High-temperature ferromagnetism in Si:Mn alloys

Abstract

A possible mechanism for high-temperature ferromagnetic order in Si:Mn alloys is proposed. These materials, which are semiconducting or metallic, depending on the Mn content, are suggested to undergo phase separation. In the phase-separated state, again depending on the Mn content, Mn atoms can be gathered within nanometer-sized particles or micrometer-sized islands composed of the MnSi 2−z precipitate with z ≈ (0.25-0.30), which are embedded in the Mn-poor silicon matrix. We consider the MnSi 2−z precipitate to be the MnSi 1.7 silicide host containing a certain amount of magnetic defects associated with unbound Mn 3d orbitals. The MnSi 1.7 silicide is considered to be a weak itinerant ferromagnet, where sizable spin fluctuations (paramagnons) exist far above its intrinsic Curie temperature, leading to a strong enhancement of the exchange coupling between the local moments of the defects. As a result, a significant enhancement of the temperature of onset of long-range order among the local moments may be achieved. We associate this temperature with the global Curie temperature of the precipitate. A phenomenological model is developed to determine the spatial structures and characteristics of ferromagnetic order for the cases of a bulk precipitate and of precipitate particles of various shapes. Moreover, allowing for the presence of strong quenched disorder in the precipitate, we describe short-range ferromagnetic order in the system. Experimental data on Si:Mn alloys are interpreted on the basis of our theoretical results.

Ferromagnetism of MnxSi1-x(x ∼ 0.5) films grown in the shadow geometry by pulsed laser deposition method

“…Early in Ref. 12 in the frame of the spin-fluctuation model of FM ordering, we have analyzed the role of dimension effects in granular dilute Si-Mn alloys. We considered the precipitate nanoparticles of MnSi 1.7 type in the Si matrix and estimated variation of the Curie temperature as a function of the shape and size of these precipitates.…”

“…3,4 We argued that the observed HT FM has a defect-induced nature: it is due to formation of local magnetic moments on the Si vacancies inside the MnSi matrix and the strong exchange coupling between these moments mediated by spin fluctuations of itinerant carriers. 12 The Mn x Si 1-x films in 3,4 were deposited at a relatively slow deposition rate (∼2 nm/min) using PLD method in a conventional "direct" geometry (DG) when the surface of Al 2 O 3 (0001) substrate is exposed to the Mn-Si laser plume. Accordingly to atomic-force microscopy (AFM) measurements, the structure of thus grown films is mosaic, with the crystallite size ∼0.5-1 µm.…”

Ab initiostudy of the magnetic ordering in Si/Mn digital alloys

“…[10][11][12][13] Until now, the mechanism of FM ordering in Si:Mn dilute alloys is far from being understood (see detailed discussion in Ref. 14). Since a FM state was also observed in Si after the implantation of nonmagnetic ions (Ar, Si) or irradiation by neutrons, some authors argued that the ferromagnetism in these alloys is due to paramagnetic defects.…”

Local structure and magnetic properties of B2- and B20-like ultrathin Mn films grown on Si(001)

“…14,15 One model attempts to explain the high T C in this condensed magnetic semiconductor as due to an enhancement of the coupling between Mn spins in the Si matrix due to spin-fluctuations in the itinerant MnSi 1.7 precipitates. 16 A second proposal to create high T C ferromagnetic Mn-Si systems is to stabilize new metastable phases with epitaxially induced strain. While bulk MnSi has a B20 crystal structure with a lattice constant a = 0.4558 nm, which has a 3% mismatch to the Si(111) surface for MnSi[110] || Si [112], the B20 phase is much less favorable on Si(001) owing to a 16% lattice mismatch.…”