Effects of the inhomogenous co doping on the magnetoresistance of Zn1-xCo(x)O epitaxial films

A series of Zn1-xCo(x)O epitaxial films around 100 nm with nominal Co concentration from 5% to 15% was prepared by ultra high vacuum (UHV) magnetron reactive sputtering. The optical, magnetic and magneto-transport properties of this series of Zn1-xCo(x)O epitaxial films were investigated, respectively. Resonant Raman spectra indicate the high structural and crystalline quality of these Zn1-xCo(x)O (5 < or = x < or = 15%) films, and confirm a consistent correlation between the substituting Co ions content with the Co doping concentration as well. Paramagnetism, superparamagnetism and ferromagnetism with altered Curie temperature from low temperatures to above room temperatures have been observed in these films by SQUID magnetometry. The broad blocking temperature range indicates the presence of inhomogenous distribution of the magnetic nano-clusters in the superparamagnetic films. However, the magneto-transport behaviors do not strongly respond to the change of the magnetic properties from paramagnetism to ferromagnetism of these Zn1-xCo(x)O films. The lack of efficient coupling between the inhomogenous magnetic nanoclusters and the carrier system in ferromagnetic Zn1-xCo(x)O films highlights the absence of the intrinsic magnetic origins in high structural quality Zn1-xCo(x)O (5 < or = x < or = 15%) epitaxial films. On the other hand, the competition between the spin alignments and the inhomogenous local disorder effect by magnetic ions is suggested to be responsible for the carrier properties and the oberseved magnetoresistance in these Co doping Zn1-xCo(x)O (5 < or = x < or = 15%) epitaxial films.     

Influence of Chromium Doping on Electrical and Magnetic Behavior of Nd<Subscript>0.5</Subscript>Sr<Subscript>0.5</Subscript>MnO<Subscript>3</Subscript> System

With a view to understand the influence of chromium doping at the Mn site on the electrical and magnetic behavior of the Nd_0.5Sr_0.5MnO₃ manganite system, a series of samples were prepared by the citrate sol–gel route method. The samples were characterized structurally by XRD. A systematic investigation of electrical resistivity over a temperature range 5–300 K was carried out mainly to understand the magneto-transport behavior in these materials. Studies on the variation of magnetization with temperature over a temperature range 80–330 K were undertaken. Investigation of magnetization at different magnetic fields at two different temperatures, viz. 80 and 300 K, was also carried out. The results show that chromium doping gave typical electrical and magnetic properties. It has been concluded that the coexistence of charge ordered and ferromagnetic phases induced by chromium doping plays an important role in the low-temperature behavior of the system.     

Effect of Nickel Doping on the Magnetotransport Properties of?Sm0.55Sr0.45MnO3 Manganites

We studied the effects of nickel (Ni) doping on the magneto-transport properties of Sm_0.55Sr_0.45MnO₃ manganites near the metal-insulator transition. Various concentrations of Ni-doped Sm_0.55Sr_0.45MnO₃ samples up to 10% were prepared (Ni was partially substituted at the Mn-site). The temperature dependence of resistivity and magnetoresistance were measured as a function of Ni concentrations at various applied magnetic fields. We observed a nonlinear reduction of the metal-insulator transition temperature (MIT) with increasing concentration of Ni, 5% of Ni was sufficient to completely suppress the insulator-metal transition. Moreover, we observed dramatic increases of the resistance of the doped material with an increasing Ni-doping (5% of Ni increases R by more than 1000 times). The resistivity peaks at various magnetic fields collapses on themselves at the high temperature ends above the MIT. We also performed magnetization versus temperature measurements on both Ni-free the Ni-doped samples for FC and ZFC states. The FC and ZFC curves rapidly decrease to paramagnetic state at 175 K and 130 K for ZFC and FC states, respectively. For other Ni-doped samples, we observed a reduction in the paramagnetic transition temperature with increasing Ni concentration.     

Combined effects of heat treatment and seed layer materials on magnetic properties of CoCrPt perpendicular media

Samples of glass/Ti(20 nm)/CoCrPt (20 nm)/Ti (3.2 nm) and glass/seed (Cr or Cu)/Ti(20 nm)/CoCrPt(20 nm)/Ti (3.2 nm) were deposited by magnetron sputtering at ambient temperature or 200 °C. All samples were post-annealed at 400 °C for 15 min. For either Cu or Cr seed layer, post-annealing can enhance the out-of-plane coercivity. For post-annealed samples with either Cr or Cu seed layers, the ambient temperature growth induce an enhancement in the coercivity but a reduction in the slope of hysteresis loop at the coercivity, in comparison with that of the elevated temperature growth. With identical post-annealing and growth conditions, Cr or Cu seed layers can enhance the out-of-plane coercivity, in comparison with those grown on bare substrates. The nucleation field is negative for Cr seed layer and positive for the Cu seed layer although they have close coercivity. With the same growth and post-annealing conditions, the magnetic properties of CoCrPt layers depend on the seed layer thickness. Evolution of magnetic properties can be explained in terms of changes of structural properties of constituent layers.     

Transport Properties of Magnetic Tunnel Junctions Comprising NiFeSiB/CoFeB Hybrid Free Layers

We report on the magneto-transport measurements of MgO magnetic tunnel junctions (MTJs) composed of NiFeSiB/CoFeB as the free layer for two different structures (top-type and bottom-type pinning). The magneto-transport properties of these MTJs were investigated by varying the thickness of the amorphous NiFeSiB layer for a fixed CoFeB thickness. The tunnel magnetoresistance (TMR), measured in both type of structures, exhibit the same or a higher amplitude (up to 230% measured at room temperature in the case of top-type device), comparing to the case of a single CoFeB free layer. These results suggest that hybrids free layers can be used as good candidates for MTJs with reduced saturation magnetization while keeping a high TMR ratio.      

Magneto-transport properties of (Cu)x/CuTl-1223 nanoparticles-superconductor composites

Copper (Cu) nanoparticles and Cu_0.5Tl_0.5Ba₂Ca₂Cu₃O_10−δ (CuTl-1223) superconducting phase were synthesized by sol-gel and solid-state reaction, respectively. These metallic Cu nanoparticles were added in CuTl-1223 superconducting matrix to get (Cu)_x/CuTl-1223; x = 0–4.0 wt% nanoparticles-superconductor composites and their temperature dependent magneto-transport properties were studied. The zero-field-cooled (ZFC) and field-cooled (FC) temperature dependent magnetization (M-T) measurements of (Cu)_x/CuTl-1223 samples showed an increase in transition temperature and in amplitude of diamagnetic signal after the inclusion of Cu nanoparticles in the host CuTl-1223 matrix. The improvement in these magneto-transport properties can be attributed to the increase in number of efficient pinning centres in CuTl-1223 matrix after addition of Cu nanoparticles. Magnetization hysteresis (M-H) loops were obtained at various operating temperatures from which the magnetization critical current density (J_c) was estimated using Bean's critical state model. M-H loops indicated the combined superconducting and ferromagnetic behaviour up to 90 K in all (Cu)_x/CuTl-1223 samples. Improvement in J_c could also be due to increase in number of pinning centres with addition of Cu nanoparticles in CuTl-1223 matrix. Maximum improvement in magneto-transport properties of (Cu)_x/CuTl-1223 samples was observed for x = 1.0 wt%, which had specified the optimum content level of Cu nanoparticles in CuTl-1223 phase.     

Effects of Os inserted layers on the microstructures and magnetic properties of the FePt films

The microstructure and magnetic properties of multilayer [Os(t)/FePt(x)]n films on a glass substrate with a 10 nm Os buffer layer by ion beam sputtering have been studied as a function of the annealing temperatures between 300 and 800 degrees C. Here, t = 0.2, 1 or 5 nm and x varied from 10, 20, 25, 50, to 100 nm with its associated n value of 10, 5, 4, 2, and 1, respectively. No diffusion evidence was found in samples with a thin Os layer and t > or = 1 nm. The average grain size of the multilayer films can be well controlled by both annealing temperature and thickness of the FePt layer by a very thin Os space layer with t > or = 1 nm. The enhancement of H(c) can be understood from the fact that for a FePt film with an Os spacer layers, the increasing number of Os layer will inhibit the grain growth of FePt grains and enriches the grain boundary. We have experimentally demonstrated that even with a very thin 1 nm Os spacer layers, the [Os(t)/FePt(x)]n multilayer films can exhibit good hard magnetic properties and are attractive candidates for ultrahigh density magnetic recording media.     

MgO/Pd底层对CoSiB/Pd多层膜垂直磁各向异性及热稳定性的影响EI北大核心CSCD

采用磁控溅射方法在玻璃基片上制备以MgO/Pd为底层的CoSiB/Pd多层膜样品,研究MgO底层厚度t对CoSiB/Pd多层膜垂直磁各向异性(perpendicular magnetic anisotropy,PMA)的影响,分析具有MgO/Pd底层的多层膜的热稳定性。通过对样品的反常霍尔效应的测试分析发现,底层中引入MgO层能够提高其PMA性能,当t为3.5 nm时,样品的矩形度最好。对最佳样品MgO(3.5 nm)/Pd(3 nm)/[CoSiB(0.5 nm)/Pd(0.8 nm)]2/Ta(2 nm)的磁滞回线进行测试,其有效磁各向异性常数K eff达到2.0×10^5 J/m^3。热稳定性分析发现,当退火温度为200℃时,样品的K eff达到最大值2.6×10^5 J/m^3;当退火温度达到400℃时,样品仍能保持良好的PMA性能。     

Influence of particle size on spin switching properties and magnetoelectric coupling in SmFeO<Subscript>3</Subscript>

Controlling the magnetic properties of a material is of great importance for spintronics and magnetoelastic devices. We studied effect of reduced particle size on structural, dielectric and magnetic properties of SmFeO₃ nanoparticles prepared by co-precipitation method (SFO-C) and by combustion (SFO-S). Reduced particle size modified interesting magnetic features of SmFeO₃. Temperature dependent magnetic study reveal significant enhancement in magnetization reversal temperature and drop in spin reorientation transition temperature. The signature of spin reorientation transition for SFO-C (~?300 nm) is marked at ~?450 K, while this temperature drops down to ~?400 K for SFO-C (~?50 nm). The magnetization reversal temperature is achieved at 30.5 K for SFO-C, much higher than 4 K, reported for the single crystal and bulk SmFeO₃. The presence significant anomalies in the temperature dependent dielectric behavior of SmFeO₃ samples across spin reorientation transition temperature indicate magneto electrical coupling. Strong exchange–bias effect is observed at low temperature for both the samples. The lowering of spin reorientation/switching transition temperature due to reduction in particle size and the signature of magnetoelectric coupling at this temperature are useful for room temperature devices. The observed experimental results establish that the spin switching properties of SmFeO₃ can be modified for practical applications in devices.     

Magnetic properties of oxidized iron thin films grown by sputtering at very low temperatures

Iron thin films have been grown by DC magnetron sputtering using Si(100) wafers as substrates, and then oxidized in a well-controlled oxygen atmosphere in the vacuum chamber. Film thickness is about 50 nm, and grains forming these samples do not exceed 20 nm. In order to control structural properties such as size and shape of these grains, growth conditions can be modified, like deposition rate or substrate temperature, varying from 150 to 300 K. Two sets of samples have been prepared considering deposition rate: (i) films grown at 0.6 nm/min and (ii) at 1.2 nm/min. In order to prevent iron films from natural oxidation, all the sample series were covered with a gold layer. Analysis of their magnetic behaviour shows a strong dependence on grain size and temperature, resulting in a more effective oxidation for samples prepared at higher deposition rates and lower substrate temperatures, which behaves as a Fe/Fe oxide granular system.     

Effect of sintering temperature on magneto-transport properties of La0.67Ba0.33MnO3/Ba0.7Sr0.3TiO3 composites

La_0.67Ba_0.33MnO₃-20 wt.%-Ba_0.7Sr_0.3TiO₃ composites were sintered at different temperatures in order to explore the possibility of improving the magneto-transport properties of the composites. Detail studies on the magnetic and electrical transport properties for the sintered composite samples have been performed. Results show that the sintered composites have identical ferromagnetic to paramagnetic transition temperature and filamentary feature of metallic phase. When sintering temperature higher than 1300 °C, the composites show Efros-Shklovskii-like variable-range hopping in the temperature range lower than Curie temperature. For samples sintered lower than 1100 °C, a dome-like resistance peak appears at a temperature well below the Curie temperature. Magnetoresistance behavior indicates the existence of spin polarized tunneling in the low temperature range. Considering the contributions from Efros-Shklovskii-like variable-range hopping and spin polarized tunneling, the resistance peak can be well fitted.     

Spin polarized transports through a narrow-gap semiconductor wire with ferromagnetic contacts formed on InAlAs step-graded buffer layers

We investigated the transport properties of ferromagnetic/semiconductor hybrid structures utilizing an InAs/In_0.75Al_0.25As modulation-doped heterostructures formed on a GaAs (001) substrate with In_xAl_1−xAs step-graded buffer layers. We used NiFe as ferromagnetic electrodes for injection/detection of spin-polarized electrons, which were formed on side walls of the semiconductor mesa to contact electron channel directly. We measured magneto-transport properties of the samples with current flow between the ferromagnetic electrodes at low temperatures. Under vertical magnetic fields, magneto-resistance oscillations were clearly observed, thus the ferromagnetic electrodes worked as ohmic contacts. In addition, we successfully found spin-valve properties under parallel magnetic fields. Furthermore, we observed the enhancement of spin-valve properties by squeezing the channel width.     

Investigation of Ti layer thickness dependent structural, magnetic, and photoemission study of nanometer range Ti/Ni multilayer structures

Structural, magnetic, and electronic properties of Ti/Ni multilayer (ML) samples as a function of Ti layer thickness are studied and reported in this paper. For this purpose [Ti (t nm)/Ni (5 nm)] x 10 ML samples, where t = 3, 5, and 7 nm have been deposited by using electron beam evaporation technique under UHV conditions at room temperature. Structure of ML samples were determined by using XRD (X-ray diffraction) technique and observed that Titanium is deposited mainly in amorphous nature with FCC structure at lower Ti layer thickness of 3 nm, which transform to crystalline HCP structures above than this Ti layer thickness. Corresponding fitted GIXRR (grazing incidence X-ray reflectivity) patterns shows asymmetric nature of Ti-Ni and Ni-Ti interfaces because of heavy intermixing and interdiffusion of Ni and Ti atoms at Ti-Ni interfaces at lower Ti layer thickness. The depth profiling core level and valence band measurements carried out by using XPS (X-ray photoelectron spectroscopy) technique confirms the interdiffusion and intermixing leading to Ti-Ni alloy phase formation at interfaces during deposition, particularly at lower Ti layer thickness of 3 nm. The corresponding magnetization behavior of ML samples has been investigated using Magneto-Optical Kerr Effect (MOKE) technique and observed that, coercitivity decreases while saturation magnetization increases with Ti layer thickness variations. These results are interpreted and discussed in terms of observed micro-structural changes due to Ti layer thickness vitiations in Ti/Ni multilayer samples.     

Structural Stability and Magnetic Properties of the TbCu$_{7}$ -Type SmCo$_{x - 0.4}$Ti$_{0.4} (x = 5.0hbox{–}8.5)$ Alloys

We have studied the composition dependence, thermal stability, long-term stability at 500°C, and magnetic properties of the nanostructural TbCu₇-type (1:7) Sm-Co-Ti alloys. We prepared the SmCox-0.4Ti_0.4 alloys with a wide composition range from x = 5.0 to x = 8.5 by high-energy ball-milling, followed by annealing at 700-1100°C for 2 h. After annealing at 700°C, the powders with x = 7.0-8.5 showed a single 1:7 structure, while the powders with x = 5.0-6.5 presented the 1:7 plus CaCu₅-type (1:5) structure. At an annealing temperature higher than 800°C, a minor Th₂Zn₁₇-type (2:17) phase precipitated in the matrix of the 1:7 phase. Intrinsic coercivity iHc exhibits a maximum of 2.3 T at room temperature and 0.4 T at 500°C in the x = 7.0 samples annealed at 700°C. The temperature coefficient of iHc seems stable as the Sm/Co ratio changes from 1/6.5 to 1/7.5. The coercivity decreased with increasing annealing temperature Ta, from 2.3 T at Ta = 700°C to 1.3 T at Ta = 1100°C, which is mainly attributed to the grain growth from 35 nm for Ta = 700°C to 1 ?m for Ta = 1100°C. After holding at 500°C for up to 360 h, the microstructure and magnetic properties of the 1:7-type nanograin alloys remained almost unchanged, indicating a structurally and magnetically long-term stabilization at the potential high-temperature application environment.     

Fe:O:C grown by focused-electron-beam-induced deposition: magnetic and electric properties

We systematically study the effect of oxygen content on the magneto-transport and microstructure of Fe:O:C nanowires deposited by focused-electron-beam-induced (FEBID) deposition. The Fe/O ratio can be varied with an Fe content varying between ～ 50 and 80 at.% with overall low C content (≈16 ± 3 at.%) by adding H(2)O during the deposition while keeping the beam parameters constant as measured by energy dispersive x-ray (EDX) spectroscopy. The room-temperature magnetic properties for deposits with an Fe content of 66-71 at.% are investigated using the magneto-optical Kerr effect (MOKE) and electric magneto-transport measurements. The nanostructure of the deposits is investigated through cross-sectional high-resolution transmission electron microscopy (HRTEM) imaging, allowing us to link the observed magneto-resistance and resistivity to the transport mechanism in the deposits. These results demonstrate that functional magnetic nanostructures can be created, paving the way for new magnetic or even spintronics devices.     

A study of the influence of crystallite size on the electrical and magnetic properties of CuFe2O4

An attempt has been made to clarify the fundamental assumption that the properties of materials change as the crystallite size of the material is reduced below 100 nm. CuFe₂O₄ samples of different crystallite sizes were prepared by the sol–gel and combustion methods and then analyzed by X-ray diffraction (XRD), thermal analyses (TGA/DTG) and scanning electron microscopy (SEM) techniques. The magnetic properties were studied by measuring the AC magnetic susceptibility (χ) and the Mössbauer spectroscopy. The DC electrical resistivity, dielectric constant, dielectric loss tangent, Curie temperature and hyperfine splitting of the samples change with the crystallite size. The change in the electrical properties is attributed to the formation of discrete energy levels instead of the bands. However, the magnetic parameters change due to the existence of non magnetic surface layers. The isomer shift and the hyperfine splitting show gradual increase with the increase in crystallite sizes.     

Synthesis of tunable sized capped magnetic iron oxide nanoparticles highly soluble in organic solvents

Surface modification of magnetic nanoparticles by organic surfactants is known to provide them with solubility in organic solvents (ferrofluids), which undoubtedly is an important property in several applications and studies. In this report, the main interest is focused on structural, magnetic and adsorption properties of iron oxide nanoparticles that are derived under water/toluene biphase conditions in the presence of oleic acid or oleylamine as the capping agents. The surfactants provide them with excellent stability and solubility in organic solvents like toluene or chloroform. Furthermore, by adding the appropriate surfactant or altering the temperature of the aqueous phase at the initial stage of the reaction we achieve a size control of the nanoparticles within the range 6–18 nm. The presence of capping agents or high reaction temperatures favours the formation of smaller nanoparticles. The adsorption of the surfactants (chemisorption) was identified with FT-IR spectroscopy, while Mössbauer studies have been performed to representative samples in order to identify the presence of either γ-Fe₂O₃ or Fe₃O₄, depending on the reaction temperature. Finally, the magnetic properties of representative samples have been studied at 5 K and room temperature.     

Structure and magnetic properties of directionally solidified Bi-MnBi eutectic alloys

An alloy containing about 2 at% Mn and 98 at% Bi should solidify at 230°C to give a eutectic consisting of about 3 vol % ferromagnetic MnBi in a matrix of diamagnetic Bi. Directional solidification of this alloy produces a structure of elongated parallel MnBi rods with diameters from 0.1 to 1.5 μm, decreasing with increasing growth rate. Samples solidified slowly (growth rate less than 5 cm/h) show magnetic properties at room temperature and down to 4.2 K that are consistent with the known magnetic properties of bulk equilibrium MnBi. Samples solidified more rapidly (20-80 cm/h) show considerably more complicated magnetic behavior. At room temperature they have less than half the moment expected for the equilibrium MnBi composition, and a second magnetic phase appears when the temperature is lowered below about 240 K. The coercive field of this phase rises rapidly with decreasing temperature, reaching the limit of our experimental apparatus (about 120 kOe) near 100 K. Magnetic measurements with the field applied perpendicular to the growth axis show the effects of anisotropy, due to particle alignment, although samples prepared at very rapid freezing rates (300 cm/h) appear to be poorly aligned. Samples prepared by arc melting on a water-cooled hearth are similar to high-growth-rate samples. Annealing at 200°C was necessary in order to obtain reproducible magnetic data. The results are generally consistent with the presence in the samples of a mixture of the normal equilibrium MnBi phase and a variant of the known high-temperature phase (HTP) with a substantially depressed Curie temperature. Structural studies using electron diffraction from thinned sections indicate the presence of both normal (LTP) and high-temperature (HTP) MnBi.     

Microstructure, electrical and magnetic properties of polycrystalline La0.85K0.15MnO3 manganites prepared by different synthesis routes

Influence of different synthesis techniques (solid state reaction, sol–gel and co-precipitation) on the structure, microstructure, magnetic and electrical properties of polycrystalline La_0.85K_0.15MnO₃ (LKMO) sintered at 900 °C is investigated. All the as-synthesized compounds were confirmed as single phase and hexagonal structure at room temperature. The nano-crystallite size and average grain size were increased from the sample synthesized through solid state, sol–gel and co-precipitation techniques. The electrical and magneto-transport properties of polycrystalline LKMO was relied on the synthesis method. Significant decreases in metal–insulator transition temperature (T_p) with the increment of resistivity were observed for co-precipitation synthesized sample when comparing with solid state and sol–gel synthesized samples. Magnetization was decreased while ferro-paramagnetic transition temperature (T _c) was shifted toward lower temperature from solid state synthesized sample to co-precipitation synthesized sample. Furthermore, co-precipitation synthesized sample achieved the highest negative magnetoresistance at room temperature.     

Effect of finite size on the magnetization behavior of nanostructured nickel oxide

Nanostructured nickel oxide samples having different average particle sizes are synthesized through a wet chemical route. Room temperature magnetic hysteresis of the samples are recorded using a vibrating sample magnetometer. The magnetic properties of the samples are found to be markedly different from those of single crystalline nickel oxide. The sample with an average particle size of 2-3 nm showed superparamagnetism with magnetization curves defined by the Langevin function. Anomalously large uncompensated magnetic moment associated with this sample is attributed to the multisublattice magnetic structure. Interestingly, samples with larger average particle sizes of 13 and 18 nm exhibited superantiferromagnetism with the magnetization curves varying linearly with applied field and susceptibility values larger than that of bulk nickel oxide. The results highlight the importance of surface atoms and surface driven spin rearrangements in determining the magnetic properties of nanostructured nickel oxide.