J. Żukrowski

Co–NC–W and Fe–NC–W Electron‐Transfer Channels for Thermal Bistability in Trimetallic {Fe6Co3[W(CN)8]6} Cyanido‐Bridged Cluster

The design and construction of switchable materials attracts tremendous interest owing to the potential in information storing and processing or molecular sensing. The archetypal examples involve a diversity of Fe-, 6] Feor Cobased spin-crossover (SCO) compounds, Co-catecholate/semiquinone systems, 11] as well as d-d bimetallic and sd-d trimetallic cyanide-bridged systems revealing chargetransfer-induced spin transitions (CTIST). Some of these compounds, for example Prussian blue analogues, are particularly promising from the point of view of photoswitching between nonmagnetic and magnetized (that is, TB, TC) states, owing to magnetic coupling through molecular bridges in discrete species 15] and extended networks. 18] Such bistability also emerged in the magnetochemistry of octacyanidometalates, exploiting metal-to-metal electron transfer in CoL[W(CN)8] 3 (L = pyrimidine, 4-methylpyridine) or canonical SCO in FeL[Nb(CN)8] 4 extended networks (L = 4-pyridinealdoxime). A magnetic hysteresis loop with a coercivity of 1–3 T were observed in an optically excited low-temperature metastable phase. As a continuing effort to obtain innovative bistable systems, we explored the simultaneous embedding of Co and Fe cations into one octacyanido-bridged coordination skeleton. We have engineered and isolated the novel trimetallic {Co3Fe II 6[W (CN)8]6(MeOH)24}·x MeOH (1) material built of nanosized (ca. 20 ) pentadecanuclear six-capped body-centered cubic Co3Fe6W6 clusters with MeOH molecules of crystallization (see the Supporting Information). The {M9M’ V 6(CN)48(L)24}·n solv compound family (M = Mn, Co, Ni; M’= Mo, W; L = blocking ligands; Figure 1) reveal high-

The influence of interstitial hydrogen, carbon and nitrogen atoms on the yttrium hyperfine field in Y2Fe17 and Y2Co17

Abstract The nuclear magnetic resonance spin echo spectra of Y 2 Fe 17 A x (A ≡ N, C, H) and Y 2 Co 17 C x are presented. Going from hydrogen through nitrogen to carbon, an increasing influence of the interstitial atoms on the yttrium hyperfine field is found. A remarkable increase of the iron hyperfine field caused by nitrogen is observed. The above effects have strongly local character and are attributed to the influence of nitrogen, carbon and hydrogen atoms on the local electronic structure at rare earth and transition metal sites in the structure

Shape of spin density wave versus temperature in AFe 2As 2 (A=Ca, Ba, Eu): A Mössbauer study

Parent compounds AFe2As2 (A=Ca, Ba, Eu) of the 122 family of the iron-based superconductors have been studied by 57Fe Moessbauer spectroscopy in the temperature range 4.2 K - ~300 K. Spin density waves (SDW) have been found with some confidence. They are either incommensurate with the lattice period or the ratio of the respective periods is far away from ratio of small integers. SDW shape is very unconventional i.e. differs from the sinusoidal shape. Magnetic order starts with lowered temperature as narrow sheets of the significant electron spin density separated by areas with very small spin density. Magnetic sheets are likely to be ordered in the alternate anti-ferromagnetic fashion as the material as whole behaves similarly to collinear anti-ferromagnet. Further lowering of temperature simply expands sheet thickness leading to the near triangular SDW. Finally, sheets fill the whole available space and almost rectangular shape of SDW is reached. Substantial maximum amplitude of SDW appears at the temperature just below the magnetic onset temperature, and this maximum amplitude increases slightly with lowering temperature. The square root from the mean squared hyperfine field behaves versus temperature according to the universality class (1, 2), i.e., with the electronic spin space having dimensionality equal unity and the real space having dimensionality equal two. The more or less pronounced tail above transition temperature due to the development of incoherent SDW is seen.

Structural and magnetic properties of TbMn2Hx hydrides

Abstract Powder samples of TbMn 2 H x hydrides, with 0≤ x ≤4.3, are characterised by X-ray analysis and SQUID magnetometry for temperatures ranging between 2 and 375 K. We observed structural phase transformations as a function of the hydrogen concentration and temperature. In particular a spinodal decomposition for x

Structural and magnetic transformations in the GdMn2Hx hydrides

Powder samples of GdMn2Hx hydrides, with 0pxp4.3, have been characterized by X-ray analysis and SQUID magnetometry for temperatures ranging between 4 and 375 K.The observed phase transformations as a function of temperature and hydrogen concentration are discussed and explained.The correlated changes of magnetization and susceptibility have been analyzed and the temperature dependent development of magnetic ordering of Mn and Gd atoms as a function of temperature and hydrogen content has been discussed.The structural and magnetic phase diagrams have been proposed. r 2002 Elsevier Science B.V. All rights reserved.

Mössbauer study of magnetic ordering in GdMn2 and YMn2

Abstract The magnetic properties of GdMn 2 and YMn 2 were investigated by means of Mossbauer Spectroscopy at 57 Fe nuclei which substituted 0.5% of Mn atoms. Measurements were performed in the temperature range between 4.2 and 300 K. Gadolinium sublattice was monitored by 155 Gd(86.5 keV)-resonance at 4.2 K. Results show an evidence for two magnetic transitions at 40 and 100 K for GdMn 2 , but only one for YMn 2 , around 100 K. For both compounds B hf shows thermal hysteresis. A new Yafet-Kittel-type magnetic ground state ordering for GdMn 2 is proposed, which came out from the numerical solution of a set of coupled Brillouin equations written in terms of the molecular field approximation. Mossbauer effect results however are too compounded to be explained in details within the frame of this type of ordering.

Interplay between magnetism and superconductivity in EuFe2−xCoxAs2studied by57Fe and151Eu Mössbauer spectroscopy

The compound EuFe(2-x)Co(x)As2 was investigated by means of the 57Fe and 151Eu Moessbauer spectroscopy versus temperature (4.2 - 300 K) for x=0 (parent), x=0.34 - 0.39 (superconductor) and x=0.58 (overdoped). It was found that spin density wave (SDW) is suppressed by Co-substitution, however it survives in the region of superconductivity, but iron spectra exhibit some non-magnetic component in the superconducting region. Europium orders anti-ferromagnetically regardless of the Co concentration with the spin re-orientation from the a-axis in the parent compound toward c-axis with the increasing replacement of iron by cobalt. The re-orientation takes place close to the a-c plane. Some trivalent europium appears in EuFe(2-x)Co(x)As2 versus substitution due to the chemical pressure induced by Co-atoms and it experiences some transferred hyperfine field from Eu2+. Iron experiences some transferred field due to the europium ordering for substituted samples in the SDW and non-magnetic state both, while the transferred field is undetectable in the parent compound. Superconductivity coexists with the 4f-europium magnetic order within the same volume. It seems that superconductivity has some filamentary character in EuFe(2-x)Co(x)As2 and it is confined to the non-magnetic component seen by the iron Moessbauer spectroscopy.

X-Ray diffraction and 155Gd-Mössbauer effect study of GdMn2Hx (0≤x≤4.3)

Abstract X-Ray diffraction and 155Gd-Mossbauer effect were applied to study the structural, electronic and magnetic properties of GdMn2Hx with x = 0, 0.5, 1.0, 2.0, 2.5, 3.4 and 4.3. The XRD studies performed between 12 and 300 K reveal the existence of defined phases with cubic or rhombohedrally distorted C15 structure. The 155Gd isomer shift, quadrupole interaction and hyperfine field clearly reflect the electronic and magnetic modifications of the Gd site by the hydrogen uptake.

Magnetic anisotropy and lattice dynamics in FeAs studied by Mössbauer spectroscopy

Iron mono-arsenide in the powder form has been investigated by transmission 57 Fe Mossbauer spectros- copy in the temperature range 4.2-1000 K. Additional spectra have been obtained at 20 K and 100 K applying external magnetic field of 7 T. It was found that the spin spiral propagating along the c-axis leads to the complex variation of the hyperfine magnetic field amplitude with the spin orientation vary- ing in the a-b plane. The magnitude of the hyperfine field pointing in the direction of the local magnetic moment depends on the orientation of this moment in the a-b plane. Patterns are vastly different for iron located in the (0 k 0) positions and for iron in the (0 k + 1 2 0) positions within the orthorhombic cell set to the Pnma symmetry. Lattice softens upon transition to the paramagnetic state at 69.2 K primarily in the a-c plane as seen by iron atoms. This effect is quite large considering lack of the structural transition. Two previously mentioned iron sites are discernible in the paramagnetic region till 300 K by different electron densities on the iron nuclei. The anisotropy of the iron vibrations developed at the transition to the para- magnetic state increases with the temperature in accordance with the harmonic approximation, albeit tends to saturation at high temperatures indicating gradual onset of the quasi-harmonic conditions. It seems that neither hyperfine fields nor magnetic moments are correct order parameters in light of the determined static critical exponents. Sample starts to loose arsenic at about 1000 K and under vacuum. 2013 Elsevier B.V. All rights reserved.

Magnetic and structural properties of DyMn2Hx (0≤x≤4.2)

Abstract The magnetic properties of Dy( 57 Fe 0.01 Mn 0.99 ) 2 H x compounds were investigated by means of 57 Fe Mossbauer spectroscopy and temperature dependent magnetic susceptibility (DC, AC) and magnetisation measurements. Their structural properties in the temperature range 70 K≤ T ≤400 K were studied by means of powder X-ray diffraction. It was found that, at room temperature, the lattice parameters increase continuously upon hydrogen absorption in the 0.8≤ x ≤3.5 range and the cubic C15 crystal structure is preserved. At 70 K the system differentiates into several distinct hydride phases and two-phase ranges. The lattice expansion together with the chemical effect of hydrogen results in dramatic changes in the properties of the hydrides as compared to the host compound. By means of 57 Fe Mossbauer spectroscopy, values of the average isomer shifts at 4.2 and 300 K and the average hyperfine fields at 4.2 K were determined. From isomer shift changes at room temperature, the relative importance of the volume expansion and band effects due to hydrogen absorption were deduced. A nonlinear increase of the magnetic ordering temperatures T M versus x and an abrupt increase of the average hyperfine fields for x ≥2.5 were also found. Based on T M values for Y( 57 Fe 0.005 Mn 0.995 ) 2 H x as reference the magnitude of the 4f–3d indirect interaction was inferred. In addition, a strong decrease of this interaction with increasing x (resulting in its cancellation for x >2.8) and a weak dependence on the Dy–Mn distance are clearly shown.