Magnetoacoustic resonance on nuclear spin waves in the cubic antiferromagnet RbMnF3

Magnetoacoustic resonance on nuclear spin waves is measured in the cubic antiferromagnet RbMnF₃. A resonance change with respect to a constant magnetic field H ₀ with maximum damping at H ₀≈4×10³ Oe is observed in the amplitude of an acoustic pulse passing through a sample owing to excitation of nuclear spin waves under nuclear magnetoacoustic resonance conditions. A study of the angular dependence of the damping revealed a 90° periodicity consistent with the fact that the [001] direction, around which the rotation takes place, is a four-fold axis of the crystal. An analysis of the dispersion law for nuclear spin waves shows that longitudinal ultrasound propagating along the [001] axis perpendicular to H ₀ excites a branch of nuclear spin waves whose frequency depends on the magnitude of the constant magnetic field. Fiz. Tverd. Tela (St. Petersburg) 41, 297–300 (February 1999)     

Magnetic resonance of intrinsic defects in the spin-Peierls magnet CuGeO3

Magnetic resonance in pure single-crystal CuGeO₃ at frequencies 9–75 GHz in the temperature range 1.2–25 K is investigated. Splitting of the magnetic-resonance line into several spectral components is observed at temperatures below 5 K, where spin-Peierls dimerization suppresses the magnetic susceptibility and the ESR signal intensity. Analysis of the magnetic resonance spectra over a wide frequency range with different directions of the magnetic field at different temperatures makes it possible to identify among these components the ESR signals due to defects, having effective spin S=1/2 and spin S=1, in the spin-Peierls phase. The g factor corresponding to these ESR signals is the same and close to the value characteristic for the ion Cu²⁺. Another magnetic-resonance line is characterized by a strongly anisotropic g factor and an increase (at a threshold in the excitation power) in the susceptibility both at resonance and in the line wings. These signals are tentatively attributed to two possible types of planar defects arising on the walls of domains of the spin-Peierls state with different values of the dimerization phase. Zh. éksp. Teor. Fiz. 114, 1876–1896 (November 1998)     

NMR in 55Mn2+ nuclei in the quasi-one-dimensional antiferromagnetic CsMnBr3

The NMR spectrum of the quasi-one-dimensional easy-plane antiferromagnetic CsMnBr₃, which has trigonal spin lattice, is investigated in detail. The measurements were performed on a wide-band NMR decimeter microwave-band spectrometer over a wide range of magnetic fields at temperatures 1.3–4.2 K. All three branches of the NMR spectrum previously found by us [JETP Lett. 64, 225 (1996)] are severely distorted because of the dynamic interaction with the Goldstone mode in the antiferromagnetic resonance spectrum. The experimental results in fields up to 40 kOe are described satisfactorily by an equation obtained by Zaliznyak et al. [JETP Lett. 64, 473 (1996)]. Formulas are obtained in our work that agree very well with experiment at all fields up to the “collapse” field H _c of all sublattices. The unbiased NMR frequency in CsMnBr₃ is determined to be v _n0=416 MHz (T=1.3 K) in zero external magnetic field, and in this way the reduction in the spontaneous moment due to the quasi-one-dimensional nature of the system of Mn²⁺ spins, which according to our data amounts to 28%, is determined more accurately. The field dependences of the directions of the magnetic sublattices with respect to the magnetic field are obtained from the NMR spectra, confirming the equations of Chubukov [J. Phys. Condens. Matter 21, 441 (1988)]. The results on the field dependence of the width and intensities of the NMR lines are discussed, along with three observed anomalies: 1) a strong increase in the NMR frequency for nuclei in sublattices that are perpendicular to the magnetic field; 2) the nonmonotonic temperature dependence of the resonance field for the lower branch of the spectrum; 3) the presence of two branches of the NMR spectrum in large H _c fields, in which the CsMnBr₃ must be a quasi-one-dimensional antiferromagnetic. Zh. éksp. Teor. Fiz. 113, 352–368 (January 1998) Deceased.     

Strong coupling in the Kondo problem in the low-temperature region

The magnetic field dependence of the average spin of a localized electron coupled to conduction electrons with an antiferromangetic exchange interaction is found for the ground state. In the magnetic field range μH∼0.5T _c (T _c is the Kondo temperature) there is an inflection point, and in the strong magnetic field range μH≫T _c , the correction to the average spin is proportional to (T _c /μ H)². In zero magnetic field, the interaction with conduction electrons also leads to the splitting of doubly degenerate spin impurity states. Zh. éksp. Teor. Fiz. 115, 1263–1284 (April 1999) Published in English in the original Russian journal. Reproduced here with stylistic changes by the Translation Editor     

Influence of chirality on the electron spin resonance in molecular magnets [MnII(H L )(H2O)][MnIII(CN)6] · 2H2O with chiral ligands L

Chiral and racemic molecular ferrimagnets [Mn^II(HL)(H₂O)][Mn^III(CN)₆] · 2H₂O, where L = S-or R-1,2-diaminopropane for chiral samples (S-pn, R-pn) and L = rac-pn for racemic samples, are investigated by the electron spin resonance technique. It is revealed that the electron spin resonance spectra of the chiral and racemic samples differ from each other at temperatures below the Curie temperature T _C = 21 K. The maximum in the temperature dependence of the integrated magnetic susceptibility χ(T) calculated by the double integration of the line centered at a negative field of −250 Oe for the racemic samples is broadened as compared to the maxima in the corresponding dependences for the enantiomers of the chiral samples with “right” (R) and “left” (S) symmetry. The new compounds under investigation differ from the previously synthesized crystals by the strong spin-orbit interaction between Mn³⁺ ions, which leads to a dependence of their magnetic properties on the chirality of the structure. Original Russian Text ? R.B. Morgunov, F.B. Mushenok, M.V. Kirman, 2008, published in Fizika Tverdogo Tela, 2008, Vol. 50, No. 7, pp. 1252–1256.     

Spin-spin interaction of Ni2+ ions in ZnSiF6·6H2O

The angular dependences of the electron spin resonance spectrum of 1% Ni²⁺ ions in a ZnSiF₆·6H₂O matrix are investigated experimentally at 36 GHz and 4.2 K. Besides the main spectrum of the isolated ion, we observed a spectrum due to interacting pairs of Ni²⁺ ions, located in the first (nn) and second (2n) coordination spheres and coupled by, besides the magnetic dipole-dipole interaction, isotropic exchange: J _nn = (−197±1)×10⁻⁴, J _2nα = (−5±1)×10⁻⁴, and J _2nβ = (3±2)×10⁻⁴ cm⁻¹. Lines due to other isolated Ni²⁺ ions, which have a different initial splitting D, are also present in the spectrum with intensity comparable to the pair spectrum. Low-symmetry distortions of the crystal field are observed, caused by a pair of impurity ions located close to one another. It is shown that the previously proposed interpretation is incorrect. Fiz. Tverd. Tela (St. Petersburg) 41, 1602–1608 (September 1999)     

Investigation of a gap in the AFMR spectrum in the quasi-one-dimensional hexagonal antiferromagnet CsMnBr3

The low-frequency part of the magnetic resonance spectrum of the hexagonal six-sublattice quasi-one-dimensional antiferromagnet CsMnBr₃ at temperatures which are low compared with T _N =8.3 K is investigated experimentally. A temperature-dependent gap Δ(T) is found in the spectrum ω _e (H) of the lower AFMR branch; this gap is due to the hyperfine interaction of the nuclear spins with the electronic spins of the (⁵⁵Mn)²⁺ ions. The spectrum of the low-lying resonance frequencies of such a system is calculated taking this interaction into acount in the approximation of fluctuationless spin hydrodynamics for the electronic branch of the oscillations. The computational results are in good qualitative and satisfactory quantitative agreement with experiment. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 6, 433–437 (25 September 1996)     

Muons in type-II superconductors: μ+ diffusion in ultra-pure niobiumdiffusion in ultra-pure niobium

The diffusivityD _μ of positive muons (μ⁺) in the mixed state of superconducting high-purity, high-perfection niobium single crystals is investigated by measurements of the relaxation of the transverse muon spin polarization (μ⁺SR). The method makes use of the strong magnetic field gradients existing in the mixed state of Type-II superconductors and monitorsD _μ through the variation of the magnetic field felt by the μ⁺ during their diffusion through the crystals. For μ⁺ near the centres of the flux lines inNb it givesD _μ(4.6 K)=(8±2)·10⁻¹¹m²S⁻¹. The positive temperature coefficient ofD _μ indicates that at liquid-helium temperatures the diffusivity of μ⁺ inNb is mainly due to phonon-assisted tunnelling processes.     

Mössbauer Studies of Dilute 119Sn and 57Fe in SrRuO3 and Sr2FeRuO6

Nuclear magnetic resonance on oriented ¹⁷⁵Hf in iron host has been measured. Samples of ¹⁷⁵HfFe were made by recoil implantation of the precursor ¹⁷⁵Ta isotope. The resonance frequency and the resonance line width have been determined to be 139.0 (1) MHz and FWHM = 2.7 (2) MHz, respectively, in an external magnetic field of 0.1 T. The resonance width was very narrow compared with the previously reported value of 11.0 (1.1) MHz. With the known value of the magnetic moment of μ(¹⁷⁵Hf) = −0.62 (3) μ_N, the hyperfine field has been deduced as B _HF = −73.6 (3.5) T.     

Giant magnetoacoustic effect in KMnF3 due to nuclear spin waves

An explanation is proposed for the gigantic magnetoacoustic effect that we observed in KMnF₃ in previous work {Kh. G. Bogdanova, V. A. Golenishchev-Kutuzov, M. I. Kurkin et al., Zh. éksp. Teor. Fiz. 112, 1830 (1997) [JETP 85, 1001 (1997)]}. The effect entails a tenfold amplitude reduction of an acoustic pulse in a magnetic field that varies over the range 0–8 kOe. It is shown that this effect is due to the interference of two nuclear magnetoelastic waves propagating in the sample under magnetoacoustic resonance conditions, if this resonance occurs in the region of strong spatial dispersion of nuclear spin waves. The effect is said to be gigantic because it exceeds in magnitude the magnetoacoustic effects observed previously in magnetically ordered materials even though it is due to nuclear magnetism, which is 10⁵ times weaker than electronic magnetism. We observe a concomitant anomalous dependence of the dispersion of the velocity of sound on the external magnetic field. Zh. éksp. Teor. Fiz. 115, 1727–1739 (May 1999)     

Observation of spin-reduction anisotropy in the quasi-one-dimensional antiferromagnet CsMnI3

The NMR of ⁵⁵Mn in the quasi-one-dimensional noncollinear antiferromagnet CsMnI₃ at T=1.3 K is investigated in magnetic fields up to ∼40 kOe. Six NMR branches corresponding to six manganese spins per magnetic unit cell are observed. The NMR spectra correspond satisfactorily to the well-known magnetic structure of CsMnI₃, taking into account the dynamic frequency shift due to the interaction with the low-lying AFMR modes. The average spins 〈S _A〉=1.86 and 〈S _B〉=1.74 of the magnetically nonequivalent Mn²⁺ ions are determined from the measured values of the hyperfine fields. The results obtained agree qualitatively with the calculations of spin reduction in quasi-one-dimensional antiferromagnets [Y. Watabe, T. Suzuki, and Y. Natsume, Phys. Rev. B 52, 3400 (1995)]. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 9, 661–665 (10 May 1998)     

Phase transitions and magnetic-transport phenomena in the system La2/3Ba1/3(Mn1−x Cox)O3

A study is performed of the crystalline structure, magnetization, and magnetotransport properties of the system La_2/3Ba_1/3(Mn_1−x Co_x)O₃ with perovskite structure. It is shown that cubic solid solutions exist over the entire range of cobalt concentrations 0⩽x⩽1. Compositions with x⩽0.2 are ferromagnets with maximum resistance near T _C . Compositions with 0.2<x<0.4 manifest properties of inhomogeneous ferromagnets. Measurements of magnetic properties indicate the absence of long-range magnetic order in compositions with 0.5⩽x⩽0.9, which are probably spin glasses. The spontaneous magnetization of cobaltate (2μ _B per formula unit) corresponds to ferromagnetic ordering of the moments of the Co³⁺ and Co⁴⁺ ions found in the intermediate spin state. It is conjectured that the magnetoresistance consists of an extrinsic and an intrinsic contribution. The first arises as a result of intergrain transport of spin-polarized charge carriers, and the second, as a result of magnetic ordering near T _C . The magnetoresistance is essentially independent of the spontaneous magnetization and decreases abruptly as the cobalt concentration is increased with a corresponding transition from long-range to short-range magnetic order. Zh. éksp. Teor. Fiz. 116, 604–610 (August 1999)     

Observation of oriented 23 S 1 helium atoms in a sodium-helium plasma irradiated by polarization-modulated laser radiation

An experiment on the observation of spin polarization of metastable helium atoms interacting with optically oriented sodium atoms under continuous rf discharge conditions is described. Laser radiation with alternating-sign circular polarization, tuned to the resonance excitation of the 3² S _1/2–3² P _1/2 transition in Na atoms, is used as the source for optical pumping of ground-state sodium atoms. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 5, 385–387 (10 March 1997)     

Determination of the polarization of deuterium atoms following optical orientation

The redistribution of the electronic polarization in deuterium atoms is analyzed theoretically and the various polarization moments are shown to influence the magnetic resonance signal of deuterium. The analysis gives expressions that relate the amplitudes of the magnetic resonance signals for various Zeemann sublevels of the D atom to the electronic and nuclear polarizations of these atoms and their nuclear alignment. Experimental data on the optical orientation and spin exchange in a D-Cs mixture are used to determine the electronic and nuclear orientation and nuclear alignment of the D atoms, which are found to be 〈S _z〉=0.1, 〈I _z〉=0.27, and 〈Q _zz=0.027. Zh. Tekh. Fiz. 67, 22–26 (January 1997)     

Double resonance experiments in low magnetic field: Dynamic polarization of protons by N and measurement of low NQR frequencies

The possibilities of dynamically polarizing proton spin system via the quadrupole ¹⁴N spin system in low magnetic field are analyzed. The increase of the proton magnetization is calculated. The polarization rate of the proton spin system is related to the transition probabilities per unit time between the ¹⁴N quadrupole energy levels and proton energy levels. The experiments performed in 1,3,5-triazine confirm the results of the theoretical analysis. A new double resonance technique is proposed for the measurement of nuclear quadrupole resonance frequencies ν_Q of the order of 100 kHz and lower. The technique is based on magnetic field cycling between a high and a low static magnetic field and observation of the proton NMR signal in the high magnetic field. In the low magnetic field the quadrupole nuclei and protons resonantly interact at the proton Larmor frequency ν_H = ν_Q/2. The quadrupole nuclei are simultaneously excited by a resonant rf magnetic field oriented along the direction of the low static magnetic field. The experimental procedure is described and the sensitivity of the new technique is estimated. Some examples of the measurement of low ¹⁴N and ²H nuclear quadrupole resonance frequencies are presented.     

Magnetoresistance of the semiconducting spinelide Cu0.625Ga0.375Cr2Se4 having a low-temperature long-range magnetic order-spin glass transition

A study is reported of the dependence of magnetoresistance Δρ/ρ on the square of magnetization σ ² of the semiconducting spinelide Cu_0.625Ga_0.375Cr₂Se₄, which exhibits a low-temperature transition from long-range magnetic order (LRMO) to the spin glass (SG) state in strong magnetic fields. It is shown that at the freezing temperature T _f the Δρ/ρ(σ ²) relations change their slope, and that below T _f this slope is about one half that for T>T f. This finding, together with the earlier observation that the freezing temperature does not depend on the frequency of the ac magnetic field in which it was measured, suggests that the spin-glass phase consists of spins of individual Cr³⁺ ions, and that the SG-LRMO crossover is a phase transition. Fiz. Tverd. Tela (St. Petersburg) 40, 315–317 (February 1998)     

A Variable-Temperature X-Band EPR Study of the Gd<Superscript>3+</Superscript> Ion in a La<Subscript>2</Subscript>Si<Subscript>2</Subscript>O<Subscript>7</Subscript> Crystal Characterized by Monoclinic Site Symmetry

Electron paramagnetic resonance (EPR) studies on a single crystal of diamagnetic compound La₂Si₂O₇, potentially a phosphorescent/luminescent/laser material, with the Gd³⁺ ion substituting for the La³⁺ ion, were carried out at X-band (9.61 GHz) over the 4–295 K temperature range. The asymmetry exhibited by the Gd³⁺ EPR line positions for the orientations of the external magnetic field about the magnetic Z- and Y-axes in the ZY-plane was ascribed to the existence of monoclinic site symmetry at the site of the Gd³⁺ ion, as confirmed by the significant values of the spin Hamiltonian parameters g _YZ , b ₂ ⁻¹, b ₄ ^−m (m = 1, 3), b ₆ ^−m (m = 1, 3, 5), estimated by fitting all EPR line positions observed at room temperature for the orientation of the magnetic field in the magnetic ZX- and ZY-planes using a rigorous least-squares fitting procedure. At 8 K measurements were only carried out for orientation of B in the magnetic ZX-plane, due to difficulty in orientation of the crystal inside the cryostat, enabling estimation of all spin Hamiltonian parameters b _n ^m except those characterized by negative m values and g _YZ . The absolute sign of the zero-field splitting parameter b ₂ ⁰ was determined to be negative from the relative intensities of the lines at 8 K. Authors' address: Sushil K. Misra, Physics Department, Concordia University, 1455 de Maisonneuve Boulevard West, Montreal, Quebec H3G 1M8, Canada     

Collisions of alkali K and Rb atoms in ground state: Modification of interaction potentials

The interaction of alkali K and Rb atoms that reside in the ground state is considered in the range of collision energies E = 10⁻⁴ to 10⁻² au. The singlet (X ¹Σ⁺) and triplet (a ³Σ⁺) interaction potentials available in the literature are analyzed and modified. For the KRb dimer in the range of interatomic distances 15–21a ₀, we chose analytical representations of the singlet and triplet potentials that more accurately describe the interaction of alkali Rb and K atoms in the ground state. Complex cross sections of the spin exchange are calculated for the first time that permit one to calculate the processes of polarization transfer and relaxation times, as well as shifts in the magnetic resonance frequencies caused by K-Rb spin exchange collisions.     

Solution of the problem of catastrophic relaxation of homogeneous spin precession in superfluid 3He-B

The quantitative analysis of the “catastrophic relaxation” of the coherent spin precession in ³He-B is presented. This phenomenon has been observed below a temperature of about 0.5 T _c as an abrupt shortening of the induction signal decay. It is explained in terms of the decay instability of the homogeneous transverse NMR mode into spin waves of the longitudinal NMR. Recently, the cross interaction amplitude between the two modes has been calculated by Sourovtsev and Fomin [9] for the so-called Brinkman-Smith configuration, i.e., for the orientation of the orbital momentum of Cooper pairs along the magnetic field, L ‖ H. In their treatment, the interaction is caused by the anisotropy of the speed of the spin waves. We found that, in the more general case of the nonparallel orientation of L corresponding to the typical conditions of the experiment, the spin-orbital interaction provides the additional interaction between the modes. By analyzing the experimental data, we are able to distinguish which contribution is dominating in different regimes. The text was submitted by the authors in English.     

Discovery of a narrow resonance state of the system K S K S at mass 1520 MeV

Data are presented which indicate the existence of a previously unknown narrow resonance near mass 1520 MeV, discovered while analyzing the system K _S K _S . The data were obtained at ITEP using the 6-m magnetic spectrometer in a beam of 40-GeV negative particles. The resonance is observed in π⁻ p and K ⁻ p interactions with a total statistical confidence of not less than 5 standard deviations. The width of this state is comparable to the mass resolution of the spectrometer (∼5 MeV). The mass is 1521.5±2.5 MeV. Estimates of the product σ·BR (K _S K _S ) give ∼5.0 nb in π⁻ p interactions and ∼250 nb in K ⁻ p interactions. From the fact that the production cross section of this resonance in the K ⁻ beam is approximately 50 times greater than the production cross section in the π⁻ beam, it can be concluded that this new state belongs to a system of mesons with hidden strangeness. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 4, 242–246 (25 August 1999)