Structure and properties of quaternary and tetragonal Heusler compounds for spintronics and spin transfer torque applications

Diese Dissertation ist in zwei Teile aufgeteilt: Teil 1 befasst sich mit der Vorhersage von Halb-Metallizit��t in quartern��ren Heuslerverbindungen und deren Potential f��r Spintronik-Anwendungen. Teil 2 befasst sich mit den strukturellen Eigenschaften der Mn2-basierenden Heuslerverbindungen und dem Tuning von ihrer magnetischen Eigenschaften bzgl. Koerzitivfeldst��rke und Remanenz. Diese Verbindungen sind geeignet f��r Spin-Transfer Torque-Anwendungen.rnrnIn Teil 1 wurden die folgenden drei Probenserien quartern��rer Heuslerverbindungen untersucht: XX��MnGa (X = Cu, Ni und X�� = Fe, Co), CoFeMnZ (Z = Al, Ga, Si, Ge) und Co2���xRhxMnZ (Z = Ga, Sn, Sb). Abgesehen von CuCoMnGa wurden alle diese Verbindungen mittels ab-initio Bandstrukturrechnungen als halbmetallische Ferromagnete prognostiziert. In der XX��MnGa-Verbindungsklasse besitzt NiFeMnGa zwar eine zu niedrige Curie-Temperatur f��r technologische Anwendungen, jedoch NiCoMnGa mit seiner hohen Spinpolarisation, einem hohen magnetischen Moment und einer hohen Curie-Temperatur stellt ein neues Material f��r Spintronik-Anwendungen dar. Alle CoFeMnZ-Verbindungen kristallisieren in der kubischen Heuslerstruktur und ihre magnetischen Momente folgen der Slater-Pauling-Regel, was Halbmetalizit��t und eine hohe Spinpolarisation impliziert. Die ebenfalls hohen Curie-Temperaturen erm��glichen einen Einsatz weit ��ber Raumtemperatur hinaus. In der strukturellen Charakterisierung wurde festgestellt, dass s��mtliche Co2���xRhxMnZ abgesehen von CoRhMnSn verschiedene Typen von Unordnung aufweisen; daher war die ermittelte Abweichung von der Slater-Pauling-Regel sowie von der 100%-igen Spinpolarisation dieser Verbindungen zu erwarten. Die Halbmetallizit��t der geordneten CoRhMnSn-Verbindung sollte nach den durchgef��hrten magnetischen Messungen vorhanden sein.rnrnIm zweiten Teil wurden Mn3���xCoxGa und Mn2���xRh1+xSn synthetisiert und charakterisiert. Es wurde gezeigt, dass Mn3���xCoxGa im Bereich x = 0.1 ��� 0.4 in einer tetragonal verzerrten inversen Heuslerstruktur kristallisiert und im Bereich x = 0.6���1 in einer kubisch inversen Heuslerstruktur. W��hrend die tetragonalen Materialien hartmagnetisch sind und Charakeristika aufweisen, die typischerweise f��r Spin-Transfer Torque-Anwengungen attraktiv sind, repr��sentieren die weichmagnetischen kubischen Vertreter die 100% spinpolarisierten Materialien, die der Slater-Pauling-Regel folgen. Mn2RhSn kristallisiert in der inversen tetragonal verzerrten Heuslerstruktur, weist einernhartmagnetische Hystereseschleife auf und folgt nicht der Slater-Pauling-Regel. Bei hohen Rh-Gehalt wird die kubische inverse Heuslerstruktur gebildet. Alle kubischen Proben sind weichmagnetisch und folgen der Slater-Pauling-Regel.

57 Fe and 119 Sn M��ssbauer studies on new Heusler compounds for spintronic applications

Seit der Entwicklung einer gro��en Vielfalt von Anwendungsm��glichkeiten der Spintronik auf Basis von Heusler Verbindungen innerhalb der letzten Dekade kann der Forschungsfortschritt an dieser Material Klasse in einer Vielzahl von Publikationen verfolgt werden. Eine typische Heusler Verbindung X2YZ besteht aus zwei ��bergangsmetallen (X, Y) und einem Hauptgruppenelement (Z). Diese Arbeit berichtet von Heusler Verbindungen mit besonderem Augenmerk auf deren potentielle halbmetallische Eigenschaften und davon insbesondere solche, die eine richtungsabh��ngige magnetische Anisotropie (perpendicular magnetic anisotropy- PMA) zeigen k��nnten. PMA ist insbesondere f��r Spin transfer Torque (STT) Bauelemente von gro��em Interesse und tritt in tetragonalrnverzerrten Heusler Verbindungen auf. Bei STT-Elementen werden mittels spinpolarisierter Str��me die magnetische Orientierung von magnetischen Schichten beeinflusst.rnDie signifikantesten Ergebnisse dieser Arbeit sind: die Synthese neuer kubischen Heusler Phasen Fe2YZ, die theoretisch als tetragonal vorausgesagt wurden (Kapitel 1), die Synthese von Mn2FeGa, das in der tetragonal verzerrten Struktur kristallisiert und Potential f��r STT Anwendungen zeigt (Kapitel 2); die Synthese von Fe2MnGa, das einen magnetischen Phasen��bergang mit exchange-bias (EB) Effekt zeigt, der auf einer Koexistenz von ferromagnetischen (FM) und antiferromagnetischen (AFM) Phasen beruht (Kapitel 3); Schlussendlich wird in Kapitel 4 die Synthese von Mn3���xRhxSn diskutiert, in welcher insbesondere tetragonales Mn2RhSn als potentielles Material f��r Anwendungen in derrnSpintronik vorgestellt wird.rnIn dieser Arbeit wurden haupts��chlich Heusler Verbindungen mit m����baueraktiven Elementen 57Fe und 119Sn, synthetisiert und untersucht. Im Falle der hier untersuchten Heusler Verbindungen spielt die Charakterisierung durch M����bauer Spektroskopie eine entscheidende Rolle, da Heusler Verbindungen meistens ein gewisses Ma�� an Fehlordnung aufweisen, welche deren magnetischen und strukturellen Eigenschaften beeinflussen kann. Die Art der Fehlordnung jedoch kann nur schwer durch standard Pulver-R��ntgendiffraktion bestimmt werden, weshalb wir die Vorteile der M����bauer Spektroskopie als lokale Methode nutzen, um den Typ und den Grad der Fehlordnung aufzukl��ren. rnDiese Arbeit ist wie folgt gegliedert:rnIn Kapitel 1 wurden die neuen, kubisch-weichferromagnetischen Heuslerphasen Fe2NiGe, Fe2CuGa und Fe2CuAl synthetisiert und charakterisiert. In vorangegangenen theoretischen Studien wurde f��r deren Existenz in tetragonaler Heuslerstruktur vorhergesagt.rnUngeachtet dessen belegten unsere experimentellen Untersuchungen, dass diese Verbindungen haupts��chlich in der kubischen invers Heusler(X-) struktur mit unterschiedlichen Anteilen an atomarer Fehlordnung kristallisieren. Alle Verbindungen sind weiche Ferromagneten mit hoher Curietemperatur bis zu 900K, weswegen alle als potentielle Materialien f��r magnetische Anwendungen geeignet sind. In Kapitel 2 wurde Mn2FeGa synthetisiert. Es zeigte sich, dass Mn2FeGa nach Temperatur Nachbehandlung bei 400��C die invers tetragonale Struktur (I4m2) annimmt. Theoretisch wurde die Existenz in der inversen kubischen Heuslerstruktur vorausgesagt. Abh��ngig von den Synthesebedingungen ��ndern sich die magnetischen und strukturellen Eigenschaften von Mn2FeGa eklatant. Deshalb ��ndert sich die Kristallstruktur von M2FeGa bei Temperung bei 800 ��C zu einer pseudokubischen Cu3Au-artigen Struktur, in welcher Fe- und Mn-Atome statistisch verteilt vorliegen. Dieser ��bergang der Kristallstrukturen wurde durch M����bauer Spektroskopie anhand des Vorliegens oder Fehlens der Quadrupolaufspaltung im Falle der invers tetragonalen bzw. pseudokubischen Modifikation nachgewiesen. In Kapitel 3 wurde Fe2MnGa ebenfalls erfolgreich synthetisiert und durch verschiedene Methoden charakterisiert. Der Zusammenhang von Kristallstruktur und magnetischen Eigenschaften wurde durch verschiedene Temperungskonditionen und mechanischer Behandlung untersucht. Der Schwerpunkt lag auf einer geschmolzenen Probe ohne weitere Temperung, die einen FM-AFM Phasen��bergang zeigte. Diese magnetische Phasenumwandlung f��hrt zu einem starken EB-Verhalten, welches seinen Ursprung haupts��chlich in der Koexistenz von FM- und AFM-Phasen unterhalb der FMAFM- ��bergangstemperatur hat. Kapitel 4 ist den neuen Mn-basierten Heusler-Verbindungen Mn3���xRhxSn gewidmet, bei denen wir versuchten, durch den Austausch von Mn durch das gr����ere Rh eine Umwandlung zu einer tetragonalen Struktur von den hexagonalen Mn3Sn-Struktur zu erreichen. Als interessant stellten sich Mn2RhSn und Mn2.1Rh0.9Sn heraus, da sie aus nur einer Phase vorzuliegen scheinen, wohingegen die anderen Verbindungen aus gemischten Phasen mit gleichzeitiger starken Fehlordnung bestehen. Im abschlie��enden Anhang wurden die Fehlordnung und gelegentliche Mischphasen einer gro��en Auswahl von Mn3���xFexGa Materialien mit 1���x���3, dokumentiert.rn

New Data on Welshite, e.g. Ca2Mg3.8Mn0.62+Fe0.12+Sb1.55+O2 [Si2.8Be1.7Fe0.653+Al0.7As0.17O18], an Aenigmatite-Group Mineral

Electron and ion microprobe data on two samples of welshite from the type locality of Langban, Sweden, gave analytical totals of 99.38-99.57 wt.% and BeO contents of 4.82-5.11 wt.%, corresponding to 1.692-1.773 Be/20 O. Mossbauer and optical spectra of one of these samples gave Fe-[iv](3+)/Sigma Fe = 0.91, Fe-[iv](2+)/Sigma Fe = 0.09, and no evidence of Mn3+. The resulting formula for this sample is Ca2Mg3.8Mn0.62+Fe0.12+Sb1.55+O2[Si2.8Be1.7Fe0.653+Al0.7As0.17O18], and that for the second sample, Ca2Mg3.8Mn0.12+Fe0.12+F0.83+Sb1.25+O2[Si2.8Be1.8F0.653+Al0.25As0.25O18], is related by the substitution involving tetrahedral and octahedral sites: 0.59([vi,iv])(Fe,Al)(3+) approximate to 0.42([vi])(Mg,Mn,Fe)(2+) + 0.21(Sb-[vi],As-[iv])(5+), i.e. 3([vi,iv]) M3+ = 2([vi])M(2+) + M-[vi,iv](5+). WelShite is distinctive among aenigmatite-group minerals in the high proportion of Fe 3+ in tetrahedral coordination and is unique in its Be content, substantially exceeding 1Be per formula unit. Given the cation distributions in other minerals related to aenigmatite, we think it is reasonable to assume that at least one tetrahedral site is >50% occupied by Be and that one octahedral site is >50% occupied by Sb, so that welshite should be retained as a distinct species with its own name in the aenigmatite group.

Manganiferous minerals of the epidote group from the Archaean basement of West Greenland

The chemical compositions and crystal structures of Mn3+-containing minerals from the epidote group in Greenland rocks are investigated and described in detail. They occur in hydrothermally altered Archaean mafic sequences within the gneissic complex of the North Atlantic craton of West Greenland. The Mn-containing minerals have a characteristic red to pink colour. A detailed microchemical study shows a significant inter- and intra-sample variation in Mn content. The samples from different parageneses can be classified as Mn-bearing epidote and Mn-bearing clinozoisite. The intra-sample variation in the content of Al, Fe and Mn is on a very fine scale, but still allows for identification of a negative correlation between Mn and Fe. Textures indicate different stages of growth. Crystal chemical data are compared with literature data and illustrate the basic systematic differences between the influence of Fe and Mn on the crystal structure of the epidote group minerals.

Third-generation synchrotron x-ray diffraction of 6-��m crystal of raite, ���Na3Mn3Ti0.25Si8O20(OH)2���10H2O, opens up new chemistry and physics of low-temperature���minerals

The crystal structure of raite was solved and refined from data collected at Beamline Insertion Device 13 at the European Synchrotron Radiation Facility, using a 3 �� 3 �� 65 ��m single crystal. The refined lattice constants of the monoclinic unit cell are a = 15.1(1) ���; b = 17.6(1) ���; c = 5.290(4) ���; �� = 100.5(2)��; space group C2/m. The structure, including all reflections, refined to a final R = 0.07. Raite occurs in hyperalkaline rocks from the Kola peninsula, Russia. The structure consists of alternating layers of a hexagonal chicken-wire pattern of 6-membered SiO4 rings. Tetrahedral apices of a chain of Si six-rings, parallel to the c-axis, alternate in pointing up and down. Two six-ring Si layers are connected by edge-sharing octahedral bands of Na+ and Mn3+ also parallel to c. The band consists of the alternation of finite Mn���Mn and Na���Mn���Na chains. As a consequence of the misfit between octahedral and tetrahedral elements, regions of the Si���O layers are arched and form one-dimensional channels bounded by 12 Si tetrahedra and 2 Na octahedra. The channels along the short c-axis in raite are filled by isolated Na(OH,H2O)6 octahedra. The distorted octahedrally coordinated Ti4+ also resides in the channel and provides the weak linkage of these isolated Na octahedra and the mixed octahedral tetrahedral framework. Raite is structurally related to intersilite, palygorskite, sepiolite, and amphibole.

Invariant scaling relationships for interspecific plant biomass production rates and body size

The allometric relationships for plant annualized biomass production (���growth���) rates, different measures of body size (dry weight and length), and photosynthetic biomass (or pigment concentration) per plant (or cell) are reported for multicellular and unicellular plants representing three algal phyla; aquatic ferns; aquatic and terrestrial herbaceous dicots; and arborescent monocots, dicots, and conifers. Annualized rates of growth G scale as the 3/4-power of body mass M over 20 orders of magnitude of M (i.e., G ��� M3/4); plant body length L (i.e., cell length or plant height) scales, on average, as the 1/4-power of M over 22 orders of magnitude of M (i.e., L ��� M1/4); and photosynthetic biomass Mp scales as the 3/4-power of nonphotosynthetic biomass Mn (i.e., Mp ��� Mn3/4). Because these scaling relationships are indifferent to phylogenetic affiliation and habitat, they have far-reaching ecological and evolutionary implications (e.g., net primary productivity is predicted to be largely insensitive to community species composition or geological age).

Heterometallic hybrids of homometallic human hemoglobins.

Hybridization experiments between normal Hb tetramers (Fe2+ Hb) and those with four metal-substituted hemes (i.e., replacement of Fe2+ by Co2+, Mg2+, Mn2+, Mn3+, Ni2+, or Zn2+) have revealed unexpected behavior. These homometallic Hbs have previously served as models that mimic the deoxy or oxy properties of normal Fe2+ Hb. In this study, hybrids were composed of one alpha 1 beta 1 dimer that is metal-substituted at both hemes, in association with a second dimer alpha 2 beta 2 that has normal Fe2+ hemes. Both metal-substituted subunits are unligated, whereas the two Fe2+ subunits either are both unligated or both ligated with O2, CO, or CN. It was found that four of the metal-substituted Hbs (Mg2+ Hb, Mn2+ Hb, Ni2+ Hb, and Zn2+ Hb) did not form detectable amounts of heterometallic hybrids with normal Fe2+ Hb even though (i) their homometallic parents formed tight tetrameric complexes with stabilities similar to that of Fe2+ Hb and (ii) hybrids with metal substitution at both alpha sites or both beta sites are known to form readily. This striking positional effect was independent of whether the normal Fe2+ hemes were ligated and of which ligand was used. These findings indicate that surprisingly large changes in tetramer behavior can arise from small and subtle perturbations at the heme sites. Possible origins of these effects are considered.

Origin of the ESR spectrum in the Prussian Blue analogue RbMn[Fe(CN)6]*H2O

We present an ESR study at excitation frequencies of 9.4 GHz and 222.4 GHz of powders and single crystals of a Prussian Blue analogue (PBA), RbMn[Fe(CN)6]*H2O in which Fe and Mn undergoes a charge transfer transition between 175 and 300 K. The ESR of PBA powders, also reported by Pregelj et al. (JMMM, 316, E680 (2007)) is assigned to cubic magnetic clusters of Mn2+ ions surrounding Fe(CN)6 vacancies. The clusters are well isolated from the bulk and are superparamagnetic below 50 K. In single crystals various defects with lower symmetry are also observed. Spin-lattice relaxation broadens the bulk ESR beyond observability. This strong spin relaxation is unexpected above the charge transfer transition and is attributed to a mixing of the Mn3+ - Fe2+ state into the prevalent Mn2+ - Fe3+ state.

New negative temperature coefficient thermistor ceramics in Mn-doped CaCu3-xMnxTi4O12 (0���x���1) system

New negative temperature coefficient (NTC) ceramics based on CaCu 3-xMnxTi4O12 (0���x���1) compositions have been investigated. The grain size of the CaCu 3-xMnxTi4O12 samples decreases at first and then increases with increasing Mn content. The X-ray photoelectron spectroscopy analysis corroborates the presence of Mn3+ and Mn 4+ in Mn-doped samples, which results in a decrease in the activation energy. All the NTC thermistors prepared show a linear relationship between the natural logarithm of the resistivity and the reciprocal temperature, indicative of NTC characteristics. The Mn-doped CaCu3-xMnxTi 4O12 NTC thermistors provide various electrical properties, depending on Mn content. The values of ��25, B constant and activation energy of the NTC thermistors are in the range of 2.22��106-3.22��108 �� cm, 5488-8031 K, and 0.473-0.692 eV, respectively. �� 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Synthesis and thermoelectric properties of Yb-doped Ca0.9-x Yb x La0.1MnO3 ceramics

The microstructure and thermoelectric properties of Yb-doped Ca0.9-x Yb x La0.1 MnO3 (0 ��� x ��� 0.05) ceramics prepared by using the Pechini method derived powders have been investigated. X-ray diffraction analysis has shown that all samples exhibit single phase with orthorhombic perovskite structure. All ceramic samples possess high relative densities, ranging from 97.04% to 98.65%. The Seebeck coefficient is negative, indicating n-type conduction in all samples. The substitution of Yb for Ca leads to a marked decrease in the electrical resistivity, along with a moderate decrease in the absolute value of the Seebeck coefficient. The highest power factor is obtained for the sample with x = 0.05. The electrical conduction in these compounds is due to electrons hopping between Mn3+ and Mn4+, which is enhanced by increasing Yb content.

S��ntese e caracteriza����es estrutural e magn��tica das ferritas de cobalto-mangan��s (Co1-xMnxFe2O4 E Co1,2Fe1,8-xMnxO4)

The cobalt-manganese ferrites (Co1��xMnxFe2O4 and Co1,2Fe1,8��xMnxO4) has a mixed structure of spinel type and it has been regarded as one of candidates for petitive wide variety of applications in devices from ultrasonic generation and detection, sensors, transformers, as well as in medical industry. Ferrites cobalt-manganese nanostructured were produced via mechanical alloying with subsequent heat treatment and were characterized by X-ray diffraction, X-ray fluorescence, scanning electron microscopy and magnetization. Samples of Co1��xMnxFe2O4 and Co1,2Fe1,8��xMnxO4 were obtained from the precursor powders Fe3O4, Co3O4 and Mn3O4 which were stoichiometrically mixed and ground by 10h and heat treated at 900��C for 2h. The diffraction confirmed the formation of the pure nanocrystalline phases to series Co1,2Fe1,8��xMnxO4 with an average diameter of about 94nm. It was found that the lattice parameter increases with the substitution of Fe3�� by Mn3��. The x-ray fluorescence revealed that the portions of metals in samples were close to the nominal stoichiometric compositions. The microstructural features observed in micrographs showed that the particles formed show very different morphology and particle size. The magnetic hysteresis measurements performed at low temperature showed that the saturation magnetization and remanence increased as the concentration of manganese, while the coercive field decreased. The anisotropy constant (Ke f ), was estimated from the data adjustments the law of approaching saturation. It was found that the anisotropy decreases substantially with the substitution of Fe by Mn.

Room temperature electromechanical and magnetic investigations of ferroelectric Aurivillius phase Bi5Ti3(FexMn1���x)O15 (x���=���1 and 0.7) chemical solution deposited thin films

Aurivillius phase thin films of Bi5Ti3(FexMn1���x)O15 with x���=���1 (Bi5Ti3FeO15) and 0.7 (Bi5Ti3Fe0.7Mn0.3O15) on SiO2-Si(100) and Pt/Ti/SiO2-Si substrates were fabricated by chemical solution deposition. The method was optimized in order to suppress formation of pyrochlore phase Bi2Ti2O7 and improve crystallinity. The structuralproperties of the films were examined by x-ray diffraction, scanning electron microscopy, and atomic force microscopy. Optimum crystallinity and pyrochlore phase suppression was achieved by the addition of 15 to 25 mol. % excess bismuth to the sols. Based on this study, 17.5 mol. % excess bismuth was used in the preparation of Bi2Ti2O7-free films of Bi5Ti3FeO15 on SrTiO3(100) and NdGaO3(001) substrates, confirming the suppression of pyrochlore phase using this excess of bismuth. Thirty percent of the Fe3+ ions in Bi5Ti3FeO15 was substituted with Mn3+ ions to form Bi2Ti2O7-free thin films of Bi5Ti3Fe0.7Mn0.3O15 on Pt/Ti/SiO2-Si, SiO2-Si(100), SrTiO3(100), and NdGaO3(001) substrates. Bi5Ti3FeO15 and Bi5Ti3Fe0.7Mn0.3O15thin films on Pt/Ti/SiO2-Si and SiO2-Si(100) substrates were achieved with a higher degree of a-axis orientation compared with the films on SrTiO3(100) and NdGaO3(001) substrates. Room temperature electromechanical and magnetic properties of the thin films were investigated in order to assess the potential of these materials for piezoelectric,ferroelectric, and multiferroic applications. Vertical piezoresponse force microscopy measurements of the films demonstrate that Bi5Ti3FeO15 and Bi5Ti3Fe0.7Mn0.3O15thin films are piezoelectric at room temperature. Room temperature switching spectroscopy-piezoresponse force microscopy measurements in the presence and absence of an applied bias demonstrate local ferroelectric switching behaviour (180��) in the films. Superconducting quantum interference device magnetometry measurements do not show any room temperature ferromagnetic hysteresis down to an upper detection limit of 2.53��������10���3 emu; and it is concluded, therefore, that such films are not mutiferroic at room temperature. Piezoresponse force microscopy lithography images of Bi5Ti3Fe0.7Mn0.3O15thin films are presented.