2020 Volume 61 Issue 8 Pages 1473-1475
The magnetization measurements were carried out for polycrystalline Au4V in magnetic fields up to 130 kOe. The spontaneous magnetic moment ps was determined to be 0.255 μB/V at 4 K by high field magnetization. The magnetization process was analyzed on the basis of Takahashi’s spin fluctuation theory for weak itinerant electron ferromagnets. The characteristic parameters of the spin fluctuation were estimated to be TA = 1.3 × 103 K and T0 = 2.3 × 103 K using the data for 110–130 kOe.
Fig. 4 Temperature dependence of the reduced spontaneous magnetic moment: ps(T)2/ps(4 K)2 vs. T4/3 (a) and ps(T)2/ps(4 K)2 vs. T2 (b). Here, ps(T) and ps(4 K) were evaluated from M2 vs. H/M for 10 ≤ H ≤ 30 kOe. The broken line is a least-squares fit to the data for 4 ≤ T ≤ 20 K.
Atomically ordered Au4V with a Ni4Mo-type body-centered tetragonal structure is a ferromagnet below a Curie temperature TC of 42 K.1) The magnetic moment and effective magnetic moment have been reported to be MS = 0.2–0.4 μB/V and peff = 1.2–1.5 μB/V, respectively.1) Generally, the ratios of peff/MS for the weak itinerant electron ferromagnets (WIEF) such as Sc3In, ZrZn2, Ni3Al, MnSi, etc. are greater than 1.2) This phenomenon is not able to be explained by a localized model. Therefore, above magnetic properties suggested that ordered Au4V was one of WIEF.
Moriya firstly proposed a self-consistently renormalized (SCR) spin fluctuation theory for explaining the magnetism of WIEF.3) At present, however, it is known that SCR theory was not enough to explain the magnetism of some WIEF such as FeSi.4) The SCR theory only considered thermal spin fluctuations.2,5) Takahashi pointed out that not only thermal spin fluctuation but also the quantum spin fluctuation plays important roles in the magnetic properties of WIEF. Takahashi’s spin fluctuation theory for WIEF explained some experimental results in the wide temperature region from ground state to the paramagnetic state.2,5,6) Takahashi’s theory also suggests that the magnetization process is important for estimating an energy width (T0) of the dynamical spin fluctuation and a dispersion (TA) of the static magnetic susceptibility in the wave vector space. However, the best of our knowledge, the magnetic properties of ordered Au4V in high magnetic fields have not been discussed on the basis of the Takahashi’s spin fluctuation theory.
In this study, we measured magnetization processes of polycrystalline Au4V in magnetic fields H up to 130 kOe to study the magnetic behavior and to estimate T0 and TA under high magnetic fields.
Polycrystalline Au4V was prepared by arc-melting a mixture of stoichiometric amounts of pure elements (Au, 3N; V, 3N) in an argon atmosphere. The obtained button ingot was turned over and re-melted several times. After that, the ingot was annealed at 773 K for 1200 h in a quartz tube with a vacuum and then slowly cooled to room temperature. The sample was confirmed to be a single phase of Au4V by X-ray powder diffraction measurements. The magnetization M measurements were carried out using a vibrating sample magnetometer (VSM) (Oxford Instruments, Mag. Lab. VSM) for 4 ≤ T ≤ 60 K and H ≤ 130 kOe.
Figure 1 shows the magnetization (M-H) curves for 4 ≤ T ≤ 55 K. The inset of Fig. 1 shows the M-H curves at 4 K for the magnetization and demagnetization processes. As seen in this inset, Au4V exhibited a large magnetic hysteresis, a large coercivity and a high residual magnetization below TC, which is consistent with previous report.1) In this study, therefore, we used data of demagnetization processes for analyzing the magnetic properties. The magnetization was not saturated even in a magnetic field of 130 kOe. The magnetic moment of 130 kOe was 3.98 emu/g (0.586 μB/V) at 4 K. This value is consistent with that traced at 130 kOe in pulsed fields up to 350 kOe.8) A magnetic field of 270 kOe was required to obtain the saturation magnetization (6.7 emu/g) of Au4V.8)
Magnetization curves of Au4V. The inset shows magnetization and demagnetization curves of Au4V at 4 K.
Figure 2 shows the isothermal M2 vs. H/M plots (Arrott plot) for 4 ≤ T ≤ 60 K. The spontaneous magnetic moment ps was determined by the linear extrapolation to H/M = 0 for the M2 vs. H/M plots for the data of 110 ≤ H ≤ 130 kOe, because the linearity of the M2-H/M curves for 110 ≤ H ≤ 130 kOe was better than that for lower magnetic fields, as shown in the inset of Fig. 2. ps at 4 K was determined to be 1.70 emu/g (0.255 μB/V).
Isothermal M2 vs. H/M plots (Arrott plot) of Au4V. The inset is data at 4 K. The broken line in the inset is a least-squares fit to the data.
The magnetic field dependence of TC was examined by extrapolating the line representing ps2 vs. T toward ps2 = 0. Figure 3 shows the ps2 vs. T plots deduced from data for 10 ≤ H ≤ 30 kOe (solid circles) and 110 ≤ H ≤ 130 kOe (open circles). When data of the M2 vs. H/M plots for 10 ≤ H ≤ 30 kOe was used, TC was evaluated to be 41.6 K. This value is in good agreement with the reported value (TC = 42 K) deduced from the M-H data using a field of 1.65 kOe.1) For using data of 110 ≤ H ≤ 130 kOe, TC was determined to be 23.8 K. As seen in Fig. 3, the curvature of the M2 vs. H/M plots is positive, so that the determined TC for the lower magnetic field region is higher than that for high magnetic field.
ps2 vs. T plots for 10–30 kOe (solid circles) and 110–130 kOe (open circles). The solid lines are least-squares fits to ps2.
Takahashi proposed a new spin fluctuation theory considering the quantum spin fluctuation for WIEF.2,6) He pointed out that the magnetization process of WIEF was affected by quantum spin fluctuation and a liner M2-H/M plots. That is, the M2-H/M plots of WIEF is well expressed by the following equation;6)
| \begin{equation} -2c\eta^{4}k_{B}T_{A} + \frac{4k_{B}T_{A}{}^{2}}{15T_{0}}\frac{p^{2}}{8} = \frac{h}{p}, \end{equation} | (1) |
According to the Takahashi’s theory for WIEF, the reduced spontaneous magnetic moment: ps(T)2/ps(0)2 follows the T4/3-dependences in the wide temperature range below TC except for very low temperature at which the T2-dependences is observed.2) Figure 4 shows the temperature dependence of the reduced spontaneous magnetic moment: ps(T)2/ps(4 K)2 vs. T4/3 (a) and ps(T)2/ps(4 K)2 vs. T2 (b). Here, ps(T) and ps(4 K) were determined using the data for 10 ≤ H ≤ 30 kOe. In Fig. 4(a), the straight line is a least-squares fit to the data. It is confirmed that ps(T)2/ps(4 K)2 shows a T4/3-dependences in the wide temperature range.
Temperature dependence of the reduced spontaneous magnetic moment: ps(T)2/ps(4 K)2 vs. T4/3 (a) and ps(T)2/ps(4 K)2 vs. T2 (b). Here, ps(T) and ps(4 K) were evaluated from M2 vs. H/M for 10 ≤ H ≤ 30 kOe. The broken line is a least-squares fit to the data for 4 ≤ T ≤ 20 K.
At low temperature, the temperature dependence of ps(T)2/ps(0)2 is given by
| \begin{equation} \frac{p_{s}(T)^{2}}{p_{s}(0)^{2}} = 1 - \alpha_{0}T^{2}, \end{equation} | (2) |
| \begin{equation} \alpha_{0} = \frac{112.1}{p_{s}(0)^{4}T_{A}{}^{2}}. \end{equation} | (3) |
From eqs. (2) and (3) by using data at 4 K, 10 K and 20 K, TA was estimated to be 6.1 × 103 K. The broken line (α0 = 0.00072) in Fig. 4(b) is a least-squares fit for these data. This value of TA is about three times larger than that estimated from the M2 vs. H/M plots at 4 K using eq. (1). The solid line in Fig. 4(b) indicates a calculated line for α0 = 0.0020. In this case, TA = 3.6 × 103 K is obtained, which is more comparable to the value evaluated from the M2 vs. H/M plots at 4 K. This indicates that ps(T)2/ps(4 K)2 of WIEF shows a T2-dependences for very low temperature reasion. Our result for 10 ≤ H ≤ 30 kOe is consistent with the Takahashi’s theory. It was difficult to discuss the behaviors under high magnetic fields at present, because the only three data of ps(T)2 could be evaluated for T < TC in 110 ≤ H ≤ 130 kOe, as seen in Fig. 3.
Consequently, the characteristic parameters of the spin fluctuation for Au4V depended on the strength of magnetic fields, but obtained magnetic data were consistent with Takahashi’s spin fluctuation theory for WIEF.
The magnetization measurements were performed for polycrystalline Au4V for 4 ≤ T ≤ 60 K and 0 ≤ H ≤ 130 kOe. ps was deduced to be 0.255 μB/V at 4 K. TC was determined to be 23.8 K for data of 110 ≤ H ≤ 130 kOe and 41.6 K for data of 10 ≤ H ≤ 30 kOe. The characteristic parameters of Takahashi’s spin fluctuation theory for WIEF were estimated to be TA = 1.3 × 103 K and T0 = 2.3 × 103 K for data of 110 ≤ H ≤ 130 kOe, and T0 = 9.2 × 102 K and TA = 2.2 × 103 K for data of 10 ≤ H ≤ 30 kOe.
Magnetization measurements were carried out at Laboratory of Low Temperature Materials Science, Institute for Materials Research, Tohoku University.
