Colossal Magnetoresistance is a Griffiths Singularity

Abstract

It is now widely accepted that the magnetic transition in doped manganites that show large magnetoresistance is a type of percolation effect. This paper demonstrates that the transition should be viewed in the context of the Griffiths phase that arises when disorder suppresses a magnetic transition. This approach explains unusual aspects of susceptibility and heat capacity data from a single crystal of La0.7Ca0.3MnO3.The term "colossal magnetoresistance" (CMR) has commonly been used to describe the very large, magneticfield driven changes in electrical resistivity in oxides based on LaMnO 3 near their second-order, ferromagnetic transitions. The largest CMR effects are accompanied by other anomalies in magnetic and thermodynamic properties. Among these are the failure of the magnetic correlation length to increase strongly as the transition temperature T C is approached from above, the persistence of a well defined spin-wave dispersion close to the transition [1], and an unusual shift in the heat capacity peak to higher temperatures in applied magnetic fields.[2] An explanation of this unusual behavior of the heat capacity in the context of a Griffiths singularity [3] is the focus of the present paper.There is now general consensus that the CMR transition is a type of percolation in which, due to the doubleexchange process [4], bonds become metallic as neighboring spins tend to align. The strength of the CMR effect (along with the transition temperature) depends strongly on the ionic size and concentration of the divalent atom that substitutes for La in LaMnO 3 . The effect is nearly absent for La 2/3 Sr 1/3 MnO 3 (T C = 360 K) but quite strong in the present sample La 0.7 Ca 0.3 MnO 3 (T C = 218 K) [5]. In the low temperature metallic phase, the exchange interactions, as measured by the spin-wave stiffness, are the same in Sr-snd Ca-doped crystals [6], demonstrating that the key to the CMR effect is to be found in the non-metallic regime. The ionic size of the divalent substituent exerts its effect on magnetic and electronic properties through local tilting of the oxygen octahedra [7] and the concurrent bending of the active Mn-OMn bonds. This inhibits the formation of metallic bonds and leads to charge localization, polaron formation, and possible charge segregation [8,9]. Evidence in favor of a percolation picture comes from a Monte-Carlo simulation of a random field Ising model that assigns conductivity zero and unity to bonds between neighboring antiparallel and parallel spins respectively [10]. It both produces CMR and emphasizes the importance of randomness. Experimental evidence was provided by Jaime et al.[11] who extracted the field and temperature dependent metallic-bond concentration c(H, T ) from the resistivity via the effective medium approximation, and showed that it also describes the thermoelectric power data. Direct evidence for coexisting polaronic/insulating and metallic components have been reported from neutron [12] Discussions of the CMR effect in percolation terms [16][17][18] have gener...

Influence of Ca-deficiency on the magneto-transport properties in La0.8Ca0.2MnO3 perovskite and estimation of magnetic entropy change

“…The down turn in the magnetic susceptibility at temperatures above T C for the sample with x ¼ 0.20 is the signature of the "Griffiths phase." 22 The deviation from Curie-Weiss law in the PM region is due to the presence of FM clusters within the PM region.…”

Spontaneous magnetization aboveTCin polycrystallineLa0.7Ca0.3MnO3Turcaud1, Pereira2, Sandeman3 et al. 2014Phys. Rev. B394191View full textAdd to dashboardCite

“…Further studies concluded that the local distortion revealed by an anomalous thermal expansion between the Curie-Weiss transition temperature T C and a higher temperature T * pointed towards the presence of ferromagnetic clusters or polaronic interactions being responsible for the electrical conduction [8][9][10]. The presence of clusters in the paramagnetic (PM) region was associated with signature magnetic behavior observed in Griffiths phases [11][12][13][14].…”

Magnetic anomalies in the spin-chain compoundSr3CuRhO6: Griffiths-phase-like behavior of magnetic susceptibility

“…The intra-cluster magnetic ordering temperature above experimentally observable bulk T C varies depending on the size of the cluster and therefore there is a spectrum of T C below the characteristic Griffiths temperature, T G . Though the existence of such a magnetic phase due to quenched disorder has been discussed in connection with giant magnetoresistance behavior of manganites [6,[11][12][13], non-Fermi liquid behavior in f-electron systems [14,15], and dilute magnetic semiconductors [16] as well as for a Tb-based intermetallic compound [7], it is believed that the experimental evidence in favor of the existence of Griffiths phase is still inadequate [17].…”