Dynamic response in Fe-doped La 0.05 Ca 0.35 Mn 1-x Fe x O 3 : Rare-earth manganites
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We have studied the dynamic response of Fe-doped manganites with ac susceptibility measurements in La0.65Ca0.35Mn1xFexO3 with 0.01 # x # 0.10 as functions of temperature and dc magnetic field. It is observed that the in-phase part of susceptibility goes through a maximum that is removed on the application of moderate dc field. DC fields suppress both the components (real and imaginary parts) with the strongest effects being at or close to TC. Conduction and ferromagnetism have been consistently suppressed by Fe substitution. Increased spin disorder and decrease in TC with increasing Fe content are evident. The effect of Fe is seen to be consistent with the disruption of the Mn–Mn exchange possibly due to the formation of magnetic clusters. There is clear correlation between the resistivity and susceptibility. Low-temperature dissipation in high-Fe-doped samples is observed; that is, increasing the Fe leads to increased spin disorder and dissipation at low temperature. The effect of the dc field is discussed in terms of the suppression of spin fluctuations close to TC.
I. INTRODUCTION
During the last few years, perovskite manganites have attracted broad research interest because of their unusual magnetic and transport characteristics. The magnetic and resistive behavior in the colossal magnetoresistance compounds1,2 is well known to be a sensitive function of the lattice strain produced, for example, by doping on various sites.3–5 The effects of substituting on the Mn sites by other transition elements are further known6 to affect the properties because of the changes produced in the average electron concentration and the shifts in the positions of the eg and tg subbands. A number of studies6–10 have been conducted on the effects of replacement of Mn by various transition elements, which have included both global and local measurements such as Mossbauer spectroscopy. The case of Fe is particularly interesting because of the large extent to which it can replace Mn. It is apparent from various studies that the Fe atom substitutes for Mn in the +3 state and does not take part in the double exchange (DE) process, thereby leading to a lowering of TC. It has also been shown10,11 that Fe and Co doping leads to the formation of locally antiferromagnetically coupled spins or clusters with localized spin excitations. Several recent studies including those by neutron scattering have shown that the onset of the paramagnetic (PM) to ferromagnetic (FM) transition is accompanied by a divergence of the correlation length, signaling the growth of the infinite correlation and an inhomogeneous12 type of FM transition. a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2011.228 J. Mater. Res., Vol. 26, No. 20, Oct 28, 2011
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It can therefore be expected that the effect of the Fe doping, which results in the formation of clusters and suppression of conductivity, would also be reflected in the effective activation energies for hopping transport
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