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ABSTRACT: We have investigated the magnetic and magnetotransport properties of Ge1-xMnxTe (x = 0.1) grown by molecular-beam epitaxy. Our results show that the sample exhibits two ferromagnetic transition temperatures at TC = 34 K and TC* = 100 K. We infer that Tc is a long-range ferromagnetic ordering in view of sufficient carriers generating uniform ferromagnetism, whereas TC* is a short-range ferromagnetic ordering due to ferromagnetic clusters. The temperature dependence of the resistivity ρ(T) curve exhibits a shallow minimum near TC. The upturn of ρ(T) toward the low temperature (T < TC) is well described by a weak-localization model, whereas in the high-temperature regime (T > TC), the phonon scattering dominates.
Journal of Applied Physics 03/2011; 109(7):07C314-07C314-3. · 2.17 Impact Factor
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ABSTRACT: We report the formation of two-dimensional electron gas (2DEG) at the Zn1−xMgxO/ZnO interface grown by metal-organic vapor phase epitaxy on sapphire substrates. The existence of the 2DEG is confirmed by the observation of Shubnikov–de Haas oscillations and the integer quantum Hall effect. In particular, the Zn0.8Mg0.2O/ZnO heterostructure shows a high Hall mobility of 2138 cm2/V s with a carrier sheet density of 3.51×1012 cm−2 at 1.4 K. We attribute the origin of 2DEG to be the donor states on ZnMgO surface. The dependence of carrier sheet density of 2DEG on ZnMgO layer thickness and Mg composition (x) are also investigated.
Applied Physics Letters 09/2010; 97(11):111908-111908-3. · 3.84 Impact Factor
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ABSTRACT: We present the results on the exchange coupling in the zinc-blende CrTe/spacer/MnTe structure with ZnTe acting as a spacer layer (SL). The magnetic hysteresis (M-H) loops show significant shifts in both horizontal and vertical directions. A nonmonotonic variation of the exchange bias field (H<sub>E</sub>) and coercivity (H<sub>C</sub>) with the SL thickness is observed. The Curie temperature decreases as the SL thickness increases. The maximum of H<sub>E</sub> and H<sub>C</sub> values occur at a SL thickness of 12 ML (monolayer). Strikingly, the exchange coupling remains noticeable even the thickness of SL reaches 24 ML. The exchange bias field and vertical shift are discussed on the basis of frozen ferromagnetic moment model.
Journal of Applied Physics 06/2010; · 2.17 Impact Factor
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ABSTRACT: We report the antiferromagnetic (AFM) thickness dependence study of zinc-blende CrTe/ZnTe/MnTe layered structure grown on GaAs (100) by low temperature molecular beam epitaxy. The coercivity (H<sub>C</sub>) and exchange bias field (H<sub>E</sub>) show a nonmonotonic dependence on the AFM thickness. Both (H<sub>C</sub>) and H<sub>E</sub> exhibit a maximum value at the AFM thickness of ∼15 nm . With decreasing AFM thickness, we found that the blocking temperature drops from 70 (Néel temperature of bulk MnTe) to 22 K, while the Curie temperature is found to increase from 100 to 190 K. Additionally, a vertical magnetization shift is observed in the M-H loop, which can be attributed to the presence of frozen ferromagnetic spins at the interface.
Journal of Applied Physics 06/2010; · 2.17 Impact Factor
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ABSTRACT: We utilize the effect of hydrostatic pressure to investigate the magnetotransport properties of degenerate p-Ge1−xMnxTe (x = 0.10) ferromagnetic semiconductor. The Curie temperature was found to increase with pressure as 0.27 K/kbar, which can be understood on the basis of the Ruderman–Kittel–Kasuya–Yosida (RKKY) interaction mechanism. For sufficiently high carrier concentration of po ∼ 1021 cm−3, both the light holes from the L valence band and the heavy holes from the Σ valence band contribute to the RKKY interaction. Additionally, a negative magnetoresistance is observed at low temperature and is found to decrease with pressure.
Applied Physics Letters 08/2009; 95(7):072510-072510-3. · 3.84 Impact Factor
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ABSTRACT: We have studied the exchange bias at the ferromagnetic (FM)/antiferromagnetic interface in the zinc-blende transition-metal chalcogenides, CrTe (5 nm)/MnTe(40 nm) bilayer grown on GaAs (100) substrate by molecular-beam epitaxy. A negative exchange bias shift in the hysteresis loop is observed when the bilayer is cooled in the applied magnetic field. The temperature-dependent remanent magnetization shows a clear enhancement of the Curie temperature and magnetization in the bilayer as compared to a single FM layer. The effects of temperature, cooling field, and angular dependence on the exchange bias have been investigated.
Applied Physics Letters 06/2009; 94(25):252504-252504-3. · 3.84 Impact Factor
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ABSTRACT: The optical, magnetic, and transport behaviors of Ge <sub>1-x</sub> Mn <sub>x</sub> Te ( x=0.24 and 0.55) grown by solid-source molecular-beam epitaxy are investigated. X-ray diffraction shows that Ge <sub>1-x</sub> Mn <sub>x</sub> Te crystallizes in rocksalt structure. The temperature-dependent magnetization (M-T) for x=0.55 sample gives a Curie paramagnetic temperature of θ<sub>p</sub>∼180 K , which is consistent with the temperature-dependent resistivity ρ(T) measurement. Anomalous Hall effect is clearly observed in the samples and can be attributed to extrinsic skew scattering based on the scaling relationship of ρ<sub>xy</sub>∝ρ<sub>xx</sub><sup>1.06</sup> . The magnetoresistance of Ge <sub>1-x</sub> Mn <sub>x</sub> Te is isotropic and displays a clear hysterestic loop at low temperature, which resembles that of giant-magnetoresistance granular system in solids.
Journal of Applied Physics 10/2008; · 2.17 Impact Factor
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ABSTRACT: We report on magnetic imaging study of hexagonal shaped ferromagnetic elements arranged in a ring network. Magnetic force microscopy images revealed the existence of vortex states in the hexagons, which agrees well with the micromagnetic modeling results. By making use of the stray field of moderate strength and well-defined symmetry, which is lacking in vortices formed in circular elements, we show that it is possible to realize vortex chains with well-defined alternative arrangement of chirality in a ring network. The chirality can be switched forth and back using a magnetic field applied along selected edges of the hexagons.
Applied Physics Letters 09/2008; 93(12):122504-122504-3. · 3.84 Impact Factor
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ABSTRACT: We present results of the magnetic, structural, and chemical characterizations of Mn <sup>+</sup> -implanted Si displaying n -type semiconducting behavior and ferromagnetic ordering with Curie temperature, T<sub>C</sub> , well above room temperature. The temperature-dependent magnetization measured by superconducting quantum interference device from 5 to 800 K was characterized by three different critical temperatures ( T<sub>C</sub><sup>*</sup>∼45 K , T<sub>C1</sub>∼630–650 K , and T<sub>C2</sub>∼805–825 K ). Their origins were investigated using dynamic secondary ion mass spectroscopy and transmission electron microscopy (TEM) techniques, including electron energy loss spectroscopy, Z -contrast scanning TEM imaging, and electron diffraction. We provided direct evidences of the presence of a small amount of Fe and Cr impurities which were unintentionally doped into the samples together with the Mn <sup>+</sup> ions as well as the formation of Mn-rich precipitates embedded in a Mn-poor matrix. The observed T<sub>C</sub><sup>*</sup> is attributed to the Mn <sub>4</sub> Si <sub>7</sub> precipitates identified by electron diffraction. Possible origins of T<sub>C1</sub> and T<sub>C2</sub> are also discussed. Our findings raise questions regarding the origin of the high- T<sub>C</sub> ferromagnetism reported in many material systems without a careful chemical analysis.
Journal of Applied Physics 09/2008; · 2.17 Impact Factor
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ABSTRACT: We present a comprehensive study relating the magnetic properties to structural properties of Mn+-implanted Si1−xGex films as a function of Ge content (x = 0–0.5). Ferromagnetic ordering with three critical temperatures, TB ∼ 10–16 K, TC1 ∼ 650–780 K, and TC2 ∼ 825–860 K, are reported in this material system. Element specific x-ray absorption fine structure results show that the majority of the Mn ions are nonsubstitutional in all samples. The transmission-electron microscopy coupled with z contrast and chemical analysis reveals the presence of Mn-rich nanosized clusters including Mn4Si7 in Si-rich samples and Mn7Ge3 phases in Ge-rich samples. A composition transition occurred at x ∼ 0.2–0.3, where we observe a change in bond lengths and defect structures. Additionally, an enhancement in magnetizations with an increase in both TB and TC1 as well as a conversion from n-type to p-type conduction are also detected.
Journal of Applied Physics 03/2008; 103(5):053912-053912-7. · 2.17 Impact Factor
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ABSTRACT: We report a systematic investigation on the structural and magnetic properties of molecular-beam epitaxial growth of CrTe thin films with different layer thicknesses and Cr/Te flux ratios. A phase diagram of the growth parameters is established based on the detailed analyses of the reflection high-energy electron diffraction patterns, atomic force microscopy, and magnetization. Our high-resolution transmission electron microscopy results show that under appropriate growth conditions, a metastable zinc-blende (ZB) phase of CrTe film can be achieved with a nominal thickness of 5 nm. The magnetic properties of ZB CrTe exhibit a strong in-plane anisotropy with an easy axis along the [001] direction and hard axes along the [011] and [01] directions. Correspondingly, the uniaxial (KU) and cubic (KC) anisotropy constants are obtained through the fitting of the [011] hard-axis direction. The temperature dependence of the remanent magnetization indicates the TC ∼ 100 K of ZB CrTe is attained.
Journal of Applied Physics 02/2008; 103(4):043908-043908-5. · 2.17 Impact Factor
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ABSTRACT: We report the growth of zinc-blende CrTe thin films with a nominal thickness of 5 nm by low-temperature molecular-beam epitaxy. Reflection high-energy electron diffraction patterns, atomic force microscopy and high-resolution transmission electron microscopy analyses established that a quasi-two-dimensional layer-by-layer growth mode is achieved. We observe a strong magnetic anisotropy in the film with an easy axis along the [0 0 1] direction. The uniaxial (KU) and cubic (KC) anisotropy constants are obtained through the fitting of the hard axes along the [0 1 1] and directions of the magnetization curves. A Curie temperature of 100 K is obtained from the temperature-dependent remanent-magnetization measurement.
Journal of Physics D Applied Physics 01/2008; 41(4):045002. · 2.54 Impact Factor
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ABSTRACT: We investigate the structural, transport, and magnetic behaviors of (Zn,Cr)Te system without additional carrier doping grown by low temperature molecular-beam epitaxy on GaAs (100) substrates. For the growth of Zn1−xCrxTe with x = 0.14, high-resolution transmission electron microscopy shows the possibility of zinc-blende (ZB)-CrTe phase being embedded in the Zn1−xCrxTe matrix. Our detailed analysis of the magnetization results in Zn0.86Cr0.14Te using scaling theory and modified Arrott plots suggests that the ferromagnetic ordering cannot be described by mean-field theory. Additionally, the resistivity behavior indicates that ZB-CrTe clusters could possibly provide a percolative conduction path that gives rise to high Curie temperature. We discussed the ferromagnetism in (Zn,Cr)Te system on the basis of clustering effect.
Journal of Applied Physics 09/2007; 102(5):053702-053702-5. · 2.17 Impact Factor
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ABSTRACT: We report on a systematic electrical transport study of the effect of an exchange tab on the magnetization switching process of magnetic nanowires. The exchange tab consists of a small segment of a ferromagnetic layer coupled antiferromagnetically to a small portion of the magnetic wire via a thin Ru spacer. The magnetic force microscopy image shows that the exchange tab has a single domain structure. The magnetoresistance of exchange tab stabilized nanowires is characterized by a reversible switching process in both the longitudinal and the transverse field directions. The former is in sharp contrast to the magnetoresistance curve of nanowires with the same geometry but without an exchange tab. The dependence of the exchange field on the tab size has also been investigated, which revealed that a shorter tab generally leads to a stronger coupling between the tab and the nanowire. Micromagnetic modelling has been carried out to interpret the experimental results.
Journal of Physics D Applied Physics 05/2007; 40(10):3011. · 2.54 Impact Factor
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ABSTRACT: The authors investigate the magnetic and transport behaviors of Ge <sub>1-x</sub> Mn <sub>x</sub> Te thin films with high Mn composition (x=0.98) grown by solid-source molecular-beam epitaxy. The temperature-dependent magnetization (M-T) gives a Curie paramagnetic temperature θ<sub>p</sub>∼120 K , in contrast to the Curie temperature of T<sub>C</sub>∼95 K obtained from the Arrott plot and temperature-dependent resistivity measurement. The resistivity and M-T behaviors can be attributed to weak localization effect of disordering. The authors discussed the ferromagnetism in Ge <sub>0.02</sub> Mn <sub>0.98</sub> Te on the basis of the Ruderman-Kittel-Kasuya-Yoshida interaction and clustering effect.
Applied Physics Letters 05/2007; · 3.84 Impact Factor
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ABSTRACT: Ferromagnetic behaviour above room temperature has been observed in Mn‐ions implanted Si0.75Ge0.25. The 900°C annealed sample with Mn dosage of 2×1016 cm−2 exhibit saturation magnetization, saturation field and coercive field of ∼9 emu/cm3 and ∼1500 Oe and ∼60 Oe respectively at 300K. © 2007 American Institute of Physics
AIP Conference Proceedings. 04/2007; 893(1):1229-1230.
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ABSTRACT: We report the synthesis of zinc-blende CrTe (zb-CrTe) as highly oriented crystalline grains in films of 100-nm thickness. The structural properties of the film were characterized using high-resolution transmission electron microscopy (HRTEM) and selective-area electron diffraction (SAED). Temperature-dependent magnetization measurements show that the Curie temperature of the film is ~327 K. Angular M-H measurements indicate that the CrTe films have an in-plane easy axis of magnetization along the [01macr1] direction of GaAs
IEEE Transactions on Magnetics 11/2006; · 1.36 Impact Factor
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ABSTRACT: In this paper, we propose an alternative method to improve magnetoresistance by inserting a thin ferromagnetic (FM) layer into the nonmagnetic (NM) spacer of a basic spin-valve (SV) trilayer (FM1-NM-FM1), thus creating a penta-layer SV structure (FM1-NM-FM2-NM-FM1). We investigated the effect of increasing the resistivity (rho<sub>F2</sub>) of the FM2 on overall magnetoresistance (MR). We performed both analytical and numerical studies on the MR of the current perpendicular-to-plane (CPP) structure using the phenomenological spin drift-diffusion models. For finite rho<sub>F2 </sub>, the MR profile is dependent on the intrinsic conductance polarization (alpha<sub>F2</sub>) of FM2. We found that inserting FM2 enhances MR when alpha<sub>F2</sub> exceeds a critical value of alpha<sub>2C</sub>. It is found that MR can be doubled by inserting a FM layer with high alpha<sub>F2</sub>, such as the half-metallic Cr <sub>2</sub>O. We have numerically calculated MR<sub>max</sub> and the corresponding rho<sub>F20</sub> values for different alpha<sub>F2 </sub> values. Finally, we studied the effect of spin relaxation on the MR of the CPP SV
IEEE Transactions on Magnetics 11/2006; · 1.36 Impact Factor
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ABSTRACT: We investigated the effect of magnetic-electric barriers on electron transport in the two-dimensional electron gas (2DEG) of a AlGaAs-GaAs heterostructure. It is found that electric potential can be controlled to achieve both spin and charge conductance modulation in this material system, with the requisite condition being the presence of magnetic barriers to break the time reversal symmetry of the system. The use of electrical potential rather than applied magnetic fields as an external control parameter to achieve spin/charge conductance modulation promises a simpler device design for practical realization
IEEE Transactions on Magnetics 11/2006; · 1.36 Impact Factor
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ABSTRACT: We report the growth of Co-doped Ge ( Co <sub>x</sub> Ge <sub>1-x</sub>) thin films by low-temperature molecular-beam epitaxy and the ferromagnetic properties without any additional carrier doping. The as-grown Co <sub>0.02</sub> Ge <sub>0.98</sub> has a Curie temperature, T<sub>C</sub>∼15 K , while those Co <sub>x</sub> Ge <sub>1-x</sub> with x≥4.0 at. % are ferromagnetic above room temperature. On the other hand, Co <sub>0.02</sub> Ge <sub>0.98</sub> exhibit ferromagnetic ordering up to T<sub>C</sub>∼150±10 K after a low-temperature annealing. A redshift in the Raman Ge–Ge mode was observed, indicating the substitution of Ge with Co atoms. The measured Co <sub>0.02</sub> Ge <sub>0.98</sub> are of p type and exhibit pronounced anomalous Hall effects.
Applied Physics Letters 08/2006; · 3.84 Impact Factor
