Thermal properties of [Cr(NH3)6](BF4)3[Cr(NH_3)_6](BF_4)_3 studied by adiabatic and relaxation calorimetry

Four (solid–solid) phase transitions were detected in the temperature range of (9 to 300) K in polycrystalline [Cr(NH3)6](BF4)3 at TC1 = 240.7 K, TC2 = 108.0 K, TC3 = 91.9 K, and TC4 = 61.3 K by adiabatic calorimetry. The measurements by relaxation calorimetry were followed on lowering temperature from 20 K down to 0.35 K under six different external magnetic field values (9, 7, 5, 3, 1 and 0) T. For non-zero values of applied magnetic field well-defined Schottky anomaly appears. Magnetic heat capacity was calculated assuming the zero-field splitting for the decoupled Cr(III) ions. There is no discrepancy between the observed and calculated values. Isothermal magnetization curve recorded up to 5 T was measured at temperature of 1.8 K

Crystal Structure and Magnetocaloric Effect of La0.80Ag0.15MnO3 Nanoparticles

Annealing of_ non stoichiometric La0.80Ag0.15MnO3 nanoparticles leads to the change of the crystal symmetry from Pnma to R3c and to almost doubling of its Curie temperature from 117 K to 317 K. This effect is due to the releasing of lattice distortions and strengthening of double exchange interaction. The large values of the magnetocaloric entropy of the order of ∆S = 6.19 J/(kg K) were observedat ∆µ0H = 5 T and T = 315 K.Proceedings of the 17th Czech and Slovak Conference on Magnetism, Košice, Slovakia, June 3–7, 2019

Strain relaxation dynamics of multiferroic orthorhombic manganites.

Resonant Ultrasound Spectroscopy has been used to characterise strain coupling and relaxation behaviour associated with magnetic/magnetoelectric phase transitions in GdMnO3, TbMnO3 and TbMn0.98Fe0.02O3 through their influence on elastic/anelastic properties. Acoustic attenuation ahead of the paramagnetic colinear-sinusoidal/incommensurate/antiferromagnetic transition at ~41 K correlates with anomalies in dielectric properties and is interpreted in terms of Debye-like freezing processes. A loss peak at ~150 K is related to a steep increase in electrical conductivity with a polaron mechanism. The activation energy, Ea, of ≥ ~0.04 eV from a loss peak at ~80 K is consistent with the existence of a well-defined temperature interval in which the paramagnetic structure is stabilised by local, dynamic correlations of electric and magnetic polarisation that couple with strain and have relaxation times in the vicinity of ~10-6 s. Comparison with previously published data for Sm0.6Y0.4MnO3 confirms that this pattern may be typical for multiferroic orthorhombic RMnO3 perovskites (R = Gd, Tb, Dy). A frequency-dependent loss peak near 10 K observed for TbMnO3 and TbMn0.98Fe0.02O3, but not for GdMnO3, yielded Ea ≥ ~0.002 eV and is interpreted as freezing of some magnetoelastic component of the cycloid structure. Small anomalies in elastic properties associated with the incommensurate and cycloidal magnetic transitions confirm results from thermal expansion data that the magnetic order parameters have weak but significant coupling with strain. Even at strain magnitudes of ~0.1-1 ‰, polaron-like strain effects are clearly important in defining the development and evolution of magnetoelectric properties in these materials. Strains associated with the cubic - orthorhombic transition due to the combined Jahn-Teller/octahedral tilting transition in the vicinity of 1500 K are 2-3 orders of magnitude greater. It is inevitable that ferroelastic twin walls due to this transition would have significantly different magnetoelectric properties from homogeneous domains due to magnetoelastic coupling with steep strain gradients.This work was funded by EPSRC Grant No. EP/ P024904/1 (UK). RUS facilities were established through grants from the Natural Environment Research Council (Grants No. NE/B505738/1 and No. NE/F017081/1) and the Engineering and Physical Sciences Research Council (Grant No. EP/I036079/1) to MAC. The work at the University of Warwick was supported by EPSRC,UK through Grant EP/T005963/1

Strain relaxation dynamics of multiferroic orthorhombic manganites

Resonant Ultrasound Spectroscopy has been used to characterise strain coupling and relaxation behaviour associated with magnetic/magnetoelectric phase transitions in GdMnO3, TbMnO3 and TbMn0.98Fe0.02O3 through their influence on elastic/anelastic properties. Acoustic attenuation ahead of the paramagnetic colinear-sinusoidal/incommensurate/antiferromagnetic transition at ~41 K correlates with anomalies in dielectric properties and is interpreted in terms of Debye-like freezing processes. A loss peak at ~150 K is related to a steep increase in electrical conductivity with a polaron mechanism. The activation energy, Ea, of ≥ ~0.04 eV from a loss peak at ~80 K is consistent with the existence of a well-defined temperature interval in which the paramagnetic structure is stabilised by local, dynamic correlations of electric and magnetic polarisation that couple with strain and have relaxation times in the vicinity of ~10-6 s. Comparison with previously published data for Sm0.6Y0.4MnO3 confirms that this pattern may be typical for multiferroic orthorhombic RMnO3 perovskites (R = Gd, Tb, Dy). A frequency-dependent loss peak near 10 K observed for TbMnO3 and TbMn0.98Fe0.02O3, but not for GdMnO3, yielded Ea ≥ ~0.002 eV and is interpreted as freezing of some magnetoelastic component of the cycloid structure. Small anomalies in elastic properties associated with the incommensurate and cycloidal magnetic transitions confirm results from thermal expansion data that the magnetic order parameters have weak but significant coupling with strain. Even at strain magnitudes of ~0.1-1 ‰, polaron-like strain effects are clearly important in defining the development and evolution of magnetoelectric properties in these materials. Strains associated with the cubic – orthorhombic transition due to the combined Jahn-Teller/octahedral tilting transition in the vicinity of 1500 K are 2-3 orders of magnitude greater. It is inevitable that ferroelastic twin walls due to this transition would have significantly different magnetoelectric properties from homogeneous domains due to magnetoelastic coupling with steep strain gradients

The Effect of Pressure on Magnetic Properties of Prussian Blue Analogues

We present the review of pressure effect on the crystal structure and magnetic properties of Cr(CN)6-based Prussian blue analogues (PBs). The lattice volume of the fcc crystal structure space group Fm 3 ¯ m in the Mn-Cr-CN-PBs linearly decreases for p ≤ 1.7 GPa, the change of lattice size levels off at 3.2 GPa, and above 4.2 GPa an amorphous-like structure appears. The crystal structure recovers after removal of pressure as high as 4.5 GPa. The effect of pressure on magnetic properties follows the non-monotonous pressure dependence of the crystal lattice. The amorphous like structure is accompanied with reduction of the Curie temperature (TC) to zero and a corresponding collapse of the ferrimagnetic moment at 10 GPa. The cell volume of Ni-Cr-CN-PBs decreases linearly and is isotropic in the range of 0⁻3.1 GPa. The Raman spectra can indicate a weak linkage isomerisation induced by pressure. The Curie temperature in Mn2+-CrIII-PBs and Cr2+-CrIII-PBs with dominant antiferromagnetic super-exchange interaction increases with pressure in comparison with decrease of TC in Ni2+-CrIII-PBs and Co2+-CrIII-PBs ferromagnets. TC increases with increasing pressure for ferrimagnetic systems due to the strengthening of magnetic interaction because pressure, which enlarges the monoelectronic overlap integral S and energy gap ∆ between the mixed molecular orbitals. The reduction of bonding angles between magnetic ions connected by the CN group leads to a small decrease of magnetic coupling. Such a reduction can be expected on both compounds with ferromagnetic and ferrimagnetic ordering. In the second case this effect is masked by the increase of coupling caused by the enlarged overlap between magnetic orbitals. In the case of mixed ferro⁻ferromagnetic systems, pressure affects μ(T) by a different method in Mn2+⁻N≡C⁻CrIII subsystem and CrIII⁻C≡N⁻Ni2+ subsystem, and as a consequence Tcomp decreases when the pressure is applied. The pressure changes magnetization processes in both systems, but we expect that spontaneous magnetization is not affected in Mn2+-CrIII-PBs, Ni2+-CrIII-PBs, and Co2+-CrIII-PBs. Pressure-induced magnetic hardening is attributed to a change in magneto-crystalline anisotropy induced by pressure. The applied pressure reduces saturated magnetization of Cr2+-CrIII-PBs. The applied pressure p = 0.84 GPa induces high spin⁻low spin transition of cca 4.5% of high spin Cr2+. The pressure effect on magnetic properties of PBs nano powders and core⁻shell heterostructures follows tendencies known from bulk parent PBs

Synthesis of hexagonal YMnO3 from precursor obtained by the glycine-nitrate process

A new synthesis route for obtaining a single-phase hexagonal YMnO3 was developed, based on the annealing of the amorphous precursor powder obtained by the glycine-nitrate combustion method. The process was monitored by XRD as well as by magnetic and heat capacity measurements. The analysis of these data shows that precursor powder undergoes a gradual phase transformation that depends on the annealing temperature. The metastable orthorhombic YMnO3 phase is the first to appear at temperatures below 700 degrees C, while a mixture of ortho- and hexa-phases was found to exist within the 700-900 degrees C range. An almost complete conversion to hexaphase occurs at 900-1000 degrees C, but to obtain the pure, well crystallized h-YMnO3 phase an annealing temperature of 1300 degrees C was necessary. The synthesis method applied in this work has been proved to be more convenient for obtaining single-phase h-YMnO3 than by a solid state reaction. In addition, it is capable of producing sample free of parasitic phases that are often present in specimens obtained by other synthesis methods. (C) 2012 Elsevier Ltd and Techna Group S.r.l. All rights reserved

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 {[Mn II (pydz)(H 2 O) 2 ][Mn II (H 2 O) 2 ][Nb IV (CN) 8 ] ? 3H 2 O} n A 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

Magnetocaloric effect in M-pyrazole-[Nb(CN)8][Nb(CN)_{8}] (M=Ni, Mn) molecular compounds

We report a study of magnetocaloric effect (MCE) in cyanido-bridged {[MII(pyrazole)4]2[NbIV(CN)8]⋅4H2O}n molecular compounds where M = Ni, Mn, pyrazole = C3H4N2. 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 (Tc = 13.4 K) and ferrimagnetic coupling for the Mn-based sample 2 (Tc = 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 μ0ΔH = 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(Tc) 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/Tc)2/3 dependence of the maximum entropy change has been tested in the ferrimagnetic Mn2–L–[Nb(CN)8] (L = C3H4N2, C4H4N2) series

Magnetocaloric effect and critical behavior in Mn2−imidazole−[Nb(CN)8]Mn_2-imidazole-[Nb(CN)_8] molecular magnetic sponge

A comprehensive study of magnetocaloric effect (MCE) and critical behavior in the {Mn2(imH)2(H2O)4[Nb(CN)8]·4H2O}n molecular magnet is reported. The compound is an example of a magnetic sponge, where structural changes provoked by dehydration process lead to the increase of Tc critical temperature from 25 K for the as-synthesized sample (1) up to 60 K for the anhydrous one (2). MCE and critical behavior were investigated by magnetization measurements. The maximum value of magnetic entropy change ΔS, determined by the magnetization measurements for 1 is 6.70 J mol−1 K−1 (8.95 J kg−1 K−1) at µ0ΔH=5 T, while for 2 it is equal to 4.02 J mol−1 K−1 (7.73 J kg−1 K−1) at the same magnetic field change. The field dependence of MCE at Tc for 1 and 2 was consistent with critical exponents, which allowed to classify both phases to 3D Heisenberg universality class. The View the MathML sourceTc-2/3 dependence of the maximum entropy change has been tested using data of 1 and 2 together with MCE data previously reported for other members of the ferrimagnetic Mn2-l-[Nb(CN)8] (L=imidazole, pyridazine and pyrazole) series. Experimental MCE results have been compared with the spin contribution to the magnetic entropy change estimated using a molecular field approximation