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Ferromagnetism in Ga1-xMnxP: evidence for inter-Mn exchange mediated by localized holes within a detached impurity band

We demonstrate that in ferromagnetic Ga1-xMnxP exchange is mediated by holes localized in a Mn-derived band. For x<0.06, infrared absorption and photoconductivity spectra indicate the presence of a Mn impurity band which is not merged with the valence band. At temperatures above TC (<65 K) electrical transport is dominated by excitation across this energy gap while nearest neighbor hopping dominates below TC. Magnetization measurements reveal a moment of 3.5 Bohr magnetons per substitutional Mn, while the large anomalous Hall signal unambiguously demonstrates that the ferromagnetism is carrier-mediated.

Diluted magnetic semiconductors (DMSs) are materials where a few atomic percent of a magnetic element is added to a non-magnetic semiconductor. Because of their unique combinations of magnetic and semiconducting properties and their potential for use as both injection sources and filters for spin-polarized carriers, these materials have been suggested for use in spin-based electronics, or spintronics. The discovery of ferromagnetism in the III-Mn-V systems In 1-x Mn x As and Ga 1-x Mn x As [1,2] ushered in more than a decade of intense experimental and theoretical research [3][4][5][6].
There is general consensus that the inter-Mn exchange in Ga 1-x Mn x As and other ferromagnetic DMSs such as Zn 1-x Mn x Te [7] and Ga 1-x Mn x Sb [8] is mediated by holes in extended or weakly localized states [9]. In Ga 1-x Mn x As, a large central-cell potential arising from the Mn 3d states results in both a greater acceptor binding energy (~110 meV) and a more localized ground-state than for shallow acceptors [10]. The character of the mediating holes-valence band-like, (Mn) impurity band-like, or mixed-is central to establishing accurate models for III-Mn-V ferromagnetism; yet this has not been conclusively established even for Ga 1-x Mn x As [11,12]. With a larger hole binding energy by Mn acceptors (~400 meV) [13], Mn-doped GaP displays greater hole localization and is expected to have stronger p-d hybridization due to its shorter bond length. Ga 1-x Mn x P is thus an important test bed for understanding the interplay between localization and carrier-mediated exchange [14].
In this Letter, we present experimental evidence demonstrating carrier-mediated ferromagnetism in Ga 1-x Mn x P and the presence of a gap separating a Mn-derived band from the valence band. At low temperatures (i.e. < T C ), holes are highly localized; despite their strongly insulating nature, Ga 1-x Mn x P films with nominal x~0.06 exhibit T C s above 60 K. Previous reports [15] have found an unusual ferromagnetism in polycrystalline GaP layers containing Mn; however, the origin of the ferromagnetism was not conclusively established.
Samples for this study were prepared by ion implantation followed by pulsed-laser melting (II-PLM), which we have used previously to synthesize Ga 1-x Mn x As layers with T C up to 137 K [16][17][18][19]. (Note: to date maximum reported T C for films grown by molecular beam epitaxy and subjected to extended low-temperature furnace annealing is 173 K [20].) Briefly, an unintentionally-doped (n-type, 10 16 -10 17 /cm 3 ) GaP (001) wafer was implanted with 50 keV Mn + to a dose of 2x10 16 /cm 2 . Each implanted sample was irradiated in air with a single 0.3-0.4 J/cm 2 pulse (FWHM = 19 ns) from a KrF excimer laser (λ = 248 nm) homogenized to a spatial uniformity of ±5 % by a crossed-cylindrical lens homogenizer. Extended etching in concentrated HCl removed a poorly-regrown surface layer [17]. Ion channeling techniques including particle induced X-ray emission (PIXE) were used to assess the crystalline quality, total Mn dose, and substitutional Mn fraction.
The solid line in Fig. 1 presents the magnetization of a Ga 0.94 Mn 0.06 P sample irradiated at 0.4 J/cm 2 . A linear dependence of magnetization on temperature up to its T C of 60 K is observed, which can be understood in terms of a small carrier density and/or carrier localization [18,21]. Samples from the same Mn implantation irradiated between 0.3-0.35 J/cm 2 show a maximum T C of 65 K while Ga 0.97 Mn 0.03 P films display T C up to 23 K [17]. At 5 K and in a field of 5 T, the magnetization of the Ga 0.94 Mn 0.06 P film saturates at a value corresponding to 3.5 µ B per substitutional Mn (and 2.3 µ B per total Mn), reflecting the measured Mn substitutionality of ~67% (and thus an effective composition for the dilute alloy x eff = 0.04). Because of the rapid regrowth from the liquid phase involved in our process, none of the Mn is present as interstitials as determined by PIXE measurements; the remaining 33% of Mn atoms likely exist as small clusters [22]. We interpret the decreased magnetization and T C from the sample coimplanted with Mn and Te ions as being due to the reduced hole concentration (p) resulting from partial compensation by Te donors. The changes in T C accompanying variations of x and p are evidence that the ferromagnetism in Ga 1-x Mn x P is due to a carrier-mediated phase.
The filled circles in Fig. 2 represent the sheet resistivity of a Ga 0.94 Mn 0.06 P sample with T C = 65 K as a function of inverse temperature. The sample is clearly insulating and its resistivity is well described by a model of two thermally-activated processes: The free parameters are the activation energies E 1,2 and the pre-exponential constants C 1,2 . Fitting the data to this simple model gives the high-and low-temperature activation energies as ~31 and ~6 meV, respectively. In p-type semiconductors, thermally-activated resistivity in the high-temperature range of the extrinsic regime is typically associated with hole transitions between the valence band and bound-acceptor states. Based on this and the spectroscopic data discussed below, we assign the high-temperature ~31 meV activation energy to excitation across a gap between a Mn impurity band and the valence band. The fact that the change in slope occurs near T C is consistent with the formation of a continuous hopping transport path at a percolation transition of magnetic polarons [23].
The low-temperature 6 meV process is also consistent with these notions; however, other explanations may also be possible. The open circles represent the resistivity of the sample in an applied field of 7 T, while the lower-right inset shows that the magnetoresistivity ratio, MR = ρ(7 T)/ ρ(0)-1, is negative, reaching its largest magnitude of -48 % at the T C of 65 K. Figure   Our results indicating a lower T C for Ga 1-x Mn x P than for Ga 1-x Mn x As of similar Mn composition do not necessarily suggest that the maximum attainable T C in this system will also be lower; it is possible that stronger ferromagnetism may be achieved by incorporating higher concentrations of Mn via optimization of II-PLM.
The fundamental issue of whether the inter-Mn exchange (and hence T C ) changes monotonically across the Ga-Mn-pnictide series [25][26][27] from Ga 1-x Mn x Sb (maximum reported T C 25 K) [8] to Ga 1-x Mn x N (maximum reported T C 940 K) [28] is still unresolved.
The shorter bond length in GaP should lead to greater p-d exchange between holes and Mn ions than in GaAs. However, this increased exchange energy could contribute to the already significant localization of hole states leading to less overlap of states on different Mn and overall weaker inter-Mn exchange [14]. The presence of a distinct Mn impurity band and the ability to vary the Fermi energy via compensation in Ga 1-x Mn x P enables testing theories of ferromagnetism and spinpolarized transport including the prediction that a change in sign of the anomalous Hall coefficient as the Fermi level moves within an impurity band [29].
In summary, Ga 1-x Mn x P represents a novel DMS alloy system where strongly localized carriers in a detached impurity band stabilize ferromagnetism. The unique electrical, magnetic, and optical properties displayed by this material make it a model system for investigating the rich interplay between bandstructure, carrier localization, and mechanisms of ferromagnetic exchange.   Magnetoresistive ratio between 7 T and zero-field data as a function of temperature.