2020 Volume 61 Issue 8 Pages 1492-1495
The spin reorientation and Curie temperature of Mn2−xFexSb1−ySny (0 ≤ x, y ≤ 0.15) compound were investigated by magnetization measurements. Spin reorientation temperature increased from 255 K at x = 0 to 383 K at x = 0.15, whereas it almost unchanged by Sn substitution. Curie temperature decreased down to 518 K with both Fe and Sn substitution. Substitution of Fe stabilized the ferrimagnetic state with magnetic moment lying in c-plane. Substitution of Sn induced the antiferromagnetic phase at low temperature. It was found that the magnetic hysteresis derived from quasi first-order magnetic phase transition exhibited at x ≥ 0.10 or T ≥ 300 K.
Fig. 5 Magnetic phase diagrams for Mn2−xFexSb1−ySny in function of x (a) and y (b). Open symbols indicate the composition which exhibited QFOMT.
Ferrimagnetic (FRI) Mn2Sb-based compounds with a tetragonal Cu2Sb-type structure have been paid attention due to the first-order magnetic transition (FOMT)1–3) with small volume change.4,5) The configuration of magnetic moment in Mn2Sb and related compounds with Cu2Sb-type structure is shown in Fig. 1. There are magnetic moments m of two Mn atoms, which were called mMn(I) at 2a-site and mMn(II) at 2c-site. mMn(I) and mMn(II) are reported to be 2.1 μB and 3.9 μB, respectively, and aligned antiparallel each other.6) The triple layer of magnetic moments, mMn(II)-mMn(I)-mMn(II), aligned parallel along the c-axis (FRI(II) state). The triple layer of magnetic moments aligned perpendicular to c-axis (FRI(I) state). This change of configuration of magnetic moments was spin reorientation (SR), which occurred in the vicinity of TSR = 250 K.1) The SR in Mn2Sb-based compound is related to sign of magnetic crystalline anisotropy constants K1 and K2.
Crystal structure and magnetic configuration of Mn(I) and Mn(II) moments of Mn2Sb compound. The arrows indicate the magnetic moments at Mn(I) and Mn(II) sites.
Substituting 3d-elements (Cr, Co and V) for Mn or Sn/Ge for Sb result in the first-order magnetic transition (FOMT) from the FRI to antiferromagnetic (AFM) state with discontinuous volume change.1–4,7,8) Meanwhile, FOMT does not appear for Ti- and Fe-substitution.2,9) As seen in Fig. 1, the mMn(II)-mMn(I)-mMn(II) triple layers align antiparallel each other in the AFM state. FOMT, which is accompanied with the small change of unit cell, is benefit for the reversibility on the magnetic cooling materials based on magnetocaloric effect. That is because the loss due to small thermal hysteresis.10)
Recently, Nwodo et al. reported that another magnetic hysteresis in the vicinity of 320 K in Mn1.9Fe0.1Sb0.9Sn0.1.11,12) At the transition temperature, latent heat and discontinuous change of lattice parameters were not observed. Therefore, it was called quasi first-order phase transition (QFOMT). Since there is a report that the spin reorientation from easy axis to an easy-plane13) was first-order transition, Nwodo et al. proposed that the QFOMT of Mn1.9Fe0.1Sb0.9Sn0.1 was derived from SR.11,12) However, although the Mn1.9Fe0.1Sb0.9Sn0.1 exhibited QFOMT, the important factor for exhibiting QFOMT is unclear. In this paper, for investigating the factor for exhibiting QFOMT in Fe- and Sn-modified Mn2Sb, magnetic phase diagram of Mn2−xFexSb1−ySny was produced. In particularly, substitution effects on the TC and TSR were focused on.
Polycrystalline Mn2−xFexSb1−ySny (0 ≤ x, y ≤ 0.15) was prepared as follows. A mixture of nominal amounts of pure elements (3N–Mn, 4N–Fe, 4N–Sb, and 5N–Sn) was arc-melted in an argon atmosphere. The obtained samples were sealed in a quartz tube in an Ar atmosphere, and were annealed at 923 K for 1 day. After that, the sample was quenched into ice water by breaking the quartz tube. X-ray powder diffraction (XRD) experiments were performed using Cu Kα radiation at room temperature. Magnetization measurements were carried out by a superconducting quantum interference device (SQUID) magnetometer for 10 ≤ T ≤ 360 K and vibrating sample magnetometer (VSM) for 300 ≤ T ≤ 770 K. In this study, thermomagnetization curves using SQUID magnetometer were performed by the warming process after zero-field cooling (ZFCW) and field cooling (FC) protocol. AFM-FRI transition temperature Tt was determined by the cross section of the largest slope of the tangent line and the baseline for ZFCW process. TSR and TC were determined by the peak of the dM/dT curve.
Figure 2 shows the typical XRD patterns for Mn2−xFexSb1−ySny at x = 0.05 (a) and 0.15 (b). The main diffraction peaks were indexed by the tetragonal Cu2Sb-type structure. The weak peaks derived from MnSb phase with hexagonal NiAs-type structure was detected. The stronger peaks of MnSb were observed at y-rich region, such as Mn1.85Fe0.15Sn0.90Sn0.10 and Mn1.95Fe0.05Sb0.85Sn0.15.
XRD patterns for Mn2−xFexSb1−ySny at x = 0.05 (a) and 0.15 (b). hkl denotes the Miller indices of Mn2Sb phase with the triangles indicated the diffraction derived from MnSb phase.
Figure 3 shows the temperature dependence of the magnetization (M-T curve) of Mn2−xFexSb1−ySny (0 ≤ x ≤ 0.15, 0 ≤ y ≤ 0.15). In Fig. 3(a), the reduction of M due to the FOMT between the AFM and FRI state was observed for 10 ≤ T ≤ 150 K. With increasing y, the Tt shifted to higher temperature. The FOMT suppressed with increasing x. However, small drops in M-T curves were observed for T < 50 K for y = 0.15 even in high x (see Fig. 3(c) and (d)). TSR of Mn2Sb was determined to be 255 K, which is consistent with previous reports.1) The magnetic hysteresis of QFOMT was observed at TSR for higher x than 0.1.11) It is found that QFOMT appeared when TSR existed in high temperature region over 300 K. Meanwhile, y-TSR relation was not observed clearly.
Temperature dependence of magnetization of Mn2−xFexSb1−ySny at 0.1 T. Solid and dashed line indicate the ZFCW and FC processes, respectively. The inset shows enlarged view in the vicinity of TSR.
Figure 4 shows the M-T curve for 300 ≤ T ≤ 770 K in a magnetic field of 0.1 T. TC of Mn2Sb was evaluated to be 549 K, which was good agreement with Ref. 6). TC of impurity MnSb-based phase was about 587 K14) and it was not observed. TC decreased monotonically with increasing both x and y. At x ≠ 0 and small y region, a cusp was observed just below the drop of M around TC. The origin of this cusp was not clarified. However, the cusp disappeared by increasing y.
M-T curves of Mn2−xFexSb1−ySny at 0.1 T (300 ≤ T ≤ 770 K). The inset shows the enlarged view in the vicinity of TC, which indicated by the arrows.
Figure 5 shows the magnetic phase diagram of Mn2−xFexSb1−ySny in the function of x (a) and y (b). The data for y = 0 was in good agreement with previous phase diagram.9) As seen in Fig. 5, the AFM state appeared only low x and high y region. Comparing the slope of x-TC and y-TC lines, Sn-substitution was more effective for the reduction of TC, although both Fe and Sn-substitution decreased TC. The reduction rates of TC at constant y and x were 70 K/x and 194 K/y, respectively. The increase of x led to enhancement of TSR at the rate of 465 K/x, whereas TSR did not clearly changed by substituting Sn. As shown in Fig. 5(a) and (b), it was found that TSR strongly depended on x and continuously increased with increasing x. QFOMT appeared when x was over 0.1 and when TSR was higher than 300 K. Therefore, it is suggested that the QFOMT was related to SR.
Magnetic phase diagrams for Mn2−xFexSb1−ySny in function of x (a) and y (b). Open symbols indicate the composition which exhibited QFOMT.
It was considered that Fe-substitution changed the magnetic crystalline anisotropy constants. According to Mössbauer spectroscopy,15) the value of quadrupole splitting in Mn1.98Fe0.02Sb at 300 K and 80 K are reported to be −0.12 mm/s and 0.10 mm/s, respectively. On the other hand, hyperfine fields in Mn1.98Fe0.02Sb at 300 K and 80 K are reported to be 80 kOe and 50 kOe, respectively.15) Since SR depend on the magnetic crystalline anisotropy constant K1 and K2,1) Mössbauer spectroscopy experiment for Mn2−xFexSb1−ySny is necessary for investigating the relationship between SR and QFOMT in microscopic viewpoint.
It is clearly observed that Fe and Sn stabilized the FRI and AFM phases, respectively. Meanwhile, both Fe- and Sn-substitution reduced TC. According to the first-principle calculation study for As-modified Mn2Sb,16) interatomic distances between the Mn atoms and their the nearest neighbors were more effective for stability of AFM and FRI states than composition change. It is suggested that exchange interaction decreased due to the modification of Fe and Sn substitution, leading to the reduction of TC. Meanwhile, stabilization of AFM or FRI state was due to the change of the lattice parameters.
The effects of Fe- and Sn-substitution on the magnetic properties of Mn2−xFexSb1−ySny was investigated. The magnetic phase diagrams at x- and y-section were presented. Substitution of Sn atom did not change TSR, although TC monotonously decreased with increasing Sn content at the rate of 194 K/y. TSR increased with increasing x and QFOMT appeared for x ≥ 0.1, suggesting the relation between QFOMT and SR.
The magnetization measurements were performed at the Institute for Solid State physics, the University of Tokyo and Institute for Materials Research, Tohoku University.
