Chemical Manipulation of the Collective Superspin Dynamics in Heat-Generating Superparamagnetic Fluids: An AC-Susceptibility Study
We use Co doping to alter the magnetic relaxation dynamics in superparamagnetic nanofluids made of 18 nm average diameter Fe<sub>3</sub>O<sub>4</sub> nanoparticles immersed in Isopar M. Ac-susceptibility data recorded at different frequencies and temperatures, χ″vs. T|<sub...
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| Main Authors: | , |
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| Format: | Article |
| Language: | English |
| Published: |
MDPI AG
2025-07-01
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| Series: | Crystals |
| Subjects: | |
| Online Access: | https://www.mdpi.com/2073-4352/15/7/631 |
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| Summary: | We use Co doping to alter the magnetic relaxation dynamics in superparamagnetic nanofluids made of 18 nm average diameter Fe<sub>3</sub>O<sub>4</sub> nanoparticles immersed in Isopar M. Ac-susceptibility data recorded at different frequencies and temperatures, χ″vs. T|<sub>f</sub>, reveals a major (~100 K) increase in the superspin blocking temperature of the Co<sub>0.2</sub>Fe<sub>2.8</sub>O<sub>4</sub>-based fluid (CFO) compared to its Fe<sub>3</sub>O<sub>4</sub> 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 <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mi mathvariant="sans-serif">Φ</mi><mo>=</mo><mstyle scriptlevel="0" displaystyle="true"><mfrac><mrow><mo>∆</mo><mi mathvariant="normal">T</mi></mrow><mrow><mi mathvariant="normal">T</mi><mo>·</mo><mo>∆</mo><mi mathvariant="normal">l</mi><mi mathvariant="normal">o</mi><mi mathvariant="normal">g</mi><mo>(</mo><mi mathvariant="normal">f</mi><mo>)</mo></mrow></mfrac></mstyle></mrow></semantics></math></inline-formula>, which decreases from Φ~0.15 in FO to Φ~0.025 in CFO. In addition, χ″vs. T|<sub>f</sub> datasets from the CFO fluid reveal two magnetic events at temperatures T<sub>p1</sub> = 240 K and T<sub>p2</sub> = 275 K, both above the fluid’s freezing point (T<sub>F</sub> = 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<sub>p1</sub>, whereas the Néel mechanism, the superspin flip across an energy barrier within the particle, is dominant at T<sub>p2</sub>. We confirm this finding through fits of models that describe the temperature dependence of the relaxation time via the two mechanisms: <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msub><mrow><mi mathvariant="sans-serif">τ</mi></mrow><mrow><mi mathvariant="normal">B</mi></mrow></msub><mo stretchy="false">(</mo><mi mathvariant="normal">T</mi><mo stretchy="false">)</mo><mo>=</mo><mstyle scriptlevel="0" displaystyle="true"><mfrac><mrow><mn>3</mn><msub><mrow><mi mathvariant="sans-serif">η</mi></mrow><mrow><mn>0</mn></mrow></msub><msub><mrow><mi mathvariant="normal">V</mi></mrow><mrow><mi mathvariant="normal">H</mi></mrow></msub></mrow><mrow><msub><mrow><mi mathvariant="normal">k</mi></mrow><mrow><mi mathvariant="normal">B</mi></mrow></msub><mi mathvariant="normal">T</mi></mrow></mfrac></mstyle><mrow><mrow><mi mathvariant="normal">exp</mi></mrow><mrow><mfenced open="[" close="]" separators="|"><mrow><mstyle scriptlevel="0" displaystyle="true"><mfrac><mrow><msup><mi mathvariant="normal">E</mi><mo>′</mo></msup></mrow><mrow><msub><mrow><mi mathvariant="normal">k</mi></mrow><mrow><mi mathvariant="normal">B</mi></mrow></msub><mfenced separators="|"><mrow><mi mathvariant="normal">T</mi><mo>−</mo><msup><msub><mrow><mi mathvariant="normal">T</mi></mrow><mrow><mn>0</mn></mrow></msub><mo>′</mo></msup></mrow></mfenced></mrow></mfrac></mstyle></mrow></mfenced></mrow></mrow></mrow></semantics></math></inline-formula> and <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msub><mrow><mi mathvariant="sans-serif">τ</mi></mrow><mrow><mi mathvariant="normal">N</mi></mrow></msub><mfenced separators="|"><mrow><mi mathvariant="normal">T</mi></mrow></mfenced><mo>=</mo><msub><mrow><mi mathvariant="sans-serif">τ</mi></mrow><mrow><mn>0</mn></mrow></msub><mrow><mrow><mi mathvariant="normal">exp</mi></mrow><mrow><mfenced open="[" close="]" separators="|"><mrow><mstyle scriptlevel="0" displaystyle="true"><mfrac><mrow><msub><mrow><mi mathvariant="normal">E</mi></mrow><mrow><mi mathvariant="normal">B</mi></mrow></msub></mrow><mrow><msub><mrow><mi mathvariant="normal">k</mi></mrow><mrow><mi mathvariant="normal">B</mi></mrow></msub><mfenced separators="|"><mrow><mi mathvariant="normal">T</mi><mo>−</mo><msub><mrow><mi mathvariant="normal">T</mi></mrow><mrow><mn>0</mn></mrow></msub></mrow></mfenced></mrow></mfrac></mstyle></mrow></mfenced></mrow></mrow></mrow></semantics></math></inline-formula>. The best fits yield <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msub><mrow><mi mathvariant="sans-serif">γ</mi></mrow><mrow><mn>0</mn></mrow></msub><mo>=</mo><mstyle scriptlevel="0" displaystyle="true"><mfrac><mrow><mn>3</mn><msub><mrow><mi mathvariant="sans-serif">η</mi></mrow><mrow><mn>0</mn></mrow></msub><msub><mrow><mi mathvariant="normal">V</mi></mrow><mrow><mi mathvariant="normal">H</mi></mrow></msub></mrow><mrow><msub><mrow><mi mathvariant="normal">k</mi></mrow><mrow><mi mathvariant="normal">B</mi></mrow></msub></mrow></mfrac></mstyle><mtext> </mtext></mrow></semantics></math></inline-formula>= 1.5 × 10<sup>−8</sup> s·K, E′/k<sub>B</sub> = 7 03 K, and T<sub>0</sub>′ = 201 K for the Brown relaxation, and E<sub>B</sub>/k<sub>B</sub> = 2818 K and T<sub>0</sub> = 143 K for the Néel relaxation. |
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| ISSN: | 2073-4352 |