Nuclear spin relaxation in itinerant electron antiferromagnetic Cr1−xVx system

Nuclear spin relaxation rate T^?1₁ for ⁵¹V in an incommensurate antiferromagnetic Cr_1?xV_x system has been measured in a temperature range between 1.3 and 4.2 K and in a range of magnetic field from 0 to 13.3 kOe by using a field-cycling nuclear magnetic resonance technique. In the (T₁T)^?1 vs x curve a pronounced maximum was observed near the critical concentration (x_c～0.040). Furthermore for alloys with x = 0.038 and 0.040 a deviation from the Korringa relation, T₁T = constant, was observed. The experimental results of (T₁T)^?1 are interpreted in terms of the spin-fluctuation and d-orbital contributions.     

Phenomenological trends for heavy fermi liquids and narrow-band metals III: Characteristic temperatures

A correlation between the ratio of the Fermi temperature T_F¹ and the Debye temperature θ_D and the occurrence of superconductivity or magnetic order is described. Values of T_F¹/θ_D⪢ 1 are found to favor “conventional” superconductivity, whereas T_F¹/θ_D≲ 1 favors magnetic order. “Heavy fermion” superconductivity is found to be restricted to the case T_F¹/θ_D < 1.     

Magnetic properties of Heisenberg triangular antiferromagnet V15 cluster studied by NMR

The magnetic properties of an s?=?1/2 Heisenberg triangular antiferromagnet V15 have been studied by proton nuclear magnetic resonance (NMR) at very low temperature down to 100 mK using a He³-He⁴ dilution refrigerator. In total spin S _T?=?3/2 ground state above 2.7 Tesla, proton spin-lattice relaxation rate (1/T₁) shows thermally activated behavior as a function of temperature. On the other hand, a temperature independent behavior of 1/T₁ at very low temperatures is observed in frustrated S _T?=?1/2 ground state below 2.7 Tesla. Possible origins for the peculiar behavior of 1/T₁ will be discussed in terms of magnetic fluctuations due to spin frustration.     

Magnetic circular dichroism of transition metal ions in solids-I. MgO:Cr3+

Absorption and magnetic circular dichroism (MCD) spectra of MgO: Cr³⁺ have been studied over the temperature range 5–300 °K and with magnetic fields up to 77 kG. A theory for intensity calculation using a weak field coupling scheme is discussed. Spin-forbidden transitions to ²E and ²T₁, are observed in MCD and are interpreted as magnetic dipole zero-phonon lines. The spin allowed bands ⁴T₂ and ⁴T₁ are found to be vibration-induced electric dipole transitions in which the t_1u lattice modes are the dominant contributors to the intensity.     

Magnetic properties and peculiarity of magnetic states in dilute antiferromagnets Mn1−x Zn x F2

The dependence of magnetic moment and susceptibility on temperature, magnetic field and frequency of some single crystals Mn_1?x Zn _x F₂ (x≈x _e=0.75—percolation limit) were experimentally investigated. Our experiments show that (Bazhan and Petrov 1984; Cowleyet al 1984; Villain 1984) in these crystals the nonequilibrium magnetic state of spinglass type with finite correlation length appears as temperature decreasesT<T in weak magnetic fields. This state is determined by fluctuation magnetic moments √nμ (wheren is the number of magnetic ions, corresponding to finite correlation length andμ the magnetic moment Mn⁺¹). In the experiments in low magnetic fields and frequencies there are no peculiarities in the magnetic susceptibility temperature dependence atT≠T _f. At temperaturesT>T _f andT<T _f magnetic susceptibility is determined by 1 $$\chi \left( {T > T_f } \right) = \frac{{N\left\langle \mu \right\rangle ^2 }}{{3k\left( {T + \theta } \right)}} = \frac{N}{n}\frac{{\left\langle {\sqrt n \mu } \right\rangle ^2 }}{{3k\left( {T + \theta } \right)}} = \chi \left( {T< T_f } \right)$$ . In strong magnetic fields and large frequencies there are peculiarities in thex(T) dependence atT=T _f. AtT<T _f and strong magnetic fieldsX(T)=x ₀ andT<T _f and at large frequenciesx(T)=x ₀+α/T. The dependences of magnetic susceptibility on the frequency are determined by the magnetic system relaxation. Calculations and comparison with experiments show that the relaxation of the investigated magnetic systems atT<T _f follows the relaxation lawM(t)=M(0) exp[?(t/τ) ^r ], suggested in Palmeret al (1984) for spin-glasses relaxation taking into account the time relaxation distributionτ ₀....τ _max in the system and its ‘hierarchically’ dynamics.     

Nuclear magnetic resonance studies of CePd2In and LaPd2In at low temperatures

We report on In nuclear magnetic resonance (NMR) and nuclear quadrupolar resonance (NQR) measurements on the new heavy-electron compound CePdZIn over a wide temperature range, from 45 mK up to 30 K. CePd₂In undergoes an antiferromagnetic (AF) phase transition at T _N = 1.23 K involving small localised Ce moments of 0.11 μ_B. In zero field, the spin-lattice relaxation rate T ₁ ^?1 (T) shows remarkable changes in its temperature dependence. Above 3 K, T ₁ ^?1 is constant and 850 sec^?1. Between T _N and 2T _N, (T ₁ T)^?1 = 330 (Ksec)^?1, but rapidly decreases below T _N. A Korringatype relaxation, characteristic for simple metals at low temperatures, with (T ₂ T)^?1 = 17(Ksec)¹ is resumed below 0.6 K. This value is an order of magnitude larger than (T ₁ T)^?1 for LaPd₂In and therefore is associated with low-energy excitations of the itinerant charge carriers with 4f symmetry. The T ₁ ^?1 data at various non-zero magnetic fields fall on a single curve when plotted as a function of (T/H) if H exceeds 3.5 T. Thus the AF ordering, the 4f moment fluctuations and the Kondo screening are drastically suppressed by the application of fields H of the order of 3.5 T.     

Room temperature magnetic entropy change and magnetoresistance in La0.70(Ca0.30−xSrx)MnO3:Ag 10% (x=0.0−0.10)

The magnetic and magnetocaloric properties of polycrystalline La_0.70(Ca_0.30−xSr_x)MnO₃:Ag 10% manganite have been investigated. All compositions are crystallized in single phase orthorhombic Pbnm space group. Both, the insulator–metal transition temperature (T^IM) and Curie temperature (T_c) are observed at 298 K for x=0.10 composition. Though both T^IM and T_c are nearly unchanged with Ag addition, the MR is increased. The MR at 300 K is found to be as large as 31% with magnetic field change of 1 T, whereas it reaches up to 49% at magnetic field of 3 T for the La_0.70Ca_0.20Sr_0.10MnO₃:Ag_0.10 sample. The maximum entropy change (ΔS_Mmax) at near its T_c (300.5 K) is 7.6 J kg⁻¹ K⁻¹ upon the magnetic field change of 5 T. The La_0.70Ca_0.20Sr_0.10MnO₃:Ag_0.10 sample having good MR (31%^1 T, 49%^3 T) and reasonable change in magnetic entropy (7.6 J kg⁻¹.K⁻¹, 5 T) at 300 K can be a potential magnetic refrigerant material at ambient temperatures.     

Spatially modulated magnetic structure of AgFeO2: Mössbauer study on 57Fe nuclei

The results of the Mössbauer study of ferrite AgFeO₂ manifesting multiferroic properties (at T ≤ T _N2) have been presented. The hyperfine interaction parameters of ⁵⁷Fe nuclei have been analyzed in a wide temperature range including the points of two magnetic phase transitions (T _N2 ≈ 7–9 K and T _N1 ≈ 15–16 K). It has been shown that the Mössbauer spectra of the ⁵⁷Fe nuclei are sensitive to the variations of the character of the magnetic ordering of Fe³⁺ ions in the studied ferrite. The results of the model identification of a series of spectra (4.7 K ≤ T ≤ T _N2) under the assumption of the cycloid magnetic structure of ferrite AgFeO₂ have been presented. The analysis of the results has been performed in comparison with the literature data for other oxide multiferroics.     

Effect of antiferromagnetic ordering on the optical transitions in MnF2

Two optical absorption bands (viz. C and F) in MnF₂, whose line positions are nearly temperature-independent above T_N = 67.3 K, are investigated for the effect of magnetic ordering. Both bands are blue-shifted by ΔE (≌90 cm^?1 for C and 120 cm^?1 for F at 10 K) and the lines show additional narrowing on cooling through T_N. It is found that ΔE varies as the sublattice lattice magnetization below T_N and nearly as the magnetic energy above T_N.     

Investigation of the GdMn2O5 multiferroic by the μSR method

The GdMn₂O₅ multiferroic (a ceramic sample and a sample consisting of a large array of randomly oriented single crystals with linear dimensions 2–3 mm) has been studied by the μSR method within the temperature range 10–300 K. Three anomalies in the temperature behavior of the parameters of the muon polarization relaxation function, namely, close to the phase transition driven by the onset of long-range magnetic order in the manganese ion subsystem (T _N1 = 40–41 K), near the lock-in transition initiated by an abrupt change of the wave vector of magnetic order (T _L = 35 K), and close to the Gd³⁺ ion ordering temperature (T _N2 = 15 K), have been found. An analysis of the time spectra of muon spin precession in the internal magnetic field of the samples has revealed two positions of preferable muon localization sites in samples, which differ in precession frequencies and the character of their behavior with temperature. The lower-frequency precession driven by Mn⁴⁺ ions, ferromagnetic Mn4⁺-Mn⁴⁺ + muonium complexes, and Gd³⁺ions is observed throughout the temperature region T < T _N1 and is practically independent of temperature. At temperatures T < T _L = 35 K, a higher-frequency precession associated with Mn³⁺ ions appears also. It is characterized by a temperature dependence ～(T/T _N1)^β with the index β = 0.39, which is typical of Heisenberg-type 3D magnets. For T < T _N1, a deficiency of the rest total asymmetry is observed. This phenomenon can probably be assigned to formation of muonium, which suggests that charge transfer processes play an important role in formation of long-range magnetic order.     

Relaxation phenomena and nuclear magnetic resonance of116In(T 1/2=14s) produced by capture of polarized neutrons in the indium III–V compounds

Polarized¹¹⁶In nuclei have been produced by capture of polarized thermal neutrons in several In compounds. At temperaturesT below 77 °K and magnetic field strengthsH ₀ of several kOe, asymmetries of a few percent of the β^? decay of the¹¹⁶In ground state could be observed in polycrystalline InP, InAs and InSb, thus indicating the nuclear polarization. Nuclear magnetic resonance signals have been measured with the result for the magnetic moment μ _i (¹¹⁶In)=2.7723 (10) nm (uncorrected). β^? decay asymmetry and spin lattice relaxation timeT ₁ have been studied as a function ofH ₀ andT. The effect ofH ₀ is to decouple the hyperfine interaction caused by the capture-γ recoil process. However,H ₀ has no influence uponT ₁, which demonstrates the absence of nuclear relaxation due to paramagnetic impurities.T ₁ is determined by quadrupolar relaxation. A quadrupole momentQ(¹¹⁶In)=0.09 (2) b was calculated by comparison of the¹¹⁶In relaxation rates with those of the stable¹¹⁵In isotope in the same compounds. Above 30 °K the temperature dependence of 1/T ₁ agrees with a recent theoretical investigation. Below 30 °K the relaxation rate shows an anomalous behaviour, which can be explained by resonance modes due to recoil lattice defects.     

Quasiparticle lifetime of electron–electron interactions for two-dimensional electron gases with magnetic field

We theoretically study the electron–electron scattering rate τ_ee⁻¹for electrons in a two-dimensional electron gas with a perpendicular magnetic field, within theGWand plasmon-pole approximations, as functions of temperatureT, impurity scattering rate Γ and magnetic fieldB. The τ_ee⁻¹increases with increasingTand increasing Γ, and shows the structure of the Landau levels asBis changed.     

57Fe Mössbauer study of new multiferroic AgFeO2

The present work reports results of the ⁵⁷Fe Mössbauer measurements on AgFeO₂ powder sample recorded at various temperatures including the points of both magnetic phase transitions. The ⁵⁷Fe Mössbauer spectra of AgFeO₂ measured in the paramagnetic range (T > T _N1) consist of one quadrupole doublet with rather high quadrupole splitting of Δ_300K = 0.66 ± 0.01 mm/s for Fe³⁺ ions. In order to predict the sign of electric field gradient (EFG) at ⁵⁷Fe nuclei, we calculated the lattice contribution to the electric field gradient (EFG) at ⁵⁷Fe nuclei, which emphasized the importance of the dipolar contributions, with resultant oxygen polarizabilities in the range of α _O = 0.83 ų, in agreement with the results obtained previously for other delafossite-like oxides. In the temperature range of T _N2 < T < T _N1, Mössbauer spectra gave clear evidence for the existence of a distribution of the hyperfine magnetic fields H _hf at ⁵⁷Fe nuclei. We present the results of a model fitting of the spectra based on an assumption of the cycloid magnetic structure of AgFeO₂ at T < T _N2. The obtained data were analysed in comparison with published data on Mössbauer studies of oxide multiferroics.     

Spin crossover in the magnetic phase transition in YBa2(Cu1−x Fex)3O7±δ superconductors

All iron ions in the Cu1 and Cu2 local lattice sites of the YBa₂(Cu_0.9 ⁵⁷Fe_0.1)₃O_7.01 superconductor with T _c=31 K experienced magnetic ordering below T _m=22 K. Therefore, at T < T _m, magnetic ordering coexisted with superconductivity. According to the Mössbauer spectroscopy data, iron ions in Cu2 (Fe2) sites were in the low-spin state at T < T _m(S= 3/2 or 1/2), whereas an equal number of iron ions in Cu1 (Fe1) sites were in the high-spin Fe³⁺ state (S=5/2). The magnetic transition near T _m changed iron ion spin states-low-spin ions turned into high-spin ions, and vice versa. This preserved the spin balance between iron ions in the Cu1 and Cu2 layers. Control measurements on other samples of the YBa₂(Cu_{1? x} Fe_x)₃O_7±δ series substantiated these conclusions.     

Spin correlations in Ni-Mn-Ga

Results of measuring small-angle neutron scattering and neutron depolarization in a Ni_49.1Mn_29.4Ga_21.5 single crystal in the temperature range 15<T<400 K and in the range of magnetic fields 0<H<4.5 kOe are presented. The characteristic temperatures of the alloy were found to be as follows: T _C=373.7 K and the martensite transition temperature T _m=301–310 K. The magnetic critical scattering at T _C and the scattering at T<T _C were adequately described by the relationship I _m=A(q ²+κ²)^?2 (q is the transferred wave vector and R _c=1/κ is the correlation radius), and the temperature dependences of the A and R _c scattering parameters were determined. Left-right asymmetry was observed at 150<T< T _m in the scattering of neutrons polarized along or opposite to the applied field. This asymmetry was due to the inelastic magnetic interaction of neutrons in the sample. The magnetization of the alloy at T _m, critical scattering at T?T _C, anomalies in scattering, and the softening of magnetic excitations at 150 <T<T _m are discussed.     

Phase separation, ferromagnetism and magnetic irreversibility in La1−xSrxMn1−yFeyO3

Magnetic susceptibility, χ(T), is investigated in ceramic La_1−xSr_xMn_1−yFe_yO₃ (LSMFO) samples with x=0.3 and y=0.15−0.25. A ferromagnetic (FM) transition observed in LSMFO is accompanied with an appreciable decrease of the transition temperature with increasing y, which is connected to breaking of the FM double-exchange interaction by doping with Fe. Strong magnetic irreversibility, observed in low (B=10 G) field, gives evidence for frustration of the magnetic state of LSMFO. The FM transition, which is expanded with increasing B, is more pronounced in the samples with y=0.15-0.20 and broadens considerably at y=0.25, where the irreversibility is increased. Well above the transition, χ(T) exhibits a Curie-Weiss asymptotic behavior, yielding very large values of the effective Bohr magneton number per magnetic ion, incompatible with those of Mn or Fe single ions. At y=0.15 and 0.20 a critical behavior of χ⁻¹(T)∼(T/T_C−1)^γ in the region of the FM transition is characterized by influence of two different magnetic systems, a 3D percolative one with γ₌γ_p≈1.8 and T_C=T_C^(p), and a non-percolative 3D Heisenberg spin system, with γ=γ_H≈1.4 and T_C=T_C^(H), where T_C^(p)<T_C^(H). At y=0.25 the percolative contribution to the critical behavior of χ(T) is not observed. The dependence of χ on T and y gives evidence for phase separation, with onset already near the room temperature, leading to generation of nanosize FM particles in the paramagnetic host matrix of LSMFO. The ferromagnetism of LSMFO is attributable to percolation over the system of such particles and generation of large FM clusters, whereas the frustration is governed presumably by a system of smaller weakly-correlated magnetic units, which do not enter the percolative FM clusters.     

Spin-lattice relaxation in solid 3He in weak magnetic fields

Spin-lattice relaxation times T ₁ were measured for solid ³He at temperatures of 0.22 to 0.73 K in a 44-Oe magnetic field. An increase in T ₁ at temperatures higher than approximately 0.4 K was related to switching on the vacancy mechanism of atomic mobility in the crystal. At a melting curve minimum, in the region of predominance of exchange motions of atoms in the crystal, measurements of T ₁ were performed in magnetic fields of 2 to 71 Oe. The data obtained in fields higher than 5 Oe were in agreement with the Cowan-Fardis theory.     

Phase transition and relaxation processes by 87Rb and 39K nuclear magnetic measurements of RbKSO4 single crystals

⁸⁷Rb and ³⁹K nuclear magnetic resonance (NMR) spectra of RbKSO₄ single crystals were measured at room temperature. ⁸⁷Rb central line has the angular dependences of second-order quadrupolar shifts. From these results, the quadrupole coupling constant and the asymmetry parameter were determined at room temperature. In addition, the spin–lattice relaxation rate, 1/T₁, and the spin–spin relaxation rate, 1/T₂, were measured as a function of temperature. The values of 1/T₁ for the ⁸⁷Rb and ³⁹K nuclei were found to increase with increasing temperature, and 1/T₁ was determined to be proportional to Tⁿ. Therefore, for the ⁸⁷Rb and ³⁹K nuclei, Raman processes with n=2 are more significantly in nuclear quadrupole relaxation than direct processes.     

Specific heats and phase transition in TMMC

The specific heats of the nearly one-dimensional antiferromagnet TMMC showed a sharp discontinuity in the temperature range ?(≡∥1?T/T_N∥) ≈ 2 × 10^-3 at T_N =0.835 ± 0.010 K. The remaining magnetic entropy below T_N is about 1% of its total for S = $\text{5}\text{2}$.     

Nuclear Spin relaxation mediated by Fermi-edge electrons in n-type GaAs

A method based on the optical orientation technique was developed to measure the nuclear-spin lattice relaxation time T ₁ in semiconductors. It was applied to bulk n-type GaAs, where T ₁ was measured after switching off the optical excitation in magnetic fields from 400 to 1200 G at low (< 30 K) temperatures. The spin-lattice relaxation of nuclei in the studied sample with n _D = 9 × 10¹⁶ cm^?3 was found to be determined by hyperfine scattering of itinerant electrons (Korringa mechanism) which predicts invariability of T ₁ with the change in magnetic field and linear dependence of the relaxation rate on temperature. This result extends the experimentally verified applicability of the Korringa relaxation law in degenerate semiconductors, previously studied in strong magnetic fields (several Tesla), to the moderate field range.