Magnetic properties of nearly defect-free maghemite nanocrystals

Abstract

Temperature variations of the magnetization $M$ and the electron magnetic resonance (EMR) parameters of maghemite $(\ensuremath{\gamma}\text{\ensuremath{-}}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3})$ nanocrystals are reported for the $4\phantom{\rule{0.3em}{0ex}}\mathrm{K}--300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ range. Transmission electron microscopy of the nanocrystals shows them to be nearly spherical samples (aspect ratio $a∕b=1.15$) with diameter $D=7(1)\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$, and analysis of x-ray diffraction lines yields $D=6.4\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ with negligible strain. $M$ versus $T$ data show blocking temperatures ${T}_{B}\ensuremath{\simeq}101\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, $89\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, and $68\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ in measuring fields $H=50$, 100, and $200\phantom{\rule{0.3em}{0ex}}\mathrm{Oe}$ respectively. $M$ versus $H$ data for $T>{T}_{B}$ fits the modified Langevin function $M={M}_{s}\mathcal{L}({\ensuremath{\mu}}_{p}H∕{k}_{B}T)+{\ensuremath{\chi}}_{a}H$ with ${\ensuremath{\mu}}_{p}=8000(500){\ensuremath{\mu}}_{B}∕\text{particle}$ and ${M}_{s}=80\phantom{\rule{0.3em}{0ex}}\mathrm{emu}∕\mathrm{g}$, identical to ${M}_{s}$ for bulk $\ensuremath{\gamma}\text{\ensuremath{-}}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$. It is argued that this large value of ${M}_{s}$, the small value of coercivity ${H}_{c}\ensuremath{\simeq}20\phantom{\rule{0.3em}{0ex}}\mathrm{Oe}$ at $5\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, 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 $(\ensuremath{\simeq}400\phantom{\rule{0.3em}{0ex}}\mathrm{Oe})$ of the two lines in nearly defect-free nanocrystals facilitated their observation.