Magnetism and the magnetocaloric effect in PrMn1.6Fe0.4Ge2

The magnetic and magnetocaloric properties of PrMn_1.6Fe_0.4Ge₂around the ferromagnetic transitions T _C ^inter ~ 230 K and T _C ^Pr ~ 30 K have been investigated by magnetisation, ⁵⁷Fe Mössbauer spectroscopy and electron paramagnetic resonance (EPR) measurements over the temperature range 5–300 K. The broad peaks in magnetic entropy around T_C ^inter (intralayer antiferromagnetism of the Mn sublattice to canted ferromagnetism) and T_C ^Pr (onset of ferromagnetic order of Pr sublattice in addition to ferromagnetically ordered Mn sublattice) are typical of second order transitions with maximum entropy values of -ΔS _M ~ 2.0 J/kg K and -ΔS _M ~ 2.2 J/kg K respectively for ΔB = 0–6 T. The EPR signal around T = 48 K of g value g ~ 0.8 is consistent with paramagnetic free ion Pr^3?+?. Below T_C ^Pr ~ 30 K the g value increases steadily to g ~ 2.5 at 8 K as saturation of the Pr^3?+? ion is approached. The EPR measurements indicate additional effects in this system below T ~ 20 K with the appearance of EPR signals of low g value g ~ 0.6.     

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.     

Proton NMR study of α-MnH0.06

Proton nuclear magnetic resonance (NMR) spectra and spin-lattice relaxation rates for the solid solution α-MnH_0.06 have been measured over the temperature range 11-297 K and the resonance frequency range 20-90 MHz. A considerable shift and broadening of the proton NMR line and a sharp peak of the spin-lattice relaxation rate are observed near 130 K. These effects are attributed to the onset of antiferromagnetic ordering below the Néel temperature T_N≈130 K. The proton NMR line does not disappear in the antiferromagnetic phase; this suggests a small magnitude of the local magnetic fields at H-sites in α-MnH_0.06. The spin-lattice relaxation rate in the paramagnetic phase is dominated by the effects of spin fluctuations.     

Optical and EPR Properties of Mn- and Eu-Activated LaMgAl11O19 Phosphor

LaMgAl₁₁O₁₉ phosphor doped with Eu and Mn ions has been prepared by the urea combustion route. The as-prepared phosphor was studied using X-ray diffraction, electron paramagnetic resonance (EPR), diffuse reflectance and photoluminescence studies. The EPR spectra of LaMgAl₁₁O₁₉:Eu, Mn phosphor exhibit signals with the effective g values at g = 1.98, 4.29 and 7.23. The resonance signals at g = 1.98 and 4.29 were attributed to Mn²⁺ ions in tetrahedral and rhombic environment, respectively. The resonance signal at g = 7.23 was attributed to Eu²⁺ ions. The optical spectrum of this phosphor exhibits an intense band in the visible region and this band has been attributed to spin-allowed ⁵E_g → ⁵T_2g transition of Mn³⁺ ions. Upon excitation at 324 nm, the material displays emission in the blue, green and red spectral region.     

Magnetic and Magnetoresonance Properties of the Solid Solution Hg0.5Cd0.4Cr0.1Se

The results of studying the magnetic and magnetoresonance properties of the diluted magnetic semiconductor Hg_0.5Cd_0.4Cr_0.1Se are presented. Microanalysis of the samples shows that the introduction of cadmium and chromium elements into the host HgSe matrix leads to the formation in the crystal of the four-component compound HgCdCrSe with the high chromium content [Cr (18.96 %)] and the three-component compound HgCdSe. The measured temperature dependence of the crystal magnetization illustrates the transition to ferromagnetic ordering at the Curie temperature T _C = 126 K. It is noted that the measured magnetization value points out the indicates the presence of both Cr³⁺ and Cr²⁺ ions in the compound HgCdCrSe, which is responsible for the magnetic and magnetoresonance properties of the sample under test. The electron paramagnetic resonance studies are carried out on the an X-band spectrometer in the temperature range 77 K < T < 300 K. The angular dependences of electron paramagnetic resonance spectra are shown in the paramagnetic and ferromagnetic temperature ranges. As follows from the analysis of experimental data, the aforementioned transition is accompanied by the evolution of the electron paramagnetic resonance spectrum at changing the temperature and the orientation of the sample relative to the static magnetic field in the ferromagnetic temperature range. In the assumption of the g-tensor axial symmetry the components of the latter are determined and the different law of their temperature changing is revealed in the ferromagnetic ordering state of the sample.     

Mössbauer and magnetic characterization of TbRhSn

The intermetallic compound TbRhSn was investigated in detail by X-ray, magnetic susceptibility measurements and ¹¹⁹Sn Mössbauer spectroscopy. This compound undergoes a transition from a paramagnetic to an antiferromagnetic state at T _N = 20.8(2) K. The ¹¹⁹Sn Mössbauer spectrum recorded at 4.2 K can be well fitted as a composition of three subspectra with the same intensities, magnitudes of H _hf, ΔE _Q, and δ _is, in agreement with the model of triangular-like antiferromagnetic arrangements of equal magnetic Tb moments lying in the basal ab-plane deduced from neutron diffraction studies (Szytu?a et al., J Alloys Compd 244:94–98, 1996).     

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.     

Synthesis, Characterization and Reactivity Study of a New Penta-Coordinated Mn(II) Complex

A penta-coordinated Mn(II) compound [dqpMnCl₂] (1) (dqp = 2,6-di-(8-quinoline-yl)-pyridine) has been synthesized and its X-ray crystallographic structure is reported here. Magnetic susceptibility measurements confirmed a high-spin Mn(II) (S = 5/2) center in 1. The X-band EPR spectrum of 1 in dimethylformamide solution exhibits widely distributed transitions in the spectral range from 0 to 700 mT with particularly well-resolved hyperfine lines due to the ⁵⁵Mn (I = 5/2) nucleus. The abundance of highly resolved transition lines in the spectrum facilitated the electron paramagnetic resonance spectral simulation which revealed large zero-field splitting and g-anisotropies. When dissolved, 1 exists in equilibrium with a hexa-coordinated species, the latter probably resulting from disassociation of one chlorido-ligand allowing ligation of two solvent molecules. The redox behavior of 1 was studied and was compared to that of a structural analog for which water oxidation in the presence of a chemical oxidant has been shown. The results from water oxidation trials of 1 are discussed.     

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.     

Observation of magnetic Mn sites in cubic and hexagonal HoMn2: 55Mn NMR study

⁵⁵Mn nuclear magnetic resonance has been measured for both cubic C15 and hexagonal C14 HoMn₂. In the ordered state, we found a high-frequency signal, which can be assigned to magnetic Mn atoms, for both C15 and C14 phases together with a low-frequency signal from non-magnetic Mn atoms. The results of the spin-spin relaxation time T₂ in the ordered state and the NMR spectra in the paramagnetic state are also given to discuss the magnetic instability and the magnetic structure.     

Magnetization and Mössbauer study of double layer perovskites La1.5Ca1.5Mn2 − x Fe x O7 prepared by sol–gel method

Magnetization and Mössbauer studies have been made for understanding magnetic behavior of three double perovskite systems La_1.5Ca_1.5Mn_2???x Fe _x O₇ corresponding to x = 0.05, 0.10 and 0.50. These have been prepared following sol–gel route. Substitution of Fe does not lead to any major change in the tetragonal cell but increased iron leads to greater distortion in octahedral site. The three samples undergo paramagnetic–ferromagnetic transition. Curie temperature (T _c) for the system with 0.05 Fe is ～150 K which is lower than (190 K) for the system without iron; with 0.50 Fe T _c goes below 50 K. Iron goes as Fe^3?+? and replaces Mn in ab plane. With increasing Fe the valence states of Mn get re-distributed in a way that number of the Jahn–Teller ions Mn^3?+? increases and that of the pairs of Mn^3?+?–O–Mn^4?+? experiencing double exchange decreases.     

Characterization of Fe states in dilute 57 Mn implanted SnO 2 film

States of dilute Fe in SnO₂ have been monitored using ⁵⁷Fe emission Mössbauer spectroscopy following implantation of ⁵⁷Mn (T _1/2 = 85.4 s) in the temperature range from 143 K to 711 K. A sharp annealing stage is observed at ~330 K where the Fe^3?+?/Fe^2?+? ratio shows a marked increase. It is suggested that this annealing stage is due to the dissociation of Mn-V_O pairs during the lifetime of ⁵⁷Mn; the activation energy for this dissociation is estimated to be 0.9(1) eV. Fe^3?+? is found in a paramagnetic state showing spin-lattice relaxation rates consistent with an expected T ² dependence derived for a Raman process. In addition, a sharp lined doublet in the Mössbauer spectra is interpreted as due to recoil produced interstitial Fe.     

Nuclear magnetic resonance of 55Mn and 75As in MnAs

Four sets of NMR signals, two each, from ⁵⁵Mn and ⁷⁵As nuclei have been observed. The temperature dependences of ⁵⁵Mn resonances have been studied from 77 to 311 K and that of ⁷⁵As, from 77 K to about 250 K. The results show that there is a phase transition at T₁ ≈ 220 K. This transition may be due to introduction of a local spontaneous distortion in the region of the domain walls in the lattice, resulting in lowering of symmetry at low temperatures. Another possibility is the canting of spins which would lower the magnetic group symmetry. The observed resonances have been assigned to arise from the nuclei at the edge and the centre of the domain walls at temperatures T >T₁ and from two types of wall edges with inequivalent orientation of atomic spins at T < T₁. The isotropic hyperfine field at 0 K obtained by extrapolating the resonance frequencies are 227 and 285.1 kOe at ⁵⁵As nuclei, respectively. The anisotropy in the hyperfine field is nearly zero at ⁵⁵Mn nuclei and about 5.8 kOe at ⁷⁵As nuclei at 0 K.     

Critical nuclear relaxation in cobalt metal

Nuclear magnetic resonance of cobalt metal was investigated in the paramagnetic and ferromagnetic states and in the critical region below T_c. The Knight shift and spin lattice relaxation times were measured in the paramagnetic phase in the solid and liquid states from 1578 K to 1825 K. The resonant frequency, spin-lattice and spin-spin relaxation times were measured in the ferromagnetic phase from room temperature to 1385 K. The main part of (T₁T)^-1 results from fluctuating orbital moments in both phases except near T_c where this process forms the background for critical spin relaxation. The critical exponents for T^-1₁ and for the magnetization in the ferromagnetic state were found to be n' = 0.96 ± 0.07 and β = 0.308 ± 0.012, respectively.     

(139La, 55Mn) NMR and magnetic susceptibility data on microscopic phase separation in La0.9MnO3 single crystal

Magnetic susceptibility of the La_0.9MnO₃ single crystal was measured and its (¹³⁹La, ⁵⁵Mn) NMR spectra were recorded. The data obtained indicate that the areas with an A-type antiferromagnetic order (T _N=140 K) and magnetic moments aligned with the b axis occupy a major part of the sample volume in manganite with a considerable concentration of cationic vacancies; simultaneously, the clusters with a canted magnetic sublattice and ferromagnetic interaction between magnetic moments are formed near the vacancies. Charge distribution in these clusters is materially different from that in the antiferromagnetic areas. Magnetic state and relative concentration of the clusters are discussed.     

Electronic Structure of Fe3+ at a Metal-Binding Site Introduced in Modified Bacterial Reaction Centers

The electronic structure of Fe³⁺ was studied in a mutant that has been modified to bind manganese or iron at a site corresponding to the manganese-binding site of photosystem II (Kálmán et al., Biochemistry 45:13869–13874, 2006). Using electron paramagnetic resonance spectroscopy, the presence of the oxidized state of the bacteriochlorophyll dimer, P^·+, was detected in the light when no metal was added. When iron was bound to the modified reaction centers in the presence of bicarbonate, the contribution of P^·+ was greatly reduced and a signal characteristic of Fe³⁺ was evident. To characterize the electronic structure of this ferric ion, the electron paramagnetic resonance spectrum was measured at X-band at temperatures from 4 to 200 K. The major contribution to the spectrum at 4 K is from Fe³⁺ with a spin 3/2 in a rhombic coordination and E/D ratio of 0.1914 and g _eff values of 6.0, 2.9, and 2.0. As the temperature increases from 4 to 200 K, the signal shifts, with the central g _eff value changing from 2.9 to 2.2. This change with temperature may result from alterations in the interaction with the bicarbonate coordinated to the iron as the temperature increases.     

Incommensurate Phases of the MgSiF6·6H2O Crystals: EPR and Group-Theoretical Studies

The group-theoretical study of the structural phase transition to incommensurate state of MgSiF₆·6H₂O crystals, revealed by the electron paramagnetic resonance (EPR) method, as well as analysis of the EPR results, are presented. The consideration of temperature dependences of Mn²⁺ admixture ion EPR spectrum symmetry and parameters leads to the conclusion that at T _i1 = 370 ± 0.3 K they undergo second-order structural phase transition to incommensurately modulated state, the order parameter of this transition may be the angle of [Mg(H₂O)₆]²⁺ octahedra rotation around crystal C ₃ axis. At temperature decreasing below T _i1 the gradual transformation of plane-wave modulation of lattice displacements into soliton mode occurs, which is interrupted by the first-order phase transition at T _i2 = 343 ± 0.3 K accompanied by abrupt decrease in modulation amplitude. At T _c = 298.5 ± 0.3 K the first-order improper ferroelastic phase transition into monoclinic phase occurs. The group-theoretical analysis of the phase transition at T _i1 in the investigated crystals, carried out for the first time, has shown that the existence of the incommensurately modulated phase is conditioned by the fundamental reasons (presence of Lifshitz invariant). The conclusions of this analysis on the nature of order parameter, the structural motifs of incommensurate phase and the possible character of temperature evolution of the structure are in agreement with the EPR investigation data.     

New aspects on URu2Si2 and CeTIn5 (T=Rh, Ir, Co) observed by high pressure NMR and NQR

NMR and NQR studies on two interesting systems (URu₂Si₂, CeTIn₅) were performed under high pressure. (1) URu₂Si₂: In the pressure range 3.0 to 8.3 kbar, we have observed new ²⁹Si NMR signals arising from the antiferromagnetic (AF) region besides the previously observed ²⁹Si NMR signals which come from the paramagnetic (PM) region in the sample. This gives definite evidence for spatially-inhomogeneous development of AF ordering below T ₀ of 17.5 K. The volume fraction is enhanced by applied pressure, whereas the value of internal field (∼91 mT) remains constant up to 8.3 kbar. In the AF region, the ordered moment is about one order of magnitude larger than 0.03 μB. (2) CeTIn₅: The pressure and temperature (T) dependences of nuclear spin-lattice relaxation rate 1/T ₁ of ¹¹⁵In in CeTIn₅ have shown that the superconductivity (SC) occurs close to an AF instability. From the T dependences of 1/T ₁ and Knight shift below T _c. CeTIn₅ has been found to exhibit non-s wave (probable d wave) SC with even parity and line nodes in the SC energy gap.     

Magnetic properties of the compounds N(CH3)4MnIIMIII(CN)6 ·8 H2O(MIII = Mn,Fe)

In the isostructural cyanobridged chain compounds N(CH₃)₄Mn^IIM^III(CN)₆ · 8H₂O high spin Mn(II) ions couple antiferromagnetically to low spin Mn(III) of Fe(III) ions. The Mn^II–Mn^III compound orders ferrimagnetically below T_N = 28.5 ± 1 K. The tetragonal a and b axes are easy ones for the magnetic moments. In the Mn^II–Fe^III compound antiferromagnetic order occurs below T_N = 9.3 K, with spins aligned along the tetragonal c axis. The compound undergoes a meta-magnetic transition from the antiferromagnetic to a ferrimagnetic phase. This occurs at 2 K for a field H_crit ≈ 1.2 T. The temperature dependence of H_crit, which vanishes at T_N, is followed. The tricritical temperature T¹ is ～ 5 K.     

NMR on 55Mn in the ferromagnetic Ni-substituted manganite La0.6Pb0.4Mn0.86Ni0.14O3

NMR on ⁵⁵Mn in the single-crystal manganite La_0.6Pb_0.4Mn_0.86Ni_0.14O₃ (T _C =242 K), which exhibits metallic conductivity below T _C , is investigated in the temperature range 61–215 K. At low temperatures, together with a line corresponding to the averaged hyperfine field at the ⁵⁵Mn nuclei (the averaging is due to the motion of electronic holes along Mn sites), the NMR spectrum also contains two lines corresponding to localized states Mn⁴⁺ and Mn³⁺. In the temperature range 100–200 K it is found that the complicated NMR spectrum is transformed into a single line on account of a delocalization of the holes in the e _g orbitals of manganese. A comparison of the NMR data with the temperature dependence of the resistivity suggsets that a wide distribution of charge-carrier mobilities exists in the crystal. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 8, 522–527 (25 October 1999)