The electron paramagnetic resonance spectra of Gd3+ in single crystals of (Gd0.5Eu0.5)2CuO4 show an anomalous anisotropy for temperatures below the magnetic ordering of the Cu ions (T approximately=280 K). The authors have performed a detailed experimental study of this feature, and they discuss their results in terms of a model that assumes a weak Heisenberg interaction between Cu and Gd moments. The authors show that that anisotropy is a consequence of the dynamic coupling of the Gd3+ paramagnetic mode with a weak ferromagnetic mode originating in the Cu magnetic moments.

ESR measurements of the microwave absorption signal associated with weak ferromagnetism in single crystals of Eu2-xGdxCuO4 are presented for X band (9.5 GHz) and L band (1.2 GHz) as a function of the Gd concentration. The strong absorption observed at low magnetic fields was interpreted, for samples with low Gd concentration, as due to a field-induced spin reorientation transition occurring at a critical field Hc, coincident with the in-plane magnetic anisotropy effective field Heffy. For larger x the Cu-Gd magnetic interaction needs to be considered leading to smaller Hc values. Our measurements show that for Gd concentrations in the range 0≤x≤1 the experimental data can be very well fitted with parameters derived from previous measurements. This fact indicates that these compounds have nearly the same in-plane anisotropy effective field, in spite of the small changes in lattice parameters. For x=2 lattice distortions increase causing an Hc larger than the expected one. © 1996 American Institute of Physics.

In the heart, molecular signaling studies are usually performed in isolated myocytes. However, many pathological situations such as ischemia and arrhythmias can only be fully understood at the whole organ level. Here, we present the spectroscopic technique of local field fluorescence microscopy (LFFM) that allows the measurement of cellular signals in the intact heart. The technique is based on a combination of a Langendorff perfused heart and optical fibers to record fluorescent signals. LFFM has various applications in the field of cardiovascular physiology to study the heart under normal and pathological conditions. Multiple cardiac variables can be monitored using different fluorescent indicators. These include cytosolic [Ca2+], intra-sarcoplasmic reticulum [Ca2+] and membrane potentials. The exogenous fluorescent probes are excited and the emitted fluorescence detected with three different arrangements of LFFM epifluorescence techniques presented in this paper. The central differences among these techniques are the type of light source used for excitation and on the way the excitation light is modulated. The pulsed LFFM (PLFFM) uses laser light pulses while continuous wave LFFM (CLFFM) uses continuous laser light for excitation. Finally, light-emitting diodes (LEDs) were used as a third light source. This non-coherent arrangement is called pulsed LED fluorescence microscopy (PLEDFM).

We have measured the specific heat of GdxEu1-xBa2Cu3O7-δ (0 ≤ × ≤ 1). The data show a λ-type anomaly at high concentrations and a broad Schottky type anomaly at intermediate and low concentrations. We compare this behavior with theoretical predictions for a 2-dimensional Ising system. © 1988.

We report a magnetic-resonance study of Gd-doped Eu2CuO4 single crystals. Cooling the samples in a magnetic field HFC, induces weak ferromagnetism (WF), with a strong out-of-plane anisotropy determined by the Dzyaloshinsky-Moriya (DM) interaction. In addition, there is in-plane anisotropy with an easy-axis parallel to the [110] crystal axis closest to HFC. An intense resonance mode is observed at the X band (9.5 GHz) when HFC is applied parallel to one of the 110 axes and the measuring field is rotated by 90°in the CuO2 plane. At the Q band (35 GHz), the in-plane resonance modes strongly depend on angle and temperature. We analyze the experimental results in terms of a phenomenological model for the magnetic free energy, which predicts a reorientation transition of the WF component of the magnetization mWF induced by the external field. Associated with this transition, a softening of the WF magnetic resonance mode occurs when the external field is applied perpendicular to the easy magnetization axis. The resulting angular variation of the resonance modes depends on whether the energy gap for the magnetic excitations is larger or smaller than the microwave energy. From the resonance data we have determined both the out-of-plane and in-plane anisotropy fields, HDM(T) and Hax(T), respectively. The extrapolated values for T=0 are HDM(0)=3.5(5)×105 G and Hax(0)=12(2) G. Both anisotropy fields decrease with increasing T, vanishing around TN243 K. The temperature dependence of the peak-to-peak linewidths, ΔHpp, measured at the X and Q bands is explained in terms of a temperature-independent frequency linewidth, Δω1/2/γ=1.6(2) kG. Nonresonant absorption losses around the maxima and minima of the ω/γ vs H curves are also described in terms of this finite width for the resonance modes. © 1993 The American Physical Society.

We report here experimental data on the crystal-field interaction of gadolinium ions with their environment in GdxEu1-xBa2Cu3O7-. The ESR spectrum and a Schottky anomaly in the specific heat of dilute samples (x1) indicate the existence of a crystal-field splitting of the S728 ground state of Gd3+ ions of about 1.5 K. We discuss the parametrization of the measured splitting in terms of an effective Hamiltonian, and the microscopic origin of the interaction within the framework of a superposition model. Since the single-ion energies involved are of the same order of magnitude as the energies associated with the magnetic ordering of the Gd3+ moments in GdBa2Cu3O7- (TN 2.24 K), we analyze the effects of the crystal-field interaction on the magnetic transition. © 1988 The American Physical Society.