Search

We report measurements of magnetic, magnetothermal and magnetoelastic properties of a new Cu(II)-coordination polymer Cu(II)-2,5-bis(pyrazol-1-yl)-1,4-dihydroxybenzene (CuCCP). According to our results which cover wide ranges of temperatures 0.06 K ⩽ T ⩽ 320 K and magnetic fields B ⩽50 T, this material presents an almost ideal realization of an one-dimensional uniform S = 1 2 Heisenberg antiferromagnet. Owing to the moderate exchange-coupling constant of | J | / k B = 21.5 K , it was possible to study the system in its interesting high-field range, i.e., across the saturation field g μ B B s = 2 | J | , which, at T = 0 , marks the endpoint of a quantum critical line. Using pulse-field techniques the high-field magnetization and elastic constant have been measured. A comparison with calculated magnetization curves reveals a distinct magnetocaloric effect at high fields for T 7.5 K , which grows upon cooling. In addition, at temperatures T | J | / k B , a pronounced acoustic anomaly has been found close to B s and identified as a generic property of the uniform antiferromagnetic Heisenberg chain with a finite spin–lattice interaction.

The ESR experiment provides us with much information on the ground and excited states of Haldane systems in a magnetic field. In this paper recent experimental results are reported on two typical Haldane materials, NENP and NINO. Then a theoretical interpretation is given on the spectrum; it is shown in particular how the ESR spectrum is related to the nature of the ground and excited states, which change with the applied magnetic field.

In the antiferromagnetic heavy fermion superconductor URu 2 Si 2 we observe the spin flop transition around 39 T below the Neel temperature T N using longitudinal sound. The c 33 -mode exhibits only one-step like anomaly at the spin flop transition in contrast to the three steps found in magnetization, magnetoresistance measurements and the longitudinal c 11 mode. This behaviour can be explained using the Gruneisen parameter coupling for the field dependence of the elastic constants. Above the Neel temperature of 17.5 K we find a softening of the c 33 elastic constants in high magnetic fields.

We present sound velocity data in high fields for the two plateau substances SrCu 2 (BO 3 ) 2 and NH 4 CuCl 3 . Pronounced sound velocity and attenuation anomalies are observed in the region between the magnetization plateaus. We interpret these as a resonant interaction between the sound wave mode and the soft magnetic excitations.

Metamagnetic behaviour exists in various compounds with highly correlated electrons. In La 2 CuO 4 it can be understood with a spin model including intra-, interlayer and asymmetric exchange contributions and their various anisotropies. For B¦b the anisotropy of the interlayer exchange leads to the metamagnetic transition at 5.3 T. For B¦c two spin flop transitions at 9.1 and 20.4 T are obtained. The field dependence of k = 0 spin waves is calculated. In heavy fermion compounds e.g. CeRu 2 Si 2 the metamagnetic transition at B c is driven by a strong Gruneisen-parameter coupling of volume strains and heavy electron bands.

Results of ultrasonic studies of the magnetoresistive compound Nd 0.5 Sr 0.5 MnO 3 in a broad temperature range (1.5–180 K) are reported. With decreasing temperature the pronounced acoustic mode hardening was observed at about T CO =145 K. The change of sound velocity around T CO was explained by a strong coupling between the acoustic phonons and the charge-ordered states.

EPR experiments were performed on powder samples of the quasi-one-dimensional S = 1 spin systems NENC and TMNIN in the frequency range 55–400 GHz in magnetic fields up to 14 T. In both materials broad and asymmetric line shapes were observed pointing to a strong planar anisotropy. We interpreted the measured line shapes using a single-ion Hamiltonian. Transition matrix elements and occupation number of energy levels are taken into account in these calculations. In this way we obtain a good estimate for both the planar anisotropy D and the in-plane anisotropy E.

We present high-field Electron Spin Resonance (ESR) measurements on the spin-Peierls compound CuGeO3. We distinguish between two types of ESR transitions in the dimerized phase. They are associated with two energy gaps located at different points in the Brillouin zone. The renormalization of a q = 0 gap at E′g = 64 K as a function of temperature is measured.

Sound propagation experiments are used to investigate the superconducting (sc) phases of Heavy Fermion (HF) compounds. Velocity measurements lead to accurate B−T phase diagrams and results for the temperature dependent attenuation of all modes in UPt3 are presented. In addition to the strain-order parameter interaction for longitudinal modes there is a Lorentz force coupling of transverse modes to the normal electrons. It leads to a B-dependence of velocity and attenuation from which information about both London and skin penetration depth in URu2Si2 and UBe13 may be obtained. In UBe13 magnetoacoustic quantum oscillations are observed which prove the coexistence of light and heavy electrons in this compound.

We present magnetic resonance experiments in the frequency range of 35 to 460 GHz at low temperatures (1.5 K) for the two compounds CsNiCl 3 and (CH 3 ) 4 NMnCl 3 (TMMC). In both cases the large frequency range enables us to make a precise determination of the various coupling constants (exchange interactions and anisotropy parameters). For CsNiCl 3 we found a larger interchain exchange than previously determined. These constants can then be used for a critical evaluation of other related experiments. In TMMC we found resonances which cannot be explained by existing models.

Magnetoacoustic oscillations in LaAg at low temperatures and high magnetic fields are presented. The de Haas-Shubnikov frequencies are interpreted using a Fermi surface model deduced from a band structure calculation.

A survey is given of crystal field effects in intermetallic rare earth compounds. The origin of this CEF is briefly discussed. The various mechanisms of the coupling of electrons and phonons to the rare earth ion are mentioned and various applications are given. The coupling between magnetic ions, arising from these mechanisms, can lead to magnetic and structural phase transitions and to a variety of collective effects.

We present results of elastic measurements for UN, UPd 3 , UBe 13 and UPt 3 . Exchange striction effects are seen for UN and crystal field anomalies for UPd 3 . Polycrystalline UBe 13 exhibits an anomalously low Poisson ratio. UPt 3 shows low temperature anomalies in various elastic constants and strong anharmonic effects for the c 44 mode.

We give a review on electron-phonon effects. Thermal expansion, elastic constant and ultrasonic attenuation give evidence of a strong electronic Gruneisen parameter leading to a strong strain dependence of the width of the heavy fermion band. One can account for the observed effects in terms of a two band model. Ultrasonic attenuation phenomena in the superconducting phase are discussed.

We present calculations for dipolar surface spin waves in antiferromagnets at low temperatures. We calculate bulk and surface spin wave spectra for long wavelengths in the magnetostatic mode region. The different phases in an applied field along the easy axis are discussed with special emphasis on the spin flop- and paramagnetic phase. Nonreciprocal features of the surface spin waves are discussed. Numerical applications for GdAlO 3 are given.

Sound waves propagating in intermetallic 4f- or 5f-compounds can couple to localized f-electrons (magnetoelastic interaction) or to 5d-, 4f- and 5f-band states (deformation potential coupling). In this review we concentrate on electron-phonon coupling in systems with unstable magnetic ions (intermediate valence compounds, Kondo lattices, heavy fermion uranium compounds). We discuss magnetoelastic phenomena for CeAl 2 , CeB 6 , CeAl 3 , deformation potential coupling for CeAl 3 , UPt 3 , intermediate valence compounds, magnetoacoustic quantum oscillations for several rare earth compounds and ultrasonic attenuation in UPt 3 .

We give a discussion of the magnetic field-temperature phase diagram and of the temperature-pressure phase diagram of CeAl 2 . Experimental results of sound velocity, thermal expansion and specific heat measurements are used to determine the different magnetically ordered phases.

TmCd and TmZn exhibit pure quadrupolar structural phase transitions at 3.16 K and 8.55 K respectively. The magneto-elastic and quadrupolar coupling constants for these two compounds have been determined by different methods. The structural phase transition temperature T a is strongly field dependent. We show the measured magnetic phase diagram T a ( B ) together with the calculated one. The strong first order phase transitions at T a are due to the Γ 5 groundstate of the Tm 3+ -ion in these materials.

Absolute sound velocity measurements lead to large differences in bulk moduli and Poisson ratios for superconducting and non-superconducting compounds of copper-oxyde compounds. This is direct evidence for the importance of valence fluctuations in these systems. In addition the temperature dependence of sound velocity and attenuation is discussed.

The elastic constants of TmAl 2 have been determined as a function of temperature. The crystal field effects on the elastic constants are clearly observed. A Comparison of elastic and magnetoelastic coupling constants for the REAl 2 compounds is also given.