Search Results (6)

We investigated the effects of chemical and physical changes on the interplay between the Néel and Brown superspin relaxation mechanisms in ferrofluids containing 18 nm-diameter Co_0.2Fe_2.8O₄ magnetic nanoparticles. We attempted to tune the ferrofluid’s magnetization dynamics via three methods: (i) changing the carrier fluid from Isopar M to kerosene (ii) doubling the Co-doping level from x = 0.2 to x = 0.4, and (iii) diluting the Co_0.2Fe_2.8O₄/Isopar M nanomagnetic fluid from δ = 1 mg/mL to δ = 0.1 mg/mL. We used temperature-resolved ac-susceptibility measurements at different frequencies, χ″ vs. T|_f, to gain insight into the thermally driven superspin dynamics of the nanoparticles within the ferrofluid. Our data demonstrates that both increasing x and using a different carrier fluid quantitatively alter the temperature dependence of the Néel and Brown relaxation frequency (f_N vs. T and f_B vs. T) by changing the nanoparticles’ magnetic moments and the fluid’s viscosity. Yet, the two mechanisms remain decoupled, as indicated by the presence of two magnetic events (peaks in the χ″ vs. T|_f datasets) one corresponding to the Néel and the other to Brown relaxation. On the other hand, diluting the ferrofluid leads to a qualitative change in the collective superspin dynamics behavior. Indeed, there is just one χ″-peak in the data from the δ = 0.1 mg/mL nanofluid, and its f vs. T dependence is well-described by a model that includes coupled contributions from both the Néel and Brown relaxation: $\mathrm{f}\left(\mathrm{T}\right)=\mathrm{p}·{\displaystyle \frac{T}{{\gamma }_0}}·\mathrm{e}\mathrm{x}\mathrm{p}\left[-{\displaystyle \frac{{\mathrm{E}}^{\prime }}{{\mathrm{k}}_{\mathrm{B}}\left(\mathrm{T}-{\mathrm{T}}_0^{\prime }\right)}}\right]+$ (1 − p) ${\mathrm{f}}_0\mathrm{e}\mathrm{x}\mathrm{p}\left[-{\displaystyle \frac{{\mathrm{E}}_{\mathrm{B}}}{{\mathrm{k}}_{\mathrm{B}}\left(\mathrm{T}-{\mathrm{T}}_0\right)}}\right]$. This is a remarkable behavior that demonstrates the ability to control a ferrofluids magnetization dynamics through simple chemical and physical changes. Full article

We use Co doping to alter the magnetic relaxation dynamics in superparamagnetic nanofluids made of 18 nm average diameter Fe₃O₄ nanoparticles immersed in Isopar M. Ac-susceptibility data recorded at different frequencies and temperatures, χ″vs. T|_f, reveals a major (~100 K) increase in the superspin blocking temperature of the Co_0.2Fe_2.8O₄-based fluid (CFO) compared to its Fe₃O₄ counterpart (FO). We ascribe this behavior to the strengthening of the interparticle magnetic dipole interactions upon Co doping, as demonstrated by the relative χ″-peak temperature variation per frequency decade $\mathsf{\Phi }={\displaystyle \frac{∆\mathrm{T}}{\mathrm{T}·∆\mathrm{l}\mathrm{o}\mathrm{g}\left(\mathrm{f}\right)}}$, which decreases from Φ~0.15 in FO to Φ~0.025 in CFO. In addition, χ″vs. T|_f datasets from the CFO fluid reveal two magnetic events at temperatures T_p1 = 240 K and T_p2 = 275 K, both above the fluid’s freezing point (T_F = 197 K). We demonstrate that the physical rotation of the nanoparticles within the fluid, the Brown mechanism, is entirely responsible for the collective superspin relaxation observed at T_p1, whereas the Néel mechanism, the superspin flip across an energy barrier within the particle, is dominant at T_p2. We confirm this finding through fits of models that describe the temperature dependence of the relaxation time via the two mechanisms: ${\mathsf{\tau }}_{\mathrm{B}}(\mathrm{T})={\displaystyle \frac{3{\mathsf{\eta }}_0{\mathrm{V}}_{\mathrm{H}}}{{\mathrm{k}}_{\mathrm{B}}\mathrm{T}}}\exp \left[{\displaystyle \frac{{\mathrm{E}}^{\prime }}{{\mathrm{k}}_{\mathrm{B}}\left(\mathrm{T}-{{\mathrm{T}}_0}^{\prime }\right)}}\right]$ and ${\mathsf{\tau }}_{\mathrm{N}}\left(\mathrm{T}\right)={\mathsf{\tau }}_0\exp \left[{\displaystyle \frac{{\mathrm{E}}_{\mathrm{B}}}{{\mathrm{k}}_{\mathrm{B}}\left(\mathrm{T}-{\mathrm{T}}_0\right)}}\right]$. The best fits yield ${\mathsf{\gamma }}_0={\displaystyle \frac{3{\mathsf{\eta }}_0{\mathrm{V}}_{\mathrm{H}}}{{\mathrm{k}}_{\mathrm{B}}}}$= 1.5 × 10⁻⁸ s·K, E′/k_B = 7 03 K, and T₀′ = 201 K for the Brown relaxation, and E_B/k_B = 2818 K and T₀ = 143 K for the Néel relaxation. Full article

We used magnetic and synchrotron X-ray diffraction measurements to investigate the possibility of tuning the strength of magnetic interparticle interactions in nanoparticle ensembles via chemical manipulation. Our main result comes from temperature-resolved in-phase ac-susceptibility data collected on 8 nm average-diameter Ni_0.25Zn_0.75Fe₂O₄ (Ni25) and Ni_0.5Zn_0.5Fe₂O₄ (Ni50) nanoparticles at different frequencies, χ′ vs. T|_f. We found that the relative peak temperature variation per frequency decade, $\mathsf{\varphi }=\frac{∆\mathrm{T}}{\mathrm{T}·∆\mathrm{l}\mathrm{o}\mathrm{g}\left(\mathrm{f}\right)}$—a known measure of interparticle interaction strength—exhibits a four-fold increase, from ϕ = 0.04 in Ni50 to ϕ = 0.16 in Ni25. This corresponds to a fundamental change in the nanoparticles’ superspin dynamics, as proven by the fit of phenomenological models to magnetic relaxation data. Indeed, the Ni25 ensemble exhibits superparamagnetic behavior, where the temperature dependence of the superspin relaxation time, τ, is described in the Dorman–Bessais–Fiorani (DBF) model: $\tau \left(\mathrm{T}\right)={\tau }_{\mathrm{r}}\exp \left(\frac{{\mathrm{E}}_{\mathrm{B}}+{\mathrm{E}}_{\mathrm{a}\mathrm{d}}}{{\mathrm{k}}_{\mathrm{B}}\mathrm{T}}\right),$ with parameters τ_r = 4 × 10⁻¹² s, and (E_B + E_ad)/k_B = 1473 K. On the other hand, the nanoparticles in the Ni50 ensemble freeze collectively upon cooling in a spin-glass fashion according to a critical dynamics law: $\tau (\mathrm{T})=\frac{{\tau }_0}{{\left[\frac{\mathrm{T}}{{\mathrm{T}}_{\mathrm{g}}}-1\right]}^{\mathrm{z}\mathsf{\nu }}}$, with τ₀ = 4 × 10⁻⁸ s, T_g = 145 K, and zν = 7.2. Rietveld refinements against powder X-ray diffraction data reveal the structural details that underlie the observed magnetic behavior: an indirect cation replacement mechanism by which non-magnetic Zn ions are incorporated in the tetrahedral sites of the inverse spinel. Full article

We used ac-susceptibility to measure the blocking temperature, T_B, and energy barrier to the magnetization reversal, E_B, of nanomagnetic fluids of different concentrations, c. We collected data on five samples synthesized by dispersing Fe₃O₄ nanoparticles of average diameter ⟨D⟩ = 8 nm in different volumes of carrier fluid (hexane). We found that T_B increases with the increase in c, a behavior predicted by the Dormann–Bessais–Fiorani (DBF) theory. In addition, our observed T_B vs. c dependence is excellently described by a power law T_B = A∙c^γ, with A = 64 K and γ = 0.056. Our data also show that a Néel–Brown activation law $\mathrm{\tau }\left(\mathrm{T}\right)={\mathrm{\tau }}_0\exp \left(\frac{{\mathrm{E}}_{\mathrm{B}}}{{\mathrm{k}}_{\mathrm{B}}\mathrm{T}}\right)$ describes the superspin dynamics in the most diluted sample, whereas an additional energy barrier term, E_ad, is needed at higher concentrations, according to the DBF model: $\mathrm{\tau }\left(\mathrm{T}\right)={\mathrm{\tau }}_{\mathrm{r}}\exp \left(\frac{{\mathrm{E}}_{\mathrm{B}}+{\mathrm{E}}_{\mathrm{a}\mathrm{d}}}{{\mathrm{k}}_{\mathrm{B}}\mathrm{T}}\right).$ We found E_B/k_B = 366 K and additional energy barriers E_ad/k_B that increase linearly with the common logarithm of the volume concentration, from 138 K at c = 8.3 × 10⁻⁴% to 745 K at c = 4 × 10⁻²%. These results add to our understanding of the contributions by different factors to the superspin dynamics. In addition, the quantitative relations that we established between the T_B, E_ad, and c support the current efforts towards the rational design of functional nanomaterials. Full article

We used dc magnetization and ac susceptibility to investigate the magnetic relaxation of ferrofluids made of 8 nm average-diameter Fe₃O₄ nanoparticles dispersed in hexane. Samples of different concentrations (δ) spanning two orders of magnitude ranging from 0.66 to 0.005 mg (Fe₃O₄)/mL (hexane) were used to vary the interparticle interaction strength. Our data reveal a critical concentration, δ_c = 0.02 mg/mL, below which the ferrofluid behaves like an ideal nanoparticle ensemble where the superspins relax individually according to a Néel–Brown activation law $\mathsf{\tau }(\mathrm{T})={\mathsf{\tau }}_0\exp \left(\frac{{\mathrm{E}}_{\mathrm{B}}}{{\mathrm{k}}_{\mathrm{B}}\mathrm{T}}\right)$ with a characteristic time τ_o ~10⁻⁹ s. That is further confirmed by the observed invariance of the relative peak temperature variation per frequency decade $∆=\frac{∆\mathrm{T}}{\mathrm{T}·∆\mathrm{l}\mathrm{o}\mathrm{g}\left(\mathrm{f}\right)}$, which stays constant at ~0.185 when δ < δ_c. At higher concentrations, between 0.02 and 0.66 mg/mL, we found that Δ exhibits a monotonic increase with the inverse concentration, $\frac{1}{\delta }$, and the collective superspin dynamics is described by a Vogel–Fulcher law, $\mathsf{\tau }(\mathrm{T})={\mathsf{\tau }}_0\exp \left[\frac{{\mathrm{E}}_{\mathrm{B}}}{{\mathrm{k}}_{\mathrm{B}}\left(\mathrm{T}-{\mathrm{T}}_0\right)}\right]$. Within this regime, the dipolar interaction strength parameter T₀ increases from T₀ = 0 K at δ_c = 0.02 mg/mL to T₀ = 14.7 K at δ = 0.66 mg/mL. Full article

This paper reports the influence of dysprosium ion (Dy³⁺) substitution on the structural and magnetic properties of NiDy_xFe_2−xO₄ (0.0 ≤ x ≤ 0.1) nanoparticles (NPs) prepared using a hydrothermal method. The structure and morphology of the as-synthesized NPs were characterized via X-ray diffraction (XRD), scanning and transmission electron microscope (SEM, and TEM) analyses. ⁵⁷Fe Mössbauer spectra were recorded to determine the Dy³⁺ content dependent variation in the line width, isomer shift, quadrupole splitting, and hyperfine magnetic fields. Furthermore, the magnetic properties of the prepared NPs were also investigated by zero-field cooled (ZFC) and field cooled (FC) magnetizations and AC susceptibility measurements. The M_ZFC (T) results showed a blocking temperature (T_B). Below T_B, the products behave as ferromagnetic (FM) and act superparamagnetic (SPM) above T_B. The M_FC (T) curves indicated the existence of super-spin glass (SSG) behavior below T_s (spin-glass freezing temperature). The AC susceptibility measurements confirmed the existence of the two transition temperatures (i.e., T_B and T_s). Numerous models, e.g., Neel–Arrhenius (N–A), Vogel–Fulcher (V–F), and critical slowing down (CSD), were used to investigate the dynamics of the systems. It was found that the Dy substitution enhanced the magnetic interactions. Full article