First-order field-induced transition, magnetoresistance, and giant magnetostriction in single crystals of(La0.4Pr0.6

Abstract

The well-established paraferromagnetic transition (T c ) accompanied by a semiconductor-metal transition in the bilayer manganite La 1.2 Sr 1.8 Mn 2 O 7 was found to be suppressed in single crystals of (La 0.4 Pr 0.6 ͒ 1.2 Sr 1.8 Mn 2 O 7 . However, a first-order field-induced transition was observed accompanied by a huge decrease of the c-axis resistance by a factor of one million. The magnetoresistance data correlate well with an equally large negative magnetostriction. In addition, strong anisotropic properties are observed. The data reflect the presence of unusual interplay among spin, charge, and orbital degrees of freedom in this layered structure.The discovery of a colossal magnetoresistance ͑CMR͒ effect followed by the observation of other interesting phenomena such as charge ordering, isotope effect, etc., in the Mn perovskites Ln 1Ϫx D x MnO 3 ͑Lnϭrare earths; DϭCa, Sr, Ba, Pb͒ has stimulated further research into these fascinating group of materials. 1 Several models 1-5 based on the double exchange ͑DE͒ interaction, Jahn-Teller distortion, antiferromagnetic ͑AF͒ superexchange, charge-orbital ordering interaction, phase separation, etc., are presently proposed to account for some of the unusual properties observed. In parallel to the data relative to this group of compounds, interesting results have been reported on the nϭ2 compounds of the Ruddlesden-Popper series generally described as (Ln 1Ϫx D x ) nϩ1 Mn n O 3nϩ1 . In the nϭ2 bilayer Mn perovskite, two MnO 6 layers are alternately stacked with (Ln,D) 2 O 2 layers along the c axis of the structure whereas in the case of Ln 1Ϫx D x MnO 3 , which is the nϭϱ term of the series, the MnO 6 octahedra extend all over the space. The reduced dimensionality of the nϭ2 compounds has been shown to have interesting consequences on their physical properties. Thus, the compound La 1.2 Sr 1.8 Mn 2 O 7 exhibits 6 a paramagnetic-to-ferromagnetic transition ͑PFT͒ at T c ϳ125 K, accompanied by a semiconductor to metal transition ͑SMT͒ and the CMR reaches 98% near T c . Further, in this compound, there is a possibility of interplay between hybridization and chemical potential. At room temperature, the Mn-O bonds are found to be longer in the z direction than in the x-y plane. 7 This would imply occupation of the 3d z 2 Ϫr 2 orbital while the hybridization would favor the occupation of the 3d x 2 Ϫy 2 one. This would affect the magnetic, transport and magnetoelastic properties as was also pointed out by Kimura et al. 8 in their studies wherein they varied the nominal hole concentration x between 0.3 and 0.45 in La 2Ϫ2x Sr 1ϩ2x Mn 2 O 7 . We have, in contrast, kept the hole density constant at xϭ0.4 and investigated the effect of the ionic radius at the rare-earth site on the physical properties of (La 1Ϫz Pr z ) 1.2 Sr 1.8 Mn 2 O 7 compounds.We have found recently 9,10 that for Pr(zϭ0.2), T c decreases from 125 to 98 K, this change being accompanied by a SMT at 98 K and that the magnitude and sign of the lattice striction were strongly affected. As we further increased ...

Magneto-optical investigation of the field-induced spin-glass-insulator to ferromagnetic-metal transition in the bilayer manganite(La0.4Pr0.6)1.2Cao1, Haraldsen2, Rai3 et al. 2006Phys. Rev. BSelf Cite

“…With increasing Pr substitution, T c decreases and is eventually quenched. 5,26 Despite the suppression of T c at z=0.6, (La 0.4 Pr 0.6 ) 1.2 Sr 1.8 Mn 2 O 7 is still in close proximity to the FMM phase boundary. Thus, a small local structure variation can modify r A and σ 2 , pushing the system back into the FMM state.…”

“…5 Upon increasing Pr substitution (z=0, 0.2, and 0.4), the paramagnetic insulator (PMI) to ferromagnetic metal (FMM) transition temperature, T c , decreases (120, 90, and 60 K, respectively). 26 At z=0.6, the transition is quenched.…”

“…O occupies three different sites, and rare earth/alkaline earth metal have different coordination numbers depending on whether they occupy perovskite (P) or rock salt (R) sites. 4,5 Note that the unit cell along the b axis has been replicated to highlight the layered structure.…”

“…Can we understand bulk property trends in (La 0.4 Pr 0.6 ) 1.2 Sr 1.8 Mn 2 O 7 within this picture? Table I shows the correlation between the PMI to FMM transition temperature (T c ), 5,26 electronic bandwidth, and quenched disorder. With increasing Pr substitution, T c decreases and is eventually quenched.…”

“…The results demonstrate that the lattice responds more slowly than charge and spin over various length and time scales. The strong hysteresis in the low temperature physical properties 5,13,29,31,32,51 of (La 0.4 Pr 0.6 ) 1.2 Sr 1.8 Mn 2 O 7 derives from these differences. …”