MAGNETO-CALORIC AND MAGNETO-RESISTANCE EFFECTS IN HEUSLER TYPE FERROMAGNETIC SHAPE MEMORY ALLOYS

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1 Scientific report over the period December 214- December 21 Project PN-II-ID-PCE Nr. /213. Project title: MAGNETO-CALORIC AND MAGNETO-RESISTANCE EFFECTS IN HEUSLER TYPE FERROMAGNETIC SHAPE MEMORY ALLOYS The objective of year 21 was the evaluation of MCE and refrigerant capacity of the alloys (as bulk and ribbons) with simultaneous or delayed magnetic and structural transitions with the following activities: A1. Magnetic measurements M(T), M(H) and hysteretic losses on alloys as bulk and ribbons in the range of magnetic and structural transition. Sm and refrigerant capacity (RC) calculation. A2.Magnetic relaxation by 7 Fe Mossbauer measurements A3. Writing of 2 articles that will be sent for publication at an ISI journal, preparation of a presentation for 2 international conferences and a rapport of activities carried out during this stage. The magnetocaloric effect can be characterized directly by measuring adiabatic change in temperature ( Tad) upon application of magnetic field or indirectly from heat capacity measurements or by estimating the isothermal entropy changes ( SM) due to changes in applied magnetic field [J. S. Amaral and V. S. Amaral, J. Magn. Magn. Mater. 322, 12 (21)]. The last method is H based on Maxwell relation ( Sm( T, H) ( M / T ) H dh ) and may be applied by measuring the magnetization isotherms at discrete field Suitable materials for magnetic refrigeration must present a series of properties; among them, the most important being discussed in the following: i) a large variation of magnetization with respect to temperature, which is provided by structural, or/and magnetic phase transition; ii) a high refrigerant capacity. iii) a low magnetic hysteresis, to avoid magnetic losses due to the rotation of magnetic domains during a magnetic-refrigeration cycle. iv) a low heat capacity Cp, since a high Cp increases the thermal load and more energy is required to heat the sample itself resulting in entropy losses (for a given Sm, Tad will be lower).

2 S[J/kgK] DSC[W/g] S[J/kgK] DSC[W/g] The ferromagnetic shape memory alloys exhibit simultaneously or successively thermoelastic structural (martensitic) and magnetic order-disorder transitions promoting these alloys as potential performing magnetic refrigerant materials. The first studied system was Ni Nd 2 Fe 18 Ga 2 and the magnetic entropy variation have been evaluated for the bulk alloy as well as on melt spun ribbons as prepared or after different thermal treatments. To our knowledge there are no reported results regarding Ni-Fe-Ga based alloys with Rare Earth additions. In order to improve the magnetic anisotropy and also the ductility of Ni- Mn-Ga ingots different substitutions of Rare Earth (RE) elements [K. Tsuchiya, A. Tsutsumi, H. Ohtsuka, M. Umemoto, Mater. Sci. Eng. A 378 (24) ] were investigated. It was concluded that the solubility of RE elements in this Heusler type matrix is low (possibly less than.1 at %), there are no major changes in the MT characteristics but through segregation, new phases precipitate out at the grains boundaries. We succeeded to prepare Ni7-xNdxFe18Ga2 as single phase of Heusler type up to x=2. For higher Nd concentration a secondary phase precipitate that was recognized, from the XRD pattern, as Nd 2 Fe 17. In order to completely characterize Ni Nd 2 Fe 18 Ga 2 alloy, the magnetic entropy variation was calculated from the thermo-magnetic measurements in fields up to 7T, during heating and the results are shown in fig 1 for bulk alloy and for ribbons as prepared and after two thermal treatments (2 min at 9 o C and 2 hours at 4 o C). The highest entropy variation was obtained on the as prepared ribbons, for which the two transformations may be considered practically concomitant. This is reflected in the highest refrigerant capacity value. It may be observed that a higher refrigerant capacity is associated with a second order phase transition. In table 1 are given the values for the experimentally measured entropy variation, as well as the other data for a complete characterization of the structural and magnetic transitions T 2T 3T 4T T Fig1 a T -1.2 Nd2 Bulk T A T -2. Nd2 AQ ribbons koe 1T 2T 3T -2. 4T T 7T Fig 1b T T A

3 DS [J/kG K] DSC [W/g] DSC[W/g] Fig1c TT2h/4C -1.8 koe 1T 2T 3T -2. T A 4T 7T A Fig 1d Nd2 TT ribbons 2min/9C koe 1T 2T 3T T T 7T T A T Tabel 1 Ni Nd 2 Fe 18 Ga 2 S mag [J/Kg K] RC [J/kg] Bulk AQ o C o C Ms Af Qm (J/g) T Apeak RC [J/kg] Selected results relating to Magneto-caloric effect in Ni-Fe-Ga-Nd Heusler alloys have been presented (as Oral presentation) at Energy, Science and Technology Conference & Exhibition EST 21, in Karlsruhe, Germany. In addition a manuscript with a full characterization of a Heusler alloy with rare earth additions is ready to be submitted to a ISI journal. From the previous results on the magnetic and structural characterization of NiFeGa alloys doped with Al an Co, the most appropriate material for magnetic refrigeration turned out to be the Ni2Co2Fe2Ga23Al3 alloy, prepared as ribbons and annealed at 4 o C, which shows relatively close magnetic and structural transitions near room temperature. Table 2 presents important data on the magnetic and structural transitions and in fig 2 b and 2.d are shown the entropy changes for an applied magnetic field up to 7T calculated from the M(T) measurements given in Fig 2a and 2.c

4 Magnetization [emu/g] Magnetization [emu/g] Table 2 Alloy e/a T Mp/ T Ap M T S /7T [J/Kg K] -T RC TR mart [J/K] RC [J/K] Co2Al3 AP / Co2Al3 TT1 244/ Ni2Fe2Ga23Al3Co2 Aq Ribbons Co2Al3_bzAQ_2 Oe Co2Al3_bzAQ_ Oe 1 Co2Al3_bzAQ_1 koe Co2Al3_bzAQ_ koe 1 Co2Al3_bzAQ_1T Co2Al3_bzAQ_2T Co2Al3_bzAQ_4T Co2Al3_bzAQ_7T Co2Al3_bzAQ_3T Fig 2a -1. cooling heating -1. Oe 1 koe koe 1T 2T 3T -2. 4T 7T -2. Co2Al3 AQ Ribbons Temperature Fig 2b Ni2Fe2Ga23Al3Co2 TT_1 2 Oe Oe 1 1 k Oe koe 1T 2T 3T 4T 7T Fig 2c -1. S-2Oe S-Oe S-1Oe -1.2 S-Oe S-1T T A -1.4 S-2T S-3T S-4T -1.6 S_7T Co2Al3_bz_2min_4C-heating T(K) Fig 2d As for the alloy with Nd substitution, the largest entropy variation is associated with the structural transition and is higher for the as quenched ribbons; conversely the refrigerant capacity is higher for the magnetic transition in thermal treated ribbons. Data on the magneto-caloric properties of this system and compared with those obtained on Ni2Co2Fe2Ga26 (with the same valance electron concentration) thermal treated ribbons were presented at the 1th European Symposium on Martensitic Transformations, ESOMAT 21, in Antwerp, as oral presentation and publishd in DOI: 1.11/matecconf/21338 In order to increase the ductility of the Heusler alloys two compositions with Cu substitution NiFe2Ga27Cu3 si NiMn2Ga27Cu3 were prepared and analyzed. Low field magnetic measurements performed on thermal treated ribbons (Fig 3a) evidenced for both alloys a premartensitic transformation. Such a transformation, on our knowledge, was never observed on

5 Magnetization [emu/g] DS [J/kg K] Magnetization in 2 Oe [emu/g] Ni-Fe-Ga based alloys. The temperature dependences of the magnetic entropy change for different magnetic fields calculated for the iron based alloy as bulk and thermal treated ribbons are shown in fig 3b and 3c. The entropy curves show a peak in the pre-martensite transformation range observed in M(T) curves ( ~2K). 3 2 NiMn2Ga27Cu3 TT 2 Oe NiFe2Ga27Cu3 TT 2 Oe T M Fig.3a S 1T -.9 S2T S 3T S 4T -1. Bulk NiFeGaCu T Fig 3.b Heating Cooling 1kOe.kOe T koe 2T 1T 7T 3 T NiFeCu3 R TT 2min 4C Fig 3.c A In order to get more information on the premartensitic transformation Mössbauer spectroscopy measurements have been performed on NiFeGaCu3 thermal treated ribbons. Mössbauer spectra collected at different temperatures are shown in Fig. 4. The changes in the neighborhood configurations of iron atoms can be followed via the evolution of the hyperfine field distributions, shown on the right hand of the spectra. At 29K are observed configurations with small hyperfine field of 12 to 21 T, indicating proximity to the Curie temperature K. At 22K, a more pronounced peak in the distribution probability, located at about 27T, suggests the presence of a new atomic configuration belonging to a pre-martensitic phase already evidenced by magnetic measurements. Additional studies are in progress for a compet characterization of these systems with premartensitic transformation. The last system prepared and discussed is Ni47Co3Mn3Ga2. For this stoichiometry it is expected the extra Mn atoms which occupy the Ga sites to reduce the distance between Mn atoms, leading to the magnetic moments partially align antiferromagnetically in martensite and austenite too. On the other hand, the Co addition can further change the magnetic ordering in the

6 Magnetization [emu/g] M (emu/g) Magnetization [emu/g] austenitic state from antiferromagnetic to ferromagnetic so that the system is highly frustrated. Some preliminary results are given in fig Fig.a Ni47Co3Mn3Ga2 2 Oe T heating 2 Oe T cooling Ni47Co3Mn3Ga2 33K 33K K 34K 3K 3K 2 Magnetic loss =.733 J/kg 36K 4 Magnetic loss = 3.71 J/kg 36K 37K 1 37K 38K K Field (Oe) Fig.b Field (Oe) Fig.c Ni 47 Co 3 Mn 3 Ga 2 Temerature.8kOe 1T T 4T 3T 2T S from MvsT The martensitic transformation temperatures are influenced by the magnetic field as evidenced the thermomagnetic measurements in 2 Oe and T (Fig a). Based on the M(H) measurements (fig.b), the temperature variation of the magnetic entropy change under magnetic field up to T was calculated (Fig c). The ΔS, large and positive ( +4. J/Kg K) within the martensite transformation temperature range, attest an inverse magnetocaloric effect in this alloy. The positive sign of ΔS exists even under high magnetic field, showing the mechanism involved is different from that of Co-free Ni-Mn-Ga alloys which is related to the coupling between the martensitic and magnetic domains. The refrigerant capacity in T is 18 J/kg but by taking into account the magnetic losses the RC is slightly reduced ( ~16J/Kg). Investigations related to the magnetoresistence effect in such a frustrated system are in progress. Conclusions: Some Fe or Mn based Heusler alloys as Ni-Fe-Ga-X (with X=Nd, Co, Al or Cu) and Ni-Mn-Ga- Y (with Y= Cu, Co) were analyzed in view of their magnetocaloric effect (MCE). The magnetic entropy change for applied magnetic field up to 7T was evaluated and compared on the alloys prepared as bulk and on melt spun ribbons as quenched or thermal treated. In the iron based Heusler alloys, a higher entropy change related to the structural transformation was obtained on as prepared ribbons, whereas for the bulk alloys, the magnetic transition promotes a higher entropy variation. The refrigerant capacity, being related to the temperature range of the transformation, is higher at. The highest entropy variation was obtained on Ni Nd 2 Fe 18 Ga 2 as prepared ribbons for which the structural and magnetic transformations are concomitant. A pre-martensitic transformation in Ni-Fe-Ga-Cu alloy was evidenced by magnetic measurements, Mossbauer spectroscopy and by a third peak in the temperature variation of the magnetic entropy change besides those at T M and T C. A large inverse magnetocaloric effect has been observed on Ni47Co3Mn3Ga2 within the martensitic transformation temperature range, which is originated from different type of

7 magnetic interactions in the martensite and austenite phase. In addition the structural transition temperatures are slightly influenced by the magnetic field. The scientific publications and participation at international conferences: ISI papers: Effect of thermal treatments on the structural and magnetic transitions in melt-spun Ni- Fe-Ga-(Co) ribbons; Tolea, F.; Sofronie, M.; Crisan, A. D.; Enculescu, M. ; Kuncser, V, M Valeanu; JOURNAL OF ALLOYS AND COMPOUNDS, Volume: 6 Pages: Published: NOV 2 21 Distribution of plates sizes tell the thermal history in a simulated martensitic-like phase transition Tolea F. ; Tolea M. ; Sofronie M. ; M. Valeanu; SOLID STATE COMMUNICATIONS Volume: 213 Pages: Published: JUL 21 Shape Memory properties of FeNiCoTi ribbons evidenced by magnetic measurements; Tolea F. ; Sofronie M. ; Tolea M. ;Kuncser V. ; Valeanu M. DIGEST JOURNAL OF NANOMATERIALS AND BIOSTRUCTURES Volume: 1 Issue: 2 Pages: 67-7 Published: APR-Jun 21 Magneto-structural properties and magnetic behavior of Fe-Pd ribbons M. Sofronie, F. Ţolea, V. Kuncser, M. Valeanu, G. Filoti; IEEE TRANSACTIONS ON MAGNETICS, Volume: 1 Issue: 1 Article Number: 244 Published: JAN 21 Non-ISI papers: Magnetic and Martensitic Transformations in the bulk and melt spun ribbons of Ni7- xndxfe18ga2 Ferromagnetic Shape Memory Alloys, F. Ţolea*, A. D. Crişan, M. Sofronie, M. Ţolea, M. Văleanu Materials Today: Proceedings 2S ( 21 ) S87 S878 Magnetocaloric effect in Ni-Fe-Ga Heusler alloys with Co and Al substitutions, F. Tolea a, M. Sofronie, A. D. Crisan, M. Tolea, M. Valeanu; MATEC Web of Conferences, 33, 8 (21) DOI: 1.11/matecconf/21338 Internationale conferences: Energy, Science and Technology Conference & Exhibition EST 21, 2-22 May, Karlsruhe, Germany Magnetocaloric effect in Ni-Fe-Ga-Nd Heusler alloys (Oral presentation) Dr. Felicia Tolea, Dr. Mihaela Sofronie, Dr. Mihaela Valeanu Karlsruhe Institute of Technology (KIT)

8 The 2th International Conference on Magnetism ICM 21, -1 July Barcelona, Spain Magnetocaloric effect in Ni-Fe-Ga-Co-Al Heusler alloys (Poster presentation) Dr. Felicia Tolea, Dr. Mihaela Sofronie, Dr. Alina Daniela Crisan, Dr. Mihaela Valeanu, and Temperature dependent magnetostrains in Mn and Ga-doped Fe-Pd ferromagnetic shape memory ribbons (Poster presentation) M. Sofronie, F. Tolea, A. Crisan, M. Enculescu, M. Valeanu The 1th European Symposium on Martensitic Transformations, ESOMAT 21, September, Antwerp, Belgium, Magnetocaloric effect in Ni-Fe-Ga Heusler alloys with Co, Al or Nd substitutions, (Oral presentation) F. Tolea *, M. Sofronie, A. D. Crisan, M, Valeanu