Magnetic properties of Mn_1-xTM_xAlGe (TM = V, Fe, Cu) with Cu₂Sb-type structure

Ferromagnetic MnAlGe has a tetragonal Cu₂Sb-type structure (P4/nmm) with a = 0.3913 nm and c = 0.5933 nm.¹ Magnetic Mn atoms occupy 2a-site and form the base-centered tetragonal sublattice. Two nonmagnetic Al/Ge-layers exist between the Mn-planes (c-plane). The shortest atomic distances of Mn are 0.2767 nm in the c-plane and 0.5933 nm along c-axis. This indicates that MnAlGe has pseudo two-dimensional (P2D) magnetic structure.

The spontaneous magnetic moment M_s and Curie temperature T_C of MnAlGe are 1.70 μ_B/f.u. and 503 K, respectively.² MnAlGe exhibits a strong uniaxial anisotropy along c-axis. The magnetic crystalline anisotropy (MCA) energy was reported to be K_u = 9.7 × 10⁵ J/m³ at 0 K and 5.2 × 10⁵ J/m³ at room temperature.² Therefore, MnAlGe-based ferromagnet has been paid attention as magnetic recording materials.^3,4

So far, substitution effects on the magnetic properties of MnAlGe have been studied for controlling the magnetic properties.^5–8 Ido et al. reported that T_C of Mn_1-xTM_xAlGe decreased by substitution of TM (3d metals) without TM = Cu and Cr.⁵ However, Goodenough et al. reported that substitution of TM reduced T_C of MnGaGe with the Cu₂Sb-type structure.⁸ For explaining the substitution effects on the magnetism, the relationship between magnetic properties and lattice parameters of MnAlGe has been discussed.^7,9,11 Shibata et al. reported that the exchange interactions between the Mn moments m_Mn are very sensitive in the c-plane rather than along the c-axis. The Mn-Mn exchange interaction in c-plane was about 10 times larger than that along c-axis.¹¹ Umetsu et al. also indicated that T_C and the c-axis related to positive correlation.⁷ 

Quite recently, Masumitsu et al. suggested that Cr-substitution effects on magnetic properties of Mn_1-xCr_xAlGe could not be explained by only the change of the lattice parameters.¹² Although the lattice expansion enhanced the m_Mn, the Cr moment m_Cr and the antiferromagnetic (AF) coupling of m_Mn-m_Cr played a key role for magnetic properties.¹² The reciprocal susceptibility 1/χ vs. temperature T curve of Mn_1-xCr_xAlGe did not follow Curie-Weiss law, indicating that the AF coupling of m_Mn-m_Cr existed.¹² This result was similar to the result for MnGaGe system, which was also P2D ferromagnet with Cu₂Sb-type structure, suggesting the AF m_Mn-m_Mn interaction and m_Mn-m_TM (m_TM is TM moment) ones by TM-substitution.⁸ It is necessary to study substitution effects on the magnetic properties of MnAlGe from a new model viewpoint.

In this study, the magnetic measurements of Mn_1-xTM_xAlGe (TM = V, Fe, Cu) alloys were carried out for 5 ≤ T ≤ 680 K and μ₀H ≤ 5 T in order to investigate the magnetism and substitution effect of MnAlGe-based system.

The polycrystalline Mn_1-xTM_xAlGe (TM = V, Fe, Cu, 0 ≤ x ≤ 0.2) were prepared by arc-melting, a mixture of nominal amounts of pure elements (Mn, 5N; Al, 4N; Ge, 5N; V, 3N; Fe, 4N; Cu, 3N) in an argon atmosphere. The obtained samples were annealed at 873 K for 3 days, and subsequently quenched into ice water. Powder X-ray diffraction (XRD) experiments with Cu Kα radiation was carried at room temperature (RT). The magnetization M measurements were performed by using a vibrating sample magnetometer for 300 ≤ T ≤ 680 K and a superconducting quantum interference device magnetometer for 5 ≤ T ≤ 300 K.

FIG. 1 (a) shows the XRD patterns for Mn_0.8TM_0.2AlGe at RT. It is confirmed that the main diffraction peaks of Mn_1-xTM_xAlGe (0 ≤ x ≤ 0.2) were indexed by the Cu₂Sb-type structure. Small diffraction peaks were detected, which were probably due to small amount of Ge. The influence of the impurity phase on the magnetic properties is negligible because Ge is nonmagnetic element.

The lattice parameters a, c, c/a, and lattice volume V in the function of x are shown in FIG. 1(b), (c), (d) and (e) respectively. The data of TM = Cr¹² are also plotted in FIG. 1. With increasing x, a increased for all TM, but x dependence of c was varied. V showed similar tendency to c and c/a. The enhancement in c/a were observed for TM = V. As the MCA of MnAlGe is closely related to the P2D structure, the improvement of K_u is expected for TM = V.

FIG. 2 shows the M-H curves at 5 K (a) and M-T curves at 0.05 T (b). M_s was evaluated by extrapolating to M² = 0 in H/M-M² plot. T_C was estimated the M-T curve. The obtained M_s and T_C for x are shown in FIG. 3(a) and (b), respectively. The M_s and T_C of TM = V, Fe and Cu decreased with increasing x. A remarkable reduction in M_s and T_C were detected for TM = Fe from 1.58 μ_B/f.u. and 507 K (x = 0) to 1.15 μ_B/f.u. and 354 K (x = 0.2), respectively. The data of TM = Cr¹² are also presented in the FIG. 3. Our results are different from that of TM = Cr. The M_s and T_C of TM = Cr increase with increasing x.

FIG. 4 shows the lattice parameter a (a) and c (b) in the function of T_C and a (c) and c (d) in the function of M_S. In this figure, the data of TM = Cr¹² are also presented. Although the relationship between T_C and c were not apparent for TM = V, Fe and Cu, the relationship between T_C and a showed a negative correlation. The data of M_S show same behavior. These negative correlations are explained by change of the exchange interactions between c-plane and along c-axis induced by anisotropic strain.¹⁰ Furthermore, it can be expressed by universal T_C-a and M_S-a lines for TM = V, Fe and Cu, which is different trend from TM = Cr.

The 1/χ − T curves for Mn_0.8TM_0.2AlGe are shown in FIG. 2(c). The good linearity was observed for TM = Mn and Cu, following Curie-Weiss law. A bent with upward convex appeared for TM = V and Fe, Herein, according to the spin fluctuation theory, the 1/χ − T curve also bent for two-dimensional magnetic material.¹³ However, the bent of the curve was downward convex and at T ∼ T_C, which was opposite phenomena induced by spin fluctuation. Therefore, the bent of the 1/χ − T curve is probably due to the AF coupling between m_Mn and m_TM.¹⁴ These behaviors are similar to TM = Cr.¹² 

A possible scenario of the reductions in M_s and T_C are considered as follows. The substitution of TM for Mn leads the reduction of Mn atom with a large m_Mn. It is reported that the Fe moment m_Fe was none by Mössbauer spectroscopy.¹⁵ On the other hand, in the different reports, m_Fe aligned antiparallel to m_Mn,^8,12 which are the reason for reduction of M_s. Meanwhile, based on molecular-field theory, T_C was determined by the number of nearest atoms z and the mean exchange interaction J. When m_TM is zero, the effective z decreased. If m_TM aligns antiparallel to m_Mn, J between m_TM and m_Mn becomes negative, suggesting the reduction of effective J. As a result, T_C decreased for both cases.

Based on the above scenario, TM-substitution effects were discussed from the aspect of 1/χ - T curve. In the case of m_TM = 0, the 1/χ – T curve does not bend. When the AF coupling between m_TM and m_Mn exists, the 1/χ – T curve will bend. The bending 1/χ – T curves in the present study suggested that the m_Fe and V moment m_V aligned antiparallel to m_Mn, whereas Cu has almost no magnetic moment. As the Mn-Mn distance in the c-plane is much smaller than that along the c-axis, the TM-substitution on the magnetic moments in Mn-plane were more effective than c-axis. Therefore, this assumption can also explain the same tendency for T_C-a and M_S-a relations in TM = V, Fe, and Cu. Further study is necessary to clarify the substitution effect on the MnAlGe magnetic materials.

The substitution effects on magnetic properties of Mn_1-xTM_xAlGe (TM = V, Fe, Cu, 0 ≤ x ≤ 0.2) were investigated. T_C and M_s reduced regardless of TM. The reduction of T_C and M_s were remarkably for TM = Fe and slightly for TM = Cu. The T_C-a and M_S-a have negative correlation, and the relationship was expressed by a single-linear function regardless of TM. The magnetic properties of Mn_1-xTM_xAlGe were discussed by localized model. The reciprocal susceptibility suggested the AF coupling between m_TM and m_Mn existed. In addition to this, lack of m_Mn due to the substitution probably explained reduction of M_s and T_C.

The M-H measurements were performed at the ISSP, The University of Tokyo. The M-T data were collected at IMR, Tohoku University.