M. Zentková

Magnetocaloric effect in M–pyrazole–[Nb(CN)8] (M = Ni, Mn) molecular compounds

We report a study of magnetocaloric effect (MCE) in cyanido-bridged {[M(II)(pyrazole)(4)](2)[Nb(IV)(CN)(8)]·4H(2)O}(n) molecular compounds where M = Ni, Mn, pyrazole = C(3)H(4)N(2). The substances show a sharp phase transition to a long range magnetically ordered state, with ferromagnetic coupling between M and Nb sublattices in the case of the Ni-based sample 1 (T(c) = 13.4 K) and ferrimagnetic coupling for the Mn-based sample 2 (T(c) = 23.8 K). The magnetic entropy change ΔS due to applied field change ΔH as a function of temperature was determined by the magnetization and heat capacity measurements. The maximum value of ΔS at μ(0)ΔH = 5 T is 6.1 J mol(-1) K(-1) (5.9 J kg(-1) K(-1)) for 1 at T = 14 K and 6.7 J mol(-1) K(-1) (6.5 J kg(-1) K(-1)) for 2 at T = 25 K. MCE data at different applied fields have been presented as one universal curve, which confirms magnetic transitions in 1 and 2 to be of second order. The temperature dependences of the n exponent characterizing the dependence of ΔS on ΔH have been obtained. The n(T(c)) values, consistent with the shape of the magnetization curves, pointed to the 3D Heisenberg behaviour for 2 and some anisotropy, probably of the XY type, for 1. The (H/T(c))(2/3) dependence of the maximum entropy change has been tested in the ferrimagnetic Mn(2)-L-[Nb(CN)(8)] (L = C(3)H(4)N(2), C(4)H(4)N(2)) series.

High-pressure single-crystal XRD and magnetic study of a octacyanoniobate-based magnetic sponge

In the present study we report the structural and magnetic response of the 3-D cyano-bridged magnetic sponge-like system {[MnII(pydz)(H2O)2][MnII(H2O)2][NbIV(CN)8]·3H2O}nA to external pressure up to 1.8 GPa using single-crystal crystallography and magnetic measurements. The observed pressure-induced structural changes: shrinkage of the Nb–C bonds and bending of Mn–NC–Nb linkages are responsible for the strengthening of the antiferromagnetic interactions between Mn and Nb centers and consequently for the substantial increase of the magnetic ordering temperature from 43 K to 51 K under 0.57 GPa. The observed magneto-structural response to external pressure is similar in nature to the removal of guest molecules and confirms the significant susceptibility of molecular magnetic sponges to mechanical stress.

Magnetorheological properties of some ferrofluids

The effect of an external magnetic field, velocity of flow and concentration of the magnetite on the rheological properties of some mineral oil based ferrofluids have been investigated. It has been shown that the increase of viscosity in magnetic field depends upon the velocity flow of ferrofluid and concentration of magnetite. >

Lightweight x-ray optics for future space missions

The future X-ray astrophysics space missions require very light-weight but large and precise X-ray mirrors shells. Clearly, developments of innovative techniques and approaches are necessary. We discuss the possible alternative techniques with focus on the technologies and experience available in the Czech Republic. They include light ceramics replication by plasma spraying as well as by CVD and PVD technologies, SiC, thin glass technology, improved electroforming, glossy carbon, as well as glossy metals.

On the ferroelectric and magnetoelectric mechanisms in low Fe3+ doped TbMnO3

Abstract This work addresses the effect of substituting Mn 3+ by Fe 3+ at the octahedral site of TbMnO 3 on the magnetic phase sequence, ferroelectric and magnetoelectric properties, keeping the Fe 3+ concentration below 5%. The temperature dependence of the specific heat, dielectric permittivity and electric polarization was studied as a function of Fe 3+ concentration and applied magnetic field. From the experimental results a strong decrease of the electric polarization with increasing Fe 3+ substitution is observed, vanishing above a concentration of 4%. However, within this range, a significant increase of the magnetic sensitivity of the electric polarization is obtained by increasing Fe 3+ concentration. For Fe 3+ concentration above 4%, a non-polar phase emerges, whose spin structure prevents ferroelectricity according to the Dzyaloshinskii-Moriya model. The experimental results here reported reveal the crucial effect of B-site substitution on the magnetic phase sequence, as well as, on the polar and magnetoelectric properties, evidencing the important role played by the e g electrons on the stabilization of the magnetic structures that are suitable for the emergence of electric polarization.

Heat capacity, magnetic and lattice dynamic properties of TbMn1-xFexO3

The effect of substitution of Fe3+ ions by Mn3+ ions on crystal structure, lattice dynamic, heat capacity and magnetic properties in TbMn1-xFexO3 ceramics has been studied. X-ray powder diffraction and Raman spectroscopy revealed that lattice distortion can be mainly attributed to Jahn-Teller distortion and tilting of octahedrons for samples with x < 0.4; for higher Fe concentration, the distortions are dominated by the octahedra tilting with less contribution of the Jahn-Teller effect. The anomalies in heat capacity of parent compounds (TbMnO3 and TbFeO3), which are associated with magnetic transitions, are smeared out by ion substitution. Magnetization measurements indicate that magnetic ordering persists in whole concentration range. The butterfly-type magnetic hysteresis loops suggest that the magnetic ground state of the whole system is complex and thence interesting for next experimental and theoretical studies.

Magnetic Properties of (Cu_{x}Mn_{1-x})_3[Cr(CN)_6]_2·zH_2O Complexes

Magnetic Properties of (CuxMn1−x)3[Cr(CN)6]2·zH2O Complexes M. Vavra, M. Antoňák, Z. Jagličić, M. Mihalika,∗, M. Mihalik, K. Csach and M. Zentková Institute of Experimental Physics, SAS, Watsonova 47, 040 01 Košice, Slovakia Institute of Chemistry, P.J. Šafárik University, Moyzesova 11, 041 54 Košice, Slovakia Institute of Mathematics, Physics and Mechanics, University of Ljubljana, Jadranska 19, 1000 Ljubljana, Slovenia BENSC — Helmholtz-Zentrum, Glienicker Str. 100, 141 09 Berlin, Germany Magnetization measurements were performed on the (CuxMn1−x)3[Cr(CN)6]2·zH2O molecule-based magnets where x = 0.0, 0.2, 0.25, 0.3, 0.35 0.4, 0.6, 0.8 and 1.0. Both the Curie temperature and saturated magnetization at first decrease with increasing value of x reaching the minimal value of TC = 49.7 K and 0.17 μB for x = 0.2 and then increase with substitution. The pronounced hysteretic behavior between zero-field cooled and field cooled regimes was observed for all samples. Magnetization changes the sign of magnetic polarization in zero-field cooled magnetization curve at the compensation temperature Tcomp = 16 K for sample with x = 0.4. Our results indicate that the system behaves as mixed-ferri-ferromagnetic system.

Structure and Magnetism of Rare-Earth Ferricyanides and the Role of 4f Electrons

The Pr, Sm, Gd, Dy, and Ho ferricyanides have been prepared. These compounds were characterized structurally and magnetically. It has been shown that their magnetic properties are dominantly associated with the electronic structure of trivalent rare-earth ion. The details of the crystal structure were determined using x-ray diffraction technique by means of the Rietveld structure refinement method. Magnetic properties have been investigated by means of vibrating sample magnetometer in the magnetic fields up to 6 T and temperatures from 4.2 to 100 K.

Magnetic properties of uranium ferrocyanides and ferricyanides

The uranium ferricyanide and ferrocyanide have been synthesized. It has been shown that the prepared compounds are ferromagnetically ordered in the low temperature region with the critical temperature Tc = 4.3K and Tc = 4.41 K, respectively.

Pressure effect on magnetic properties of La0.67Ca0.33(Co x Mn1−x )O3 ceramics

In this communication, we compare effects of hydrostatic and chemical pressure on the magnetic properties of La0.67Ca0.33(Co x Mn1−x )O3 ceramics. For ceramics with x=0.03, the Curie temperature T C increases under hydrostatic pressure with a slope d T c/d p=+23.4 K/GPa. The saturated magnetization μs does not change with applied hydrostatic pressure. Chemical pressure was induced by substitution of Co for Mn. The substitution from x=0.03 to x=0.1 reduces the volume of the elementary cell by about 0.28%, but T C decreases from 203.4 to 167 K. The saturated magnetization μs is not affected by this substitution.