Variable range hopping transport in ferromagnetic GaGdN epitaxial layers

Abstract

Electrical-transport properties of ferromagnetic GaGdN layers grown by molecular-beam epitaxy on highly resistive 6H-SiC͑0001͒ substrates have been investigated. It is found that doping with low concentrations of Gd increases the resistivity by several orders of magnitude as compared to unintentionally doped GaN. In the measurable temperature range between 5 and 120 K two different temperature dependences of the resistivity ͑T −1/2 and T −1/4 ͒ are observed, both of which are characteristic of variable-range-hopping conductivity in an impurity band of localized states. The experimental evidence, that room-temperature ferromagnetism and impurity-band hopping transport are coexistent, is discussed in connection with recent models proposing defect-induced magnetism in wide-band-gap III-Nitrides.

Structural, magnetic and electrical transport properties of Co doping in LaKFeMoO6 double perovskite

“…For La 1.05 K 0.95 Fe 0.9 Co 0.1 MoO 6 and La 1.1 K 0.9 Fe 0.8 Co 0.2 MoO 6 samples the resistivities follow models from Mott VRH in high temperature range transition to ES VRH in low temperature range. This behavior is ascribed to the electron interactions playing an important role, which results in the density of states quadratically decreasing with energy toward the middle of the impurity band, creating a soft gap in the density of states at the Fermi level, the socalled Coulomb gap [18]. The T ES values obtained from fittings is presented in Table 1, and the tendency of T ES to change with Co content is consistent with that of T M .…”

Tracking defect-induced ferromagnetism in GaN:Gd

“…It should be noted that attributing the favorable effect of oxygen codoping to carrier induced ferromagnetism due to O N donors seems rather unlikely considering the high resistivity of ferromagnetic GaN:Gd. 9,20 In conclusion, room-temperature ferromagnetism and colossal magnetic moments of dilute GaN:Gd layers grown by MBE are confirmed. However, the reproducibility and the long-term stability of the magnetic properties are poor.…”

“…20 The observed variable-range hopping in an impurity band of localized states is another hint that deep defects form in a large number, even for low Gd concentrations.…”

“…Both configurations would be consistent with the observed compensation of the unintentional n-type dopants in GaN:Gd and with the electrical transport by hopping. 20 Due to the large affinity of Gd to O, it is also possible that oxygen donors are attracted into interstitial positions by the Gd, thus further contributing to the compensation and providing extra localized magnetic moments. The efficient gettering of residual donors in GaN by Gd was also reported for bulk samples grown under high pressure 26 and was theoretically confirmed by Mitra and Lambrecht for the case of substitutional oxygen.…”

GaN:Co epitaxial layers grown by MOVPE

“…On the other hand, many groups have already reported ferromagnetism in various transition and rare earth metal doped AIII nitride layers [6][7][8][9][10][11]. Ferromagnetic ordering above room temperature was obs erved for GaN layers doped with Mn [12,13], Cr [14,15], Fe [16] and lanthanide atoms, like Gd [17,18] and Dy [19][20][21]. Both MOVPE and MBE techniques were used for the growth of these layers.…”