An exchange-coupled FePt/ Fe 3 Pt nanocomposite system with well defined hard/soft two-phase morphology was prepared by reducing chemically synthesized FePt/ Fe 3 O 4 bimagnetic nanoparticles. It was interesting to observe that the recoil loop openness is strongly dependent on the sweep rate of applied fields. A quantitative analysis shows a linear relation between the openness of recoil loops and the reciprocal activation volume, indicating that the recoil loops are intimately related to thermal fluctuation. The large open area of the recoil loops for nanocomposite magnets compared to that of single phase magnets is attributed to the unstable magnetic moments in the soft phase.
It has been well known that recoil loop openness is related to soft-phase presence in exchange-coupled hard-soft nanocomposite magnets. Our study on recoil loop openness of exchange-coupled nanocrystalline magnets ͑both single-phase and composite͒ using a micromagnetic finite-element method has revealed that the recoil loop openness is also due to decreased grain size. Open recoil loops exist in single-phase magnets as well. Simulation of magnetization distribution in both nanocrystalline single-phase magnets and nanocomposite magnets shows that the openness of the recoil loops is correlated with unstable magnetization behavior in grain boundary and soft-phase regions, which is attributed to high energy state caused by exchange coupling in these regions. The simulation results are supported by experimental data. © 2009 American Institute of Physics. ͓DOI: 10.1063/1.3127226͔Magnetically hard/soft exchange-coupled nanocomposites show enhanced magnetic properties due to intergrain exchange interactions. 1-3 Strength of the intergranular exchange coupling in these types of materials can be evaluated by measuring the remanence ratio or recoil loops. Analysis of recoil loops can provide deep physical insight into magnetization reversal behavior in nanocomposite magnets. [4][5][6][7][8][9][10][11][12][13] However, physical origin of the open recoil loops in nanocrystalline magnets is still not well understood. [11][12][13] It is commonly accepted that the open recoil loop phenomena were resulted from exchange decoupled soft phase in a nanocomposite magnet. [4][5][6][7][8][9][10] Recently, the recoil loop openness was found to be related to the inhomogeneity in the magnetic anisotropy, 11 unstable magnetic moments affected by thermal fluctuation, 12 and variations of exchange interactions in nanocomposite magnets. 13 It seems that the origin of open recoil loops in nanocrystalline magnets is more complex than what was previously thought and needs to be further studied.Micromagnetic simulation is an effective method to study magnetization behaviors inside nanostructured magnets since it gives a clear physics explanation on mechanisms of magnetization distribution and reversal. [14][15][16][17][18][19] In this work, we extend our study from nanocomposite magnets to singlephase magnets as well, to investigate the origin of recoil loop openness in exchange-coupled nanocrystalline magnets by using a micromagnetic finite-element method. It is revealed that not only the soft phases but also grain boundaries contribute to openness of the recoil loops.The equilibrium magnetization distribution under external magnetic field is calculated by the minimization of the total magnetic Gibbs free energy. It should be mentioned here that a magnetic vector potential was introduced to calculate the upper bound of the stray field energy, which was suggested by Brown. 20 The resulting open-boundary problem can be treated by a parallelepipedic shell transformation, which transforms the infinite exterior space of the magnet on a shell around t...
The correlation between the recoil loop openness and the activation volume has been studied for the SmCo5/α-Fe nanocomposite powder material prepared by mechanical alloying. The α-Fe phase content in the nanocomposite powders was adjusted by varying the amount of α-Fe addition in raw materials and the postannealing temperature after mechanical alloying. It was found that the recoil loop openness increases with increasing α-Fe phase content. More interestingly, there is a linear relation between the openness of recoil loops and the reciprocal activation volume, indicating that the recoil loops are related to thermal fluctuation. The large open area of the recoil loops for nanocomposite magnets is attributed to the fact that low anisotropy in the α-Fe phase leads to unstable magnetization under thermal fluctuation.
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