Z. Kąkol

Magnetostriction of First and Second Order Magnetite Samples and its Relation to the Magnetic Easy Axis Switching

Magnetic field (up to 8T) and temperature (10-300K) dependence of size of Zn doped magnetite samples Fe3-xZnxO4 (x=0.008, 0.022, i.e. falling within first and second order Verwey transition regime) were measured by the strain gauge method. Both samples experienced shrinking on cooling through the Verwey transition along monoclinic c axis, while the strong expansion was found for the 1st order sample in the a-b direction, unlike in the 2nd order sample. Magnetostriction of both samples is very small and limited to low fields only, concomitant with magnetization process. However, field application perpendicular to c axis and at T slightly below the Verwey transition temperature TV results in a large change of dimensions, coinciding with the axis switching process.

Charge rearrangement in magnetite: from magnetic field induced easy axis switching to femtoseconds electronic processes

Magnetite is the oldest magnet and the first material where the concept of a strong correlations driven metal–insulator transition was suggested and found at TV = 124 K in the so-called Verwey phase transformation. Recently, the structure below TV was solved revealing subtle electronic structure in the form of trimeron lattice that, according to yet another recent communication, may be switched within femtosecond range. In this review article, we argue that the same change of trimeron lattice can be achieved by a magnetic field, in the phenomenon called the easy axis switching. The results of many of our experiments show that although this process is best viewed by magnetization studies, it is also reflected in magnetostriction, causes some changes in electronic transport and can be observed microscopically by NMR that proved electronic order alteration. All those facts suggest that the axis switching process observed and studied by us is intimately linked with the fast change of electronic trimeron order mentioned above.