Flux Growth and Magnetic Properties of CaB6 Crystals
Flux Growth and Magnetic Properties of CaB6 Crystals
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DOI：
10.1143/jpsj.71.1791
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发表时间：
2002-07
影响因子：
1.7
通讯作者：
S. Otani;T. Mori
中科院分区：
文献类型：
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作者：
S. Otani;T. Mori
La掺杂的六硼化钙Ca1 xLaxB6虽然不含磁性元素，但因其高温弱铁磁性而引起人们的极大兴趣。然而，人们对这一材料一直存在一些怀疑。例如，有些样品不具有铁磁性。尽管样品的铁磁性很弱，但杂质的检测并不是很好。此外，报告中没有详细报告样品制备的程序。因此，我们制备了Ca1xLaxB6单晶，同时注意到了主要的杂质铁，并研究了杂质与铁磁性的关系。用助熔剂法制备了未掺杂和掺La的CaB6晶体。以氢氧化钙和硼为原料，合成了CaB6起始原料。3)铝金属Al的标称纯度为4N。将总重量为50-52g的CaB6和铝放入AlN坩埚中。CaB6与Al的摩尔比为0.6%。为了抑制AlB_2晶体的形成，还将Ca(OH)_2以Ca(OH)_2的形式加入到坩埚中。钙的加入量为CaB6溶质的20摩尔%。通过添加商品化的LaB6粉末对La进行掺杂。样品在1450℃的氩气中保持24小时，每小时以15℃的速度缓慢冷却至800℃，然后淬火。在氢氧化钠溶液中除去铝助熔剂。用SQUID磁强计测量了未掺杂的六硼化钙CaB6和掺La的Ca1xLaxB6(x1⁄4 0：0 0 5 1)单晶的磁化强度，最大磁场为5 5 kg。得到的晶体尺寸小于几毫米，厚度0.2-1毫米。形状为长方形、板状或立方体。大晶体如图1所示。La的掺杂不影响CaB6晶体的大小和形状。超过一半的未掺杂和La掺杂晶体都被磁铁吸引或轻微移动。尤其是细小的晶体受到强烈的吸引。立方晶体几乎没有被吸引。然而，在盐酸溶液中放置几个小时后，所有的晶体都没有被吸引。表一显示了来自铝熔剂的铁杂质的含量。用于磁性测量的未掺杂和掺La的CaB6晶体在HCl处理前分别为190ppm和100ppm。通过盐酸处理去除了铁，这意味着铁存在于CaB6晶体的表面。这种现象可以理解为以下几点。在用氢氧化钠溶液溶解铝熔剂的过程中，铝基质起到阴极的作用。保持在铝表面的晶体作为阳极，铁离子在晶体表面被还原为铁。也就是说，这些晶体被电化学镀上了铁。因此，由于铁在氢氧化钠溶液中是稳定的，所以晶体上覆盖着铁。在盐酸处理中，铁被去除，因为铁与盐酸溶液发生反应。La掺杂的六硼化钙Ca0：0049La0：0051B6在300K的磁化强度如图2所示。与以前的报道类似，观察到具有磁滞的弱铁磁性。当La的掺杂量为0：51%时，与所报道的铁磁性成分最大的样品的化学计量比接近。铁磁性的大小(2emu/molF.U..)与之前报道的数据相当。然而，我们发现，通过洗涤这些样品。图1。原始生长的CaB6晶体。一分为1毫米。
Lanthanum doped calcium hexaboride, Ca1 xLaxB6, attracts a lot of interest because of high temperature weak ferromagnetism though the material consists of no magnetic elements. However, there have been some doubts about this material. For example, some of the samples did not have ferromagnetism. Impurities in the samples were not examined well in spite of weak ferromagnetism. In addition, procedures of the sample preparation were not reported in detail in the reports. Therefore, we prepared single crystals of Ca1 xLaxB6 while paying attention to a main impurity, iron, and examined the relation between the impurity and the ferromagnetism. Undoped and lanthanum-doped CaB6 crystals were prepared by the flux method. The CaB6 starting material was synthesized by reaction of Ca(OH)2 and boron. 3) Aluminium metal, Al, was of nominal purity of 4N. The CaB6 and aluminium, 50–52 g in a total weight, were put into an AlN crucible. A molar ratio of CaB6 to Al was 0.6%. Ca was also added as a form of Ca(OH)2 into the crucible in order to suppress formation of AlB2 crystals. The amount of Ca was 20mol% of the CaB6 solute. Lanthanum was doped by adding commercial LaB6 powder. The samples were kept at 1450 C in a flow of argon for 24 h, slowly cooled to 800 C by 15 C every hour, and then quenched. The aluminium flux was removed in NaOH solution. Magnetization of undoped calcium hexaboride CaB6 and lanthanum doped calcium hexaboride Ca1 xLaxB6 (x 1⁄4 0:0051) single crystals were measured by using a SQUID magnetometer up to fields of 55 kG. The obtained crystals were less than several mm in size and 0.2–1mm thick. The shapes were rectangular plate-like or cubic. The large crystals were shown in Fig. 1. The dope of lanthanum did not influence the size and shape of the CaB6 crystals. More than half of both the undoped and La-doped crystals were attracted or slightly moved by a magnet. Especially, the thin crystals were strongly attracted. The cubic crystals were hardly attracted. However, all the crystals were not attracted at all after being kept in HCl solution for a few hours. Table I shows contents of iron impurity, which came from the aluminium flux. The undoped and La-doped CaB6 crystals used for magnetic measurement contained 190 ppm and 100 ppm, respectively, before the HCl treatment. The iron was removed by the HCl treatment, which means that the iron existed on the surface of the CaB6 crystals. This phenomena can be understood as follows. In the process of dissolving the aluminium flux by the NaOH solution, the aluminium matrix acts as a cathode. The crystals which are held on the surface of the aluminium acts as an anode and iron ions were reducted to iron on the crystal surface. That is, the crystals were electrochemically plated with iron. Consequently, the crystals were covered with iron because iron is stable in the NaOH solution. The iron was removed in the HCl treatment because iron reacts with HCl solution. The magnetization of lanthanum doped calcium hexaboride Ca0:0049La0:0051B6 at 300K is shown in Fig. 2. Similar to what has been reported previously, a weak ferromagnetism is observed with hysteresis. The lanthanum doping of 0:51% of our sample is close to the stoichiometry reported for the sample exhibiting the largest magnitude of ferromagnetic component. The magnitude of ferromagnetism ( 2 emu/mol f.u.) is comparable to that reported previously. However, we find that by washing these samples Fig. 1. As-grown CaB6 crystals. One division is 1mm.