A temperature and magnetic field dependence Mössbauer study of ɛ-Fe2O3

ɛ-Fe2O3 was synthesized as nanoparticles by a pre-vacuum heat treatment of yttrium iron garnet (Y3Fe5O12) in a silica matrix at 300-C followed by sintering in air at 1,000-C for up to 10 h. It displays complex magnetic properties that are characterized by two transitions, one at 480 K from a paramagnet (P) to canted antiferromagnet (CAF1) and the second at ca. 120 K from the canted antiferromagnet (CAF1) to another canted antiferromagnet (CAF2). CAF2 has a smaller resultant magnetic moment (i.e. smaller canting angle) than CAF1. Analysis of the zero-field Mossbauer spectra at different temperatures shows an associated discontinuity of the hyperfine field around 120 K. In an applied field, the different magnetic sublattices were identified and the directions of their moments were assigned. The moments of the two sublattices are antiparallel and collinear at 160 K but are at right angle to each other at 4.2 K

Eur. J. Solid State Inorg. Chem.

The location of Eu3+ in alpha-cordierite, synthesized by a sol-gel route, was investigated by using both site-selective spectroscopy and time-resolved spectroscopy. F-7(0) --> D-5(1,2) excitation and D-5(0) --> F-7(0,1,2) emission spectra were performed at 77 K. The Eu3+ ions are distributed in two families : the A family in the structure channels and the B family at (4c) position of D-2h(16) in substitution for Mg2+. Four B subfamilies were characterized and understood as a difference at the second neighbours, i.e., the four possible occupations of the surrounding tetrahedra by Al and Si. The A family was characterized by anomalous spectra : strong D-5(0) --> F-7(0) transition, high position of the D-5(0), D-5(1) and D-5(2) levels and very targe F-7(1,2) splittings. By comparison with Eu-doped apatites, in which similar features occur, a predominant Eu-O bonding of covalent character with one of the oxygen ions bordering the channel was assumed. When the annealing temperature increases, Eu3+ ions migrate from the channel to the (4c) position.The location of Eu3+ in alpha-cordierite, synthesized by a sol-gel route, was investigated by using both site-selective spectroscopy and time-resolved spectroscopy. F-7(0) --> D-5(1,2) excitation and D-5(0) --> F-7(0,1,2) emission spectra were performed at 77 K. The Eu3+ ions are distributed in two families : the A family in the structure channels and the B family at (4c) position of D-2h(16) in substitution for Mg2+. Four B subfamilies were characterized and understood as a difference at the second neighbours, i.e., the four possible occupations of the surrounding tetrahedra by Al and Si. The A family was characterized by anomalous spectra : strong D-5(0) --> F-7(0) transition, high position of the D-5(0), D-5(1) and D-5(2) levels and very targe F-7(1,2) splittings. By comparison with Eu-doped apatites, in which similar features occur, a predominant Eu-O bonding of covalent character with one of the oxygen ions bordering the channel was assumed. When the annealing temperature increases, Eu3+ ions migrate from the channel to the (4c) position

Magnetic phase transitions in transition-metal complexes with triazole derivatives

Three divalent transition-metal (Co, Ni and Cu) complexes with the organic anion, 1,2,4-triazolato (tr), as a ligand molecule were prepared by means of hydrothermal syntheses and their magnetic properties were investigated by SQUID magnetometry. The Co(tr)2 and Cu(tr)2 complexes exhibit long range ordering below 8 and 30 K, respectively, while the Ni(tr)2 complex does not show any magnetic phase transition down to 1.8 K. The magnetization isotherms of Co(tr)2 and Cu(tr)2 measured at 2.0 K show hysteresis loops with the coercive fields of 0.5 and 4.7 kOe, respectively. At temperatures higher than about 50 K, the temperature dependence of the magnetic susceptibility of Co(tr)2, Ni(tr)2 and Cu(tr)2 follows the Curie-Weiss law with the Curie constants of 2.95, 0.945 and 0.420 emu K mol-1 and the Weiss temperatures of -62, -74 and -97 K, respectively. These results suggest that the magnetically ordered phases observed in Co(tr)2 and Cu(tr)2 at low temperatures come from antiferromagnetic interactions resulting in canted arrangements of magnetic moments of the transition-metal cations. We discuss here the magnetic interactions in these transition-metal complexes by referring the results of the magnetization measurements. © 2011 Elsevier B.V. All rights reserved

Étude des propriétés optiques linéaires et non linéaires du fluoroberyllate de lithium hydrazine

Second harmonic generation in the ultraviolet range has lead to the study of the optical properties of complex fluorides in non centered crystalline phase. An account is given of the preparation, and structure of the lithium hydrazine fluoroberyllate. Some measurements of the linear optical properties are exposed, such as transparency, refraction indexes at various frequencies and phase-matching properties. The measurement of second order non linear properties at 1.06 μ, (YAG : Nd3+ laser) makes use of the Maker fringes method. An analysis allowing the prediction of the number of fringes observed is developped and applied to measure and confirm the relative sign between two elements of the second harmonic generation macroscopic tensor. In spite of the poor birefringency, the comparatively good values of S. H. G. tensor elements and the quality of the transparency confirm the interest of further investigating the field of complex fluorides crystals.La génération de second harmonique dans l'ultraviolet, amène à étudier les propriétés de fluorures complexes sous forme de cristaux centrosymétriques. Après une description de la préparation et de la structure du fluorobéryllate de lithium hydrazine, certains résultats de mesures d'optique linéaire sont présentés tels que la transparence, les indices de réfraction à différentes fréquences, les possibilités d'accord de phase. Les propriétés non linéaires d'ordre deux mettent en jeu, pour leur mesure à 1,06 μ (laser YAG : Nd3+) la méthode des franges de Maker. Une analyse permettant de prévoir le nombre des franges observées est développée et a permis une mesure de signe relatif d'éléments du tenseur de génération de second harmonique macroscopique dij, mesure confirmée par ailleurs. Malgré sa médiocre biréfringence, la valeur des coefficients non linéaires et la bonne qualité de transmission sont encourageantes

Magnetic structure and magnetic properties of synthetic lindgrenite, Cu-3(OH)(2)(MoO4)(2)

International audienceSynthetic Cu3(OH)2(MoO4)2 consists of Cu3(OH)2 brucite ribbons of edge-sharing copper octahedra connected by MoO4 into a 3D network as in the mineral, lindgrenite, for all temperatures between 1.5 and 300 K. Each ribbon consists of a triangular connection between two different types of copper atom (Cu(1) and 2 Cu(2)) via í3-OH. The MoO4 acts both as one- and three-atom bridges to connect six Cu atoms belonging to three adjacent ribbons. The magnetic properties are consistent with those of ferrimagnetic chains, and the resulting moment of each chain is parallel below the long-range magnetic ordering at 13 K. The Curie constant is 0.468(1) emu K mol-1 of Cu; the Weiss temperature is -14.2(2) K, and the saturation magnetization at 2 K in 50 kOe is 0.41 N íB mol-1 of Cu. Analyses of the neutron powder diffraction reveal an ordered magnetic state where the moment of Cu(1) is antiparallel to those of the two Cu(2); all of them point along the a axis without any sign of geometrical frustration. Any degeneracy that may be present because of the triangular topology of the Cu atoms (s ) 1/2) appears to be lifted by the distortion from an ideal equilateral geometry of the triangle. The entropy, estimated from the heat capacity measurements, attains 50% of the total of 17.7 J K-1 mol-1, close to that expected for three Cu atoms (3R ln 2), up to the long range ordering temperature, and the remaining is associated with the low dimensionality of the material

Giant Magnetic Hardness in the Synthetic Mineral Ferrimagnet K2Co3II(OH)(2)(SO4)(3)(H2O)(2)

Wepresent the synthesis, single-crystal X-ray (173 K) and powder neutron (2-30 K) structures and its thermal, optical and magnetic properties of K 2CoII3(OH)2(SO4) 2(H2O)2. It is a ferrimagnet (TC= 29.7 K) constructed of Co3(OH)2 diamond chains connected by sulfate and it displays hysteresis loops ranging from being soft with nearly zero coercivity between 29 and 10 K to very hard reaching coercive field exceeding 70 kOe at 1.8 K. This dramatic change is associated with the changes in domain shape due to the strong exchange anisotropy. Considerable frequency dependence of the acsusceptibilities is observed in the ordered state.Measurements on a single crystal have established the magnetic axes to be a-axis (easy), b-axis (intermediate), and c-axis (hard). © 2010 American Chemical Society