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

Abstract

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 T V = 124 K in the so-called Verwey phase transformation. Recently, the structure below T V 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.Probably none of the naturally existing minerals was utilized by mankind and nature more in its pristine form than magnetite. Navigation in the world, both by artificial and natural compasses, storage media and, finally, the tool in the emerging field of spin electronics are a few examples only. But despite its role in civilization, magnetite displays yet another fascinating face: the Verwey transition (VT). In this first-order phase transformation, the resistivity drops two orders of magnitude when heated above T V = 124 K. Surprisingly, all groups of magnetite "application" and the Verwey transition are interwoven: cooling down magnetite below T V results in a drastic drop of electric conductivity, affecting its spintronic application and the thermal cycling across the transition greatly diminishes coercive force, i.e. magnetic memory (see e.g. [1]). This transition is the subject of our extensive studied described below.Not only resistivity jumps at T V ; the spectacular anomalies are observed practically in all physical characteristics. In particular, there is a huge peak in heat capacity, proving first-order character of the transition and the structure symmetry changes. The hightemperature magnetite structure was found of inverse spinel type, with Fd 3m symmetry and with Fe residing in tetrahedral and octahedral positions. When cooled below T V ,

Magnetic field induced structural changes in magnetite observed by resonant x-ray diffraction and Mössbauer spectroscopy

“…The intensity of this reflection vs increasing magnetic field at 110 K is shown in Fig. 6 and is correlated with sample resistance that also changes once AS occurs [31,20]. However, the peak almost totally recovers after magnetic field release (Fig.…”

“…The small difference in T V is likely due to a slight nonstoichiometry of the latter. Based on the universal curve [20], we expect the nonstoichiometry parameter 3δ in Fe 3(1−δ) O 4 of the order of 7 × 10 −4 for sample 2. Despite the small variation in the stoichiometry, no significant differences in the electronic properties were observed between the two samples.…”

“…We have performed extensive studies of the AS phenomenon by magnetism-oriented experiments [17][18][19][20] and by resistivity experiments [31]. It was demonstrated [18] that not only the magnetic but also the crystallographic c M axis is forced in this particular direction.…”

“…Sample resistance is sensitive to AS [20,31] and thus it was monitored in situ throughout the experiment. A standard four-point resistance measurement technique was used with the RS830 lock-in amplifier employed as the current source and for reading the voltage on the sample.…”

“…With such a variety of phenomena and mutually cooperating subsystems, it is interesting to see to what extent those subsystems can be manipulated by an external trigger, such as an electric field [13], light [14], or a magnetic field [15,16]. Here we exploit the phenomenon discovered by Calhoun [15], an easy magnetization axis switching (AS) caused by the magnetic field, the phenomenon closely linked with crystal structure rearrangements [17,18] and visible by several bulk [18][19][20] and local [21] techniques. Here we use resonant x-ray scattering (RXS) and Mössbauer spectroscopy (MS) as two complementary techniques to probe the changes occurring in the system during the AS process in the temperature range below but close to T V .…”