锌离子掺杂钴基金属有机材料[(CH_(3))_(2)NH_(2)]Co_(1-x)Zn_(x)(HCOO)_(3)中的低温反常磁现象

Abnormal magnetic phenomenon at low temperature in Zn doped[(CH_(3))_(2)NH_(2)]Co_xZn_(1-x)(HCOO)_(3) frameworks

金属有机框架材料(MOFs)是一种有机配体桥接金属离子组成的新型无机-有机杂化多孔材料,它具有功能可调、稳定性好以及多孔性等特点,受到人们的广泛关注.本文利用水热法制备了高质量的锌离子掺杂的钴基[(CH_(3))_(2)NH_(2)]Co_(1-x)Zn_(x)(HCOO_(3))单晶样品(x=0,0.1,0.2,0.3,0.4,0.5).单晶衍射、摇摆曲线、能量色散X射线光谱的实验结果表明,锌离子均匀掺杂进了钴基金属有机框架材料中,没有出现局部团簇等现象.低温场冷曲线和比热曲线的测量结果表明,非磁锌离子的掺杂减弱了Co基MOFs材料中Co离子之间的长程反铁磁相互作用,使得Co-MOF的反铁磁相变温度由纯钴的15 K变为14.2 K(x=0.1),12.8 K(x=0.2).通过对掺锌样品低温下的磁滞回线的细致研究发现,掺锌样品相对于纯钴样品在低温下具有更大的磁滞损耗和矫顽场.相比于纯钴样品450 Oe(1 Oe=10^(3)/(4π)A/m)的矫顽场,掺锌样品的矫顽场最高达到3600 Oe,并且磁滞面积也为纯钴样品的3倍以上.另一方面,在DMCo_(0.9)Zn_(0.1)F样品的磁滞回线上发现一系列台阶,这一台阶现象随着温度升高而逐渐消失,类似于单分子磁体的量子隧穿现象.以往研究表明,在这一类钙钛矿结构的金属有机骨架材料中,存在长程磁相互作用和磁单离子行为的竞争.我们认为非磁锌离子的掺杂减弱了Co离子之间的长程相互作用,使得Co离子在低温下显示出量子隧穿引起的台阶效应.

Metal-organic framework(MOF)is a new type of inorganic-organic hybrid porous material composed of organic ligands bridging metal ions,and it has the characteristics of tunable functions,good stability and porosity.In this study,Zn doped Co-based metal organic frame works single-crystal samples[(CH_(3))_(2)NH_(2)]Co_(1-x)Zn_(x)[HCOO]_(3)are synthesized by the solvothermal method with normal ratio x=0,0.1,0.2,0.3,0.4,0.5.Single crystal diffraction,scanning electron microscope and energy dispersive X-ray spectroscopy results show that Zn ions are uniformly doped into Co-based MOFs crystals.The field cooling curves show that antiferromagnetic phase transition temperature of Co-based MOFs decreases from 15 K for pure Co-MOFx=0 to 12.8 K for x=0.2.Abnormal large magnetic hysteresis is obtained for Zn doped crystals with large coercive field 3600 Oe(x=0.3)compared with 450 Oe coercive field for pure Co-MOF(x=0),and the hysteresis area of Zinc-doped sample is more than 3 times that of pure cobalt sample.On the other hand,we find a series of steps on the hysteresis loop of DMCo_(0.9)Zn_(0.1)F sample,which gradually disappears with the increase of temperature,similar to the quantum tunneling phenomenon of a single molecule magnet.Previous studies have shown that the long range magnetic interaction and the magnetic single-ion behavior competition coexist in these systems.It is believed that the doping of non-magnetic zinc ions weakens the long-range interaction between Co ions and makes Co ions show the step effect caused by quantum tunneling at low temperature.