Multiferroic composites of NiFe 2 O 4 /(0.5BaZr 0.2 Ti 0.8 O 3 -0.5Ba 0.7 Ca 0.3 TiO 3 ). Improvement the ferroelectric and ferromagnetic properties. Strong absorbing electromagnetic waves in the frequency range of 6–18 GHz.

Abstract Reduced graphene oxide/maghemite (RGO/γ-Fe2O3) material was successfully synthesized by combining the modified Hummers method with co-precipitation (RGO 10 wt.%). γ-Fe2O3 nanoparticles with a particle size of ∼14.8 nm were distributed on the surface of RGO sheets. Results of Brunauer–Emmett–Teller analysis showed that RGO/γ-Fe2O3 had a mesoporous structure and a narrow capillary size distribution curve at about 13 nm. The specific surface area of the RGO/γ-Fe2O3 was 168 m2·g−1. The RGO/γ-Fe2O3 nanocomposite was used to adsorb arsenic As(V) and a mixture of heavy metals (As(V), Cr(VI), Pb(II), and Fe(III)) in water. The maximum adsorption efficiency of As(V) reached 98.9% after 45 min with an adsorption capacity of 5.93 mg·g−1, higher than the simultaneous adsorption of the four metal ions. Competitive adsorption decreased in the order As(V), Cr(VI), Pb(II), and Fe(III). Therefore, RGO/γ-Fe2O3 could be used as an effective adsorbent to remove heavy metals from aqueous solutions.

The magnetic and absorbing properties of La0.7Sr0.3MnO3 nanoparticles created by ceramic and high-energy ball milling methods were investigated in detail in this study. The x-ray diffraction data show that they belong to a rhombohedral structure with an average crystalline size of ⟨D⟩ = 41 ± 2 nm. The field emission scanning electron microscope image showed that the La0.7Sr0.3MnO3 particles have a pseudo-spherical shape with particle sizes ranging from 20 to 100 nm. The Curie temperature (TC), effective paramagnetic moment, coercivity (Hc), and saturation magnetization (Ms) of the sample were determined using the temperature and magnetic field dependences of the magnetization. It shows that La0.7Sr0.3MnO3 nanoparticles exhibit a soft ferromagnetism with TC = 358 K and Hc ≈ 15 Oe and Ms ≈ 60 emu/g at 300 K. The microwave reflection loss (RL) of La0.7Sr0.3MnO3–paraffin composites with thicknesses t = 1.5–3.5 mm at room temperature was measured at frequencies 4–18 GHz using a vector network analyzer. The largest negative RL values obtained for un-backed and Al-backed samples are about −14.07 dB at 17.97 GHz and −24.87 dB at 14.71 GHz corresponding to the absorbing thicknesses t = 1.5 and 2.0 mm, respectively. These resonant effects are explained by the impedance matching mechanism in the samples.

Vật liệu nano tổ hợp đa pha điện từ có công thức thành phần xNi0.6 Zn0.4Fe2O4/(1-x)BaTiO3 (x = 0; 0,1; 0,3; 0,5) (xNZFO/(1-x) BTO) với kích thước hạt cỡ 80-100 nm được chế tạo bằng phương pháp nghiền cơ năng lượng cao kết hợp xử lý nhiệt. Các đặc trưng cấu trúc, tính chất điện, từ và quang đã được khảo sát. Giản đồ nhiễu xa tia X thể hiện vật liệu tồn tại hai pha độc lập của Ni0.6Zn0.4Fe2O4 (NZFO) và BaTiO3 (BTO). Ở nhiệt độ phòng, do ảnh hưởng của việc tăng hàm lượng của pha sắt từ NZFO từ x = 0 đến x = 0,5, trong dải điện trường cực đại cỡ 10 kV/cm, giá trị của độ phân cực điện dư (Pr), lực kháng điện (Ec) và độ từ hóa bão hòa (Ms) tăng mạnh, có giá trị tương ướng từ 0,0055-0,0158 µC/cm2, 1,05-3,2 kV/cm, và 0,6 -31,2 emu/g. Bên cạnh đó, những ảnh hưởng của hàm lượng pha sắt từ NZFO đến tính chất quang của vật liệu đã được nghiên cứu hệ thống. Khi hàm lượng NZFO tăng từ x = 0 đến x = 0,5, giá trị năng lượng vùng cấm của vật liệu đã suy giảm từ 3,2 xuống 2,65 eV.

We have investigated the structural and magnetic properties of $$\hbox {Ge}_{0.94}\hbox {Mn}_{0.06-\delta }\hbox {C}_{\delta }$$ films $$(\delta = 0.005, 0.01\ \hbox {and} 0.02)$$ using reflexion high-energy electron diffraction (RHEED) technique, transmission electron microscopy (TEM) and superconducting quantum interference device magnetometer. All films have been prepared by co-depositing Ge, Mn and C by molecular beam epitaxy. RHEED pattern shows the increase in sample surface roughness when doping carbon into the GeMn films. TEM analyses indicate that adding carbon greatly reduces the surface diffusion of both Ge and Mn elements. Ferromagnetic ordering in samples containing carbon contents of 0.01 and 0.02, persists at temperatures $${>}400\hbox { K}$$. An increase in net magnetization is found for carbon-doped samples with increasing carbon content from 0.01 to 0.02. However, we found a decrease in the net magnetization and the Curie temperature of the samples after annealing at 450 and 650°C. The Curie temperature reduces down to about 300 K, which is comparable to the value of the free-carbon sample showing a harmful effect of the post-annealing on the magnetic properties of carbon-doped GeMn nanocolumns.

Structural and magnetic characterisations along with first-principles total energy calculations based on the density functional theory have been combined to investigate the effects of (001) and (111)-oriented Ge substrates on the formation of GeMn nanocolumns. The samples were grown by means of molecular beam epitaxy (MBE) at the growth temperature (T-S) of 130 degrees C, the Mn concentration of similar to 6% and the film thickness of similar to 80 nm. We found that due to the surfactant effect of Mn atoms, Ge0,94Mn0,06 films grown on the Ge(001) substrate exhibit the nanocolumn structure along the growth direction and the Curie temperature (T-C) is higher than 400 K. On the other hand, for the Ge0,94Mn0,06 films grown on the Ge(111) substrate, Mn adatoms can easily diffuse into deeper layers through the interstitial sites, resulting in the formation of Ge3Mn5 streaks along the preferred direction [110] and surrounded by diluted matrix. The physical origin of the contrasting behaviour of Mn atoms is therefore strongly related to the different surface reconstruction.

This chapter presents the results of growing GeMn nanocolumns on Ge(001) substrates by means of molecular beam epitaxy (MBE). The samples have been prepared by co-depositing Ge and Mn at growth temperature of 130°C and Mn at concentration of ~6% to ensure the reproduction of GeMn nanocolumns. Based on the observation of changes in reflection high-energy electron diffraction (RHEED) patterns during nanocolumn growth, surface signals of GeMn nanocolumn formation have been identified. Structural analysis using transmission electron microscopy (TEM) show the self-assembled nanocolumns with core-shell structure extend through the whole thickness of the GeMn layer. Most of nanocolumns are oriented perpendicular to the interface along the growth direction. The nanocolumn size has been determined to be about 5–8 nm in diameter and a maximum height of 80 nm. A phenomenological model has been proposed to explain the driving force for self-assembly and growth mechanisms of GeMn nanocolumns. The in-plane or lateral Mn diffusion/segregation is driven by a low solubility of Mn in Ge while the driving force of Mn vertical segregation is induced by the surfactant effect along the [001] direction.

Ge0.94Mn0.06 nanocolumn thin film is a unique phase of GeMn diluted magnetic semiconductors (DMS) which exhibit Curie temperature (TC) > 400 K. The multilayers of Ge0.94Mn0.06 nanocolumns separated by nano-scaled spacers represent great interests for spintronic applications, such as spin valves or giant magneto-resistance (GMR) multilayers. In this article, we present the results obtained from the preliminary study on the exchange coupling in two types of GeMn nanocolumn/Ge multilayers. All the samples have been grown using molecular beam epitaxy (MBE). The superconducting quantum interference device (SQUID) magnetometer has been used to determine the magnetic properties of the samples. In the multilayer system Ge/[Ge0.94Mn0.06(40 nm)/Ge(d nm)]9/Ge0.94Mn0.06(40 nm)/Ge, no exchange coupling can be observed. Inversely, exchange coupling between the layers exists and depends on the thickness of the Ge spacers for the GeMn nanocolumns/Ge multilayer spin valve systems. The exchange coupling in the nanocolumns multilayer systems has been shown to be complex due to the leakage field induced by neighboring nanocolumns and the magnetic anisotropy of nanocolumns.

The critical exponents and magnetocaloric properties of La 0.7 Sr 0.3 Mn 1− x Ti x O 3 with x = 0 and 0.05 have been explored via magnetic measurements. The magnetic data analyzed in the critical region by using modified Arrott plots, the Kouvel–Fisher method, and the scaling hypothesis reveal values for the critical parameters of β = 0.3165 and 0.3027, γ = 1.1121 and 1.203, and δ = 4.7556 and 4.833 for x = 0 and x = 0.05 respectively. These values are in agreement with the three-dimensional (3D) Ising model. The n values obtained from the law Δ S_max = a(μ_0 H)^n are 0.648 and 0.685. The maximum values of the magnetic entropy change are 1.65 J/kg K and 1.46 J/kg K under a magnetic field change of 10 kOe for x = 0.00 and 0.05, respectively. These results can be explained in the scenario of the short-range interaction and the phase separation as well as the diluted magnetic effect in manganites.

The critical exponents and magnetocaloric properties of La0.7Sr0.3Mn1−xTixO3 with x = 0 and 0.05 have been explored via magnetic measurements. The magnetic data analyzed in the critical region by using modified Arrott plots, the Kouvel–Fisher method, and the scaling hypothesis reveal values for the critical parameters of β = 0.3165 and 0.3027, γ = 1.1121 and 1.203, and δ = 4.7556 and 4.833 for x = 0 and x = 0.05 respectively. These values are in agreement with the three-dimensional (3D) Ising model. The n values obtained from the law \( \Delta S_{\rm{max} } = a(\mu_{0} H)^{n} \) are 0.648 and 0.685. The maximum values of the magnetic entropy change are 1.65 J/kg K and 1.46 J/kg K under a magnetic field change of 10 kOe for x = 0.00 and 0.05, respectively. These results can be explained in the scenario of the short-range interaction and the phase separation as well as the diluted magnetic effect in manganites.

High-resolution transmission electron microscopy (HR-TEM) has been used to investigate the structural properties of GeMn/Ge nanocolumns multilayer samples grown on Ge(001) substrates by means of molecular beam epitaxy (MBE) system. Four bilayers with the spacer thickness in the range between 6 nm and 15 nm and 10 periods of bilayers of Ge0.94Mn0.06/Ge nanocolumn are presented. A simplified 2D model based on the theory of elastic constant interactions has been used to provide reasonable explanations to the vertical self-organization of GeMn nanocolumns in multilayers.

Molecular Beam Epitaxy (MBE) system equipped with in-situ Reflection High-Energy Electron Diffraction (RHEED) has been used for (Ge, Mn) thin film growth and monitoring the surface morphology and crystal structure of thin films. Based on the observation of changes in RHEED patterns during nanocolumn growth, we used a real-time control approach to realize multilayer structures that consist of two nanocolumn layers separated by a Ge barrier layer. Transmission Electron Microscopy (TEM) has been used to investigate the structural properties of the GeMn nanocolumns and GeMn/Ge nanocolumns bi-layers samples.

By mean of molecular beam epitaxy (MBE) equipped with a reflexion high-energy electron diffraction (RHEED) technique, we have chosen an intermediate and appropriate substrate temperature of 130\(\r{}\)C to reproducibly synthetize high-T\(_{C}\) Ge\(_{1 - x}\)Mn\(_{x}\) nanocolumns phase. Laser Pulse Atom Probe Tomography (LP-APT) technique have been used to determine at atomic scale the chemical composition inside nanocolumns and also in the surrounding diluted matrix. The Mn concentration inside nanocolumns is found to be highly inhomogeneous, it is about 20\({\%}\) at the bottom and can increase up to \(\sim 40{\%}\) in the top near the surface region. The Mn concentration in the matrix is about 0.25\({\%}\) at the surface and can reach a highest value of $\sim $1{\%} in regions close to the interface.

We have combined numerous characterization techniques to investigate the growth of tensile-strained and n-doped Ge films on Si(001) substrates by means of solid-source molecular-beam epitaxy. The Ge growth was carried out using a two-step growth method: a low-temperature growth to produce strain relaxed and smooth buffer layers, followed by a high-temperature growth to get high crystalline quality Ge layers. It is shown that the Ge/Si Stranski–Krastanov growth mode can be completely suppressed when the growth is performed at substrate temperatures ranging between 260°C and 300°C. X-ray diffraction measurements indicate that the Ge films grown at temperatures of 700–770°C are tensile-strained with typical values lying in the range of 0.22–0.24%. Cyclic annealing allows further increase in the tensile strain up to 0.30%, which represents the highest value ever reported in the Ge/Si system. n-Doping of Ge was carried out using a GaP decomposition source. It is shown that heavy n-doping levels are obtained at low substrate temperatures (210–250°C). For a GaP source temperature of 725°C and a substrate temperature of 210°C, a phosphorus concentration of about 1019cm−3 can be obtained. Photoluminescence measurements reveal an intensity enhancement of about 16 times of the direct band gap emission and display a redshift of 25meV that can be attributed to band gap narrowing due to a high n-doping level. Finally, we discuss about growth strategies allowing optimizing the Ge growth/doping process for optoelectronic applications.

Structural and magnetic characterizations have been combined to investigate the growth kinetics of Ge 1−x Mn x diluted magnetic semiconductors (DMSs) on Ge(001) substrates by means of molecular beam epitaxy (MBE). We have identified the growth process window allowing stabilization of a high Curie temperature (T C ) nanocolumn phase and provide evidence that the growth of semiconducting Ge 1−x Mn x nanocolumns and metallic Mn 5 Ge 3 clusters is a competing process. Due to a continuous increase of the Mn concentration inside nanocolumns, induced by Mn segregation along the growth direction from the interface toward the film surface, nanocolumns become unstable when the Mn concentration reaches a value of ∼40 at.% then transform into Mn 5 Ge 3 clusters. We propose a real-time approach to realize stacked layers consisting of nanocolumns separated by a Ge barrier layer, allowing exploitation of the effect of giant magneto-resistance in multilayer structures

The Mn5Ge3 compound, thanks to its room-temperature ferromagnetism, metallic character and ability to epitaxially grow on germanium, acts as a potential candidate for spin injection into group-IV semiconductors. Understanding and controlling Ge overgrowth behaviour on Mn5Ge3/Ge heterostructures represents a crucial step to realize Ge/Mn5Ge3/Ge multilayers for numerous spintronic applications. Here, we have combined structural and morphological characterizations with magnetic analyses to study the mechanisms of Ge overgrowth on epitaxial Mn5Ge3 layers in the temperature range of 450-550 degrees C. It is found that deposited Ge instantly reacts with Mn to form a Mn5Ge3 surface layer, which, acting as a surfactant, continuously floats upwards from the growing surface to a distance larger than 70 nm. New Ge layers are successively formed underneath, allowing such a floating Mn5Ge3 surface layer to be stabilized by epitaxy. These observations can be considered as a typical example in which the stabilization of metastable thin films by epitaxy can overcome thermodynamic equilibrium. We have also investigated the effect of carbon adsorption on the top of the Mn5Ge3 layer prior to Ge deposition to control the Mn:Ge reaction. It is shown that adsorbed carbon effectively reduces the out-diffusion of Mn from Mn5Ge3, allowing Ge layers to stack up on top of Mn5Ge3. However, at temperatures of 450-550 degrees C, carbon may react with Mn to form manganese carbides and the resulting Ge overlayers are found to change their orientation from the (111) plane to the (001) plane, which has a higher surface energy. Finally, a strategy to realize Ge/Mn5Ge3/Ge multilayers will be addressed.