XPS and Magnetization Measurements Study of CrMn Nanoparticles
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XPS and Magnetization Measurements Study of CrMn Nanoparticles Wathiq Abdul-Razzaq1 and Unchul Lee U.S. Army research Laboratory, Adelphi, MD 20783, U.S.A. 1 Physics Department, West Virginia University, Morgantown, WV 26506, U.S.A. ABSTRACT Cr1-xMnx (x < 6.3 %) alloy in the bulk form is an interesting complex system that behaves in many but not all respects like a spin-glass system. In this study, 14 nm-size CrMn nanoparticles with 5% Mn, were synthesized by ball milling of Cr and Mn particles in Ar atmosphere. After 50 hours of milling, the x-ray diffraction pattern indicated alloying of the two metals Cr and Mn. However XPS data indicated the formation of MnO2 and Cr2O3 oxides on the surface of the chromium core in the nanoparticle. These oxides on the surface, like the Cr core, are antiferromagnets. Magnetization (M) measurement at 200 G as a function of temperature (T), after zerofield-cooling, showed that M increased as T increased from 5 K all the way to 330 K, beyond the Neel temperatures of the Cr and the oxides. Even though the magnetization value is in the range of a typical antiferromagnet at all temperatures from 5 to 330 K, the shape of M versus T curve is neither that of an antiferromagnet nor of a spin-glass. It appears that the behavior of the magnetization curve is due to the frustration of the spins by weak fields associated with spin-density-wave and by the existence of the oxides. This frustration should be different from the frustration of the spins due to the ferromagnetic-antiferromagnetic competition in spin-glass systems. INTRODUCTION Chromium and its alloys have rich antiferromagnetic behavior and have been of great interest in recent years [1,2]. Cr1-xMnx (x < 6.3 %) alloys in the bulk form have unusual magnetic properties that in many aspects resemble the properties of the prototype spin-glass CuMn alloy which also exhibits spin-density wave (SDW) [3]. However, there are differences from spin-glass behavior. For example, the temperature at which the maximum occurs in the zero-field-cooled magnetization-versus-temperature curve is independent of Mn concentration [4,5]. The unusual magnetic behavior of Cr1-xMnx with x < 6.3 % has been explained through the frustration of the moments associated with the SDW of the host Cr and not through the frustration of the magnetic impurity moments as in spin-glass cases [4,5]. We attempted in this study to synthesize Cr1-xMnx nanoparticles with x =5 at. % by mechanical alloying of Mn and Cr fine particles. Magnetization and XPS measurements are presented below.
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EXPERIMENTAL DETAILS The nanoparticles of CrMn were produced by ball milling Cr and Mn fine particles using tungsten-carbide vial and balls and the Spex Model 8000 Mixer-Mill. To reduce oxidation, the whole machine was placed in a glove box filled with Argon gas, although the system was by no means completely devoid of some oxygen. The starting materials were a commercial Cr powder and a commercial Mn powder from Fisher Scientific Company. Samples were taken out after 2, 4, and 50 hou
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