Thermal and magnetic field-induced martensite-austenite transition in Ni 50 . 3 Mn 35 . 3 Sn 14 . 4 ribbons

Thermal and field-induced martensite-austenite transition was studied in melt spun Ni50.3Mn35.3Sn14.4 ribbons. Its distinct highly ordered columnarlike microstructure normal to ribbon plane allows the direct observation of critical fields at which field-induced and highly hysteretic reverse transformation starts H=17 kOe at 240 K , and easy magnetization direction for austenite and martensite phases with respect to the rolling direction. Single phase L21 bcc austenite with TC of 313 K transforms into a 7M orthorhombic martensite with thermal hysteresis of 21 K and transformation temperatures of MS=226 K, Mf =218 K, AS=237 K, and Af =244 K. © 2008 American Institute of Physics. DOI: 10.1063/1.2838356

][4][5][6][7][8][9][10][11] The studies concluded that these materials are prospective for the development of magnetically driven actuators and working substances for magnetic refrigeration.The occurrence of ferromagnetism in both phases has been only found in the narrow composition range of 13ഛ x ഛ 15, 2 and the different magnetization values between martensite and austenite phases leads to a large magnetocaloric effect around the martensitic transition. 7,9Their crystal structures, as well as the characteristic temperatures of its mutual reversible transformation ͑the starting and finish martensitic and austenitic temperatures, M S , M f , A S , and A f , respectively͒, are very sensitive to small variations in the valence electron concentration per atom e / a, 2,3 and consequently to chemical composition and also depends on the magnetic applied field.The magnetic field can also induce a reverse structural transformation as has been unambiguously demonstrated studying field dependence of x-ray diffraction ͑XRD͒ profiles in bulk polycrystalline Ni 50 Mn 36 Sn 14 alloys, 6 where a field higher than 50 kOe was required to induce around A S a complete phase transition.
Rapid quenching by melt spinning offers two potential advantages for the fabrication of these magnetic shape memory alloys: the avoiding, or reduction, of the annealing to reach a homogeneous single phase alloy, and the synthesis of highly textured polycrystalline ribbons.In addition, ribbon shape can be also more appropriate for use in practical de-vices.Recently, we reported that it was an effective singlestep production process for obtaining Ni 50 Mn 37 Sn 13 ribbons with homogenous chemical composition and strongly ordered microstructure. 12In this letter, we report on the thermal and magnetic field-induced martensite-austenite transformation, microstructural, and magnetic properties of Ni 50.3 Mn 35.3 Sn 14.4 alloy ribbons.
Ribbon flakes of width about 1.5-2.0mm and length of 6 -7 mm were produced by melt spinning in argon atmosphere at a wheel linear speed of 48 ms −1 starting from arc melted alloys prepared from highly pure elements ͑Ͼ99.9%͒.Samples were annealed in high vacuum for 2 h at 1073 K. Annealing was followed by water quenching.Microstructure and elemental compositions were examined by using a scanning electron microscope ͑SEM͒ equipped with an x-ray energy dispersive spectroscopy ͑EDS͒ system.SEM examinations in backscattering emission mode revealed that chemical elements tend to distribute homogeneously in the alloy.EDS analyses performed on the cross section and both ribbon surfaces of different flakes gave the average composition Ni 50.3 Mn 35.3 Sn 14.4 .XRD analyses were performed using Cu K␣ radiation in a low temperature diffractometer.Zero-field cooling ͑ZFC͒, field cooling ͑FC͒, and field heating ͑FH͒ thermomagnetic curves were recorded from 4.2 to 350 K for applied fields of 50 and 50 kOe, along the ribbon axis ͑i.e., rolling direction͒, with a heating or cooling rate of 2 K / min, using a physical properties measuring system platform with a vibrating-sample magnetometer module.Curie point T C was inferred from the minimum in the dM / dT versus T curve.Hysteresis loops were measured up to 10 kOe using a superconducting quantum interference device.
SEM micrographs of the fracture morphology and free surface of ribbons are shown in Fig. 1, where a highly ordered columnar microstructure is observed.Individual col-umns grow perpendicular to the ribbons plane from a thin layer of small equiaxed crystallized grains at the surface in contact with the wheel.They run through the entire ribbon thickness around 7 -10 m.Ribbons are brittle and easy to cleave in such direction.
M͑T͒ curves recorded at 50 Oe in ZFC, FC, and FH modes with those recorded at 50 kOe on heating and cooling are compared in Fig. 2. The inset shows the corresponding dM / dT versus T curve at low field.In the ZFC curve, M͑T͒ is zero with the increasing temperature up to 100 K, afterward a quasilinear monotonic increase starts.At 236 K, an abrupt and sharp increase in the magnetization suggests the occurrence of martensite-austenite transformation.X-ray diffraction patterns were obtained at a temperature below and above the transformation for phase identification ͑Fig.3͒.At 270 K, the sample is a single phase austenite with cubic bcc L2 1 crystal structure and lattice parameter a = 0.5977 nm, while the obtained one at 150 K was indexed on the basis of a modulated 7M orthorhombic martensite showing lattice parameters a = 0.6162 nm, b = 0.6048 nm, and c = 0.5633 nm.Traces of secondary or spurious phases were not detected.Structures of both phases coincide with the found ones in ribbons, 12 but the modulation of the orthorhombic martensite is different to that found in bulk materials with closer compositions such as Ni 50 Mn 36 Sn 14 ͑4O͒ ͑Refs.The maximum value of magnetic entropy change ⌬S M at a field of 20 kOe obtained from the magnetization isotherms ͑ϳ5 J kg −1 K −1 ͒ is, in fact, large and comparable to that found in bulk materials of similar composition. 7ysteresis loops measured at 270 and 150 K ͑Fig.4͒, underline the ferromagnetic ordering of austenite and martensite.When the magnetic field is applied parallel to the rolling direction, M͑H͒ curve for austenite shows high susceptibility at low fields and fast approach to saturation ͓Fig.4͑a͔͒.This magnetically soft behavior is a consequence of the low anisotropy of austenite in agreement with its cubic symmetry.Domain walls displacement would be the domi-  nant magnetization process in this phase.Furthermore, the loop shape suggests that the easy magnetization direction lies along the rolling direction and therefore perpendicular to the major axis of columnar grains.A different situation is found for martensite, since the influence of rotation process can be observed from low fields up to 5 kOe when M͑H͒ approaches its saturation value, indicating that anisotropy axis for this phase tilts from the rolling direction.The inset of Fig. 4͑a͒ zooms into the low magnetic field region.Coercive field H C for austenite is 28 Oe, while H C for martensite is 52 Oe.Hysteresis loops for both phases do not saturate up to near 10 kOe when the magnetic field is applied perpendicular to the ribbon plane ͓Fig.4͑b͔͒.The loop is anhysteretic for austenite but H C is 88 Oe for martensite and harder magnetic properties are found ͑as the inset shows͒, confirming that magnetization easy axes are in the ribbon plane for both phases.
The effect of magnetic field on reverse transformation was studied by measuring magnetization isotherms, under increasing and decreasing the field, in the transformation temperature range.Before recording each isotherm, the sample was cooled down to 200 K ͑to ensure the start of measurement in martensitic state͒, and then heated to the measuring temperature in zero field.After temperature stabilization, the isotherm is measured and the cycle is repeated.M͑H , T͒ curves are shown in Fig. 5.All of them have the signature of metamagnetism owing to the effect of the magnetic field on the transformation.Consequently, a hysteresis between field-up and field-down curves is observed.The behavior is emphasized as the temperature approaches to 240 K, where the inflection point of the low-field M͑T͒ curve was obtained ͑see inset of Fig. 2͒.At this temperature, a well defined discontinuity in the first derivative of the field-up M͑H͒ curve is observed at H = 17.3 kOe.Over this field, a progressive magnetic field induced transformation from martensite to austenite is observed.As temperature approaches to "pure" austenitic or martensitic phase existence regions, these effects vanish.
In summary, martensitic transformation from cubic bcc L2 1 to 7M orthorhombic structure was observed in Ni 50.3 Mn 35.3 Sn 14.4 melt spun ribbons exhibiting highly ordered columnar grains running through the ribbon thickness.Magnetization easy axis lies along the rolling direction for austenite while anisotropy direction for martensite tilts out of it.Around A S , a field over 17 kOe induces a progressive reverse martensitic transformation.The large inverse magnetocaloric effect ͑comparable to that of bulk materials͒ originated from the abrupt change in magnetization around structural transition, together with advantages of the single step production by melt spinning, points to the fact that the NiMnSn Heusler ribbons could be of potential interest as magnetocaloric material.
4 and 5͒ or Ni 50 Mn 37 Sn 13 ͑10M͒.2It is worthy of mention the magnetically soft behavior of austenite that reaches a large magnetization value at 50 Oe.The transition from ferromagnetic to paramagnetic state for this phase occurs at 313 K.The structural transformation shows start and finish temperatures of M S = 226 K, M f = 218 K, A S = 237 K, and A f = 244 K, while a thermal hysteresis of 21 K between cooling and heating curves was measured.These values are close to those reported for the bulk Ni 50 Mn 36 Sn 14 alloy.4,6M͑T͒ at the FH curve slightly decreases monotonously until the transformation starts, and coincides with the magnetization value for the FC curve in the 10-215 K range.At 50 kOe, heating and cooling M͑T͒ show two well distinct ferromagnetic regions illustrating a reversible and abrupt change in magnetization as well as the field dependence of the martensitic transformation.A decrease in the characteristic temperatures of the structural transformation is observed ͑estimated values: ⌬M S =−11 K, ⌬M f =−8 K, ⌬A S = −7 K, and ⌬A f =−7 K͒.The abrupt change of M͑T͒, displayed in the inset of Fig.2around crystal transition temperatures, suggests that significant and positive magnetic entropy change ⌬S M associated with the inverse magnetocaloric effect could be expected.