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Thermal Diffusivity Study of Weak Ferromagnetism in Zn_(1−x)Co_xO
Khusan T. Igamberdiev,Shavkat U. Yuldashev,권영해,강태원,Vasiliy O. Pelenovich,Anatoly G. Shashkov 한국물리학회 2011 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.58 No.51
Thermal diffusivity measurements have been conducted in order to study the origin of ferromagnetism in Zn_(1−x)Co_xO. Thin films of Zn_(1−x)Co_xO (x = 0.03) were deposited on Si (100) substrates by using ultrasonic spray pyrolysis. Magnetization M(H) measurements at low temperaturs show a hysteresis loop, which indicates the existence of ferromagnetic ordering in Zn<SUB>0.97</SUB>Co<SUB>0.03</SUB>O. However, the magnetic moment per Co ion is much lower than expected. A comparison of M(T) measured at zero-field-cooled (ZFC) and at field-cooled (FC) conditions shows a superparamagnetic-like behavior, and the blocking temperature is about 130 K. The temperature dependence of the thermal diffusivity of Zn_(0.97)Co_(0.03)O shows a pronounced lambda-shaped minimum at 130 K, which indicates the existence of a second-order phase transition at that temperature. The weak ferromagnetism in Zn_(0.97)Co_(0.03)O with a Curie temperature of 130 K is ascribed to an uncompensated magnetic moment at the surface of CoO nanoclusters with radii of about 1 nm.
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.