Magnetic properties of nearly defect-free maghemite nanocrystals

Temperature variations of the magnetization M and the electron magnetic resonance (EMR) parameters of maghemite (gamma-Fe2O3) nanocrystals are reported for the 4 K-300 K range. Transmission electron microscopy of the nanocrystals shows them to be nearly spherical samples (aspect ratio a/b=1.15) with diameter D=7(1) nm, and analysis of x-ray diffraction lines yields D=6.4 nm with negligible strain. M versus T data show blocking temperatures T(B)similar or equal to101 K, 89 K, and 68 K in measuring fields H=50, 100, and 200 Oe respectively. M versus H data for T>T-B fits the modified Langevin function M=MsL(mu(p)H/k(B)T)+chi(a)H with mu(p)=8000(500)mu(B)/particle and M-s=80 emu/g, identical to M-s for bulk gamma-Fe2O3. It is argued that this large value of M-s, the small value of coercivity H(c)similar or equal to20 Oe at 5 K, the lack of exchange bias in a field-cooled sample, and negligible strain point to nearly defect-free nanocrystals. In the EMR studies, two resonance lines are observed, one with the resonance field H-r greater than that for the free-electron value and the other smaller. From the temperature variations of H-r and the linewidths of the two lines, it is argued that the two lines are, respectively, due to nanocrystals with easy-axis aligned perpendicular and parallel to the applied field. The relatively narrow intrinsic linewidths (similar or equal to400 Oe) of the two lines in nearly defect-free nanocrystals facilitated their observation.