Magnetization Measurements of Weak Itinerant Electron Ferromagnet Ni–Pt Alloy
Magnetization Measurements of Weak Itinerant Electron Ferromagnet Ni–Pt Alloy
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弱巡回电子铁磁体镍铂合金的磁化强度测量
DOI：
10.1143/jpsj.72.767
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发表时间：
2003
影响因子：
1.7
通讯作者：
M. Motokawa
中科院分区：
文献类型：
--
作者：
K. Koyama;H. Sasaki;T. Kanomata;Kazuo Watanabe;M. Motokawa
具有面心立方结构的无序Ni-Pt合金可以以任意比例合成。随着Pt掺杂浓度的增加，样品的磁矩和居里温度降低，铁磁性在58 at.%左右消失PT.在铁磁起始的临界浓度区，合金被认为是研究弱巡游电子铁磁体的良好参考材料。迄今为止，Ni-Pt合金的磁性数据几乎都是用基于单粒子激发的WIEF模型（所谓的Stoner-Wohlfarth模型）的热力学处理来讨论的。然而，仍然存在一些与实验不一致的地方，这些差异被认为是由于自旋涨落的影响。最近，Takahashi指出，不仅热自旋涨落，而且“量子自旋涨落”在WIEF的磁性中起着重要作用，并解释了从基态到顺磁态的宽温度范围内的一些实验结果。Ni-Pt合金的详细磁性能还没有使用Takahashi的理论进行讨论。在这项工作中，我们进行了精确的磁化强度测量和高场磁化强度测量附近的临界浓度的Ni 0：45 Pt 0：55合金研究的量子自旋涨落的影响，Takahashi提出的。采用重复电弧熔炼法制备了纯度为99.99%的Ni 0：45 Pt 0：55多晶样品。由于熔化后的重量损失可忽略不计，因此认为标称组成是准确的。为了得到均匀化的样品，将熔化的锭在1000 ℃下退火3天，然后在水中淬火。通过X射线粉末衍射分析，证实了样品为单相无序Ni-Pt合金，具有面心立方结构。确定的晶格参数为1/4 3：7807 A。在东北大学低温科学中心，使用SQUID磁力计（Quantum Design）和VSM（Oxford）分别在高达10 kOe和高达130 kOe的磁场H中以及在5 K至280 K的温度T范围内测量磁化强度。图1显示了Ni 0：45 Pt 0：55在100 T 280 K下的磁化曲线。在200 K以上，随着H的增加而线性增加。我们确定的T-依赖的磁化率从200 K以上的磁化曲线的斜率。1=-T曲线符合Cuire-Weiss定律，顺磁居里温度p = 57.6 K，有效磁矩peff = 2：21 B/Ni。图2显示了5 T 60 K下的典型磁化曲线。低场区（H 10 kOe）的数据如图2的插图所示。如该图所示，即使在高达130 kOe的磁场中，磁化强度也不饱和。图1为等温H= H; T = H; T = H的关系图。Ni 0：45 Pt 0：55在100至280 K不同温度下的磁化曲线。图中示出了磁化率的倒数1= .插图中的直线是对数据的最小二乘拟合。
Disordered Ni–Pt alloys with a face-centered cubic structure can be synthesized in any proportions. With increasing the concentration of Pt, the magnetic moment and the Curie temperature decrease, and then the ferromagnetism vanishes at about 58 at.% Pt. It has been supposed that the alloys in the critical concentration region for the onset of ferromagnetism are good reference materials for studying weak itinerant electron ferromagnets (WIEF). Almost all the magnetic data of Ni–Pt alloys have been discussed using a thermodynamic treatment based on WIEF model with single-particle excitation (so-called Stoner– Wohlfarth model) so far. However, some inconsistencies with experiments remain, and these discrepancies have been considered to be due to the influence of spin fluctuations. Recently, Takahashi pointed out that not only the thermal spin fluctuation but also the ‘‘quantum spin fluctuation’’ plays important roles in the magnetic properties of WIEF and explained some experimental results in the wide temperature range from the ground state to the paramagnetic state. The detailed magnetic properties of Ni–Pt alloys has not been discussed using Takahashi’s theory. In this work, we have performed the precise magnetization measurement and high field magnetization measurements on a Ni0:45Pt0:55 alloy near the critical concentration to study the effect of the quantum spin fluctuation proposed by Takahashi. The polycrystalline sample of Ni0:45Pt0:55 was prepared by repeated arc-melting the elements of 99.99% purity. Since the weight loss after melting was negligible, the nominal composition was accepted as being accurate. To get the homogenized sample, the as-melted ingot was annealed at 1000 C for 3 days, and then quenched in water. By powder x-ray diffraction, the sample was confirmed to be a single phase of the disordered Ni–Pt alloy with the face-centered cubic structure. The determined lattice parameter is a 1⁄4 3:7807 A. Magnetization were measured using a SQUID magnetometer (Quantum Design) and a VSM (Oxford) in magnetic fields H up to 10 kOe and up to 130 kOe, respectively, and in the temperature T range from 5K to 280K at Center for Low Temperature Science, Tohoku University. Figure 1 shows the magnetization curves of Ni0:45Pt0:55 in 100 T 280K. above 200K increases linearly with increasing H. We determined the T-dependence of the susceptibility from the slope of the magnetization curves above 200K. The observed 1= vs. T plot is well expressed by the Cuire–Weiss law, and the paramagnetic Curie temperature p and the effective magnetic moment peff were determined to be 57.6 K and 2:21 B/Ni, respectively. Figure 2 shows the typical magnetization curves in 5 T 60K. The data in lower field region (H 10 kOe) is shown in the inset of Fig. 2. As seen in this figure, the magnetization is not saturated even in the fields up to 130 kOe. The isothermal ðH;TÞ vs. H= ðH;TÞ plot is Fig. 1. Magnetization curves of Ni0:45Pt0:55 at various temperatures from 100 to 280K. The inst shows the temperature dependence of the inverse susceptibility 1= . The straight line in the inset is least-squares fits to the data.