In-field Mossbauer study of disordered surface spins in core/shell ferrite nanoparticles

Abstract

Structural evolution and the kinetics of Cu clustering in the amorphous phase of Fe-Cu-Nb-Si-B alloy J. Appl. Phys. 110, 033537 (2011) Room temperature long range ferromagnetic ordering in (BiFeO3)1x (PbTiO3)x nanocrystallites J. Appl. Phys. 109, 123911 (2011) Hafnium oxide thin films studied by time differential perturbed angular correlations J. Appl. Phys. 109, 113918 (2011) Slow magnetic relaxation and electron delocalization in an S = 9/2 iron(II/III) complex with two crystallographically inequivalent iron sites J. Chem. Phys. 134, 174507 (2011) Additional information on J. Appl. Phys. Magnetization and Mossbauer spectroscopy measurements are performed at low temperature under high field, on nanoparticles with a nickel ferrite core and a maghemite shell. These nanoparticles present finite size and surface effects, together with exchange anisotropy. High field magnetization brings the evidences of a monodomain ordered core and surface spins freezing in disorder at low temperature. Mossbauer spectra at 4.2 K present an extra contribution from the disordered surface which is field dependent. Field and size dependences of this latter show a progressive spin alignment along the ferrite core which is size dependent. The weak surface pinning condition of the nanoparticles confirms that the spin disorder is localized in the external shell. The underfield decrease in the mean canting angle in the superficial shell is then directly related to the unidirectional exchange anisotropy through the interface between the ordered core and the disordered shell. The obtained anisotropy field H Ea scales as the inverse of the nanoparticle diameter, validating its interfacial origin. The associated anisotropy constant K Ea equals 2.5ϫ 10 −4 J / m 2 .

Core/Shell Nanoparticles of Non-Stoichiometric Zn–Mn and Zn–Co Ferrites as Thermosensitive Heat Sources for Magnetic Fluid Hyperthermia

“…Even though the surface layer is not homogeneous and presents some remaining content of core divalent metals in the composition, the obtention of the core-shell structure by hydrothermal soft chemistry is remarkable. Indeed, our chemical core/shell approach well accounts for several structural and magnetic properties, such as coordination of core and shell metal ions 71 and intrinsic NPs' magnetization 63,72,73 . Figure 2b), the solid line represents the best magnetization fit calculated using Eq.…”

Effect of surface pinning on magnetic nanostuctures

“…Obviously, the spins interact ferromagnetically (antiferromagnetically) when the corresponding value of interaction strength is positive (negative); for example when J c < 0, and sites i and j are within the core (i ∈ C and j ∈ C) the spins s i and s j interact antiferromagnetically. The magnetic field H is experienced by all the spins, both in core C and shell S. In addition, to mimic surface pinning which has been observed in magnetic nanoparticles in contact of organic liquids [41], in ferromagnetic thin films [42] and core-shell structures [38] etc., we assume that η-fraction of boundary spins are pinned; a parameter 0 ≤ r ≤ 1 controls the fractions of ↑ spins among pinned ones. We proceed by assuming the interaction of spins within the core is antiferromagnetic (J c < 0) whereas the interaction of spins in the shell is ferromagnetic (J sh > 0) and the interaction at the interface can be ferro-or antiferromagnetic; this is not the usual scenario but observed in several experimental systems [43].…”

“…The microscopic origin of the exchange bias however depends on many details, like the thickness of core and shell [34], anisotropic spin interaction [35] and interaction in the core-shell interface [36]. In addition, pinning of spins at the interface [37] or at the the surface [38] can affect the magnetic behavior strongly.…”

Exchange-bias and magnetic anisotropy fields in core–shell ferrite nanoparticles

“… 29 and 27 , in chemically homogeneous particles this anisotropy field is imposed on the core magnetic moment by outer surface spins. In CS case, the structure border between different ferrites may also affect the symmetry of anisotropy 19 , 33 . Similar observations are obtained for CuFe O @ –Fe O and NiFe O @ –Fe O core–shell nanoparticles with a mean NP diameter slightly larger than that of sample S1 (data not shown).…”