Crystals 2021, 11, 1115. https://doi.org/10.3390/cryst11091115 www.mdpi.com/journal/crystals

Article

Tunable Martensitic Transformation and Magnetic Properties

of Sm-Doped NiMnSn Ferromagnetic Shape Memory Alloys

Najam ul Hassan 1,2, Mohsan Jelani 3, Ishfaq Ahmad Shah 2, Khalil Ur Rehman 4,5, Abdul Qayyum Khan 5,

Shania Rehman 6, Muhammad Jamil 7,8, Deok-kee Kim 6,* and Muhammad Farooq Khan 6,*

1 Department of Physics, University of Central Punjab, Sargodha 40100, Pakistan; najamphys@gmail.com 2 School of Materials Science and Engineering, Nanjing University of Science and Technology, Nanjing

210000, China; ishfaq.pak@gmail.com 3 Department of Physics, University of Kotli Azad Jammu and Kashmir, Kotli 11100, Pakistan;

mohsanjelani@uokajk.edu.pk 4 Department of Physics, University of Lahore, Lahore 40050, Pakistan; khalilpcsir@gmail.com 5 Applied Physics, Computer and Instrumentation Centre, PCSIR Laboratories Complex Ferozpur Road,

Lahore 54600, Pakistan; q_khan_uet@hotmail.com 6 Department of Electrical Engineering, Sejong University, 209 Neungdong-ro, Gwangjin-gu, Seoul 05006,

Korea; shania.rehman19@gmail.com 7 Division of Global Business, University College, Konkuk University, Seoul 05029, Korea;

mjamil@konkuk.ac.kr 8 Department of Physics, Konkuk University, Seoul 05029, Korea

* Correspondence: deokkeekim@sejong.ac.kr (D.-k.K.); mfk@sejong.ac.kr (M.F.K.)

Abstract: NiMnSn ferromagnetic shape memory alloys exhibit martensitic transformation at low

temperatures, restricting their applications. Therefore, this is a key factor in improving the marten-

sitic transformation temperature, which is effectively carried out by proper element doping. In this

research, we investigated the martensitic transformation and magnetic properties of Ni43Mn46-x

SmxSn11 (x = 0, 1, 2, 3) alloys on the basis of structural and magnetic measurements. X-ray diffraction

showed that the crystal structure transforms from the cubic L21 to the orthorhombic martensite and

gamma (γ) phases. The reverse martensitic and martensitic transformations were indicated by exo-

thermic and endothermic peaks in differential scanning calorimetry. The martensitic transformation

temperature increased considerably with Sm doping and exceeded room temperature for Sm = 3 at.

%. The Ni43Mn45SmSn11 alloy exhibited magnetostructural transformation, leading to a large mag-

netocaloric effect near room temperature. The existence of thermal hysteresis and the metamagnetic

behavior of Ni43Mn45SmSn11 confirm the first-order magnetostructural transition. The magnetic en-

tropy change reached 20 J·kg−1·K−1 at 266 K, and the refrigeration capacity reached ~162 J·Kg−1, for

Ni43Mn45SmSn11 under a magnetic field variation of 0–5 T.

Keywords: martensitic transformation; X-ray diffraction; magnetocaloric effect; magnetic entropy

changes

1. Introduction

The materials showing martensitic transformation (MT) exhibit various multifunc-

tional phenomena, such as the magnetocaloric effect (MCE) [1,2], exchange bias (EB) [3–

5], magnetothermal conductivity (MC) [6,7] and magnetoresistance (MR) [8–10]. This cou-

pled transition is obtained around the MT temperature (Tt). These multifunctional prop-

erties show application in vast areas such as magnetic refrigeration, energy harvesting,

magneto-mechanical devices and sensors [11–14]. The MCE is a magneto-thermodynamic

phenomenon in which the temperature is changed in the material when exposed to an

external non-constant magnetic field [15]. MCE-based magnetic refrigeration has many

advantages over conventional gas cooling technology, such as being environmentally

Citation: Hassan, N.u.; Jelani, M.;

Shah, I.A.; Rehman, K.U.; Khan, A.Q.;

Rehman, S.; Jamil, M.; Kim, D.-k.;

Khan, M.F. Tunable Martensitic

Transformation and Magnetic

Properties of Sm-Doped NiMnSn

Ferromagnetic Shape Memory

Alloys. Crystals 2021, 11, 1115.

https://doi.org/10.3390/cryst11091115

Academic Editor: Cyril Cayron

Received: 12 August 2021

Accepted: 9 September 2021

Published: 13 September 2021

Publisher’s Note: MDPI stays neu-

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Copyright: © 2021 by the authors. Li-

censee MDPI, Basel, Switzerland.

This article is an open access article

distributed under the terms and con-

ditions of the Creative Commons At-

tribution (CC BY) license (http://crea-

tivecommons.org/licenses/by/4.0/).

Crystals 2021, 11, 1115 2 of 10

friendly, having high refrigerant efficiency and low cast measures, occupying less space,

possessing low mechanical vibration and being harmless [16–19]. MCE materials are uti-

lized in magnetic refrigeration technology. The materials showing first-order magnetic

transitions are widely used for magnetic cooling and investigated due to their large MCE,

e.g., LeFeSi [20–22], MnFePAs [23], GdSiGe [24], MMnX (M = Ni or Co, X = Ge or Si) [25]

and NiMnZ (Z = Sn, In, Sb) ferromagnetic shape memory alloys (FMSMAs) [26].

FMSMAs exhibit first-order magnetostructural transformation from high-tempera-

ture austenite to low-temperature martensite upon cooling, with considerable thermal

hysteresis and other effects such as lattice distortion and abrupt changes in magnetization.

The magnetic field induces a first-order magnetostructural transition in these alloys, gen-

erating the MCE [27]. It is promising to tune the Tt in a wide temperature range, endowing

FMSMAs with the ability to exhibit large multifunctional effects under a low applied mag-

netic field for magnetic applications. It is reported that the Tt can be tuned by changing

the valence electron concentration (e/a) [28], grain size [29], external parameters such as

the magnetic field and applied pressure [30], volume of the unit cell [14] and heat treat-

ment [31], while adjustable elemental composition and preparation conditions are very

important to achieve the MCE for the desired applications [25–27].

NiMnSn-based alloys undergo a reverse martensitic transformation due to magneto-

structural coupling and exhibit a variety of multifunctional properties, which indicates a

potential for many applications. The utilization of MCE materials strongly depends upon

the mechanical properties and magnetostructural coupling. Unfortunately, the highly

brittle nature and poor mechanical properties of NiMnSn alloys are major drawbacks. Ad-

ditionally, NiMnSn alloys show a low working temperature due to coupling between MT

and ferromagnetic transition [32].

Only a few studies have reported on the enhancement of the working temperature of

these alloys, which is the main point of application. Moreover, it is well known that a large

MCE has been reported in rare-earth metal-based materials, i.e., Gd5Si2Ge2 [24]. Mean-

while, it has also been reported that the addition of rare-earth elements could enhance the

Tt in these types of alloys. Therefore, it can be anticipated that the addition of proper rare-

earth elements in NiMnSn FMSMAs can tune the Tt to a higher temperature, resulting in

the MCE near room temperature (RT). There is still a lack of information and knowledge

about the rare-earth element Sm after it has been doped. In this research, we doped Sm in

Ni43Mn46-x SmxSn11 (x = 0, 1, 2, 3) alloys which increases the Tt considerably, and a large

MCE is obtained near RT for the Ni43Mn45SmSn11 alloy sample.

2. Materials and Methods

Polycrystalline Ni43Mn46-x SmxSn11 (x = 0, 1, 2, 3) alloy samples were prepared by arc

melting 99.99% pure Ni, Mn, Sn and Sm using a water-cooled copper crucible in an argon

atmosphere. The melting process was repeated four times to ensure the purity and uni-

formity of the alloys. Homogeneity is an important factor to enhance the properties of the

materials [31]. Therefore, ingots were annealed at 1123 K for 2 days followed by quenching

in ice water. The elemental composition of the samples was determined by X-ray energy-

dispersive spectroscopy (EDS, Thermo system 7, Malvern UK), and powder X-ray diffrac-

tion (XRD, Bruker, D8 Advance, Madison, WI, US) was used to identify the crystal struc-

ture at room temperature. The structural transitions were investigated using differential

scanning calorimetry (Mettler Toledo, DSC 3, Nanterre, France), while the magnetic prop-

erties were examined by a magnetic property measurement system (MPMS-7, Quantum

Design, San Diego, CA, USA) and physical property measurement system (PPMS, Quan-

tum Design, Dynacool, San Diego, CA, USA). The so-called loop method was used to

measure isothermal magnetization (MH) curves to avoid irreversibility [33].

Crystals 2021, 11, 1115 3 of 10

3. Results and Discussion

Energy-dispersive X-ray spectroscopy (EDS) was used to check the appropriate

amount and homogeneity of the constituent elements in the prepared alloys. The EDS

results of the Ni43Mn46–xSmxSn11 (x = 0, 1, 2, 3) alloys are shown in Table 1, confirming the

presence of all precursor elements. The obtained compositions of the alloys were found to

be very close to the nominal composition. Here, a small deviation in the composition of

the elements is very common due to various factors such as weighing the precursor ele-

ments, melting and annealing. The electronic concentration (e/a) was measured using

Equation (1) [34].

e/a = (7 × at. % Mn + 10 × at. % Ni + 4 × at. % Sn + 8 × at. % Sm)/10 (1)

Table 1. Atomic weight percentage and electronic concentration of Ni43Mn46-xSmxSn11 (x = 0, 1, 2 and

3) alloys.

Sample Atomic Weight Percentage (%) Electronic Concentration

(e/a) Ni Mn Sn Sm

x = 0 42.96 46.09 11.05 0 79.643

x = 1 42.92 45.11 10.90 1.07 79.713

x = 2 42.90 43.96 11.03 2.11 79.772

x = 3 43.05 43.02 10.90 3.03 79.794

It is well known that martensites with twin boundaries play an important role in the

magnetic field-induced strain of FMSMAs, depending upon the twin boundary migration.

The X-ray diffraction patterns of the Ni43Mn46-x SmxSn11 (x = 0, 1, 2, 3) alloy samples are

shown in Figure 1.

Figure 1. XRD patterns of Ni43Mn46-xSmxSn11 (x = 0, 1, 2, 3) alloys at room temperature.

The reflection peaks corresponding to the cubic austenite phase are indicated by

green squares. On comparing the NiMnSn alloys reported by Krenke et al. [35], the main

reflection peaks corresponding to the martensitic phase are indicated by red circles, and

extra peaks which indicate the presence of an extra γ phase are represented by blue trian-

gles. The XRD pattern for x = 0 shows typical diffraction peaks from the austenite phase,

indicating a single cubic L21 crystal structure. As MT is closely linked to the crystal struc-

ture, the presence of the L21 cubic structure for x = 0 indicates that its Tt is below the RT.

For the x = 1 and 2 Sm-doped samples, a mixed complicated structure of the cubic L21

Crystals 2021, 11, 1115 4 of 10

phase, orthorhombic martensite phase and γ phase is exhibited. The coexistence of aus-

tenite and martensite phases indicates that the Tt lies near RT for these samples.

Ni43Mn43Sn11Sm3 presents a two-phase structure, i.e., orthorhombic martensite phase and

γ phase. The disappearance of the austenite phase in Ni43Mn43Sm3Sn11 indicates that the Tt

is increased from RT. This is in good agreement with the DSC measurement, which is

discussed in the next session. For x = 0, the alloy sample crystallizes in the cubic L21 struc-

ture with lattice parameters a = b = c = 5.986 Å. In the case of x = 1 and 2, there is the

appearance of an orthorhombic martensite phase with dominating cubic austenite with

lattice parameters a = b = c = 5.784 Å. It is well known that the crystal structure of NiMnSn

alloys is sensitive to the composition. With Sm doping, the 220 peak shifts minutely to-

wards a higher angle, and the cell volume decreases. When Sm is increased to 3 at. %, the

crystal structure changes completely to the 10 M orthorhombic phase with lattice param-

eters a = 476 Å, b = 5.64 Å and c = 26 Å.

Furthermore, it can also be seen that as the Sm content is increased, the γ peak be-

comes stronger. Therefore, it can be speculated that the appearance of unknown peaks is

due to the presence of Sm, as reported earlier for rear-earth metals [36]. A detailed inves-

tigation about the γ phase may be obtained from the temperature-dependent XRD and

TEM analyses. Therefore, XRD patterns show that with increasing Sm contents, the struc-

tural transformation from austenite to martensite takes place, which is responsible for an

increase in the Tt.

The stability of the martensite temperature is a key factor in the application temper-

ature of FMSMAs based on twin boundary migration [34]. Therefore, it is very important

to tune the Tt temperature in a wide stable temperature range. The thermal-induced struc-

tural transformations in the Ni43Mn46-x SmxSn11 (x = 0, 1, 2, 3) alloys were studied by DSC

analysis from 160 to 360 K at a ramp rate of 10 K/min during heating and cooling cycles,

as shown in Figure 2a. The existence of large exothermic/endothermic peaks during the

heating/cooling process is assigned to the occurrence of reverse martensitic/martensitic

transformation in the alloys. These peaks confirm the structural transformation between

the austenite and martensite phases.

The first-order nature of the transformation is evident by the presence of thermal

hysteresis among the heating and cooling processes [37]. The characteristic MT tempera-

tures are the austenite start temperature As, austenite finish temperature Af, martensite

start temperature Ms and martensite finish temperature Mf. The Tt in heating and cooling

processes can be calculated using the equations TtH = (As + Af)/2 and TtC = (Ms + Mf)/2. The

characteristic temperatures of the MT of the Ni43Mn46-xSmxSn11 (x = 0, 1 and 3) alloys are

listed in Table 2. The sample without Sm doping (x = 0) is found far below the RT (As = 200

K, Af = 214 K, Ms = 197 K and Mf = 184 K). With a small amount of Sm doping (x = 1), the Tt

increases, with a range of 62 to 69 K, i.e., the As, Af, Ms and Mf are increased by 69, 62, 64

and 65 K, respectively. An increase in the Tt has already been reported in FMSMAs by Gd

(rare element substitution) [36–38]. With the further increase, the Mt shows an overall in-

creasing tendency.

Crystals 2021, 11, 1115 5 of 10

Figure 2. (a) DSC curves of Ni43Mn46-xSmxSn11 (x = 0, 1, 2, 3) alloys during heating and cooling processes. (b) The effect of

the Sm content on the phase transformation temperatures of Ni43Mn46-xSmxSn11 (x = 0, 1, 2, 3) alloys.

Many investigations on FMSMA alloys show that the composition of the constituents

influences the Tt. The results reveal that the valance electron concentration (e/a) is the most

significant factor affecting the martensitic transformation, and that there is a linear rela-

tionship between the Tt and e/a [34]. The second important factor that affects the MT is the

change in the matrix composition. It is agreed that the substitution with atoms having

different sizes from their parent atoms results in the expansion or contraction of the unit

cell, and hence lattice distortion. Figure 2b shows that as the e/a is increased, the charac-

teristic temperatures of martensitic transformation increase linearly, which is in accord-

ance with previous reports [32].

Table 2. The measured values of characteristic temperatures for Ni43Mn46-xSmxSn11 (x = 0, 1, 2 and 3)

alloys.

x As

(K)

Af

(K)

Ms

(K)

Mf

(K)

Tt (Heating)

(K)

Tt (Cooling)

(K)

0 200 214 197 184 207 190

1 269 276 261 247 272 254

2 294 305 292 278 299 285

3 314 327 311 294 320 302

The temperature dependence of the magnetization (M–T) curves of the Ni43Mn46-

xSmxSn11 (x = 0, 1, 2 and 3) alloys were measured to investigate the magnetic phase transi-

tion. M–T curves were measured under the applied magnetic field of 0.01 T, with a tem-

perature range of 150–400 K, during heating and cooling processes, as shown in Figure

3a–d. It can be seen that for x = 0, the alloy sample experiences a magnetic order–disorder

transition of martensite at about ≈175 K, followed by reverse martensitic transformation

to ferromagnetic austenite at ≈200 K during heating.

With a further increase in the temperature to about ≈310 K, the magnetization grad-

ually decreases to zero, which corresponds to the Curie temperature of austenite. During

the cooling process, however, a sudden drop in magnetization can be observed at about

≈197 K, corresponding to the occurrence of MT from the martensite to the austenite phase.

Irreversibility during the heating and cooling processes is observed, which is the signature

of the first-order transition and is frequently seen in these alloys [16]. It is seen that the Tt

increases remarkably with the Sm addition. For x = 1, martensitic transformation occurs

between the paramagnetic austenite and ferromagnetic martensite, and the Tt increases

by ≈ 65 K, but the Curie temperature of austenite almost remains the same. As shown in

Figure 3c,d, the behavior is different for x = 2 and 3: here, the structural transition occurs

Crystals 2021, 11, 1115 6 of 10

in the paramagnetic state, and magnetic transition disappears, indicating that the mag-

netic field cannot induce the transition in x = 2 and 3. The characteristic temperatures

measured in the M–T curves agree well with the DSC measurements.

Figure 3. (a–d) The temperature-dependent magnetization (M–T) curves of Ni43Mn46-xSmxSn11 (x = 0, 1, 2, 3) alloys under

an applied magnetic field of 0.01 T during the heating process.

It can be seen from the DSC and M–T measurements that the Ni43Mn45SmSn11 alloy

undergoes structural as well as magnetic transition near the Tt. Therefore, isothermal mag-

netization (M–B) curves under the applied magnetic field of 5 T around MT were meas-

ured to investigate the field-induced transition, as shown in Figure 4a. Initially, the sample

was cooled down to complete the martensitic state and then slowly heated to the target

temperature before starting each M–B measurement, known as the loop method [33]. This

alloy exhibited a weak magnetic behavior at temperatures below the Tt (248 and 257 K),

while near the Tt (260, 263 and 266 K), it showed a metamagnetic behavior, which indicates

that the magnetic field can induce the transition and then transform to a strong magnetic

state at 269 K. Hence, it can be concluded from the M–B curves that the magnetic field

induced MT between the weak magnetic austenite and strong magnetic martensite state.

A magnetization difference (∆M) of about 38 Am2/kg under a field of 5 T was observed in

this alloy. This ∆M is attributed to the field driving capacity [39].

Crystals 2021, 11, 1115 7 of 10

Figure 4. (a) Isothermal magnetization (M–B) curves of Ni43Mn45SmSn11 alloy at different temperatures around martensitic

transformation. (b) High-field M–T curves for Ni43Mn45SmSn11 alloy with a magnetic field of 5 T during heating (solid

symbols) and cooling (open symbols) processes.

The structural transformation between the austenite and martensite phases in the

Ni43Mn45SmSn11 alloy can be driven by the magnetic field, indicated by the magnetization

difference. The M–T curves of the Ni43Mn45SmSn11 alloy were measured under a field of 5

T, as shown in Figure 4b. If a comparison is made with M–T at 0.01 T, the Tt decreased by

about ≈9 K, and ΔM increased by ≈37 Am2/kg, with the rate of As shifted by the magnetic

field, i.e., dAs/dB, being −1.8 K·T−1, confirming the field-induced MT. This magnetic field

driving capacity for the Ni43Mn45SmSn11 alloy can be attributed to the larger value of ΔM.

Commonly, M–T curves at different magnetic fields can be used to obtain the values of

magnetic entropy change (ΔSM). The transformation hysteresis defined as Af–Ms [39] was

18 and 16 K under the applied magnetic fields of 5 and 0.01 T. Here, we calculated the ΔSM

of the Ni43Mn45SmSn11 alloy using the Clausius–Clapeyron equation (ΔS = −ΔM·dB/dT)

from the M–T curves at 0.01 and 5 T. The obtained value of ΔS was 20.51 J·K−1·kg−1 (using

ΔM = 37 Am2/kg, change in magnetic field (ΔB) = 4.99 T and temperature change (ΔT) = 9

K), which is in good agreement with the ∆SM obtained from the M–B curves using Maxwell

relations.

The temperature-dependent ∆SM of the Ni43Mn45SmSn11 alloy was calculated by the

M–B curves using the Maxwell relation given in Equation (2). The measured ∆SM for the

Ni43Mn45SmSn11 alloy was 20 J·kg−1·K−1 at a temperature of 266 K, with a magnetic field

variation of 0–5 T, as shown in Figure 5. The ∆SM originates from a magnetic field-induced

magnetostructural transition. It can be seen that with 1% doping of Sm, the working tem-

perature of the NiMnSn alloy increased to 266 from ~205 K, showing potential for mag-

netic refrigeration applications near RT. Moreover, the ∆SM obtained for Ni43Mn45SmSn11 at

266 K is comparable to many other first-order magnetocaloric materials [16,34,39–41].

B

M

0

δM(B,T)S = dB

δT (2)

The maximum value of the ∆SM derived from Equation (2) is smaller than the ΔST

calculated from the DSC measurements because the magnetic field of 5 T is unable to un-

dergo a complete phase transition.

The refrigeration capacity (RC) is a very important factor to assess the MCE and can

be calculated by integration of the area under the ∆SM–T curves, using the temperature at

half maximum of the peak as the upper and lower integration limits [34]. The calculated

RC for the Ni43Mn45SmSn11 alloy under the 5 T field was 262 J·Kg–1. The high MCE and RC

values near RT show that the Ni43Mn45SmSn11 alloy can be used in magnetic refrigeration.

Crystals 2021, 11, 1115 8 of 10

Figure 5. The temperature dependence of ∆SM of Ni43Mn45SmSn11 alloy under an applied magnetic

field of 5 T measured upon heating.

4. Conclusions

The effect of Sm doping on martensitic transformation and the magnetic properties

of Ni43Mn46-xSmxSn11 ferromagnetic shape memory alloys was investigated. The results

show that Sm doping changes the crystal structure from cubic L21 to orthorhombic mar-

tensite, with a fraction of the γ phase. The DSC measurements showed endothermic/exo-

thermic peaks, indicating the martensitic and reverse martensitic transformation. The Tt

increased significantly with the Sm content and exceeded RT for Sm = 3 at. %. The

Ni43Mn45SmSn11 alloy exhibited a magnetostructural transition, and a magnetic field-in-

duced transition was confirmed by the high-field M–T and M–B measurements. Moreo-

ver, a magnetic entropy change (∆SM) value of 20 J·kg−1·K−1 was reached at a temperature

of 266 K, and the RC was about 166 J·kg−1, obtained for the Ni43Mn45SmSn11 alloy under an

applied magnetic field of 5 T. The large ∆SM value near RT for the Ni43Mn45SmSn11 alloy

shows its potential for magnetic refrigeration application.

Author Contributions: Conceptualization, N.u.H. and M.J. (Mosin Jelani); methodology, N.u.H.

and I.A.S.; formal analysis, N.u.H. and K.U.R.; investigation, N.u.H., M.J. and I.A.S.; resources,

M.F.K. and A.Q.K.; data curation, K.U.R. and A.Q.K.; writing—original draft preparation, N.u.H.;

writing—review and editing, N.u.H., M.J. (Muhamma Jamil), S.R. and M.F.K.; supervision, M.F.K.

and D.-k.K.; project administration, D.-k.K.; funding acquisition, M.F.K. and D.-k.K. All authors

have read and agreed to the published version of the manuscript.

Funding: This research was supported by the National Research Foundation of Korea (NRF), ICT

(2020R1G1A1012022). This research was also supported by the MOTIE (Ministry of Trade, Industry

and Energy (10080581)) and KSRC (Korea Semiconductor Research Consortium) support program

for the development of future semiconductor devices.

Data Availability Statement: All data is provided in the manuscript.

Conflicts of Interest: The authors declare no conflict of interest.

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