Strong Spin-phonon Interaction Research Articles

Theoretical Study of the Phonon and Electrical Conductivity Properties of Pure and Sr-Doped LaMnO3 Thin Films.

The film thickness, temperature, substrate and doping dependence of the phonon energy ω and damping γ, as well as the electrical conductivity, of pure and Sr-doped LaMnO3 thin films near the phase transition temperature TN are investigated using a microscopic model and the Green's function technique. Due to the strong spin-phonon interaction, there appears a kink at TN in the temperature dependence of ω(T) and γ(T). The softening and hardening of the ω = 495 cm-1 (A1g) and ω = 614 cm-1 (B2g) modes is explained by the different sign of the anharmonic spin-phonon interaction constant R. The damping increases with T for both cases because it is proportional to R2. ω decreases whereas γ increases with an increasing Sr concentration. This is due to the strain caused by the difference between the ionic radii of the La and Sr ions. The film thickness dependence is also considered. ω and γ increase strongly with the decreasing film thickness. The electrical conductivity is enhanced after the doping of the LMO thin films with Sr, which could be used for energy storage applications. The observed results are in good qualitative agreement with the experimental data.

Size, external fields and ion doping effects on the multiferroic properties of hexagonal YMnO[formula omitted] nanoparticles

The size, external field and doping dependence of different properties of hexagonal YMnO3 nanoparticles (NPs) is theoretically investigated. The spontaneous magnetization Ms increases with increasing electric field and decreasing NP size N. The polarization decreases with increasing magnetic field h. The band gap increases with decreasing N. The phonon mode ω̄0 = 167 cm−1 shows a kink at TN due to strong spin–phonon interaction. Applying h the kink disappears. By doping with different ions at the Mn site, due to the observed different strains Ms can increase or decrease. By large Ti ion doping, above a critical x value, is taken into account that there appears a hexagonal–orthorhombic phase transition. The Gd and Sr substitution on the Y place in YMnO3 NPs is also discussed. The dielectric function as a function of temperature, NP size and h is studied. The electron and phonon contributions to the specific heat Cp are shown.

Multiferroic and phonon properties near the phase transitions of pure and ion-doped Ca3Mn2O7

ABSTRACT The magnetic, dielectric and phonon properties of CaMnO are investigated using a microscopic model and the Green's function technique. Electrically induced decrease of the magnetization M is observed which is an indirect evidence that CaMnO is a multiferroic material. The dielectric function has an anomaly at the Nel temperature and a broad peak at the Curie temperature . increases with increasing magnetic field. M and are enhanced by Li ion doping, whereas reduced by Ti ion doping due to different strain caused by the doped ions which changes the exchange interaction constants. increases with increasing Ti concentration. The temperature and magnetic field dependence of the phonon energy and damping for the A mode = 624 cm are studied. Both show a kink at due to the strong spin–phonon interaction. The kink disappears applying a magnetic field.

Multiferroic and phonon properties at the phase transition of S = 1/2 chain cuprates NaCu2O2. Comparison with LiCu2O2

ABSTRACT The multiferroic properties of NaCuO (NCO) around the phase transition temperature are studied theoretically. The total polarization P is zero without magnetic field h due to the antiferroelectric state induced by the alternate stacking of CuO layer with opposite spin vector chirality. Applying h P increases, goes through a maximum and vanishes at . The magnetic field dependence of is an evidence that NCO is multiferroic. For h = 0 there is no anomaly in the dielectric constant at which is an evidence for the antiferroelectric behavior of the compound. It is observed that and in NCO is quite different compared to that of LiCuO. Around the Neel temperature we observe in NCO an anomaly in the specific heat and the phonon energy due to strong anharmonic spin-phonon interaction. This kink disappears by applying an external magnetic field h.

EPR of Dy3+ ions in YAl3(BO3)4 and EuAl3(BO3)4 aluminoborates

The EPR spectra of impurity Dy3+ ions in YAl3(BO3)4 and EuAl3(BO3)4 aluminoborates were investigated. The signals from the ground and excited doublets were observed and the g-factors and the hyperfine interaction constants were determined. From the temperature dependence of the signal intensity of the excited doublet, the energy distances between the ground and excited doublets were obtained as (3.26 ± 0.13) cm–1 and (2.54 ± 0.16) cm–1 for YAl3(BO3)4 and EuAl3(BO3)4, respectively. A broadening of the absorption line with increasing temperature is related with the strong spin-phonon interaction and is described by the Orbach–Aminov process via the excited doublet. It was found that the decrease in g-factor is caused by the spin-phonon interaction.

Magneto-Elastic Effects in Tb3Ga5O12

We report new results for the elastic constants studied in Faraday and Cotton–Mouton geometry in Tb3Ga5O12 (TGG), a frustrated magnetic substance with strong spin–phonon interaction and remarkable crystal-electric-field (CEF) effects. We analyze the data in the framework of CEF theory taking into account the individual surroundings of the six inequivalent Tb3+-ion positions. This theory describes both, elastic constants in the magnetic field and as a function of temperature. Moreover we present sound-attenuation data for the acoustic Cotton–Mouton effect in TGG.

Low-temperature thermal conductivity of antiferromagnetic S = 1/2 chain material CuCl2·2((CH3)2SO)

We study the heat transport of S = 1/2 chain compound CuCl2·2((CH3)2SO) along the b axis (vertical to the chain direction) at very low temperatures. The zero-field thermal conductivity (κ) shows a distinct kink at about 0.9 K, which is related to the long-range antiferromagnetic (AF) transition. With applying magnetic field along the c axis, κ(H) curves also show distinct changes at the phase boundaries between the AF and the high-field disordered states. These results indicate a strong spin-phonon interaction and the magnetic excitations play a role in the b-axis heat transport as phonon scatterers.

Spin–phonon interaction effects in pure and Fe-doped antiferromagnetic Cr2O3 nanoparticles

We have studied the temperature and size dependence of the magnetic and phonon properties of pure and Fe-doped antiferromagnetic Cr2O3 nanoparticles using a microscopic model and a Green's function technique. The magnetization M increases whereas the Neel temperature TN decreases with decreasing of particle size. The phonon energy ω has a kink at TN due to the strong spin–phonon interaction. It increases with decreasing of particle size. Fe-dopant concentration reduces ω.

EPR of Nd3+ and Er3+ ions in aluminum borates YAl3(BO3)4 and EuAl3(BO3)4

EPR spectra of impurity Nd3+ and Er3+ ions in aluminum borates YAl3(BO3)4 and EuAl3(BO3)4 have been studied. Dependences of g-factors and linewidth as a function of temperature are established, and the hyperfine interaction constants are determined. Temperature dependences of linewidth of Nd3+ and Er3+ ions in both crystals are identical. The absorption line broadening with increasing temperature is caused by strong spin–phonon interaction and is described by the Orbach–Aminov process. Unlike Nd3+ ion, an anisotropy of temperature dependence of Er3+ ion linewidth is observed. The relaxation velocity of Er3+ ions is lower then that of Nd3+ ions. The anisotropy of spin–lattice relaxation observed for Er3 ion is due to a large g-factor anisotropy which is larger in the EuAl3(BO3)4 crystal than in YAl3(BO3)4 one. A decrease of g-factor of Er3+ ion in both crystals with increasing temperature is shown to result from spin–phonon interaction.

Low-temperature thermal conductivity of terbium-gallium garnet

Thermal conductivity of paramagnetic Tb3Ga5O12 (TbGG) terbium-gallium garnet single crystals is investigated at temperatures from 0.4 to 300 K in magnetic fields up to 3.25 T. A minimum is observed in the temperature dependence κ(T) of thermal conductivity at Tmin = 0.52 K. This and other singularities on the κ(T) dependence are associated with scattering of phonons from terbium ions. The thermal conductivity at T = 5.1 K strongly depends on the magnetic field direction relative to the crystallographic axes of the crystal. Experimental data are considered using the Debye theory of thermal conductivity taking into account resonance scattering of phonons from Tb3+ ions. Analysis of the temperature and field dependences of the thermal conductivity indicates the existence of a strong spin-phonon interaction in TbGG. The low-temperature behavior of the thermal conductivity (field and angular dependences) is mainly determined by resonance scattering of phonons at the first quasi-doublet of the electron spectrum of Tb3+ ion.

Spin-Phonon Coupling Effects in Antiferromagnetic Cr<SUB>2</SUB>O<SUB>3</SUB> Nanoparticles

In the present work we use Raman microscopy to investigate the size effect of spin-phonon coupling in antiferromagnetic Cr2O3 nanoparticles. The peculiarities of the dependency of the phonon wave number on temperature (with a relatively weak peak at 293 cm(-1)) can be attributed to the spin-phonon coupling. The variation with temperature of the spin-phonon mode develops when the antiferromagnetic state is near the ordering temperature of the Cr spins. The observations can be reasonably well interpreted by describing the order parameter. A shift in frequency is caused by a strong spin-phonon interaction in Cr2O3. The obtained s-ph coefficients are found to be consistent with the strain, where raising the strain results in weakening of the s-ph coupling.

Impact of the spin–phonon interaction on the phonon properties of multiferroic hexagonalRMnO3 thin films

The phonon properties of multiferroic hexagonalRMnO3 thin films are studied based on a microscopic model including anharmonic spin–phononinteractions. We obtain an anomaly near the magnetic phase transition temperatureTN which can be attributed to the magnetoelectric nature ofRMnO3 and a strong spin–phonon interaction. The size and surface effects on the phononproperties are discussed. It is demonstrated that the phonon energies can be larger orsmaller for different substrates or doping ions, whereas their damping is alwaysenhanced. The influence of an external magnetic field on the phonon properties isobserved.

Ultrasonic study on RuSr2GdCu2O8

AbstractThe physical properties of RuSr2GdCu2O8 have been investigated by ultrasonic measurements. The huge stiffening of the velocity and the sharp elastic energy loss peak with the asymmetric shape near 220 K both indicate a phase transformation in the sample, which is induced by the disorder to order transformation of the orientation of RuO6 octahedra. The softening of the velocity and the elastic energy loss peak near 132 K may give further evidence to the strong spin–phonon interaction in the compound, and suggest a complex interplay of spin and lattice degrees of freedom in RuSr2GdCu2O8. A velocity jump and a decrease of the elastic energy loss are observed around the onset of the superconducting transition temperature TC,onset ∼ 50 K. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

(Sr,Mn)TiO3: A Magnetoelectric Multiglass

By close analogy with multiferroic materials with coexisting long-range electric and magnetic orders a "multiglass" scenario of two different glassy states is observed in Sr(0.98)Mn(0.02)TiO(3) ceramics. Sr-site substituted Mn2+ ions are at the origin of both a polar and a spin glass with glass temperatures T(g) approximately equal to 38 K and < or =34 K, respectively. The structural freezing triggers that of the spins, and both glassy systems show individual memory effects. Thanks to strong spin-phonon interaction within the incipient ferroelectric host crystal SrTiO3, large higher order magnetoelectric coupling occurs between both glass systems.

Theoretical study of the phonon spectra of hexagonal multiferroicsRMnO3

The phonon properties of hexagonal multiferroicRMnO3 materials are studied using a Green’s function technique. The calculations are performed onthe basis of the Heisenberg and the transverse Ising model taking into account anharmonicspin–phonon and phonon–phonon interaction terms. The strong spin–phononinteraction leads to an anomaly in the phonon energy and the damping around themagnetic and ferroelectric phase transitions. The phonon spectrum is discussed fordifferent exchange, magnetoelectric and spin–phonon interaction constants. Itis shown that the phonon energy depends on the radius of the rare earth ionrR. The influence of an applied magnetic field on the phonon spectrum is studied, too. Thepredictions are consistent with experimental results.

Electron spin resonance study of Nd0.7Sr0.3MnO3 single crystals

Electron spin resonance (ESR) measurements in Nd0.7Sr0.3MnO3 single crystals provide evidence for the existence of spin and lattice correlated clusters above and below TC=205K. The ESR linewidth deviates from the quasilinear “universal” temperature dependence of the linewidth observed in several manganites. A linear temperature dependence of the linewidth is attributed to the spin-phonon interaction. Thus the linewidth of the paramagnetic resonance line indicates the presence of a strong spin-phonon interaction rather than the spin-spin interaction seen in other manganites. The gradual increase observed in the g value above TC is attributed to the presence of orbital correlations. The split in the ESR spectra only below 180K (TC=205K) and more noise in the spectra between 150 and 180K are ascribed to the strong competition between localization due to lattice distortions and delocalization of charge carriers.

Comparative Raman studies of [formula omitted], [formula omitted] and [formula omitted

The polarized Raman spectra of layered ruthenates of the Sr n + 1 Ru n O 3 n + 1 ( n = 1 , 2 , 3 ) Ruddlesden–Popper series were measured between 10 and 300 K. The phonon spectra of Sr 3 Ru 2 O 7 and Sr 4 Ru 3 O 10 confirmed earlier reports for correlated rotations of neighboring RuO 6 octahedra within double or triple perovskite blocks. The observed Raman lines of A g or B 1 g symmetry were assigned to particular atomic vibrations by considering the Raman modes in simplified structures with only one double or triple RuO 6 layer per unit cell and by comparison to the predictions of lattice dynamical calculations for the real Pban and Pbam structures. Along with discrete phonon lines, a continuum scattering, presumably of electronic origin, is present in the zz, xx and xy, but not in the x ′ y ′ and zx spectra. Its interference with phonons results in Fano shape for some of the lines in the xx and xy spectra. The temperature dependences of phonon parameters of Sr 3 Ru 2 O 7 exhibit no anomaly between 10 and 300 K where no magnetic transition occur. In contrast, two B 1 g lines in the spectra of Sr 4 Ru 3 O 10 , corresponding to oxygen vibrations modulating the Ru–O–Ru bond angle, show noticeable hardening with ferromagnetic ordering at 105 K, thus indicating strong spin-phonon interaction.

Infrared spectroscopic study of CuO: Signatures of strong spin-phonon interaction and structural distortion

Optical properties of single-crystal monoclinic CuO in the range $70--6000 {\mathrm{cm}}^{\ensuremath{-}1}$ were studied at temperatures from 7 to 300 K. Normal reflection spectra were obtained from the (001) and (010) crystal faces thus giving separate data for the ${A}_{u}$ and ${B}_{u}$ phonon modes excited in the purely transverse way (TO modes). Mode parameters, including polarizations of the ${B}_{u}$ modes not determined by the crystal symmetry, were extracted by the dispersion analysis of reflectivity curves as a function of temperature. Spectra of all the components of the optical conductivity tensor were obtained using the Kramers-Kronig method recently extended to the case of the low-symmetry crystals. The number of strong phonon modes is in agreement with the factor-group analysis for the crystal structure currently accepted for the CuO. However, several ``extra'' modes of minor intensity are detected; some of them are observed in the whole studied temperature range, while existence of others becomes evident at low temperatures. Comparison of frequencies of ``extra'' modes with the available phonon dispersion curves points to possible ``diagonal'' doubling of the unit cell ${\mathbf{a},\mathbf{b},\mathbf{c}\stackrel{\ensuremath{\rightarrow}}{}}{\mathbf{a}+\mathbf{c},\mathbf{b},\mathbf{a}\ensuremath{-}\mathbf{c}}$ and formation of the superlattice. The previously reported softening of the ${A}_{u}^{3}$ mode $(\ensuremath{\sim}400 {\mathrm{cm}}^{\ensuremath{-}1})$ with cooling at ${T}_{N}$ is found to be $\ensuremath{\sim}10%$ for the TO mode. The mode is very broad at high temperatures and strongly narrows in the antiferromagnetic phase. We attribute this effect to strong resonance coupling of this mode to optical or acoustic bimagnons and reconstruction of the magnetic excitations spectrum at the N\'eel point. A significant anisotropy of ${\ensuremath{\epsilon}}^{\ensuremath{\infty}}$ is observed: it was found to be 5.9 along the $\mathbf{b}$ axis, 6.2 along the [101] chains, and 7.8 along the $[101\ifmmode\bar\else\textasciimacron\fi{}]$ chains. The transverse effective charge ${e}_{\mathrm{T}}^{*}$ is more or less isotropic; its value is about two electrons.

Spin-order-dependent Raman scattering inRuSr2GdCu2O8

A detailed study of the magnetic ordering effects on the Raman spectra of the ferromagnetic superconductor ${\mathrm{RuSr}}_{2}{\mathrm{GdCu}}_{2}{\mathrm{O}}_{8}$ is presented. As the temperature is lowered, additional phonon peaks are observed and assigned to folded phonons rendered active by the doubling of the unit cell consequent with rotations of the ${\mathrm{RuO}}_{6}$ octahedra. The intensity of these modes strengthens very rapidly as the sample is cooled below the Curie temperature, evidencing the importance of spin-dependent phonon Raman scattering in this material. In addition, all phonon lines display either an ``anomalous'' softening or hardening below ${T}_{C},$ also indicative of a strong spin-phonon interaction. Most interestingly, a different low energy peak which is not seen in the related ferromagnet ${\mathrm{SrRuO}}_{3}$ develops and shifts to higher energies below ${T}_{C}.$ A tentative explanation is proposed in that this latter feature reflects the two-dimensional density of ${\mathrm{RuO}}_{2}$-layer magnon states, observable by Raman scattering through an ``exchange-scattering'' mechanism that involves the ${\mathrm{Gd}}^{3+}$ paramagnetic spins. This conclusion and some of the phonon peak assignments are partly supported by complementary measurements on a ${\mathrm{RuSr}}_{2}{\mathrm{EuCu}}_{2}{\mathrm{O}}_{8}$ sample where ${\mathrm{Gd}}^{3+}(S=7/2)$ has been fully replaced by the $J=0$ ground-state ${\mathrm{Eu}}^{3+}$ ion.

Elastic-moduli and ultrasonic-attenuation anomalies near antiferromagnetic phase transitions in La1−xCaxMnO3

Both the longitudinal and transverse ultrasonic sound velocities and attenuations in the single-phase polycrystalline compound La1−xCaxMnO3 (x=0.63,0.83) have been measured by a conventional pulse-echo-overlap technique at a frequency of 10 MHz, between 50 and 300 K. The variations with temperature of the Young modulus E, shear modulus G, bulk modulus B, Poisson’s ratio σ, and attenuation α have been determined. Anomalies in E, G, B, σ, and α were observed near the temperature of the charge-ordering transition TCO and antiferromagnetic (AFM) transition TN. The results imply a very strong spin–phonon interaction caused by linear magnetostriction near the AFM transition. The detailed form of the anomalies is different for different types of phase transitions.