AF Transition Research Articles

Effect of Pressure on Antiferromagnetic Transition in Alkali-Electro-Sodalite

Pressure (0–19 kbar) and temperature (4–300 K) dependent EPR study of Sodium-Electro-Sodalite (SES) is presented. SES, which consists of a bcc sub-lattice of F-centers supported by a zeolite-like framework, is known to be a Mott insulator at room temperature. On cooling, SES undergoes an AF transition at 48±2 K providing the first example of an s-electron antiferromagnet. We find that the width of the EPR resonance above T N is influenced not only by a strong exchange interaction, but also by a fast spin-lattice relaxation. Also, with increasing pressure, T N decreases linearly and extrapolates to 0 K at about 65 kbar. The reason for this seemingly unexpected behavior is briefly discussed.

Phase transition in alkali-electro-sodalite studied by ESR

AES (A= Na, K) is a Mott insulator at room temperature consisting of a bcc sub-lattice of F-centers, supported by a zeolite-like framework. Each sodalite cage contains an s-like electron trapped by a spherical potential due to four tetrahedrally arranged alkali cations. On cooling, SES and KES undergo an AF transition at about 50 and 70 K respectively, providing the first example of an s-electron antiferromagnet, ESR linewidth, lineshape and g-shift were measured, which shows unique feature of these s-electron lattices. The linewidth is narrow above T N due to exchange narrowing and rapidly broadens below T N. Under high pressures AES could be metal.

Low-temperature magnetoresistance measurements of YbBe 13

The temperature dependence of the resistance R( T) of YbBe 13 is strongly influenced by a magnetic field. The maximum in R( T) at 2 K vanishes at fields above 2 T whereas the maximum at 22 K broadens and shifts with increasing field to about 60 K at 20 T. The kink observed in R( T) as well as in the magnetoresistance R( B) corresponding to the AF transition allowed us to determine a magnetic phase diagram of YbBe 13 showing a mean-field behaviour with B c ≃0.67 T and T N ≃1.05 K . To explain our results the Kondo scattering modified by crystal-field effects is suggested.

Neutron diffraction study of the La 1− xY xMn 2Si 2 solid solution (0≤ x≤1)

The La 1− x Y x Mn 2Si 2 compounds (0< x≤1) have been investigated by powder neutron diffraction. For x<0.5, the low temperature magnetic structures are characterized by mixed planes where both ferro- and antiferromagnetic components coexist while for x≥0.5, ferromagnetic Mn planes prevail. The in-plane ferromagnetic component couples ferromagnetically with those of adjacent planes for x<0.2 and antiferromagnetically for x>0.2. The La 0.8Y 0.2Mn 2Si 2 compound displays a F–AF transition as a function of the temperature. The in-plane antiferromagnetic Mn moment components are smaller than those measured in the corresponding germanides. These results are discussed and compared with previous results of the La 1− x Y x Mn 2Ge 2 compounds. The Mn magnetic behaviour is consistent with bibliographic data on NMR experiments and Mössbauer spectroscopy.

Physical properties of (DMCTTF)2X salts and their selenium analogues

The authors report results on a new series of compounds: the (DMCTYF)2X salts (where Y identical to S or Se). Conducting and magnetic properties are presented in relation to the structural data of these salts which give evidence of two classes of compounds depending on the anion geometry. The structure of most of the octahedral compounds is reminiscent of that of the Bechgaard salts. The tetrahedral salts exhibit a more complex unit cell containing two organic stacks. These two stacks are either symmetry related or independent and a classification of these compounds into two phases labelled alpha and beta is proposed. For all the salts of the series an electronic localisation becomes dominant at low temperature. The magnetic properties have been studied using electron spin resonance (ESR). For all the sulphur compounds an antiferromagnetic ground state is observed. With tetrahedral anions the two chains can be studied separately and the data rationalise the structural classification; the two chains of the alpha -samples undergo an AF transition at the same temperature (T approximately=20 K) whereas the chains of the beta -samples which are not equivalent present an ACF ground state at two different temperatures (T approximately=10 and 20 K).

A method for determining a solid solution of the Pb(Hf1−yZry)1−xTixO3 type used for electromechanical energy conversion

The mechanical stress-forced Ferroelectric F→Antiferroelectric AF transition energy conversion is reviewed. The temperature-composition phase diagram of PbHf1−xTixO3+1%La2O3 is established. The composition of a suitable material such as a ternary solid solution of the Pb(Hf1−yZry)1−xTixO3+1%La2O3 type, characterized by a low transition pressure, is theoretically determined by a graphic construction using the Goldschmidt factor. Experimental results on the prepared material are given.