Dresden 2009 – scientific programme

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MA: Fachverband Magnetismus

MA 3: Magnetic Half Metals and Oxides

MA 3.9: Talk

Monday, March 23, 2009, 13:00–13:15, HSZ 401

Ultrafine nc-NiO: a Néel-type Random Ferrimagnet? — •Marek Petrik and Bernd Harbrecht — Chemistry Department and Center of Materials Science, Philipps University, Hans-Meerwein-Strasse, 35032 Marburg, Germany

Is ultrafine nanocrystalline nickel oxide nc-NiO an intrinsic random ferrimagnet as hypothesized by Louis Néel in 1962 [1]? Or is the often reported uncompensated magnetic moment of this archetypal antiferromagnet in the finely divided state due to some other cause? The question has never been conclusively settled in spite of efforts by the groups of Richardson [2], Mørup [3], Seehra [4], and others. Unable to locate the uncompensated spins in nc-NiO directly, we have resorted to indirect means of identifying their origin [5]. Since the magnetism is strongly correlated with the crystallite size and shape, we control these two in a novel solid-state synthesis in the full mesoscopic range (2,5-50 nm). An analysis of the Langevin-type superparamagnetism and the Néel-Arrhenius thermal relaxation, as a function of size, for a large number of samples indicates that nc-NiO not only is a random ferrimagnet as considered by Néel, but also that it could either be subject to a size-dependent magneto-crystalline anisotropy or even consist of multiple antiferromagnetic nanodomains. -- [1] L. Néel, in Low Temperature Physics, C. De Witt et al., eds., Gordon and Breach, New York, 1962, p. 412. - [2] J. T. Richardson et al, J. Appl. Phys., 70 (1991) 6977. - [3] C. R. H. Bahl et al., J. Phys.: Condens. Matter, 18 (2006) 4161. - [4] H. Shim et al., Solid State Comm., 145 (2008) 192. - [5] M. Petrik, B. Harbrecht, Z. Anorg. Allg. Chem., 634 (2008) 2069.