Martin Misek

Enforcing Multifunctionality: A Pressure-Induced Spin-Crossover Photomagnet

Photomagnetic compounds are usually achieved by assembling preorganized individual molecules into rationally designed molecular architectures via the bottom-up approach. Here we show that a magnetic response to light can also be enforced in a nonphotomagnetic compound by applying mechanical stress. The nonphotomagnetic cyano-bridged Fe(II)-Nb(IV) coordination polymer {[Fe(II)(pyrazole)4]2[Nb(IV)(CN)8]·4H2O}n (FeNb) has been subjected to high-pressure structural, magnetic and photomagnetic studies at low temperature, which revealed a wide spectrum of pressure-related functionalities including the light-induced magnetization. The multifunctionality of FeNb is compared with a simple structural and magnetic pressure response of its analog {[Mn(II)(pyrazole)4]2[Nb(IV)(CN)8]·4H2O}n (MnNb). The FeNb coordination polymer is the first pressure-induced spin-crossover photomagnet.

High-pressure effect on the superconductivity of YB6

Pressure effects on superconducting properties of two YB6 samples (T-c = 5.9 and 7.5 K) were investigated by measurements of electrical resistivity, magnetic susceptibility, and x-ray diffraction in the pressure range up to 320 kbar. Magnetoresistivity measurements down to 60 mK and up to 47 kbar have shown a negative pressure effect on T-c as well as on the third critical field H-c3 with the slopes dlnT(c)/dp = -0.59%/kbar and dlnH(c3)/dp = -1.1%/kbar, respectively. The magnetic susceptibility measurements evidenced that the slope of dlnT(c)/dp gradually decreases with pressure reaching a value three times smaller at 112 kbar. The lattice parameter measurements revealed the volume reduction of 14% at 320 kbar. The pressure-volume dependence is described by the Rose-Vinet equation of state. The obtained relative volume dependence dlnT(c)/dlnV analyzed by the McMillan formula for T-c indicates that the reduction of the superconducting transition temperature is mainly due to hardening of the Einstein-like phonon mode responsible for the superconducting coupling. This is confirmed by the analysis of the resistivity measurements in the normal state up to T = 300 K performed at pressures up to 28 kbar.

Low-temperature spin dynamics of a valence bond glass in Ba2YMoO6

We carried out ac magnetic susceptibility measurements and muon spin relaxation spectroscopy on the cubic double perovskite Ba2YMoO6, down to 50mK. Below 1K the muon relaxation is typical of a magnetic insulator with a spin-liquid type ground state, i.e. without broken symmetries or frozen moments. However, the ac susceptibility revealed a dilute-spin-glass-like transition below 1K. Antiferromagnetically coupled Mo 5+ 4d 1 electrons in triply degenerate t2g orbitals are in this material arranged in a geometrically frustrated fcc lattice. Bulk magnetic susceptibility data has previously been interpreted in terms of a freezing to a heterogeneous state with non-magnetic sites where 4d 1 electrons have paired in spin-singlets dimers, and residual unpaired Mo 5+ 4d 1 electron spins. Based on the magnetic heat capacity data it has been suggested that this heterogeneity is the result of kinetic constraints intrinsic to the physics

Synthesis and Properties of Lanthanide Ruthenium(III) Oxide Perovskites

An extensive series of new LnRuO3 perovskites has been synthesized at high pressure. These ruthenium(III)-based oxides are ruthenium deficient, and high-pressure samples have compositions close to LnRu(0.9)O3. These phases stabilize ruthenium(III) which is very unusual in oxides. X-ray and neutron powder diffraction studies show that the materials adopt orthorhombic perovskite superstructures in which the RuO6 octahedra are tetragonally compressed. These distortions, and the Mott insulator properties of the materials, are driven by strong spin-orbit coupling.

Raman spectroscopy of NdFeO3 at pressures up to 11 GPa

We report the theoretical and experimental study of evolution of the first-order Raman-active phonons in NdFeO3 with pressure up to 11 GPa at room temperature. With non-polarized light, we have observed 10 Raman-active modes. Our study confirmed no structural phase transition in the studied pressure range. We have calculated that weighted average Grüneisen parameter is ⟨γ⟩  = 1.19.

Use of an advanced composite material in construction of a high pressure cell for magnetic ac susceptibility measurements

The applicability of fibre-reinforced polymers for fabrication of high pressure cells was assessed using finite element analysis and experimental testing. Performance and failure modes for the key components of the cell working in tension and in compression were evaluated and the ways for optimising the designs were established. These models were used in construction of a miniature fully non-metallic diamond anvil cell for magnetic ac susceptibility measurements in a magnetic property measurement system. The cell is approximately 14 mm long, 8.5 mm in diameter and was demonstrated to reach a pressure of 5.6 GPa. AC susceptibility data collected on Dy2O3 demonstrate the performance of the cell in magnetic property measurements and confirm that there is no screening of the sample by the environment which typically accompanies the use of conventional metallic high pressure cells in oscillating magnetic fields.

Raman spectroscopy and magnetic properties of KMCr(CN)6 under pressure

We study the effect of pressure on Raman spectra as well on magnetic properties of molecule-based magnets KNiCr(CN)6 and KMnCr(CN)6. The effect of pressure on the ν[C≡N] vibration band which is located in the 2100–2200 cm−1 spectral range is relatively weak. Hydrostatic pressure has small almost a negligible effect on the Curie temperature of ferromagnetic KNiCr(CN)6 while leads to a pronounced reversible Curie temperature increase occurrence of new magnetic phase under pressure in the case of KMnCr(CN)6. Applied pressure affects magnetization curves only marginally. All pressure-induced changes are reversible.

Direct measurement of magnetization isotherms of Fe64Ni36 Invar alloy in a diamond anvil cell

Magnetization isotherms of the Fe64Ni36 Invar alloy have been measured under pressure up to 5.3 GPa in magnetic field up to 5 T using a diamond anvil cell and SQUID magnetometer. The unambiguous change of the pressure parameter dlnMS/dP (from −9 to −13×10−2 GPa−1) has been observed in a narrow pressure interval from 2.5 to 3.5 GPa at all temperatures in the range from 5 to 300 K. The pressure interval, where the sharp decrease in magnetization was observed, coincides with the critical pressures of the pressure-induced decrease in Fe-moment that were determined by the X-ray Magnetic Circular Dichroism and the X-ray Emission Spectra studies, recently. The pronounced decrease in the Curie temperature of the Fe64Ni36 alloy under pressure, dTC/dP = −44 ±2 K/GPa, has been confirmed.