Enhancement of Ferromagnetism in the Colossal-Magnetoresistance Layered Manganite La^sub 1.2^Ba^sub 1.8^Mn^sub 2-x^Ru^sub x^O^sub 7^ (x = 0, 0.1, 0.5, and 1)

Journal of Electronic Materials, Nov 2004 by Sudhakar, Nori, Rajeev, K P, Nigam, A K

Electrical transport and magnetic measurements of the Ru-doped, layered manganite system, La^sub 1.2^Ba^sub 1.8^Mn^sub 2-x^Ru^sub x^O^sub 7^ (x = 0, 0.1, 0.5, and 1), have been carried out in the temperature range of 5-310 K and in the presence of magnetic fields up to 10 T. The magnetic transition temperatures are found to be above room temperature, and colossal magnetoresistance (CMR) is present at low temperatures and even close to room temperature. Magnetoresistance (MR) is found to obey power-law behavior as a function of applied field with an exponent close to 0.5 at low temperatures.

Key words: Manganites, spin glasses, colossal magnetoresistance (CMR), magnetization curves

INTRODUCTION

Jonker and van Santcn1 were the first to report the fascinating electrical and magnetic properties of rare-earth manganites as early as 1950. Zener2 tried to explain the observed behavior by introducing the concept of double exchange. Subsequently, this paved the way for some of the classic theories developed by Anderson and Hasegawa,3 Goodenough,4 and de Gennes5 to explain the physical properties of manganites. All the theories attempted to explain the different exotic antiferromagnetic phases formed in certain compositional ranges of the rareearth ion concentration caused by a superexchange interaction between the Mn ions. The mixed valency exhibited by Mn plays a crucial role in determining the properties to a great extent. The Mn^sup 3+^ and Mn^sup 4+^ ratio holds the key and dictates the magnetic phase and nature of the system. Goodenough,4 in his seminal work, discussed in great detail the formation of various magnetic phases in the phase diagram of A^sub 1-x^B^sub x^MnO^sub 3^ (where A is a rare-earth element and B is an alkaline-earth element), namely, ferromagnetic metal (FMM) at low temperatures, ferromagnetic insulator, antiferromagnetic insulator, and antiferromagnetic phases with canted spins.

It took almost a good half century before these materials were explored sufficiently to be regarded as technologically important materials. In 1993, von Helmholt et al.6 reported the dramatic changes in many physical properties of these rare-earth-based (trivalent) manganites when doped with an alkaline-earth metal (or a divalent ion), such as Ca or Ba, and was followed by Jin et al.7 who showed remarkable magnetoresistance (MR) effects near Tc in optimized thin films. For small concentrations (up to x ≤ 0.3), they found that these materials show a property known as colossal magnetoresistance (CMR), which signifies a dramatic drop in the electrical resistivity, below the magnetic transition temperature (T^sub c^) on the application of a magnetic field. Below the Curie temperature, often the material behaves like a FMM and above T^sub c^, like a paramagnetic insulator. Moritomo et al.8 and several other researchers9"11 demonstrated similar properties in the bilayered manganites belonging to the Ruddlesden-Popper homologous series (Re,A)^sub n+1^ Mn^sub n^O^sub 3n + 1^, with n = 2, where Re is a rare-earth element, such as La, Pr, Sm, or Nd, and A is a divalent cation (Ca, Sr, or Ba).

The wide range of physical phenomena, such as CMR, metal insulator transition, the occurrence of simultaneous ferromagnetism (FM) and metallicity, microscopic phase separation of metallic and insulating phases, and charge and orbital orderings, have been intriguing researchers over the past decade or so, resulting in the synthesis of a host of new materials and a continuous refinement of the existing theories, aiming to increase and stabilize the CMR properties. Recently, Ramakrishnan et al. have come out with yet another new theory for manganites that addresses poorly understood phenomena, such as metal insulator transition, CMR, and Jahn-Teller distortion.12'13

The main aim of this report is to explore the complex electronic transport and the ground-state magnetic ordering in the layered-manganite La^sub 1.2^Ba^sub 1.8^ Mn^sub 2-x^Ru^sub x^O^sub 7^ and the effects that are brought about by Ru doping. One of the important factors determining the characteristics of CMR systems is the Mn^sup 3+^/Mn^sup 4+^ ratio, which has a direct bearing on the double exchange mechanism. Although there are many systematic studies on the hole-doped manganites that vary the concentration of the alkaline earth ions that affects the Mn^sup 3+^/Mn^sup 4+^ ratio, there are very few studies on the doping at the Mn site in these systems. Notable among them are the ones that study doping with various transition elements, such as Fe, Cr, Ni, Co, and Ru, at the Mn site.14"20 Only Ru doping enhances FM and metallicity,17"20 while the rest of the dopants destroy FM. Also, to the best of our knowledge, the reports on Ba-based layered manganites are nonexistent.

EXPERIMENTAL DETAILS

Polycrystalline samples of the series La^sub 1.2^Ba^sub 1.8^ Mn^sub 2-x^Ru^sub x^O^sub 7^ for x = 0,0.1,0.5, and 1 were prepared by the conventional solid-state ceramic route. High purity (99.99%, Aldrich, St. Louis, MO) starting compounds, La^sub 2^O^sub 3^, BaCO^sub 3^, MnO^sub 2^, and RuO^sub 2^, were taken in stoichiometric proportions, and the mixtures were thoroughly ground, calcined at 1,000°C for 24 h, and pressed into pellets with several intermediate grindings to achieve homogenization. The samples were then finally sintered at 1,350°C for 48 h in air. The powder x-ray diffraction (XRD) patterns were recorded in the range 300 -80° with a Cu K^sub α^ radiation source using a Rich Seifert x-ray diffractometer (model 2002, Ahrensburg, Germany). The electrical resistivity measurements in the range 4.2-320 K were done by employing a standard four-probe configuration. The field-cooled and the zero field-cooled (ZFC) direct-current (DC) magnetization measurements (M versus T) were done in the temperature range of 4-320 K using a Magnetic Property Measurement System XL SQUID magnetometer (Quantum Design, San Diego, CA, USA) in a magnetic filed of 100 Oe. The field dependence of magnetization data were collected using a vibrating sample magnetometer (Maglab 2000 VSM, Oxford Instruments, U.K.) in magnetic fields up to ±7 T (M versus H).