Uncompensated Surface Spins Research Articles

Superstructures of self-assembled cobalt nanocrystals

Uniform three-dimensional superstructures of spherical cobalt nanocrystals are produced by the interplay between dipolar interaction and applied magnetic field. An anomalous low-temperature magnetic behavior is observed, indicating that uncompensated surface spins become ordered below 10 K, as evidenced by the presence of two magnetic phases that superimpose in hysteresis loops as compared to measurements at 20 K. The approach discussed here provides a framework for applications such as high-performance mesomagnets, microelectronic and magnetic devices fabrication, and can be extended to other nanocomposite materials fabrication if cobalt particles can act as carriers for other nanoparticles.

Magnetic properties of mechanically milled nanosized cupric oxide

Detailed magnetic measurements have been carried out in order to understand the effects of ball-mill-induced defects on the magnetic properties of cupric oxide nanoparticles. A progressive decrease in the crystallite size and a concomitant increase in the induced strain have been observed with the milling times. The field-cooled (FC) and zero-field-cooled (ZFC) measurements indicate a presence of an exchange bias field, whose value increases with the increase in the milling time, reaching a maximum value of 750 Oe at the 50 h milling time. However, the ZFC coercivity first increases up to the 30 h milling time and then decreases upon prolonged milling. The temperature-dependent magnetization measurements indicate the presence of a hysteresis loop even at room temperature. The presence of an exchange bias field is indicative of the presence of an exchange interaction between the ferromagnetic surface, arising from uncompensated moments generated by mechanical strain, and the antiferromagnetic (AFM) core. The exchange bias field reduces to zero at about the Néel temperature of the AFM core of the nanoparticles. The Néel temperature of the AFM core is milling time dependent; for example, the 50 h milled sample shows a higher Néel temperature ( T N∼100 K) than the 30 h ( T N∼30 K) milled sample. The presence of weak ferromagnetism and an exchange bias field is attributed to the uncompensated surface spins resulting from induced defects via mechanical milling.

Hysteresis anomalies and exchange bias in 6.6 nm CuO nanoparticles

In the 6.6 nm particles of CuO with Néel temperature TN≃40 K, hysteresis loops are observed up to 330 K. With decrease in temperature, the coercivity Hc increases slowly up to TN below which rapid increases in Hc and exchange bias He are observed. At 5 K, the ratio He/Hc≃2, decreasing to zero as T→TN. At 10 K, the variation of He, Hc, and the ratio He/Hc with the cooling field show behavior similar to that of the weak ferromagnetic moment resulting from the uncompensated surface spins.

A microsatellite polymorphism in the CRP gene is associated with susceptibility to invasive pneumococcal disease

Detailed magnetic measurements have been carried out in order to understand the effects of ball-mill-induced defects on the magnetic properties of cupric oxide nanoparticles. A progressive decrease in the crystallite size and a concomitant increase in the induced strain have been observed with the milling times. The field-cooled (FC) and zero-field-cooled (ZFC) measurements indicate a presence of an exchange bias field, whose value increases with the increase in the milling time, reaching a maximum value of 750 Oe at the 50 h milling time. However, the ZFC coercivity first increases up to the 30 h milling time and then decreases upon prolonged milling. The temperature-dependent magnetization measurements indicate the presence of a hysteresis loop even at room temperature. The presence of an exchange bias field is indicative of the presence of an exchange interaction between the ferromagnetic surface, arising from uncompensated moments generated by mechanical strain, and the antiferromagnetic (AFM) core. The exchange bias field reduces to zero at about the Néel temperature of the AFM core of the nanoparticles. The Néel temperature of the AFM core is milling time dependent; for example, the 50 h milled sample shows a higher Néel temperature (TN∼100 K) than the 30 h (TN∼30 K) milled sample. The presence of weak ferromagnetism and an exchange bias field is attributed to the uncompensated surface spins resulting from induced defects via mechanical milling.

Neutron scattering and magnetic studies of ferrihydrite nanoparticles

Magnetic properties of two-line ferrihydrite $(\mathrm{FeOOD}\ensuremath{\cdot}n{\mathrm{D}}_{2}\mathrm{O})$ nanoparticles with an average size \ensuremath{\simeq}4 nm are investigated using neutron scattering and magnetometry. Comparison of the neutron scattering and x-ray diffraction patterns identifies the (002) peak at $Q=1.3{\AA{}}^{\ensuremath{-}1}$ as predominantly magnetic. The intensity of this peak, measured from 10 to 450 K, decreases almost linearly with temperature until 350 K, becoming temperature independent above 350 K. From this, ${T}_{N}\ensuremath{\simeq}350\mathrm{K}$ is identified to be the ordering temperature of the core spins of the nanoparticles. The width of the line is temperature independent, yielding a magnetic coherence $\mathrm{length}\mathrm{\ensuremath{\simeq}}\mathrm{particle}$ size. The temperature variations (5--300 K) of the initial susceptibility \ensuremath{\chi} for the field-cooled (FC) and zero-field-cooled (ZFC) cases yield a peak at ${T}_{p}(m)\ensuremath{\simeq}65\mathrm{K},$ below which $\ensuremath{\chi}(\mathrm{FC})g\ensuremath{\chi}(\mathrm{ZFC}).$ For $Tg{T}_{p}(m),$ the variation of ${\ensuremath{\chi}}^{\ensuremath{-}1}$ vs T is analyzed in terms of the model of El-Hilo et al., involving particle-size distribution and interparticle interactions, and substantial interparticle interactions are inferred. Following the observations in ferritin, the field dependence of the magnetization M for $Tg{T}_{p}(m)$ is analyzed in terms of the modified Langevin variation: ${M=M}_{o}\mathcal{L}({\ensuremath{\mu}}_{p}H/kT)+{\ensuremath{\chi}}_{a}H,$ where ${\ensuremath{\mu}}_{p}$ is the magnetic moment/particle. The fit at 100 K yields ${\ensuremath{\mu}}_{p}\ensuremath{\simeq}250{\ensuremath{\mu}}_{B},$ consistent with the theoretical estimates based on uncompensated surface spins of ${\mathrm{Fe}}^{3+}.$

Magnetic anomalies in NiO nanoparticles

As first noted by Néel, antiferromagnetic nanoparticles could exhibit superparamagnetic relaxation of their spin lattices as well as permanent moments arising from uncompensated surface spins. Several samples of antiferromagnetic NiO nanoparticles with average sizes ranging from 50 to >800 Å were investigated in the present study. In addition to the inverse dependence on average particle size of the susceptibility predicted by Néel, and previously reported, some unusual behavior was observed. Above the blocking temperatures (TB) of the particles, the reversible magnetization could not be fit with a Langevin function that was consistent with the physically reasonable moment representing the uncompensated spins. For the 53 Å diameter particles, both zero-field-cooled (ZFC) and field-cooled (FC) loops below TB exhibit large coercive forces (several kOe) and the loops showed irreversibility up to 50 kOe. In addition, in the FC state below TB the hysteresis loops were strongly shifted. The latter behavior may be due to exchange coupling of the uncompensated spins with the antiferromagnetic core. However, the large coercive forces and high field irreversibility which accompany the shifted loops raise new questions about the nature of the uncompensated spins. The magnetization relaxation of these particles is also discussed.

Magnetic properties and microstructure of Fe-O and Co-O thin films

Iron oxide and cobalt oxide thin films were prepared by reactive DC magnetron sputtering in a mixture of Ar and O/sub 2/ gases. The stoichiometry of the oxide films was studied as a function of the amount of O/sub 2/ during deposition. Sputtering conditions, including dc power and oxygen-to-argon ratio, were found which allowed pure FeO, Fe/sub 3/O/sub 4/, /spl alpha/-Fe/sub 2/O/sub 3/, CoO, and Co/sub 3/O/sub 4/ films to be fabricated. The microstructure of the films, and especially the grain shape and size distribution, were quite different for oxide films with different stoichiometry. The crystallographic axes of Fe-O films were randomly oriented in contrast to Co-O films in which a strong texture was found. A strong ferromagnetic-like behavior was observed in FeO and /spl alpha/-Fe/sub 2/O/sub 3/ films, in contrast to the well-established antiferromagnetic behavior in bulk. The reason for this anomalous behavior is attributed to clusters of defects in FeO films and uncompensated surface spins in /spl alpha/-Fe/sub 2/O/sub 3/ films. A large shift (3800 Oe) was observed in the low temperature hysteresis loop of a field cooled sample consisting of 50% Co and 50% CoO. A similar but less pronounced effect was observed in FeO and /spl alpha/-Fe/sub 2/O/sub 3/ films. This result is believed to be caused by a strong exchange coupling between the ferromagnetic and antiferromagnetic constituents in these composite films.

Stoichiometry and Magnetic Properties of Iron Oxide Films

ABSTRACTThe stoichiometry, structural and magnetic properties, and Mössbauer spectra of reactively sputtered Fe-O films were studied as a function of the O2 partial pressure during the deposition. By increasing the amount of O2 films with the following crystallographic structures and stoichiometry were fabricated; amorphous Fe-O, mixture of Fe and FeO, offstoichiometric single-phase FexO, mixture of FeO and Fe3O4, single-phase Fe3O4, mixture of Fe3O4 and γ-Fe2O3, mixture of Fe3O4 and α-Fe2O3, and single-phase α-Fe2O3. The Verwey transition in Fe3O4 films was observed in the coercivity versus temperature curve and in the thermomagnetic data. FeO and α-Fe2O3 films showed anomalous ferromagnetic-like behavior, in contrast to their anti ferromagnetic nature in bulk. The unusual magnetic properties were attributed to the formation of clusters of tetrahedraly coordinated Fe3+ ions in FeO and to uncompensated surface spins, in α - Fe2O3 films.