J.C. Gómez Sal

Magnetic properties of RNi2Si2 compounds (R= rre earth)

Abstract We report an extensive study of the magnetic properties of tetragonal RNi 2 Si 2 compounds (R=Pr, Nd, Gd, Tb, Dy, Ho, Er, Tm), through resistivity, neutron diffraction, susceptibility and magnetization experiments. All compounds exhibit complex incommensurate antiferromagnetic structures, while a transition occurs in TbNi 2 Si 2 between a modulated phase and a simple antiferromagnetic structure, stable at low temperature. The magnitude of the bilinear exchange interactions deviates from the Gennes law and the direction of the ordered magnetic moments presents anomalies across the series, including the probable existence of other types of interactions between the rare earth ions.

Resistivity changes of some amorphous alloys undergoing nanocrystallization

Abstract The electrical resistivity of amorphous alloys with compositions: Fe73.5Nb3Cu1Si13.5B9, Fe86Zr7Cu1B6 and Co80Nb8B12 has been studied in the temperature range from 300 to 1100 K, where crystallization occurs. The products of crystallization and the grain size have been studied by X-ray diffraction. In a first step, all the alloys crystallize with small grains of a few nanometers in diameter (nanocrystalline state), and the resistivity behavior at this process accounts for the difference between the amorphous and nanocrystalline phases. The nanocrystalline phases are: α-Fe-Si, α-Fe and fcc Co for the three compounds studied respectively. A second process, at which grain growth and precipitation of intermetallic compounds and borides takes place, has been found for all the alloys. The resistivity is sensitive, not only to the total transformed sample amount, but to the topological distribution of the crystalline phases, and therefore shows a more complex behavior than other well established techniques, as differential scanning calorimetry. This supplementary information given by the resistivity is also discussed.

Metamagnetism and thermodynamical properties in modulated systems modelisation and application to PrNi2Si2

Abstract A periodic field model taking into account explicitly the crystal field anisotropy is presented, in order to describe the thermodynamical properties and the magnetization process of incommensurate modulated magnetic systems. An application is made to the tetragonal PrNi 2 Si 2 compound, where the crystal field and exchange parameters have been unambiguously determined, from a careful joint analysis of specific heat, magnetization, magnetic susceptibility and inelastic neutron scattering experiments. The ground state is a nonmagnetic singlet well isolated from another singlet and a magnetic doublet as the first two excited states. The existence of an amplitude modulated magnetic structure persisting from T N = 20 K down to 0 K is then well explained. It is shown that the periodic field model does not require any additional parameter to account quantitatively very well for the observed properties in the modulated ordered phase, in particular the variation of the specific heat and the metamagnetic process along the [001] easy magnetization direction.

Magnetic properties of PrNi2Si2

Abstract Resistivity and magnetic measurements as well as neutron diffraction studies have been performed on the tetragonal PrNi2Si2 compound. Below TN = 18 K a colinear modulated antiferromagnetic structure with Qm = (0, 0, 0.87) is observed down to the lowest temperature investigated. Mi is non-magnetic and Pr moments are along the c-axis with a maximum value of 2.6 ± 0.1μB at 5.5 K. Magnetic measurements performed on a single crystal reveal a large uniaxial magnetocrystalline anisotropy. From the anisotropy of the paramagnetic susceptibility, the second-order crystal field parameter V 2 0 = + 190 K . is deduced. The stability of the modulated structure at low temperature should be associated with a crystal field splitting of the Pr3+ multiplet which gives rise to a non magnetic singlet ground state.

Magnetic and electrical properties of GdNi1-xCux compounds

The magnetic properties of the orthorhombic compounds GdNi1-xCux have been studied by means of magnetization, resistivity and neutron diffraction measurements. GdNi and GdNi0.7Cu0.3 show ferromagnetic structures while for GdNi0.4Cu0.6 the authors propose a helimagnetic structure. The link between the macroscopic magnetic properties in the ordered phase and the magnetic structures is also stressed. Comparison with other RNi1-xCux compounds with strong magnetocrystalline anisotropy allows one to clarify the role of the magnetic interactions as well as the importance of the magnetocrystalline anisotropy in all these pseudo-binary compounds.

Magnetic and transport properties of Fe-Zr-B-(Cu) amorphous alloys

FeZrB metallic glasses present magnetic properties that are enhanced compared to the pure FeZr ones. In particular, a large increase of the Curie temperature has been found. Magnetic and Mossbauer measurements show a decrease of the spin-glass character and a parallel homogenization of the hyperfine-field distribution as the boron concentration increases. Resistivity versus temperature measurements show a change in behaviour with B content: in the samples with small amounts of boron, a minimum in the resistivity versus temperature curves appears near the Curie temperature, while samples with high boron content show a low-temperature minimum, characteristic of most metallic glasses. The analysis of the results suggests that the evolution of the magnetic behaviour is related to changes in the density of states at the Fermi level, rather than to changes in the Fe - Fe distances. This is in agreement with published data on the specific heat of FeZr and FeB glasses. The influence of boron is shown to greatly enhance the weak itinerant ferromagnetism of FeZr glasses, leading to stronger ferromagnetic behaviour. The characteristic features of the resistivity are analysed in terms of localization effects on the conduction electrons, which extend to higher temperatures in the low-boron-content alloys.

On the conduction band polarization in metallic systems with a periodic array of localized magnetic moments

It is shown that the indirect exchange between magnetic moments distributed in a periodic array and carried out through the conduction band results from the balance of a ferromagnetic component, static effect, and an antiferromagnetic one, polarization effect. The sign of the coupling mainly depends on the density of states at the Fermi level. For filled bands only the antiferromagnetic component exists (metallic superexchange) and, in particular, only for free electrons both components exactly compensate to each other, remaining the typical Ruderman–Kittel–Kasuya–Yosida ripple due to the oscillating character of the polarization.

Influence of boron on the magnetic and transport properties of FeZr amorphous and nanocrystalline alloys

The magnetic properties and electrical resistivity of amorphous and nanocrystalline FeZr and FeZrB(Cu) alloys are compared in a wide range of temperatures (4 to 1000 K). The addition of boron increases the Curie temperature of the alloys and induces a broad minimum in the resistivity vs temperatures. A first step of crystallization occurs around 700 K in all the alloys, giving rise to /spl alpha/-Fe crystallites of very small size. Small amounts of boron greatly influence the exchange interactions, enhancing the ferromagnetic character of these compounds. >

Crystal structure and magnetic behaviour of nanocrystalline Fe-Nb-Cu-Si-B alloys studied by means of in situ neutron diffraction

Two Fe-Nb-Cu-Si-B alloys, (B9) and (B6), prepared with the isotope, have been analysed using data obtained by means of in situ neutron diffraction. This technique allows one to scrutinize crystallographic phases during thermal treatments, avoiding problems due to sample handling. The B9 sample develops Fe(Si) nanometric crystals (10 nm) with 19 at.% Si in the phase when it is annealed at for one hour. An increase to favours the growth of Fe(Si) grains and the crystallization of other phases, mostly Fe borides. A Rietveld analysis of these phases results in a good reproduction of the nominal composition of the alloy. It also elucidates the crystallographic structure of the Fe(Si) phase. This is similar to the structure, but with some of the Fe atoms occupying some (45%) of the Si 4a sites. The compositions and amounts of the phases derived are in agreement with Mossbauer spectroscopy results for the same sample. Knowledge of the Fe(Si) composition enables one to compare the different magnetic behaviours observed for bulk and nanocrystalline alloys. By contrast, B6 alloy does not show the presence of a Fe(Si) structure, presumably due to the lower amount of Si in the Fe(Si) phase. The thermal expansion of the phases that appear is fairly linear and the corresponding thermal expansion coefficients for the different phases have been extracted. The magnetic structure of the Fe(Si) phase is ferromagnetic collinear, without any trace of antiferromagnetic ordering. The thermal variation of the (1, 1, 1) magnetic peak intensity of the Fe(Si) phase matches well with reported DC magnetization results.

Magnetic ground state of CeNi1-xCux : A calorimetric investigation

We present a detailed specific heat study of the CeNi