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...

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A magnetic, neutron diffraction, and Mössbauer spectral study of the Ce2Fe17−xAlx solid solutions

Combining different experimental techniques, investigations in hexagonal $P{6}_{3}/mmc$ Er${}_{2}$Fe${}_{17}$ show remarkable magnetovolume anomalies below the Curie temperature, ${T}_{C}$. The spontaneous magnetostriction reaches $1.6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}2}$ at 5 K and falls to zero well above ${T}_{C}$, owing to short-range magnetic correlations. Moreover, Er${}_{2}$Fe${}_{17}$ exhibits direct and inverse magnetocaloric effects (MCE) with moderate isothermal magnetic entropy $\ensuremath{\Delta}{S}_{M}$, and adiabatic temperature $\ensuremath{\Delta}{T}_{\mathrm{ad}}$ changes [$\ensuremath{\Delta}{S}_{M}\ensuremath{\sim}\ensuremath{-}4.7$ J(kgK)${}^{\ensuremath{-}1}$ and $\ensuremath{\Delta}{T}_{\mathrm{ad}}\ensuremath{\sim}2.5$ K near the ${T}_{C}$, and $\ensuremath{\Delta}{S}_{M}\ensuremath{\sim}1.3$ J(kgK)${}^{\ensuremath{-}1}$ and $\ensuremath{\Delta}{T}_{\mathrm{ad}}\ensuremath{\sim}\ensuremath{-}0.6$ K at 40 K for $\ensuremath{\Delta}H=80$ kOe, respectively, determined from magnetization measurements]. The existence of an inverse MCE seems to be related to a crystalline electric field-level crossover in the Er sublattice and the ferrimagnetic arrangement between the magnetic moments of the Er and Fe sublattice. The main trends found experimentally for the temperature dependence of $\ensuremath{\Delta}{S}_{M}$ and $\ensuremath{\Delta}{T}_{\mathrm{ad}}$ as well as for the atomic magnetic moments are qualitatively well described considering a mean-field Hamiltonian that incorporates both crystalline electric field and exchange interactions. $\ensuremath{\Delta}{S}_{M}(T)$ and $\ensuremath{\Delta}{T}_{\mathrm{ad}}(T)$ curves are essentially zero at $\ensuremath{\sim}$150 K, the temperature where the transition from direct to inverse MCE occurs. A possible interplay between the MCE and the magnetovolume anomalies is also discussed.

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Magnetovolume and magnetocaloric effects in Er2Fe17

Nanocrystalline Nd2Fe17 powders have been obtained by means of high-energy ball milling from nearly single-phase bulk alloys produced by arc melting and high temperature homogenization annealing. The rhombohedral Th2Zn17-type crystal structure of the bulk alloy remains unaltered after the milling process, with almost unchanged values for the cell parameters. However, the severe mechanical processing induces drastic microstructural changes. A decrease of the mean crystalline size down to around 10 nm is observed, giving rise to a considerable augmentation of the disordered inter-grain boundaries. This modification of the microstructure affects the magnetic behaviour of the milled powders, although the magnetic structure remains collinear ferromagnetic. While a unique ferro-to-paramagnetic transition temperature, TC = 339 ± 2 K, is observed in the bulk alloy, the nanocrystalline samples exhibit a more likely distribution of TC values. The latter seems to be responsible for the significant broadening of the temperature range in which magneto-caloric effect is observed, and the lowering of the maximum value of the magnetic entropy change. (Some figures in this article are in colour only in the electronic version)

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Nanocrystalline Nd2Fe17 synthesized by high-energy ball milling: crystal structure, microstructure and magnetic properties.

Rietveld analysis of neutron‐diffraction data from Nd3Fe29−xTx (T=Ti, Cr, and Mn) has been used to determine the location of the substitutional atoms and the magnetic moments. Reanalysis of the T=Ti data confirms that the space group A2/m is a better choice than P21/c, which had previously been used to describe the structure. The Ti atom locations and concentrations remain unaffected in the two space groups, but for the other substituents the refined concentrations are well behaved in A2/m, whereas in P21/c the refinements were unstable due to the symmetry relations between certain substituted sites. The site occupancies are analyzed in terms of steric and environment effects. A possible explanation for the high Curie temperature of the Cr compound is proposed.

Neutron diffraction analysis of Nd3Fe29−xTx (T=Ti, Cr, Mn)

In order to understand the magnetic properties of the substituted rare‐earth‐iron alloys, it is especially important to know the location of the substitutional atoms within the iron lattice. The site distributions of some nontransition‐metal substituents in the substituted Nd2Fe17−x T x alloys have previously been reported. Here we report the site distributions of some transition‐metal substituents (Cu,Zr,Nb,Ti,V) in the Nd2Fe17−x T x alloys and compare them with those of the nontransition‐metal substituted compounds.Rietveldanalysis of neutronpowderdiffraction data indicates that the nontransition‐metal substituents show very similar site affinity at low substituent content. For example Al, Ga, and Si all prefer the 18h sites. The transition‐metal substituents show a more complex site affinity. Ti and V atoms strongly prefer the 6c sites, Cu atoms prefer the 9d and 18f sites, Nb atoms prefer the 6c and 18h sites, and Zr atoms prefer 6c and 18f sites. It was also noted that the site affinity can change if carbon is included in the melting procedure of the sample preparation. The superconducting quantum interference device measurements show that all the substituted compounds have a Curie temperature higher than the unsubstituted parent compound. The relationship between the site distribution of substituents and the magnetic properties of the substituted Nd2Fe17−x T x alloys is discussed.

Site affinity of substituents in Nd2Fe17−xTx (T=Cu,Zr,Nb,Ti,V) alloys

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...

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A magnetic, neutron diffraction, and Mössbauer spectral study of the Nd2Fe17−xAlx solid solutions

Three samples of Nd2Fe16TiCx, with x equal to 0.0, 0.3, and 2.8, with the Th2Zn17‐type rhombohedral structure, have been studied by powder x‐ray and neutron diffraction, magnetic measurements, and Mossbauer spectroscopy. Nd2Fe16Ti and Nd2Fe16TiC0.3 were synthesized by induction melting stoichiometric amounts of the constituent elements, whereas Nd2Fe16TiC2.8 was synthesized by methane‐derived gas phase insertion of carbon into finely ground Nd2Fe16Ti at 600 K. The neutron diffraction determined titanium site occupancies are similar in both Nd2Fe16Ti and Nd2Fe16TiC2.8 in which titanium preferentially occupies the 6c transition metal site. In contrast, the titanium occupancies in Nd2Fe16TiC0.3 are markedly different in that titanium avoids the 6c transition metal site and randomly occupies the other three transition metal sites. This difference in occupancies most likely occurs because the titanium diffusion rate during the quenching of Nd2Fe16TiC0.3 is affected by the presence of carbon in the melt. Even t...

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Zhongbo Hu

Papers

A magnetic, neutron diffraction, and Mössbauer spectral study of the Nd2Fe17−xAlx solid solutions

A magnetic, neutron diffraction, and Mössbauer spectral study of the Ce2Fe17−xAlx solid solutions

Neutron diffraction and Mössbauer spectral study of the Nd2Fe16TiCx solid solutions