Intermixing enables strong exchange coupling in nanocomposites: Magnetism through the interfacial ferrite in γ−Fe2O3/NiO

E. Skoropata, T. T. Su, H. Ouyang, J. W. Freeland, J. van Lierop
2017 Physical review B  
6 We quantified M s (T ) by fitting the high-field region of the loops and verifying the result by extrapolating from 100 M (µ 0 H) at 1/µ 0 H=0. In nanoparticles, M s (T ) is typically described by a Bloch T 3/2 dependence 24 that is modified 101 to include a term 12,24 A exp −T /T f that describes qualitatively the "freezing out" of disordered surface spins that the Bloch constant, B∝1/J describes 103 the average exchange strength. Fits to this function (solid lines in Figs. 4c-d) describe M
more » ... (T ) well with A=0.21±0.04, 104 T f =3.3±0.4 K and B=3.19±0.06×10 −5 K −3/2 for γ-Fe 2 O 3 nanoparticles, and A=0.42±0.05, T f =3.2±0.5 K and 105 B=3.31±0.05×10 −5 K −3/2 for γ-Fe 2 O 3 /NiO nanoparticles. The fit results reveal γ-Fe 2 O 3 /NiO nanoparticles' disor- 106 dered surface spin population makes up a larger fraction of the low T M s while T f is unaffected. However, reconciling 107 the much lower H ex of the γ-Fe 2 O 3 /NiO nanoparticles with this result suggests strongly that uncompensated Ni 2+ 108 spins from the NiO contribute to the low T M s (T ) (e.g. the more pronounced upturn at 5 K ). The larger B indicates 109 a weaker overall J amongst spins which contribute to M s for T T f (i.e. the "bulk" ordered spins). Stronger exchange 110 interactions are expected between Fe spins at the γ-Fe 2 O 3 /NiO interface compared to those at the γ-Fe 2 O 3 surface 111 due to (better) filled coordination. However, a lower exchange strength compared to the ordered interior spins of the 112 γ-Fe 2 O 3 core is expected for coupling through Ni 2+ (providing a weaker superexchange path compared to Fe 3+ -O 2− -113 Fe 3+ ) or if some degree of disorder is retained. The larger B for γ-Fe 2 O 3 /NiO nanoparticles points to the recapture 114 of γ-Fe 2 O 3 surface spins, increasing the "effective magnetic volume" via an interfacial population with J<J core but 115 with significantly larger exchange strength compared to J surf of bare γ-Fe 2 O 3 . 116 C. Atomic magnetism 117 Clearly, a better microscopic understanding of the Fe and Ni spin composition and magnetism is necessary to 118 identify the origin of the changes to H c , H ex , and surface magnetism from the strong exchange coupling enabled by 119 the NiO crystallites. Mössbauer spectroscopy at 10 K ( T B where superparamagnetism does not alter the hyperfine 120 parameters) provides each unique magnetic and electronic environment (site), described by a sextet characterized 121 by a Lorentzian (FWHM) linewidth Γ, hyperfine field B hf , isomer shift δ, and quadrupole splitting ∆, with the 122 relative abundance of each site proportional to the respective spectral areas. The majority of the spectrum of γ-123 Fe 2 O 3 /NiO at 10 K is described by components (labeled A and B I ) with hyperfine parameters typical of the B-sites 124 (B hf,B I =53.32±0.06 T, δ B I =0.532±0.007 mm/s) and T d A-sites (B hf,A =50.93±0.05 T, δ A =0.393±0.007 mm/s) 13 125 with Γ=0.26±0.01 mm/s. Assuming (as usual) that the recoil-free fractions of the A and B-sites are equal at 10 K 26 , 126 30% and 44% is the site abundance of the Fe-ions (versus 62% and 38% for stoichiometric γ-Fe 2 O 3 ). An additional 127 component with B hf,B II =49.7±0.1 T, δ B II =0.70±0.03 mm/s and Γ=0.45±0.05 mm/s was necessary to fully describe 128 the spectrum, indicating a change in the environment of some of the Fe-ions occurred after adding the NiO shells, 129 comprising 22% of the Fe-sites. These hyperfine parameters are consistent with the B-sites of non-stoichiometric 130 Ni-ferrite, existing at the interface. The larger δ represents a lower Fe-valence, so that the B II -site is from Fe 2+ -131 ions. The lower B hf identifies fewer (or weakened) nearest-neighbour J's, in keeping with the M s (T ) analysis. Also, 132 the v=0, B hf =0 of paramagnetic surface spins 25 of the γ-Fe 2 O 3 nanoparticles is not present in the γ-Fe 2 O 3 /NiO 133 nanoparticles' spectrum, replaced with an interfacial component (observable most clearly as absorption at ∼-3 mm/s) 134 with B hf,int =22.1±0.01 T, due to a recapture of the (now) interfacial spins. B hf,int is lower than the ∼50 T of 135 FIG. 5. Mössbauer spectra measured at various temperatures for γ-Fe2O3/NiO and the subspectrum component due to individual Fe-sites determined from the 10 K spectrum. Theoretical spectra which well describe the measured spectra for the γ-Fe2O3 cores 25 are indicated with a dashed line for comparison.
doi:10.1103/physrevb.96.024447 fatcat:cqfiwe5ukfglbpgk5uxk7wbima