Specific heat of Fe₃O₄

The Verwey transition - a topical review

“…[82] clear results concerning this problem, cf section 3.2.2, the relevance of a second peak near 110 K, besides the major one at T v , remained under discussion for many further years. Whereas Rigo et al [84,179,180] regarded this second peak-obtained on powdered material-as purely intrinsic, Gmelin et al [181,182] and Shepherd et al [183], on well prepared single crystals, could not detect any trace of this 110 K anomaly. The final 'knockout' for the thermal bifurcation, as an intrisic process of perfect magnetite, was accomplished by means of the MAE [184].…”

“…(v) Evidently, owing to these uncertainties, resulting from (i) lack of perfect crystals and (ii) absence of reliable quality control, it would be unrealistic to expect the definite solution of the magnetite problem during era (II). (a) Multi-stage transitions in the neighbourhood of T v have been reported from specific heat [79][80][81][82][83][84] and spontaneous magnetization in combination with Mössbauer-effect measurements [85][86][87][88]. Whereas in early specific heat investigations only one single anomaly, somewhat below 120 K, had been observed [19][20][21][22], later on the appearance of double peaks (pronounced bifurcation) near 113 and 119 K have been reported by Westrum et al [79][80][81] [NSC/SSC, 119 K], Matsui et al [82] [SSC, 118.4 K] and Rigo et al [83,84] [SSC, powder, 123 K].…”

“…(a) Multi-stage transitions in the neighbourhood of T v have been reported from specific heat [79][80][81][82][83][84] and spontaneous magnetization in combination with Mössbauer-effect measurements [85][86][87][88]. Whereas in early specific heat investigations only one single anomaly, somewhat below 120 K, had been observed [19][20][21][22], later on the appearance of double peaks (pronounced bifurcation) near 113 and 119 K have been reported by Westrum et al [79][80][81] [NSC/SSC, 119 K], Matsui et al [82] [SSC, 118.4 K] and Rigo et al [83,84] [SSC, powder, 123 K]. For some time the phenomenon of a multi-stage-type Verwey transition has been under serious discussion, even more since Hirsch et al [85][86][87][88] [SPC, 116.6 K], too, promoted the idea of an extended multi-stage transition, composed of up to 12 stages within the temperature range 15 K < T < 151 K (!)…”

Suppressing the charge-ordering transition in LiMn2O4 through substitution of Li by Mg

“…22 The effect of electron doping on the Verwey transition in Fe 3 O 4 has been examined in systems where F substitutes O. 24 The authors ®nd for the compositions x~0.1 and x~0.25 in Fe 3 O 42x F x , corresponding to Fe valence changes of 0.03 and 0.08 respectively, the main peak in the heat capacity is no longer observed, though the second peak is retained. This peak is claimed by the authors to correspond to a lowering of crystallographic symmetry to a compound that is not charge ordered.…”