sign reversal of the magnetocrystalline Co-sublattice anisotropy from easy-plane anisotropy for low Al concentration to easy-axis anisotropy for higher Al concentration. In the Pr2Co17-xAlx compounds, the competition between the Pr-and Co-sublattice anisotropies leads to spin-reorientation transitions at temperatures that decrease strongly with increasing Al concentration. There is an almost linear decrease of the Curie temperature with Al concentration. Also the saturation moments at 5 K decrease strongly with Al concentration although this decrease is less strong in Pr2Co17-xAlx than in Y2Co17-xAlx. (C) 1997 Elsevier Science S.A.

The magnetic properties of the novel compound DyGe 3 have been studied by neutron diffraction and magnetic measurements. DyGe 3 (orthorhombic Cmcm, Z = 4) orders antiferromagnetically at T N = 24 K. The superstructure lines observed in the neutron powder data require a sixfold cell enlargement of the primitive chemical cell upon magnetic ordering (2 a , ( a + b )/2, 3 c ) and a twofold splitting of the atomic positions. The magnetic space group is P a 2 1 /m. The two Dy sublattices have a uniaxial moment arrangement along c and are coupled antiferromagnetically with the same ordered moment values 8.1 μ B below 8 K. Their different temperature dependencies are discussed in terms of a Fourier analysis of the observed data over the whole magnetically ordered region.

We have studied the magnetic properties of the compound TbMn 4 Al 8 by means of neutron diffraction and magnetic measurements. From the magnetic measurements it is concluded that the magnetic interactions in this compound are very weak and do not lead to magnetic ordering above 4.2 K. From the neutron diffraction measurements it is concluded that magnetic ordering is absent even down to 1.5 K. The influence of atomic-site disorder on the magnetic properties is discussed.

We present Si L2,3 emission-band spectra of a series of 3d and 4d transition-metal (TM) silicides, together wtih Si K emission-band spectra of four 3d TM disilicides. The data are compared with augmented-spherical-wave density-of-states (DOS) calculations, and good agreement is found. The trends we find are explained with a general scheme for chemical bonding in TM silicides. The differences between the experimental data and the calculated DOS curves are tentatively attributed to self-energy effects.

Investigations of magnetic and related electronic properties of UTX compounds have revealed a variety of magnetic phenomena ranging from weak paramagnetism to various types of magnetic ordering. The observed systematics of ground-state properties is discussed with respect to the development of the electronic structure related to the expected variations of the 5f-ligand hybridization. The huge magnetic anisotropy observed in all studied materials is tentatively attributed mainly to the hybridization-mediated anisotropic two-ion interaction.

The valence bands and conduction bands of about 30 transition metal silicides (of which we concentrate on 4 here) have been investigated by measurements of Si X-ray emission bandsspectra, X-ray absorption spectra near the Si K (1s) edge, photoemission spectra, and Bremsstrahlung Isochromat spectra. The densities of states have also been calculated for the materials in their real crystal structures. The influence of the core hole on some spectra has been investigated using supercell calculations, a (Greens function) generalized Clogston-Wolff model, and Auger spectroscopy. A selection of results is presented to illustrate the utility of site and selective methods in investigations of the electronic structure of silicides and the nature of the "quasi-gap" of the partial density of Si p states in the region of the transition metal d bands.

We have investigated the magnetic properties of UFe 10 Si 2 by means of high-field magnetic measurements and specific-heat measurements. The spontaneous moment of UFe 10 Si 2 at 4.2 K equals18.33 μ B /f.u. The electronic specific-heat coefficient γ equals 77 mJ/mol K 2 . Our data show that the 5f electrons in UFe 10 Si 2 have to be regarded as fairly localised in character.

We have studied the effect of Ga substitution on the magnetic and transport properties of MnPtAl1-xGax compounds with x = 0, 0.05, 0.1, 0.15 and 0.2. With increasing Ga concentration, we find a rather weak linear reduction of the Néel temperature. However, the metamagnetic field associated with the field-induced ferromagnetic alignment is strongly reduced. The results are discussed in the framework of competing Mn-Mn interactions.

The quadrupole splitting in alloys is determined by the structural and electronic properties of the alloy. Only in cases where the density of states at the Fermi level is constant or known as a function of the concentration, relevant structural information can be obtained from the quadrupole splitting.

It is shown that the major contribution to the high uniaxial anisotropy of the compound SmCo5 arises from the action of crystalline and exchange fields on the Sm3+ ion.

The authors report ellipsometry measurements on polycrystalline samples of NiMnSb and PtMnSb at room temperature under ultra-high vacuum conditions. From these measurements the optical constants of the two materials were determined in the energy range 0.5-5.3 eV. The measured optical conductivities are in reasonable agreement with the optical conductivities calculated from the band structures by de Groot (1983). The band gap for the minority-spin direction is clearly observed and can be brought into agreement with the band-structure calculations by consideration of spin-orbit splitting.

The resistivity of the equiatomic ternary compounds UTSn, UTSb and ThTSn is up to two orders of magnitude larger than Mooij's criterion: ϱ max =250 μΩcm. An exponential decrease of the resistivity at high temperatures suggest a band-gap in spite of the relatively high values for the linear specific heat coefficient γ.

The compounds NiMnSb and IrMnGa were studied by neutron diffraction. The former compound has the cubic Cl b crystal structure with an ideal site occupancy of the atoms. Below 750 K the compound is ferromagnetically ordered. The magnetic moments are confined to the Mn atoms (4.0μ B /Mn). The compound IrMnGa has a crystal structure related to the C1 b type but characterized by a high atomic disorder. Results of magnetic measurements are presented for NiMnSb, CuMnSb and IrMnGa. The results of these measurements are in accord with the occurrence of high atomic ordering in the first two compounds but point to high atomic disorder in the latter material. Total energies and Mn moments derived from band-structure calculations are presented for three different types of site occupancies in NiMnSb. The lowest total energy as well as the correct value for the Mn moment correspond to the site occupancy found by means of the neutron diffraction.

The magnetic properties of the cubic NaZn13 type pseudobinary compounds LaFexAl13-x were studied in the temperature range T=4.2 - 300 K by means of 57Fe-Mössbauer spectroscopy, magnetization and zero-field susceptibility measurements. The compounds LaFexAl13-x show a rather peculiar concentration dependence of the type of magnetic ordering as well as of the ordering temperature.

We have measured the electrical-transport and magnetic properties of several intermetallic compounds UTSn and UTSb, where T is a transition metal. The electrical resistivity is up to three orders of magnitude larger than usually found for U-based compounds. This is ascribed to the occurrence of a spin polarized energy gap related to the MgAgAs-type crystal structure. For UNiSn at least one spin band closes at low temperature, resulting in half-metallic behavior. Interestingly, the magnetic properties exhibit Kondo-lattice character and weak-moment ordering.

The magnetic properties of the cubic NaZn 13 type pseudobinary compounds La(T x Si 1- x ) 13 were studied for T = Fe, Co and Ni in the temperature range 4.2–300 K. 57 Fe Mossbauer spectroscopy was performed on LaFe 11 Si 2 and 57 Fe-doped LaCo 11 Si 2 . The compounds with T = Fe or Co are ferromagnetic, while the compound LaNi 11 Si 2 is a Pauli paramagnet. The critical behaviour around the Curie temperature T c was studied in La(Fe x Si 1- x ) 13 by measuring the temperature dependences on the zero-field susceptibility and the electrical resistivity. The critical exponent γ in the expression χ ∝ ( T - T c ) -γ was found to be close to 1.38 corresponding to an isotropic Heisenberg ferromagnet. The anomalous critical behaviour shown by electrical resistivity can be explained in terms of lattice softening associated with the Invar effect.

The ferromagnetic (FM) La(Fe0.86Al0.14)13 Invar alloy displays a pressure-induced FM to antiferromagnetic (AF) phase transition at a pressure as low as p⪅0.1 GPa. A quantitative analysis of the results shows that the FM → AF magnetic phase transition recently observed in La(FexAl1−x)13 at ambient pressure is governed by the decrease of the unit cell volume rather than the increase of the number of Fe nearest neighbor atoms.

Magnetization and susceptibility data on PrCo 2 and PrCo 2 H 4 are presented. The ac susceptibility of PrCo 2 measured in zero dc field displays a sharp and high peak at T c = (39.9 ± 0.2) K. The magnetization versus temperature curves show ferromagnetic behaviour for B >1 T, but display a maximum at lower values of the applied field. These results, together with the behaviour of the hysteresis loops at different temperatures below T c , indicate that PrCo 2 orders ferromagnetically, the magnetic hardness increasing strongly for T → 0. The saturation moment at 4.2 K equals 3.9 μ B per formula unit, as found from the magnetization curve measured in a pulsed-field magnet up to B = 30 T. Similar experiments on PrCo 2 H 4 provide evidence that the introduction of hydrogen in PrCo 2 not only destroys the long-range atomic order, but also considerably reduces the ferromagnetic interactions. Such an effect of the hydrogen is commonly observed in cobalt intermetallics. Part of the PrCo 2 H 4 is found to have decomposed into PrH 2 and free Co. The clusters of free Co atoms give rise to a maximum in the zero-field ac susceptibility versus temperature curves, similar as observed in spin glasses or magnetic glasses. By increasing the ac frequency, the maximum shifts to higher temperatures. The behavior can be explained in terms of the Neel model for superparamagnetic particles with randomly oriented local anisotropy axes.

We have determined the magnetic and crystallographic properties of intermetallic compounds of the type Ni1-xFexMnSb, using standard X-ray diffraction techniques, magnetic measurements and 57Fe Mössbauer spectroscopy. These materials were found to have the cubic MgAgAs structure (C1b) for concentrations x < 0.1 and the CoMnSb structure related to C1b for concentrations 0.7 < x < 0.9. Lattice constants, saturation moments and Curie temperatures were found to vary smoothly with x, making it possible to derive values for all these physical constants of FeMnSb by extrapolation. Calculation of the electronic band structure of FeMnSb revealed that this compound can be regarded as a novel type of material. Like the half-metallic ferromagnet NiMnSb, the compound FeMnSn has a band gap near the Fermi energy in the band structure of the minority spin electrons. This band gap stabilizes an antiparallel coupling between the Mn and Fe moment, so that one may characterize this compound as a half-metallic ferrimagnet.