P.V. Vanitha

Barkhausen jumps and related magnetic properties of iron nanowires encapsulated in aligned carbon nanotube bundles

Abstract Iron nanowires encapsulated in aligned carbon nanotube bundles show interesting magnetic properties. Besides the increased coercivity, Barkhausen jumps with 5 emu/g steps in magnetization are observed due to magnetization reversal or depinning of domains.

Photoluminescence spectra and ferromagnetic properties of GaMnN nanowires

Abstract Mn-doped GaN nanowires have been prepared by reacting a mixture of acetylacetonates with NH 3 at 950 °C in the presence of multi-walled (MWNTs) and single-walled (SWNTs) carbon nanotubes, the nanowires prepared with SWNTs being considerably smaller in diameter. GaMnN nanowires with 1%, 3% and 5% Mn so obtained have been characterized by X-ray diffraction, EDAX analysis and photoluminescence (PL) spectra. The GaMnN nanowires are all ferromagnetic even at 300 K, exhibiting magnetic hysteresis. The PL spectra of the GaMnN nanowires prepared with SWNTs show a large blue-shift of the Mn 2+ emission.

Phase separation and segregation in rare earth manganates: the experimental situation

Rare earth manganates of the general formula, Ln1−xAxMnO3 (Ln=rare earth, A=alkaline earth) exhibit phase separation due to the occurrence of tiny clusters or small nanometric regions of one type of magnetic phase in the matrix of another (e.g., ferromagnetic metallic clusters in an insulating antiferromagnetic matrix) or phase segregation due to the coexistence of large domains of two phases. The phenomenon is nearly universal in the manganates and crucially depends on the composition, temperature, external magnetic field, dopant substitution in the Mn site and related factors. Percolative transport has been considered to result from the coexistence of the ferromagnetic metallic and insulating phases, but it is necessary to ensure whether these phases form large domains or remain as clusters (<100 nm) in the relevant composition.

Effect of substitution of Mn3+ by Ni3+ and Co3+ on the charge-ordered states of the rare earth manganates, Ln0.5A0.5MnO3

Abstract Substitution of Mn 3+ in Nd 0.5 Sr 0.5 MnO 3 by Ni 3+ and Co 3+ upto 5% destroys the charge-ordered (CO) state and renders the material ferromagnetic and metallic. In Nd 0.5 Ca 0.5 MnO 3 , the charge-ordering transition temperature, T CO , decreases with increase in the Ni 3+ or Co 3+ content and the ferromagnetic metallic state occurs below T CO . Substitution of Ni 3+ or Co 3+ has no effect on the CO state of Y 0.5 Ca 0.5 MnO 3 , showing thereby that such substitution effects delineate the different types of CO states caused by differences in the average size of the A-site ions. The presence of empty e g orbitals in the substituent transition metal ion appears to be necessary for destroying the CO state in these manganates.

An investigation of the re-entrant ferromagnetic transition in rare earth manganates in the regime of competing charge-ordering and ferromagnetic interactions

Manganates of the compositions Nd0.5-xLaxCa0.5MnO3 and Pr0.5-xLaxCa0.5 MnO3, with the average A-site cation radius, rA, in the range 1.17-1.20??, are charge ordered at ordinary temperatures (TCO~240?K), and undergo a re-entrant transition to a ferromagnetic state on cooling (TC TC. Site disorder due to the size mismatch of the A-site cations, ?2, has a marked effect on the re-entrant transition temperature, TC. Thus, in a series of manganates of the type Ln0.5A0.5MnO3 with a fixed rA value of 1.185??, the TC decreases markedly with increase in site disorder, suggesting that the re-entrant transition can be entirely suppressed at a sufficiently high value of ?2. Between TCO and TC, the CO and FM states are likely to coexist, the coexistence temperature regime decreasing with increasing rA, and increasing with ?2 at a fixed rA.

Effect of substituting Ru4+ and other tetravalent ions in the B-site of rare earth manganates on the magneto-transport properties and charge-ordering

Abstract Substitution of Ti and Zr in the B-site of La 0.7 Ca 0.3 MnO 3 lowers the T C and the temperature of insulator—metal transition. Substitution of Ru in the B-site of La 0.7 Ca 0.3 MnO 3 , however, does not alter the T C or T IM significantly. More importantly, the effect of substitution of Ti and Zr in the B-site of the charge-ordered Nd 0.5 Ca 0.5 MnO 3 progressively destroys the charge-ordering in this insulating material. In contrast, substitution by Ru in the B-site renders the material ferromagnetic with the T C increasing with the Ru content and the material also shows insulator-metal transition. This marked effect of Ru substitution on the charge-ordering is also seen in Nd 0.5 Sr 0.5 MnO 3 where the T C increases with Ru content while charge-ordering gets destroyed. The extraordinary effect of Ru in favouring the ferromagnetic metallic state and destroying the charge-ordering is attributed mainly to its unique electronic configuration ( t 2 g 4 e g 0 .

Occurrence of re-entrant ferromagnetic transitions in rare-earth manganates on cooling the charge-ordered states

Some of the compositions of the half-doped rare-earth manganates, La0.52xLnxCa0.5MnO3 (Ln ¼ Nd, Pr) and Nd0.5Ca0.52x SrxMnO3 with relatively small A-cation radii, krAl; show an unusual behavior wherein they become ferromagnetic (FM) on cooling the charge ordered (CO) state ðTCO . TCÞ: With increase in krAl; however, the TC becomes greater than TCO. Thus, plots of TC and TCO against krAl for La0.52xLnxCa0.5MnO3 (Ln ¼ Nd, Pr) and Nd0.5Ca0.52xSrxMnO3 show cross-over from the TCO . TC regime to the TC . TCO regime around krAl values of 1:195 ^ 0:003 and 1:200 ^ 0:005 A ˚ , respectively. Between TC and TCO, the CO and FM phases are likely to coexist. In Nd0.5Ca0.5Mn12xMxO3 (M ¼ Cr, Ru), TCO . TC when x # 0:10; suggesting the re-entrant nature of the FM transition. q 2002 Elsevier Science Ltd. All rights reserved.

A comparative study of thin films of hole-doped Pr0.6Ca0.4MnO3 and electron-doped Pr0.4Ca0.6MnO3

Abstract Properties of thin films of hole-doped Pr0.6Ca0.4MnO3 and electron-doped Pr0.4Ca0.6MnO3 are compared. While both are charge-ordered insulators, the hole-doped manganate undergoes an insulator–metal (I–M) transition on the application of magnetic-fields, but the electron-doped manganate does not. Substitution of 3% Cr3+ or Ru4+ in the Mn site has greater effect on the hole-doped manganate. Electrical fields, however, have similar effects on the hole-doped and electron-doped manganates, both exhibiting current-induced I–M transitions. The study not only establishes that the mechanism of the I–M transition brought about by electric and magnetic fields are different, but also suggests that the electronic structures of the hole-doped and electron-doped manganates have basic differences.

Charge ordering in electron-doped manganates

Electron-doped rare-earth manganates of the type Ca1-xLnxMnO3 (Ln = La, Nd, Gd or Y) with x = 0.2 and 0.3 show charge ordering in the 150 - 270 K range, but the charge-ordering transition temperature, Tco, generally decreases with the decrease in the size of the A-site cations, a trend exactly opposite to that for hole-doped manganates. On the other hand, Tco increases with x or the electron concentration. These trends are also seen for Ca1-xLnxMnO3 compounds (Tco = 300K for x = 0.3) which show transitions to a more distorted orthorhombic structure below Tco. In Ca1-xLnxMnO3, Cr doping does not melt the charge-ordered state, unlike the case for the hole-doped systems. CaMnO2.82, for which electron doping is affected by anion vacancies, appears to show charge ordering at around 200 K.