Search

We have studied the effect of the atomic disorder–order transformation on remanence enhancement and coercivity in CoPt ingots by isothermal annealing at various temperatures well below the transformation point. The relative amount of the low-temperature hard-magnetic face-centered-tetragonal (fct) phase precipitated in the high-temperature magnetically soft face-centered-cubic (fcc) phase was determined by means of x-ray diffraction. As a function of annealing temperature, the mean grain size and the relative amount of the fct phase increase at the cost of the fcc phase. These changes were followed by means of magnetic measurements. We observed a continuous increase in coercivity with increasing annealing temperature, eventually reaching a maximum. The Kneller–Hawig model was used to explain the occurrence of remanence enhancement and the continuously changing degree of exchange coupling between the magnetically soft and hard phases.

A comparative study has been performed concerning the microstructure and the magnetization reversal behavior of nearly single-phase Sm2Fe14.5Cu0.5Ga2C2 and two-phase exchange-coupled Sm2Fe14.5Cu0.5Ga2C2/α-Fe and Sm2Fe14.25Zr0.25Cu0.5Ga2C2/α-Fe synthesized by melt spinning and annealing. A few percent of Zr substitution proved to be effective in refining the microstructure and enhancing exchange-spring behavior after melt spinning and annealing. All samples show remanence enhancement, the value of Mr/Ms being the largest for the sample containing Zr. Measurements of the reversible susceptibility show a large reversible magnetization in the two-phase materials. For the Zr-containing sample, a more detailed analysis of the temperature dependence of the exchange-spring behavior was made.

The structure, magnetic properties and microstructure of Sm2Fe15−xCuxGa2C (x=0 and 0.5) ribbons prepared by melt spinning and subsequent heat treatment were studied. A significant increase in coercivity has been found for ribbons prepared with the addition of a small amount of Cu. The maximum coercivity μ0Hc is 1.4 and 2.0 T at room temperature for ribbons with x=0 and 0.5, respectively, after annealing in vacuum at 1043 K for 15 min. A small amount of Cu addition has little effect on the lattice parameters, Curie temperature TC, and magnetocrystalline anisotropy field HA at room temperature, while it decreases the saturation magnetization Ms. The magnetocrystalline anisotropy constants K1 and K2 for the x=0 and 0.5 samples were determined in the temperature range between 50 and 293 K, using magnetically aligned samples. The variations of coercivity, magnetization, and remanence as functions of applied field and temperature are consistent with the view that the magnetization of Sm2Fe15−xCuxGa2C ribbons wi...

A detailed investigation of the structure and magnetic properties of Tm2Fe17−xGax (x=0, 1, 2, 3, 4, 5, 6, 7, and 8) compounds has been performed by means of x-ray-diffraction, neutron-diffraction, magnetization, and ac-susceptibility measurements. Crystal-structure studies have shown that the prepared samples are single phase with the hexagonal Th2Ni17 for x⩽3 and the rhombohedral Th2Zn17 structure for x⩾5. In Tm2Fe13Ga4 the Th2Zn17 structure coexists with the Th2Ni17 structure. Substitution of Ga for Fe in Tm2Fe17 leads to an increase of the unit-cell volume, which is linear with the Ga concentration. In Tm2Fe17−xGax, the saturation magnetization at 1.5 K decreases linearly with increasing Ga content with a rate of 2.3 μB per substituted Ga atom. The Curie temperature is found first to increase with increasing Ga content, going through a maximum value of 485 K at about x=3, then to decrease. Between x=6 and 7, a minimum value of TC is reached and for higher x values TC increases again. X-ray-diffraction ...

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 ...

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 properties of a series of Ce2Fe17−xAlx solid solutions with x equal to 0.00, 0.88, 2.06, 2.81, 3.98, 5.15, 6.08, 7.21, 8.20, 9.08, 9.84, and 10.62 have been studied by magnetic measurements, neutron diffraction, and Mossbauer spectroscopy. The compounds crystallize in the rhombohedral Th2Zn17‐type structure. Magnetization studies indicate that the Curie temperature increases uniformly from 238 K for Ce2Fe17 to 384 K for Ce2Fe14Al3 and then decreases at higher aluminum content. Powder neutron diffraction results, obtained at 295 K, indicate that aluminum avoids the 9d site for all x values and preferentially occupies the 18h site at low aluminum content. Aluminum shows a marked preference for the 6c site for x≳6. The room‐temperature iron magnetic moments increase from x=0 to 2 and then decrease for x≳2. The Mossbauer spectra have been fit with a binomial distribution of the near‐neighbor environments in terms of a maximum hyperfine field, Hmax, for an iron with zero aluminum near neighbors, a...

The kinetics of the face-centered-cubic to face-centered-tetragonal phase transition in Fe59.7Pt39.8Nb0.5 and Fe59.85Pt39.9Al0.25 alloys was studied using data of differential-scanning calorimetry in conjunction with the Johnson–Mehl–Avrami model. The results show that in the Nb-containing alloy, the transition includes both nucleation and growth processes. In the Al-containing alloy, the growth process is dominant during the transition. These differences can be related to the microstructure of the studied alloys.

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 magnetic properties of a series of Nd2Fe17−xAlx solid solutions, with x equal to 2.04, 4.01, 5.97, 7.94, and 9.06, have been studied by magnetic measurements, neutron diffraction, and Mossbauer spectroscopy. Magnetization studies indicate that the Curie temperature increases from 330 K in Nd2Fe17 to a maximum of ∼470 K at an x of 3.5. The compounds crystallize in the Th2Zn17 structure with lattice parameters and unit cell volumes which increase linearly with increasing aluminum content. The neutron diffraction results indicate that aluminum atoms are excluded from the 9d site, prefer the 18h site at low aluminum content, and prefer the 6c and 18f sites at high aluminum content. At 10 K the magnetic moments of the iron and neodymium atoms are collinear and take up a basal orientation at all aluminum contents. The moments decrease with increasing aluminum content and the magnetic moments per unit cell at 10 K are in excellent agreement with the 4.2 K saturation magnetization values. At 295 K the Nd2Fe17...

An x‐ray diffraction study of the substitution of gallium in Tb2Fe17 to form the Tb2Fe17−xGax solid solutions indicates that the compounds adopt the rhombohedral Th2Zn17 structure. The unit cell volume and the a‐axis lattice parameter increase linearly with increasing gallium content. The c‐axis lattice parameter increases linearly from x=0 to 6 and then decreases between x=7 and 8. Magnetic studies show the Curie temperature increases by ∼150° above that of Tb2Fe17 to reach a maximum between x=3 and 4, and then decreases with further increases in x. Neutron diffraction studies of Nd2Fe15Ga2 and Tb2Fe17−xGax, with x equal to 5, 6, and 8, indicate that the gallium completely avoids the 9d site, occupies the 6c ‘‘dumbell’’ site only at high values of x and strongly prefers the 18f site at high values of x. The magnetic neutron scattering indicates both that the terbium sublattice magnetization couples antiferromagnetically with the iron sublattice and that there is a change in easy magnetization direction f...

The Mossbauer spectra of Th2Fe17 and Th2Fe17N2.6 have been measured at various temperatures between 85 and 295 K and analyzed with a model that is based on the Wigner–Seitz cell environment of each iron site, the orientation of the magnetization, and the magnetic moments as determined from either neutron‐diffraction measurements or band‐structure calculations. Upon nitrogenation of Th2Fe17, the 85 K weighted average isomer shift increases from 0.037 to 0.156 mm/s and further the isomer shifts of the four crystallographically distinct sites increase in agreement with the increase observed in their Wigner–Seitz cell volumes and the presence of a nitrogen near neighbor for two of the sites. The nonmagnetic thorium in Th2Fe17N2.6 yields an easy axis of magnetization which, in contrast to magnetic rare earths in R2Fe17 and R2Fe17Nx, is in a general basal direction and not oriented along one of the basal axes. The isomer shift and its temperature dependence for both Th2Fe17 and its nitride are very similar to those observed in related rare‐earth R2Fe17 compounds and their nitrides. Upon nitrogenation of Th2Fe17, the 85 K weighted average hyperfine field increases from 256.3 to 336.5 kOe. However, the increases on the 6c and 18f sites are smaller than those observed on the 9d and 18h sites; changes which are in agreement with changes in the magnetic moments observed by neutron diffraction and calculated for the nitrogenation of Nd2Fe17, Gd2Fe17, and Y2Fe17.The Mossbauer spectra of Th2Fe17 and Th2Fe17N2.6 have been measured at various temperatures between 85 and 295 K and analyzed with a model that is based on the Wigner–Seitz cell environment of each iron site, the orientation of the magnetization, and the magnetic moments as determined from either neutron‐diffraction measurements or band‐structure calculations. Upon nitrogenation of Th2Fe17, the 85 K weighted average isomer shift increases from 0.037 to 0.156 mm/s and further the isomer shifts of the four crystallographically distinct sites increase in agreement with the increase observed in their Wigner–Seitz cell volumes and the presence of a nitrogen near neighbor for two of the sites. The nonmagnetic thorium in Th2Fe17N2.6 yields an easy axis of magnetization which, in contrast to magnetic rare earths in R2Fe17 and R2Fe17Nx, is in a general basal direction and not oriented along one of the basal axes. The isomer shift and its temperature dependence for both Th2Fe17 and its nitride are very similar to t...

The effect of the atomic disorder–order transformation on remanence enhancement and coercivity in isotropic CoPt magnets has been studied by isothermal annealing at 675 °C, well below the transformation temperature, for 10–80 min after quenching from 900 °C. The results of x-ray diffraction, alternating-current susceptibility, and magnetization measurements indicate that the strong remanence enhancement effect originates from exchange coupling between the face-centered-tetragonal ordered hard-magnetic precipitated phase and the face-centered-cubic disordered soft-magnetic matrix phase during the initial ordering transformation. The nonuniform distribution of the magnetic anisotropy results in a lower reversibility, unlike the so-called exchange spring magnet.

The neutron diffraction powder patterns of Pr2Fe17 and Pr2Fe17N2.6 have been measured at 295 K and refined within the Th2Zn17 structure type to give the structural changes which occur upon nitrogenation; changes which include a 6.4% expansion of the unit cell volume. The Mossbauer spectra of Pr2Fe17 and Pr2Fe17N2.6 have been measured between 85 and 295 K and analyzed with a model which is based on the Wigner–Seitz cell environment of each iron site, the basal orientation of the magnetization, and a comparison with the Mossbauer spectra of Nd2Fe17 and Nd2Fe17N2.6. Upon nitrogenation of Pr2Fe17, the 85 K‐weighted average isomer shift increases from 0.049 to 0.156 mm/s, and the isomer shifts of the four crystallographically distinct iron sites increase in agreement with the increase found in their Wigner–Seitz cell volumes and the presence of a nitrogen near‐neighbor for the 18f and 18h sites. The temperature dependence of the isomer shifts indicates an increase in covalency upon the formation of the nitride...

The Mossbauer spectra of Nd2Fe17 and Nd2Fe17N2.6 have been measured at various temperatures between 78 and 295 K and analyzed with a model that is based on the Wigner–Seitz cell environment of each iron site, the orientation of the magnetization, and the magnetic moments as determined from either neutron‐diffraction measurements or band‐structure calculations. Upon nitrogenation of Nd2Fe17, the weighted average isomer shift increases from 0.060 to 0.164 mm/s and further the isomer shifts of the four crystallographically distinct sites increase in agreement with the increase observed in their Wigner–Seitz cell volumes and the presence of a nitrogen near neighbor for two of the sites. Upon nitrogenation of Nd2Fe17, the weighted average hyperfine field increases from 292.3 to 333.8 kOe. However, the increases on the 6c and 18f sites are much smaller than those observed on the 9d and 18h sites; changes which are in agreement with calculated changes in the magnetic moments upon nitrogenation of Nd2Fe17, Gd2Fe17, and Y2Fe17. The temperature dependence of the isomer shifts indicates an increase in covalency upon the formation of the nitride.

The magnetic properties and specific heat of ErMn4Al8 and ErCr4Al8 compounds have been studied between 0.5 and 30 K in magnetic fields up to 12 T. We find clear indications that the specific heat is dominated by CEF effects rather than by magnetic ordering. The very high low-temperature specific heat of ErMn4Al8 may be attributed to an isolated |Jz=±3/2〉 doublet, which is strongly affected by the applied magnetic field. For ErCr4Al8, the zero-field data very much resemble the ErMn4Al8 data but the influence of the magnetic field is quite different, which excludes a possible explanation in terms of a doublet ground state.

The magnetic structure at 15 K of LaFe10.8Al2.2 and its ternary nitride LaFe10.8Al2.2N3 have been determined by high resolution time-of-flight neutron powder diffraction. Magnetism in these materials is determined by clusters formed by a central Fe atom surrounded by Fe atoms with icosahedral symmetry. This icosahedral symmetry is not affected by N uptake since both compounds are confirmed to crystallize in the cubic NaZn13-type structure. The Al atoms are statistically distributed over the 96i cluster site only, while N atoms are found to fully occupy the highly symmetrical 24d interstitial hole site. Charging with N gives rise to an 8% increase in the unit cell volume of the parent compound. The magnetic structures are overall ferromagnetic for both compounds, with each of the two Fe sublattices aligned parallel to each other. The influence of N uptake on the magnitudes of the cluster Fe moments will be discussed.

Samples of R2Fe17C(y)N(x) (R = Y, Sm, Er, Tm) were prepared by arc melting appropriate amount of R, Fe, and C, vacuum annealing at 1373 K and finally annealing at 740 K in nitrogen for 10 h. The magnetic properties of these compounds were investigated by means of ac initial susceptibility, magnetization measurements, and x-ray diffraction. The thermal stability of the nitride phase was studied by differential scanning calorimetry. It was found that, when heated above 600 K, R2Fe17C(y)N(x) irreversibly decomposes N which is irrespective of the carbon concentration and rare-earth element. The Curie temperatures of R2Fe17C(y)N(x) are independent of the carbon concentration and are approximately 400 K higher than those of the corresponding pure R2Fe17 compounds. However, the Curie temperatures cannot be correlated to the composition x of the initial R2Fe17C(y)N(x) compounds at room temperature because some N was lost during the heating to T(c). In the Er and Tm compounds spin reorientation transitions were found, marking the change of the easy magnetization direction from the c axis to the basal plane with increasing temperature. The Tm compounds show an additional magnetic transition at low temperatures (below 40 K). A coexistence of the hexagonal and the rhombohedral structural modifications was found in Er2Fe17C(y)N(x) when y < 1.5, characterized by two different spin reorientation temperatures. The anisotropy fields of Sm2Fe17C(y)N(x) are higher than that of Sm2Fe17N(x). Indications of a magnetic phase transition were found also in Sm2Fe17C0.7N(x) and Sm2Fe17C0.9N(x).

The ferromagnetic to antiferromagnetic transition in Fe3Ga4 is studied by means of ac-susceptibility measurements under chemical and external hydrostatic pressure. Substitution of Al for Ga results in an increase of the ferromagnetic to antiferromagnetic transition temperature (T0) of 4.1(4) K/at. % Al. Application of hydrostatic pressures up to 7 kbar also yields a positive effect: dT0/dp=3.4(4) K/kbar. The external-pressure-induced increase of T0 in Fe3(Ga0.9Al0.1)4 is considerably weaker: dT0/dp=0.93(10) K/kbar. Thermal-expansion measurements reveal that the volume change at the ferromagnetic to antiferromagnetic transition ΔV/V amounts to only 0.02%. We believe that the volume effect in conjunction with a shift in the Fermi level drives the ferromagnetic to antiferromagnetic transition. The details of the band structure at the Fermi surface determine the magnetic properties and may give rise to the strong pressure dependence of T0.

The Mossbauer spectra of a series of rhombohedral Tb2Fe17−xSix solid solutions, with x equal to 0, 1, 2, and 3, have been measured as a function of temperature. Although the spectra of Tb2Fe17 change substantially upon cooling from 295 to 85 K, it has been possible to fit them with a consistent seven sextet model corresponding to a basal magnetization. The spectral analysis yields reasonable hyperfine parameters and the expected changes with temperature. The resulting weighted average effective iron recoil mass of 66 g/mol and the Mossbauer temperature of 395 K are typical of this type of intermetallic compound. In addition, the isomer shifts and the hyperfine fields observed for the crystallographically distinct iron sites in Tb2Fe17 agree well with those expected from the differences in the Wigner–Seitz cell volumes and the near-neighbor environments of the four sites. The spectra of the silicon substituted solid solutions have been fit with the same model and similar hyperfine parameters, but with a bi...

Specific-heat measurements have been performed on the tetragonal ThMn12-type compound ErCu4Al8 and YCu4Al8 in the temperature range 1.5–200 K. A sharp peak in the specific heat, signals the antiferromagnetic transitions at TN=5.18 K. In order to separate out the effect of the Er sublattice we have also performed measurements on the YCu4Al8 compound in the same temperature range. From the temperature dependence of the magnetic contribution to the specific heat of ErCu4Al8 we obtained the temperature dependence on the magnetic entropy and discuss our results in terms of the crystal field level scheme proposed earlier from inelastic neutron scattering.

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 effect of substitution of Ti for Co in compounds R2Co17−xTix (R=Pr or Nd, and x=0.0–2.0) on the structure and the magnetic properties has been investigated. It is found that the solubility of Ti in the compounds R2Co17−xTix (R=Pr or Nd) is around 1.0. The R2Co16Ti (R=Pr or Nd) are isostructures to their parent compounds (rhombohedral), and its unit cell is expanded. The spontaneous magnetization μs and Curie temperature TC decrease with increasing Ti content, owing to the magnetic dilution and the reduction of the moment of Co sublattice caused by Ti substitution. Moreover, an anomaly in the M-T curves of Pr2Co16Ti and Nd2Co17−xTix compounds with x=0.5 and 1.0 may be attributed to a spin reorientation. This transition may originate from the change of crystal electron field due to the preferential occupation of Ti atoms on specific crystallographic sites.

The Mossbauer spectra of Nd6Fe13X, where X is Si, Cu, Ag, and Au, have been measured between 85 and 435 K. An analysis of the spectra of Nd6Fe13X, where X is Cu, Ag, and Au in terms of five sextets, indicates that the iron moments lie in the basal plane of the unit cell. The assignment of the five sextets to the five magnetically inequivalent sites in the structure is based on a correlation between the isomer shift and the Wigner–Seitz cell volume for all iron sites, between the isomer shift and the hyperfine field for each site, and the relationship between the hyperfine field at a given iron site and its number of iron near-neighbors. An analysis of the spectra of Nd6Fe13Si reveals that a high temperature magnetic phase, with its moments parallel to the c axis, coexists between 295 and 85 K with a low temperature magnetic phase, with its moments oriented in the basal plane. Hence, Nd6Fe13Si shows a continuous magnetic spin-reorientation from axial towards basal alignment upon cooling.

Magnetization and resistivity measurements of poly- and single-crystalline samples of Hf1−xTaxFe2 are presented. At the antiferromagnetic to ferromagnetic transition we find a large change of resistance which can be induced as a function of temperature as well as field. For single-crystalline Hf0.86Ta0.14Fe2 we observe at room temperature a giant magnetoresistance effect of 6%. From neutron-powder-diffraction measurements, possible magnetic structures in the different magnetic states are constructed.

Crystal-structure studies on Tm2Fe17-xGax (x = O, 1, 2, 3, 4, 5, 6, 7, and 8) compounds have shown that the prepared samples are single phase with the 2:17-type structure. Both the unit-cell volume and the saturation magnetization at 1.5 K decrease linearly with increasing Ga content. The Curie temperature is found to increase with increasing x; it goes through a maximum value of 485 K at about x = 3 to lower values and then it rises again. The Tm2Fe17-xGax compounds exhibit a spin-reorientaiton transition at low temperature. X-ray diffraction measurements on magnetically aligned Tm2Fe17-xGax powders show that the compounds with x less than or equal to 6 have the easy-plane type of magnetic anisotropy, whereas the compounds with x less than or equal to 7 exhibit the easy c-axis type of magnetic anisotropy at room temperature. (C) 1997 American Institute of Physics.

Electrical resistivity measurements have been carried out between 20 and 300 K on Ce2Fe16.8, on the Ce2Fe17−xAlx solid solutions, with x=0.4, 1, 2, 5, 6, 8, and 9, and on the Ce2Fe17−xSix solid solutions, with x=0.2, 0.4, 1, and 2, with the four probe method. The temperature dependence of the resistivity of Ce2Fe16.8 shows an inflection at 110 K, a feature which is also observed in the temperature dependence of the magnetization and is related to a magnetic phase transition from a helical structure, above 110 K, to a fan structure, below 110 K. The temperature dependence of the resistivity of Ce2Fe16.8 is characteristic over the investigated temperature range of weak antiferromagnetic behavior with incommensurate periodicity as has been observed in the earlier neutron diffraction study. The resistivity of Ce2Fe17−xAlx increases with temperature in agreement with a density of states at the Fermi level dominated by the d band. For a given temperature, the resistivity increases substantially and regularly wi...

The magnetic properties of Pr2Co17−xAlx compounds with 0⩽x⩽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⩽x⩽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.

The temperature dependence of the magnetization of DyCo10−xNixSi2 compounds with x=0, 2, 4, and 6 was measured in fields up to 5.0 T and in the temperature range from 4.2 to 300 K in a SQUID magnetometer. Compensation temperatures were observed for the compounds with x=4 and 6. The values of the R‐T exchange interaction obtained by fitting the M‐T curves on the basis of two‐sublattice molecular‐field theory are compared with the values obtained from high‐field magnetization measurements on powder particles free to be oriented by the applied field. In all four compounds magnetic transitions are found in the ac‐susceptibility measurements which were carried out between 4.2 K and room temperature. The types of magnetic anisotropy were investigated by magnetization measurements on samples that were magnetically aligned at room temperature.

A simple two‐sublattice model is used to analyze the magnetization process of anisotropic antiferromagnets at T=0. The magnetization is calculated for samples free to be oriented by the applied field (free single crystals or powders consisting of single‐grain particles). We find that the anisotropy does not necessarily lead to a nonlinear M(B) relation, but that the magnetic isotherms are broken into two linear sections separated by a discontinuity. The difference between the slopes in the two regions enables a determination of the anisotropy constant K1. The results will be discussed in relation to our findings for the ferrimagnetic case and to the behavior of oriented antiferromagnetic crystals.

The magnetic properties of ferromagnetic compounds of Sm2Fe17, Sm2Fe17C0.6, and Sm2Fe17C0.6H4.4 were investigated. Sm2Fe17 nitrides, carbides, and carbohydrides are suitable for making high performance permanent magnets. The high field magnetization measurements performed up to 200 kOe in continuous fields are reported. In this article, the isothermal magnetization curves measured between 2 and 300 K on powder samples embedded in a resin and then aligned under a magnetic field of about 10 kOe in order to get oriented samples are presented. The magnetic anisotropy constants K1 and K2 are determined taking into account the angular distribution of the grains axis. The results are discussed in light of previously reported data and then the effects of C or H on the Sm contribution to the anisotropy is compared to that of N. It is shown that carbon and hydrogen have opposite influence on the Sm anisotropy and it is suggested that a wide range of magnetic anisotropy can be obtained depending on the H and C conte...

The magnetic properties of a series of Tb2Fe17−xAlx solid solutions, with nominal x compositions of 0, 2, 3, 4, 5, 6, 7, and 8, have been studied by neutron diffraction and Mossbauer spectroscopy. Neutron‐diffraction data indicate that the compounds all crystallize with the Th2Zn17 structure and that the aluminum atoms are excluded from the 9d site and show a distinct preference for the 6c site only for an aluminum content greater than 6. The unit‐cell volume increases by approximately 1% per aluminum atom substituted in the formula unit. The magnetic moment per formula unit, measured at 295 K, shows very little change for x less than or equal to 4, but decreases rapidly with increasing aluminum content for higher values of x. Mossbauer spectral results indicate that all the samples are ferromagnetically ordered at 85 K. However, at 295 K Tb2Fe9Al8 is paramagnetic and Tb2Fe10Al7 is either paramagnetic or has at most very small ferromagnetic moments. An analysis of the magnetic spectra with a basal magneti...

The Mossbauer spectra of Sm2Fe17 and Ho2Fe17 and their nitrides have been measured between 295 and 85 K and analyzed with a model which is consistent with our earlier work on R2Fe17 and R2Fe17Nx compounds, where R is Pr, Nd, and Th. This model is completely consistent throughout these rare‐earth compounds and is in agreement with the crystallographic changes occurring upon nitrogenation and with the prediction of band structure calculations. The dramatic increase in Curie temperature in the nitrides results from the expansion of the crystallographic lattice, an expansion which is mainly centered on the 9d and 18h iron sites as is indicated by the increase of their Wigner–Seitz cell volumes upon nitrogenation. The 9d and 18h sites show a larger enhancement of their hyperfine fields as compared to the 6c and 18f sites as a result of improved ferromagnetic exchange between these sites and their near neighbors because of the lattice expansion and the consequent reduced iron 3d‐iron 3d overlap.

The possibility of fenimagnetic coupling between the magnetic moments of the light-rare-earth (Nd, Pr) ions and of the transition-metal (Fe) ions in NdrGa3Fe11- and Pr6Ga3Fe11-based compounds is deduced from the high-field magnetization curves measured at 4.2 K on magnetically aligned and on free-powder samples and from the temperature dependence of the magnetization. The ferrimagnetic coupling is very weak: at 4.2 K, the rare-earth moments and the transition-metal moments can be forced to ferromagnetic alignment by application of an external magnetic field of about 4 T. Generally, in intermetallic compounds, the magnetic coupling is ferromagnetic between light-rare-earth and transition-metal ions and ferrimagnetic between heavy-rare-earth and transition-metal ions. The present results suggest that in some crystal structures ferrimagnetic coupling also may exist in the compounds formed by light-rare-earth and transition-metal elements.

We have studied two new tetragonal compounds, CeCuAl3 and CeCuGa3, which both display very large specific‐heat, c(T), values at low temperature. CeCuAl3 orders antiferromagnetically below TN=3.0 K, while CeCuGa3 remains paramagnetic down to 0.4 K. The large values of c(T) in these and other cerium intermetallics are interpreted as resulting from an unusually small crystal‐field splitting and the possible presence of atomic disorder.

The magnetic properties of R2Co14B1−xCx (x=0, 0.5 and R=Y, Sm) compounds have been studied by measuring the temperature dependence of the easy‐ and hard‐magnetization curves on magnetically aligned samples between 1.5 and 300 K for Y2Co14(B,C) and at 4.2 K for Sm2Co14(B,C). The magnetic anisotropy of Y2Co14B increases due to the substitution of C for B, whereas the saturation magnetization decreases. Between 1.5 and 300 K, the anisotropy field of Y2Co14B0.5C0.5 increases about 2 T and the Co moment decreases about 0.05μB. The anisotropy field of Sm2Co14B also increases upon C substitution and the saturation magnetization decreases slightly. The ac susceptibilities of both Sm2Co14B and Sm2Co14B0.5C0.5 exhibit anomalies that may arise from a spin reorientation within the basal plane.

Magnetization curves have been calculated for single crystals of ferrimagnetic rare‐earth transition‐metal compounds that are free to rotate in the applied magnetic field. The calculations have been performed taking into account the magnetic‐anisotropy constants of the rare‐earth and transition‐metal sublattices up to the fourth order. Seven different types of rotation processes can be distinguished.

Alternating current (ac) susceptibility and high magnetic field measurements were performed to investigate the magnetic phase transitions and the magnetic anisotropy fields in Sm2Fe17CxNy with x=0, 0.4, 0.7, and 0.9. An unidentified magnetic phase transition is observed in all Sm2Fe17CxNy compounds, which is neither due to a spin reorientation nor due to a first‐order magnetization process (FOMP) transition. Samples of Sm2Fe17CxNy (x≳0) have even higher anisotropy fields than are found in Sm2Fe17Ny.

The changes in magnetic and crystallographic properties in the series Th2Fe17Cx and Th2Fe17Nx have been studied. The changes in the latter series were also studied by 57Fe Mossbauer spectroscopy. The ultimate enhancements of the saturation moments in these series are 13% and 24%, respectively. This is much larger than observed previously in the corresponding rare‐earth compounds, although the Curie temperature enhancements are of comparable magnitude. In the Th–Fe–C system a novel compound of the BaCd11 type was found, having a Curie temperature around 370 K.

The composition of rare‐earth cobalt compounds of the hexagonal CaCu5‐structure type varies from RCo5 to approximately RCo6 when passing through the lanthanide series. We have studied the field dependence (up to 35 T) at 4.2 K of the magnetization of several compounds of this series, including various pseudobinary compounds of the type R1−xYxCo5+δ and GdCo5−rNir. The magnetic isotherms at 4.2 K were analyzed with a mean‐field model. The magnetic coupling constant JRT between the moments of the rare‐earth (R) and 3d (T) element in the Hamiltonian H = −Σ2JRTSR⋅ST derived from these measurements was found to have a tendency to decrease with increasing atomic number of the lanthanide involved.

From first‐principles band‐structure calculations, the electric‐field gradient Vzz at Gd nuclei in Gd2Fe14B, Gd2Co14B, Gd2Fe17, Gd2Co17, GdCo5, GdNi5, GdFe3, and elemental Gd has been calculated. The crystal‐field parameter A02 has been calculated for Gd2Fe14B. A02 and Vzz are mainly determined by the asphericity of the charge density of the 5d and 6p valence shells of the Gd atoms. We discuss the validity of a frequently used proportionality relation between A02 and Vzz.

Magnetic properties and specific heat are reported on compounds of the type UTSi, where T is a transition metal. A tendency towards magnetism is found for the compounds with the late transition metals: UCoSi, URuSi, and UIrSi are nonmagnetic, whereas UNiSi, URhSi, UPdSi, and UPtSi are magnetically ordered. This observation agrees well with the expected behavior on the basis of 5f‐d hybridization.

The 57Fe Mossbauer‐effect spectra of the series of hard magnetic materials, R2Fe14C, where R is Nd, Gd, Tb, Dy, Ho, and Lu, have been measured at 295 K. All of these carbides exhibit uniaxial magnetic anisotropy. The spectra resemble those obtained for the related R2Fe14B compounds and have been fit with the model used earlier for Nd2Fe14B. The magnitude of the hyperfine field on each site, as a function of rare earth, parallels the Curie temperature; the maximum hyperfine fields and the maximum Curie temperature are observed for Gd2Fe14C. A linear correlation is observed between the hyperfine fields on the six sites in the analogous borides and carbides; however, these fields are systematically smaller in the carbides. The decrease in the tetragonal unit cell c‐axis length in the carbides apparently reduces the exchange interactions between the 8j and 16k iron layers and hence reduces the moments. The isomer shift on each site decreases as the atomic number of the rare earth increases, whereas the quadru...

The temperature dependence of the anisotropy field of Sm2Fe17C(x) and Tm2Fe17C(x) has been determined over the entire uniaxial ferro- or ferrimagnetic temperature range using the singularity point detection technique. The temperature dependence of the ac initial susceptibility was measured on these two systems from 4.2 to 300 K. The Curie temperature, lattice constants, spin-reorientation temperature (of Tm2Fe17C(x)) and the magnetic anisotropy field are increasing with increasing carbon concentration. A "magnetic-phase-like transition" is observed in Sm2Fe17C(x) for x < 0.7 at low temperatures, which is proved not to be a spin reorientation. Additionally, the transition temperature increases systematically with increasing carbon concentration. However, the nature of this transition is not yet well understood.

The crystallographic and magnetic properties of the compounds Ho2Fe14−xMnxC have been investigated by means of x‐ray diffraction and magnetic measurements. Magnetization measurements made in fields up to 38 T showed that the saturation moment exhibits a minimum at a Mn concentration corresponding to about x=2. The occurrence of this minimum is explained by antiparallel coupling between the Ho and 3d sublattice magnetization, the latter becoming strongly reduced with increasing Mn concentration. Most of the Ho2Fe14−xMnxC compounds investigated have a break in the magnetization curves measured at 4.2 K. This break is interpreted as the onset of decoupling between the antiparallel coupling between the rare‐earth and 3d sublattice. A mean‐field analysis was used to derive the intersublattice molecular‐field coefficients nRT.

59Co NMR was studied in a series of Nd2(Co1−xFex)14B compounds with x=0.001, 0.01, 0.02, 0.05, 0.10 between 4.2 and 77 K. A fast increase of the canting angle between the magnetization and c axis upon Fe substitution has been found. Nonregular local environment effects in the form of rapidly developing satellite structure with significant intensity have been observed and tentatively attributed to the Fe influence on the crystal field experienced by the Nd sublattice.

MnFeP1-xSix compounds with x=0.10,0.20,0.24,0.28,...,0.80,1 were prepared by high-energy ball milling and solid-state reaction. The structural, magnetic, and magnetocaloric properties are investigated as a function of temperature and magnetic field. X-ray diffraction studies show that the samples in the range from x=0.28 to 0.64 adopt the hexagonal Fe2P-type structure with a small amount of second phase which increases with increasing Si content. The samples with lower Si content show the orthorhombic Co2P-type structure. Magnetic measurements show that the paramagnetic-ferromagnetic transition temperatures range from 214 to 377 K. Of much importance is the fact that these compounds do not contain any toxic components and exhibit excellent magnetocaloric properties.

Magnetic measurements are carried out on polycrystalline Th2Fe17Cx (x=0,0.6,0.9,1.1) samples between 4 and 850 K and magnetic field up to 7 T. This wide temperature range covers both the ferromagnetically ordered region and the paramagnetic region. The ordering temperature, but also the magnetization is 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 magnetization is also found to depend upon the carbon concentration. Mean magnetic moment values of 1.79 and 1.98 μB per iron atom are observed for Th2Fe17 and Th2Fe17C1.1, respectively. The effective magnetic moment is found to decrease from 4.40 to 4.09 μB per iron atom for Th2Fe17 and Th2Fe17C1.1, respectively. The increase of the carbon concentration induces a progressive evolution toward a more local...

The effects of Si substitution for As on the magnetic and magnetocaloric properties of MnFeP0.5As0.5−xSix compounds have been investigated. The Si-substituted compounds MnFeP0.5As0.5−xSix with x=0, 0.1, 0.2, and 0.3 have been prepared by high-energy ball milling and solid-state reaction. X-ray diffraction shows that all the MnFeP0.5As0.5−xSix compounds crystallize in the Fe2P-type structure. Magnetic measurements show that the compounds are ferromagnetic with a nonlinear dependence of TC on Si content. All compounds exhibit a field-induced first-order magnetic phase transition. The magnetic-entropy change is derived from the magnetization data by using the Maxwell relations. The maximum magnetic-entropy change observed in this system reaches about 32J∕kgK for a field change from 0to2T for x=0.2, at temperature around 294K.