Magnetization-driven metal-insulator transition in strongly disordered Ge:Mn magnetic semiconductors

Abstract

We report on the temperature-and field-driven metal-insulator transition in disordered Ge:Mn magnetic semiconductors accompanied by magnetic ordering, magnetoresistance reaching thousands of percents, and suppression of the extraordinary Hall effect by a magnetic field. Magnetoresistance isotherms are shown to obey a universal scaling law with a single scaling parameter depending on temperature and fabrication. We argue that the strong magnetic disorder leads to localization of charge carriers and is the origin of the unusual properties of Ge:Mn alloys.Interest in magnetic semiconductors was triggered by the development of spintronics in metallic magnetic materials and prospects to empower the semiconducting electronics by the spin-dependent degree of freedom. Compatibility with the existing silicon technology recently attracted much attention to the group-IV semiconductors doped with magnetic impurities. The novel materials appeared remarkably interesting and revealed a number of unusual properties not yet well understood. Huge positive magnetoresistance ͑MR͒ of hundreds to thousands of percent, 1-4 reversal of magnetoresistance from positive to negative after annealing, 4 and large Hall effect with nonmonotonic field dependence 5-7 are among them. Interpretation of the data is difficult owing to the complicated structure of these alloys. As a result, almost no general relations were derived from the experiments, and only special models were proposed to explain the data in each case.In this Rapid Communication we report on the high-field magnetic and magnetotransport properties of several Ge:Mn samples produced by ion implantation. The material was found to pass from metallic-like to insulator-like state, either at low temperatures in zero field or under an applied field at elevated temperatures. We show that all the magnetoresistance data can be scaled on a universal curve with a single parameter H s , which tends to zero at the paramagnet-toferromagnet transition. The same scaling procedure is applicable to the published data 4 obtained in samples produced by the molecular-beam epitaxy, which indicates the generality of the scaling approach. We argue that establishment of an inhomogeneous magnetization landscape leads to localization of the charge carriers and, respectively, to a huge positive magnetoresistance and a total suppression of the extraordinary Hall effect.Two Ge:Mn samples discussed here were fabricated by implanting Mn + ions into commercial single-crystalline Ge͑100͒ wafers with resistivity of 40-57 ⍀ cm. Mn + ions were implanted with an energy of 100 keV at fluences of 1 ϫ 10 16 and 2 ϫ 10 16 that produce average volume concentrations of Mn of about 2 and 4 at. % in the projected depth range of about 120 nm. During the implantation, the samples were held at 300°C to avoid amorphization. Structure of the samples is strongly nonuniform, depending on the concentration, and contains diluted Mn, amorphous semiconducting Mn-rich nanoclusters, and ferromagnetic metallic Mn 5 Ge 3 clusters. Detailed str...

Mn-doped Ge and Si: A Review of the Experimental Status

“…By scrutinizing the published data on Ge:Mn, one can observe three features in the reported AHE. First, most of the AHE curves shown were recorded at temperatures above 10 K. Indeed, Riss et al [50] reveal only ordinary Hall effect below 10 K. Second, no hysteresis in AHE curves has been observed, despite the observation of a clear hysteresis in magnetization, which is much different from the case of III-Mn-V and ZnMnTe [45,46,47]. Third, the Hall curve changes its sign of slope at lower temperatures, usually between 10 K and 50 K. Obviously, the correlation between magnetization, MR, and AHE, which is a hallmark of III-Mn-V and ZnMnTe FMS [45,46,47], has not been proven for Ge:Mn so far.…”

Interplay between electrical transport properties of GeMn thin films and Ge substrates

“…We can rather identify two distinct temperature regimes with different properties below and above 9 K. Similar results for the Hall effect and field dependence of longitudinal resistivity, which can be separated into two temperature regimes, were found for the GeMn material systems in degenerate thin films prepared by ion implantation. 12 They were interpreted in terms of a two-band-like conduction scheme, accounting for possible electronic ground and excited states of Mn in Ge. 9 In contrast, our results in the two distinct regimes are naturally explained by assuming parallel conduction through the substrate: Below 9 K parallel conduction is not present, since the substrate resistance gets very large, whereas the resistance of the metallic epilayer does not change.…”

Memory effect of Mn5Ge3 nanomagnets embedded inside a Mn-diluted Ge matrix

“…1, beside the main peaks from Ge͑004͒ and Ge͑002͒, the diffraction peaks of Mn 5 Ge 3 ͑111͒, ͑002͒, ͑310͒, ͑222͒, and ͑004͒ are clearly visible. 13 The ferromagnetic properties measured in this sample are due to the formation of Mn 5 Ge 3 nanocrystals. The dilution of Mn ions inside Ge has been evidenced by photoemission spectroscopy 5 and by Hall measurements.…”

“…As shown in Fig. 13 Mn ion implantation results in p-type doping of Ge. Note that compared to the work by Ottaviano et al 5 SR-XRD reveals more Mn 5 Ge 3 peaks even for a much smaller Mn-ion fluence due to the large flux of x rays.…”