| Popis: |
Magnetic measurements for stoichiometric and nonstoichiometric BaLn 2 Mn 2 O 7 were performed from 5 to 100K using a SQUID magnetometer (Ln = Sm, Eu, Gd and Tb) and from 77 to 800K with the Faraday method (Ln = Eu,Gd and Tb). These compounds have essentially paramagnetism in this temperature range. With the Faraday method the magnetic susceptibility curve for each stoichiometric compound shows a similar pattern of anomaly at high temperatures, corresponding to the phase transitions, caused by the liberation from the constraint of the Jahn-Teller effect, which were found by the authors. The transition temperatures determined from an anomalous point of the magnetic susceptibility measurements, 640K for BaGd 2 Mn 2 O 7 and 725K for BaTb 2 Mn 2 O 7 are also in good agreement with previous results from other methods. Especially in BaEu 2 Mn 2 O 7 there are two anomalies, at 420K and at 550K, the latter of which is in good agreement with the transition temperature from the orthorhombic to tetragonal phase. At 420K a thermal anomaly also exists in the DSC curve. The existence of two kinds of phase transitions, orthorhombic (Sm-type) to another type of orthorhombic (Tb-type) and subsequently to a tetragonal phase is suggested in BaEu 2 Mn 2 O 7 . The phase diagram for the nonstoichiometric compound of BaEu 2 Mn 2 O 7 is proposed. |
