B.V. Mill

Magnetic and spectral studies of Er2BaNiO5 and other rare earth nickelates

Abstract Optical absorption spectra, magnetic susceptibility and magnetization of nickelates R 2 BaNiO 5 (R=Nd-Gd, Dy-Tm) containing isolated NONi chains have been measured. Magnetic ordering temperatures were found to be considerably higher than the temperatures of the maxima in the magnetic susceptibility. Most of the compounds R 2 BaNiO 5 exhibit two metamagnetic transitions. By comparison with neutron diffraction data for Er 2 BaCuO 5 it was shown that the interaction R-Ni dominates and that the magnetic behavior of the rare earth subsystem can be satisfactorily described in a molecular-field framework.

Two magnetic transitions and metamagnetism in the R2BaCuO5 (R = Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb) compounds

Specific heat, ac magnetic susceptibility and magnetization in static and in pulse fields were measured for polycrystalline “green phase” R 2 BaCuO 5 (R = Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb) samples. Two magnetic phase transitions were found in these compounds at temperatures T N1 and T N2 . An assumption is made that the antiferromagnetic ordering of copper and rare earth subsystem occurs at T N1 and T N2 , respectively. The Dy, Ho and Er compounds have been shown to be metamagnetic at temperatures below T N2 .

Spectral studies of magnetic ordering in the cuprates R2BaCuO5 (R=Sm, Eu, Tm, Yb, Lu)

Abstract High resolution optical absorption spectra of an intrinsic R 3+ ion or Er 3+ probe have been measured in the “green phases” R 2 BaCuO 5 related to 123 high- T c superconductors. Evidence is given that the phase transitions observed in these compounds earlier are of magnetic nature. Additional spin-reorientation transitions were found in Yb 2 BaCuO 5 and Lu 2 BaCuO 5 . The suggestion is made that the magnetic structures of the copper subsystem are the same in each of the following two groups: (1) Y 2 BaCuO 5 and Lu 2 BaCuO 5 at T R N , Eu 2 BaCuO 5 ; (2) Yb 2 BaCuO 5 at T R N , Y 2 BaCuO 5 at T R , Tm 2 BaCuO 5 .

Magnetic interactions in R2BaCuO5 (R=Y, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, Lu) compounds

Abstract Complementary specific heat data on “green phase” R 2 BaCuO 5 (R=Y, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, Lu) compounds and Zn-based Gd 2 BaZnO 5 and Y 2 BaZnO 5 are obtained. Details of phase transition in Dy 2 BaCuO 5 , specific heat upturn in the Yb 2 BaCuO 5 , dimensionality of magnetic interactions in the Y 2 BaCuO 5 and Lu 2 BaCuO 5 , the role of Cu 2+ - O 2− - R 3+ - O 2− - Cu 2+ chains in magnetic exchange are discussed.

Low temperature specific heat of R2Cu2O5 (R = Y, Tb, Dy, Ho, Er, Tm, Yb, Lu) compounds

Specific heat of rare earth cuprates R 2 Cu 2 O 5 (R = Y, Tb-Lu) was measured in the temperature range 2.0 to 30 K. Specific heat anomalies corresponding to antiferromagnetic ordering of Cu 2+ ions have been found. The Neel temperatures vary from T N =10 K (R = Dy) to T N = 25 K (R = Er). For Yb 2 Cu 2 O 5 and Dy 2 Cu 2 O 5 compounds additional sharp peaks are observed at temperatures T = 9.5 K (R = Dy) and T = 6.8 K (R = Yb). These peaks may be associated with a spin-reorientation transition.

Magnetic phase transitions in the chain nickelates R2BaNiO5 (R = Sm, Eu, Tm) by optical spectroscopy

Abstract Magnetic ordering of Sm2BaNiO5 at 55 ± 4 K, of Eu2BaNiO5 at 23 ± 2 K and of Tm2BaNiO5 at 13 ±1 K was found by high resolution Fourier transform spectroscopy. The Sm3+ ground Kramers doublet is split by Δ=28 cm-1 in the magnetically ordered state of Sm2BaNiO5 while the first excited crystal field level lies at 25 cm-1. Low-dimensional magnetic correlations are present in all the compounds studied.

Magnetic ordering and metamagnetism in Dy2BaCuO5

Abstract Specific heat, ac magnetic susceptibility and magnetization of the “green phase” Dy 2 BaCuO 5 polycrystalline samples were measured. Two magnetic transitions have been found: antiferromagnetic ordering of Cu 2+ and Dy 3+ magnetic moments at 18 K and 10 K, respectively. The Dy 3+ magnetic ordering is the first order phase transition. The metamagnetic transition in dysprosium subsystem, induced by magnetic field, is observed at temperatures lower than 10 K.

Stark structure and exchange splittings of Nd3+ ion levels in chain nickelate Nd2BaNiO5

Diffuse transmittance spectra of polycrystalline samples of chain nickelate Nd2BaNiO5 were measured with high resolution (0.1 cm-1) over wide ranges of wavenumbers (1500–20000 cm-1) and temperatures (4.2–300 K). The energies of 54 Stark sublevels of the Nd3+ ion and exchange splittings of some of them were determined in the magnetically ordered state of Nd2BaNiO5 (TN = 47.5 ± 1 K). It was shown that the low-temperature magnetic properties of Nd2BaNiO5 are determined by exchange splitting (32 cm-1) of the ground state.

Two magnetic transitions in Gd2BaNiO5. Complex nature of the low-temperature transition

Abstract Magnetic ordering of the chain nickelate Gd2BaNiO5 at 58 ± 2 K was found from high resolution optical spectra of the Er3+ probe in it. Both Ni and Gd magnetic subsystems order simultaneously to some high temperature magnetic phase (HTP). Quite different, a low temperature magnetic phase (LTP), is observed below 21 K. The transition from the HTP to the LTP is complex and occurs through intermediate, in particular, incommensurate structures.

Anisotropy of the magnetic properties of the cuprates Dy2BaCuO5 and Ho2BaCuO5: Magnetic and spectroscopic investigations

The magnetization of single crystals of the cuprates Dy2BaCuO5 and Ho2BaCuO5 along different crystallographic directions is measured using a SQUID magnetometer in fields up to 54 kOe. Spectroscopic investigations of the exchange splitting of the levels of Er3+ probe ions in these cuprates are also performed. It is shown that the cuprates investigated are highly anisotropic antiferromagnets, which display quasi-Ising behavior in a magnetic field. The nature of the two spontaneous magnetic phase transitions detected in Dy2BaCuO5 and Ho2BaCuO5 is analyzed. It is shown that the low-temperature transition is due to an increase in rare-earth-copper exchange.