Irradiation-Induced Electron Tunnelling and Small-Polaron Hopping in Single-Crystalline YIG

Abstract

Subject classification: 72.20.Ee; 75.60.Lr; S11.2In electron (e À )-irradiated (>10 23 e À m À2 ) single crystalline yttrium-ion Garnet (YIG), after moderate annealing, low-temperature (T < 125 K) magnetic after-effect (MAE) spectra are observed which are of striking similarity to the well investigated spectra occurring in magnetite (Fe 3 O 4 ) below the Verwey transition (T V % 125 K). Our analysis shows that this similarity is the result of corresponding relaxation mechanisms in both systems, i.e. electron tunnelling and small-polaron hopping. Deeper insights into these mechanisms are obtained from a thorough inspection of these spectra and their physical preconditions in the two, a-priori, completely different ferrimagnetic systems: (i) semiconductor, transformed into a quasi-insulating state due to long-range ionic ordering at low-temperatures (Fe 3 O 4 ) and (ii) insulator, brought into a state of low resistivity due to radiation-induced perturbations of the charge equilibrium (YIG).In Elektronen (e À )-bestrahltem (>10 23 e À m À2 ) einkristallinem Eisen-Granat (YIG) wird, nach mä ßiger Erholung, bei tiefen Temperaturen (T < 125 K) ein magnetisches Nachwirkungsspektrum beobachtet, das dem gut untersuchten Spektrum in Magnetit (Fe 3 O 4 ), unterhalb des Verwey-Ûber-gangs (T V % 125 K) verblü ffend ä hnelt. Unsere Untersuchung zeigt, daß diese Øhnlichkeit auf ü bereinstimmenden Nachwirkungsmechanismen in beiden Systemen beruht, nä mlich Elektron-Tunneln und Klein-Polaron-Hü pfen. Neue Einsichten hinsichtlich dieser Mechanismen erbringt ein sorgfä ltiger Vergleich dieser Spektren und der sie bedingenden physikalischen Randbedingungen in den, a-priori, vollstä ndig unterschiedlichen Ferrit-Systemen: (i) Halbleiter, in den Quasi-Isolator Zustand versetzt infolge Ausbildung eines weitreichenden ionischen Ordnungszustands bei tiefen Temperaturen (Fe 3 O 4 ) und (ii) Isolator, schwach leitend geworden, unter dem Einfluß strahlungsinduzierter Stö rungen des ursprü nglichen Ladungsgleichgewichts (YIG).

Comparison of low‐temperature magnetic relaxations in two systems GdAl2Dx (C15 Laves phase) and Fe3O4 (Inverse spinel)

“…1b) identified to result from anisotropy transport due to (i) incoherent, inter-atomic groundstate tunnelling according to Eq. (1) (plateau, extending within 5 K < T < 25 K) and (ii) intra-atomic thermal excitations of electrons within the split level system (Debye peak, near 30 K), [3][4][5][6][7][8]. In the following we shall show that the low-temperature spectrum in D-charged GdAl 2 (5 K < T < 45 K, cf.…”

“…Interestingly, there exists a third candidate displaying this same Fe 3 O 4 specific MAE spectrum, namely e À -irradiated Yttrium-Iron-Garnet (YIG). This latter spectrum, occurring in the pre-annealed state of the system (350 K < T a < 800 K) has been shown [7] to result from valency exchange mechanisms between radiation-induced Fe 2þ and Fe 3þ ions according to (1), similarly as in Fe 3 O 4 .…”

“…Fig. 1a, 1b; [3][4][5][6][7][8]), suggesting these two processes to be governed by related relaxation mechanisms. Generally, all MAE mechanisms have as common source the occurrence of time-dependent interactions between locally reorientable magneto-crystalline and magneto-elastic anisotropies and the spontaneous magnetization within the domain wall.…”

“…Generally, all MAE mechanisms have as common source the occurrence of time-dependent interactions between locally reorientable magneto-crystalline and magneto-elastic anisotropies and the spontaneous magnetization within the domain wall. Thus, in the low-temperature phase of magnetite (T < 45 K) the induction of magnetic anisotropies is associated with incoherent ground-state tunnelling of electrons between differently valent Fe ions, according to [3][4][5][6][7][8]:…”