Magnetization Measurements of Weak Itinerant Electron Ferromagnet Ni–Pt Alloy

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.