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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 ...

We report on the low-temperature resistance and magnetization of a single crystal of the bilayer manganite (La 0.4 Pr 0.6 ) 1.2 Sr 1.8 Mn 2 O 7 . In zero field the sample is an insulator at all temperatures while at fields larger than 3 T a transition to a ferromagnetic metallic state is observed. Below 50 K the sample shows a pronounced memory effect, in which both resistivity and magnetization become dependent on the magnetic history and clearly demonstrate hysteretic behavior. At these low temperatures a difference in zero-field resistivity before and after magnetization of six orders of magnitude has been measured. The relaxation of the memory effect follows a stretched exponential function with a characteristic decay time of 5 hours at 10 K, which decreases to less than 1 ms above 40 K. These low-temperature properties can be related to magnetostriction data, indicating the presence of a field-induced change in the orbital occupancy of the e g electrons of Mn 3ϩ .