Dilution of two-dimensional antiferromagnetism by Mn site substitution inLa1Sr2Mn2−xAlx<

Abstract

We report the effect of Al substitution on the Mn site of the bilayered half doped manganite La1Sr2Mn2O7. This substitution dilutes the magnetically active Mn-O-Mn network without introducing an appreciable distortion in the lattice and ionic considerations leads to a predominant reduction of Mn 4+ with increasing Al. The rate of fall of the long range antiferromagnetic transition temperature as a function of substitution is seen to match well with established quasi 2D Heisenberg systems indicating that the nature of magnetic interactions in this quasi 2-D system is of the short range Heisenberg type. The magnetic contribution of the specific heat estimated using Fisher's relation could be fitted with a function incorporating the presence of a gapped Fermi surface appropriate for this type of systems. The resistivity was seen to increase as a function of substitution due to the weakening of the double exchange within the ferromagnetic layers of these A type of antiferromagnets and justifiably, in the paramagnetic region the data could be fitted to Mott's equation for the variable range hopping of polarons in 2 dimensions. O 3n+1 are known to have a layered structure in which n-MnO 3 blocks are seperated by an additional (La 1−x Sr x )O layer. The perovskite structure corresponds to the n = ∞ and the K 2 NiF 4 structure to n = 1 members of this series. The bilayered (n = 2) manganites La 2−2x Sr 1+2x Mn 2 O 7 have been extensively investigated ever since the x= 40% member of this series showed collosal magnetoresistance[2], albeit at lower temperatures. The structural consequence of moving from the perovskite (n= ∞) to the bilayer (n = 2) is the introduction of a 2 dimensional character to the system. The number of next neighbour Mn cations around each transition metal site reduces from 6 to 5, thus providing an anisotropic reduction in the one electron (e g ) bandwidth, as a consequence of which the electronic and transport properties are interestingly modified.One of the most intruiging phenomenon observed in perovskite manganites is the real space ordering of [10] of powder XRD data is used to determine the structural parameters tabulated in Table.1. As is clearly seen, the variation in the structural parameters is < 1% indicating that Al doping does not introduce any major structural distortion. The values of the mean Mn valence determined by using iodometric redox titrations (using sodium thiosulphate and pottasium iodide) unambiguously indicate a preferential replacement of Mn 4+ with increasing Al substitution. The presence of stacking faults in the double layered structure is known to result in the formation of parasitic

Scaling behaviour of magnetic transitions in Ni3V2O8

“…The initial suppression rate for T H of 2.2 is somewhat larger than the expected values for 2D Ising spins, which are generally in the range of 1.6-1.9, although it is smaller than typical values for 2D Heisenberg spins (approximately 3) [21]. The initial suppression rate for T L (4.3) is larger even that computed for 2D Heisenberg spins (% 3), although it is roughly consistent with that observed in other systems, including La 2 CuO 4 doped with Mg and Zn [21] and Al doped LaSr 2 Mn 2 O 7 [24]. These dilution studies suggest that the HTI phase acts like a 2D Ising spin system, while the LTI phase acts like a 2D Heisenberg spin system.…”

Effects of pH and sintering temperature on the synthesis and electrical properties of the bilayered LaSr2Mn2O7 manganite prepared by the sol–gel process

“…The calculated results for lattice parameters are collected in Table 2. The lattice parameters are in good agreement with the earlier studies [24][25][26]. A monotonic incensement trend was seen in the a-lattice parameter and the unit cell volume of samples (Fig.…”

Electrical transport and magnetoresistance of double layered CMR manganites R1.2Sr1.8Mn2O7(R = La, Pr, Sm)

“…The decreasing value of N(E F ) with the decrease of the size of lanthanide ion also indicates that the ability to conduct decreases from LSMO to SSMO. The obtained best fit parameters are in good agreement with the previous reports on similar DL manganites [21,[29][30][31][32]36]. All the fitting parameters in Mott 2D and 3D VRH models vary in a similar manner with lanthanide ion and these results suggest the reliability of the performed fittings.…”

“…In case of DL manganites, the conduction at T > T IM has been explained by Matsukawa et al [29], Zhu et al [30] and others [21,31,32] using Mott type VRH while Chatterjee et al [12] and others [33,34] suggested SPH model. Chen et al [35] suggested ES VRH type of conduction in similar DL manganites.…”