Temperature variations of the magnetization $M$ and the electron magnetic resonance (EMR) parameters of maghemite $\left(\gamma \text{−}{\mathrm{Fe}}_2{\mathrm{O}}_3\right)$ nanocrystals are reported for the $4\mathrm{K}–300\mathrm{K}$ range. Transmission electron microscopy of the nanocrystals shows them to be nearly spherical samples (aspect ratio $a∕b=1.15$) with diameter $D=7\left(1\right)\mathrm{nm}$, and analysis of x-ray diffraction lines yields $D=6.4\mathrm{nm}$ with negligible strain. $M$ versus $T$ data show blocking temperatures $T_B\simeq 101\mathrm{K}$, $89\mathrm{K}$, and $68\mathrm{K}$ in measuring fields $H=50$, 100, and $200\mathrm{Oe}$ respectively. $M$ versus $H$ data for $T>T_B$ fits the modified Langevin function $M=M_s\mathcal{L}({\mu }_{p}H∕k_{B}T)+{\chi }_{a}H$ with ${\mu }_p=8000\left(500\right){\mu }_B∕\text{particle}$ and $M_s=80\mathrm{emu}∕\mathrm{g}$, identical to $M_s$ for bulk $\gamma \text{−}{\mathrm{Fe}}_2{\mathrm{O}}_3$. It is argued that this large value of $M_s$, the small value of coercivity $H_c\simeq 20\mathrm{Oe}$ at $5\mathrm{K}$, 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 $(\simeq 400\mathrm{Oe})$ of the two lines in nearly defect-free nanocrystals facilitated their observation.

• Received 19 April 2004

DOI:https://doi.org/10.1103/PhysRevB.70.174428