Electrical probing of magnetic phase transition and domain wall motion in single-crystalline Mn5Ge3 nanowire.

In this Letter, the magnetic phase transition and domain wall motion in a single-crystalline Mn(5)Ge(3) nanowire were investigated by temperature-dependent magneto-transport measurements. The ferromagnetic Mn(5)Ge(3) nanowire was fabricated by fully germaniding a single-crystalline Ge nanowire through the solid-state reaction with Mn contacts upon thermal annealing at 450 °C. Temperature-dependent four-probe resistance measurements on the Mn(5)Ge(3) nanowire showed a clear slope change near 300 K accompanied by a magnetic phase transition from ferromagnetism to paramagnetism. The transition temperature was able to be controlled by both axial and radial magnetic fields as the external magnetic field helped maintain the magnetization aligned in the Mn(5)Ge(3) nanowire. Near the magnetic phase transition, the critical behavior in the 1D system was characterized by a power-law relation with a critical exponent of α = 0.07 ± 0.01. Besides, another interesting feature was revealed as a cusp at about 67 K in the first-order derivative of the nanowire resistance, which was attributed to a possible magnetic transition between two noncollinear and collinear ferromagnetic states in the Mn(5)Ge(3) lattice. Furthermore, temperature-dependent magneto-transport measurements demonstrated a hysteretic, symmetric, and stepwise axial magnetoresistance of the Mn(5)Ge(3) nanowire. The interesting features of abrupt jumps indicated the presence of multiple domain walls in the Mn(5)Ge(3) nanowire and the annihilation of domain walls driven by the magnetic field. The Kurkijärvi model was used to describe the domain wall depinning as thermally assisted escape from a single energy barrier, and the fitting on the temperature-dependent depinning magnetic fields yielded an energy barrier of 0.166 eV.

Ferromagnetic germanide in Ge nanowire transistors for spintronics application.

To explore spintronics applications for Ge nanowire heterostructures formed by thermal annealing, it is critical to develop a ferromagnetic germanide with high Curie temperature and take advantage of the high-quality interface between Ge and the formed ferromagnetic germanide. In this work, we report, for the first time, the formation and characterization of Mn(5)Ge(3)/Ge/Mn(5)Ge(3) nanowire transistors, in which the room-temperature ferromagnetic germanide was found through the solid-state reaction between a single-crystalline Ge nanowire and Mn contact pads upon thermal annealing. The atomically clean interface between Mn(5)Ge(3) and Ge with a relatively small lattice mismatch of 10.6% indicates that Mn(5)Ge(3) is a high-quality ferromagnetic contact to Ge. Temperature-dependent I-V measurements on the Mn(5)Ge(3)/Ge/Mn(5)Ge(3) nanowire heterostructure reveal a Schottky barrier height of 0.25 eV for the Mn(5)Ge(3) contact to p-type Ge. The Ge nanowire field-effect transistors built on the Mn(5)Ge(3)/Ge/Mn(5)Ge(3) heterostructure exhibit a high-performance p-type behavior with a current on/off ratio close to 10(5), and a hole mobility of 150-200 cm(2)/(V s). Temperature-dependent resistance of a fully germanided Mn(5)Ge(3) nanowire shows a clear transition behavior near the Curie temperature of Mn(5)Ge(3) at about 300 K. Our findings of the high-quality room-temperature ferromagnetic Mn(5)Ge(3) contact represent a promising step toward electrical spin injection into Ge nanowires and thus the realization of high-efficiency spintronic devices for room-temperature applications.

Free-standing and single-crystalline Fe(1-x)Mn(x)Si nanowires with room-temperature ferromagnetism and excellent magnetic response.

High-aspect-ratio Fe(1-x)Mn(x)Si nanowires with room-temperature ferromagnetism were synthesized by a chemical vapor deposition (CVD) method in one step. This is the first report of ternary silicide nanowires using magnetic Mn ions to partially replace metal sites in the host matrix. Here we report the excellent magnetic characteristics of Fe(1-x)Mn(x)Si nanowires, which exhibit strong ferromagnetism at room temperature and high magnetoresistance (MR) variation. As-synthesized Fe(1-x)Mn(x)Si nanowires show a hyperbranched morphology and a spin-disorder behavior. The strong spin interaction in Fe(1-x)Mn(x)Si nanowires, induced by the substitution of Fe sublattices for magnetic Mn ions, was revealed in the hysteresis loops. The magnetization versus magnetic field (M-H) curves of Fe(1-x)Mn(x)Si nanowires are much less sensitive to the temperature variation from 10 to 300 K than those of FeSi nanowires. Remarkably, the excellent MR performance, -41.6% at 25 K with a magnetic field of 9 T, was demonstrated in an individual Fe(0.88)Mn(0.12)Si nanowire.

Electrical properties and magnetic response of cobalt germanosilicide nanowires.

The effects of partial substitution of Ge for Si in cobalt germanosilicide (CoSi(1-x)Ge(x) and Co(2)Si(1-x)Ge(x)) nanowires (NWs) on the electrical transport, magnetic properties, and magnetoresistance (MR) have been investigated. Cobalt germanosilicide NWs were synthesized by a spontaneous chemical vapor transport growth method. The Ge concentration can be selectively controlled from 0 to 15% and 0-50% for CoSi(1-x)Ge(x) and Co(2)Si(1-x)Ge(x) NWs, respectively, by varying the reaction temperature. Electrical measurements showed that the resistivities of CoSi(1-x)Ge(x) NWs are 90, 60, 30, and 23 µΩ-cm for x = 0, 0.01, 0.05, and 0.15, respectively. Therefore, the electrical resistivity of CoSi(1-x)Ge(x) NWs was found to decrease significantly with an increasing Ge concentration, which is believed to be a result of the band gap narrowing. On the other hand, the Co(2)Si(1-x)Ge(x) (x ≤ 0.5) NWs exhibited ferromagnetism at 300 K, which is attributed to the uncoordinated Co atoms on the NW surface and spin-glass behavior at low temperature. The highest MR response of Co(2)Si(1-x)Ge(x) NWs occurred at x = 0.5, where a MR ratio of 11.7% can be obtained at 10-25 K with a magnetic field of 8 T. The enhanced physical properties of cobalt germanosilicide NWs with Ge substitution shall lead to promising application in the fabrication of nanodevices, including spintronics and serving as the gate and interconnect material.