Synthesis, Structure and Magnetic Properties of Sm_6−xLa_xMn₂₃ (0.5 ≤ x ≤ 4) Alloys

Abstract

The structure and magnetic properties of Sm_6−xLa_xMn₂₃ (x = 0.5, 1, 2, 3 and 4) alloys have been studied systematically. We found that the Th₆Mn₂₃-type Sm_6−xLa_xMn₂₃ alloys become less stable with increasing La content, and α-Mn becomes the dominant phase at x = 4. More impurities were found to present in Sm₅LaMn₂₃ samples prepared by a rapid solidification process than those present in the as-cast ingots. The coercivity of Sm₄La₂Mn₂₃ induction-melted ingots and Sm₅LaMn₂₃ melt-spun ribbons reached up to 0.47 T and 0.53 T, respectively, indicating potential applications of this alloy in hard magnetic materials. The Curie temperature of Sm_6−xLa_xMn₂₃ falls in the range of 398 K for x = 1 to 438 K for x = 3. The La-substitution results in a reduced saturation magnetization of Sm_6−xLa_xMn₂₃, owing to a reduced total-magnetic-moment contribution of the Sm-sublattices. This work provides us a deeper understanding of the effect of La-substitution on the structure and magnetic properties of the ternary La-Sm-Mn alloys.

1. Introduction

A number of Fe-based and Co-based rare earth (R) intermetallic compounds have been studied systematically in terms of their structure and magnetic properties [1,2,3,4]. However, studies on Mn-based rare earth intermetallic alloys are limited. Although some scattered studies on R-Mn intermetallic systems can be traced back to the middle of the last century, the magnetic properties of R-Mn alloys remain unclear due to their high complexity in crystallographic and magnetic structures. Binary Sm₆Mn₂₃ crystallizes in a face-centered Th₆Mn₂₃-type cubic structure in which there are five crystallographically non-equivalent positions (24e, 4b, 24d, 32f1 and 32f2) [5]. The Sm atoms occupy the 24e positions while the Mn atoms occupy the four remaining non-equivalent positions. The interactions between the localized magnetic moments of the R atoms, originating from the 4f shell, and the magnetic moments of the Mn atoms, due to the 3d shell, are very complex [5]. A net antiparallel coupling of Sm moments and Mn moments is found to exist in the Sm₆Mn₂₃ alloys, which arises from a non-collinear rare earth magnetic structure and a ferrimagnetic Mn magnetic structure [6]. Moreover, Sm in Sm₆Mn₂₃ was found to have about the same crystal field parameter as that in SmCo₅, about −200 K, which could lead to high magnetic anisotropy and thus the development of the large coercivity needed for permanent magnet materials [6].

Although the Sm₆Mn₂₃ phase is a thermal stable phase, there is no thermally stable La-Mn binary alloyed phase according to the La-Mn binary phase diagram [7]. It is known that elemental substitution usually has substantial effects on the structure and magnetic properties of a compound. The effect of the substitution of Mn with Fe on the magnetic properties of Sm₆Mn₂₃ has been studied previously [8,9], while the studies on the substitution of Sm in Sm₆Mn₂₃ with other rare earth elements are relatively rare. It should be noted that the effects of La-substitution on the magnetic properties of Nd-Fe-B have been studied systematically [10,11]. After La-substitution, melt-spun (Nd_1−xLa_x)₁₃Fe₈₁B₆ (0 ≤ x ≤ 0.5) alloys and the alloy containing 40 at. % La have more uniform and refined microstructures, resulting in enhanced magnetic properties and thermal stability [11]. An enhanced coercivity in a sample of Sm₅LaMn₂₃ induction-melted ingots was observed in our previous work but no systematic studies of La-substitution have been conducted [12]. The investigations of Sm_6−xLa_xMn₂₃ compounds with an extended composition range and a different preparation method in this work provide us a deeper understanding of the effect of La-substitution on the structure and magnetic properties of the ternary alloys of Sm_6−xLa_xMn₂₃.

2. Materials and Methods

The Sm_6−xLa_xMn₂₃ master alloys with nominal composition of x = 0.5, 1, 2, 3, and 4 were prepared by induction melting manganese (iTASCO, Seoul, Republic of Korea), lanthanum, and samarium metals of 99.95% purity under an argon atmosphere and then cooling down naturally. The as-prepared Sm₅LaMn₂₃ ingot was placed in a quartz tube with an orifice at the bottom. After that, the Sm₅LaMn₂₃ alloy was induction-melted again and ejected through the quartz tube orifice with argon onto a rotating water-cooling copper wheel. The as-prepared melt-spun ribbons were annealed in vacuum at 1023 K for 20 min.

The structure of the as-prepared alloys and LaSm₅Mn₂₃ melt-spun ribbons was characterized by using an X-ray diffractometer (XRD: Rigaku A/Max 2500, Tokyo, Japan) operating at 40 kV with Cu-Kα radiation. The scan range is 2θ = 20–80°, with a step size of 0.02° and a scan speed of 7° min⁻¹. The microstructure of the samples was analyzed by using a scanning electron microscope (SEM, JSM-6610LV, Tokyo, Japan) equipped with an energy-dispersive X-ray spectrometer (EDX). The magnetic properties of the samples were assessed by using a Quantum Design physical property measurement system (PPMS, San Diego, CA, USA) under an applied magnetic field up to 6 Tesla.

3. Results and Discussion

Figure 1 shows the room temperature X-ray power patterns of Sm_6−xLa_xMn₂₃ (x = 0.5, 2, 3 and 4) induction-melted ingots and Sm_6−xLa_xMn₂₃ (x = 1) melt-spun ribbons after annealing. The main phase of Sm_6−xLa_xMn₂₃ (x = 0.5, 2, 3) ingots and the x = 1 melt-spun ribbons could be indexed with the Th₆Mn₂₃ cubic structure. It seems that the Th₆Mn₂₃ phase in the x = 1 melt-spun ribbons segregates into two phases with two varied lattice parameters due to compositional inhomogeneity. In our previous work, an Sm₅LaMn₂₃ ingot was synthesized by the induction-melting method, with a mainly cubic Th₆Mn₂₃ structure and a small presence of α-Mn and Sm₂O₃ [12]. It is known that the lattice parameters of R₆Mn₂₃ vary slightly with varying R elements. The presence of two kinds of Th₆Mn₂₃-phase with varied lattice parameters in the rapidly solidified samples was ascribed to the thermally meta-stable characteristic of the rapidly quenched samples, in which the (Sm, La)₆Mn₂₃ phase may exhibit a locally varying La/Mn ratio and thus a varied lattice parameter. An annealing process can usually promote the increased growth of the rapidly quenched ribbons, but may also promote phase decomposition of meta-stable phases. In addition, when the melt is cooling down slowly, as in the case of an ingot in [12], the (Sm, La)₆Mn₂₃ phase may have enough time to form a relatively homogenous phase. The relative intensity of the peaks of the Th₆Mn₂₃ phase at 37.5 and 40.4 reverse gradually with an increasing x substitution, as seen in x = 0.5, 2, 3. We speculate that such gradual change might be due to the change in the atomic scattering factor of the X-ray after substitution. The high peak at 42.5 of the x = 3 sample was ascribed to the formation of Mn instead of the Th₆Mn₂₃ phase.

The minor phases in Sm_6−xLa_xMn₂₃ (x = 0.5, 1, 2, and 3) ingots/ribbons could be indexed with Sm₂O₃ and α-Mn, which increases in proportion with an increasing x, indicating the decreasing stability of the Sm_6−xLa_xMn₂₃ phase with an increasing x. Noticeably, the (3, 1, −3) plane at 41.93° of Sm₂O₃ (space group C12/m1) in the x = 3 ingot seems to overlap with 42.01° of the Th₆Mn₂₃ phase in the x = 2 ingot. We ascribe the appearance of α-Mn in Sm_6−xLa_xMn₂₃ (x = 0.5, 2) to Sm evaporation and the precipitation of the unreacted Mn when the high temperature melt was cooling down. Since a thermodynamically stable La-Sm binary phase does not exist, an increasing La content would also increase the fraction of the unreacted Mn phase. The formation of Sm₂O₃ was ascribed to the oxidation of the fine powders of the samples during XRD experiments when exposed to air. The lattice parameters of Sm_6−xLa_xMn₂₃ with x = 0.5, 2, and 3 are estimated to be 12.616 Å, 12.631 Å, and 12.656 Å, respectively. The lattice parameter of Sm₆Mn₂₃ was reported to be 12.594 Å in a previous study [13], indicating increasing lattice parameters of Sm_6−xLa_xMn₂₃ with increasing La content. We ascribe the increasing lattice parameters of Sm_6−xLa_xMn₂₃ with an increasing x to the larger ionic radius of La over that of Sm. The Th₆Mn₂₃ cubic structured phase almost disappears in Sm_6−xLa_xMn₂₃ when x = 4, where α-Mn is the main phase, as shown in Figure 1. Owing to the increasing La content and the inhomogeneity in Sm_6−xLa_xMn₂₃ (x = 1 and 4), a trace amount of La₂O₃ and LaMnO₃ could be found.

Figure 2 shows the typical morphological microstructure of the fractured surface of Sm_6−xLa_xMn₂₃ (x = 0.5, 2, and 3) alloys. The homogeneity of Sm_5.5La_0.5Mn₂₃ and Sm₄La₂Mn₂₃ seem better than that of Sm₃La₃Mn₂₃, in agreement with the XRD results that showing less stable Th₆Mn₂₃ structure and more impurities in the samples with an increasing x. The grain size of the melt-spun ribbons with x = 1 is much smaller than that of the other samples, as manifested by a broadening of the XRD peaks, owing to the rapid solidification process of the melt-spun ribbons.

A detailed compositional analysis was conducted by using the energy-dispersive X-ray spectrum. The atomic ratio of La:Sm:Mn was estimated to be ~0.575:5.75:23 (a1), in good agreement with the nominal composition in La_0.5Sm_5.5Mn₂₃ within the experimental uncertainties. It should be noted that a Mn-rich region (a2) was also observed by EDX and the atomic percentage of Mn was estimated to be 90.46% while no La element was observed. We ascribe the slight compositional deviation of Sm_5.5La_0.5Mn₂₃ to the evaporation of the constituents during melting and the inhomogeneity of the samples during the cooling process. The Sm₄La₂Mn₂₃ showed an atomic ratio of 1.53:5.5:23 at b1. The Sm₃La₃Mn₂₃ showed a presence of matrix regions (c1) with a composition close to the nominal composition, Mn-rich regions (c2) and rare-earth-rich region (c3), as shown in Figure 2b. The atomic ratio of the rare-earth-rich region c3 was estimated to be La: Sm: Mn ~10.7:8.2:23, while the atomic percentage of Mn in Mn-rich region was almost 100 % Mn at c2, corresponding to the minor phase of the α-Mn phase, as confirmed by XRD.

The morphological observations of the Sm₅LaMn₂₃ melt-spun ribbons and the Sm₂La₄Mn₂₃ ingot are shown in Figure 3. A clear phase separation of the main particulate phase separated by the inter-grain phase was observed in the melt spun ribbons, in agreement with the XRD results. The size of a domain which contains many grains with different orientations is around 3 μm or less, resulting in a broadening of the XRD diffraction peaks. It is interesting that the Sm₂La₄Mn₂₃ ingots show a quite different morphology of well-ordered particles with a smooth surface and spherical dimensions of 15 μm. The formation of the unique shape of the Sm₂La₄Mn₂₃ alloy is not clear. As mentioned above, the spherical micro-particles are mainly composed of α-Mn, owing to the instability of the Th₆Mn₂₃ phase at this composition.

The normalized temperature dependence of magnetization plots of Sm_6−xLa_xMn₂₃ (x = 0.5, 1, 2, 3) are shown in Figure 4a. The Curie temperature (T_C)—defined as the smallest of the dM/dT values—of the Sm_6−xLa_xMn₂₃ ingot/ribbons was measured to be 413 K for x = 0.5, 398 K for x = 1 ribbons, 426 K for x = 2, and 438 K for x = 3 ingots, respectively. Noticeably, the T_C of Sm₅LaMn₂₃ and Sm₆Mn₂₃ alloys prepared by induction melting in our previous work [12] was measured to be 403 K and 432 K, which agrees well with our results within experimental uncertainties. However, the mechanism of Tc variation with La-substitution remains unclear. It was suggested that the Mn–Mn exchange interactions in R₆Mn₂₃ may largely dominate the magnetic ordering near T_C with a collinear ferrimagnetic structure and the R–Mn magnetic interactions, positive for light lanthanides, are also non-negligible at high temperatures [14]. The magnetic parameters of the samples are listed in

Table 1. The composition, magnetic, and lattice parameters of Sm_6−xLa_xMn₂₃ (x = 0, 0.5, 1, 2, 3) with a Th₆Mn₂₃-type major phase. The x = 4 sample is not listed below because its major phase is α-Mn. The data for x = 0 and x = 1 ingots were from Ref. [12].

x	0	0.5	1	2	3
Lattice parameter (Å)	12.6	12.616	Ingot 12.67	12.631	12.656
			Ribbon 12.69/12.34
Ms (Am2/kg)	13	16.3	Ingot 16	15.8	5.6
			Ribbon 6
Hc at 5 K (T)	0.29	0.44	Ingot 0.74	0.47	0.32
			Ribbon 0.53
Tc (K)	432	413	Ingot 403 K	426	438
La/Sm/Mn	0/5.9/23-a1	0.575/5.75/23-a1		1.53/5.5/23-b1	0/0/1-c2
(atomic ratio)	0/0/1-a2	0/2.6/23-a2			10.7/8.2/23-c3

for comparison.

The temperature dependence of magnetizations of Sm_6−xLa_xMn₂₃ (x = 0.5, 1, 2 and 3) measured under an applied magnetic field of 6 T and 5 mT is shown in Figure 4b,c. Under an applied field of 6 T, the temperature of peak magnetization of La_xSm_6−xMn₂₃ (x = 0.5, 2 and 3) ranges from 71 K to 98 K, and that of Sm_6−xLa_xMn₂₃ (x = 0.5, 2 and 3) ranges from 207 K to 336 K at 5mT. The temperature shift in peak magnetization under a varied magnetic field may indicate the different M-T behaviors of different magnetic sublattices under varied applied magnetic fields. The maximum magnetization of Sm_5.5La_0.5Mn₂₃ reaches 18.15 Am²/kg at 6 T and 98 K. Noticeably, the melt-spun Sm₅LaMn₂₃ ribbons show no obvious maximum magnetization at 6 T, while it exhibits a peak magnetization at the temperature of 207 K at 5 mT, owing to the field effect.

The magnetic hysteresis loops of Sm_6−xLa_xMn₂₃ are shown in Figure 5. The saturation magnetization (Ms, defined as the magnetization value at 6 T) of Sm_6−xLa_xMn₂₃ at 5 K corresponds to 6.308 μ_B for x = 0.5, 1.732 μ_B for x = 1, 6.115 μ_B for x = 2 and 2.157 μ_B per unit cell for x = 3. The coercivity of Sm_6−xLa_xMn₂₃ at 5 K reached up to 0.44 T for x = 0.5, 0.53 T for x = 1, 0.47 for x = 2 and 0.32 T for x = 3, respectively. The saturation magnetization of the samples is closely related to the fraction of the Th₆Mn₂₃ magnetic phase in the samples. The magnetization of Sm_5.5La_0.5Mn₂₃ and Sm₄La₂Mn₂₃ was measured to be 11.45 Am²/kg and 10.6 Am²/kg, respectively; both are much larger than that of Sm_6−xLa_xMn₂₃ with x = 1 and x = 3.

4. Conclusions

The structural and magnetic properties of Th₆Mn₂₃-type Sm_6−xLa_xMn₂₃ (x = 0.5, 1, 2, 3 and 4) alloys were systematically investigated. The fraction of impurities increases with increasing La content and the Th₆Mn₂₃-type cubic structure almost disappears in Sm_6−xLa_xMn₂₃ when x = 4. The melt-spun LaSm₅Mn₂₃ shows lower structural stability and saturation magnetization than that of Sm_6−xLa_xMn₂₃ (x = 0.5, 1 and 2) prepared by induction melting, owing to the presence of impurities and inhomogeneous phases. The Curie temperature of Sm_6−xLa_xMn₂₃ falls in the range from 398 K for x = 1 to 438 K for x = 3, owing to the La-substitution to Sm in the lattices. The Sm_5.5La_0.5Mn₂₃ ingots shows a maximum Ms of 6.308 μ_B per unit cell and a maximum Mr of 11.45 Am²/kg.