Charge and spin state of dilute Fe in NaCl and LiF

There is an apparent mismatch between electron paramagnetic resonance and Mössbauer spectroscopy results on the charge and spin states of dilute Fe impurities in NaCl; Mössbauer spectroscopy data have been interpreted in terms of high-spin Fe2+, while electron paramagnetic resonance studies suggest low-spin Fe1+. In the present study, the charge and spin states of dilute substitutional Fe impurities in NaCl and LiF have been investigated with 57Mn→57Fe emission Mössbauer spectroscopy. A scheme is proposed which takes into account the effects of nearest-neighbor distances and electronegativity difference of the host atoms on the Mössbauer isomer shift and allows for the unequivocal differentiation between high-spin Fe2+ and high/low-spin Fe1+ in Mössbauer spectroscopy. From these considerations, the Mössbauer results are found to be consistent with dilute Fe impurities in NaCl and LiF in a low-spin Fe1+ state. These conclusions are supported by theoretical calculations of isomer shifts and formation energies based on the density-functional theory. The experimental results furthermore suggest that charge compensation of dilute Mn2+ dopants in NaCl and LiF is achieved by Na vacancies and F− interstitials, respectively.We acknowledge the support of the ISOLDE Collaboration and technical teams and cluster resources provided by CERN (HTC/Condor). G. Marschick (Vienna University of Technology, Austria; University Duisburg-Essen, Germany & CERN, Switzerland) is acknowledged for experimental help. B. Qi (University of Iceland, Iceland) is acknowledged for fruitful discussions. S.O. acknowledges support from the University of Iceland Research Fund. R.A. acknowledges the funding of the Austrian Science Fund (FWF) through Projects No. P26830 and No. P31423. The Federal Ministry of Education and Research (BMBF) through Grants No. 05K16PGA and No. 05K19SI1 "eMMA" is acknowledged. The European Commission through the Horizon 2020 programme (Grant No. 654002, ENSAR 2) is acknowledged. K.B.-R., H.M., D.N., K.N., and G.P. acknowledge support of the Department of Science & Innovation (South Africa) within the SA-CERN Program. I.U. acknowledges the support of the Ministry of Economy and Competitiveness (MINECO/FEDER) under Project No. RTI2018-094683-B-C55 and Basque Government Grant No. IT-1500-22

Mikroegiturako akatsek, ni-oinarridun heusler aleazioen transformazio martensitikoan eta propietate magnetikoetan duten eraginaren azterketaa

187 p.(eusk) 189 p. (eng.)The thesis deals with the effect that microstructural defects have on the magneticproperties and martensitic transformation in Ni-Mn-Z (Z = Ga, In, Sn)and Ni-Fe-Ga alloys. For this purpose, along with the standard characterizationtechniques, two additional specific nuclear techniques have been employed; 119SnMössbauer Spectroscopy and Positron Annihilation Lifetime Spectroscopy, PALS.For the consecution of this thesis one PALS spectrometer has been designed, builtand optimized. For offline emission Mössbuauer experiments, a setup has been alsobuilt for measuring ion implanted samples. Finally, a high temperature annealingfurnace has been designed and built to carry out the annealing of powdered samplesunder controllable atmosphere

Observation of a charge delocalization from Se vacancies in Bi2Se3: A positron annihilation study of native defects

[EN] By means of positron annihilation lifetime spectroscopy, we have investigated the native defects present in Bi2Se3, which belongs to the family of topological insulators. We experimentally demonstrate that selenium vacancy defects (VSe1) are present in Bi2Se3 as-grown samples, and that their charge is delocalized as temperature increases. At least from 100 K up to room temperature both V0Se1and V+Se1 charge states coexist. The observed charge delocalization determines the contribution of VSe1defects to the n-type conductivity of Bi2Se3. These findings are supported by theoretical calculations, which show that vacancies of nonequivalent Se1 and Se2 selenium atoms are clearly differentiated by positron annihilation lifetime spectroscopy, enabling us to directly detect and quantify the most favorable type of selenium vacancy. In addition to open-volume defects, experimental data indicate the presence of defects that act as shallow traps, suggesting that more than one type of native defects coexist in Bi2Se3. As will be discussed, the presence of a dislocation density around 1010cm−2 could be the source of the detected shallow traps. Understanding the one-dimensional defects and the origin of the charge delocalization that leads Bi2Se3 to be an n-type semiconductor will help in the development of high-quality topological insulators based on this material.This work is supported for the Basque Government Grant IT-443-10 and partially supported by the Spanish Ministry of Economy and Competitiveness (MINECO) under the project TEC2014-60173 and by the Generalitat Valenciana under the projects Prometeo II 2015/004 and ISIC/2012/008. I. Unzueta also wants to acknowledge financial support from the Basque Government Grant PRE-2014-214. V. Marín-Borrás thanks the University of Valencia for its pre-doctoral fellowships. Finally we would also like to thank BCMaterials for its economic support

Identification of Ni-vacancy defect in Ni-Mn-Z (Z = Ga, Sn, In): An experimental and DFT positron annihilation study

[EN] By means of experimental positron-annihilation-lifetime measurements and theoretical density functional theory (DFT) positron-lifetime calculations, vacancy-type defects in Ni50Mn50−xSnx (x=25, 20,15,13,10) and Ni50Mn50−xInx (x=25,20,16,13) systems are systematically studied. The study is extended to Ni-Mn-Ga systems as well. Experimental results are complemented with electron-positron DFT calculations carried out within the local density approximation and generalized gradient approximation, where five different parametrizations accounting for the γ(r) enhancement factor are analyzed. Theoretical results indicate that the Boronski-Nieminen parametrization of γ(r) is the one that best predicts the experimental results, which ultimately enables us to identify VNi as the vacancy present in the studied samples. The characteristic positron lifetime related to VNi ranges between 181 and 191 ps in Ni-Mn-Sn/In systems. Positron-annihilation-lifetime spectroscopy results in these two systems delimit the lower bound of the achievable vacancy concentration, which is much larger compared with the reported values in Ni-Mn-Ga systems. The present work, along with setting the basis for positron simulations in Ni-Mn based Heusler alloys, delimits the effect that the variation of vacancies has in the martensitic transformation in Ni-Mn-Sn systemsThis work is supported by the Basque Government and Spanish Ministry of Economy and Competitiveness under grants IT-1005-16, IT-756-13 and MAT2015-65165-C2-R (MINECO/FEDER). I. Unzueta also wants to acknowledge the Basque Government Grant PRE-2014-214

Vacancies mediated ordering in Ni-Mn-Ga shape memory alloys

[EN] Vacancy mediated diffusion is the main atomic mechanism that controls the ordering of substitutional alloys. In particular, in Ni-Mn-Ga shape memory alloys vacancies control the evolution of the L21 atomic order degree and consequently most of the order-dependent properties of the martensitic transformation. This work adapts a general phenomenological model to quantify the evolution of the vacancy concentration during the ordering process for three different Ni-Mn-Ga alloys. The results show that out of equilibrium vacancies are responsible of the ordering process, fixing the set of parameters governing it. The present work shows the suitability of the model to determine the vacancy dynamics throughout any process in which they intervene.This research was funded by Projects RTI2018-094683-B-C5 (4,5) (MCIU/AEI/FEDER, UE) and Basque Government Grant IT-1005-16

Low temperature magnetic properties of a Ni50Mn34In16 ball-milled metamagnetic shape memory alloy

[EN] The effect of the atomic disorder induced by ball-milling on the structure and the magnetic properties has been analyzed in a Ni50Mn34In16 metamagnetic shape memory alloy. The as-milled samples displayed an amorphous structure which crystallizes to a disordered B2 structure on annealing. On further annealing, several recovery processes leading to the austenitic L21 structure and the subsequent martensitic transformation are observed. As a result of the recovery processes, the magnetic order drastically varies concurrent with the long-range atomic order. In particular, the magnetism evolves from a frustrated magnetic state compatible with a canonical spin-glass, observed in the amorphous structure, to the well-developed ferromagnetic state.This work has been carried out with the financial support of the Spanish “Ministerio de Economía y Competitividad” and FEDER funding, projects no. MAT2012-37923 and MAT2015-65165-C2-R. I. Unzueta acknowledges financial support from the Basque Government Grant nos. IT-443-10 and PRE_2014_214

Experimental observation of vacancy assisted martensitic transformation shift in Ni-Fe-Ga alloys

[EN] Positron annihilation lifetime spectroscopy is used to experimentally demonstrate the direct relationship between vacancies and the shift of the martensitic transformation temperature in a Ni55Fe17Ga28 alloy. The evolution of vacancies assisting the ordering enables shifts of the martensitic transformation up to 50 K. Our results confirm the role that both vacancy concentration and different vacancy dynamics play in samples quenched from the L21 and B2 phases, which dictate the martensitic transformation temperature and its subsequent evolution. Finally, by electron-positron density functional calculations VNi is identified as the most probable vacancy present in Ni55Fe17Ga28. This work evidences the capability of vacancies for the fine-tuning of the martensitic transformation temperature, paving the way for defect engineering of multifunctional properties.This work is supported for the Basque Government Grant IT-1005-16 and by the Spanish Ministry of Economy and Competitiveness under the projects MAT2015- 65165-C2-R (MINECO/FEDER), MAT2014-56116-C4- 1-R and GIC1585. I. Unzueta wants to acknowledge the Basque Government Grant PRE-2014-214

Mechanically induced disorder and crystallization process in Ni-Mn-In ball-milled alloys

[EN] High mechanical deformation has been induced in a Ni-Mn-In metamagnetic shape memory alloy by means of ball milling. The evolution of both the martensitic transformation and the magnetic properties associated to the microstructural variations has been characterized. The as-milled nanometric particles display an amorphous structure with a frustrated magnetic state compatible with a canonical spin-glass. On heating, an abrupt crystallization process occurs around 500 K leading to a cubic B2 structure, which, in turn, does not show martensitic transformation. Modified Arrott plots point to competing long- and short-range magnetic couplings in the B2 structure. On further heating, a relaxation process takes place above 700 K concurrently with a B2-L21 atomic ordering, giving rise to an anomalous two-step thermal expansion. The combined effect of both processes makes possible the subsequent occurrence of a martensitic transformation, which takes place at the same temperature than in the bulk. The large relative-cooling-power linked to the magnetocaloric effect at the martensitic transformation in the annealed powder makes it interesting for practical applications of magnetic refrigeration at nanoscale.This work has been carried out with the financial support of the Spanish “Ministerio de Economía y Competitividad” (Projects number MAT2012-37923-C02 and MAT2015-65165-C2-R). We also acknowledge ILL and SpINS for beam time allocation (experiment CRG-2158). RCF acknowledges a Postdoctoral fellowship from the Univeridad Pública de Navarra (grant number: 1081/2015). JARV acknowledges CSIC for a JAEdoc contract. J. Pons is acknowledged for TEM observations

Testing the Applicability of 119Sn Mössbauer Spectroscopy for the Internal Stress Study in Ternary and Co-Doped Ni-Mn-Sn Metamagnetic Alloys

The influence of both the Co addition and the internal stress on the atomic level magnetism is comparatively studied in Ni50Mn37Sn13 and Ni45Mn38Sn13Co4 alloys by magnetic measurements and 119Sn Mössbauer spectroscopy. The results show that the saturation magnetization and the hyperfine field follow the same temperature trend. The internal stress state is investigated by subjecting the samples to milling and annealing treatments, and tracking the singlet component revealed by 119Sn Mössbauer spectroscopy. Contrary to what was expected, in the Co-doped Ni-Mn-Sn sample the singlet component can be resolved between the milled and annealed states in both martensite and austenite phases. Therefore, the results demonstrate the feasibility of tracking the singlet component upon the structural recovery in Co-doped Ni-Mn-Sn alloys in a much wider range than in ternary alloys. In addition, it is concluded that the transferred dipolar field at Sn from the neighbor magnetic atoms depends very strongly on the stress field and on the microstructural order surrounding Sn atoms. The observed sensitivity of Sn Mössbauer probe atoms to slight microstructural distortions make 119 Sn a powerful technique for the characterization of the stress present in Sn containing metamagnetic shape memory alloys.This research was funded by Projects RTI2018-094683-B-C5 (4,5) (MCIU/AEI/FEDER, UE) and Basque Government Grant IT-1005–16

Magnetocaloric effect enhancement driven by intrinsic defects in Ni-Mn-Sn- Co alloys

[EN] The influence of mechanically-induced defects on the magnetostructural properties is analyzed in a Ni-Co-Mn-Sn alloy subjected to soft milling and subsequent annealing treatments. It is found that, opposite to what occurs in Ni-Mn-Sn ternary alloys, the annealing treatment affects the magnetic properties in a different way in martensite and in austenite. In particular, the saturation magnetization significantly increases in martensite after annealing whereas just a very slight variation is observed in austenite. This leads to the interesting fact that the presence of microstructural defects, far for worsening, makes the magnetocaloric effect to be higher in the as-milled state than after annealing. This behavior is explained as the result of the combination of the effect of defects on the Mn-Mn distance, the effect of Co on the magnetic exchange coupling between Mn atoms, and the effect of defects on the vibrational entropy change at the martensitic transformation.This work has been carried out with the financial support of the Spanish “Ministerio de Economía y Competitividad” (Projects number MAT2015-65165-C2-R) and of the Basque Government (Grant No. IT-1005-16). We also acknowledge ILL and SpINS for beam time allocation: experiments 5-24-591 (https://doi.org/10.5291/ILL-DATA.5-24-591) and CRG-2352. J. López-García acknowledges ILL for his Ph. D. contract and I. Unzueta also wants to thank the Basque Government Grant No. PRE-2014-1-214