Ferromagnetic Mn5Ge3 Research Articles

Lateral Mn5Ge3 spin-valve in contact with a high-mobility Ge two-dimensional hole gas

Abstract Ge two-dimensional hole gases (2DHG) in strained modulation-doped quantum-wells represent a promising material platform for future spintronic applications due to their excellent spin transport properties and the theoretical possibility of efficient spin manipulation. Due to the continuous development of epitaxial growth recipes extreme high hole mobilities and low effective masses can be achieved, promising an efficient spin transport. Furthermore, the Ge 2DHG can be integrated in the well-established industrial complementary metal-oxide-semiconductor (CMOS) devices technology. However, efficient electrical spin injection into a Ge 2DHG—an essential prerequisite for the realization of spintronic devices—has not yet been demonstrated. In this work, we report the fabrication and low-temperature magnetoresistance (MR) measurements of a laterally structured Mn5Ge3/Ge 2DHG/ Mn5Ge3 device. The ferromagnetic Mn5Ge3 contacts are grown directly into the Ge quantum well by means of an interdiffusion process with a spacing of approximately 130 nm, forming a direct electrical contact between the ferromagnetic metal and the Ge 2DHG. Here, we report for the first time a clear MR signal for temperatures below 13 K possibly arising from successful spin injection into the high mobility Ge 2DHG. The results represent a step forward toward the realization of CMOS compatible spintronic devices based on a 2DHG.

Epitaxial growth and magnetic properties of Mn5(SixGe1-x)3 thin films

Structural and magnetic properties of Mn5(SixGe1-x)3 thin films were investigated. Ferromagnetic Mn5Ge3 and anti-ferromagnetic Mn5Si3 thin films have been synthesized and characterized as these compounds exhibit interesting features for the development of spintronics. Here, Mn5(SixGe1-x)3 thin films were grown on Ge(111) substrates by co-deposition using molecular beam epitaxy. Crystalline thin films can be produced with controlled Si concentrations ranging from 0 to 1. The thin films were relaxed by dislocations at the interface with the substrate. A lattice parameter variation was observed as the Si content increased, which is comparable to previous works done in bulk. Reflection high-energy electron diffraction diagrams and X-ray diffraction profiles showed that lattice parameters a and c are shrinking and that the surface roughness and crystallinity degrade as the Si amount increases. Magnetometric measurements revealed a ferromagnetic behavior for all Si concentrations. The measured average ferromagnetic moment per manganese atom decreased from 2.33 to 0.05 μB/Mn atom. No ferro to anti-ferromagnetic transition was observed contrary to the bulk Mn5(SixGe1-x)3 compound.

Realization of a New Graphene-Ferromagnet Interface with Dirac Linear Band Dispersion.

The integration of graphene in spintronics applications requires its close contact with ferromagnetic materials, promoting effective spin injection. At the same time, the linear energy vs wave-vector dependence for the charge carriers in the vicinity of the Fermi level for graphene has to be conserved. Here, motivated by recent theoretical predictions, we present the experimental realization on the synthesis of graphene/ferromagnetic-Mn5Ge3/semiconducting-Ge heterostructures using the intercalation of Mn in the epitaxial graphene/Ge interfaces. Different in situ and ex situ methods confirm the formation of such heterosystems, where graphene is in close contact with ferromagnetic Mn5Ge3, as the Curie temperature reaches room temperature. Despite the expected small distance between graphene and Mn5Ge3 causing the strong interaction at interfaces, our angle-resolved photoelectron spectroscopy experiments for the formed graphene/Mn5Ge3 interfaces confirm the linear band dispersion around the Fermi level for the carriers in graphene. These findings open up an interesting perspective for the integration of graphene in modern semiconductor technology with possible implications for spintronics device fabrication.

Solid-state synthesis and characterization of ferromagnetic Mn5Ge3 nanoclusters in GeO/Mn thin films

Mn5Ge3 films are promising materials for spintronic applications due to their high spin polarization and a Curie temperature above room temperature. However, non-magnetic elements such as oxygen, carbon and nitrogen may unpredictably change the structural and magnetic properties of Mn5Ge3 films. Here, we use the solid-state reaction between Mn and GeO thin films to describe the synthesis and the structural and magnetic characterization of Mn5Ge3(Mn5Ge3Oy)-GeO2(GeOx) nanocomposite materials. Our results show that the synthesis of these nanocomposites starts at 180°С when the GeO decomposes into elemental germanium and oxygen and the resulting Ge atoms immediately migrate into the Mn layer to form ferromagnetic Mn5Ge3 nanoclusters. At the same time the oxygen atoms take part in the synthesis of GeOx and GeO2 oxides and also migrate into the Mn5Ge3 lattice to form Mn5Ge3Oy Nowotny nanoclusters. Magnetic analysis assumes the general nature of the Curie temperature increase in carbon-doped Mn5Ge3Cx and Mn5Ge3Oy films. Our findings prove that not only carbon, but oxygen may contribute to the increase of the saturation magnetization and Curie temperature of Mn5Ge3-based nanostructures.

Effect of Ge layer thickness on the formation of Mn5Ge3 thin film on Ge/Si (1 1 1)

Abstract The effect of Ge buffer layer thickness on the formation of epitaxial Mn5Ge3 films and their magnetic properties has been investigated. Epitaxial ferromagnetic Mn5Ge3 thin films were deposited on Ge/Si (1 1 1) substrates using solid phase epitaxy. The crystalline quality, chemical composition and magnetic properties of the films were investigated by x-ray diffraction, transmission electron microscopy, x-ray photoelectron spectroscopy, electron spin resonance. Increasing the thickness of the Ge layer significantly enhanced the crystallinity and magnetic homogeneity of Mn5Ge3 films. Both structural and magnetic investigations show substantial diffusion of Mn atoms through Ge buffer layer which results in the formation of Mn5Si3 and a delay in Mn5Ge3 formation until the thickness of the Ge layer reaches to 70 nm. The observed magnetic behavior for the films grown on different Ge thicknesses is interpreted in terms of changes in the magnetic phases and surface properties. These results show that Mn5Ge3 can be epitaxially grown on Si substrate with Ge buffer layer thicknesses of above 140 nm, despite ∼8% lattice mismatch between Mn5Ge3 and Si (1 1 1).

Formation of Mn5Ge3 by thermal annealing of evaporated Mn on doped Ge on Si(111)

Mn5Ge3 can be used as a ferromagnetic contact material to fabricate spintronic devices. Here, we show that Mn5Ge3 can be fabricated with a simple germanidation process by evaporating Mn on undoped and doped Ge on Si followed by a thermal annealing step to form the ferromagnetic Mn5Ge3 phase. This solid phase preparation of Mn5Ge3 is a robust process with a minor dependence on the annealing parameters. The formation of Mn5Ge3 can be realized using undoped as well as highly doped p-Ge and n-Ge with different doping levels. The interface of Mn5Ge3 is atomically sharp which leads to very low contact resistivities < 1 × 10−7 Ω cm2.

Solid state synthesis of Mn5Ge3 in Ge/Ag/Mn trilayers: Structural and magnetic studies

The thin-film solid-state reaction between elemental Ge and Mn across chemically inert Ag layers with thicknesses of (0, 0.3, 1 and 2.2µm) in Ge/Ag/Mn trilayers was studied for the first time. The initial samples were annealed at temperatures between 50 and 500°C at 50°C intervals for 1h. The initiation temperature of the reaction for Ge/Mn (without a Ag barrier layer) was ~ 120°C and increased slightly up to ~ 250°C when the Ag barrier layer thickness increased up to 2.2µm. In spite of the Ag layer, only the ferromagnetic Mn5Ge3 compound and the Nowotny phase were observed in the initial stage of the reaction after annealing at 500°C. The cross-sectional studies show that during Mn5Ge3 formation the Ge is the sole diffusing species. The magnetic and cross-sectional transmission electron microscopy (TEM) studies show an almost complete transfer of Ge atoms from the Ge film, via a 2.2µm Ag barrier layer, into the Mn layer. We attribute the driving force of the long-range transfer to the long-range chemical interactions between reacting Mn and Ge atoms.

Research Update: Strain and composition effects on ferromagnetism of Mn0.05Ge0.95 quantum dots

Mn0.05Ge0.95 quantum dots (QDs) samples were grown by molecular beam epitaxy on Si substrates and 15-nm-thick fully strained Si0.8Ge0.2 virtual substrates, respectively. The QDs samples grown on the Si0.8Ge0.2 virtual substrates show a significant ferromagnetism with a Curie temperature of 227 K, while the QDs samples grown on the Si substrates are non-ferromagnetic. Microstructures of the QDs samples were characterized by high resolution transmission electron microscopy and synchrotron radiation X-ray diffraction. Interdependence between microstructure and ferromagnetism of Mn-doped Ge QDs was investigated. For the QDs sample grown on the strained Si0.8Ge0.2 virtual substrate, although the ferromagnetic phase Mn5Ge3 clusters were found to be formed in small dome-shaped dots, the significant ferromagnetism observed in that sample is attributed to ferromagnetic phase Mn-doped large dome-shaped Ge QDs, rather than to the ferromagnetic phase Mn5Ge3 clusters. The fully strained Si0.8Ge0.2 virtual substrates would result in a residual strain into the QDs and an increase in Ge composition in the QDs. Both consequences favor the formations of ferromagnetic phase Mn-doped Ge QDs from points of view of quantum confinement effect as well as Mn doping at substitutional sites.

Spin Transport in Ge Nanowires for Diluted Magnetic Semiconductor-Based Nonvolatile Transpinor

Spintronic devices, in particular spin field-effect transistors (spinFETs), have been researched for decades as a promising candidate to replace Si transistors with potentially low power dissipation and low variability. In this paper, we propose a nonvolatile spin-base transistor (transpinor) based on diluted magnetic semiconductor (DMS) with electric field-controlled paramagnetism-to-ferromagnetism phase transition. To realize the transpinor, we demonstrate the electrical spin injection into Ge nanowires using both ferromagnetic Mn5Ge3 Schottky contacts and Fe/MgO tunnel junctions. We observe much longer spin lifetimes and diffusion lengths in Ge nanowires compared with bulk Ge. Furthermore, we successfully grow the single-crystalline MnxGe1-x DMS nanowires using pattern-assisted molecular beam epitaxy (MBE). The MnxGe1-x nanowires exhibit a Curie temperature above 400 K, and the ferromagnetism can be further modulated by an external gate voltage. The demonstrated electric field-controlled ferromagnetism, along with the successful spin injection into Ge nanowires, paves the road to build the proposed transpinor.

Spin Transport in Ge Nanowires for Diluted Magnetic Semiconductor-Based Nonvolatile Transpinor

Spintronic devices, in particular spin field-effect transistors (spinFETs), have been studied for decades as a promising candidate to replace Si CMOS devices. Several variants of spinFETs have been proposed to use the spin of electrons as another degree of freedom for information processing. However, all the previous proposals still use charge current, which generates power dissipation. To eliminate the charge current, we propose a novel spin-based transistor (transpinor) based on diluted magnetic semiconductor (DMS) with electric field-controlled paramagnetism-to-ferromagnetism phase transition. One promising prototype is to integrate a MnxGe1-x DMS nanowire, whose ferromagnetism can be effectively controlled by a gate electrode, with Mn5Ge3ferromagnetic contacts for spin injection (Figure 1). Another variant of the ferromagnetic contacts could be epitaxial Fe/MgO tunnel junctions. In this transpinor device, the magnetic moments are transferred from the source to the channel, and then to the drain through spin injection and exchange interaction. As the gate electrode manipulates the ferromagnetism of the DMS channel, it controls the communication between the source and drain.In order to realize the proposed transpinor device, we first studied the spin transport in Ge nanowires and demonstrated the electrical spin injection and detection using both ferromagnetic Mn5Ge3 Schottky contacts and Fe/MgO tunnel junctions (Figure 2). The Mn5Ge3 contacts were fabricated by germanidation through rapid thermal annealing. This convenient approach produced single-crystalline Mn5Ge3 contacts with a controlled channel length and atomically clean interfaces to the Ge nanowire, which helped alleviate Fermi level pinning at the interface. Subsequently, electrical spin injection and detection was successfully demonstrated in the Mn5Ge3/Ge/Mn5Ge3 nanowire transistor. A spin diffusion length of λsf = 480 nm and a spin flip lifetime of τsf = 244 ps are revealed in p-type Ge nanowires at T = 10 K, which were much larger than those reported for bulk p-type Ge. Meanwhile, spin injection into Ge nanowires with epitaxial Fe/MgO tunneling contacts was performed in nonlocal spin valve measurements. The spin diffusion length of λsf = 2.57 μm and the spin lifetime of τsf = 7.2 ns were obtained in n-type Ge nanowires at T = 40 K, which are again much larger than those in bulk n-type Ge. The significant enhancement in the spin lifetime and diffusion length in nanowires suggested that the spin relaxation was suppressed in nanowires compared with bulk materials.Furthermore, we successfully demonstrated the pattern-assisted growth of MnxGe1-x DMS nanowires (Figure 3), which serve as the transpinor channel. The growth was performed using molecular beam epitaxy (MBE) chamber with SiO2 nanotrenches as the mask. The ferromagnetic MnxGe1-xnanowires showed a Curie temperature above 400 K without any precipitate. The ferromagnetism can be further modulated by a gate voltage at 100 K in a metal-oxide-semiconductor capacitor structure, demonstrating electric field control of ferromagnetism. This result, along with the electrical spin injection into Ge nanowires, lays the foundation to build the transpinor device as a possible realization of all-spin logic devices with built-in memory for low-power applications. Acknowledgement: This work was in part supported by Western Institution of Nanoelectronics (WIN) and National Science Foundation through grant ECCS 1308358.

Synthesis of Ferromagnetic Germanides in 40Ge/60Mn Films: Magnetic and Structural Properties

Solid-state reactions between Ge and Mn films are systematically examined using X-ray diffraction, photoelectron spectroscopy and magnetic measurements. The films have a nominal atomic ratio Ge:Mn = 40:60 and are investigated at temperatures from 50 to 500 °С. It is established that after annealing at ~120 °С, the ferromagnetic Mn5Ge3 phase is the first phase to form at the 40Ge/60Mn interface. Increasing the annealing temperature to 500 °С leads to the formation of the ferromagnetic phase with a Curie temperature TC ~ 360 K and magnetization MS ~ 140-200 emu/cc at room temperature. Analysis of X-ray diffraction patterns and the photoelectron spectra suggests that the increased Curie temperature and magnetization are related to the migration of C and O atoms into the Mn5Ge3 lattice and the formation of the Nowotny phase Mn5Ge3СxOy. The initiation temperature (~120 °С) of the Mn5Ge3 phase is the same both for solid-state reactions in Ge/Mn films, as well as for phase separation in GexMn1-x diluted semiconductors. We conclude that the synthesis of the Mn5Ge3 phase is the moving force for the spinodal decomposition of the GexMn1-x diluted semiconductors.

Formation of ferromagnetic germanides by solid-state reactions in 20Ge/80Mn films

Solid state reactions between Ge and Mn films are systematically examined using X-ray diffraction, photoelectron spectroscopy, and magnetic and electrical measurements. The films have a nominal atomic ratio Ge:Mn=20:80 and are investigated at temperatures from 50 to 500ºC. It is established that after annealing at ~120ºC, the ferromagnetic Mn5Ge3 phase is the first phase to form at the 20Ge/80Mn interface. As the annealing temperature increases to 300ºC, the weak magnetic Mn5Ge2+Mn3Ge phases simultaneously begin to grow and they become dominant at 400ºC. Increasing the annealing temperature to 500ºC leads to the formation of the ferromagnetic phase with a Curie temperature TC~350–360K and magnetization 14–25kA/m at room temperature. The X-ray diffraction study of the samples shows the reflections from the Mn5Ge3 phase, and the photoelectron spectra contain the oxygen and carbon peaks. The homogeneous distribution of oxygen and carbon over the sample thickness suggests that the increased Curie temperature and magnetization are related to the migration of C and O atoms into the Mn5Ge3 lattice and the formation of the Nowotny phase Mn5Ge3СxOy.The initiation temperature (~120°С) is the same in the Mn5Ge3 phase with the solid-state reactions in the Ge/Mn films as well as in the phase separation in the GexMn1−x diluted semiconductors. Thus, we conclude that the synthesis of the Mn5Ge3 phase is the moving force for the spinodal decomposition of the GexMn1−x diluted semiconductors.

Strain-induced modification in the magnetic properties of Mn5Ge3 thin films

Epitaxial ferromagnetic Mn5Ge3 thin films were stabilized on GaSb(001) and GaAs(001) substrates using molecular beam epitaxy. Compared to bulk Mn5Ge3 materials, an enhancement of the Curie temperature above 350 K and about 320 K was observed for Mn5Ge3/GaAs(001) and Mn5Ge3/GaSb(001) heterostructures, respectively. The magnetization was found to decrease from 323 to 245 emu/cm3 for films grown on GaSb(001) and GaAs(001). Anomalous Hall effect measurements provide evidence of the strain-induced large spin polarization from density-functional study. Furthermore, our calculated results in bulk Mn5Ge3 under strain indicate that the strain is the origin of different physical properties of Mn5Ge3 grown on different substrates.

Electrical Spin Injection and Detection in Mn5Ge3/Ge/Mn5Ge3 Nanowire Transistors

In this Letter, we report the electrical spin injection and detection in Ge nanowire transistors with single-crystalline ferromagnetic Mn5Ge3 as source/drain contacts formed by thermal reactions. Degenerate indium dopants were successfully incorporated into as-grown Ge nanowires as p-type doping to alleviate the conductivity mismatch between Ge and Mn5Ge3. The magnetoresistance (MR) of the Mn5Ge3/Ge/Mn5Ge3 nanowire transistor was found to be largely affected by the applied bias. Specifically, negative and hysteretic MR curves were observed under a large current bias in the temperature range from T = 2 K up to T = 50 K, which clearly indicated the electrical spin injection from ferromagnetic Mn5Ge3 contacts into Ge nanowires. In addition to the bias effect, the MR amplitude was found to exponentially decay with the Ge nanowire channel length; this fact was explained by the dominated Elliot-Yafet spin-relaxation mechanism. The fitting of MR further revealed a spin diffusion length of lsf = 480 ± 13 nm and a spin lifetime exceeding 244 ps at T = 10 K in p-type Ge nanowires, and they showed a weak temperature dependence between 2 and 50 K. Ge nanowires showed a significant enhancement in the measured spin diffusion length and spin lifetime compared with those reported for bulk p-type Ge. Our study of the spin transport in the Mn5Ge3/Ge/Mn5Ge3 nanowire transistor points to a possible realization of spin-based transistors; it may also open up new opportunities to create novel Ge nanowire-based spintronic devices. Furthermore, the simple fabrication process promises a compatible integration into standard Si technology in the future.

Magnetic Mn5Ge3 nanocrystals embedded in crystalline Ge: a magnet/semiconductor hybrid synthesized by ion implantation.

The integration of ferromagnetic Mn5Ge3 with the Ge matrix is promising for spin injection in a silicon-compatible geometry. In this paper, we report the preparation of magnetic Mn5Ge3 nanocrystals embedded inside the Ge matrix by Mn ion implantation at elevated temperature. By X-ray diffraction and transmission electron microscopy, we observe crystalline Mn5Ge3 with variable size depending on the Mn ion fluence. The electronic structure of Mn in Mn5Ge3 nanocrystals is a 3d6 configuration, which is the same as that in bulk Mn5Ge3. A large positive magnetoresistance has been observed at low temperatures. It can be explained by the conductivity inhomogeneity in the magnetic/semiconductor hybrid system.

Magneto-Caloric Effect in Ge0.95Mn0.05 Films

In this paper, a phenomenological model is used to calculate magneto-caloric properties of Ge0.95Mn0.05 films fabricated with substrate temperatures of 85, 100, and 120 ∘C. Calculations showed that the ferromagnetic Mn5Ge3 phase improves magneto-caloric properties of the Ge0.95Mn0.05 films. It is suggested that Ge0.95Mn0.05 films are suitable candidates as refrigerants near room-temperature region.

Phase transformations in the Mn-Ge system and in Ge x Mn1 − x diluted semiconductors

Results of an X-ray diffraction study as well as magnetic and electrical measurements of the solid-state reactions in Ge/Mn polycrystalline films of an 80/20 atomic composition have been presented. It has been shown that the ferromagnetic Mn5Ge3 phase is formed first on the Ge/Mn interface after annealing at ∼120°C. The further increase in the annealing temperature to 300°C leads to the beginning of the synthesis of the Mn11Ge8 phase, which becomes dominating at 400°C. The existence of new structural transitions in the Mn-Ge system in the region of ∼120 and ∼300°C has been predicted on the basis of the presented results and results obtained earlier when studying solid-state reactions in different film structures. The supposition about the general chemical mechanisms of the synthesis of the Mn5Ge3 and Mn11Ge8 phases during the solid-state reactions in the Ge/Mn films of the 80/20 atomic composition and the phase separation in GexMn1 − x (x > 0.95) diluted semiconductors has been substantiated.

Structural and magnetic properties of Ge1−xMnx thin films grown on Ge (001) substrates

We investigate the structural and magneto-optical properties of Mn-doped Ge (Ge1−xMnx) films with self-organized nanocolumns, grown on Ge (001) substrates by molecular beam epitaxy (MBE), in which the substrate temperature (TS) and growth rate (RG) are varied. Transmission electron microscopy (TEM) observations and magnetic circular dichroism (MCD) measurements reveal that Mn-rich nanocolumnar precipitation is formed in the Ge1−xMnx films grown at TS ≤ 100 °C, with keeping the size and spacing. At higher TS (≥150 °C), ferromagnetic Mn5Ge3 clusters are formed. It is also found that the Mn distribution in the Ge1−xMnx films can be controlled: By lowering TS or increasing RG, the Mn content xnc in the nanocolumns decreases and Mn atoms are more distributed into the Ge matrix, and eventually the magnetic properties are changed. The formation of the nanocolumns is explained by the spinodal decomposition in the layer-by-layer growth mode. We analyzed the periodicity and Mn content xnc of nanocolumns by using the Cahn-Hilliard equation.

Thermal stability of epitaxial Mn5Ge3 and carbon-doped Mn5Ge3 films

The thermal stability of epitaxial Mn5Ge3 and carbon-doped Mn5Ge3 films was investigated by combining structural and magnetic characterizations. It is shown that ferromagnetic Mn5Ge3 films transform into antiferromagnetic Mn11Ge8 upon post-annealing at 650 °C whereas carbon-doped Mn5Ge3 conserves its ferromagnetism up to 850 °C. The magnetic properties of Mn5Ge3 (Curie temperature, magnetization at saturation, and magnetic moment per Mn atom) are found to undergo a reversible transition upon carbon doping and its extraction during thermal annealing. The stability of carbon-doped Mn5Ge3 films will be discussed in terms of carbon filling of interstitial sites of Mn5Ge3, which prevents Ge diffusion from the substrate.

Relationship between the Spontaneous Magnetization, Spontaneous Magnetostriction and Heat Capacity in the Compound Mn&lt;sub&gt;5&lt;/sub&gt;Ge&lt;sub&gt;3&lt;/sub&gt;

Spontaneous magnetization Ms of the ferromagnetic compound Mn5Ge3 with the Curie temperature TC=305 K has been studied in the temperature range from 2 to 400 K. The relationships between the magnetic part of the heat capacity (CMAG), the magnetic part of the volume thermal expansion coefficient (TEC) MAG, and the Ms for the compound Mn5Ge3 have been considered. It has been found that the magnetic part of the heat capacity can be well approximated by the dependence CMAG(T)=a×|dMs2/dT|–b×|dMs4/dT| with a&gt;0 and b&gt;0, and the magnetic part of the TEC, by the dependence  MAG=C1×dMs2/dT+C2×dMs4/dT with C1&lt;0 and C2&gt;0 in the whole temperature. It is assumed that the main sources in the formation of the observed CMAG(T) and MAG(T) dependences are the changes in the elastic, magnetoelastic, magnetovolume, and magnetostatic energies of the sample that appear as a result of the spontaneous magnetostrictive deformation with temperature changing.