A. G. Kuchin

Neutron diffraction studies of the magnetic phase transitions in Ce2Fe17 compound under pressure

The influence of hydrostatic pressure (up to 5 kbar) on the magnetic structure of Ce2Fe17 was investigated using neutron diffraction in the temperature range from 2 to 300 K. The existence of a collinear ferromagnetic phase below 95 K with a magnetic moment of Fe, mFe=2.0 μB, was confirmed at ambient pressure. Magnetic peaks present between 95 and 205 K correspond to an incommensurate antiferromagnetic structure with a wave vector changing its value from τ1=0.026 A−1 at 100 K to τ1=0.034 A−1 at 205 K. A helical model is used to describe the magnetic structure. Application of high pressures leads to significant changes of the magnetic structure. The ferromagnetic phase, suppressed in the studied temperature range by pressures higher than 3 kbar, gets substituted by a new incommensurate antiferromagnetic phase. This phase can be described as a superposition of the helical structure with a second antiferromagnetic coupling with propagation vector τ2≈0.078 A−1 at 40 K under pressures above 3 kbar. The correla...

Magnetic and structural properties of Ce2Fe17−xMnx compounds

Abstract The compounds Ce 2 Fe 17− x Mn x ( x =0–3) were synthesized and their structure and magnetic properties were studied. The compounds crystallize in the rhombohedral Th 2 Zn 17 -type structure. Neutron diffraction study of Ce 2 Fe 17− x Mn x solid solutions shows that Mn prefers the 6 c site and avoids completely the 9 d site, whereas the 18 f and 18 h sites are occupied almost randomly. The compositional dependences of the unit cell parameters at 293 K are non-monotonous. The binary compound Ce 2 Fe 17 shows ferromagnetic behavior below Θ=94 K and helicoidal antiferromagnetic structure with T N =206 K. The Neel temperature of the compounds Ce 2 Fe 17− x Mn x decreases almost monotonously with increasing Mn content, whereas Θ( x ) dependence is non-monotonous. Ferromagnetic order disappears in the range x =0.5–1. Probably, the compounds with 1 x ≤2 at low temperatures are inhomogenous ferromagnets consisting of microscopical regions with ferromagnetic ordering. Simultaneously, a short-range helicoidal antiferromagnetic state exists in the latter compounds. A magnetic phase ( x − T ) diagram is presented. The magnetization curves of the compounds with x =0.5–1 show a metamagnetic transition at ∼(4–7) kOe.

High pressure effect on magnetic properties and volume anomalies of Ce2Fe17

We present the results of high pressure studies of magnetization, magnetic phase transitions, and related volume anomalies for the intermetallic compound Ce2Fe17 under hydrostatic pressures up to 10 kbar. The giant negative pressure effect on the ferromagnetic ordering temperature ΘT=94 K was determined: dΘT/dP=−(38±2) K/kbar. This extraordinary decrease of ΘT under pressure is not accompanied by the related pronounced decrease of the saturated magnetization under pressure below 2.5 kbar. The effect of pressure on the Neel temperature TN=206 K is less pronounced: dTN/dP=−(1.7±0.2) K/kbar. A large positive spontaneous volume magnetostriction that is observed below TN is suppressed by the application of high pressure and it disappears above 2.5 kbar. A competition of positive and negative exchange interactions between iron atoms is discussed and is thought to be the main reason for the volume anomalies observed. The role of the f electrons of Ce, their possible hybridization with d states of Fe, and their r...

Original magnetic behaviour observed in RNi5−xCux alloys (R = Pr, Gd or Y)

Abstract The Curie temperatures, spontaneous magnetizations and initial susceptibilities were investigated in the temperature interval from 1.5 to 40 K for the pseudobinary alloys RNi5−xCux, where R = Pr, Gd and Y, x ≤ 2.6. It was discovered that the solid solutions PrNi5−xCux between the PrNi5 and PrCu5 Van Vleck paramagnets are ferromagnets for intermediate Cu concentrations. It was shown that the ferromagnetic ordering in PrNi5−xCux alloys can be explained by concentrational enhancement of 3d band susceptibility which in its turn leads to the enhancement of f-f exchange interactions. The maxima on the concentration dependences of both Curie temperatures for the RNi5−xCux ferromagnets with R = Pr, Gd and susceptibility for the YNi5−xCux Pauli paramagnets are manifestations of 3d band magnetism.

Mechanisms controlling magnetic properties of pseudobinary compounds TbNi5−xMx (M=Cu or Al)

Abstract Magnetic properties and heat capacity of the pseudobinary TbNi 5− x Cu x and TbNi 5− x Al x alloys have been studied for x ⩽2.5 and x ⩽1.5, respectively. The substitution of Cu or Al for Ni decreases sharply the spontaneous magnetic moment, increases the magnetic anisotropy in the “easy” basal plane, and leads to strong magnetic domain wall pinning. The Curie temperature T C in TbNi 5− x Cu x depends on x non-monotonously and has a maximum value at x =1, while T C in TbNi 5− x Al x does not depend on composition up to x =1 and sharply decreases at x >1. These results are explained by the effects of random local crystal fields, band magnetism and heterogeneous polarization of the mixed 3d band.

Sm2Fe17 and Tm2Fe17: electronic structure, magnetic and optical properties

For two intermetallics, the results of experimental measurements of optical and magnetic properties together with calculations in the frame of the LSDA+U method are reported. The calculations of the electronic structure allow one to interpret the curves of optical conductivity extracted from direct experimental ellipsometry measurements of optical constants. A detailed analysis shows that, whereas experimental curves of the optical conductivity for both compounds are similar to each other, the contributions of various interband transitions are slightly different. Also band structure calculations give magnetic structure and values of magnetic moments for each ion in Sm2Fe17 and Tm2Fe17 as well as the total one in good agreement with experimental values if the orbital moment is taken into account. Obtained optical theoretical and experimental results demonstrate good agreement with each other for both intermetallics.

Electronic structure, magnetic, and optical properties of the intermetallic compounds R2Fe17 (R=Pr,Gd)

In this paper we report comprehensive experimental and theoretical investigation of magnetic and electronic properties of the intermetallic compounds Pr2Fe17 and Gd2Fe17. For the first time electronic structure of these two systems was probed by optical measurements in the spectral range of 0.22-15 micrometers. On top of that charge carriers parameters (plasma frequency and relaxation frequency) and optical conductivity s(w) were determined. Self-consistent spin-resolved bandstructure calculations within the conventional LSDA+U method were performed. Theoretical interpetation of the experimental s(w) dispersions indicates transitions between 3d and 4p states of Fe ions to be the biggest ones. Qualitatively the line shape of the theoretical optical conductivity coincides well with our experimental data. Calculated by LSDA+U method magnetic moments per formula unit are found to be in good agreement with observed experimental values of saturation magnetization.

Helimagnetic order in the re-entrant ferromagnet Ce2Fe15.3Mn1.7

Substitution of Mn for Fe in Ce2Fe17 strongly suppresses the ferromagnetic ground state, changing it into an antiferromagnetic one. However, a state of spontaneous magnetization is re-entered in the Ce2Fe17−xMnx compounds when the Mn content reaches x=1.3. Magnetic structures and magnetic interactions in the re-entrant ferromagnetic Ce2Fe15.3Mn1.7 compound have been studied using both microscopic (neutron powder diffraction in magnetic fields of up to 0.7T) and macroscopic (ac susceptibility and magnetization measurements under high pressures of up to 8kbar in magnetic fields of up to 5T) techniques. A spiral magnetic structure distorted by the presence of an in-plane ferromagnetic component is found below TN=182K. Enhancement of the ferromagnetic interactions leads to a steep transformation of the magnetic spiral to a magnetic structure with nonzero spontaneous magnetization below ΘT=59K. This structure is described by a distorted elliptical spiral model with an incommensurate propagation vector along th...

Real disordered crystal structure and Curie temperature of intermetallic compounds Y2Fe17−xMx (M=Si or Al)

Abstract The crystal structure of intermetallic compounds Y 2 Fe 17 , Y 2 Fe 15.3 Al 1.7 and Y 2 Fe 15.3 Si 1.7 was studied by powder X-ray and neutron diffraction. The real disordered structure of the compounds was found to be a disordered variant of the Th 2 Ni 17 -type structure, in which the Y atoms from the 2b sites substitute in part the Fe “dumbbells” on 4f sites and the Fe atoms occupy part of vacant 4e sites. Besides, the Y atoms implant in 2c sites in the iron plane of the compounds, which causes plane distortions and splitting of the 12j Fe sites into two subsites. The substitution of Fe by Si or Al was found to have a preferential character for sites in the order: 12k, 4f, 4e, 6g and 12j. Partial substitution of Al or Si for Fe is established as resulting in an increase in the atomic Fe–Fe spacing in the 4f “dumbbell” site. The Curie temperature of the compounds Y 2 Fe 17 , Y 2 Fe 15.3 Al 1.7 and Y 2 Fe 15.3 Si 1.7 is proportional to the atomic Fe–Fe spacing in the “dumbbell” site.

Effect of pressure on the magnetic properties of YNi5, LaNi5, and CeNi5

The effect of pressure on the electronic structure and magnetic properties of the compounds YNi5, LaNi5, and CeNi5 is studied. Large magnetovolume effects of magnitude dlnχ∕dlnV≃4–7 are found for the magnetic susceptibility χ of these systems at low temperatures. Experimental data and “first principles” calculations of the electronic structure and paramagnetic contributions to the susceptibility indicate that YNi5, LaNi5, and CeNi5 are close to a quantum critical point. It is found that the van Vleck orbital contribution to the magnetic susceptibility is 15–20% in these exchange-enhanced paramagnetic materials and should be taken into account in analyzing experimental data on χ and dlnχ∕dlnV. Calculations of the magnetic-field induced moments for atoms in the unit cell of YNi5 reveal an inhomogeneous distribution of the magnetization density and a nontrivial competition between the spin and orbital moments.