Antiferromagnetic spin correlations in MnO nanoparticles

We have investigated nearly monodispersive MnO nanoparticles of 10 nm diameter by polarised neutron diffraction with XYZ-polarisation analysis. We found no long-range ordering down to 3.5K. A broad magnetic peak appeared close to Q1⁄4 (1/2,1/2,1/2) signifying short-range antiferromagnetic correlations. The correlation length was found to be about 2.4 nm at T1⁄43.5 K. The correlation length decreases rapidly with increasing temperature and becomes about 0.7 nm at T1⁄4250K. & 2010 Elsevier B.V. All rights reserved. Size-dependent scaling laws and the magnetic and optical properties of small particles and nanostructured assemblies, as a function of size, shape, dimensionality, morphology and interparticle interactions, are increasingly of fundamental, technological and biomedical interest [1–8]. There are clearly two limits to the magnetic behavior as a function of size and dimensionality. At one end of the spectrum (bulk) the microstructure determines the magnetic (hard and soft) behavior. This is a function of the processing method and our understanding of it is qualitative and empirical at best. At the other end, as the length scales approach the size of domain wall-widths (nanostructures), lateral confinement (shape and size) and inter-particle exchange effects dominate, rendering classical descriptions grossly inadequate. Moreover for nanoscale structures, surface effects due to the lack of translation symmetry, reduced coordination number and broken bonds, possible at the particle boundary, can lead to surface spin disorder and frustration [9,10]. These surface effects can dominate the magnetic behavior of fine particles, especially with decreasing particle size (note that, for example, 60% of the atomic spins of a cobalt nanoparticle, 1.6 nm in diameter, are on the surface). As a result, the ideal model of a single domain particle or a giant superspin, with the same orientation and magnetic moment as the bulk material and reversing coherently is no longer valid. In other words, the combination of finite size, structural details of the core/surface and their interactions influence the magnetic ground state, including the non-uniform magnetization profile across the particle. Nanoparticles, prepared ll rights reserved. [email protected] by chemical routes, are excellent materials to study such magnetic correlations on the nanometer scale, provided they can be synthesized with narrow size distributions. Beginning with cobalt as a model system, we have developed a comprehensive chemical method for the reproducible synthesis of monodisperse, passivated nanocrystals (metals [11], alloys [12], oxides [13] and core-shell structures [14]) with good size/shape control. The nanoparticles are well-characterized magnetically (a.c. and d.c magnetometry over a wide temperature range, high resolution electron holography and X-ray magnetic scattering), optically (VUV-VIS spectroscopy), structurally (transmission electron microscopy, dynamic light and small angle X-ray scattering) and chemically (electron energy-loss spectroscopy). However, there still remain fundamental questions about magnetic correlations in such materials. In order to get microscopic information about the magnetic correlations in magnetic nanoparticles in general and MnO nanoparticles in particular we have undertaken systematic neutron diffraction investigations. Antiferromagnetic nanoparticles have been reported to have a net magnetic moment due to the uncompensated surface spins. The magnetic moment becomes larger with the decrease in particle size implying increase in the surface-to-volume ratio. This surface magnetism affects the magnetic properties such as the superparamagnetic blocking of the spin magnetisation direction of the nanoparticles. The resulting phenomenon is a shift of the peak Tp in the zero-field cooled (ZFC) magnetisation of MnO nanoparticles towards higher temperatures with decreasing particle size [15–18]. This temperature dependence is opposite to that of most other antiferromagnetic nanoparticles, including the isostructural NiO [19–21]. The fundamental problem of investigating magnetic ordering or short-range magnetic correlations in nanoparticles is the high T. Chatterji et al. / Journal of Magnetism and Magnetic Materials 322 (2010) 3333–3336 3334 incoherent background coming from the hydrogen atoms of surfactants with which the nanoparticles are coated in order to achieve narrow size dispersions and also to keep them apart. The incoherent background is so high that any reasonable determination of the relatively weak magnetic scattering becomes impossible by unpolarised neutron diffraction. However, magnetic information can be obtained by polarised neutron diffraction with proper analysis in magnetic fields. The XYZ-method using polarised neutrons for the separation of coherent, spin-incoherent, and magnetic scattering was developed by Schärpf et al. [22]. We exploited this method to extract magnetic scattering and determine magnetic correlations in MnO nanopartices. We have prepared high quality, monodisperse, 10nm dia., antiferromagnetic MnO nanoparticles. In the bulk, MnO is a cubic antiferromagnet with a Néel temperature, TN 122K. We have extensively characterised structure and magnetic behavior of these nanoparticles [23]. Fig. 1 shows the X-ray y22y scans from the bulk as well as from the nanoparticles. One immediately notices that the Bragg peaks from the bulk sample has smaller 2y values compared to those of the nanoparticles. This shows that the room temperature lattice parameter of the nanopartices is smaller than that of the bulk. The bulk lattice parameter is a1⁄44.436 Å whereas that for nanoparticles is a1⁄44.37 Å. The reduction of the unit cell volume with decreasing particle size is, by now, a well-known effect. This can naively be explained by the fact that the nanoparticle of diameter d is under effective pressure P1⁄42S/d due to surface tension S. The reduction of the cell volume in nanoparticles of half-doped manganites has been reported [24]. The inset of Fig. 1 shows a TEM picture of a single Fig. 1. X-ray diffraction diagram from bulk (dashed line) and nanoparticles (solid line) of MnO. The inset shows the TEM picture of a nanoparticle. The scale bar