Materials Science and Engineering B 176 (2011) 1322–1325 Contents lists available at ScienceDirect Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb Short communication Structural, magnetic and magnetocaloric properties of Heusler alloys Ni50 Mn38 Sb12 with boron addition N.V Nong a,∗ , L.T Tai b , N.T Huy c , N.T Trung d , C.R.H Bahl a , R Venkatesh a , F.W Poulsen a , N Pryds a a Fuel Cells and Solid State Chemistry Division, Risø National Laboratory for Sustainable Energy, Technical University of Denmark, 4000 Roskilde, Denmark Cryogenic Laboratory, Faculty of Physics, Hanoi University of Science, Vietnam National University, 334 Nguyen Trai, Hanoi, Viet Nam PetroVietnam University, 173 Trung Kinh, Hanoi, Viet Nam d Fundamental Aspects of Materials and Energy, Faculty of Applied Sciences, TU Delft, Mekelweg 15, 2629 JB Delft, The Netherlands b c a r t i c l e i n f o Article history: Received March 2011 Received in revised form June 2011 Accepted 11 July 2011 Available online 27 July 2011 Keywords: Heusler alloys Martensitic transition Magnetocaloric effect a b s t r a c t We report on the structural, magnetic and magnetocaloric properties of the Ni50 Mn38 Sb12 Bx alloys in term of boron addition with x = 1, and We have found that both the paramagnetic–ferromagnetic austenitic transition (TC ) and the ferromagnetic–antiferromagnetic martensitic transition (TM ) are sensitively influenced by the boron addition: TC tends to increase, while TM decreases with increasing boron concentration Temperature dependent X-ray diffraction in the range of 200–500 K clearly shows an evolution of the structural transformation from orthorhombic to cubic structure phase transition on heating for the x = and samples Strikingly, the addition of boron atoms into the lattice favours the ferromagnetic ordering relatively to the antiferromagnetic arrangement below TM This consequently affects on the magneto-structural transition as well as on the size of magnetocaloric effect © 2011 Elsevier B.V All rights reserved Introduction An external magnetic field can strongly affect the magnetic order of a material, and also its temperature if phonon and magnetic excitations are well coupled via spin–lattice coupling In most of the magnetocaloric systems, although the magnetic entropy change ( SM ) is dominant the entropy change associated with the lattice vibration is comparably large, and the lattice entropy change can resonate the magnetic entropy change one in some systems (e.g., in the Gd5 Si4−x Gex family), while it counteracts in other ones (for example in the LaFe13−x Six system) [1] The resulting change of temperature of the material ( Tad ) is known as magnetocaloric effect (MCE) [2] Although the principle of magnetic cooling by means of adiabatic demagnetization has been used to achieve MilliKelvin temperatures for more than seventy years [3], the search for suitable magnetocaloric materials for domestic applications of magnetic refrigeration is still in progress Nowadays, most studies devoted to room-temperature applications of magnetic refrigeration are focused on materials showing a giant MCE associated with a first-order magneto-structural transition (FOMST) After the major breakthrough when the so-called “giant” MCE was discovered in the pseudo binary system Gd5 (Six Ge1−x )4 [4], it soon appeared that other materials such as MnFeP1−x Asx [5], MnAs1−x Sbx [6], La(Fe1−x Six )13 [7] and ∗ Corresponding author Tel.: +45 5142 3023 E-mail address: ngno@risoe.dtu.dk (N.V Nong) 0921-5107/$ – see front matter © 2011 Elsevier B.V All rights reserved doi:10.1016/j.mseb.2011.07.013 Ni0.50 Mn0.50−x Snx [8] also exhibited giant MCEs in conjunction with a FOMST Among the materials reported in Refs [4–8], the Heusler-type Ni–Mn–Sn alloys exhibit a conventional MCE around the second-order paramagnetic–ferromagnetic (PM–FM) transition They further exhibit an inverse giant MCE involving the first-order magneto-structural transition from the high-symmetry ferromagnetic (FM) austenite to the low-symmetry antiferromagnetic (AFM) martensite [8] Zhang et al recently suggested that the efficiency of a specialized magnetic refrigerator using materials with a positive SM value followed by a negative SM value can be strongly increased [9] Thus, the study on Ni–Mn–Sn and related Heusler alloys Ni–Mn–X (X = Ga, In, Sb) is very attractive not only for the fundamental aspects of multiple magnetic phase transitions but also for the search of novel magnetic refrigerants that can be utilized to enhance the efficiency of a magnetic refrigerator Recently, it has been reported that the transformation temperatures, especially the first-order one could be tunable by: (i) modifying the valence electron concentration per atom (e/a) via substituting other 3d metals such as Cu, Cr, Co, Fe, Al, or Si for Mn-, Ni- or X-sites [10–15]; (ii) changing the unit cell volume by off-stoichiometric composition or by adding small size atoms such as boron, hydrogen, carbon into interstitial site [16–18] Following these approaches, the characteristic temperatures (TM and TC ) of the Ni43 Mn46 Sn11 Bx alloys were found to remarkably increase with increasing concentration of adding interstitial boron atoms, and MCE was significantly improved for the alloy with x = [16] Very recently, a large inverse MCE around room temperature was also detected in Ni50−x Mn38+x Sb12 alloys, promoting another promising N.V Nong et al / Materials Science and Engineering B 176 (2011) 1322–1325 1323 Fig X-ray diffraction patterns of Ni50 Mn38 Sb12 Bx (x = 1, 3, 5) alloys at room temperature (291 K) The Rietveld refined profile plots (observed, calculated, and difference) shows for the x = system having both MCE effects in one [19] Therefore, the ability to tune the FOMST temperature of these materials while preserving the high MCE value would be an interesting and significant outcome of further research In this paper, we report the studies on the structural, magnetic and magnetocaloric properties of Ni50 Mn38 Sb12 Bx alloys with x = 1, and The influence of additional boron atoms on the magnetic properties of this alloy will also be discussed Experimental procedures Polycrystalline ingots of Ni50 Mn38 Sb12 Bx with x = 1, 3, and were prepared by the conventional arc-melting the stoichiometric amount of Ni, Mn, Sb, and B of at least 99.9% purity under argon atmosphere in a water cooled Cu crucible The ingots were remelted several times in order to ensure the homogeneity of the samples The ingots were sealed in evacuated quartz tube, subsequently annealed at 1173 K for days and then quenched in ice-water to obtain pure phase as confirm by X-ray diffraction Xray diffraction (XRD) for all the investigated samples was recorded at room temperature, and at selective temperatures between 200 K and 500 K using a Bruker D8 diffractrometer with Cu-K␣ radiation A Lynx-Eye PSD detector was used The crystal structures were refined from the whole diffraction profiles by the Fullprof Rietveld method [20] The magnetic properties were measured using a superconducting quantum interference device (SQUID) magnetometer made by Quantum Design, Inc with magnetic field up to T and a Lakeshore 7400 vibrating sample magnetometer (VSM) at low field Results and discussion Fig shows the XRD patterns of Ni50 Mn38 Sb12 Bx alloys with x = 1, 3, and at room temperature (291 K) It is evident from the figure that the crystal structure changes depending on the boron concentration A profile refinement of the sample with x = reveals an orthorhombic four-layered (4O) structure with lattice parame˚ b = 5.625(3) A, ˚ and c = 4.348(1) A ˚ The goodness ters: a = 8.574(8) A, of fit of the models was assessed by calculating and was found to be 1.4 In this structure, the space group is Pmma, Ni atoms occupy the 4h and 4k sites, while Mn atoms occupy the 2a and 2f sites, and Sb atoms occupy the 2b and 2e sites [21] Comparing with the sample without boron addition, Ni50 Mn38 Sb12 reported in Ref [22], the lattice of the Ni50 Mn38 Sb12 Bx sample with x = are found to be slightly distorted: a and b are expanded, while c is con- Fig X-ray diffraction patterns at various temperatures for Ni50 Mn38 Sb12 Bx : (a) and (b) are for x = and 3, respectively tracted As for Ni50 Mn38 Sb12 B3 and Ni50 Mn38 Sb12 B5 samples, most the strong XRD peaks e.g., at 2Â = 25.886◦ , 29.957◦ , 42.855◦ , 62.183◦ , and 78.437◦ can be analyzed and indexed in line with the cubic L21 phase, as previously reported in couple of Ni–Mn–X (X = In, Sn, Sb) Heusler alloys [14–16,22–24] However, beside the L21 structure, a considerable number of XRD peaks were found to belong to the orthorhombic 4O phase, as indexed in Fig By carefully analyzing the XRD data of the sample with x = 5, we found a peak at 2Â ≈ 44.9◦ , which could be attributed to an impurity phase, Mn2 B A similar trace was also found with the boron adding Ni50 Mn38 Sn12 Bx system for x = and 5, as previously reported [16] The obtained result suggests that for the Ni50 Mn38 Sb12 Bx system the boron content with x = may be the upper limit of additional boron To elucidate the structural changes with different boron concentration, X-ray analysis was performed at various selected temperatures varying from 200 K to 500 K for two typical samples with x = and Fig 2a and b illustrates the temperature dependent X-ray diffraction patterns of Ni50 Mn38 Sb12 Bx alloys with x = and An evolution of the structural transformation from orthorhombic to cubic structure can be clearly observed from the XRD spectrums when the temperature increases, particularly in the 2Â ranges of 40◦ –45◦ and 70◦ –85◦ However, in the temperature region of the austenitic phase, beside main peaks which belong to the cubic L21 structure, a peak at 2Â = 44.3◦ belong to the orthorhombic four layered (4O) phase still exists even at the temperature much higher than TC , i.e., at 400 and 500 K for the x = sample This peak appears in all the X-ray spectrums of the x = sample over the whole measured temperature range It seems that 1324 N.V Nong et al / Materials Science and Engineering B 176 (2011) 1322–1325 Fig Thermal magnetization curves during heating and cooling of Ni50 Mn38 Sb12 B3 alloy in different applied magnetic fields of 0.1, 1, 3, and T Fig Magnetization as a function of temperature for Ni50 Mn38 Sb12 Bx (x = 1, 3, and 5) measured at low magnetic field of 0.01 T under the ZFC and FC processes The inset shows the first-order differential of thermal magnetization, dM/dT the structural transformation to a pure cubic phase was not fully completed over the investigated temperature range, or it was obstructed by the additional boron Although the XRD patterns vs temperature for both the Ni50 Mn38 Sb12 B1 and Ni50 Mn38 Sb12 B3 samples exhibited a similar phenomenon the detailed evolution of the transformation from martensitic to austenitic phase were different These results suggest that boron atoms might exist at the interstitial site, and that influences the progress of structural transformation Fig displays the magnetization as a function of temperature (M–T) for Ni50 Mn38 Sb12 Bx with x = 1, 3, and 5, measured at low magnetic field of 0.01 T under the ZFC and FC processes In this measurement, the samples were cooled from 400 K to 100 K in zero field (ZFC) and in a field of 0.01 T (FC) Both the ZFC and FC thermal magnetization curves undergo two sharp transitions for the samples with x = and 3, while the first one seems to be suppressed for the sample with x = 5, which only exhibits a small hump at about 175 K (see Fig 3) During heating, the first transition is attributed to the martensitic–austenitic phase transition at TM from antiferromagnetic to ferromagnetic state, and the second one is from ferromagnetic to paramagnetic state at TC Although M–T of the sample with x = also exhibits a conventional magnetic transition at TC its magnetization is not going to zero in the paramagnetic temperature region A possible reason to explain this phenomenon may be due to the existence of impurity (Mn2 B) phase To determine the exact transition temperatures, the firstorder differential of magnetization (dM/dT) was performed and shown in Fig inset The TM and TC values were 300 K and 330 K for the x = 1, 262 K and 345 K for the x = 3, respectively In comparison with the sample without boron x = (TM starts at 315 K, and TC at 335 K as reported in Ref [25]), it is clear that the martensitic transformation temperature (TM ) tends to decrease, while the Curie temperature (TC ) increases with increasing boron concentration This evidently reveals the influence of the boron addition into the lattice In addition, the magnitude of the magnetization of the samples is different with varying boron concentration It is assumed that in the Ni–Mn based Heusler alloys, the magnetic moments are localized mainly on the Mn atoms and the exchange interaction is very sensitive to the Mn–Mn distance Hence, any change in the distance due to the change in the crystallographic configuration can affect the strength of interactions, leading to the different magnetic exchanges [21,23] The changes in TM , TC , and the difference in the magnetization of the Ni50 Mn38 Sb12 with boron addition could be attributed to the distortion of the lattice parameters, which may modify the Mn–Mn distance Accordingly, our data from the structure refinement revealed that lattice of the samples with boron addition were distorted, as mentioned above for the case of the x = sample Another feature worth noting in Fig is the splitting of the ZFC and FC curves in low temperature region (i.e., in the martensitic phase region), which indicates the presence of magnetic frustration It has been reported that in off-stoichiometric NiMnSn alloys, part of the Mn ions residing in the Sn site, the magnetic coupling between the Mn ions is antiferromagnetic (AFM), while that between the Mn ions in the regular Mn site is ferromagnetic (FM) [24] In the Ni50 Mn38 Sb12 Bx case, a similar scenario may occur, and the competition between the AFM and FM interactions below the martensitic phase at low temperature changes due to the concentration of the boron addition Domain wall pinning of the FM component due to the larger anisotropy of orthorhombic structure (martensitic phase) also contribute to the splitting between the ZFC and FC curves This origin anisotropy of the FM component is reduced when boron content increases because the splitting degree reduces Notably, the magnetization of the x = sample suddenly dropped at the temperatures just before the martensitic transition occurred, indicating that the low-symmetry AFM ordering starts breaking, and the high-symmetry FM ordering is established Fig shows the thermal magnetization curves (M–T) of Ni50 Mn38 Sb12 B3 on heating and cooling in the temperature region below and above the martensitic transformation under various applied magnetic fields of 0.1, 1, 3, and T In all the applied magnetic fields, the M–T curves showed a thermal hysteresis associated with the first-order structural transition, and the value of hysteresis remained constant at about 6.5 K On the other hand, the thermal hysteresis curve shifts towards the low temperatures with increasing applied magnetic field, which indicates that the magnetic field also tries to suppress the martensitic transformation The magnetization curves as a function of magnetic field at various temperatures in the vicinity of the structural transitions were measured and shown in Fig 5a for the x = sample is as an example A field-induced magnetic transition from the martensite (with a low magnetization) to the austenite phase (with a high magnetization) is obviously observed at around TM = 300 K The isothermal magnetization curves then exhibited a transition from ferromagnetic to paramagnetic when temperature increased To determine the magnetic entropy changes, the isothermal magnetization curves have been obtained in an interval of K (with high field in SQUID) and K (with low field in VSM) in the increasing temperature mode The magnetocaloric effect ( SM ) was calculated from the isothermal magnetization curve, around the martensitic and magnetic transition temperature using the Maxwell relation N.V Nong et al / Materials Science and Engineering B 176 (2011) 1322–1325 1325 to decrease with further increasing boron addition (i.e., for x > 1) the maximum obtained SM is still in the consideration range, as compared to that of the Ni50 Mn38−x Sbx (x = 12, 13, 14) system [25,26] Conclusions We have investigated the effects of additional boron on the crystal structure, magnetic and magnetocaloric properties of Heusler alloys Ni50 Mn38 Sb12 Bx with x = 1, 3, and The results revealed the evidence of the boron addition into the interstitial site, and it affected the structural and magnetic characteristics of the alloys The possibility of tuning the martensitic transition temperature by the interstitial addition of the atoms with small ionic radii like boron, and the relative large MCE in these alloys make this system quite interesting from the points of view of fundamental aspects as well as of the search for new materials utilizing in practical applications Acknowledgements This work was supported by Trig A program of Vietnam National University, Hanoi (VNU-HN) and the Fuel Cells and Solid State Chemistry Division, Risø National Laboratory for Sustainable Energy, Technical University of Denmark References Fig (a) Magnetization as a function of magnetic field measured by VSM in the vicinity of transition temperatures for a typical x = sample; (b) temperature dependence of magnetic entropy change ( SM ) of Ni50 Mn38 Sb12 Bx alloys with x = and in H = 0.8 T, and in H = 2, 3, and T for the sample with x = (dS/dH)T = (dM/dT)H The magnetic entropy changes can be calculated as: SM (T, H) = SM (T, H) − SM (T, 0) H = ∂SM ∂H H dH = T ∂M ∂T dH H Fig 5b shows the SM as function of temperature in an applied magnetic field change ( H = 0.8 T) for Ni50 Mn38 Sb12 B3 and Ni50 Mn38 Sb12 B1 , and SM at H = 2, 3, and T is only for the Ni50 Mn38 Sb12 B3 sample Here, SM shows positive sign in a narrow temperature range around TM , indicating an inverse MCE and that its values become negative around TC The maximum SM values were 1.4 and 2.1 J/kg K at 262 K and 300 K in H = 0.8 T for the Ni50 Mn38 Sb12 B3 and Ni50 Mn38 Sb12 B1 samples, respectively To find out the influence of applied magnetic field to SM value in this system, the isothermal magnetization curve with high applied magnetic field changes ( H) of Ni50 Mn38 Sb12 B3 were performed and the calculated SM also shown in Fig 5b The magnetic entropy changes increased rapidly as expected with increasing the magnetic field changes and the maximum SM values reached 3.2, 5.3, 7.4 and 9.2 J/kg K at H = 2, 3, and T, respectively The maximum SM value was reported about J/kg K at H = T for the 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boron, and. .. interesting and significant outcome of further research In this paper, we report the studies on the structural, magnetic and magnetocaloric properties of Ni50 Mn38 Sb12 Bx alloys with x = 1, and The