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La1-xCaxMnO3 belongs to the family of manganites, with generalized chemical formula R1-xAxMnO3, where R element is a rare earth element, and A is alkaline, alkaline earth or Pb. These compounds became very interesting in 1994, due to the discovery of colossal magnetoresistance (CMR). CMR was originally found for 0.2 < x < 0.5, and therefore that part of the phase diagram was mostly investigated. Here we report our results obtained in the overdoped part of the diagram 0.5 ≤ x ≤ 0.75 on La1-xCaxMnO3 thin films grown on MgO substrate. We have found CMR and metastable behavior deep in the overdoped part, which we attributed the existence of FM clusters in the AFM / CO insulator state.

We investigate the dc resistivity and Hall effect of the quasi-two-dimensional organic materials κ–(ET)2X, where X=Ag2(CN)3 and B(CN)4 and compare them with the results for X=Cu2(CN)3. All three compounds are considered to be quantum-spin-disordered Mott insulators. Despite high similarities in chemical composition and crystal structure, large differences in the dc resistivity and Hall coefficient are found. While around room temperature the dc transport properties are dominantly determined by the strength of the electron correlations, upon reducing the temperature, dc transport happens by hopping due to inherent disorder. The most disordered compound with X=Cu2(CN)3 turns out to have the lowest dc resistivity and the highest charge carrier density, i.e., in the phase diagram it is located closest to the metal-insulator transition. The least disordered compound with X=B(CN)4 shows the highest resistivity and the lowest carrier density, i.e., lies farthest from the metal-insulator transition. We explain such counterintuitive behavior within the theory of Mott-Anderson localization as a consequence of disorder-induced localized states within the correlation gap.

Heuslerove slitine na bazi Co privukle su veliki interes kao kandidati za primjenu u spintronici. Neki od ovih materijala karakteriziraju se kao polu-metalni (half-metal) feromagneti s visokom Curievom temperaturom i polarizacijom spina struje [1]. Jedan od takvih je Co_2TiSn [2, 3]. Za ovaj materijal teorijska istraživanja predvidjela su postojanje topološkog semimetalnog stanja s Weylovim čvorovima koji se javljaju za određene smjerove magnetizacije, što bi trebalo rezultirati velikim magnetootporom i anomalnim Hallovim efektom [4] te da je glavni doprinos intrinsičnom anomalnom Hallovom efektu rezultat pojave Berryeve zakrivljenosti [5]. U ovom radu predstavljamo sistematsko istraživanje otpornosti, magnetootpora i Hallovog efekta Co_2TiSn i Co_2Ti_0.6V_0.4Sn tankih filmova na više temperatura i za magnetska polja do 16 T primijenjena duž odabranih kristalografskih smjerova. Oba sustava pokazuju veliki anomalni Hallov efekt, ali ipak manji nego što je za neke standardne feromagnetske slitine. Izmjerena anomalna Hallova vodljivost za Co_2TiSn manja je od drugih rezultata [5], ali još uvijek u skladu sa teorijski izračunatom vrijednošću [5]. Za Co_2Ti_0.6V_0.4Sn anomalna Hallova vodljivost je znatno niža. Za oba sustava dominantni doprinos anomalnom Hallovom efektu dolazi od intrinsičnog side-jump efekta, što je u skladu sa teorijskim predviđanjima [6] za sustave s visokom otpornošću. Magnetoopor za oba sustava je malog iznosa (

Manganites R1−xAxMnO3, R and A being rare-earth and alkaline-earth ions respectively, show a wide variety of intriguing phenomena such as colossal magnetoresistance (CMR), multiferroicity, charge ordering, electronic phase separation, etc. Strong coupling of the spin, charge, orbital, and lattice degrees of freedom and the intense competition between the ferromagnetic (FM) metallic and antiferromagnetic (AFM) insulating phases cause very complicated phase diagrams. The exact phase depends on chemical substitution, level of doping, magnetic field, pressure, grain size, etc. La1−xCaxMnO3 is the most investigated member of the manganites family. Two phases dominate the well known phase diagram of La1−xCaxMnO3: a ferromagnetic metallic ground state for doping 0.2 < x < 0.5 and an antiferromagnetic (AFM) insulating charge order (CO) ground state for 0.5 < x < 0.9. However, the exact boundary between FM and CO/AFM phases is somehow sample dependent and it is believed that close to x = 0.5 the two dissimilar phases can coexist. In this work we show characterization of ceramic polycrystalline samples of La1-xCaxMnO3 with x = 0.5 using dc conductivity, ac susceptibility, magnetotransport and dielectric spectroscopy at low temperatures. Most importantly we find two prominent phase transitions, a high-temperature one corresponding to CO and a low-temperature transition with a FM fingerprint. Between these two features a metastable region is present both in charge as well as the spin sector. We discuss these findings in the context of phase separation and compare them with other La/Ca or Pr/Ca substitutions x.

The family of compounds based on bis-(ethylenedithio)tetrathiafulvalene (BEDT-TTF) molecule shows intriguing phenomena such as unconventional superconductivity, charge ordering, Mott transition, antiferromagnetism, incoherent metallic state etc. κ-(BEDT-TTF)2Cu2(CN)3 member is especially interesting because it is considered as one of the first realizations of a quantum spin liquid. It has a layered crystal structure consisting of conducting BEDT-TTF layers sandwiched between insulating Cu2(CN)3 layers and according to the phase diagram is in a Mott insulating state. However, approaching the spin liquid ground state various anomalies were observed showing that κ-(BEDT-TTF)2Cu2(CN)3 is not an ordinary Mott insulator. Here we present DC resistivity, Hall effect and magnetoresistance measurements for 10 K < T < 300 K and in magnetic fields up to 5 T on high quality κ-(BEDT-TTF)2Cu2(CN)3 single crystals. Our results clearly show that at lower temperatures (T < 130 K) charge transport takes place via variable range hopping, a mechanism typical for disordered systems. The origin of disorder, in nominally clean single crystals, can be found in insulating Cu2(CN)3 layers since some CN groups lie in centers of inversion and are therefore crystallographically disordered. The CN disorder affects conducting layers via hydrogen bonds between CN groups and ethylene groups of the BEDT-TTF molecules.

La1-xCaxMnO3 pripada obitelji manganovih oksida, preciznije manganita, s općenitom kemijskom formulom R1-xAxMnO3, gdje je R element rijetke zemlje, a A alkalni, zemnoalkalni metal ili Pb. Ovi spojevi se istražuju još od 1950., ali postaju posebno interesantni 1994., zbog otkrića kolosalnog magnetootpora (CMR). Osim toga, manganiti imaju vrlo bogate fazne dijagrame, s mnogim zanimljivim fenomenima. Istraživani spoj La1-xCaxMnO3 ima tri najvažnija (i najveća) dijela faznog dijagrama: paramagnetsko (PM) izolatorsko stanje na visokom temperaturama, feromagnetsko (FM) metalno stanjeza 0.2 < x < 0.5 (koje je povezano s CMR) i antiferomagnetsko (AFM) izolatorsko stanje, popraćeno uređenjem naboja (CO) za 0.5 < x < 0.87. Danas se vjeruje da u nekim dijelovima faznog dijagrama može doći do koegzistencije faza, posebno u blizini granice faza. Najzanimljivija je granica između FM metalnog stanja i AFM/CO izolatorskog stanja na x = 0.5. Nedavno smo objavili naše rezultate istraživanja magnetotransportnih svojstva tankih filmova manganita La1-xCaxMnO3, za x = 0.52, 0.58 i 0.75. Za koncentracije x = 0.52 i 0.58, opazili smo pojavu CMR (karakterističnu za x < 0.5), a koju smo pripisali postojanju FM klastera u AFM/CO izolatorskom stanju. Efekt je najizraženiji za x = 0.52, gdje dolazi do prijelaza u metalno stanje pod utjecajem magnetskog polja. Inače, ovisnost otpornosti o temperaturi bez primjene magnetskog polja, za sve tri koncentracije i na nižim temperaturama (T < TCO), slijedi režim trodimenzionalnog (3D) Mottovog preskoka promjenjivog dosega (VRH). Objavljeni rezultati su prošireni mjerenjima do nižih temperatura i jačih magnetskih polja, a također smo obuhvatili još različitih koncentracija, pri čemu su opažene i neke nove pojave: velike razlike između otpornih krivulja mjerenih u magnetskom polju, u slučajevima kad je uzorak hlađen u magnetskom polju (FC) i bez magnetskog polja (ZFC). Osim toga otporne krivulje pokazuju značajne efekte ovisne o povijesti mjerenja i neobične vremenske relaksacije.

Ferromagnetic Heusler alloys are characterized by high Curie temperatures and a great variety of electronic and magnetic properties. The Co-based Heusler alloys are of a particular interest, as half-metallic ferromagnets with a high spin polarization at Fermi energy they have a great potential for spintronic applications (for example, as magnetic electrodes in magnetic tunnel junctions). Recently, a theoretical study [1] has predicted a topological semimetal state in Co2TiSn with Weyl nodes appearing for certain magnetization directions, which could result in a large magnetoresistance and anomalous Hall effect. We have performed a detailed study of the resistivity, magnetoresistance, and Hall effect of Co2TiSn and Co2Ti0.6V0.4Sn Heusler alloys at several temperatures and with the magnetic field applied along selected crystallographic directions. Our data show that the anomalous Hall effect conductivity is large, in the range of the AHE conductivity found in Mn3Ge [2], while the magnetoresistance is small and quasi-isotropic without any unusual dependences on the field orientation with respect to the crystal axes. Although the results do not show a specific and clear signature of a Weyl character, they recognize the potential of the Co2TixV1-xSn system for future spintronic device applications. [1] G. Chang et al, Sci. Rep. 6 (2016) 38839. [2] A. K. Nayak et al, Sci. Adv. 2 (2016) e1501870.

La1-xCaxMnO3 belongs to the family of manganese oxides, known as manganites, with a general chemical formula R1-xAxMnO3, where R is a rare earth element or Bi and A an alkali or alkaline earth metal or Pb. These compounds have been investigated since 1950., but a huge interest started in 1994. with the discovery of the famous colossal magnetoresistance (CMR). Besides CMR, manganites are interesting because of their very rich phase diagrams which show plenty of intriguing phenomena such as metal- insulator transition, charge ordering, multiferroicity, magnetocaloric effect, colossal thermoelectric power, phase coexistence, etc. In the case of the investigated compound La1-xCaxMnO3 the phase diagram consists of the three large parts: a paramagnetic (PM) insulating phase at high temperatures for all x, a ferromagnetic (FM) metallic ground state for 0.2 < x < 0.5 and an antiferromagnetic (AFM) charge order (CO) insulating ground state for 0.5 < x < 0.87. CMR effect is closely related to the FM phase and has maximum value at the Curie temperature. At present, it is believed that across the phase diagram there are areas where two phases can coexist, particularly close to the boundary between the two phases. Especially interesting is the coexistence between the FM metallic and CO/AFM insulating phase which is believed to be the key for understanding the CMR effect. Our recent work on magnetotransport properties of manganites thin films La1-xCaxMnO3 for x > 0.5 has shown the existence of CMR, FM and metal- insulator transition, properties characteristic for x < 0.5, which has been ascribed to the presence of FM clusters in the CO/AFM background. Here we present a continuation of this research to lower temperatures and higher magnetic fields which reveals a huge difference between field- and zero-field cooled resistivity curve and unusual time relaxation of the resistivity after application of an external magnetic field. We discuss this features in terms of the FM/AFM phase separation scenario.

Manganites La1-xCaxMnO3 belong to the family of manganese oxides with a general chemical formula R1-xAxMnO3, where R represents a rare earth element or Bi and A an alkali or alkaline earth metal or Pb. Replacement of trivalent R and divalent A introduces a mixed valence state Mn3+/Mn4+ and depending on the concentration x causes a presence of various phases in the phase diagrams of R1-xAxMnO3. A huge interest for this family of compounds started in 1994. when Jin et al. found a huge decrease of resistivity in external magnetic field close to Curie temperature known as colossal magnetoresistance (CMR). CMR is present in the part of the phase diagram with ferromagnetic metallic ground state which for the investigated system La1-xCaxMnO3 appears for concentrations 0.2 < x < 0.5. Here we focused on the poorly explored part of the phase diagram x > 0.5 with a paramagnetic insulating phase at high, charge order phase at intermediate and antiferromagnetic insulating phase at low temperatures. We performed magnetotransport and magnetization measurements in the temperature interval 4.2-300 K and magnetic fields up to 5 T on thin film manganites for different concentrations 0.52 ≤ x ≤ 0.75. Our results show traces of CMR and ferromagnetism and a strong coupling between magnetic and transport properties which can be interpreted in terms of spin dependent variable range hopping mechanism and a coexistence of ferromagnetic and antiferromagnetic phase.

Manganites La1-xCaxMnO3 belong to the family of manganese oxides with a general chemical formula R1- xAxMnO3, where R represents a rare earth element or Bi and A an alkali or alkaline earth metal or Pb. Replacement of trivalent R and divalent A introduces a mixed valence state Mn3+/Mn4+ and depending on the concentration x causes a presence of various phases in the phase diagrams of R1-xAxMnO3. A huge interest for this family of compounds started in 1994. when Jin et al. found a huge decrease of resistivity in external magnetic field close to Curie temperature known as colossal magnetoresistance (CMR). CMR is present in the part of the phase diagram with ferromagnetic metallic ground state which for the investigated system La1-xCaxMnO3 appears for concentrations 0.2 < x < 0.5. Here we focused on the poorly explored part of the phase diagram x > 0.5 [3] with a paramagnetic insulating phase at high, charge order phase at intermediate and antiferromagnetic insulating phase at low temperatures. We performed magnetotransport and magnetization measurements in the temperature interval 4.2-300 K and magnetic fields up to 5 T on thin film manganites for different concentrations 0.5 < x < 0.8. Our results show traces of CMR and ferromagnetism and a strong coupling between magnetic and transport properties which can be interpreted in terms of spin dependent variable range hopping mechanism and a coexistence of ferromagnetic and antiferromagnetic phase.

alpha-(BEDT-TTF)2I3 pripada familiji spojeva temeljenoj na organskoj molekuli bis-(ethylene-dithio)-tetrathiafulvalene (BEDT-TTF) koja se u novije vrijeme intenzivno istražuje zbog nekonvencionalne supravodljivosti, uređenja naboja, elektronske feroelektričnosti, stanja kvantne spinske tekućine itd. Kao i ostali pripadnici ove familije, i alpha-(BEDT-TTF)2I3 ima slojevitu kristalnu strukturu koja uzrokuje kvazidvodimenzionalna elektronska svojstva. Pri atmosferskom tlaku alpha-(BEDT-TTF)2I3 na visokim temperaturama pokazuje polumetalna svojstva s malim džepovima elektrona i šupljina, a na temperaturi TCO = 136 K prelazi u stanje uređenja naboja (CO) s feroelektričnim oznakama. Primjena tlaka uzrokuje postepeno potiskivanje CO faznog prijelaza, a pri određenim uvjetima i pojavu supravodljive faze ispod 7 K. Mjerenja Hallovog efekta i magnetootpora pri tlakovima većim od 1.5 GPa, kada je CO faza u potpunosti potisnuta, pokazuju da se koncentracija slobodnih nosioca naboja od 300 K do 2 K smanjuje 6 redova veličine, dok njihova pokretljivost raste 6 redova veličine. Ovi rezultati pokazuju da je alpha-(BEDT-TTF)2I3 iznad 1.5 GPa poluvodič s nultim energijskim procjepom čiji se nosioci naboja ponašaju kao Diracovi fermioni što je u skladu s najnovijim proračunima vrpci. Budući da su zbog spomenutih izrazito zanimljivih fizikalnih svojstava magnetotransportna istraživanja u literaturi uglavnom provedena uz primjenu visokih tlakova, proveli smo mjerenja pri atmosferskom tlaku koja nedostaju: dc otpornost, anizotropija dc otpornosti, magnetootpor i Hallov efekt u temperaturnom intervalu 40-300 K i u magnetskim poljima do 5 T. Dobivene rezultate usporedit ćemo s rezultatima magnetotransportnih istraživanja alpha-(BEDT-TTF)2I3 pod tlakom kao i s našim rezultatima magnetotransportnih istraživanja srodnog spoja kappa-(BEDT-TTF)2Cu2(CN)3 u kojem CO faza ne postoji.

(TMTTF)2ReO4 i (TMTTF)2BF4 pripadaju u skupinu takozvanih Fabreovih soli koje pri sobnoj temperaturi većinski pokazuju kvazi 1D metalnu vodljivost. Variranjem raznih parametara poput tlaka, temperature te vrste aniona, dobiven je vrlo bogat fazni dijagram Fabreovih soli u kojem su neka od mogućih stanja: metalno stanje, stanje uređenja naboja, spin-Peierlsovo stanje, supravodljivo stanje i stanje vala gustoće spina[1]. Od prije je poznato da navedena dva spoja pri sobnoj temperaturi pokazuju kvazi 1D metalnu vodljivost te da spuštanjem temperature ispod 230 K za (TMTTF)2ReO4, odnosno 84 K za (TMTTF)2BF4, oba spoja prolaze kroz metal-izolatorski fazni prijelaz u izolatorsko stanje uređenja naboja. Također, oba spoja spadaju u skupinu soli s nesimetričnim anionima pa daljnjim spuštanjem temperature ispod 157 K za (TMTTF)2ReO4, odnosno 41.5 K za (TMTTF)2BF4, dolazi do zamrzavanja orijentacije aniona i loma simetrije pa oba spoja prolaze kroz izolator-izolatorski fazni prijelaz u stanje uređenja aniona[2]. U cilju boljeg razumijevanja transportnih mehanizama, mjerili smo otpornost i Hallov koeficijent RH u ovisnosti o temperaturi. Otpornost je mjerena u smjeru najveće vodljivosti, kao i RH (magnetsko polje je okomito na smjer struje) koji je mjeren AC kompenzacijskom metodom u magnetskim poljima do 9 T te za temperature 45 K< T < 300 K. Oba spoja pri sobnoj temperaturi imaju približno jednaku vrijednost RH koja je oko 3 puta veća od teorijski predviđene vrijednosti. RH je u oba spoja pozitivan što je u skladu s činjenicom da su nosioci naboja šupljine. Fazni prijelaz u stanje uređenja naboja posebno je naglašen u (TMTTF)2ReO4 gdje smo detektirali naglu promjenu predznaka RH u uskom intervalu temperatura. Daljnjim opadanjem temperature ispod faznog prijelaza u stanje uređenja naboja, porast RH u oba spoja prati eksponencijalni porast otpornosti što odgovara ponašanju izolatora kod kojih je ovakvo ponašanje posljedica smanjenja broja nosioca naboja sa snižavanjem temperature. Dobivene rezultate ćemo usporediti sa rezultatima srodnih spojeva te interpretirati u okviru postojećih teorijskih modela. [1] M. Dressel, Naturwissenschaften 94, 527 (2007). [2] B. Köhler, E. Rose, M. Dumm, G. Untereiner, and M. Dressel, Phys. Rev. B 84, 035124 (2011)

alpha-(BEDT-TTF)2I3 is a quasi-2D organic conductor most famous for its prominent charge order within molecular planes below TCO =136 K [1]. Due to the breaking of an inversion center, the response at the charge ordering transition should have a ferroelectric character [2, 3]. Puzzlingly, at the moment it is unclear whether the ferroelectric peak appears in the dielectric properties at all [4]. Furthermore, the low-lying excitations above the ground state seem to bring forth both a phason-like as well as a soliton-like relaxation, which have so far not been reported to appear together in similar systems [5]. In order to explore these open questions, we have carefully determined the direction-dependent (in-plane and out-of-plane) dielectric response in alpha-(BEDT-TTF)2I3 together with resistance and Hall effect measurements down to lowest temperatures. The temperature dependence of the Hall coefficient absolute value follows a similar behavior as the resistivity. An intriguing feature is found at TCO where the Hall coefficient changes sign. Another interesting property is a change in the in-plane transport activation energy around 60 K. On the other hand, dielectric measurements give strongly anisotropic and complex spectra. Our results are discussed within the wider context of strongly correlated systems with charge order. [1] T. Takahashi, Y. Nogami, and Kyuya Yakushi, J. Phys. Soc. Jpn. 75, 051008 (2006). [2] T. Kakiuchi, Y. Wakabayashi, H. Sawa, T. Takahashi, and T. Nakamura, J. Phys. Soc. Jpn. 76, 113702 (2007) [3] K. Yamamoto, A. A. Kowalska, and K. Yakushi, Appl. Phys. Lett. 96, 122901 (2010). [4] T. Ivek, B. Korin-Hamzić, O. Milat, S. Tomić, C. Clauss, N. Drichko, D. Schweitzer, and M. Dressel, Phys. Rev. Lett. 104, 206406 (2010) ; P. Monceau, private communication (2014). [5] T. Ivek, B. Korin-Hamzić, O. Milat, S. Tomić, C. Clauss, N. Drichko, D. Schweitzer, and M. Dressel, Phys. Rev. B 83, 165128 (2011).

Upon cooling the highly anisotropic organic conductors (TMTTF) 2 X undergo transitions to various ordered states in the charge and spin degrees of freedom and therefore turn out to be an excellent choice for exploring the phenomena of low-dimensional physics. In order to study the transport mechanism of these quasi-one-dimensional (1D) organic salts, the temperature-dependent Hall effect of (TMTTF) 2 X (X=BF 4 , ReO 4 , AsF 6 , SbF 6 ) has been measured by an AC compensation technique with current injection along the high-conducting a-axis. At elevated temperatures the Hall coefficient R H of all investigated compounds seems to be constant, positive and somehow enhanced compared to the calculated value for one hole per unit cell. Beyond the charge order phase transition the Hall coefficient follows similar exponential laws as the resistivity data. This indicates the behavior of a semiconductor, where the change in ρ(T) mainly originates from the change in the number of carriers. Here we discuss how the Hall coefficient is influenced by anisotropy, localization effects, anion symmetry and charge- as well as anion-ordering phenomena at low temperatures. We compare the results to resistivity studies and theoretical expectations.

We have in detail characterized the anisotropic charge response of the dimer Mott insulator κ−(BEDT-TTF)2Cu2(CN)3 by dc conductivity, Hall effect, and dielectric spectroscopy. At room temperature, the Hall coefficient is positive and close to the value expected from stoichiometry; the temperature behavior follows the dc resistivity ρ(T). Within the planes the dc conductivity is well described by variable-range hopping in two dimensions; this model, however, fails for the out-of-plane direction. An unusually broad in-plane dielectric relaxation is detected below about 60 K; it slows down much faster than the dc conductivity following an Arrhenius law. At around 17 K, we can identify a pronounced dielectric anomaly concomitantly with anomalous features in the mean relaxation time and spectral broadening. The out-of-plane relaxation, on the other hand, shows a much weaker dielectric anomaly; it closely follows the temperature behavior of the respective dc resistivity. At lower temperatures, the dielectric constant becomes smaller both within and perpendicular to the planes; also, the relaxation levels off. The observed behavior bears features of relaxorlike ferroelectricity. Because heterogeneities impede its long-range development, only a weak-tunneling-like dynamics persists at low temperatures. We suggest that the random potential and domain structure gradually emerge due to the coupling to the anion network.

Novel forms of the low-temperature phases in the two-dimensional molecular solids with competing interactions between charges, spins and lattice, in particular those featuring anomalous dielectric relaxation, have been the focus of intense activity in recent years. Open issues concern the nature of collective charge excitations as well as their coupling to applied ac and dc electric fields. The charge response is reasonably well understood by now in the charge-ordered phase with the formation of ferroelectric-like domains below the metal-to-insulator phase transition [1, 2]. Conversely, the dielectric response observed in dimer Mott insulator phases with no charge ordering [3] is rather intriguing [4, 5, 6, 7]. In this presentation we review our in-depth study (dc conductivity, Hall effect and dielectric spectroscopy) of the anisotropic charge response in an unconventional Mott insulator with spin liquid kappa-(BEDT-TTF)2Cu2(CN)3. Variable-range hopping transport within molecular planes in dc limit and an anomalously broad anisotropic dielectric relaxation in audio-frequency range demonstrate the inherent heterogeneity present in nominally pure single crystals of kappa-(BEDT-TTF)2Cu2(CN)3. The relaxation bears fingerprints of relaxor ferroelectricity: a gradual slowing down becomes dominated by tunneling at low temperatures and at the bifurcation temperature anomalous features in dielectric constant, mean relaxation time and spectral broadening are detected. We argue that the observed behavior is due to a strong charge-spin-lattice coupling reflecting the gradual emergence of random potential and domain structure originated in the anion network. [1] T.Ivek et al., Phys.Rev.B83, 165128 (2011). [2] T.Ivek et al., Phys.Rev.B86, 245125 (2012). [3] K.Sedlmeier et al., Phys.Rev.B86, 245103 (2012). [4] M.Abdel-Jawad et al., Phys.Rev.B82, 125119 (2010). [5] S.Iguchi et al., Phys.Rev.B87, 075107 (2013). [6] P.Lunkenheimer et al., Nat. Mater. 11, 755 (2012). [7] S.Tomic et al., J.Phys.: Condens.Matter 25, 436004 (2013). [8] M.Pinteric et al., to be submitted (2014).

U uvodniku sveska Prirode posvećenom PMF-u prikazana je prošlost fakulteta i nastave prirodoslovlja u Zagrebu, ali i suvremena istraživanja koja se obavljaju na toj znanstvenoj i visokoškolskoj ustanovi.

Manganites La1-xCaxMnO3 belong to the family of manganese oxides with a general chemical formula R1-xAxMnO3, where R stands for a rare earth element and A for an alkali or alkaline earth metal. One of the most prominent property of manganese oxides is a huge decrease of resistivity in external magnetic field close to Curie temperature known as colossal magnetoresistance (CMR). In these systems CMR can be as large as -99%, unlike magnetoresistance of most materials which is usually small (a few percent). Except CMR, manganese oxides are interesting because of their very rich phase diagrams as a consequence of the fine interplay of charge, spin, lattice and orbital interactions. CMR is present in the part of the phase diagram of manganese oxides with ferromagnetic metallic ground state and in the case of the investigated system La1-xCaxMnO3 for concentrations 0.2 < x < 0.5. While pure LaMnO3 and CaMnO3 are antiferromagnetic insulators and contain Mn3+, Mn4+ respectively, doped manganites La1-xCaxMnO3 have a variety of different phases and contain both Mn3+ and Mn4+ (a mixed valence state). Here we focused on the poorly explored part of the phase diagram x > 0.5 with a paramagnetic insulating phase at high, charge order phase (CO) at intermediate and antiferromagnetic insulating phase at low temperatures. We performed magnetotransport measurements in the temperature interval 4.2-300 K and magnetic fields up to 5 T on a series of thin film manganites (170 nm thick) of different concentrations 0.5 < x < 0.8 obtained by metalorganic aerosol deposition technique. We will show and discuss the results of magnetotransport properties by which we investigated the influence of the concentration x on resistivity, CO phase transition temperature, magnetoresistance and anisotropy of magnetoresistance.

Oksidi mangana R1−xAxMnO3 (gdje R označava element rijetke zemlje, A alkalijske elemente) predstavljaju jedinstveno područje za istraživanje utjecaja elektronskih korelacija u materijalima koje se odnose na naboj, spin, orbitalne stupnjeve slobode i stupnjeve slobode rešetke budući da su prisutne interakcije usporedive jakosti što ima za posljedicu vrlo bogati fazni dijagram. Naša istraživanja odnose se na seriju filmova manganita La1-xCaxMnO3 različitih koncentracija (x = 0.52, x = 0.58 i x = 0.75). Dok je početni spoj LaMnO3 izolator (s osnovnim antiferomagnetskim stanjem) i sadrži Mn3+ kation, dopiranje dvovalentnim Ca2+ kationom uzrokuje pojavu Mn4+ kationa te time i stanje „mješanih valencija“ Mn3+/Mn4+, a kao posljedicu veliku promjenu u fizikalnim svojstvima. Jedno od najpoznatijih svojstava manganovih oksida je kolosalni magnetootpor (CMR) koji se pojavljuje u La1-xCaxMnO3 za x ≈ 0.3 (veliko smanjenje otpornosti u vanjskom magnetskom polju u blizini Curieve temperature). Kako su do sada istraživanja uglavnom bila ograničena na koncentracije x < 0.5, a o prirodi samog spoja i pogotovo CMR još uvijek postoji čitav niz nepoznanica fokusirali smo se na koncentracije x > 0.5 gdje fazni dijagram karakterizira paramagnetsko stanje na visokim temperaturama, te sa snižavanjem temperature fazni prijelazi u stanje uređenja naboja (CO) i antiferomagnetsko stanje (AF). Mjerenja otpornosti, ovisnosti magnetootpornosti o temperaturi, jačini i smjeru vanjskog magnetskog polja te Hallovog efekta provedena su u širokom području temperatura 30 K < T < 300 K i u magnetskim poljima do 5 tesla. Prikazati ćemo i diskutirati rezultate mjerenja pomoću kojih smo odredili temperaturu faznog prijelaza u CO stanje za sve x, promjene u otpornosti u ovisnosti o x, promjene doprinosa CMR u ovisnosti o x, vrstu nosioca naboja na visokim temperaturama te promjenu predznaka Hallovog koeficijenta ispod TCO.

Hallov koeficijent RH je bitna fizikalna veličina za karakterizaciju materijala, budući da odražava strukturu vrpce, koncentraciju i predznak nosioca naboja. U jako koreliranim sustavima RH pokazuje kompleksnu ovisnost o raznim parametrima (temperatura, magnetsko polje, dimenzionalnost sustava...). U slučaju reducirane dimenzionalnosti (visoko-anizotropni sustavi), elektronske korelacije postaju još istaknutije, što se jasno pokazuje i u Hallovom efektu. Za izotropne sustave mjerenja Hallovog efekta provode se u standardnoj geometriji, dok je u slučaju visoko-anizotropnih sustava nužan uvjet za dobivanje kvalitetnih i pouzdanih rezultata korištenje adekvatne geometrije mjerenja. U tu svrhu nužno je koristiti koncept ekvivalentnog izotropnog uzorka (EIS) [1]. Ovaj smo pristup detaljno primijenili pri mjerenjima Hallovog efekta kvazi-jednodimenzionalnog organskog vodiča TTF-TCNQ, za koji u literaturi postoje samo dva rezultata iz 1977. g., dobivena u ograničenom intervalu visokih temperatura, a koji se međusobno razlikuju i u iznosu i u predznaku [2] [3]. Kristalnu strukturu spoja TTF-TCNQ karakteriziraju odvojeni lanci u koje se slažu planarne TTF i TCNQ molekule. Uslijed prijenosa naboja s molekule TTF (kation) na TCNQ (anion) oba tipa lanca pokazuju metalno ponašanje: u TTF lancu nosioci naboja su šupljine, a u TCNQ lancu elektroni. Direktna posljedica ovakve strukture je izrazita anizotropija vodljivosti: σb>σc>σa pri čemu je σb/σc ≈ 102, σb/σa ≈ 103. Dok je na visokim temperaturama vodljivost za sva tri kristalografska smjera metalnog tipa, ispod 60K dolazi do serije faznih prijelaza te materijal postepeno prelazi u stanje izolatora. Mjerenja Hallovog efekta provedena su u širokom području temperatura 30K

Spinski Hallov efekt (SHE) je pojava kod koje dolazi do akumuliranja/razdvajanja spinova u vodiču, uslijed asimetrije u udarnom presjeku raspršenja nosioca naboja različitog spina. Osim ovog direktnog SHE-a značajan je i tzv. inverzni SHE (ISHE) u kojem spinska struja -- struja u kojoj postoji tok spina ali ne i naboja -- uslijed spinski ovisnog raspršenja generira tok naboja odnosno električnu struju. S obzirom da se pomoću (I)SHE može ostvariti transformacija obične električne struje u spinsku struju i obratno, oba efekta su od vrlo velikog interesa za daljnji razvoj spintronike i uređaja koji se zasnivaju na manipulaciji spina elektrona. U našem radu proučavan je inverzni SHE injektiranjem spinske struje iz NiFe elektroda u CuIr elektrodu i njegova ovisnost o koncentraciji primjesa iridija. Pokazano je da se uslijed asimetričnog raspršenja (skew scattering) na nemagnetskim primjesama iridija generira tok naboja (inverzni SHE). Učinak je proporcionalan koncentraciji iridija i praktički temperaturno neovisan za 4.2K < T < 200K. Efikasnost pretvorbe `spinska struja' – `električna struja' (tzv. spinski Hallov kut) veća je nego u dosadašnjim mjerenjima SHE u čistim metalima, i u skladu je s vrijednostima iz literature od prije tridesetak godina. Dobiveni rezultati pokazuju da je raspršenje elektrona na nemagnetskim primjesama jedan od mogućih načina za ostvarivanje velikog iznosa spinskog Hallovog efekta.

The extrinsic spin Hall effect induced by Ir impurities in Cu is studied by injecting a pure spin current into a CuIr wire from a lateral spin valve structure. The spin Hall angle of CuIr, (2.1+/- 0.6)% throughout the concentration range between 1% and 12%, is practically independent of temperature. The results represent a clear example of predominant skew scattering extrinsic contribution to the spin Hall effect in a nonmagnetic alloy

The spin Hall effect (SHE) is one of the important phenomena in spintronics since it enables to create spin currents in non-magnetic materials without using a ferromagnet. Recently-reported large SHEs in Pt and Au have triggered the search for a larger SHE in metallic devices. The mechanism of the large SHEs, however, have not been figured out yet ; whether it is intrinsic or extrinsic, if it is extrinsic whether it stems from the skew scattering mechanism or the side jump one. In this work, we focus on the extrinsic SHE. By adding Ir impurity into pure Cu, we have studied the impurity concentration dependence of the SHE in CuIr. We have used the lateral spin valve structure which consists of two Py wires and a CuIr middle wire.This device allows to estimate precisely how much spin currents are injected into CuIr. By flowing electric current from Py to one of the Cu electrodes, pure spin currents are injected into the CuIr middle wire and we are able to measure the inverse SHE. No SHE has been observed when there is no Ir impurity. With increasing the Ir impurity, however, the SH resistivity increases linearly with the resistivity of CuIr. This linear dependence strongly indicates that the SHE in CuIr is due to the skew scattering mechanism. From the slope of the SHE resistivity vs CuIr resistivity curve, it turns out that the SH angle in this system is 1.5%, which is consistent with the previous anomalous Hall effect measurement in CuMnIr.

Due to the large diversity of their physical properties and the potentiality to modulate them, the role of oxides in electronics has continuously increased over the past few years. The field of spintronics is no exception. This functional character is further increased by the possibility to obtain radically new properties at their interfaces. To illustrate our work in this field, we will present the results on all manganite tunnel junctions in which ferromagnetism is induced in the barrier by proximity effect resulting in a related exchange splitting of the conduction band and the ensuing spin filtering phenomena. Also, the experiments on heterostructures combining ferroelectric tunnel barriers of BaTiO3 and ferromagnetic electrodes (Fe or Co) will be discussed. This kind of heterostructures allows to generate, within a single device, not only a tunnel magnetoresistance (TMR) phenomena related to the relative orientation of the magnetization of the electrodes and a tunnel electroresistance (TER) induced by the ferroelectric polarisation of the barrier. They also give rise to an interfacial magnetoelectric coupling that results in an unusual modulation of the spin polarisation at the interface by the ferroelectricity resulting in a TEMR (Tunnel Electro MagnetoResistance) effect in these artificial multiferroic heterostructures. Finally, we will show the recent results on the conduction at the interface between two insulating materials SrTiO3 and LaAlO3 and the experiments we have performed in order to determine the extension of this electron gas.

The only published Hall effect measurements of TTF-TCNQ date back to 1977. They were performed in the high temperature metallic region above the phase transitions, and the results vary in their sign and values. The aim of the present work has been to extend the previous investigations to lower temperatures and higher magnetic fields and to compare the new data with the known results.

We present experimental results which show that doping Cu with (nonmagnetic) 6d impurities can induce a very large Spin Hall Effect. The Spin Hall conductivity can exceed 10^7 Ohm^(-1)m^(-1), to be compared to 10^4-10^6 Ohm^(-1)m^(-1) in Pt or Au (Kimura et al, PRL 2007 ; Seki et al, Nature Materials ; 2008), 10^3 Ohm^(-1)m^(-1) in Al (Valenzuela and Tinkham, Nature 2006) or 1 Ohm^(-1)m^(-1) in InGaAs (Kato et al, Science 2004). Resonant skew scattering on spin-orbit split impurity states explains the large value of the SHE induced by nonmagnetic impurities like Ir, Ta, Lu or Sb. It also explains that the different signs of the SHE we observe at the beginning (Lu) and end (Ir) of the 5d series are related to the position of the resonant 5d states with respect to the Fermi level. We will present a theoretical model of resonant skew scattering accounting for our results and discuss the advantage of resonant scattering on other mechanisms. An extension to resonant side-jump scattering will be also presented.

Proučavanje fizikalnih svojstava sistema sa jakim interakcijama posebno je važno u slučaju reducirane dimenzionalnosti gdje elektronske korelacije postaju još istaknutije. Vrlo osjetljiv odnos između elektronskih i strukturnih svojstava omogućuje da se modifikacijom strukturnih svojstava mijenja i anizotropija sistema. Istraživanje magnetotransportnih svojstava (anizotropije otpornosti, Hallovog efekta, magnetootpora) pokazuje se kao važno sredstvo za istraživanja fundamentalnih svojstava kao što su karakter vodljivosti, elektronske korelacije, stupanj dimenzionalnosti (od strogo 1D do 3D), fenomeni uređenja, fazni prijelazi, ... U tom smislu ova istraživanja obuhvaćaju organske vodiče (TMTSF)2ReO4 i (TMTTF)2AsF6, gdje je ustanovljen prijelaz u strogo 1D režim [1] ; kvazi-1D kuprate Sr14−xCaxCu24O41 (0

The future developments of new spintronic devices require high mobility 2DEG to inject and manipulate spins. Recently, such high mobility electron gas has been reported in the LaAlO_3/SrTiO_3 system and attributed to charge transfer at the polar interface. Such oxide based 2DEG is promising since it could be combined with other oxide materials taking advantage of their attractive versatility and multifunctional character. In this charge transfer mechanism the carrier concentration is very high, about 10^21 cm^(-3) and presents a moderate mobility compared to doped SrTiO3 single crystal. An alternative way to obtain high mobility electron gas while controlling the carrier concentration and its extension is by ion etching. We will present analysis of the distribution of defects and mobile carriers generated by means of preferential ion-beam etching of SrTiO_3 single crystals by combining positron annihilation spectroscopy (PAS), conductive-tip atomic force microscopy (CT-AFM) and low temperature magnetotransport. We demonstrate that vacancies generated at the surface by room-temperature ion irradiation diffuse on the micron scale and that consequently, carriers are distributed on the same scale.

We present Hall effect measurements of two-leg ladder Sr14-xCaxCu24O41 (0 10). They are the first superconducting copper oxides (superconductivity occurs under pressure for x>10) without the CuO2 planes, characteristic for high-temperature superconductors. The question of charge dynamics and hole distribution between the ladders and chains is still open. Our Hall effect results show typical semiconducting behavior for x

Ohtomo i Hwang (Nature, 427 (2004) 423) su na 6 nm tankom filmu LaAlO3 (LAO) naparenom na SrTiO3 (STO) opazili metalno ponašanje i pokretljivosti veće od 10^4 cm2/Vs. Ta svojstva bila su pripisana 2D elektronskom plinu oformljenom na granici STO i LAO, što može biti od posebnog interesa za razvoj spintroničkih elemenata (npr. Datta-Das spin-FET). Naša magnetotransportna mjerenja (T > 300 mK, B < 18 T) [1] su pokazala da se ipak radi o 3D strukturi, što smo kasnije i direktno potvrdili inovativnim korištenjem CT-AFM eksp. tehnike. Navedena svojstva posljedica su dopiranja supstrata STO šupljinama kisika za vrijeme rasta tankog filma LAO, te je pokazano da je debljina vodljivog sloja reda veličine čak nekoliko stotina μ m. Diskutirat ćemo i nove načine realizacije i karakterizacije visokopokretljivog elektronskog sloja u monokristalu STO. [1] M.Basletić, J.-L.Maurice, C.Carretero, G.Herranz, O.Copie, M.Bibes, E.Jacquet, K.Bouzehouane, S.Fusil, A.Barthelemy, Nature Mater. 7 (2008) 621 ; G.Herranz, M.Basletić, M.Bibes, C.Carrétéro, E.Tafra, E.Jacquet, K.Bouzehouane, C.Deranlot, A.Hamzić, J.-M.Broto, A.Barthélémy, A.Fert, Phys. Rev. Lett. 98 (2007) 216803

We present Hall-effect measurements of two-leg ladder compounds Sr14− xCaxCu24O41 (0

La recherche portant sur les semiconducteurs magnétiques est d’ une grande importance pour l’ électronique de spin. Récemment des expériences ont aussi suggéré l’ existence de ferromagnétisme dans La_0.5Sr_0.5Ti_O_(3-delta) dopé avec du cobalt. Contrairement a d’ autres systemes dilués, celui-ci présente l’ originalité d’ avoir comme matrice hôte un métal fortement corrélé et non semiconducteurs, ce qui est susceptible de donner lieu a des mécanismes d’ échange de nature différente. Nous avons suivi cette approche en dopant SrTiO_3, préalablement dopé avec du lanthane, avec l.5% de cobalt (Co-LSTO). Les échantillons ont été fabriqués par ablation laser pulsé avec un laser Nd:YAG. Dans le but de comprendre l’ origine des propriétés magnétiques de Co-LSTO, nous avons procédé a une large étude structurale, ainsi qu’ a la mesure d’ une polarisation en spin pour des jonctions tunnels avec des électrodes de Co-LSTO et nous avons trouvé que les conditions de croissance, comme la pression partielle d’ oxyg&egrave ; ; ne, ont une influence importante sur les propriétés magnétiques, révélant le rôle des défauts (lacunes d’ oxygene, p.ex.) Pour mesurer la polarisation en spin du matériau, des jonctions tunnels &agrave ; ; base de Co-LSTO ont été fabriquées par lithographie optique. La mesure de la magnétorésistance tunnel TMR pour des jonction Co-LSTO/LAO/Co donne des valeurs de TMR de l’ ordre de 20% ce qui nous permettent de déduire une polarisation en spin pour Co-LSTO de l’ ordre de 80%. Les expériences de spectroscopie d’ absorption de rayon X et de dichro&iuml ; ; sme circulaire magnétique nous ont permis de sonder les états électroniques des atomes de cobalt et de titane, important pour comprendre les origines microscopique des propriétés magnétiques nos films de Co-LSTO. En particulier, les signatures spectrales du XAS et de l’ XMCD indiquent que le cobalt est dans un état ionique, coexistant dans certains cas avec du Co métallique. Au vu de l’ ensemble de ces résultats, nous tenterons de dégager des conclusions quant a la nature du ferromagnétisme observé dans les couches minces de Co-LSTO.

I will present our comparative study of the magnetotransport properties of organic conductors (TMTCF)2X (C= Se, S). Their phase diagram spanning from more anisotropic C = S to C = Se series shows the increasing interchain coupling that leads to the crossing-over of the dimensionality. Our results of the conductivity anisotropy, Hall effect and magnetoresistance enabled us to gain information about the spin density wave ground state at low temperature, Fermi liquid or Luttinger liquid behavior in the normal phase at high temperatures, possible dimensional crossover and phase transitions. The results are discussed in the framework of the existing theoretical models. The evidence that the 1D Luttinger liquid features appear in the transport properties at high temperatures in C=S series will be addressed. I will also present our results of the Hall effect measurements of quasi-one-dimensional cuprates Sr14-xCaxCu24O41 ( 0 ≤ x ≤ 11.5) that have been conducted in order to determine the number of carriers participating in dc transport, and its relationship with doping. Namely, a fair number of investigations were dedicated to the distribution of holes in the parent system at room temperature with contradictory results ; the reason for that has not been clarified.

Ru(1-x)SnxSr2EuCu2O8 and Ru(1-x)SnxSr2GdCu2O8 have been comprehensively studied by microwave and dc resistivity and magnetoresistivity and by the Hall measurements. The ruthenium magnetic ordering temperature Tm is considerably reduced with increasing Sn content. However, doping with Sn leads to only slight reduction of the superconducting critical temperature Tc accompanied with the increase of the upper critical field Bc2, indicating an increased disorder in the system and a reduced scattering length of the conducting holes in CuO2 layers. In spite of the increased scattering rate, the normal state resistivity and the Hall resistivity are reduced with respect to the pure compound, due to the increased number of itinerant holes in CuO2 layers, which represent the main conductivity channel. Most of the electrons in RuO2 layers are presumably localized, but the observed negative magnetoresistance and the extraordinary Hall e® ; ; ; ect lead to the conclusion that there exists a small number of itinerant electrons in RuO2 layers that exhibit colossal magnetoresistance.

We present Hall effect measurements of two-leg ladder quasi-one-dimensional copper oxides Sr14-xCaxCu24O41 (0 ≤ x ≤ 11.5). In these composite materials charge and spin ordering occurs within chain and ladder subsystems [1]. They have been investigated intensively during the last years due to their fascinating physical properties, especially after the discovery of superconductivity under pressure (for x > 10). They are the first superconducting copper oxides without the CuO2 planes, characteristic for high-temperature superconductors. The question of charge dynamics and hole distribution between the ladders and chains is still open, because the different experimental techniques have provided contradictory results [2-5]. Our Hall effect and electric resistivity results show typical semiconducting behavior for x < 10. We have determined the effective number of carriers and its dependence on composition (x) and temperature (T) and compared it to the number of holes in the ladders obtained by other experimental techniques [2-5]. Our results give around one carrier per formula unit for x=0, which increases slightly with x. The x=11.5 compound shows metallic behavior and our results match qualitatively Hall effect behavior found in HTC [6]. [1] T. Vuletić, et al., Phys. Rep. 428, 169 (2006). [2] T. Osafune, et al., Phys. Rev. Lett. 78, 1980 (1997). [3] N. Nücker, et al., Phys. Rev. B 62, 14384 (2000). [4] Y. Piskunov, et al., Phys. Rev. B 72, 064512 (2005). [5] A. Rusydi, et al., Phys. Rev. B 75, 104510 (2007). [6] For a review see: N. E. Hussey, J. Phys: Condens. Matter 20, 123201 (2008).

Spintronics applications require the use of highly spin-polarized current sources. For this purpose, half-metallic ferromagnets, such as various magnetic oxides, have been integrated in spintronics devices and high magnetoresistance values have been recorded. However, their Curie temperatures are too close to room temperature for technological applications. Another possibility to achieve high spin-polarized sources at high temperatures consists of doping the host materials with magnetic ions, as in diluted magnetic semiconductor. We have followed this approach by doping metallic non-magnetic (La, , Sr)TiO_3 with 1, 5% Co (CoLSTO). The samples were grown by pulsed laser deposition on STO (001) substrates. In order to understand the origin of the magnetic properties of our CoLSTO samples (they revealed magnetic hysteresis cycles), we have performed a broad study including micro- and nano-structural characterization, as well as the measurement of the spin-polarization through the analysis of the magnetotransport properties of tunnel junction devices using CoLSTO electrodes. Quite interestingly, we have found that growth conditions, such as oxygen partial pressure, have a relevant influence on the magnetic properties, probably revealing the role of defects, including oxygen vacancies, on the mechanisms leading to magnetic correlations. It is well known that phase segregation is a major concern in these magnetic diluted systems. Bearing this in mind, we have performed an extensive structural analysis of our samples combining X-ray diffraction, high-resolution transmission electron microscopy (HRTEM), electron energy-loss spectroscopy (EELS) and Auger spectroscopy. None of these experiments showed any direct evidence for the presence of Co-rich clusters. To measure the spin polarization, CoLSTO tunnel junction devices with micro- and nanometric size have been fabricated either by optical lithography or by real-time resistance-controlled nanoindentation with a conductive AFM tip. The measurements of the tunnel magnetoresistance (TMR) for the CoLSTO/LAO/Co junctions gave the TMR values up to 20%. Our TMR results enabled us to deduce that a spin polarization for the CoLSTO system is up to 80%. We will address the dependence of the TMR and spin polarization on temperature and applied voltage bias, as well as the influence of growth conditions on the magnetotransport properties of these tunnel devices, with particular emphasis on the growth oxygen pressure. In order to analyze better the origin of the ferromagnetism and spin polarization of CoLSTO films, we have carried out XMCD (X-ray Magnetic Circular Dichroism) experiments. These experiments have provided us with useful insights into the electronic states at the Co and Ti sites, which are relevant to understand the microscopic origins of the magnetic properties of the CoLSTO films. In particular, we will show that the spectral signature of both XAS (X-ray Absorption Spectroscopy) and XMCD indicates that cobalt is in ionic states.

The physical properties of many perovskite oxides are critically dependent on either changes of the stoichiometry or on pure electronic doping. These effects may become particularly significant at the interface between two different oxides because of the structural and electronic mismatches of both compounds. Usually, the disparity of their properties results in strain fields or in diverging electrical potentials that may drive to either local changes of chemical composition or to charge transfer effects at the interface. These interface effects may lead to the emergence of physical functionalities at the interface which may be different to those of bulk, adding a larger flexibility to tune physical properties and opening up promising prospects for multifunctional devices. One particularly intriguing example of multifunctional oxide is that of SrTiO_3. In its stoichiometric form is a band insulator, but for tiny amounts of some dopants, of the order of 10^17 cm^(-3), it becomes metallic with high mobility conduction at low temperature. Also recently it has been suggested that pure electron charge transfer may dope the interface between a polar oxide (as LaAlO_3) and SrTiO_3, leading also to a high-mobility conduction. To fully understand the properties of these interfaces, we have made an effort to combine different advanced characterization tools with special emphasis on the use of spatially resolved characterization techniques. One step in that direction is the direct determination of the carrier density profile through resistance profile mappings collected in cross-section LaAlO_3/SrTiO_3 samples with a conducting-tip atomic force microscope. We have found that, depending on specific growth protocols, the spatial extension of the high-mobility electron gas can be varied from hundreds of micrometers into SrTiO_3 to a few nanometers next to the LaAlO_3/SrTiO_3 interface. Our results reveal that both interface conduction and bulk conduction due to oxygen vacancies can occur depending on the growth and annealing process. By using positron annihilation spectroscopy in combination with low-temperature magnetotransport characterization we have studied the mechanisms of defect diffusion in SrTiO_3 in the regime where oxygen vacancies are relevant. We show from this study that oxygen vacancy diffusion may lead to an alternative way to carrier confinement below the Fermi wavelength. Our results emphasize the relevance of using advanced interface characterization tools able to both resolve spatially the physical properties and also to detect small changes of the stoichiometry at the interface of complex oxides.