本研究所使用樣品RVO4 (R = Tb, Ho, Lu)由高溫熔液成長法(high-temperature solution method)製成。X光繞射(X-ray diffraction)所得繞射峰的結構為鋯石型態四方晶系(zircon-type tetragonal)結構,半高寬皆在0.13˚~ 0.14˚之間,顯示樣品結晶性甚佳。掃描電子顯微鏡(scanning electron microscope, SEM)顯示表面平整,同時能量色散分析儀 (energy dispersive spectroscopy, EDS)確認樣品中無明顯雜質。磁性方面由超導量子干涉儀(superconducting quantum interference device, SQUID)測量。HoVO4在室溫下呈順磁性(paramagnetic, PM),於250 K開始有反鐵磁(antiferromagnetic, AFM)作用表現出來。外加磁場H = 0~7 T下最大磁熵改變(maximum magnetic entropy change, 〖∆S〗_M^max)位於17.9 K,其值為9.02 J/kg∙K。相對致冷功率(Relative cooling power, RCP)為251.4 J/kg。TbVO4在室溫下呈順磁性PM,本研究發現多處有趣行為。於Jahn–Teller effect相變溫度觀察到行為異常,6 K以下觀察到磁化率減弱行為。外加磁場H = 0~7 T下最大磁熵改變〖∆S〗_M^max位於34 K,其值為54.7 J/kg∙K。相對致冷功率RCP為343.1 J/kg。於6 K有明顯的極小值,暗示有反鐵磁的作用抗拒磁矩排列。LuVO4在室溫下呈順磁性PM,於150 K開始有鐵磁(ferromagnetic, FM)作用開始表現出來。外加磁場H = 0~7 T下最大磁熵位於17.9 K,其值為0.26 J/kg∙K,RCP約為12.2 J/kg。最大磁熵改變所在溫度T_(〖∆S〗_M^max )與外加磁場改變量∆H的關係表現出有趣的行為,值得更進一步探討。
vanadates RVO4 (R = Tb, Ho, Lu) were synthesized by high-temperature solution method. X-ray diffraction (XRD) measurements reveal that the structure of RVO4 is of zircon-type tetragonal lattice with the full width at half maximum (FWHM) of the main XRD peaks being around 0.13~0.14, indicating the excellent crystalline quality of the samples. Scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS) spectrum exhibited the samples are flat and pure without discernible impurities. Magnetic properties measured by superconducting quantum interference device (SQUID) indicate that HoVO4 is paramagnetic (PM) at room temperature and displays antiferromagnetic (AFM) behavior below 250 K. The magnetization versus applied field (M-H) measurements taken under applied magnetic field of H = 07 T give that the maximum magnetic entropy change (〖∆S〗_M^max) and the relative cooling power (RCP) of HoVO4 are 9.02 J/kg∙K and 251.4 J/kg, respectively. The temperature at which 〖∆S〗_M^max occurs is 17.9 K for HoVO4. For TbVO4, although it is PM at room temperature, the low temperature magnetic property exhibits some interesting behaviors, including an anomaly at the phase transition temperature of Jahn–Teller effect (JTE) and a sudden declination in the magnetic moment below 6 K. The M-H measurements on TbVO4 under H = 07 T yielded a 〖∆S〗_M^max= 54.7 J/kg∙K occurring at 34 K with a RCP of 343.1 J/kg. It is noted that the 〖∆S〗_M^max for TbVO4 is substantially larger than that of HoVO4 despite of about the same RCP was observed. The magnetization minimum around 6 K further implies that there might be an additional AFM effect competing against magnetic moment ordering. LuVO4 was PM at room temperature and displayed ferromagnetic (FM) behavior below 150 K. Similar M-H measurements and analyses give that the 〖∆S〗_M^max occurring at 17.9 K and the RCP for LuVO4 are 0.26 J/kg∙K and 12.2 J/kg, respectively. The smaller values of both 〖∆S〗_M^max and RCP may have something to do with the ferromagnetic characteristics seen in LuVO4. The rich magneto-caloric behaviors exhibited in this family of rare-earth vanadates are interesting and calling for further investigation.