問合せ 東京大学大学院工学系研究科 野村貴美
L1 Electronic phase separation as a root of colossal magnetoresistance
(Eötvös Loránd University, Budapest) Dr. Zoltan Nemeth
Modern data storage systems are dominantly based on the recording and reading of the magnetization of small magnetic domains, which represent a digital value. In order to convert the magnetic signal into electronic pulse, magnetoresistive (MR) materials are essential. The more sensitive the magnetoresistive material is, the higher the density of stored data and the faster the writing/reading process can be. In the previous decade, besides the currently used layered compounds with giant magnetoresistance, a new class of magnetoresistive materials were also discovered which show significantly higher MR around their magnetic ordering temperature . The other advantage is that this so called colossal magnetoresistance (CMR) is a bulk property of several types of compounds.
Typical example for CMR materials are the doped cobaltate perovskites , cation disordered double perovskites , or chalcogenide spinels . The most intriguing fact is, that these materials differ in almost every respect (composition, crystal structure, magnetism) from each other, still they all show a striking new phenomenon, the colossal magnetoresistance.
In order to elucidate the source of the CMR effect, and so to be able to improve magnetoresistive sensitivity, finding a common feature of CMR materials is essential. In this review we present our recent studies of the local and bulk electronic/magnetic properties of several CMR families of materials using mainly Mössbauer spectrometry and magnetization measurements [8,9,10]. Investigating the effect of small modulations (doping with ions, increasing cation disorder) and utilizing the unique advantage of Mössbauer spectrometry being sensitive to local properties, we were able to determine that the common point in the aforementioned CMR materials is an intrinsic, nanoscale electronic phase separation, which seems to be responsible for the observed magnetoresistance.
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L2 Multiple spectroscopic studies emphasizing Mössbauer of iron containing systems
(Indian Institute of Technology, Madras) Honorary Professor P.T. Manoharan, SAIF
We have been interested in the electronic and molecular structural aspects and lattice dictated dynamics in the series of Iron (II) and Iron (III) systems. While Mössbauer helps us looking at different oxidation states of Iron, NMR is helpful to detect only the diamagnetic and paramagnetic Iron (II) (diamagnetic and paramagnetic systems) and ESR is able to detect the Iron (III) states both at high and low spin states. To a large extent, combining these spectroscopic studies with magnetism and crystallography provide for a detailed understanding of the electronic and molecular structural aspects of Iron systems. In addition, variable temperature spectroscopy reveals temperature dependent population of different spin states, dynamics and any other abnormal behavior such as that arising from different conformations and phase transitions.
In this talk, the main emphasis will be placed on lattice dictated conformers of the ligand, L=[2,6-bis(3,5-dimethyl pyrazoly-1-yl)pyridine]-based Iron (II) complexes containing different counter ions such as spherical but differently sized ClO4-, PF6-, BPh4- in that order of increasing size and planar Ni(mnt)22- and Ni(mnt)23- where mnt is maleonitilodithiolate. The results of variable temperature experiments reveal the presence of high and low-spin forms both in solution and in solid states. The cation FeL22+ exhibits the thermally driven inter conversion between low-spin and high-spin structural forms- phenomena observed in solid and solution states due to ligand dynamics in contrast to the well known cross- over phenomenon. This in fact is an extension of EPR behavior of the Cu (II) systems corresponding to the same Iron family cited above with two different dynamically driven forms, one exhibiting ferromagnetic and the other, antiferromagnetic interaction. An additional example will be given from the magnetic and spectroscopic studies of ferricenium tetrabromoferrate (III), [Fe (C5H5)2].+[FeBr4].-, a soft ferromagnet and its iodo analogue. The latter material undergoes the transition at 13.2 K, traced to the near neighbour ferromagnetic interaction. Again, EPR and Infrared studies contribute to the understanding of Mössbauer-dictated mechanism.
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