Magnetization and specific heat of the dimer system CuTe2O5

We report on magnetization and specific heat measurements on single-crystalline CuTe2O5. The experimental data are directly compared to theoretical results for two different spin structures, namely an alternating spin-chain and a two-dimensional (2D) coupled dimer model, obtained by Das et al. [Phys. Rev. B 77, 224437 (2008)]. While the analysis of the specific heat does not allow to distinguish between the two models, the magnetization data is in good agreement with the 2D coupled dimer model.Comment: 5 pages, 3 figure

Destruction of long range magnetic order in an external magnetic field and the associated spin dynamics in Cu2GaBO5 and Cu2AlBO5 ludwigites

The quantum spin systems Cu2M BO5 M Al,Ga with the ludwigite crystal structure consist of a structurally ordered Cu2 sublattice in the form of three leg ladders, interpenetrated by a structurally disordered sublattice with a statistically random site occupation by magnetic Cu2 and nonmagnetic Ga3 or Al3 ions. A microscopic analysis based on density functional theory calculations for Cu2GaBO5 reveals a frustrated quasi two dimensional spin model featuring five inequivalent antiferromagnetic exchanges. A broad low temperature 11B nuclear magnetic resonance points to a considerable spin disorder in the system. In zero magnetic field, antiferromagnetic order sets in below TN approximation 4.1 K and 2.4 K for the Ga and Al compounds, respectively. From neutron diffraction, we find that the magnetic propagation vector in Cu2GaBO5 is commensurate and lies on the Brillouin zone boundary in the H0L plane, qm 0.45, 0, 0.7 , corresponding to a complex noncollinear long range ordered structure with a large magnetic unit cell. Muon spin relaxation is monotonic, consisting of a fast static component typical for complex noncollinear spin systems and a slow dynamic component originating from the relaxation on low energy spin fluctuations. Gapless spin dynamics in the form of a diffuse quasielastic peak is also evidenced by inelastic neutron scattering. Most remarkably, application of a magnetic field above 1 T destroys the static long range order, which is manifested in the gradual broadening of the magnetic Bragg peaks. We argue that such a crossover from a magnetically long range ordered state to a spin glass regime may result from orphan spins on the structurally disordered magnetic sublattice, which are polarized in magnetic field and thus act as a tuning knob for field controlled magnetic disorde

Flux crystal growth of Cu<inf>2</inf>GaBO<inf>5</inf> and Cu<inf>2</inf>AlBO<inf>5</inf>

© 2020 Elsevier B.V. The results of growth of two oxyborate single crystals, Cu2GaBO5 and Cu2AlBO5, with ludwigite structure from different flux systems using the spontaneous nucleation technique are reported. We obtained large Cu2GaBO5 and Cu2AlBO5 crystals of dimensions up to 3 × 4 × 15 mm3 and 1 × 1 × 10 mm3, respectively. Trimolybdate-bismuth fluxes diluted with the sodium carbonate for both Cu2GaBO5 and Cu2AlBO5, as well as lithium-molybdate and lithium-borate fluxes for Cu2AlBO5, were used. It was observed that the growth of Cu2GaBO5 and Cu2AlBO5 is accompanied by the growth of CuO, Cu2Al6B4O17, and Cu2Al1-xGaxBO5 crystals as secondary phases. Structural and magnetic characterization of the crystals is also performed

EPR spectroscopy of ⁵³Cr monoisotopic impurity ions in a single crystal of yttrium orthosilicate Y<inf>2</inf>SiO<inf>5</inf>

© 2020 Elsevier B.V. The electron paramagnetic resonance spectra of 53Cr monoisotopic impurity ions in yttrium orthosilicate single crystals (Y2SiO5) were studied by electron paramagnetic resonance (EPR) in the X-band. The directions of the principal magnetic axes and the parameters of the effective spin Hamiltonian, which describes magnetic characteristics of the impurity centers of chromium are determined. It is shown that the orientation dependencies of the EPR spectra are described by the second-order spin Hamiltonian corresponding to the rhombic symmetry of the local crystal field acting on the impurity ion. It was assumed that the g-tensor and A-tensor describing the Zeeman energy of electron levels in a magnetic field and the hyperfine interaction of electron and nuclear spins are isotropic, and the entire anisotropy of the EPR spectra is due to the anisotropy of the D-tensor, which describes the fine structure of electron levels in crystal electric field. A strong dependence of the probability of forbidden transitions between hyperfine sublevels of electron levels on the orientation of an external magnetic field is established. Moreover, for some orientations, the probability of forbidden transitions becomes comparable to the probability of allowed transitions

EPR Spectroscopy of ⁵³Cr ^{3 +} Monoisotopic Impurity Ions in a Single Crystal of Scandium Orthosilicate Sc <inf>2</inf>SiO <inf>5</inf>

Monoisotopic 53Cr 3 + impurity ions in scandium orthosilicate single crystal (Sc 2SiO 5) are studied by the method of electron paramagnetic resonance in the X-band frequencies. The directions of the main principal magnetic axes and the parameters of the effective spin Hamiltonian that describe the magnetic characteristics of the impurity centers of chromium, which replaces scandium in two structurally nonequivalent positions, are determined. It is shown that the orientation dependencies of the EPR spectra are well described by the second-order spin Hamiltonian corresponding to the orthorhombic symmetry of the local crystal field acting on the impurity ion. It was assumed that the g-tensor and the A-tensor determining the Zeeman energy of electronic levels in a magnetic field and the hyperfine interaction of electron and nuclear spins are isotropic, and the entire anisotropy of the EPR spectra is due to the anisotropy of the D-tensor, which describes the fine structure of electronic levels in a crystalline electric field. A strong dependence of the probability of “forbidden” transitions between hyperfine sublevels of electronic levels on the orientation of an external magnetic field is established. Moreover, for some orientations, the probability of “forbidden” transitions exceeds the probability of “allowed” transitions

Magnetic properties of ludwigite Mn<inf>2.25</inf>Co<inf>0.75</inf>BO<inf>5</inf>

Needle-shape single crystals of Mn2.25Co0.75BO5 ludwigite were synthesized by the flux technique and investigated by means of X-ray diffraction, static magnetic susceptibility and specific heat measurements. Magnetization and specific heat experiments showed the presence of ferrimagnetic ordering near TN≃61K. Below this temperature the magnetic properties demonstrated the angular dependence and the high magnetic hardness in a magnetic field applied perpendicular to the crystallographic c axis, which is similar to other cobalt-containing ludwigites. The effective magnetic moment per magnetic ion 4.8μB was calculated from the paramagnetic data. Analysis of the phonon contribution to the specific heat allowed to distinguish the magnetic contribution CM, which has two features. One of which was observed at TN and can be attributed to strong exchange interactions between spins of magnetic ions. The second one at T = 11.6 K is due to the effect of the ligands crystal field on a magnetic ion with S>1∕2

Structure, magnetic and thermodynamic properties of heterometallic ludwigites: Cu<inf>2</inf>GaBO<inf>5</inf> and Cu<inf>2</inf>AlBO<inf>5</inf>

© 2020 Elsevier B.V. We present an extensive study of the structural, magnetic and thermodynamic properties of high-quality monocrystals of two heterometallic oxyborates from the ludwigite family: Cu2GaBO5 and Cu2AlBO5 in the temperature range above 2 K. The distinctive feature of the investigated structures is the selective distribution of Cu and Ga/Al cations. The unit cell of Cu2GaBO5 and Cu2AlBO5 contains four nonequivalent crystallographic sites of metal ions. Two sites in the structure of Cu2GaBO5 are fully occupied by Cu atoms which form the quasi one-dimensional chains along the a-axis. For Cu2AlBO5 all sites are partially occupied by Al and Cu atoms. The joint analysis of low-temperature data on magnetic susceptibility and magnetic contribution to the specific heat showed that Cu2AlBO5 and Cu2GaBO5 exhibit an antiferromagnetic transition at TN=2.4 and 4.1 K, respectively. The magnetic response below TN can be represented as the sum of two contributions: the ordered component and the paramagnetic contribution from defects of the same relative weight. It is shown that the external magnetic field above 2.5 T and 2.8 T for Cu2GaBO5 and Cu2AlBO5, respectively, leads to a broadening of the magnetic phase transition indicating suppression of the long-range antiferromagnetic order

Synthesis, structure, magnetic behavior and dielectric relaxation of the La<inf>x</inf>Sr<inf>2-x</inf>Fe<inf>х</inf>Ti<inf>1-х</inf>O<inf>4</inf> (х ​= ​0.5, 0.7) oxide ceramic

© 2020 Elsevier Inc. This study is devoted to the investigation of the high dielectric constant causes in complex oxides with a structure of the K2NiF4 type. А new thermobaric treated ceramics on the basis conjugate LaxSr2-xFeхTi1-хO4 (х ​= ​0.5, 0.7) solid solutions was synthesized and the study their structure, microstructure, magnetic and dielectric properties was performed. It is shown that antiferromagnetic interactions coexist with ferromagnetic, which become dominant towards to low temperatures; the appearance of two types of magnetic interactions may be related to the presence of magnetic ions of different valences. Different values of the dielectric constants ε are observed in wide region of frequencies 10–107 ​Hz. In obtained at ambient pressure LaxSr2-xFeхTi1-хO4 (х ​= ​0.5, 0.7) ceramics the highest permittivity ε value is only 30–50 in the frequency range from 1 ​kHz to 1 ​MHz. After the samples treatment at 1273 ​K and P ​= ​4 ​GPa during 5 ​min ε increases to 5–102–103 ​at 293 ​К and independent of frequency in the range (102–106) Hz. At the temperature increase the permittivity as well increases and the ε value becomes ~106 at, approximately, f ​= ​100 ​Hz and T ​= ​750 ​K. An obvious change of samples microstructure and polyhedra structure anisotropy in LaxSr2-xFeхTi1-хO4 (х ​= ​0.5, 0.7) was observed after the thermobaric treatment. Described in this article performed dielectric properties investigations indicate that possible reasons of the high-permittivity origin are specifics of layered structure, microstructure and charge polarization associated with it, Maxwell-Wagner polarization at the grain boundaries and inhomogeneities and small polaron hopping conduction mechanism

Influence of oxygen vacancies on magnetic and dielectric properties of nanocrystaline barium titanate

© 2020 Taylor & Francis Group, LLC. Influence of thermal treatment in H2 atmosphere on magnetic and dielectric properties of nanocrystalline BaTiO3 samples was studied. It was found that increase of the concentration of oxygen vacancies during prolonged annealing in hydrogen leads to a noticeable increase in the diamagnetic response and a decrease in the magnetization and dielectric permittivity. Observed changes of magnetic properties probably are caused by the modification in a state of oxidation of titanium ions in the surface layer. Along with these the experimental results revealed that magnetization of the nanocrystalline BaTiO3 is proportional to the total surface area of nanogranules in the sample

Iron oxidation state in La<inf>0.7</inf>Sr<inf>1.3</inf>Fe<inf>0.7</inf>Ti<inf>0.3</inf>O<inf>4</inf> and La<inf>0.5</inf>Sr<inf>1.5</inf>Fe<inf>0.5</inf>Ti<inf>0.5</inf>O<inf>4</inf> layered perovskites: Magnetic properties

La0.5Sr1.5Fe0.5Ti0.5O4 and La0.7Sr1.3Fe0.7Ti0.3O4 solid solutions with the layered perovskite structure were synthesized using a solid state method. Structural properties of obtained samples were characterized using X-ray diffraction and X-ray fluorescence analyses. Magnetic properties were investigated using magnetometry, electron spin resonance (ESR) and Mössbauer spectroscopy methods. Based on magnetization and ESR measurements it was suggested the presence of Fe4+ ions in addition to trivalent iron ions that was exactly confirmed by Mössbauer spectroscopy investigations. Based on all experimental results one can suggest the presence of the electronic phase separation in the investigated samples – the simultaneous existence of the paramagnetic phase and magnetically correlated regions, which form due to the mixed-valence iron ions. So the paramagnetic phase with strong antiferromagnetic correlation exists in both samples, while the second phase is ferromagnetically and ferrimagnetically correlated regions in La0.5Sr1.5Fe0.5Ti0.5O4 and La0.7Sr1.3Fe0.7Ti0.3O4, respectively