FORMATION OF AN EXCITONIC FERROMAGNETIC MOMENT IN THE LOW DENSITY-CARRIER BORIDES

我们发现Yb_<1-x>La_xB_6 （x<0.006）与激子铁磁体Ca_<1-x>La_xB_6具有非常相似的电子结构，并且该合金体系在居里点Tc = 150 ~ 200 K处通过磁致化和μ子自旋弛豫（μSR）发生铁磁跃迁。单晶生长：采用浮动区法在装有四氙灯的成像炉上生长YbB_6单晶样品。为了消除YbB_6晶体中的杂质相YbB_<50>，烧结多晶棒选用了Yb_<1.1>B_6的标称成分进行单晶生长。这个铸态样品显示出抗磁性。热处理后引入yb缺陷，形成了较大的顺磁矩。多晶样品：烧结后的多晶样品表面呈海绵状，是用射频感应炉真空烧结而成的。高分辨率x射线衍射分析和电子能谱分析表明，多晶和单晶均不含FeB和Fe_2B杂质，除了少量的顺磁性YbB_4（<或等于0.001 %）。经酸蚀处理后，多晶样品呈现铁磁性，Tc = 150 ~ 200 K。在接下来的研究中，我们将研究该烧结样品的表面效应对磁性能的影响。μSR：我们在300 K到10 K范围内对单晶和多晶进行了μ^+SR测量。弛豫谱表现出典型的顺磁性。由弛豫率推导出的内部场在室温下为0.2 mT，在150k下没有变化。当进一步冷却时，内部磁场强烈增加，在10 k时达到0.6 mT，这种温度依赖性与磁化率非常相似。

We have found that Yb_<1-x>La_xB_6 (x<0.006) has a very similar electronic structure as that of the excitonic ferromagnet Ca_<1-x>La_xB_6 and that this alloy system shows ferromagnetic transition at Curie point Tc = 150-200 K by manetization and muon spin-relaxation (μSR).1. Single crystal growth : YbB_6 single-crystal samples were grown by a floating-zone method using an image furnace equipped with four-xenon lamps. In order to eliminate impurity phase YbB_<50> from the crystals of YbB_6, a nominal composition of Yb_<1.1>B_6 was chosen for sintered polycrystalline rods for the single-crystal growth. This as-cast sample shows diamagnetism. After heat treatment to introduce Yb-defect we have found that large paramagnetic moment was formed.2. polycrystalline sample : Sintered polycrystalline sample has a sponge-like surface that has been sintered in vacuum by using a radi-frequency induction-furnace. The high-resolution x-ray diffraction analysis and EPMA revealed that both polycrystals and single crystals have no impurities of FeB and Fe_2B except for a small amount of paramagnetic YbB_4 (【less than or equal】0.001 at%). After acid etching treatment, polycrystalline sample shows ferromagnetism with Tc = 150-200 K. In the next research, we will study the surface effect of this sintered sample for magnetic property.3. μSR : we have performed μ^+SR measurement for both single crystals and polycrystals from 300 K down to 10 K. Relaxation spectra shows typical paramagnetic pattern. Internal fields deduced from relaxation rate are 0.2 mT at room temperature, and do not change down to 150 K. On cooling further, the internal field increases strongly and reaches a value of 0.6 mT at 10 K. this temperature depence is very similar to that of the magnetic susceptibility.