ESR study of the Eu2+ g-value in the metallic phase of cubic hexaboride Ca1−xEuxB6 (0.15≲x⩽1.00) (original) (raw)
Related papers
Noncubic symmetry in Ca 1-x Eu x B 6 (0.15>~ x≤ 1.00): An electron-spin-resonance study
2006
The Eu 2+ ͑4f 7 , S =7/2͒ g value in Ca 1−x Eu x B 6 ͑0.15Շ x ഛ 1.00͒ was measured by means of electron spin resonance at two frequencies ͑fields͒, 9.4 ͑Ϸ3.4 kOe͒ and 34.4 GHz ͑Ϸ12.1 kOe͒. The g value was found to be anisotropic and magnetic-field dependent. The amplitude of the anisotropy increases at low temperatures. The observed angular and temperature dependences of the g value suggest tetragonal symmetry caused, presumably, by a distortion along a direction perpendicular to the largest crystal face, the ͓001͔ direction. Due to the platelet shape of the samples, part of the anisotropy of the g value can also be attributed to demagnetization effects. The g values decrease at higher fields, which is interpreted in terms of a two-band model involving an exchange interaction between the localized Eu 2+ 4f 7 electrons with conduction Eu 2+ 5d-like electrons and B 2p-like holes.
Noncubic symmetry in Ca[sub 1−x]Eu[sub x]B[sub 6] (0.15≲x≤1.00): An electron-spin-resonance study
Journal of Applied Physics, 2006
The Eu 2+ ͑4f 7 , S =7/2͒ g value in Ca 1−x Eu x B 6 ͑0.15Շ x ഛ 1.00͒ was measured by means of electron spin resonance at two frequencies ͑fields͒, 9.4 ͑Ϸ3.4 kOe͒ and 34.4 GHz ͑Ϸ12.1 kOe͒. The g value was found to be anisotropic and magnetic-field dependent. The amplitude of the anisotropy increases at low temperatures. The observed angular and temperature dependences of the g value suggest tetragonal symmetry caused, presumably, by a distortion along a direction perpendicular to the largest crystal face, the ͓001͔ direction. Due to the platelet shape of the samples, part of the anisotropy of the g value can also be attributed to demagnetization effects. The g values decrease at higher fields, which is interpreted in terms of a two-band model involving an exchange interaction between the localized Eu 2+ 4f 7 electrons with conduction Eu 2+ 5d-like electrons and B 2p-like holes.
2007
The environment of Gd 3þ =Eu 2þ ð4f 7 ; S ¼ 7 2 Þ in Ca 1Àx R x B 6 ðR ¼ Gd; Eu; 0:0001pxp0:30Þ is studied by electron spin resonance (ESR). For xt0:001 the spectra show Lorentzian shape (insulating phase). As x increases, the spectra present a superposition of Lorentzian and Dysonian resonances (coexistence of insulating and metallic phases). For x\0:01, the line shape becomes pure Dysonian (metallic phase). Thus, the intermediate concentration regime of Ca 1Àx R x B 6 is intrinsically inhomogeneous. These compounds show no weak ferromagnetism.
Magnetic and EPR studies of the EuFe3(BO3)4 single crystal
The European Physical Journal B, 2010
Magnetic and electron paramagnetic resonance (EPR) properties of EuFe3(BO3)4 single crystals have been studied over the temperature range of 300-4.2 K and in a magnetic field up to 5 T. The temperature, field and orientation dependences of susceptibility, magnetization and EPR spectra are presented. An antiferromagnetic ordering of the Fe subsystem occurs at about 37 K. The easy direction of magnetization perpendicular to the c axis is determined by magnetic measurements. Below 10 K, we observe an increase of susceptibility connected with the polarization of the Eu sublattice by an effective exchange field of the ordered Fe magnetic subsystem. In a magnetic field perpendicular to the c axis, we have observed an increase of magnetization at T < 10 K in the applied magnetic field, which can be attributed to the appearance of the magnetic moment induced by the magnetic field applied in the basal plane. According to EPR measurements, the distance between the maximum and minimum of derivative of absorption line of the Lorentz type is equal to 319 Gs. The anisotropy of g-factor and linewidth is due to the influence of crystalline field of trigonal symmetry. The peculiarities of temperature dependence of both intensity and linewidth are caused by the influence of excited states of europium ion (Eu 3+). It is supposed that the difference between the g-factors from EPR and the magnetic measurements is caused by exchange interaction between rare earth and Fe subsystems via anomalous Zeeman effect.
Gd3+ and Eu2+ local environment in Ca1–xEuxB6 (0.0001 ≤ x ≤ 0.30) and Ca1–xGdxB6 (0.0001 ≤ x ≤ 0.01)
Local environment of Gd3+ and Eu2+ 4f7 ions, S = 7/2, in Ca1–xEuxB6 (0.0001 ≤ x ≤ 0.30) and CaxGdxB6 (0.0001 ≤ x ≤ 0.01) is investigated by means of electron spin resonance (ESR). For x ≤ 0.001 the spectra show resolved fine structures due to the cubic crystal electric field and, in the case of Eu, the hyperfine structure due to the nuclear hyperfine field is also observed. The resonances have Lorentzian line shape, indicating insulating host for the Gd3+ and Eu2+ ions. As x increases, the ESR lines broaden due to local distortions caused by the Ca/Gd,Eu ions substitution. For Gd (x ≈ 0.001) and Eu (x ≈ 0.02), the spectra present superposition of Lorentzian and Dysonian resonances, suggesting a coexistence of insulating and metallic hosts for the Gd3+ and Eu2+ ions. The Gd3+ and Eu2+ fine structures are still observable up to x ≈ 0.003 for Gd and x ≈ 0.15 for Eu. For larger values of x the fine and hyperfine structures are no longer observed, the line width increases, and the line shape becomes pure Dysonian anticipating the metallic and semimetallic character of GdB6 and EuB6, respectively. These results clearly show that in the low concentration regime the Ca1–xRxB6 (R = Gd, Eu) systems are intrinsically inhomogeneous. No evidence of weak ferromagnetism (WF) was found in the ESR spectra of either metallic or insulating phases of these compounds, suggesting that, if WF is present in these materials, the Gd3+ and Eu2+ 4f7-electrons are shielded from the WF field.
Gradual transition from insulator to semimetal ofCa1−xEuxB6with increasing Eu concentration
Physical Review B, 2005
The local environment of Eu 2+ (4f 7 , S = 7/2) in Ca1−xEuxB6 (0.003 x 1.00) is investigated by means of electron spin resonance (ESR). For x 0.003 the spectra show resolved fine and hyperfine structures due to the cubic crystal electric field and nuclear hyperfine field, respectively. The resonances have Lorentzian line shape, indicating an insulating environment for the Eu 2+ ions. For 0.003 x 0.07, as x increases, the ESR lines broaden due to local distortions caused by the Eu/Ca ions substitution. For 0.07 x 0.30, the lines broaden further and the spectra gradually change from Lorentzian to Dysonian resonances, suggesting a coexistence of both insulating and metallic environments for the Eu 2+ ions. In contrast to Ca1−xGdxB6, the fine structure is still observable up to x ≈ 0.15. For x 0.30 the fine and hyperfine structures are no longer observed, the line width increases, and the line shape is purely Dysonian anticipating the semimetallic character of EuB6. This broadening is attributed to a spin-flip scattering relaxation process due to the exchange interaction between conduction and Eu 2+ 4f electrons. High field ESR measurements for x 0.15 reveal smaller and anisotropic line widths, which are attributed to magnetic polarons and Fermi surface effects, respectively.
Title Exchange and crystal field effects in the ESR spectra of Eu 2 + in la B 6 Permalink
2007
Electron spin resonance of Eu 2+ ͑4f 7 , S =7/2͒ in a La hexaboride ͑LaB 6 ͒ single crystal shows a single anisotropic Dysonian resonance. From the observed negative g shift of the resonance, it is inferred that the Eu 2+ ions are covalent exchange coupled to the B 2p-like host conduction electrons. From the anisotropy of the spectra ͑linewidth and field for resonance͒, we found that the S ground state of Eu 2+ ions experience a cubic crystal field of a negative fourth order crystal field parameter ͑CFP͒, b 4 = −11.5͑2.0͒ Oe, in agreement with the negative fourth order CFP, A 4 , found for the non-S ground state R hexaborides. These results support covalency as the dominant contribution to the fourth order CFP for the whole R hexaboride family.
Percolation and the Colossal Magnetoresistance of Eu-Based Hexaboride
Physical Review Letters, 2004
Upon substituting Ca for Eu in the local-moment ferromagnet EuB6, the Curie temperature TC decreases substantially with increasing dilution of the magnetic sublattice and is completely suppressed for x ≤ 0.3. The Ca substitution leads to significant changes of the electronic properties across the EuxCa1−xB6 series. Electron microscopy data for x ≈ 0.27 indicate a phase separation into Eu-and Ca-rich clusters of 5 to 10 nm diameter, leading to percolation-type phenomena in the electrical transport properties. The related critical concentration xp is approximately 0.3. For x ≈ 0.27, we observe colossal negative magnetoresistance effects at low temperatures, similar in magnitude as those reported for manganese oxides.
Unusual magnetism of hexaborides
Physica B: Condensed Matter, 2000
Various recent experimental investigations have revealed unusual magnetic properties of hexaborides with divalent cations M>. EuB is ferromagnetic below 16 K and its low-temperature properties show remarkable similarities to those of manganese oxides, exhibiting the phenomenon of colossal magnetoresistance. Close to the phase transition as well as far below the ordering temperature, EuB exhibits anomalous features, that are brie#y discussed. Alkaline-earth hexaborides are close to a metal}insulator transition and it has been found that, in a narrow range of electron doping, an itinerant-type of ferromagnetic order, stable up to temperatures of the order of 600}900 K, is established. This remarkable phenomenon is suspected to be due to the peculiar electronic band structure of these materials.