{"title":"Magnetic irreversibility and magnetization processes in Ni<sub>5</sub>Al<sub>3</sub>/NiO core/shell nanoparticle system.","authors":"P V Prakash Madduri, S N Kaul","doi":"10.1088/1361-648X/ad69ee","DOIUrl":null,"url":null,"abstract":"<p><p>This work brings out many interesting facets of magnetism in the Ni<sub>5</sub>Al<sub>3</sub>/NiO core/shell nanoparticle system. The<i>weak</i>and<i>strong</i>magnetic irreversibility lines (TWI(H)andTSI(H)) reproduce the previously reported<i>H</i> - <i>T</i>phase diagram at fieldsH⩽30 Oe, but strong departures occur for<i>H</i> > 30 Oe. Comparison with the theoretically predicted<i>H</i> - <i>T</i>phase diagram allows us to identify<i>T</i><sub><i>WI</i></sub>withTCG+SG, where the paramagnetic (PM)-chiral glass (<i>CG</i>) and PM-spin glass (<i>SG</i>) phase transitions occur<i>simultaneously</i>, and<i>T</i><sub><i>SI</i></sub>with<i>T</i><sub><i>SG</i></sub>, the temperature at which transition to the replica symmetry breaking<i>SG</i>state takes place. TheTSI(H)transition line abruptly ends at the point (H≃30 Oe,T≃90K). As<i>H</i>exceeds 30 Oe, a new transition appears which gets completely suppressed at fieldsH>1 kOewhere the magnetic irreversibility ceases to exist. No<i>intrinsic</i>long-range ferromagnetic ordering exists but fields as low as 3 kOe suffice to induce long-range ferromagnetic order. At fixed temperatures, the magnetocrystalline anisotropy fluctuations essentially govern the 'approach-to-saturation' in magnetization for fields in the range 3 - 70 kOe. The present nanocrystalline system behaves as an isotropic system with random easy axis in which the magnetization reversal occurs through the coherent rotation of the magnetizations of weakly-interacting single-domain Ni<sub>5</sub>Al<sub>3</sub>particles. Saturation magnetization, like<i>M</i>(<i>T</i>) atH⩾2 kOe, exhibits an anomalous upturn at temperatures below ≈ 30 K. This upturn is associated with the anomalous softening of spin-wave modes which results in the thermal excitation of a large number of non-equilibrium (finite lifetime) magnons. At sub-Kelvin temperatures, these magnons undergo Bose-Einstein condensation.</p>","PeriodicalId":16776,"journal":{"name":"Journal of Physics: Condensed Matter","volume":null,"pages":null},"PeriodicalIF":2.3000,"publicationDate":"2024-08-14","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":"0","resultStr":null,"platform":"Semanticscholar","paperid":null,"PeriodicalName":"Journal of Physics: Condensed Matter","FirstCategoryId":"101","ListUrlMain":"https://doi.org/10.1088/1361-648X/ad69ee","RegionNum":4,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":null,"EPubDate":"","PubModel":"","JCR":"Q3","JCRName":"PHYSICS, CONDENSED MATTER","Score":null,"Total":0}
引用次数: 0
Abstract
This work brings out many interesting facets of magnetism in the Ni5Al3/NiO core/shell nanoparticle system. Theweakandstrongmagnetic irreversibility lines (TWI(H)andTSI(H)) reproduce the previously reportedH - Tphase diagram at fieldsH⩽30 Oe, but strong departures occur forH > 30 Oe. Comparison with the theoretically predictedH - Tphase diagram allows us to identifyTWIwithTCG+SG, where the paramagnetic (PM)-chiral glass (CG) and PM-spin glass (SG) phase transitions occursimultaneously, andTSIwithTSG, the temperature at which transition to the replica symmetry breakingSGstate takes place. TheTSI(H)transition line abruptly ends at the point (H≃30 Oe,T≃90K). AsHexceeds 30 Oe, a new transition appears which gets completely suppressed at fieldsH>1 kOewhere the magnetic irreversibility ceases to exist. Nointrinsiclong-range ferromagnetic ordering exists but fields as low as 3 kOe suffice to induce long-range ferromagnetic order. At fixed temperatures, the magnetocrystalline anisotropy fluctuations essentially govern the 'approach-to-saturation' in magnetization for fields in the range 3 - 70 kOe. The present nanocrystalline system behaves as an isotropic system with random easy axis in which the magnetization reversal occurs through the coherent rotation of the magnetizations of weakly-interacting single-domain Ni5Al3particles. Saturation magnetization, likeM(T) atH⩾2 kOe, exhibits an anomalous upturn at temperatures below ≈ 30 K. This upturn is associated with the anomalous softening of spin-wave modes which results in the thermal excitation of a large number of non-equilibrium (finite lifetime) magnons. At sub-Kelvin temperatures, these magnons undergo Bose-Einstein condensation.
期刊介绍:
Journal of Physics: Condensed Matter covers the whole of condensed matter physics including soft condensed matter and nanostructures. Papers may report experimental, theoretical and simulation studies. Note that papers must contain fundamental condensed matter science: papers reporting methods of materials preparation or properties of materials without novel condensed matter content will not be accepted.