Synthesis,structure and magnetic properties of layered manganate Sr4Mn3−xCrxO10 (x=0, 0.2)

The manganates Sr₄Mn_3−xCr_xO₁₀ (x=0 and 0.2) have been synthesized by solid state reaction. Powder X-ray diffraction analysis shows orthorhombic symmetry with space group Cmca for both compounds. The magnetic susceptibility measurements show an antiferromagnetic transition at 192 and 176 K for x=0 and 0.2, respectively. The magnetic susceptibility data were estimated using a model based on spin exchange antiferromagnetic interactions in isolated (Mn⁴⁺) trimer; a paramagnetic contribution due to the chromium ions was added in the case of Cr-doped materials.     

Evidence for the spin-dependent scattering of conduction electrons on Mn2+ ions in Hg1−xMnxTe and Cd1−xMnxSe mixed crystals

Changes in the resistivity of Hg_1?xMn_xTe and Cd_1?xMn_xSe mixed crystals associated with paramagnetic resonance of the Mn²⁺ ions have been observed at liquid helium temperature in a strong magnetic field. The effect was recorded by monitoring the submillimeter radiation induced photoconductivity in a swept magnetic field. An increase in the resistivity associated with EPR of the Mn²⁺ ions is interpreted in terms of the spin- dependent scattering of electrons on magnetic impurities, the spins of which are selectively depolarised by means of paramagnetic resonance. Some additional effects influencing the experiments are also discussed.     

Peculiarities of antiferromagnetic ordering in orthorhombic LiMnO<Subscript>2</Subscript>

Data on the antiferromagnetic ordering in orthorhombic lithium manganite LiMnO₂ are obtained from magnetic-susceptibility, calorimetry, and nuclear magnetic resonance studies. The minimal hysteresis and the absence of jumps in the temperature dependences of the sublattice magnetization M(T) and the magnetic susceptibility near T _N indicate that the ordering occurs through a continuous second-order phase transition. Within the critical temperature range, the M(T?T _N) variation is satisfactorily described by a power-law dependence with a critical exponent β = 0.25(4), which is substantially smaller than that predicted for 3D magnetic systems with isotropic Heisenberg exchange. The band structure of orthorhombic LiMnO₂ is calculated using the LMTO-ASA method. Taking into account the spin states of manganese ions, an adequate pattern is obtained for the density-of-states distribution with an energy gap near the Fermi level (～0.7 eV), which is in agreement with the measured electrical parameters of lithium manganite. The calculations demonstrate that the exchange interactions between Mn³⁺ ions leading to antiferromagnetic ordering are significantly anisotropic. It is found that small paramagnetic regions persist in the manganite below the Néel temperature, and it is concluded that the reason for this is partial structural disordering of LiMnO₂. As a result, a certain fraction of the manganese positions is occupied by lithium ions (Li_Mn) and vise versa (Mn_Li). These defects are not involved in the formation of the ordered magnetic structure and compose a paramagnetic fraction.     

Antiferromagnetic Resonance in GdB<Subscript>6</Subscript>

The electron spin resonance has been measured for the first time both in the paramagnetic phase of the metallic GdB₆ antiferromagnet (T_N = 15.5K) and in the antiferromagnetic state (T < T_N). In the paramagnetic phase below T* ~ 70 K, the material is found to exhibit a pronounced increase in the resonance linewidth and a shift in the g-factor, which is proportional to the linewidth Δg(T) ~ ΔH(T). Such behavior is not characteristic of antiferromagnetic metals and seems to be due to the effects related to displacements of Gd³⁺ ions from the centrosymmetric positions in the boron cage. The transition to the antiferromagnetic phase is accompanied by an abrupt change in the position of resonance (from μ₀H₀ ≈ 1.9 T to μ₀H₀ ≈ 3.9 T at ν = 60 GHz), after which a smooth evolution of the spectrum occurs, resulting eventually in the formation of the spectrum consisting of four resonance lines. The magnetic field dependence of the frequency of the resonant modes ω₀(H₀) obtained in the range of 28–69 GHz is well interpreted within the model of ESR in an antiferromagnet with the easy anisotropy axis ω/γ = (H ₀ ² +2H_AH_E)^1/2, where H_E is the exchange field and H_A is the anisotropy field. This provides an estimate for the anisotropy field, H_A ≈ 800 Oe. This value can result from the dipole?dipole interaction related to the mutual displacement of Gd³⁺ ions, which occurs at the antiferromagnetic transition.     

Anomalies in the magnetic properties of copper impurity clusters in barium fluoride crystals

Copper impurity trimers with antiferromagnetic coupling between Cu²⁺ ions are synthesized in BaF₂ crystals. In some of the BaF₂: Cu crystals thus prepared, the concentration of copper stable trimers turns out to be higher than that of copper dimers and copper single centers. The BaF₂: Cu crystals are studied using electron paramagnetic resonance (EPR) spectroscopy at frequencies of 9.3, 23.0, and 37.0 GHz in the temperature range 4.2–77 K. It is found that the components of the effective tensor g for copper trimers depend strongly on the external magnetic field and temperature. A static model of the molecular structure of the copper trimer is proposed on the basis of the experimental data obtained.     

Antiferromagnetic Ordering of Magnetic Clusters Units in Nb6F15

We have studied the magnetic cluster compound Nb₆F₁₅ which has an odd number of 15 valence electrons per (Nb₆F₁₂)³⁺ cluster core, as a function of temperature using nuclear magnetic resonance, magnetic susceptibility, electron magnetic resonance and neutron powder diffraction. Nuclear magnetic resonance of the ¹⁹F nuclei shows two lines corresponding to the apical F^a?a nucleus, and to the inner Fⁱ nuclei. The temperature dependence of the signal from the Fⁱ nuclei reveals an antiferromagnetic ordering at T < 5 K, with a hyperfine field of ~2 mT. Magnetic susceptibility exhibits a Curie–Weiss behavior with T _N ~5 K, and μ _eff ~1.57 μ_B close to the expected theoretical value for one unpaired electron (1.73 μ_B). Electron magnetic resonance linewidth shows a transition at 5 K. Upon cooling from 10 to 1.4 K, the neutron diffraction shows a decrease in the intensity of the low-angle diffuse scattering below Q ~0.27 Å^?1. This decrease is consistent with emergence of magnetic order of large magnetic objects (clusters). This study shows that Nb₆F₁₅ is paramagnetic at RT and undergoes a transition to antiferromagnetic order at 5 K. This unique antiferromagnetic ordering results from the interaction between magnetic spins delocalized over each entire (Nb₆F ₁₂ ⁱ )³⁺ cluster core, rather than the common magnetic ordering.     

Variation du champ seuil avec une tension uniaxiale dans FeCl2 A 4,2°K

In FeCl₂, at low temperature, the Fe²⁺ ions present an antiferromagnetic order characterised by an easy magnetization axis perpendicular to the ion-containing planes. There exists a value h_s of a magnetic field parallel to the axis, which transforms the system from an antiferromagnetic order into a saturated paramagnetic order. There we present the variations of h_s at 4·2°K as well as the deformations of the sample, when we impose upon it a strain parallel to the axis without allowing its lateral dilatation. These results, when added to those already obtained with χ_i in similar experiments enable us to determine the variations of the crystalline anisotropy and anti-ferromagnetic exchange (between Fe²⁺ ions belonging to two consecutive planes) in terms of the deformations. Our results concerning h_s are different from the ones obtained when exerting a hydrostatic strain. We compare the two cases by using the relations between elastic constants of FeCl₂ obtained at 300°K and recorded in the Appendix.     

EPR and optical absorption studies of Fe ions in sodium borophosphate glasses

Electron paramagnetic resonance (EPR) and optical absorption spectral investigations have been carried out on Fe³⁺ ions doped sodium borophosphate glasses (NaH₂PO₄-B₂O₃-Fe₂O₃). The EPR spectra exhibit resonance signals with effective g values at g=2.02, g=4.2 and g=6.4. The resonance signal at g=4.2 is due to isolated Fe³⁺ ions in site with rhombic symmetry whereas the g=2.02 resonance is due to Fe³⁺ ions coupled by exchange interaction in a distorted octahedral environment. The EPR spectra at different temperatures (123-295 K) have also been studied. The intensity of the resonance signals decreases with increase in temperature whereas linewidth is found to be independent of temperature. The paramagnetic susceptibility (χ) was calculated from the EPR data at various temperatures and the Curie constant (C) and paramagnetic Curie temperature (θ_p) have been evaluated from the 1/χ versus T graph. The optical absorption spectrum exhibits bands characteristic of Fe³⁺ ions in octahedral symmetry. The crystal field parameter (Dq) and the Racah interelectronic repulsion parameters (B and C) have also been evaluated and discussed.     

High-field EPR on the cyclic spin cluster Fe6(tea)6

The eigenstates of the antiferromagnetic spin cluster Fe₆(tea)₆ are studied by electron paramagnetic resonance spectroscopy at 9.5 and 94 GHz in order to characterize the interactions which are important for the magnetic properties of the molecule. Experimental results could be obtained for the first excited spin states withS= 1, 2, 3 and 4. This enables a very accurate and detailed analysis of the intramolecular interactions. It turns out that the magnetic properties of Fe₆(tea)₆ are determined by the isotropic exchange (J/k_B = ?31.5 K), the ligand-field interaction of the Fe^III ions and the dipolar interaction. Other kinds of anisotropic interactions are of minor importance.     

Study of the Structures of the Tetragonal Paramagnetic Centers in the Mixed Fluorite Crystals with Rare-Earth Ions by EPR

Electron paramagnetic resonance (EPR) of the mixed fluorite crystals with the general formula (MeF₂)_1?x?y (REF₃) _x (RF₃) _y (Me = Ca, Sr, Ba; R = Y, La, Lu; RE—paramagnetic trivalent rare-earth ions) were studied comprehensively by different authors and several structural models of paramagnetic centers were considered. However, a lot of details of EPR spectra still remain unexplained. In this work some modifications of the simplest models are proposed which allow explaining adequately the variety of the tetragonal centers in crystals grown under the different conditions. The calculated from the proposed models components of g-factors for Ce³⁺, Nd³⁺, Sm^3+, Er³⁺ ions are in a good agreement with the experimental values.     

Structural transformations and phase transitions in aqueous solutions of gadolinium nitrate in the course of freezing

The microstructure of Gd(NO₃)₃ aqueous solutions in the course of cooling at different rates is investigated using electron paramagnetic resonance. The local concentrations of Gd³⁺ ions in microregions are estimated, the concentration boundary at which the solution is completely vitrified throughout its volume is revealed, and the temperature of vitrification of liquid microregions (eutectic temperature) and the eutectic composition are determined. It is found that the adsorption of Gd³⁺ aqua ions on the capillary surface affects the crystallization of water and the process of concentration of the solute. It is demonstrated that the anomalous experimental dependence δH(C₀), which is observed for vitrified water-glycerol solutions with a uniform distribution of paramagnetic centers over the sample, agrees well with the theoretical curve in the case when allowances are made not only for the dipole-dipole interactions but also for the inhomogeneous broadening of the EPR lines in the spectra with an incompletely resolved fine structure.     

EPR spectra of Fe centers in layered TlGaSe2 single crystal

A single-crystal TlGaSe₂ doped by paramagnetic Fe ions has been studied at room temperature by electron paramagnetic resonance (EPR) technique. The fine structure of EPR spectra of paramagnetic Fe³⁺ ions was observed. The spectra were interpreted to correspond to the transitions among spin multiplet (S=5/2, L=0) of Fe³⁺ ion, which are splitted by the local ligand crystal field (CF) of orthorhombic symmetry. Four equivalent Fe³⁺ centers have been observed in the EPR spectra and the local symmetry of crystal field at the Fe³⁺ site and CF parameters were determined. Experimental results indicate that the Fe ions substitute Ga at the center of GaSe₄ tetrahedrons, and the rhombic distortion of the CF is caused by the Tl ions located in the trigonal cavities between the tetrahedral complexes.     

NMR studies of magnetic properties in Heusler-type Mn 3 Si

In order to microscopically investigate the magnetic properties of both paramagnetic and antiferromagnetic phases in Mn₃Si (T _N?=?23 K), the ⁵⁵Mn NMR has been carried out at temperatures between 2.2 K and 300 K. The temperature dependences of the spectrum, Knight shift (or resonance frequency shift) and spin-lattice relaxation time T ₁ of ⁵⁵Mn NMR have been measured. In the paramagnetic phase, only one resonance spectrum can be obtained. The observed spectrum is identified to be a signal corresponding to the Mn(II) site. In the antiferromagnetic phase, two different spectra corresponding to the Mn(I) and Mn(II) sites are found at the resonance frequencies of 145 and 6 MHz, respectively, by the zero field NMR at 4.2 K. From these results, the internal magnetic fields on the ⁵⁵Mn(I) and ⁵⁵Mn(II) nuclei are found to be 13.6 and 0.6 T, respectively. According to the NMR results, the helical structure in incommensurate Mn spin states is better explained compared with the transverse sinusoidal structure.     

Crystal growth and magnetic properties of tin selenide-doped europium Sn1−xEuxSe

The growth and magnetic properties of Tin Selenide (SnSe) doped with Eu²⁺ Sn_1?xEu_xSe (x=2.5%) were investigated. Q-band (34 GHz) electron paramagnetic resonance measurements show that the site symmetry of Eu²⁺ at 4.2 K is orthorhombic and the Landé factor was determined to be g=1.99±0.01. The exchange coupling between nearest-neighbor (NN) Eu²⁺ ions was estimated from magnetization and magnetic-susceptibility measurements using a model that takes into account the magnetic contributions of single ions, pairs and triplets. The exchange interaction between Eu²⁺ nearest neighbors was found to be antiferromagnetic with an estimated average value of J_p/k_B=?0.18±0.03 K.     

A search for magnetic resonance of ruthenium in octahedral coordination with oxygen

We have searched, without success, for microwave-frequency magnetic resonance of Ru⁴⁺ and Ru⁵⁺ ions in a number of materials for which the Ru are in octahedral coordination with six oxygen. A number of ruthenates including RuO₂, SrRuO₃, Sr₃Ru₂O₇, Ba₃Ru₂NiO₉, Ba₂GdRuO₆, Sr₂YRuO₆, and Ba₂YRuO₆, which include paramagnetic, antiferromagnetic, and ferromagnetic spin configurations, have been examined. We present analysis which shows that the last material provides an optimized opportunity to detect antiferromagnetic Ru resonance for temperatures less than T_N=39 K; none is detected for frequencies as high as 35 GHz in magnetic fields up to μ_oH=2 T. This result indicates that the antiferromagnetic magnon energy gap exceeds the energy associated with the signal frequency. SrRuO₃ is a known ferromagnetic contaminant phase in the rutheno-cuprates. We report neutron diffraction measurements on SrRuO₃, finding it to have an appreciable local moment at low temperatures, 1.25(0.1)μ_B; this moment vanishes near 165 K. We show that it also fails to exhibit ferromagnetic resonance, at least in the range 10-35 GHz. As a result of the diffraction and resonance studies, it is concluded that the reports of ferromagnetic resonance in superconducting rutheno-cuprates are actually due to antiferromagnetically ordered Cu in these materials, and the presence of even a few percent of SrRuO₃ as a potential contaminant is of little importance.     

Influence of zeolite water on paramagnetic and ferromagnetic resonances in the Co2[Nb(CN)8] · 8H2O molecular magnet

The contributions of Co²⁺ and Nb⁴⁺ ions to the high-frequency dynamic magnetic susceptibility of the Co₂[Nb(CN)₈] · 8H₂O molecular magnet in the paramagnetic state at T > 12 K are separated. It is found that the ferromagnetic ordering, which leads to the reconstruction of the electron paramagnetic resonance spectrum into the ferromagnetic resonance spectrum, occurs at T < 12 K. The influence of zeolite water on the spectra of the paramagnetic and ferromagnetic resonances is found. Dehydration leads to a decrease in the time of the spin relaxation of the ferromagnetic system from 50 ps to 17 ps at T = 4 K and to the variation in the temperature dependences of the widths of the lines and g factors in the electron spin resonance spectra.     

Temperature dependence of the EPR lines in weakly doped LiNbO3:Yb—possible evidence of Yb ion pairs formation

The electron paramagnetic resonance (EPR) studies of LiNbO₃ single crystal doped with 1 wt% of Yb³⁺ are reported. To put the EPR results in perspective, a brief discussion of optical absorption spectroscopy investigations of LiNbO₃:Yb³⁺ is provided. The temperature behavior of the EPR lines intensity and linewidth for LiNbO₃:Yb³⁺ reveals antiferromagnetic coupling between Yb³⁺ ions. The deconvolution of the EPR lines indicates that EPR signals arise from both the isolated Yb³⁺ ions as well as the Yb³⁺-Yb³⁺ ion pairs; the latter signals dominate. Based on this indication, EPR spectra are interpreted using a spin Hamiltonian for the Yb³⁺ dissimilar ion pairs. The negative sign of the isotropic parameter J confirms the existence of the antiferromagnetic interactions within Yb³⁺-Yb³⁺ pairs. The value of J obtained based on the proposed pair model, assuming the dipole-dipole interactions, is used to identify the positions of the Yb³⁺-Yb³⁺ pairs in the unit cell. Our results suggest the ^evenYb³⁺-^evenYb³⁺ pairs are located at the neighboring Li⁺ and Nb⁵⁺ positions, whereas the pair axis is not parallel to the optical c-axis. Some alternative explanations of the observed EPR spectra are also considered.     

Evaluation of exchange interaction in binuclear copper(II) chelates of amide-based cyclophanes

Temperature dependence of the electron paramagnetic resonance (EPR) spectra of binuclear Cu(II) complexes with amide-based cyclophanes, Cu₂(bis-EDTAPDN), Cu₂(bis-EDTANAP) and Cu₂(bis-EDTABPE), in 60% methanol (pH ca. 10) has been studied at a frequency of 9.43 GHz in the temperature range of 77–4.2 K; the cyclophanes are macrocyclic compounds in which four amide bonds link two ethylenediaminetetraacetate (EDTA) units and two units of the aromatic diamine 1,4-phenylenediamine (DPN), 1,5-diaminonaphthalene (NAP) or bis(4-aminophenyl) ether (BPE). Exchange integrals found from the EPR measurements are ca. 70–105 cm^?1, 22 cm^?1 andJ ≤ 0, for Cu₂(bis-EDTAPDN), Cu₂(bis-EDTANAP) and Cu₂(bis-EDTABPE), respectively; the sign ofJ is defined to be positive for antiferromagnetic interaction. The variation of exchange interactions in these compounds is related to the nature of the bridging organic groups.     

Propriétés magnétiques et relaxation magnétique nucléaire de Be9 dans des composés intermétalliques de terres rares: GdBe13, TbBe13 et DyBe13

Be⁹ nuclear magnetic relaxation, magnetic susceptibilities and magnetic moments measurements of intermetallic compounds GdBe₁₃, TbBe₁₃ and DyBe₁₃ have been performed. The relaxation rate is mainly due to a dipolar interaction between the nuclear moments and the paramagnetic ions moments. Antiferromagnetic ordering was observed for all compounds. Curie-Weiss behaviour was observed at high temperature. The effective moments are comparable to that expected from the three positive ions.     

Pressure-induced electron spin transition in the paramagnetic phase of the GdFe<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript> Heisenberg magnet

The HS → LS spin crossover effect (high-spin → low-spin transition) induced by high pressure in the range 45–53 GPa is observed in trivalent Fe³⁺ ions in the paramagnetic phase of a Gd⁵⁷Fe₃(BO₃)₄ gadolinium iron borate crystal. This effect is studied in high-pressure diamond-anvil cells by two experimental methods using synchrotron radiation: nuclear resonant forward scattering (NFS) and Fe K _β high-resolution x-ray emission spectroscopy (XES). The manifestation of the crossover in the paramagnetic phase, which has no order parameter to distinguish between the HS and LS states, correlates with the optical-gap jump and with the insulator-semiconductor transition in the crystal. Based on a theoretical many-electron model, an explanation of this effect at high pressures is proposed.