1 Introduction

Materials with capacity of multi-magnetoresponsive effects are of broad scientific and technological interest due to the associated phenomena related to both, the magnetic order–disorder transition (i.e., Curie transition at a specific temperature Tc), and structural transformations (e.g., martensitic-austenitic transition at a particular temperature Tt), as well as the possibility to couple both phenomena to enhance functional responses such as magnetic field-induced strain, magnetoresistance, shape memory, and magnetocaloric effect [1,2,3,4,5,6]. In particular, MnCoGe-based alloys have been profusely studied due to the fact that these alloys present a diffusionless martensitic transformation from a low-temperature orthorhombic phase (with TiNiSi-type crystal structure) to a high temperature hexagonal phase (with Ni2In-type structure) [2, 7]. This transformation occurs at 650 K. On the other hand, both phases present Curie transitions at distinctive temperatures of 345 K and 275 K, respectively. In spite of the good modulability of magnetic and structural properties of MnCoGe alloys [2, 7], a significant disadvantage is their Ge content, because of the high cost and scarcity of germanium metal. Much less attention has been devoted to related MnCoSi alloys, for which Tt transformation goes through above 1100 K, but Tc transition occurs around 350 K, rendering these alloys as an attractive choice for room temperature applications [8,9,10,11] In this work, we present and discuss magnetic properties of melt-spun MnCoSiB alloys, as well as their magnetocaloric response.

2 Experimental Methods

Initial ingots of CoMnSi and MnCoSiB0.5 alloys were produced by Ar-arc melting in a titanium-gettered atmosphere. Ingots were remelted 4–5 times in order to ensure initial homogeneity. Metallic ribbons were subsequently obtained by chill block melt spinning onto a copper roll by using rotating speeds of 22 m/s (CoMnSi) and 30 m/s (CoMnSiB0.5) in a high purity argon atmosphere within a sealed chamber. Phase distribution analysis was performed by X-ray diffraction analysis (XRD) in a Bruker D8 ADVANCE diffraction equipment with Cr radiation (λ = 2.2897 Å). Further determination of unit cell parameters and crystal structure was carried out by Rietveld fitting. Microstructural features (morphology, grain size, chemical composition) were characterized by scanning electron microscopy (SEM) in a JEOL 7600 F equipment coupled with an energy-dispersive spectroscopy (EDS) unit. Magnetic properties together with magnetocaloric response were studied by means of vibrating sample magnetometry by using a Quantum Design MPMS3 equipment with maximum applied field of 2.0 Tesla. Measurements were made in the plane of the ribbon samples and transverse to the spinning direction so that no correction for self-demagnetization was necessary.

3 Results and Discussion

X-ray diffractograms for MnCoSi and MnCoSiB0.5 melt-spun alloys are shown in Fig. 1, including experimental data, Rietveld fitting, and the corresponding difference. All the peaks were identified as reflections of the orthorhombic MnCoSi phase with the following parameters a = 5.808 Å, b = 3.694 Å, c = 6.869 Å, and unit cell volume V = 147.17 Å3 for MnCoSi alloy and a = 5.791 Å, b = 3.697 Å, c = 6.873 Å, and V = 147.43 Å3 for MnCoSiB0.5 sample. The slight variation of the unit cell volume of the Boron-containing alloy reflects the incorporation of B atoms into the crystal structure of the MnCoSi phase. Both set of parameters are consistent with those reported in ICDD 00–030-0448 file (a = 5.819 Å, b = 3.691 Å, c = 6.853 Å, and V = 147.1 Å3). No secondary phases were observed for both alloy compositions.

Fig. 1
figure 1

X-ray diffractograms for a) MnCoSi and b) MnCoSiB0.5 melt-spun alloys. All reflections were associated with orthorhombic MnCoSi phase (only main peaks were labeled with their Miller indices). For both samples, the experimental data is included, together with calculated data and their corresponding difference

Full size image

SEM micrographs for both MnCoSi and MnCoSiB0.5 melt-spun alloys are displayed in Fig. 2. A polycrystalline microstructure comprising polyhedral, randomly oriented grains is evident for both samples, with distinctive average grain sizes of 2.13 ± 1.5 microns and of 0.76 ± 1.1 microns for MnCoSi and MnCoSiB0.5 alloys, respectively (see distribution histogram for each case). The noticeable grain size refinement for the B-containing alloy can be ascribed to the higher roll speed used for this simple (of 30 m/s compared with 22 m/s of MnCoSi composition), which promotes higher cooling rates and hence, higher number of nucleation sites yielding to reducing mean grain sizes. In addition, due to the very fast extraction of heat, there is no time for crystal growth at any preferred orientation, which in turn leads to random orientation of grains. This fact is supported by XRD measurements, for which no texture was detected for any diffracting plane identified in Fig. 1. The absence of significant microstructural defects (like voids or pores) indicates a well-defined single-phase crystalline structure, consistent with a microstructural homogeneity, which in turn favors a soft magnetic behavior, as discussed in the next section. Chemical homogeneity of alloys was verified by EDS mapping as shown in Fig. 3 for the MnCoSiB0.5 alloy, for which no segregation of any atomic species (Mn, Co, Si) is observed, which in turn is consistent with XRD results indicating no secondary phases formation for both alloy samples. The EDS mapping observed for the MnCoSi sample show same features (see Fig. S1 of supplementary material).

Fig. 2
figure 2

SEM micrographs for a) MnCoSi and b) MnCoSiB0.5 melt-spun alloys. Main grain sizes are of 2.13 ± 1.5 microns and of 0.76 ± 1.1 microns, respectively

Full size image
Fig. 3
figure 3

EDS mapping for the MnCoSiB0.5 alloy sample showing chemical homogeneity and single-phase formation

Full size image

Magnetization curves of magnetization M as a function of applied field H are displayed in Fig. 4 for MnCoSi and MnCoSiB0.5 melt-spun alloys. A clear soft magnetic behavior is manifested for both samples, with coercivity field Hc values of 4.0 and 12.0 Oe, respectively, as well as saturation magnetization of 80 and 78 emu/g respectively. The increment of Hc observed for the CoMnSiB0.5 alloy can be associated with its grain size refinement since Hc presents a marked dependence with microstructural features and, in particular, with grain size, for which enhanced coercivity values with decreasing grain size in soft magnetic materials has been widely reported [12,13,14]. In general, low Hc values are consistent with the microstructure homogeneity described in previous section since such structural features facilitate the formation and displacement of domain walls and hence the soft magnetic response characterized for Hc values well below 20 Oe. On the other hand, the slight decrease (2%) observed for the saturation magnetization Ms of the CoMnSiB0.5 alloy can be attributed to a dilution effect of the magnetic moment provoked by the inclusion of B atoms into the unit cell structure. This decrement is consistent with the intrinsic nature of Ms, which renders this property as strongly dependent of chemical composition.

Fig. 4
figure 4

M-H curves for both MnCoSi and MnCoSiB0.5 melt-spun alloys showing a characteristic soft magnetic response with very low coercivity fields of 4.0 and 12.0 Oe, respectively and saturation magnetization values of ~ 80 emu/g

Full size image

Thermomagnetic plots of M as a function of temperature T, for an applied filed H = 100 Oe, are displayed in Fig. 5 for MnCoSi and MnCoSiB0.5 melt-spun alloys, showing the Curie transition (Tc) for both samples at 399 and 425 K, respectively (see inset showing a well-defined peak for the derivative dM/dT at both Tc transitions). These Tc values are consistent with those reported for similar alloys, for which an interval of 390–420 K for Tc has been determined [11,12,13,14,15,16]. The significant difference observed of Tc for the B-containing alloy can be attributed to the incorporation of B atoms into the unit cell structure of the MnCoSi phase, which affects the Co–Co and Mn–Mn interatomic distance, and hence, the exchange interaction determining Tc.

Fig. 5
figure 5

Thermomagnetic M-T (with constant H = 100 Oe) plots for MnCoSi and MnCoSiB0.5 melt-spun alloys, showing Tc for both samples

Full size image

On the other hand, isotherms for initial M-H curves are illustrated in Fig. 6a, b, for which the ferromagnetic–paramagnetic transition is observed as a progressive linear tendency with increasing temperature. Full linear response indicates the Curie transition. The associated Arrott plots M2 vs (H/M) (Fig. 7a, b) show a well-defined linear tendency with positive slope at high H values, reflecting the second-order character of the magnetic transition [17].

Fig. 6
figure 6

Initial M-H isotherms for a) MnCoSi and b) MnCoSiB0.5 alloy samples

Full size image
Fig. 7
figure 7

Associated Arrott plots for a) MnCoSi and b) MnCoSiB0.5 alloys

Full size image

Within the frame of the modified Arrott plot formalism [18] the equation of state

$${\left(H/M\right)}^{\frac{1}{\gamma }}=a\frac{T-{T}_{c}}{T}+b{M}^{\frac{1}{\beta }}$$
(1)

where a and b are constants and γ and β are the critical exponents, which can be used to identify long-range exchange interactions, in particular, when the fitting exponents γ and β have values of 1.0 and ½, respectively [19]. Such conditions correspond to the Arrott plots of Fig. 7, for which Eq. (1) was fitted with γ = 1.0 and β = 1/2. These critical exponents values indicate consistency with the mean field model, and hence, long-range exchange interactions are expected in our alloys [19], which in turn is congruent with the structural and chemical homogeneity observed by XRD and SEM data.

The magnetocaloric response for MnCoSi and MnCoSiB0.5 melt-spun alloys were determined by means of the Maxwell relationships, for which the variation of magnetic entropy ΔSm as a function of temperature T can be calculated as [20].

$$\Delta {S}_{M}={\underset{{H}_{0}}{\overset{{H}_{f}}{\int }}\left(\frac{\partial M}{\partial T}\right)}_{H}dH$$
(2)

Curves for ΔSm (T) for both alloy compositions with a variation of applied field ΔH = 2.0 Tesla are displayed at Fig. 8, for which maximum entropy variation ΔSmmaxv for each alloy can be found around 1.0 J/kg K. This moderated response can be ascribed of the pure second-order character of the magnetic transition since such transition occurs at temperatures well below the structural transition reported at 1165 K for this type of alloys [11], and thus, both transformations events (magnetic and structural) occur uncoupled, yielding to the limited magnetocaloric response observed.

Fig. 8
figure 8

Magnetic entropy variation ΔSM as a function of temperature for MnCoSi and MnCoSiB0.5 alloys for ΔH = 2.0 T

Full size image

In addition, the refrigerant capacity (RC) for both alloys between Tcold and Thot sinks (calculated as RC = (Thot − Tcold) × ΔSmpeak), resulted of 108 J/kg and 95 J/kg for MnCoSi and MnCoSiB alloys, respectively. These values compare favorably with those of similar melt-spun MnCoGe alloys (of 90 J/kg)2. This property is of particular interest since it represents the capacity of the material to transport thermal energy between Tcold and Thot sinks, and hence, RC characterizes the alloy ability for technical application as cooling element within a magnetic refrigeration process.

4 Conclusions

The microstructural and magnetic properties of MnCoSi and MnCoSiB0.5 melt-spun alloys were characterized by XRD, SEM, and VSM techniques. Single-phase materials were established for both compositions. Magnetic properties were identified as characteristic of soft magnetic materials, with intermediate values of saturation magnetization (of 80 emu/g), low coercivity fields (below 20 Oe), and attractive Curie temperatures between 390 and 430 K. Magnetocaloric evaluation for both melt-spun alloys indicated a maximum magnetic entropy value limited to 1.0 J/kg K, together with a noticeable refrigerant capacity over 100 J/kg. This last property suggests an advantageous ability of our MnCoSiB alloys to transport thermal energy between cold and hot points of a magnetic refrigeration process and thus rendering these alloys as functional materials with potential interest for magnetocaloric applications.