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Subjects: Physics >> Condensed Matter: Electronic Structure, Electrical, Magnetic, and Optical Properties

Cubic helimagnet FeGe has emerged as a class of skyrmion materials near room temperature that may impact future information technology. Experimentally identifying the detailed properties of skyrmion materials enables their practical application acceleratedly. Here we study the magnetic entropy change (MEC) of single crystalline FeGe in its precursor region and clarify its close relation to the critical exponents of a second-order phase transition in this area. The maximum MEC is found to be 2.86 J/kg.K for 7.0 T magnetic field change smaller than that of common magnetocaloric materials indicating the multiplicity and complexity of the magnetic structure phases in the precursor region. This result also implies that the competition among the multimagnetic phases can partly counteract the magnetic field driven force and establishes a stable balance. Based on the obtained MEC and the critical exponents, the exact Curie temperature of single crystalline FeGe under zero magnetic field is confirmed to be 279.1 K, higher than previously reported 278.2 K. This finding pave the way for reconstruction of FeGe phase diagram in the precursor region. |

submitted time
2017-11-27
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Subjects: Physics >> Condensed Matter: Electronic Structure, Electrical, Magnetic, and Optical Properties

The magnetic entropy change [ΔSM(T;H)] around the phase transition temperature TC is investigated by the scaling method for Fe0:5Co0:5Si, which exhibits a skyrmion phase below TC. The parameters of ΔSM(T;H) exhibit field dependent behaviors. The ΔSM(T;H) curves under high field can be well scaled into a single universal curve independent of external field and temperature. However, ΔSM(T;H) curves under low field become divergent just below TC, which indicates a characteristic of first-order transition. The scaling investigation of ΔSM(T;H) curves indicates that the phase transition in Fe0:5Co0:5Si is of a weak first-order type in low field region, while it is driven into a second-order one under high field. This weak first-order phase transition in low field region resembles that in typical skyrmion system MnSi which is caused by the critical fluctuation. The result suggests that critical fluctuation plays an important role in the phase transition and formation of skyrmion state. |

submitted time
2017-11-27
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Subjects: Physics >> Condensed Matter: Electronic Structure, Electrical, Magnetic, and Optical Properties

Critical phenomenon of the noncentrosymmetric Cr11Ge19, which exhibits an itinerant ferromagnetic ground state, is investigated by scaling of the magnetic entropy change [ΔSM(T;H)]. It is found that parameters #14;FWHM (the full width at half maximum), ?ΔSmax M (the maximum of the magnetic entropy change), and RCP (the relative cooling power) of ΔSM(T) are governed by the power law of critical exponents. With the critical exponents, ΔSM(T;H) curves are scaled into a universal curve independent of temperature and field, which suggests that the magnetic transition is of a second order type. The universal collapse of ΔSM(T;H) indicates that the critical behavior of Cr11Ge19 can be well described by the scaling laws for the critical phenomenon. Moreover, the ΔSM follows the power law of Hn with n(T;H) = dlnjΔSMj=dln(H). The temperature dependence of n values reach minimum at #24; 71.5 K. Based on the magnetic specific change ΔCp(T;H), the actual magnetic transition temperature is strictly determined as TC = 71:3 #6; 0:2 K for the single crystal Cr11Ge19. |

submitted time
2017-11-27
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Scaling investigation of the magnetic entropy change in helimagnet MnSi

张蕾Subjects: Physics >> Condensed Matter: Electronic Structure, Electrical, Magnetic, and Optical Properties

The B20 compound MnSi exhibits a skyrmion phase in a certain range of temperature (T) and external magnetic ?eld (H) in addition to the helimagnetic ordering. In this work, the magnetic entropy change (?SM) in the single crystal helimagnet MnSi has been investigated by the scaling analysis. The ?SM(T;H) curves in higher ?eld range (2 kOe < H < 5.6 kOe) collapse onto an universal curve after scaling, which is independent of T and H. Based on this universal curve, the ?SM(T;H) (excluding the skyrmion entropy change) at any T and H can be calculated. On the other hand, a small valley is found on the ?SM(T) curve at ? 28.5 K in the ?eld range from ? 1 kOe to ? 2 kOe, which is resulted from the skyrmion phase. Subsequently, the magnetic entropy change of the skyrmion phase [?SskX M (T;H)] can be derived by subtraction of the calculated ?Scal M (T;H) from the experimental ?Sexp M (T;H). The obtained results show that ?SskX M reaches the maximum at ? 1.6 kOe, which indicates the highest number of skyrmion vortices at that ?eld. |

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