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Study of Ga<SUB>1-x</SUB>Mn<SUB>x</SUB>As Critical Behavior by Using Thermal Diffusivity
Shavkat U. Yuldashev,Khusan T. Igamberdiev,이세준,권영해,강태원,Anatoly G. Shashkov 한국물리학회 2011 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.59 No.21
The temperature dependence of the thermal diffusivity has been measured in the close vicinity of the magnetic phase transition in Ga_(1−x)Mn_xAs. The thermal diffusivity of Ga_(1−x)Mn_xAs samples demonstrated a pronounced -shaped peak, which indicates the existence of a second-order phase transition in the samples. The thermal diffusivity peak maximum is located near the Curie temperature;therefore, it is attributed to a ferromagnetic-paramagnetic phase transition. Taking into account that the inverse of the thermal diffusivity has the same critical behavior as the specific heat,we determine the critical exponent . The critical behavior of the specific heat of the Ga_(1−x)Mn_xAs samples is well described by the mean-field, including Gaussian fluctuations, model.
Magnetic and Optical Properties of Zn1-xMnxO Thin Films Prepared by Using Ultrasonic Spray Pyrolysis
Shavkat U. Yuldashev,전희창,강태원,율다세프,Irina V. Khvan,Vasiliy O. Pelenovich 한국물리학회 2008 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.53 No.1
Diluted magnetic semiconductor Zn1-xMnxO (x = 0.05) thin films were prepared using ultrasonic spray pyrolysis. The films were additionally doped with nitrogen in order to obtain a p-type conductivity. Aqueous solutions of zinc acetate (0.5 mol/l), manganese acetate (0.5 mol/l) and ammonium acetate (2.5 mol/l) were used as sources of Zn, Mn and N, respectively. The hole concentration in the nitrogen-doped lms was in the range of 1015 -1018 cm-3 at room temperature. The dependence of the magnetization on the magnetic field (M-H) for the highly nitrogen-doped Zn0.95Mn0.05O sample showed a hysteresis loop at low temperatures. A Curie temperature of around 80 K for the sample with a hole concentration of 1018 cm-3 was determined from the temperature dependence of the magnetization.
Study of the Magnetic Phase Transition in ZnMnO by Using Specific Heat Capacity Measurements
율다세프,Shavkat U. Yuldashev,강태원,Vasiliy O. Pelenovich 한국물리학회 2009 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.55 No.3
The magnetic phase transition in diluted magnetic semiconductor Zn1−xMnxO was studied by using a specific heat capacity method. Zn1−xMnxO (x = 0.05) thin films were prepared by using ultrasonic spray pyrolysis. The films were additionally doped by nitrogen in order to obtain a ptype conductivity. The magnetization (M-H) measurements, obtained by using a SQUID showed a hysteresis loop at low temperatures, which demonstrated the existence of ferromagnetic ordering in these samples. The temperature dependence of the specific heat capacity (Cp), measured without an external magnetic field, revealed a pronounced λ-shaped peak at 75 K. Such a Cp dependence indicates a well-defined second-order magnetic phase transition in the nitrogen-doped Zn0.95Mn0.05O samples.
The role of zinc vacancies in bipolar resistance switching of Ag/ZnO/Pt memory structures
Yalishev, Vadim Sh,Yuldashev, Shavkat U,Kim, Yeon Soo,Park, Bae Ho IOP Pub 2012 Nanotechnology Vol.23 No.37
<P>We have presented a study of the bipolar resistance switching characteristics in the Ag/ZnO/Pt cell. This switching is accompanied by a change in intensity of the photoluminescence emission at 3.33 eV which is attributed to zinc vacancy related transitions in ZnO film. Besides voltage-driven resistance switching phenomena, a transition from a high-resistance state to a lower one is observed under laser illumination at low temperature. These results demonstrate that the bipolar resistance switching can originate due to an electron trapping/de-trapping process at zinc-vacancy defects localized in the interface layer. The Mott metal–insulator transition is proposed as a possible mechanism of the memory effect. </P>
Effect of Isovalent Doping on the Magnetic Properties of ZnMnO Diluted Magnetic Semiconductors
Ziyodbek A. Yunusov,Shavkat U. Yuldashev,강태원,이승주,권영해,전희창 한국물리학회 2019 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.74 No.2
The magnetic properties of a ZnMnO diluted magnetic semiconductor isovalently doped with Mg and S have been successfully studied. ZnMnO alloys were prepared with dierent concentrations of magnesium and sulfur by using ultrasonic spray pyrolysis technique; additionally, the lms were doped for free charge carriers by using nitrogen. For ZnMnO doped with 5% of Mg, the Curie temperature reached 104 K, and second-phase magnetic precipitates were observed with increasing Mg concentration. On the other hand, the sulfur doped ZnMnO showed an increased Curie temperature higher than room temperature due to increased number of holes which mediated the magnetic exchange interaction between magnetic ions.
Yalishev, Vadim Sh.,Yuldashev, Shavkat U.,Igamberdiev, Khusan T.,Kang, Tae Won,Park, Bae Ho Korean Physical Society 2014 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.64 No.1
We studied the photoluminescence (PL) spectra in the near-band-edge region of undoped ZnO films as a function of temperature to determine the thermal quenching behavior in the emission intensity. The quenching of the PL intensity was found to change to an increasing intensity with increasing temperature in the temperature region from 40 to 60 K. On the other hand, a reduction of the vacuum in the same temperature region was observed and was attributed to desorption of oxygen molecules from the cryostat finger. Further investigation revealed that the increase in the PL intensity was caused by adsorption of oxygen on the surface of ZnO films resulting in a decrease in the number of non-radiative recombination centers.
Effects of near-surface defects on the optical, electrical and magnetic properties of ZnO films
Yalishev, Vadim Sh.,Yuldashev, Shavkat U.,Igamberdiev, Khusan T.,Kang, Tae Won,Park, Bae Ho Korean Physical Society 2014 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.64 No.10
We studied the optical, electrical and magnetic properties of ZnO films prepared by using a pulsed laser deposition method. Low-temperature photoluminescence (PL) measurements revealed a strong emission line at 3.333 eV. A dependence of this emission intensity on oxygen adsorbed on the surface was observed, indicating a near-surface origin for the emission. The transition at 3.333 eV was attributed to bound exciton, and the observed anomalously small thermal activation energy in comparison with the localization energy could be explained by a modification of the surface state. The observed resistance switching effect and ferromagnetism in the ZnO films were related to the existence of defects responsible for the PL emission at 3.333 eV.