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)     

Magnetostriction of the spin-Peierls cuprate CuGeO3

The isothermal, longitudinal magnetostriction of the spin-Peierls cuprate CuGeO₃ has been measured along the three orthorhombic directions up to 14 Tesla by means of a high resolution capacitance dilatometer. For all three axes we observe anomalies at both the dimerized/incommensurate (D/I) and the dimerized/uniform (D/U) transition whose sizes and signs differ. A precise H-T phase diagram is determined from the field and temperature dependence of the lattice constants, which roughly agrees with theoretical predictions. At the D/I transition the magnetostriction shows a jumplike behavior and a hysteresis indicating a first order transition. From the jumps of the magnetostriction at the D/I transition we estimate considerable, uniaxial stress dependences of the critical magnetic field H _c, which correlate with those of the spin-Peierls transition temperature. A finite magnetostriction is also resolved within the high temperature uniform phase of CuGeO₃ for all three lattice directions. These data show a pronounced, strongly anisotropic spin-lattice coupling in CuGeO₃ and allow to derive the uniaxial pressure dependences of the magnetic susceptibility. Based on our findings the relevance of different structural parameters for the magnetic exchange interaction is discussed.     

Magnetostriction of the spin-Peierls magnet CuGeO3

The temperature dependence of the longitudinal magnetostriction of a CuGeO₃ single crystal is measured within the temperature range 4.2–20 K in a magnetic field of 10 T. As the temperature is raised above 4.2 K, the magnetostriction at first increases from vanishingly small values, attains a maximum at a temperature of approximately 12 K, and then abruptly drops as the temperature approaches the spin-Peierls transition. The results are interpreted on the basis of a simple model utilizing the real pattern of magnetic excitations in the spin system. Fiz. Tverd. Tela (St. Petersburg) 40, 1671–1673 (September 1998)     

Magnetostriction of the spin-Peierls compound CuGeO3

The longitudinal and transverse magnetostriction λ of the spin-Peierls compound CuGeO₃ in the b-c plane is measured in magnetic fields up to 20 T both above and below the transition temperature T _sp=14.3K. It is found that for a given crystallographic direction the value of magnetostriction is weakly dependent on the magnetic field direction. In the uniform U phase at T⩾T _sp, λ is negative and approximately equal in the b and c directions, while in the dimerized D phase at T<T _sp, λ is positive and λ _b >λ _c . At low temperatures, λ increases sharply at the magnetic-field-induced transition from the dimerized to the magnetic M phase. The experimental data allow estimation of the stress derivatives of the antiferromagnetic intrachain exchange interaction parameter and of the stress dependence of the critical field of the D-Mphase transition. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 3, 156–159 (10 August 1996) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Magnetoelectric response of the spin-Peierls compound CuGeO3

The effect of an electric field on the magnetic susceptibility in a pure CuGeO₃ single crystal at microwave frequencies is investigated. A quadratic effect of the electric field on the magnetic susceptibility, which increases with decreasing temperature, is observed in the spin-Peierls state at temperatures below 5 K. The observed effect is tentatively attributed to residual magnetism, due to distortions of the regular dimerized structure at domain walls with different values of the dimerization phase, and to the effect of the electric field on the interchain exchange interaction. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 9, 646–651 (10 November 1996)     

Thermodynamic properties of the spin-Peierls transition in CuGeO3

We present an investigation of the spin-Peierls transition atT _SP=14.5 K in polycrystalline CuGeO₃ through specific-heat and thermal-expansion measurements. Clear second-order phase-transition anomalies are found in both properties atT _SP, although only a small entropy of S0.1 Rn2 is released at the transition. Most of the entropy is released atT _SP<T<150 K, where the temperature dependence of the magnetic contribution to the specific heat as well as the thermal expansion exhibit extrema atT ^*40 K. These are caused by one-dimensional antiferromagnetic fluctuations along the Cu chains, possibly accompanied by structural fluctuations. Using Ehrenfest's relation, a hydrostatic pressure coefficient (T _SP/_p)_p0 (0.45±0.06) K/kbar is derived.     

Specific heat of the spin-Peierls compound CuGeO3

The specific heat of the quasi-onedimensional magnetic compound CuGeO₃ shows a sharp anomaly at the spin-Peierls transition temperatureT _sp . The experimental decrease of the magnetic specific heat belowT _sp indicates the presence of a spin gap as observed previously with inelastic neutron scattering. A magnetic field of 6T suppressesT _sp only slightly but reduces the spin gap by a factor of two.     

Thermal conductivity of the spin-Peierls compound CuGeO3

The thermal conductivity of the pure and zinc-doped spin-Peierls compound CuGeO₃ is investigated for the first time. Characteristic features reflecting the changes in the phonon spectrum of the crystals are observed in the k(T) curves at a transition into the dimerized state in CuGeO₃ at T _sp =14.2 K and in Cu_0.98Zn_0.02GeO₃ at T _sp =10.6 K. Near the spin-Peierls transition temperature T _sp the thermal conductivity of Cu_0.98Zn_0.02GeO₃ (k=3 W/m·K) is much less than the thermal conductivity of CuGeO₃ (k=21.5 W/m·K). Pis’ma Zh. éksp. Teor. Fiz. 66, No. 12, 823–826 (25 December 1997)     

Cu-isotope effect on the spin-Peierls transition of CuGeO3

The Cu-isotope effect on the spin-Peierls transition temperature T _SP in samples of the Cu²⁺ linear-chain compound CuGeO₃ has been determined by dc-magneticsusceptibility and specific-heat measurements. A downshift of the spin-Peierls transition by about 0.1 K for the ⁶⁵Cu-isotope enriched ⁶⁵CuGeO₃ as compared to ^natCuGeO₃ containing the natural mixture of isotopes is found consistently in the magnetization and specific heat. This result suggests different isotope effects on the phonon characteristic frequency θ₀ on the one hand and on the spin-phonon coupling constant g on the other hand. However, the isotope effect on T _SP is smaller than estimated if all atoms were to contribute to ω₀ and only the Cu atoms to g.     

Effect of structural distortions on the magnetism of doped spin-Peierls CuGeO3

The chemical selectivity and great sensitivity of the Extended X-ray Absorption Spectroscopy technique allowed the determination, in the paramagnetic phase, of the structural distortions induced by doping in the spin-Peierls CuGeO₃ compound. The distorted environments were analyzed as a function of concentration, magnetic nature of impurity and the substitution site (Ni, Mn and Zn impurities on the Cu site, Si impurity on the Ge site). This has led to estimate the variation of the angles and pair distances, and hence to evaluate the magnetic coupling along the Cu chains in the vicinity of the impurities. The antiferromagnetic interaction between Cu first neighbors in the pure sample is found to be weakened around Ni, almost cancelled in the case of Mn doping, and even to change sign, producing a ferromagnetic coupling for Si doping. More generally, the structural distortions on a local scale are shown to be key parameters for the understanding of the magnetic properties of doped spin-Peierls compounds.     

Field-controlled phase separation at the impurity-induced magnetic ordering in the spin-Peierls Magnet CuGeO3

The paramagnetic fraction surviving at the impurity-induced antiferromagnetic phase transition in the spin-Peierls magnet CuGeO3 is found to increase with an external magnetic field. This effect is explained by the competition of the Zeeman interaction and of the exchange interaction of local antiferromagnetic clusters formed on the spin-gap background near impurities.     

Solitons in the spin-Peierls compound CuGeO <Subscript>3</Subscript>

We calculate the excitation gap, the soliton energy, and the soliton density at finite temperature, of the spin-1/2 one dimensional antiferromagnet coupled to phonons, using a self-consistent harmonic approximation, and the thermal-Green function technique. The spin degrees of freedom are represented by the phase Hamiltonian with the help of the boson representation of the spinless fermions. We estimate the critical field H_c above which begins the incommensurate phase. We also present a theoretical calculation for the specific heat in this phase. We use CuGeO₃ as an example of a compound where our theory could be applied. Received 22 October 2002 / Received in final form 21 January 2003 Published online 14 March 2003