Q. Y. Dong

A phenomenological fitting curve for the magnetocaloric effect of materials with a second-order phase transition

The magnetic entropy change of polycrystalline samples Gd, La(Fe0.92Co0.08)11.83Al1.17, LaFe10.8Si2.2, Mn5Ge2.7Ga0.3, Nd2AlFe13Mn3, and TbCo2 with a second-order phase transition has been investigated. A uniform phenomenological function that describes the magnetic entropy change is found for these materials. This could be of great benefit for the design of magnetic refrigerators. The field dependence of the critical exponent for the variation in the maximum entropy change with field is studied. The critical exponent value of 2/3, which is predicted by the mean field theory, is only satisfied for moderate field values. The refrigerant capacity is analyzed and compared to the predictions of the fitting function.

Giant reversible magnetocaloric effect in metamagnetic HoCuSi compound

The magnetic properties and magnetocaloric effect (MCE) of antiferromagnetic HoCuSi compound have been studied. It is found that HoCuSi undergoes a field-induced first order metamagnetic transition from antiferromagnetic (AFM) to ferromagnetic (FM) states below the Neel temperature (TN). A giant MCE without hysteresis loss is observed in HoCuSi around TN. The maximal magnetic entropy change (−ΔSM) and refrigerant capacity are 33.1 J/kgK and 385 J/kg, respectively, for a field change of 0–5 T. The excellent magnetocaloric properties can result from the field-induced AFM-FM transition below TN and the increase in magnetization change caused by the change in lattice volume at TN.

Large reversible magnetocaloric effect caused by two successive magnetic transitions in ErGa compound

Intermetallic compound ErGa exhibits two successive magnetic transitions: spin-reorientation transition at TSR=15 K and ferromagnetic-paramagnetic transition at TC=30 K. Both transitions contribute greatly to the magnetic entropy change (ΔSM), each yielding a significant peak on their ΔSM-T curve and thus a considerable value of refrigerant capacity (RC) without hysteresis loss. For a magnetic field change of 5 T, the maximal values of −ΔSM are 21.3 J/kg K at TC and 16.5 J/kg K at TSR, with an RC value of 494 J/kg. Large reversible magnetocaloric effect and RC indicate the potentiality of ErGa as a candidate magnetic refrigerant at low temperatures.

Large magnetic refrigerant capacity in Gd71Fe3Al26 and Gd65Fe20Al15 amorphous alloys

Magnetic entropy change and refrigerant capacity of Gd-based amorphous Gd71Fe3Al26 and Gd65Fe20Al15 alloys are investigated. The refrigerant capacities reach 750 and 726 J kg−1 for Gd71Fe3Al26 and Gd65Fe20Al15, respectively, which are much larger than those of all magnetocaloric materials ever reported. The peak values of magnetic entropy change under a field change of 0–5 T are 7.4 J kg−1 K−1 at 117.5 K and 5.8 J kg−1 K−1 at 182.5 K for Gd71Fe3Al26 and Gd65Fe20Al15, respectively. A very large refrigerant capacity and a considerable magnetic entropy change jointly make them attractive candidates for magnetic refrigerant.

Magnetocaloric effects in the La(Fe,Si)(13) intermetallics doped by different elements

The magnetocaloric effects (MCEs) of LaFe13−xSix compounds doped by magnetic rare earths (R=Ce, Pr, and Nd) and transition metal (Co) are analyzed. It is found that varying the contents of R and Fe produces similar effects on the MCE, both of which cause a rapid decrease in ΔS with the increase in TC. The ΔS−TC relations thus obtained coincide with each other fairly well, which indicates the equivalence of substituting R for La and Fe for Si. In contrast, partially replacing Fe by Co leads to a slow decrease in ΔS with TC. It is therefore a promising approach to maintain a large ΔS up to high temperatures. As a comparison with these element-doping compounds, the MCEs of hydrides are also discussed. Although interstitial hydrogen can also keep up a large ΔS to high temperatures, the corresponding hydrides are unfortunately unstable above 150 °C. Based on these analyses, the potential refrigerants made of LaFe13−xSix are proposed to have as low as possible Si content (or high R content) and proper Co conten...

Large reversible magnetocaloric effect in DyCuAl compound

Large reversible magnetocaloric effect, which is associated with a second-order magnetic transition at 28 K, has been observed in DyCuAl compound. The maximum values of magnetic entropy change −ΔSM and adiabatic temperature change ΔTad are 20.4 J kg−1 K−1 and 7.7 K for a field change of 0–5 T, respectively. Especially, the large values of −ΔSM (10.9 J kg−1 K−1) and ΔTad (3.6 K) with a considerable refrigerant capacity value of 150 J kg−1 are also obtained for a relatively low field change of 0–2 T, suggesting that DyCuAl compound could be considered as a good candidate for low-temperature magnetic refrigerant.

Magnetic entropy change and refrigerant capacity in GdFeAl compound

Magnetic properties and magnetocaloric effect of GdFeAl compound have been investigated. The small saturated magnetization of GdFeAl compound is caused by the antiferromagnetic coupling between the magnetic moments of Gd and Fe atoms. A second-order magnetic phase transition is confirmed around 265K. The maximum magnetic entropy change for GdFeAl compound is 3.7Jkg−1K−1 under the field change of 0–5T. However, a large refrigerant capacity of 420Jkg−1 is obtained, which is due to the large full width at half peak of the magnetic entropy change versus temperature curve in GdFeAl compound.

Large magnetocaloric effect with a wide working temperature span in the R2CoGa3 (R = Gd, Dy, and Ho) compounds

We investigate magnetic properties and magnetocaloric effects of R2CoGa3 (R = Gd, Dy, and Ho) compounds. It is found that all the compounds are ferromagnetic with the Curie temperatures of TC = 50, 17, and 10 K for R = Gd, Dy, and Ho, respectively. The R2CoGa3 have large magnetic entropy change (ΔS) that arise from the second-order ferromagnetic-to-paramagnetic phase transition. The maximum values of ΔS are found to be −12.6, −10.8, and −13.8 J/kg K with corresponding refrigerant capacity values of 382, 252, and 287 J/kg for a magnetic field change of 0–50 kOe, respectively. The large ΔS values with little or no hysteresis losses as well as wide working temperature spans imply that the R2CoGa3 compounds may serve as promising candidates for magnetic refrigeration.

Magnetic phase transition and magnetocaloric effect in Dy12Co7 compound

Magnetic and magnetocaloric properties of Dy12Co7 compound have been investigated by magnetization measurements. Its magnetization does not reach saturation even for 7 T at 2 K due to the crystalline field effect. Dy12Co7 undergoes a ferromagnetic-paramagnetic phase transition around Curie temperature TC = 64 K. The thermomagnetic irreversibility between the zero-field-cooling and field-cooling curves is detected below TC in low magnetic field, and it is attributed to the narrow domain wall pinning effect. Large magnetic entropy change of 10.0 J kg−1 K−1 and refrigerant capacity of 299 J kg−1 for a magnetic field change of 0–5 T are found around TC, resulting from the large change of magnetization during the magnetic phase transition. The nature of second-order phase transition for Dy12Co7 induces the complete reversibility of magnetic entropy change around TC, which is very favourable for the application of magnetic refrigeration.

Magnetic coupling between rare-earth and iron atoms in the La1-xRxFe11.5Si1.5 (R=Ce, Pr, and Nd) intermetallics

A systematic investigation on the effect of R doping, particularly the magnetic coupling between R and Fe, has been performed for the La1−xRxFe11.5Si1.5 intermetallics (R=Ce, Pr, and Nd). A magnetic interaction comparable to that between Fe atoms is found between the R and Fe atoms, which causes an enhancement of the Curie temperature up to ∼11% when ∼30% of the La atoms are replaced by R. The R–Fe coupling is further found to be strongly dependent of the species of the rare earths. It monotonically grows as R sweeps from Ce to Nd. This could be a consequence of the lanthanide contraction, which causes an enhancement of the intra-atomic magnetic coupling.