Quality of Heusler single crystals examined by depth-dependent positron annihilation techniques

Abstract

Abstract.Heusler compounds exhibit a wide range of different electronic ground states and are hence expected to be applicable as functional materials in novel electronic and spintronic devices. Since the growth of large and defect-free Heusler crystals is still challenging, single crystals of Fe 2 TiSn and Cu 2 MnAl were grown by the optical floating zone technique. Two positron annihilation techniques -Angular Correlation of Annihilation Radiation (ACAR) and Doppler Broadening Spectroscopy (DBS) -were applied in order to study both, the electronic structure and lattice defects. Recently, we succeeded to observe clearly the anisotropy of the Fermi surface of Cu 2 MnAl, whereas the spectra of Fe 2 TiSn were disturbed by foreign phases. In order to estimate the defect concentration in different samples of Heusler compounds the positron diffusion length was determined by DBS using a monoenergetic positron beam. IntroductionHeusler crystals comprise a large class of ternary intermetallic compounds with the L2 1 structure type and with the formula X 2 YZ where X, Y are transition metals, and Z is a non-magnetic or a nonmetallic element. The family of Heusler compounds exhibits an extraordinary wide range of different electronic ground states with metallic, semiconducting, insulating, half-metallic, ferromagnetic or superconducting behavior [1]. For novel electronic and spintronic applications great efforts are undertaken to benefit from the electronic properties of this class of materials. For this reason, it is a major task to develop new functional materials based on Heusler compounds. In order to enable the investigation of basic properties such as the electronic structure the availability of Heusler singlecrystals is of major importance. However, the growth of large and defect-free single-crystals of this class of ternary alloys remains still challenging.Within this contribution, we focus on the ternary systems Cu 2 MnAl and Fe 2 TiSn. Cu 2 MnAl is the Heusler system per se: In 1903, Fritz Heusler discovered that this compound behaves like a magnet, although none of its constituent elements is ferromagnetic by itself [2]. The Curie temperature of Cu 2 MnAl is found to be T C = 622 K. For Fe 2 TiSn various properties are theoretically predicted such as a non-magnetic ground state, a pseudogap and a very low density of states at the Fermi level [3,4].

Spin-Resolved Fermi Surface of the Localized Ferromagnetic Heusler CompoundCu2MnAlMeasured with Spin-Polarized Positron Annihilation

“…This is in good agreement with previously published values [25][26][27][28]. Complementary positron annihilation experiments performed on our sample revealed that the vacancy density is below 9.7 × 10 −5 per atom [29].…”

“…Complementary positron annihilation experiments performed on our sample revealed that the vacancy density is below 9.7 × 10 −5 per atom [29].…”

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Hugenschmidt, C. (2015). Spin-resolved fermi surface of the localized ferromagnetic Heusler compound Cu2MnAl measured with spin-polarized positron annihilation.

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Breakdown of Arrhenius law of temperature-dependent vacancy concentration in fcc lanthanum

“…For the present study, we used the CDB spectrometer [16] with a monoenergetic positron beam provided by the neutron induced positron source Munich (NEPOMUC) [17] at the research neutron source Heinz-Maier Leibnitz (FRM II). DBS at this spectrometer with an accessible temperature range of 40 − 1100 K was shown to be particularly suited for the determination of the vacancy concentration, e.g., in Heusler alloys [18] or the in-situ observation of fast defect annealing after severe plastic deformation [19]. In addition, compared to conventional PAS with β + emitters the usage of a positron beam has the advantage of simple sample heating under ultra high vacuum conditions and that the recorded signal exclusively stems from annihilation events inside the sample (absence of the so-called source component).…”

Defect imaging and detection of precipitates using a new scanning positron microbeam

“…Monoenergetic positron beams allow for depth-dependent defect investigations of samples or thin layers. The positron diffusion length can be measured by variation of the implantation energy that in turn allows to evaluate high vacancy concentrations up to the percent range [16]. DBS with energy variable positron beams is particularly suited to investigate the depth distribution of defects.…”