Supermagnetism in magnetic nanoparticle systems

Magnetic nanoparticles (NPs) form a single domain when their size falls below a certain critical diameter. These single domain magnetic NPs show superparamagnetic blocking phenomena when the time scale of the measurement technique becomes equal to the time scale of fluctuation of the net magnetization (or “superspin”) of the nanoparticles. However interparticle interactions in an ensemble of magnetic NPs can change the magnetic property of the system. When they will be non-interacting the system can show simple “superparamagnetic blocking” phenomena. For increasing concentration and hence increasing interactions the nanoparticle system can show spin glass like properties which is called as “superspin glass”. Furthermore with more increase in concentration but still below physical percolation the strong interactions between the NPs can lead to a ferromagnetic (FM) like state which is called as “superferromagnetic” (SFM).  A SFM domain is defined like a FM domain, the only difference being that the atomic moments are replaced by the supermoments of the individual nanoparticles. I will present the results obtained from soft ferromagnetic Co80Fe20 nanoparticles discontinuously embedded in an insulating matrix Al2O3 in the form of discontinuous metal-insulator multilayers (DMIMs) [Co80Fe20(tn)/Al2O3]m where tn and m denote the nominal thickness of CoFe and the number of bilayers, respectively. Here the CoFe particles are treated as superspins with random size, position and anisotropy [1, 2, 3, 4]. We will show that DMIMs represent a model system to observe supermagnetism which comprises three fascinating subjects: superparamagnetism (SPM), superspin glass (SSG) and superferromagnetism (SFM) [4]. Single particle blocking has been observed in the DMIM sample with low nominal thickness, tn = 0.5 nm, which has negligible inter-particle interaction [5]. At intermediate thickness, 0.5 < tn  1.1 nm, one observes memory and aging of a SSG [6]. At higher particle densities, but still below the conductive percolation threshold, 1.2 nm < tn < 1.4 nm, SFM behavior with domain state properties is encountered [1, 7, 8, 9]. The SFM state has been evidenced by magneto-optic Kerr magnetometry (MOKE), Cole-Cole plots drawn from the complex ac susceptibility and by polarized neutron reflectivity measurements [8]. We have shown for the first time homogeneously magnetized SFM domains forming at the magnetization reversal of DMIMs by means of X-ray photoemission electron microscopy and MOKE microscopy [9]. In this system the decisive FM inter-nanoparticle exchange is probably mediated by ultrasmall paramagnetic particles deposited between the superspin particles [5, 9]. 

[1] W. Kleemann et al., Phys. Rev. B 63, 134423 (2001); [2] S. Bedanta et al., Phys. Stat. Sol. (c) 12, 3288 (2004) ;  [3] O. Petracic et al., J. Magn. Magn. Mater. 300, 192 (2006) ; [4] S. Bedanta and W. Kleemann, J. Phys. D : Appl. Phys. 42, 013001 (2009) ; [5] X. Chen et al., Phys. Rev. B 72, 214436 (2005) ; [6] S. Sahoo et al., Phys. Rev. B 67, 214422 (2003) ; [7] X. Chen et al., Phys. Rev. Lett. 89, 137203 (2002) ; [8] S. Bedanta et al., Phys. Rev. B 72, 024419 (2005) ; [9] S. Bedanta et al., Phys. Rev. Lett. 89, 176601 (2007); [10] S. Bedanta et al., J. Appl. Phys. 105, 07C306 (2009) ; [11] S. Bedanta et al., J. Phys. D- Appl. Phys. 43, 474002 (2010).