| Magnetic refrigeration is a promising technology with the potential to replace conventional gas compression refrigeration. In recent years, the magnetocaloric materials applied in the magnetic refrigeration technology attracted broad attention.However, these materials usually suffer from their expensiveness, poisonousness and hysteresis in magnetic and thermal cycles arising from the fist-order ferromagnetic transition. For the purpose of finding magnetocaloric materials without those weaknesses, we prepared single crystals of Ca Fe0.7Co0.3O3 and Sr Fe0.8Co0.2O3 and their magnetic properties and magnetocaloric effects were investigated in detail. A“Tow step†high pressure single crystal growing method by combining floating-zone and high-pressure treatment methods was proposed.The preparing conditions of some materials are really harsh, for example, some can only be synthesized under extremely high temperature and high pressure and their single crystal growing is tougher. Due to the sample cell of traditional high pressure and high temperature technic is usually very small, single crystals grown by this method are usually small and it is very difficult for physical properties measurements.A “tow step†high pressure single crystal growing method which was first developed by us in recent years solved this problem successfully. The “tow step†method is applied to the single crystal growing of materials with nonstoichiometric property. In the first step, single crystal of oxygen deficiency single crystal is grown using a floating zone method at ambient pressure. In the second step high pressure and high temperature treatment is used with the presence of KCl O4 oxidant to replenish the deficient oxygen. The as-synthesized single crystals are highly-qualified and with high purity, confirmed by the Laue pattern and the powder XRD measurement. The“two method†high pressure single crystal growing method makes the high pressure single crystal growing comes true.The Ca Fe0.7Co0.3O3 single crystal was grown for the first time by the two-stepmethod and its magnetism and magnetocaloric effect were investigated. This compound experiences a second-order paramagnetism-to-ferromagnetism transition in a wide temperature window between 200 and 150 K due to the presence of multiple ferromagnetic interactions. Since the spin entropy is gradually released above the ferromagnetic Curie temperature(~ 177 K), no sharp l-type anomaly is observed in specific heat. On the basis of magnetization measurements, however, a considerable entropy change is found in this perovskite oxide. More interesting, this compound exhibits a broadening working temperature, and a significant refrigerant capacity(~355 J/kg at 6 T) which is comparable with those found in some giant magnetocaloric alloys is shown up. The present study therefore provides an example on how to enhance the refrigerant capacity by extending the working temperature of magnetocaloric material.A high-quality Sr Fe0.8Co0.2O3 single crystal is prepared by combining floating-zone and high-pressure treatment methods. Its Magnetocaloric effect is investigated by means of magnetic measurements. A ferromagnetism transition is found at about 270 K, and this transition is a second-order one in nature as confirmed by Arrott plots. The saturated moment obtained at 2 K and 7 T is 3.63 μB/f.u. The maximal value of magnetic entropy change measured at 5 T is about 4.0 J/kg K. The working temperatures(applied field, 0-4 T and 0-5 T) of this material are considerably large and cover the room temperature region. As a consequence, the relative cooling power value of Sr Fe0.8Co0.2O3 obtained at 5 T is 331 J/kg, which is greatly higher than those observed in other perovskite oxides. Considering the large magnetic entropy change, working temperature, refrigerant capacity and the absence of rare metals, the present Sr Fe0.8Co0.2O3 single crystal therefore provides a promising candidate for magnetic refrigeration near room temperature. |
