Search

The compound ScMn6Ge6 (HfFe6Ge6-type, P6/mmm) orders antiferromagnetically below TN=516 K. Neutron powder diffraction at various temperatures 1.5–309 K shows the existence of two distinct magnetic ordering ranges described by the commensurate q1=(0, 0, 1/2) and the incommensurate q2=(0, 0, qz) vectors: (i) the HT (high temperature) Tt≈100 K T>1.5 K range has an easy double-cone incommensurately modulated structure described by two vectors (q1, q2). At 1.5 K q2=(0, 0, 0.405(1)), the spiral angle is Φ=145.8° the cone half angle is α≈8° and the ordered moment value μMn=2.12(2) μB is the same as for the isomorphic RMn6Ge6 (R=heavy rare earth) compounds. The 1.5 K q2 satellites are very weak as the in-plane moment component measures only 0.31 μB.

TmFe4Ge 2 undergoes a double magneto-elastic first order transition at T N ,T c where the high temperature ( HT ) tetragonal phase disproportionate into two distinct orthorhombic low temperature ( LT ) phases with commensurate and incommensurate magnetic wave vectors respectively: P 4 2 / m n m ( H T ) T N , T c → C m m m q 1 = ( 0 , 1 2 , 0 ) + P n n m ( q 2 = ( 0 , q y , 0 ) , q y ≈ 2 / 11 ( L T ) Neutron diffraction shows the relative portions of the LT Cmmm and Pnnm competing phases change linearly with T. The amount of the majority HT phase Pnnm (54% at 30 K) decreases linearly to 30% down to 10 K in favour of the Cmmm phase that dominates the range 26–1.5 K. The Tm moments point along the c -axis in both phases while the Fe moments have canted arrangements. The μ Tm =3.54(3) μ B /atom at 1.5 K is strongly reduced below the Tm 3+ free ion value g J J =7 μ B for the q 1 phase. The q 2 phase corresponds to a 3D canted sinusoidal arrangement. The results are summarised on a phase diagram and compared to the findings in R Fe 4 Ge 2 ( R =Y, Lu, Er, Ho, Dy) that are reviewed. The multitude of transition paths occurring in those systems arise from the competing magnetoelastic mechanisms involving the R -crystal field anisotropy, the exchange interactions R–R , R– Fe, Fe–Fe of the two sublattices and their coupling to the lattice strain. The geometrical frustration emerging from the compact tetrahedral Fe arrangement with antiferromagnetic interactions leads to 2D and 3D canted, incommensurate and non-magnetic states. The Cmmm transition is triggered by dominating R–R and R –Fe interactions becoming stronger at LT while the Pnnm phase is promoted by Fe–Fe and R –Fe interactions that prevail at HT . Included is also the magnetic structure of the ferromagnetic impurity phase Fe 3 Ge.

The magnetic structure of the tetragonal antiferromagnetic LuFe4Ge2 and YFe4Si2 compounds has been studied by high resolution neutron and X-ray diffraction above and below the magneto-elastic transition at TN, Tc=32 K and 76 K, respectively. Below TN,Tc the tetragonal high temperature (HT) phase transforms into the low temperature (LT) orthorhombic magnetic phase: P42/mnm (HT) → Pnnm (q=0) (LT) with a 2D canted moment arrangement of the Fe moments within the (001) plane with the magnetic space group P 2 1 n ' 2 1 n ' 2 1 m ' ( S h 58 399 ) . The Fe magnetic moment value is highly reduced in both compounds due to the presence of geometrical and exchange frustration relating to the underlying crystal structure comprising columns of Fe tetrahedra expanding along the c direction with antiferromagnetic interactions. The magnetic refinements were perturbed by the presence of partly unidentified impurity phases mainly in YFe4Si2.

Permanent or hard magnetic materials containing rare-earth (RE) elements are of great technological importance. Due to the cost and availability issues related to RE elements, RE-free permanent magnets have attracted great scientific and industrial interest. This article reviews the structural compositions and magnetic properties of L10 type RE-free permanent magnets, Pt–Co, Fe–Pt, Mn–Al.

The temperature magnetic phase diagrams of the dimorphic HoSi compound were studied by neutron diffraction. The sample comprises 35.5% CrB- (Cmcm) and 64.5% FeB-type (Pnma) of structure. Both phases order antiferromagnetically below TN=25 K and undergo first-order magnetic transitions at Tic=16.5 K. Their T-phase diagrams comprise a low temperature (LT) 2.7 K−Tic and a high temperature (HT) range Tic−TN with distinct wave vectors. The LT magnetic ordering of the CrB-type HoSi with the wave vector q1=(1/2, 0, 1/2) corresponds to a uniaxial magnetic structure, with the Ho moments along the shortest axis c. At 2.7 K the ordered moment value is 8.6(2) μB/Ho atom. The HT ordering, described by the wave vector q2=(q2x, 0, q2z) with a T-variable length, corresponds to an amplitude modulated structure. The magnetic ordering of the FeB-type HoSi requires two symmetry independent vectors q3=(0, q3y, q3z) for the LT- and q4=(q4x, q4y, 0) for the HT range. Both vectors correspond to sine wave modulated structures with the Ho magnetic moments confined along the shortest axis b. The q3 vector has an almost invariable length vs. T close to ≈(0, 9/17, 1/11). At 2.7 K the amplitude of the wave is 10.9(1) μB/Ho atom. At Tic q3 jumps to the wave vector q4=(q4x, q4y, 0) with a T-variable length. At 17 K q4=(0.092(1), 0.538(3), 0). Around Tic there is a narrow coexistence range of the q3 and q4 competing phases. Various models are discussed and compared with the isomorphic RSi (R=rare earth) compounds counterparts of HoSi, a comparison that has led us to briefly review the magnetic structures available in the literature for this interesting class of compounds.

We present the magnetic temperature phase diagrams of the CrB- and FeB-type orthorhombic phases of the DySi compound, determined from high-quality powder XRPD and neutron diffraction, as well as the sample microstructure as determined by HRTEM. Both phase diagrams comprise a HT (Tc1, Tc2−TN) and a LT range (5 K–Tc1, Tc2) separated by a monoclinic phase transition at Tc1=Tc2=23.5 K well below the second-order Neel transition (TN=40 K). The transition paths are for CrB-type Cmcm (HT) Tc1→C2/m11 (LT), and for FeB Pnma (HT) Tc2→P21/n11 (LT). The transitions are related to non-monotonous changes of the lattice and structural parameters displaying anomalies at Tc1, Tc2 and slight volume changes. For the CrB-type the monoclinic angle decreases smoothly from Tc1 down to 5 K and the maximum strain experienced by the crystal lattice in the (0 2 1) direction was found at Tc1. In the FeB-type, in addition to the magneto-elastic transition at Tc2=23.5 K, minor anomalies are found at 13.5 K in the temperature dependence of the monoclinic angle and the maximum strain along (0 1 1). Both temperatures mark the sequence of changes in the magnetic domain microstructure observed in FeB: below T2=23.5 K the incommensurate HT magnetic phase with q 3 ≈ ( 0 1 2 1 7 ) disproportionates into two LT phases q 3 ≈ ( 0 1 2 1 11 ) T2 and q 2 ≈ ( 0 1 2 1 6 ) coexisting in the form of domains with portions varying with T going from T2 down to 13.5 K (q2 increasing at the cost of q3). This behaviour could be related to structural inhomogeneities below the first-order magneto-elastic transition Tc2, if one assumes a broad two phase range, where the HT (Pnma) phase coexists with (P21/n) as a metastable phase at LT in the form of domains with different magnetic behaviour.

The temperature magnetic phase diagrams, of the dimorphic DySi compound, have been studied in terms of wave vectors in the range 1.5–45 K, by neutron diffraction. The polycrystalline sample consists of 26% of CrB-type ( Cmcm no. 63, all atoms at 4c site: (0, y , 1/4)) and of 74% of FeB-type ( Pnma no. 62, all atoms at 4c site: ( x , 1/4, z )). The CrB-ordering is described by the wave vector: q 1 =(0, 0, 1/2) over the entire magnetically ordered regime with a uniaxial magnetic structure along the shortest axis c. The FeB-type magnetic phase diagram reveals three distinct regions of magnetic ordering below T N and one first order transition at T 2 =23.5 K (on heating). The ordering is described by two symmetry independent magnetic vectors q 2 =(0, 1/2, 1/6) and q 3 =(0, q 3 y , q 3 z ) with a temperature variable length. At 1.5 K q 3 y ≈1/2 and q 3 z ≈1/11. The two phases coexist in the form of domains. They differ in the moment orientation of the q 3 phase that deviates by ∼22° from the b -axis in the (0, 0, 1) plane. The low temperature range ( LT ) 1.5 K— T 2 subdivides into two regions: (i) LT -1, between 1.5 K— T 1 where the relative amount of the two phases remains unchanged and in (ii) LT -2: T 1 – T 2 where the amount of the incommensurate q 3 phase increases at the cost of the commensurate q 2 amplitude modulated structure which remains unchanged but fully disappears at the first order transition at T 2 =23.5 K. The q 3 phase undergoes minor changes until 22 K and gets destabilised at T 2 where the q 3 z component jumps from the LT value q 3 z ≈1/11 to the HT value ≈1/7 and the q 3 y component increases from 0.484(1) to 0.495(1). (iii) The high temperature ( HT ) range T 2 – T N ( T N =40±1 K) is described by a single wave vector q 3 . The disproportionation of the HT magnetic phase q 3 below T 2 into two coexisting distinct phases q 2 , q 3 down to 1.5 K is an unusual phenomenon, to our knowledge observed for the first time. Various mechanisms are discussed.

The low-temperature magnetic ordering of the dimorphic DySi compound has been studied at 1.5 K by neutron diffraction on two polycrystalline samples. The samples comprise various amounts of the two orthorhombic modifications: CrB-type (Cmcm Nr. 63, all atoms at 4c site: (0, y, 1 4 )) and FeB-type (Pnma Nr. 62, all atoms at 4c site: (x, 1 4 , z)), both order antiferromagnetically (TN≈38 K). The CrB-type phase orders with a uniaxial structure with the wave vector q1=(0, 0, 1 2 ) requiring a doubling of the c-axis. The Dy moments point along the linear chain with the shortest distance c. At 1.5 K, the ordered moment value is 8.57(1) μB/Dy atom. Two symmetry independent wave vectors describe the 1.5 K magnetic ordering of the FeB-type phase: q2=(0, 1 2 , 1 6 ) and q3=(0, 0.484(1), 0.0892(1)), coexisting in form of domains. In both structures the magnetic moments are confined to the (0 0 1) plane at an angle of 2(2)° and 22(3)° from the shortest axis b, respectively. Both structures correspond to sine wave modulations. The amplitude of the q2 wave is mo=7.5(1) μB/Dy atom and that of q3 8.2(1) μB/Dy atom. The wave vector q2 when referring to the (a, 2b, c) cell and the wave vector q=(0, 0, 1 6 ) corresponds to a transversal modulation, which by a proper origin choice can be also described as an antiphase domain structure with two amplitudes. The moments point to the b-axis and are stacked in the sequence (+mo/2, −mo/2, −mo, −mo/2, +mo/2, +mo, …) along the c-direction, while tb acts as an antitranslation. For the q3 phase, the local moment value depends on the atom position in the wave. We also discuss the case where q3 and q2 act simultaneously in physical space.

A noncollinear-ferromagnetic spin-glass-like state was observed in Tb55Co20Al25 bulk metallic glass due to the strong random magnetic anisotropy. Associated with this behavior, we observed a comparatively large magnetic entropy change (ΔSm is 9.75 J K−1 kg−1) in a field change of 7 T and a correspondingly high value of the magnetic refrigeration capacity (RC is 540 J kg−1) with almost no hysteresis loss in the vicinity of the so-called Curie temperature. This opens the possibility of using this material for magnetic cooling purposes.

The crystal and magnetic structure of TbNiSnD has been studied by neutron powder diffraction. The deuteride crystallizes with the filled TiNiSi type structure (sp. gr. Pnma; a=7.0354(1); b=4.20662(9); c=8.1808(2) A at 293 K; a=7.008(1); b=4.1954(6); c=8.153(1) A at 2 K) and is canted ferromagnetic below 10 K. The deuterium atoms occupy the tetrahedral Tb3Ni sites. The Tb atoms carry a magnetic moment of 6.6(1) μB /Tb. They order antiferromagnetically along the a-axis and ferromagnetically along the c-axis. No significant magnetic moment on the Ni atoms has been observed by neutron diffraction. Insertion of deuterium in TbNiSn induces both a change of the magnetic structure and a reduction of the Tb magnitude of the magnetic moment.

Modern society relies on readily available refrigeration. The ideal cooling machine would be a compact, solid state, silent and energy-efficient heat pump that does not require maintenance. Magnetic refrigeration has three prominent advantages compared to compressor-based refrigeration. First, there are no harmful gases involved, second it may be built more compact as the working material is a solid and third magnetic refrigerators generate much less noise. Recently, a new class of magnetic refrigerant materials for room-temperature applications was discovered. These new materials have important advantages over existing magnetic coolants: They exhibit a large magnetocaloric effect (MCE) in conjunction with a magnetic phase transition of first order. This MCE is, larger than that of Gd metal, which is used in the demonstration refrigerators built to explore the potential of this evolving technology. In the present review, we compare the different materials considering both scientific aspects and industrial applicability.

We have revised the T-magnetic phase diagram of the compound ErFe4Ge2 based on a recent X-ray powder diffraction study and software development enabling a new approach to this complex situation. Special emphasis is given to the intermediate temperature (IT) range 20 K–TN,Tc below the double first-order magneto-elastic transition where the tetragonal high-temperature (HT) phase disproportionates into two distinct orthorhombic phases: P42/mnm (HT phase) TN,Tc=44 K→Cmmm (majority low-temperature (LT) phase) q 1 = ( 0 , 1 2 , 0 ) + Pnnm (minority IT Phase) (q2=(0, qy, 0), qy≈2/11). The phase diagram comprises three distinct regions: the HT range TN,Tc–293 K where the tetragonal phase P42/mnm is stable, the IT range 20 K–TN, where the Cmmm and Pnnm phases coexist in strongly variable proportions and the LT range, 1.5–20 K, where Cmmm prevails. The Pnnm phase reaches its highest concentration (≈33%) around 30 K. We report on the multi-axial amplitude modulated incommensurate magnetic structure of the Pnnm phase with a 2D-canted Fe moment arrangement q2=(0, qy, 0) and on the coexisting Cmmm 3D-canted magnetic phase with q 1 = ( 0 , 1 2 , 0 ) . The spin reorientation transitions occurring in the Cmmm phase relate to the various structural changes both arising from competing magneto-elastic mechanisms, involving the Er crystal field anisotropy the Er–Er, Er–Fe and Fe–Fe exchange interactions and their coupling to the lattice strain. The Er–Er and Er–Fe interactions are dominating in the LT Cmmm phase while the Fe–Fe and Er–Fe interactions prevail in the Pnnm phase.

The structure and magnetic properties of Gd2Co17−xCrx (1.17 ≤ x ≤ 3.0) compounds have been investigated by means of X-ray diffraction (XRD) and magnetization measurements. The powder X-ray diffraction patterns show that all samples crystallize as a single phase with the rhombohedral Th2Zn17-type structure. The lattice parameters a and the unit cell volume V increases slightly with increasing Cr content, but the c parameter varies in a less simple way with increasing Cr content. The X-ray diffraction patterns of the magnetically aligned samples show that all compounds investigated have uniaxial anisotropy. Spin reorientation phenomena occur in all of the compounds. The Curie temperature TC, the spin reorientation temperature Tsr, the spontaneous magnetization M0 and the saturation magnetization Ms decrease with the increasing Cr content. The anisotropy constant K1 and the anisotropy field Ba of the compounds at room temperature reach a maximum for x = 1.76. The M0 and Ms increase with increasing temperature from 5 K to 300 K. The easy-axis anisotropy of all compounds changes to easy-plane anisotropy at low temperatures and the spin reorientation phenomena are more pronounced for low Cr concentration.

We present the temperature phase diagram of the compound DyFe 4 Ge 2 determined from high-quality powder XRPD for the entire magnetically ordered regime. DyFe 4 Ge 2 undergoes at T N , T c = 55 K a simultaneous structural and magnetic transition of second-order P4 2 /mnm , T N , T c = 55 K → Cmmm , followed by two subsequent isostructural first-order magnetic transitions at T ic 2 = 45 K and T ic 1 = 28 K . The structural transition at T N , T c is characterised by strong changes of the a / b ratio and of the shear strain while the T ic 1 = 28 K transition is related to a lattice expansion along c on cooling. Minor Fe parameter shifts and deformations (torsion angle and out-of plane Fe–Fe distances) of the Fe tetrahedra were found to accompany the two magnetic transitions. Using the XRPD instrumental resolution and modelling the anisotropic micro-strain enabled us to refine the observed anisotropic peak broadening associated with the presence of micro-structure effects. The maximum strain experienced by the crystal lattice during a sequence of subtle magnetic transitions was observed in the intermediate temperature interval T ic 1 = 28 K - T ic 2 = 45 K of incommensurate magnetic phases.

We present the temperature magnetic phase diagram of the compound DyFe 4 Ge 2 determined from neutron diffraction data for the entire magnetically ordered regime. DyFe 4 Ge 2 undergoes at T N , T c = 55 K a simultaneous structural and magnetic transition of second order (or weakly first order) followed by two subsequent isostructural first-order magnetic transitions at T ic 2 = 45 K and T ic 1 = 28 K : P 4 2 / mnm , T N , T c = 55 K → Cmmm , q 1 = ( 0 1 2 0 ) T ic 2 = 45 K → q 2 = ( 0 q y 0 ) T ic 1 = 28 K → q 1 . The re-entrant lock-in magnetic phase q 1 = ( 0 1 2 0 ) is stable in the high-temperature range T ic2 – T N and in the low-temperature range 1.5 K– T ic1 while the incommensurately modulated q 2 = ( 0 q y 0 ) magnetic phase is sandwiched in the intermediate range T ic1 – T ic2 between the two commensurate phases. The wave vector q 2 has a temperature-dependent length with a minimum in the middle of the incommensurate range and corresponds to a multiaxial amplitude modulated phase. Symmetry analysis leads for both propagation vectors in Cmmm to a twofold and fourfold splitting of the tetragonal Dy 2 b site and the Fe 8 i sites, respectively. The low temperature q 1 = ( 0 1 2 0 ) and the q 2 = ( 0 q y 0 ) phases correspond to 3D canted magnetic structures described by the irreducible representations (Irreps) Γ 2 + Γ 3 while the high-temperature q 1 phase to 2D canted magnetic structures described by a single Irrep Γ 2 . The T ic2 transition is connected with reorientations of both Fe and Dy moments.

The magnetization has been measured of single crystals of RMn 6 Ge 6− x Ga x (R = Tb, Er, Tm and Yb and x ≈ 1) compounds at 4.2 K in high magnetic fields up to 65 T applied along the main crystallographic directions. At low temperature, where the magnetocrystalline anisotropy is determined by the R contribution, these compounds exhibit easy-axis anisotropy with the c -axis as preferred moment direction. The magnetization display a variety of field-induced phase transitions which are particularly pronounced in the compounds with Tb, Er and Tm. The Yb compound exhibits the onset of a phase transition in the field region above 65 T. The results are discussed in terms of the interplay between the antiferromagnetic intersublattice R–Mn coupling and the magnetic anisotropy.

We have studied the magnetic properties and magnetocaloric effects in the Mn1.2Fe0.8P1−xGex compounds with x = 0.2, 0.22, 0.3, 0.4 and 0.5. X-ray diffraction patterns show that the Mn1.2Fe0.8P1−xGex compounds crystallize in the hexagonal Fe2P-type crystal structure. The magnetic moments of the Mn1.2Fe0.8P1−xGex compounds measured at 5 K and 5 T increase with increasing Ge content. The Curie temperature increases strongly and the magnetic entropy change has a maximum around 233 K for the compound with x = 0.22, which is about 19 and 31 J kg−1 K−1 for a field change of 2 and 5 T, respectively.

DyAl2 nanoparticles have been prepared by means of are discharge in a mixture of argon and hydrogen gas. The Structure of DyAl2 nanoparticles is studied by means of X-ray diffraction, X-ray photoelectron spectroscopy, and high-resolution transmission electron rnicroscopy. X-ray photoelectron spectroscopy shows that the as-prepared DyAl2 nanoparticles are coated with a layer of Al2O3 phase on the surface, and their sizes vary from 20 nm to about 100 nm. The DyAl2 nanoparticles exhibit ferromagnetic properties that are different from bulk DyAl2 compound. The gradual decrease of the magnetization with increasing temperature in a wide temperature range reveals the size distribution ofthe DyAl2 nanoparticles. The magnetic-entropy changes are derived from the isothermal magnetization curves measured at different temperatures. The magnetic-entropy change of the DyAl2 nanoparticles is lower than that of the bulk DyAl2 material but has a broadened peak. (c) 2005 Elsevier B.V. All rights reserved.

In order to investigate the behaviour of the magnetic anisotropy and the Mn–Mn exchange interaction in RMn 6 Sn 6 (R = rare earth) compounds, magnetic studies have been carried out on single crystals of RMn 6 Sn 6 compounds in which R is one of the non-magnetic elements Sc, Y and Lu. The temperature dependence of the magnetization of these compounds has been measured with the field applied parallel and perpendicular to the crystallographic c -direction. Magnetic isotherms of the three compounds were measured at 4.2 K in magnetic fields up to 50 T, applied parallel and perpendicular to the c -direction, sufficiently high to reach the forced ferromagnetic state and to detect multiple field-induced transitions. For LuMn 6 Sn 6 , magnetic isotherms were measured also at a higher temperature of 150 K. From our results we may conclude that the magnetic anisotropy as well as the antiferromagnetic coupling strength increases in the sequence of ScMn 6 Sn 6 , YMn 6 Sn 6 and LuMn 6 Sn 6 .

We have measured the temperature dependence of the magnetization on single crystals of the RMn 6 Sn 6 compounds with R = Gd, Tb, Dy and Ho. The observed magnetic-ordering temperatures and the anisotropy types agree well with earlier results from literature. Measurements of the magnetization with the field applied along the main crystallographic directions have been made at 4.2 K in magnetic fields up to 55 T. Field-induced transitions are observed that are interpreted in terms of the interplay between the antiferromagnetic intersublattice R–Mn coupling and the magnetocrystalline anisotropy.

Single crystals of TmMn 6 Sn 6− x Ga x compounds (0 ≤ x ≤ 1.22) obtained in a tin flux have been studied by thermomagnetic and high-field magnetization measurements. A collinear-antiferro to heli-magnetic transition at 324 K has been observed in TmMn 6 Sn 6 . Spin-reorientation transitions at low temperature are found in the compounds with x = 0.66 and 1.22. The magnetization curves in high fields at 4.2 K are interpreted to result from the competition between the Tm–Mn exchange interaction and the magnetocrystalline anisotropy of the Tm and Mn sublattices. A strong easy-plane anisotropy is favored for the Ga-poor compounds and a strong easy-axis anisotropy for the Ga-rich compounds while the compounds of intermediate Ga content (0.51 ≤ x ≤ 0.66) display rather weak magnetocrystalline anisotropy.

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 hexagonal TmCuAl is studied with magnetic specific heat, resistivity and Tm 169 Mossbauer spectroscopy measurements. TmCuAl orders antiferromagnetically at T N = 1.9 K. The distance between the two levels of the quasi-doublet ground state of TmCuAl, determined with the Tm 169 Mossbauer pseudo-quadrupole shift, amounts to 2.8(3) K. Based on the temperature dependence of the Tm 169 quadrupole splitting a provisional calculation has been made for the crystal field diagram with a coupling constant α , which is non zero.

The Mossbauer spectra of Nd6Fe13Ag and Nd6Fe13AgH13 have been measured between 85 and 295 K and analyzed with a model that takes into account the basal magnetic anisotropy, the iron and hydrogen near-neighbor environments of the five crystallographically and magnetically inequivalent 4d, 16k, 16k′, 16l1, and 16l2 iron sites in the former and the six crystallographically and magnetically inequivalent 4d, 16k, 16k′, 16l1, 16l2−1 and 16l2−2 iron sites in the latter, where the additional site results from a predominance of hydrogen as near-neighbors on the iron 16l2 site. The increases in hyperfine field and isomer shift observed upon hydrogenation are in agreement with the increase in Wigner–Seitz cell volumes of each site and the changes in the near-neighbor environments.

We have studied the magnetic properties and magnetic-entropy changes of Mn1.1Fe0.9P0.7As0.3–xGex compounds with x = 0, 0.05, 0.1, 0.15 and 0.3. X-ray diffraction (XRD) study shows all the compounds crystallize in the Fe2P-type structure. Magnetic measurements show that the Curie temperature increases from 150 K for Mn1.1Fe0.9P0.7As0.3 to 380 K for Mn1.1Fe0.9P0.7Ge0.3. A field-induced first-order magnetic phase transition is observed above the Curie temperature for the compounds with x up to 0.15. There exists an optimal composition in which the first-order phase transition is the sharpest. The optimal composition for this system is x = 0.1. The maximal magnetic-entropy change derived from the magnetization data is about 40 J/(kg K) for a field change from 0 to 3 T.

We have synthesized Cr nanoparticles by are-discharge and Cr2O3 nanoparticles by subsequent annealing the as-prepared Cr nanoparticles. The structure of these nanoparticles is studied by means of X-ray diffraction, X-ray photoelectron spectroscopy, and high-resolution transmission electron microscope. Most of the particles show a good crystal habit of well-defined cubic or orthorhombic shape, while some small particles show spherical shape. The as-prepared Cr nanoparticles have a BCC Cr core coated with a thin Cr2O3 layer. Cr in the core of the particles heated at 873 K for 4 h is changed to Cr2O3. The results of magnetic measurements show that the Cr nanoparticles exhibit mainly antiferromagnetic properties, in addition to a weak-ferromagnetic component at lower fields. The weak-ferromagnetic component may be ascribed to uncompensated surface spins. For the field-cooled Cr2O3 nanoparticles, an exchange bias is observed in the hysteresis loops, which can be interpreted as the exchange coupling between the uncompensated spins at the surface and the spins in the core of the Cr2O3 nanoparticles. (c) 2005 Elsevier B.V. All rights reserved.

The magnetic ordering of the hexagonal (P63/mmc) RCuSi (R=Dy, Ho) compounds has been studied by neutron powder diffraction. Magnetic ordering occurs below TN=11.9 K for R=Dy and 9.5 K for R=Ho and is associated with the wave vectors, q1=(0,0,q1z) and q2=(q2x,0,q2z) for both compounds. At 1.5 K the length of the wave vector q1 is close to the commensurate value q1z≈1/6 for both compounds and that of q2≈(1/15,0,1/6) for DyCuSi and q2≈(1/13,0,1/7) for HoCuSi. The wave vector length displays more important changes with temperature for the R=Dy compound than for R=Ho. Various models are discussed. For amplitude modulated arrangements of the R moments the amplitude of the waves (Fourier coefficients) are for the q1 structures 3.1(1)μB/Dy and 3.9(1)μB/Ho and for the q2 structures 5.9(2)μB/Dy and 7.8(1)μB/Ho. The two structures coexist in form of domains. The present investigation is based on simultaneous refinements of high-resolution and high-flux data. Our results differ in several points from recently published data.

The structural and magnetic properties of Sm2Co17-xCrx (0 less than or equal to x less than or equal to 3.0) compounds have been investigated by means of X-ray diffraction and magnetization measurements. The powder X-ray diffraction patterns show that all samples crystallize as a single phase with the rhombohedral Th2Zn17-type structure. The lattice parameters a and V increase monotonically with increasing Cr content, but the lattice parameter c increases very slowly with increasing Cr content. The X-ray diffraction patterns of the aligned powder of the samples have confirmed that all the compounds investigated have uniaxial anisotropy. The Curie temperature of the compounds decreases rapidly the spontaneous magnetization M-0, the anisotropy field B-a, and the anisotropy constant K-1 of Sm2Co17-xCrx (0 less than or equal to x less than or equal to 3.0) compounds decrease strongly with increasing Cr content. (C) 2004 Elsevier B.V. All rights reserved.

The magnetic ordering of the compound CeCoAl4 (orthorhombic; Pmma space group) has been investigated by means of neutron diffraction from powder and single-crystal samples. Only the Ce moments order antiferromagnetically below TN = 12.8(1) K, with a propagation vector . Their collinear magnetic moments point along the b axis of the magnetic unit cell, which is doubled along both the nuclear b and c axes. At 1.5 K the refined magnetic moment value is 1.29(3) μB/Ce atom.

We have studied the magnetic behaviour of a (tetragonal) TbFe3.5Al8.5 single crystal extending earlier work on a TbFe4.4Al7.6 one. At 5 K, the magnetisation curve measured along the [0 0 1] direction points to easy-plane behaviour, whereas that along the [1 0 0] direction shows a strong hysteresis, coercivity field of about 0.5 T and a remanence of 5.0 μB/f.u. A similar behaviour is observed when the field is applied along the [1 1 0] direction, the remanence now being 3.7 μB/f.u., which corresponds closely to ½√2 of the remanence measured along the [1 0 0] direction. It is concluded that the orthorhombically distorted ferrimagnetic state, found to be field-induced for TbFe4.4Al7.6, is formed spontaneously in TbFe3.5Al8.5. The magnetisation was measured at increasing temperature in a field of 0.05 T applied along the [1 0 0] direction. After field cooling (FC), the magnetisation decreases from about 4.8 μB/f.u. at 5 K down to about zero at 110 K. After zero-field cooling (ZFC), the magnetisation starts with a very small (consistently negative) value, passes through a maximum (at about 60 K) and then joins the FC curve. From a closer examination of the low-temperature ZFC virgin magnetisation curves, it was concluded that 180° walls play an important role in the initial stage.

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.

The field dependence of the magnetization in antiferromagnetic Nd6Fe13Si has been measured between 4.2 and 295 K under an applied magnetic field of between zero and 23 T. Isofield magnetization measurements at 1, 10, 15 and 20 T have also been carried out between 4.2 and 295 K. The magnetic phase diagram of Nd6Fe13Si contains three regions. Between 4.2 and 110 K, the magnetic moments are aligned within the basal plane of the tetragonal unit cell and two critical fields are observed. Between 160 and 295 K, the magnetic moments are aligned along the c-axis and one critical field is observed. Finally, between 110 and 160 K, a complex intermediate or mixed magnetic phase with up to three critical fields is observed. At 4.2 K and 23 T a hysteresis with a substantial coercive field of 0.4 T is observed. The presence of both two critical fields and hysteresis at low temperatures indicates a strong magnetic anisotropy within the basal plane. At 295 K an applied field of 23 T does not yield a complete spin flop of the axial magnetic moments, which remain canted. The critical field in the high temperature region is smaller than that observed in the low temperature region, indicating that the uniaxial antiferromagnetic exchange is weaker than the basal exchange. The temperature dependence of the magnetization under applied fields of 15 and 20 T shows the overall expected decrease with increasing temperature but with a break between 80 and 110 K in the spin reorientation region. Finally, the temperature dependence of the magnetization under applied fields of 10 T or less is very complex and results from a field induced canting of the magnetic moments below 30–40 K and a minimum in magnetic anisotropy around the spin reorientation at 110 K.

The re-entrant magneto-elastic transition of the antiferromagnetic HoFe 4 Ge 2 compound has been studied by neutron powder diffraction as a function of temperature. The magnetic phase diagram comprises the wave vectors: ( q 1o , q 2o , q 1t ) and three magnetic transitions, two of them occurring simultaneously with the structural changes at T c , T N =52 and T c′ , T i c1 =15 K, the third being purely magnetic at T i c2 =40 K. The first transition is of second order while the latter two of first order . The sequence of phases follows the path: P4 2 /mnm (HT), T c , T N =52 K→ Cmmm (IT): ( q 1o =(0,1/2,0) , T i c 2 =40 K ⇒ q 2o =(0,q y ,0)) , T c′ , T i c 1 =15 K ⇒ P4 2 /mnm (LT): q 1t =(0,1/2,0) . The magnetic structures described by the wave vectors ( q 1o , q 2o and q 1t ), where the components are referred to the reciprocal basis of the conventional Cmmm cell, correspond to canted multi-axial arrangements. The q 2o wave vector length of the amplitude modulated phase varies non-monotonously, decreasing fast just below T i c2 ,—slowly between 36 K— T c′ , T i c1 and jumping to the q 1t =(0,1/2,0) lock-in value at T c′ , T i c1 simultaneously with the first order re-entrant transition to the (LT) tetragonal phase. In the coexisting meta-stable orthorhombic phase from T c′ , T i c1 down to 1.5 K the length of the wave vector q 2o continues to decrease. To solve the magnetic structures of all the phases appearing in this complex situation, arising from competing ordering mechanisms and anisotropies of the underlying sublattices, we have used the simulated annealing method of global optimisation on high-resolution neutron powder diffraction data.

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.

We have investigated the ordering transformation and magnetic properties of Fe 59.75 Pt 39.5 Nb 0.75 bulk alloys in detail by using different high-temperature homogenization treatments, different cooling rates and different low-temperature annealing treatments to obtain samples with different microstructure and different atomically ordered states. The quenching rate after the high-temperature homogenization treatment was chosen much lower than in previous investigations. In this way, we were able to obtain nanostructured bulk alloys consisting exclusively of the hard-magnetic face-centered-tetragonal phase. A high remanence ratio was obtained by profiting from the nanocomposite exchange coupling between nearest-neighbor-ordered regions. The present results are compared with results of previous investigations in which much higher cooling rates were applied. We also discussed why the present alloy systems are less suitable for the attainment of exchange spring behavior.

High-field free-sample magnetisation results at 4.2 K on tetragonal ferrimagnetic DyCo10V2 and TbFe9Mo3 samples show a marked difference between the zero-field-cooled (ZFC) and field-cooled (FC) magnetisation isotherms, which is still present after a field as high as 38 T has been applied. The results indicate that the ZFC samples remain multidomain throughout the experiment, with a compensated magnetisation perpendicular to the magnetic-field direction, although, for instance for DyCo10V2, the coercive field (|| [0 0 1] direction) at 4.2 K is only about 2 T.

Neutron powder diffraction on the binary intermetallics Er5Mg24 and Tm5Mg24 confirms their isostructural and pure crystallization in the Ti5Re24-type structure (space group I 4 3m, Z = 2)with rare earths located in 2a(0,0,0) and 8c(x,x,x) and Mg in two different 24g(x,y,x) sites. Room temperature lattice constants are 11.263(2) Å and 11.215(1) Å for the Er and Tm compound, respectively. Atomic positions have been refined. Both compounds order ferromagnetically below Curie temperatures of 17.5(5) K and 7.5(5) K for Er5Mg24 and Tm5Mg24, respectively. The magnitudes of the Er moments at 4.2 K are 7.5(2) µB and 4.4(2)µB on the 2a and the 8c sites, respectively. The Tm moments which have been refined from 2 K measurements amount to 3.0(3)µB and 2.8(2) µB, respectively. The temperature dependencies of the magnetic Bragg intensities reveal distinct deviations from Brillouin curves for J = 15/2 (Er) and J = 6 (Tm) systems and indicate a complex magnetic exchange.

The alloy MnFeP 0.45 As 0.55 has been identified as a promising material to be used as magnetic refrigerant for room-temperature applications. This alloy, which is derived from the well-known iron pnictide Fe 2 P, exhibits a first-order temperature- and field-induced magnetic phase-transition. The Curie temperature can be varied by substitutions into the pnictide sublattice. We studied the effect of Ge substitution in this material. Most strikingly, we find a strong increase of the magnetic ordering temperature up to 570 K and a concomitant increase of the ordered magnetic moment up to 4.25 μ B /f.u. for the alloy MnFeP 0.5 Ge 0.5 . On substitution with the large Ge ions, the expected lattice expansion is very anisotropic. We observe an increase and decrease of the a and c parameter of the hexagonal lattice, respectively. Compared with the (P,As) series, for a similar volume change we find a much stronger increase of T C . This confirms the importance of competing exchange interactions in this interesting alloy system.

The magnetic structure of PrCoAl 4 has been studied by neutron diffraction from a single crystal. The Pr moments order at a temperature near to 20 K with a sine-wave longitudinal amplitude-modulated structure. The length of the wave vector q = ( 0 0 q z ) is almost temperature independent with q z = 0.4087 ( 5 ) , and only the first harmonic was observed down to 2 K. The amplitude of the wave, 2.05(3) μ B /Pr atom at 2 K, is reduced compared to the Pr 3+ free-ion moment value g J μ B = 3.2 μ B , due to strong crystal-field effects. One difference between the present and previously reported neutron powder-diffraction results concerns the wave vector length. The reason for this may lie in the different magnetic microstructure (stacking faults) of the material depending on the thermochemical history of each sample.

The magnetic structure and ordering temperatures of three intermetallic compounds which crys- tallize in the tetragonal ThCr2Si2 structure, TbCr2Si2 ,H oCr2Si2 and ErCr2Si2, have been determined by neutron diffraction, differential scanning calorimetry and magnetization measurements. The Cr-sublattice orders anti-ferromagnetically with Neel temperatures of 758 K for TbCr2Si2, 718 K for HoCr2Si2 and 692 K for ErCr2Si2. Chromium atoms located at 4d crystallographic sites are aligned anti-parallel along the c-axis, with GZ Cr magnetic modes. In contrast with metallic bcc Cr, the refined room temperature value of the ordered Cr moment is anomalously large for all three compounds. No long range magnetic order of the R sublattice in TbCr2Si2 and HoCr2Si2 is observed, whilst the Er sublattice in ErCr2Si2 orders independently of the Cr sublattice below 2.4 K with moments ferromagnetically aligned in the basal plane.

We have used neutron diffraction and magnetic measurements to investigate the changes in magnetic properties of the Kondo semimetal compound CeNiSn after charging with deuterium gas under moderate conditions. Charging with deuterium led to the formation of the monodeuteride CeNiSnD with filled TiNiSi structure. This compound gives rise to long-range antiferromagnetic ordering below T N =5.1 K. The ordered magnetic moment value equals 1.37(3) μ B per Ce atom, the moments pointing along the [001] direction. No significant ordered magnetic moment on the Ni atoms has been observed by powder neutron diffraction.

Neutron diffraction performed on deuterated powder samples of Pr 6 Fe 13 Au shows that the crystal structure of the parent compound is retained upon charging with deuterium. The refined composition of the deuteride is Pr 6 Fe 13 AuD 13 . The deuterium atoms occupy various types of interstitial sites bounded by Pr and Fe atoms. Each of these sites is surrounded by at least two Pr atoms. A collinear antiferromagnetic ordering of the four Fe and the two Pr sublattices was reported previously for the uncharged Pr 6 Fe 13 Au compound. By contrast, a collinear ferromagnetic structure has been derived from the present neutron data of the deuteride. The easy moment direction is the same in Pr 6 Fe 13 AuD 13 and Pr 6 Fe 13 Au, being perpendicular to the c-axis. For the deuteride this result was derived from a complementary synchrotron radiation X-ray study made on magnetically aligned particles of the ferromagnetic deuteride.

Neutron diffraction performed on Nd 6 Fe 13− x Al 1+ x ( x =1–3) and La 6 Fe 11 Al 3 powder samples has shown a selective distribution of the Al atoms among the five non-rare-earth positions filling up successively the 4a Si and the 16l 2 Fe4 sites of the prototype Nd 6 Fe 13 Si structure with increasing x . Traces of Al could be detected at the 16k Fe site when x =3. The magnetic ordering is associated in all cases with the wave vector q =(0 0 1) ( I p magnetic lattice with anticentering translation). All Nd compounds display almost easy axis behaviour below T N and spin reorientation transitions at lower temperatures from easy axis to easy cone. A full set of neutron data in the entire magnetically ordered regime has been analysed to derive the exact magnetic structural parameters and possible spin reorientation transitions. The largest ordered moment values were observed for Nd 6 Fe 12 Al 2 at 1.5 K (3.2(2) μ B /Nd atom, close to the free ion value, and 2.1 μ B /Fe atom) while no detectable moment value could be found for the Fe4 site for higher Al concentrations. The iron moment values are in satisfactory agreement with Mossbauer and magnetization results.

Magnetic measurements are carried out on polycrystalline ThFe 11 C x ( x =1.5,1.8) samples between 4 and 900 K and magnetic field up to 7 T. This wide temperature range covers both the ferromagnetically ordered region and the paramagnetic region. Not only the ordering temperature, but also the magnetisation are found to be very sensitive to the carbon concentration. The evolution of the magnetic properties versus the carbon content is discussed. The Curie temperature and the Fe–Fe exchange interactions increase when the carbon concentration increases. The mean ordered iron magnetic moment deduced from saturation magnetisation is also found to depend upon the carbon concentration but in the opposite direction. Mean magnetic moment values of 2.00 and 1.88 μ B per iron atom are observed for ThFe 11 C 1.5 and ThFe 11 C 1.8 , respectively. The effective magnetic moment is found to decrease from 3.91 to 3.73 μ B per iron atom for ThFe 11 C 1.5 and ThFe 11 C 1.8 , respectively. The evolution of the effective magnetic moment is discussed on the basis of the spin fluctuations theory. The iron magnetic state in the ThFe 11 C 1.5 and ThFe 11 C 1.8 present similarities with that reported for the Fe 3 C compound.

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.