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We have shown, using magnetotransport measurements in high magnetic fields (SdH oscillations and Hall effect), that LAO/STO is not a 2D system, but 3D system. High mobility in this system is due to the doping of STO substrate by oxygen vacancies, and this happens during growth, but only in low oxygen pressure. In higher oxygen pressures, LAO/STO structure is no more of high mobility type.

Nous avons examiné les propriétés de magnétotransport de couches minces de LAO sur des substrats de STO(001) terminés TiO_2 déposées dans des pressions dans une large range de pressions (10^-6 -10^-3 mbar). On a observé que seulement les échantillons déposés aux pressions d' oxygene < 10^-5 mbar possedent une haute mobilité, tandis que les échantillons déposés aux pressions supérieures ont des résistances progressivement croissantes. Ce résultat est indicatif du fait que le déficit d' oxygene joue un rôle essentiel dans la conductivité de ces systemes. Pour analyser la dimensionnalité des échantillons avec la haute mobilité (>10^4 cm&sup2 ; /Vs) on a mesuré la magnétorésistance (MR) avec le champ magnétique appliqué dans le plan de l’ échantillon ou parallele au courant dans le plan de l' échantillon. Les deux MR montrent des oscillations de Shubnikov - de Haas (SdH) pour des températures T < 4 K et des champs B > 6 T. Ces oscillations disparaissent progressivement pour T > 1.5 K, comme attendu. L' observation d' oscillations SdH avec la meme fréquence dans les deux configurations exclut absolument la possibilité de confinement des porteurs a l' interface. Les mesures d' effet SdH et d' effet Hall ont permis une mesure plus précise de la taille de l' épaisseur du systeme conducteur: quelques centaines de microns. Cette épaisseur est en accord avec des calculs basés sur les coefficients de diffusion des lacunes d'oxygene déterminés expérimentalement dans des monocristaux de STO. En fait, différents systemes basés sur STO montrent la meme dépendance en température de la mobilité et la résistance que celles des structures LAO/STO déposées &agrave ; basse pression. Ça c'est une forte évidence de l'origine commune de la haute mobilité: le dopage par des lacunes d'oxygene dans le STO, avec des concentrations entre 10^17 et 10^19 cm^-3. Ces résultats ouvrent des perspectives intéressantes pour l'électronique d'oxydes, car ils pourraient etre utilisés pour fabriquer des heterostructures intégrant des oxydes multifonctionnels avec du STO ayant haute mobilité pour des dispositifs électroniques basés entierement sur des oxydes.

LaAlO_3/SrTiO_3 (LAO/STO) structures showing high mobility conduction have recently raised great expectations as they might represent a major step towards the conception of all-oxide electronics devices. The low-temperature high mobility was found for LAO films deposited on STO substrates at oxygen pressures below 10^-4 mbar. Generally, this metallic behavior has been ascribed to charge transfer effects at the interface between the insulators LAO and STO. For the development of oxide-based electronic devices a full understanding of the dimensionality and origin of this conducting electronic system is crucial. We have investigated the magnetotransport properties of LAO films epitaxially grown on STO(001) substrates in a wide range of deposition pressures. We show that metallic behavior and high mobility are only obtained when the films are deposited at low pressure (

Zbog izolatorskog karaktera i dijamagnetskih svojstava, SrTiO3 (STO) je često korišten materijal za naparavanje višeslojnih tankih filmova, a u svrhu dobivanja dvodimenzionalnog elektronskog plina velike pokretljivosti. Ohtomo i Hwang (Nature, 427 (2004) 423) su, na 6nm tankom filmu LaAlO3 (LAO) naparenom na STO, opazili metalno ponašanje i pokretljivosti veće od 10^4 cm2/Vs ; ta svojstva su u tom radu pripisana dvodimenzionalnom elektronskom plinu, oformljenom na granici STO i LAO. Naša mjerenja transportnih svojstava (T>300mK, B

The future development of micro- and nanoelectronic devices based on oxide structures requires novel materials with high carrier mobility. In this regard, SrTiO_3 (STO) is being intensively investigated due to the dramatic sensitivity of its electronic properties to extrinsic impurity doping. Stoichiometric STO is a band insulator, but an insulator-to-metal transition is induced when STO is doped with extrinsic impurities such as La, Nb or oxygen vacancies with concentrations above 10^16 cm^ -3. In these conditions, the sheet resistance can be as low as 0.01 ohm sq and the electronic mobility can exceed 10^4 cm^2/Vs at low temperature. Vacuum annealing at high temperatures has been the traditional method to dope STO with oxygen vacancies, but recently we have demonstrated that this doping can also be achieved at a much enhanced rate by thin film deposition of oxides on STO substrates at low oxygen pressure and high temperatures. We will address both theoretically and experimentally the mechanisms of oxygen vacancy formation and diffusion in STO to provide a quantitative approach to control the electronic properties of STO. Finally, we will also present results on the generation of carriers in STO, with the objective of achieving two-dimensional conduction. For this purpose, high-magnetic field transport characterization at low temperature is a powerful tool to get insight on the electronic properties and the dimensionality of the high-mobility gas created in STO by the above-mentioned methods.

Poznavanje Hallovog koeficijenta RH omogucuje karakterizaciju metala i poluvodica, jer odražava strukturu vrpce i tip nosilaca naboja. S druge strane, kvantitativno razumijevanje Hallovog koeficijenta jako koreliranih sustava (te visokotemperaturnih supravodica) nije još u potpunosti postignuto zbog njegove kompleksne ovisnosti o temperaturi i drugim parametrima od kojih se ocekuju kljucne informacije o osnovnim nekonvencionalnim elektronskim stanjima. Sr14-xCuxCu24O41 su kvazi-1D kuprati u kojima se naboji i spinovi uredjuju unutar podsustava lanaca i ljestvica [1]. Otkrice supervodljivosti pod tlakom (za x > 10) potaknulo je intenzivna istraživanja, jer je to prvi supervodljivi kuprat koji ne sadrži CuO2 ravnine. Pitanje dinamike naboja i raspodjele dopiranih šupljina izmedju podsustava ljestvica i lanaca je još uvijek otvoreno i razlicite eksperimentalne tehnike su dale kontradiktorne rezultate. Naši rezultati mjerenja RH i elektricne otpornosti pokazuju tipicno poluvodicko ponašanje (za x < 10). Odredili smo efektivni broj nosioca naboja i njegovu ovisnost o sastavu (x) te ga usporedili s brojem šupljina u sustavu ljestvica (dobivenim drugim mjerenjima). Za x = 11.5, koji pokazuje metalno ponašanje, naš rezultat kvalitativno odgovara ponašanju Hallovog koeficijenta za visokotemperaturne supravodice. [1] T. Vuletic, et al., Phys. Rep. 428, (2006.) 169

Popularni prikaz gigantskog magnetootpora.

Metalni oksidi u obliku perovskitnih struktura vjerojatno su najproučavaniji materijali posljednjih desetlje&cent ; ; ; a [1]. Jednu klasu tih materijalačine visokotemperaturni supravodiči temeljeni na ravninama bakrovih oksida, a druga vrlo proučavana klasa materijala temelji se na ravninama oksida magnetskih iona koji su najpoznatiji po kolosalnom magnetootporu. Hibridni materijali RuSr2RCu2O8 objedinjuju te dvije skupine materijala. U njima koegzistiraju dvije vrste, naizgled antagonisti&pound ; ; ; kih, dugodosežnih uredjenja: supravodljivost i feromagnetizam. Detaljnim proučavanjem mikrovalnih svojstava, magnetotransporta te Hallova efekta na nizu uzoraka raznih dopiranja, pokazali smo da se vodljiva i supravodljiva svojstva mogu korelirati s koncentracijom slobodnih šupljina u slojevima bakrovih oksida. Medjutim, za interpretaciju ponašanja magnetootpora i anomalnog Hallova otpora nužno je u razmatranja tenzora vodljivosti uvesti i delokalizirane elektrone u ravninama rutenijevih oksida. Njihov broj je vrlo malen, ali oni osje&cent ; ; ; aju veliku magnetootpornost usporedivu s onom u manganitnim strukturama poznatim po kolosalnom magnetootporu. [1] M. Imada, A. Fujimori, and Y. Tokura, Rev. Mod. Phys. 70, 1039 (1998).

We present results on current-induced domain wall motion in Co/Cu/CoFeB and Co/Cu/NiFe trilayered stripes. We find that the threshold current densities in our CoFeB spin valve stripes are around 10^6 A/cm^2 at zero magnetic field i.e. about one order of magnitude smaller than in our NiFe spin valve stripes (that is about two orders of magnitude smaller than in single NiFe stripes). The motion of the domain wall can be reversible or irreversible depending on the experimental conditions. In the irreversible regime, we observe switching of the spin valve by irreversible domain wall displacement in the free layer. The domain wall displacement is field-assisted when the applied field acts in the same direction as the spin transfer torque. The switching current density decreases as the applied field increases. This feature can have interesting applications in spintronics devices such as magnetoresistive random access memories. When the applied field torque is opposed to the spin transfer one, and above a threshold field, we observe a reversible displacement of the domain wall (corresponding to a peak in the differential resistance measurements). This can be ascribed to the onset of domain wall fluctuations, which is confirmed by micromagnetic simulations, taking into account the non adiabaticity in the spin transfer model. To understand the origin of the reversible domain wall behavior, we have performed time dependent resistance measurements that show evidences of domain wall fluctuations dominated by telegraph noise. The domain wall fluctuations occur between two well-defined states, corresponding to domain wall pinning centers distant of 400 nm. The dwell times are measured in the two states as a function of both the magnetic field and the current. The main features of the fluctuations are probably linked to the energy form of the pinning potentials.

The research on magnetic semiconductors is one of the most relevant subjects of the nowadays spintronics. Remarkable advances have been achieved in diluted magnetic oxide systems (DMOS). Indeed, the discovery of room temperature ferromagnetism in Co-doped TiO_2 has triggered an intense research on other diluted systems, but the origin of ferromagnetism is not still well understood. However, there is a growing consensus on the role of extrinsic carriers (introduced by means of structural defects) on the mediation of ferromagnetic correlations. Most of these works have been aimed at the ferromagnetic properties, but a spin polarization of carriers is an essential requirement for the use of DMOS in spintronics. Here we report on the magnetic properties and spin polarization of (La, Sr)Ti_(1− x)Co_xO_3 (Co-LSTO) thin films, in which the host oxide is not a semiconductor but a strongly correlated metal with a high density of carriers. These films are ferromagnetic at room temperature and the magnetization is enhanced at the lowest oxygen growth pressures, indicating that oxygen vacancies may have an active role on the magnetism. As a direct way to probe the spin polarization, we have performed tunneling magnetoresistance (TMR) measurements on magnetic tunnel junctions associating Co-LSTO and Co electrodes. A large spin polarization of around -80% has been determined at low temperature, indicating that the carriers are highly spin-polarized in this system. We have carried out an extensive structural analysis of Co-LSTO combining high-resolution transmission electron microscopy, Auger electron spectroscopy and X-ray magnetic circular dichroism. As a result, we can exclude the presence of Co clusters in the Co-LSTO layer and thus, the measured ferromagnetism and high spin polarization are intrinsic properties of this DMOS. We report finally on planar magnetotransport experiments on Co-LSTO films grown on SrTiO_3 (STO) substrates. The electrical properties of these samples are strongly dependent on the growth conditions and large electronic mobilities as high as 10^4 cm^2/Vs at T = 10 K are observed for specimens grown at the lowest oxygen pressure. At high enough applied fields (above 6 Teslas) and low enough temperatures (below 4 K) Shubnikov-de Haas oscillations are also observed in the high-mobility samples. We present an extensive analysis of these quantum oscillations and relate them with the electronic properties of STO, for which we find large scattering rates up to 10 ps. Our results bring strong arguments for the DMOS approach with complex oxide materials in spintronics and give an opportunity to have a better understanding on the fundamental physical mechanisms leading to long-order magnetic order in diluted magnetic systems.

We present experiments showing back and forth domain wall displacement induced by a spin polarized current, without any applied external field, in agreement with Slonczewski and Berger predictions. We have studied domain wall motion in stripes of Co/Cu/CoFeB and Co/Cu/NiFe spin valves. Our samples were designed to allow easy magnetization control. Spin polarized current was injected along the stripe axis (CIP configuration). Electrical measurements were performed at room temperature using the spin valve effect to probe and observe the DW displacement in the magnetically soft CoFeB or NiFe. Low current densities were required to switch the CoFeB magnetization: 8.5 x10^5 A/cm^2 at almost zero fields (1 Oe). We have also obtained an interesting evidence of a new effect (domain wall oscillations) as a response to the current injection. This effect is now investigated and it requires complementary RF measurements. In addition we show a strong dependence of the critical current with the applied field, which can be alternatively described as a strong decrease of the coercive fields when the current is increased. This feature is important for application to future MRAMs based on spin transfer domain wall motion.

We present the high-temperature (70 K < T < 300 K) resistivity anisotropy and Hall effect measurements of the quasi-one-dimensional (1D) organic conductor (TMTTF)_2AsF_6. The temperature variations of the resistivity are pronouncedly different for the three different directions, with metalliclike at high temperatures for the a axis only. Above 220 K the Hall coefficient R_H is constant, positive, and strongly enhanced over the expected value ; and the corresponding carrier concentration is almost 100 times lower than calculated for one hole/unit cell. Our results give evidence for the existence of a high-temperature regime above 200 K where the 1D Luttinger liquid features appear in the transport properties. Our measurements also give strong evidence of charge ordering in (TMTTF)_2AsF_6. At the charge-ordering transition T_CO = 100 K, R_H(T) abruptly changes its behavior, switches sign, and rapidly increases with further temperature decrease.

We present the high-temperature (70 K < T < 300 K) resistivity anisotropy and Hall effect measurements of the quasi-one-dimensional (1D) organic conductor (TMTTF)_2AsF_6. The temperature variations of the resistivity are pronouncedly different for the three different directions, with metalliclike at high temperatures for the a axis only. At lower temperatures the resistivity data show a semiconducting behavior for all three directions down to T_CO=100 K, where a charge-ordering transition occurs. This is characterized by a steep increase of the conductivity gap in the resistivity measurements for all three directions, although somehow stronger for the two axis perpendicular to the chains. This may be the consequence of the uniform shift of ions yielding a macroscopic FE polarization as predicted in analysis of dielectric measurements. Also, the steep increase of the conductivity gap below T_CO in resistivity is accompanied by an abrupt change in the RH(T) behavior: at the charge-ordering transition T_CO=100 K, RH(T) switches sign, and rapidly increases with further temperature decrease, which is a strong indication that carriers condense into an insulating charge-ordered state.

We report on the growth of heterostructures composed of layers of the high-Curie temperature ferromagnet Co-doped (La, Sr)TiO_3 (Co-LSTO) with high-mobility SrTiO_3 (STO) substrates processed at low oxygen pressure. While perpendicular spin-dependent transport measurements in STO//Co-LSTO/LAO/Co tunnel junctions demonstrate the existence of a large spin polarization in Co-LSTO, planar magnetotransport experiments on STO//Co-LSTO samples evidence electronic mobilities as high as 10^-4 cm2/V s at T=10 K. At high enough applied fields and low enough temperatures (\mu H>=6 T, T

Nous avons étudié des couches minces de l oxyde métallique La_0.5Sr_0.5TiO_3 (LSTO) dopé avec des ions de Co (avec des concentrations entre 0 et 3%). La croissance des couches a été réalisée par ablation laser. Les couches sont métalliques avec des valeurs de résistivité qui dépendent fortement des conditions de dépôt, notamment de la pression d oxygè ne. Pour la détermination de la polarisation en spin d un tel systeme dilué, des jonctions de taille micronique ou nanométrique LSTO(Co 1, 5%) / STO / Co et LSTO(Co 1, 5%) / LAO / Co ont été réalisées par lithographie optique ou par nanoindentation a l aide d un AFM a pointe conductrice. La magnétorésistance tunnel inverse (TMR) indique la polarisation positive de LSTO(Co) aux deux interfaces. Nous avons pu extrapoler la valeur de polarisation en spin a 70 % pour le LSTO et 12.5% pour le Co a l interface avec LAO (STO).

Spintronics applications require the use of highly spin-polarized current sources. With this purpose in mind, half-metallic ferromagnets, such as various magnetic oxides, have been integrated in devices and high magnetoresistance values have been recorded. However, the Curie temperatures of the involved materials are too close to room temperature for technological applications. New approaches to achieve high spin-polarized sources at high temperatures involve doping host materials with magnetic ions, as in diluted magnetic semiconductors (DMS). We have followed this approach by doping metallic non-magnetic La_0.5Sr_0.5TiO_3 with Co (LSTO(Co)) in the range up to 3%. The samples were grown by pulsed laser deposition with a frequency-tripled Nd:YAG laser (355 nm) on SrTiO_3 (STO) (001) and LaAlO_3 (LAO) (001) substrates. X-ray diffraction structural characterization shows that films are epitaxial and without parasitic phases. RHEED and AFM measurements show that the growth is 3D for samples grown on LAO, while it is 2D on STO substrates, with very low roughness values. No Co-clusters are observed neither by means of high-resolution transmission electron microscopy (HRTEM), electron energy-loss spectroscopy (EELS) or Auger spectroscopy. The dependence of the resistivity and the magnetization on growth parameters such as oxygen growth pressure, film thickness and Co-content have been carefully analyzed. The films show magnetic hysteresis cycles at room temperature and the observed magnetic anisotropy supports an intrinsic origin for ferromagnetism. The largest ferromagnetic signals are measured in films grown at low oxygen pressures (< 10^-5 mtorr). The main result of the present work is the determination of a spin polarization in a diluted magnetic system. LSTO(Co 1, 5%)/STO/Co and LSTO(Co 1, 5%)/LAO/Co junctions have been fabricated either by optical lithography or by real-time resistance-controlled nanoindentation with a conductive AFM tip. Tunnel magnetoresistance values (TMR = (Rp-Rap)/Rap, Rp and Rap are the resistances in parallel and anti-parallel configurations, respectively) up to 20% are measured at low temperature. Applying Julliere's model, this implies a spin polarization of at least 50 % calculated from the highest TMR value. The analysis of the temperature dependence of the TMR reveals a fast decrease with temperature and the TMR becomes vanishingly small at T > 200 K. The origin of this TMR suppression remains to be clarified. The bias dependence of the TMR will be discussed. The present work represents an important step towards the understanding and application of the DMS approach to spintronic devices using complex oxide materials.

Spintronics applications require the use of highly spin-polarized current sources. Recently studies rely on the doping of host materials with magnetic ions (diluted magnetic semiconductors (DMS)) to achieve highly spin-polarized sources at high temperatures. We have followed this approach by doping metallic non-magnetic (La, Sr)TiO_3 with Co (Co-LSTO) in the range 0 – 3 %. We have grown Co-LSTO films by PLD on STO substrates. The films are ferromagnetic at room temperature, with a sizable magnetic anisotropy, which indicate the intrinsic nature of the ferromagnetism. To determine the spin polarization of Co-LSTO, we have grown Co-LSTO/LAO/Co heterostructures. On these samples we defined magnetic tunnel junctions of micrometric size by optical lithography. We performed tunneling magnetoresistance (TMR) experiments which demonstrate the existence of a large spin polarization (up to -80%) in Co-LSTO. We discuss both the temperature- and bias- dependence of the TMR. We have analyzed the composition and structure of Co-LSTO films at a local scale, by different techniques: High-Resolution Transmission Electron Spectroscopy (HRTEM), Electron Energy Loss Spectroscopy (EELS) and Auger Electron Spectroscopy (AES). From these experiments we conclude that the Co distribution is homogeneous in the film with no Co-rich regions larger than 10 nm. Thus, we exclude Co-segregation as the origin of the ferromagnetism and TMR in Co-LSTO, and we conclude that the spin-polarization as well as the ferromagnetism are intrinsic properties of this DMOS. The present work represents an important step towards the understanding and application of the DMS approach to spintronic devices using complex oxide materials.

The search for highly-spin polarized sources to be implemented in spintronic devices is an active field of research. One approach is to dope with magnetic elements a non-magnetic host showing the required transport properties. Here we report on the structural and magnetotransport properties of Co-doped (up to 3%) La_0.5Sr_0.5TiO_3 (LSTO(Co)) thin films grown by pulsed laser deposition. All samples appear to be ferromagnetic with T_c well above room temperature. X-ray diffraction and Auger spectroscopy do not show any traces of Co-clustering, suggesting that the ferromagnetism is intrinsic to the LSTO(Co) compound. We have performed an exhaustive study of the dependence of the magnetotransport properties on the oxygen pressure at deposition (P_O2) and on the Co-content. We have found that the samples are metallic, albeit the resistivity is remarkably dependent on the growth pressure.

Nous avons étudié les variations de la résistance de barreaux de vanne de spin du type Co/Cu/CoFeB de largeur 100, 200 et 300 nm en fonction du courant injecté. Pour contrôler le mouvement, nous avons muni les barreaux de constrictions pour piéger la paroi. Nous avons mis en évidence l existence de deux régimes : un premier régime ou le sens de déplacement de paroi est déterminé par le signe du courant et un deuxieme régime ou le courant permet de dépiéger la paroi mais la direction du champ appliqué détermine le sens du déplacement. Ces résultats sont similaires a ceux obtenus dans des barreaux de Co/Cu/NiFe.

Spintronics applications require the use of highly spin-polarized current sources. With this purpose in mind, half-metallic ferromagnets, such as various magnetic oxides, have been integrated in devices showing high magnetoresistance values. However, their Curie temperatures are too close to room temperature for technological applications. New approaches to achieve high spin-polarized sources at high temperatures involve doping host materials with magnetic ions, as in diluted magnetic semiconductors (DMS). We have followed this approach by doping metallic non-magnetic La_0.5Sr_0.5TiO_3 with Co (LSTO(Co)) in the range up to 3 %. The samples were grown by pulsed laser deposition with a frequency-tripled Nd:YAG laser (355 nm) on SrTiO_3 (STO) (001) and LaAlO_3 (LAO) (001) substrates. X-ray diffraction structural characterization shows that films are epitaxial and without parasitic phases. No Co-clusters are observed neither by means of high-resolution transmission electron microscopy (HRTEM), electron energy-loss spectroscopy (EELS) or Auger spectroscopy. The dependence of the resistivity and the magnetization on growth parameters such as oxygen growth pressure, film thickness and Co-content has been carefully analyzed. The main result is the determination of a spin polarization in a diluted magnetic system. LSTO(Co 1.5%)/STO/Co and LSTO(Co 1.5%)/LAO/Co junctions have been fabricated either by optical lithography or by real-time resistance-controlled nanoindentation with a conductive AFM tip. Tunnel magnetoresistance (TMR) values up to 20% are measured at low temperature. This implies a spin polarization of at least 50 % calculated from the highest TMR value. The analysis of the temperature dependence of the TMR reveals a fast decrease with temperature and the TMR becomes vanishingly small at T > 200 K. The origin of this TMR suppression and he bias dependence will be discussed. The present work represents an important step towards the understanding and application of the DMS approach to spintronic devices using complex oxide materials.

We report Hall effect measurements performed in the normal phase and at ambient pressure of (TMTSF)_2ReO_4 and (more anisotropic) (TMTTF)_2AsF_6 quasi-1D organic conductors. For T>200 K, the Hall coefficient of (TMTSF)_2ReO_4 is smoothly temperature-dependent, and its value corresponds to a carrier concentration of one hole/unit cell. On the other hand, the Hall coefficient of (TMTTF)_2AsF_6 is enhanced above 220 K, and the corresponding carrier concentration is almost 100 times lower than the expected one for one hole/unit cell. Our results give evidence for the existence of a high temperature regime above 200 K in (TMTTF)_2X salts, where the 1D Luttinger liquid features appear in the transport properties.

We have studied the current-induced displacement of a domain wall (DW) in the permalloy (Py) layer of a Co/Cu/Py spin-valve structure. At zero and very small applied fields (< 10 Oe), the displacement is in the opposite direction for opposite dc currents, and the current density required to move DW is of the order of a few 10^6 A/cm^2. At higher applied magnetic fields, the DW motion, even though triggered by the current, has its direction controlled by the field.

Recent experiments have shown Co-doped La_0.5Sr_0.5TiO_3 (LSTO) as a diluted magnetic systems with a high T_c. La_0.5Sr_0.5TiO_3:Co (1.5%) layers were grown by PLD on SrTiO_3 (100) at O_2 pressures ranging from 6.10^(-7) to 2.3 10^(-4) mbar. All the films are epitaxial and ferromagnetic (T_c > 300 K). Upon decreasing the pressure, the out of plane cell parameter decreases and the electrical conductivity increases, while the magnetization shows a maximum (2.38 mu_B/Co) at p = 5.1 10^(-6) mbar. Experiments are being carried out to determine the origin of ferromagnetism and to measure the carriers' spin-polarization.

Le concept de transfert de spin entre porteurs polarisés en spin et moments localisés est un mécanisme qui a été proposé pour prédire qu' un courant polarisé en spin pouvait induire le déplacement d'une paroi de domaine magnétique. Nous avons fabriqué de tricouches magnétiques CoO/Co/Cu/NiFe, qui on été ensuite découpées par lithographie électronique en forme de barreaux comportant ou non des constrictions latérales aux 2/3 de la longueur totale du barreau. Dans ces structures, la mesure GMR permet une détermination précise de la position de la paroi et de son déplacement lorsque le courant est injecté. Nous avons montré l'existence de deux régimes en fonction du champ magnétique appliqué. A champ nul (ou a champ tres faible), le sens de déplacement de la paroi dépend du signe du courant et les densités de courant critiques sont de l'ordre de 10^6 A.cm^2, i.e. un ordre de grandeur de moins que les courants critiques mesurés dans les systemes de piliers. Pour des champs magnétiques plus élevés, l'injection d'un courant permet de dépiéger la paroi mais le sens du déplacement est alors contrôlé par le champ magnétique. Nous avons pu tracer sur plusieurs types de structures le diagramme de phase champ-courant. Des résultats similaires de la dépendance en champ des courants critiques ont été obtenus lorsque l'injection de courant est effectuée sous forme de pulses ultra-courts et non en courant continu: nous avons pu montrer qu'il était possible de commuter la vanne de spin (par le déplacement de la paroi) avec des pulses de courant de seulement 0, 4 ns, et que les courants critiques restaient constants dans la gamme 0, 4 a 2 ns. Nous discuterons de l'origine des résultats expérimentaux ainsi que de la dépendance en champ des courants critiques et les comparerons aux différents modeles théoriques existants.

For spin injection into semiconductors, magnetic semiconductors are an alternative to ferromagnetic metals, with the advantage that the difficulties resulting from the conductivity mismatch just as well the chemical reactivity between metal and semiconductor can be avoided. On the other hand, magnetic tunnel junctions with magnetic semiconductor electrodes can be used to test spin injection since the existence of magnetoresistance (TMR) is a signature of the transmission os spin polarized carriers between the electrodes. We have fabricated single and double barrier magnetic tunnel junctions with GaMnAs electrodes by a low temperature molecular beam epitaxy procedure. The two magnetic electrodes (Ga_(1-x)Mn_xAs) are separated by thin AlAs (1.7 nm) tunnel barrier (single barrier MTJ) or by a AlAs (1.7 nm)/GaAs (5nm)/ AlAs (1.7 nm) trilayer (double barrier MTJ). Antiparallel arrangement of the ferromagnetic electrodes at low field is obtained by playing with the thickness and manganese concentration in the GaMnAs layers to obtain different coercive fields. GaAs layers have been inserted between electrodes and tunnel barrier in order to avoid the diffusion of the manganese into the tunnel barrier. We observe large TMR effects, 38% at low field associated with the switching between the parallel (P) and antiparallel (AP) configurations of the remanent magnetization. The existence of similar large MR ratios in the double junction is a new and interesting effect, never observed in metallic double junctions when the intermediate electrode is nonmagnetic. Our result can be accounted for the non-relaxed spin splitting of the chemical potential, that is spin accumulation, in GaAs well. We will discuss the conditions on the hole spin relaxation time in GaAs that are required to obtain the large effects we have observed.

Odnos između interakcija i dimenzionalnosti je vrlo važno pitanje za široki spektar materijala od visokotemperaturnih supervodiča do organskih vodiča. U poznatom faznom dijagramu vrlo anizotropnih, tzv. kvazi-1D (jednodimenzionalnih) organskih vodiča (koji počinje sumporovim spojevima (TMTTF)2X i nastavlja se izostrukturnim selenovim (TMTTF)2X) anizotropija sistema se mijenja promjenom aniona X (X = ReO4, AsF6, ...). Teoretski je predviđeno da na visokim temperaturama, kad termička energija prelazi transverzalno vezanje, dolazi do gubitka transverzalne koherencije, a time i do 2D → 1D prijelaza. Pri tome se očekuju drastične promjene u fizikalnim svojstvima, koje se mogu opisati u okviru teorije Luttingerove tekućine (LL), budući da se 1D interagirajući elektronski sistem ne može opisati u okviru teorije Fermijeve tekućine (FL). Dimenzionalnost, a time i primjena FL ili LL koncepta za organske vodiče je još uvijek kontroverzna. Teoretski LL modeli predvidjeli su značajnu redukciju koncentracije nosioca naboja u normalnoj fazi ovih organskih vodiča. Naši rezultati mjerenja Hallovog efekta u normalnoj fazi (TMTSF)2ReO4 i (TMTTF)2AsF6 (koji je više anizotropan) pokazali su veliku razliku u iznosu Hallovog koeficijenta te ukazuju da u (TMTTF)2X spojevima dolazi do značajne redukcije u koncentraciji nosioca naboja. Dobiveni rezultati Hallovog efekta, uz ostale transportne rezultate, ukazuju na postojanje visokotemperaturnog režima, ali samo u (TMTTF)2X spojevima, gdje se elementi 1D LL obilježja pojavljuju u transportnim svojstvima.

For spin injection into semiconductors, magnetic semiconductors are an alternative to ferromagnetic metals, with the advantage that the difficulties resulting from the conductivity mismatch between metal and semiconductor can be avoided. On the other hand, magnetic tunnel junctions with magnetic semiconductor electrodes can be used to test spin injection since the existence of magnetoresistance (TMR) is a signature of the transmission of spin polarized carriers between the electrodes. We have fabricated single and double barrier magnetic tunnel junctions with GaMnAs electrodes by a low temperature molecular beam epitaxy procedure. The two magnetic electrodes (Ga_(1-x)Mn_xAs)) are separated by thin AlAs (1.7nm) tunnel barrier (single barrier MTJ) or by a AlAs(1.7nm)/GaAs(5nm)/AlAs(1.7nm) trilayer (double barrier MTJ). Antiparallel arrangement of the ferromagnetic electrodes at low field is obtained by playing with the thickness and manganese concentration in the GaMnAs layers to obtain different coercive fields. GaAs layers have been inserted between electrodes and tunnel barrier in order to avoid the diffusion of the manganese into the tunnel barrier. We observe large TMR effects, 38% at low field and 650% at 5 T in the single junction after progressive saturation of the electrode magnetization. The existence of similar large MR ratios in the double junction is a new and interesting effect, never observed in metallic double junctions when the intermediate electrode is nonmagnetic. This can be ascribed by the non-relaxed spin splitting of the chemical potential predicted for a semiconductor intermediate electrode.

Nous présenterons des expériences de déplacement de paroi par injection d un fort courant dc effectuées sur des barreaux de CoO/Co/Cu/NiFe. Ces barreaux, de largeur 1 micron et de longueur 20 microns, ont été réalisés par lithographie électronique. Une constriction a été réalisée a 1/3 de la longueur totale du barreau. Elle agit comme centre de piégeage pour paroi magnétique, comme le prouvent les plateaux de la courbe de magnetoresistance géante (GMR) situés a un état intermédiaire (1/3 or 2/3) entre les résistances des configurations parallele (P) et antiparallele (AP). L effet de GMR présent dans de tels échantillons est utilisé pour détecter la configuration magnétique. Nous montrons par des mesures de transport électrique que, une fois piégée dans la constriction, la paroi peut etre déplacée par injection d un courant dc supérieur a un courant critique de l ordre de 10^7A/cm^2 . Nous discuterons des différentes origines possibles de l effet, i.e. le champ magnétique local créé par le courant et/ou le transfert de spin a partir d un courant polarisé en spin. Nous montrerons qu il est envisageable d utiliser cet effet pour contrôler la configuration magnétique d une vanne de spin a l échelle de temps de la nanoseconde.

In order to achieve spin dependent transport in semiconductor heterostructures, diluted magnetic semiconductors (DMS) present the advantage to be more compatible with semiconductors than metal transition. DMS allow to add a degree of freedom (the spin of carriers) in semiconductor heterostructures and open up new opportunities for spin electronic. We have studied the influence of the manganese concentration on the magnetic and transport properties of Ga_(1-x)Mn_xAs thin films and spin dependent transport in magnetic tunnel junctions Ga_(1-x)Mn_xAs/AlAs/Ga_(1-x)Mn_xAs.

Un des défis de l'électronique de spin est l'injection et la détection d'électrons polarisés en spin dans des structures semiconductrices. Les jonctions tunnel a base de semiconducteurs ferromagnétiques peuvent etre utilisées comme test d'injection de spins puisque la magnétorésistance tunnel est la signature d'une transmission de porteurs polarisés en spin entre deux electrodes. Nous avons élaboré par épitaxie par jets moléculaires a basse temperature des simples et doubles jonctions tunnel magnétiques (JTMs). Les deux électrodes de GaMnAs (semiconducteur ferromagnétique) sont séparées par une fine barriere tunnel d'AlAs (1.7nm) (simple JTMs) ou par un puits quantique AlAs(1.7nm)/GaAs(5nm)/AlAs(1.7nm) (double JTMs). A T = 4 K la magnétorésistance tunnel (TMR) pour des JTMs a bas champ est 38%, correspondant a la transition de la configuration parall&egrave ; ; le a antiparallele de l'aimantation rémanente. L'observation du meme taux de TMR dans le cas de la double JTM est un effet du a la nature semiconductrice de l'électrode centrale (faible densité d'état au niveau de Fermi). Contrairement au cas des structures métalliques, la dépendance en tension de la TMR est la meme pour les deux JTMs. Ces effets liés a la nature semiconductrice de l'électrode centrale seront discutés.

Nous présentons des résultats expérimentaux concernant le renversement d aimantation par injection d un courant polarisé en spin dans des piliers submicroniques constitués d une tricouche Co 15nm/ Cu 10nm/ Co 2.5 nm. Les expériences réalisées a champ nul sont en accord avec les résultats précédents obtenus sur des piliers similaires . De plus, nous étudions la dépendance en champ des courants critiques. Deux régimes sont clairement mis en évidence, qui dépendent de l amplitude du champ magnétique appliqué par rapport a un champ d anisotropie.

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The talk will review the experimental data and the theoretical interpretation of the spin injection in two topics in the field of spin electronics: diluted magnetic ferromagnets (GaMnAs) and magnetization reversal by spin injection in pillar-shaped Co/Cu/Co trilayers.

We present experimental results on the displacement of a domain wall by injection of a dc current through the wall. The samples are from 1 micron down to 0, 3 micron wide long stripes of a CoO/Co/Cu/NiFe classical spin valve structure. The stripes have been patterned by electron beam lithography. A set of two triangular-shaped constrictions are placed at 1/3 of the total length on either side of the stripe and is a pinning center for the magnetic domain wall, as shown by the steps of the giant magnetoresistance curves at intermediate levels (1/3 or 2/3) between the resistances corresponding to the parallel and antiparallel configurations. The GMR signal is used to detect the position of the domain wall along the stripe. We show by electric transport measurements that, once a wall is trapped, it can be moved by injecting a dc current higher than a threshold current of the order of magnitude of 10^7 A/cm^2. The striking result is that the direction of the domain wall displacement does not depend on the sign of the injected current. This effect has been observed at both low (3K) and room temperature, which excludes a purely thermally activated mechanism. We discuss the different possible origins of this effect, i.e. the local magnetic field created by the current and/or spin transfer exerted by the spin-polarized conduction electrons

We present experimental results on the magnetization reversal driven by a spin polarized current in pillar-shaped Co (15nm)/Cu (10 nm)/Co (2.5 nm) trilayers. Besides experiments performed at zero field in agreement with previous results on similar pillars, we also study the field dependence of the critical currents and clearly evidence the existence of two regimes according to the amplitude of the applied field H_appl compared to an anisotropy field H_an. For H_appl < H_an, the reversals from parallel to antiparallel and vice versa, are sharp as the respective critical currents are reached and the resulting R(I) curve is hysteretic. For H_appl > H_an, the reversals are much smoother and fully reversible. Up to now, two theoretical approaches have been proposed in order to calculate the current induced torque. The model based on a direct interaction between the magnetic moment carried by the conduction electrons and the local magnetization is unable to explain even qualitatively these later results. On the contrary, we are able to interpret them correctly in the frame of the spin transfer model using a diffusive approach. We calculate in detail the spin polarization of the conduction electron using the Valet-Fert model developed for the CPP-GMR and the critical currents extracted from the Landau-Lifschitz-Gilbert equation in presence of an external applied field. The comparison with our experimental results leads us to the conclusion that only the spin transfer approach is able to describe properly the properties of the spin current induced magnetization reversal.

Sustavno je proučavana temperaturna i magnetska ovisnost mikrovalne apsorpcije, dc i ac susceptibilnosti, dc otpora, magnetootpora i Hallovog efekta u keramičkom i praškastom uzorku supravodiljivih hibridnih rutenokuprata RuSr2EuCu2O8 i RuSr2GdCu2O8. Mikrovalna mjerenja su napravljena novom metodom za precizno određivanje Q-faktora i frekvencije rezonantne mikrovalne šupljine. U oba keramička uzorka detektiran je lokalni maksimum na temperaturi magnetskog uređenja oko 130 K. On je superponiran na maksimum širine 50 K iznad i ispod 130 K. Mikrovalni otpor u tom području pada s povećanjem vanjskog magnetskog polja. Pojave među zrnima su razdvojene od pojava u zrnima usporedbom mikrovalnih mjerenja u keramičkom i praškastom uzorku u epoxy smoli. Praškasti uzorak nema poluvodičkog porasta sniženjem temperature ispod 120 K, što dokazuje da je taj porast posljedica transporta između zrna. Lokalni maksimum na oko 130 K je prisutan i u praškastom uzorku, što dokazuje da se radi o intrinsičnom svojstvu magnetskog prijelaza. U supravodljivom stanju RuSr2EuCu2O8 (Tc=32 K) pronašli smo dokaz supravodljivih slabih veza u keramičkom uzorku koje prelaze u normalno stanje na poljima iznad 0.5 T. U supravodljivom stanju RuSr2GdCu2O8 (Tc=50 K) mikrovalna apsorpcija na 5 K u nultom polju je veća nego na 8 T. Pokazali smo da se radi o ESR liniji gadolinija koja je superponirana na mikrovalni otpor, kojim na višim poljima dominira gibanje virova u miješanom stanju. Rezultati mjerenja pokazuju da su RuO2 ravnine vodljive, ali ne postaju supravodljive.

Le renversement de l'aimantation d'une couche magnétique, par injection d'un courant polarisé en spin à partir d'une autre couche magnétique, était observé dans le Co(15nm)/Cu(10nm)/Co(2.5nm) piliers. Le renversement d'aimantation par injection de courant résulte d'effets d'accumulation de spin directement impliqués dans la GMR en courant perpendiculaire.

We present experiments of the magnetization reversal by spin injection obtained on the pillar-shaped Co/Cu/Co trilayers. The pillars are fabricated by e- beam lithography and reactive ion etching. Their small section (200 x 600 nm^2) allowed the injection a current density of about 10^7 A/cm^2. Our data confirm similar previous results by the Cornell's group. In addition, we present a new type of experiment, which also evidences clearly the control of the magnetic configuration by the current intensity. The interpretation is based on the Slonczewski’s equations with a calculation of the current spin polarization based on the VF model of CPP-GMR. This type of calculation predicts right order of magnitude of the critical currents, but some discrepancy between calculation and experiments for the critical current asymmetry and their field dependence remains.

Mogućnost promjene smjera magnetizacije feromagneta pomoću injektirane istosmjerne struje (i bez vanjskog magnetskog polja) predložena je 1996. Efekt se zasniva na transferu spina između debelog (spinski polarizator) i tankog (spinski senzor) feromagnetskog sloja (koji su međusobno odvojeni tankim nemagnetskim slojem), a potvrđen je na troslojnim magnetskim nanostrukturama Co/Cu/Co na Cornell University i u Orsayu. Moguće magnetske konfiguracije dva Co sloja (P – paralelna ili AP – antiparalelna) detektirane su mjerenjem gigantskog magnetootpora u tzv. CPP konfiguraciji (sa strujom koja teče okomito na slojeve). Pozitivna struja dovodi do orijentacije tankog Co sloja u P konfiguraciju, a negativna struja u AP konfiguraciju. Kritične gustoće struje (za H = 0) za reorijentaciju magnetizacije su reda 10^7 A/cm2. Postojeći teorijski modeli još u potpunosti ne opisuju sve karakteristike dobivenih rezultata. Optimizacija parametara uzoraka trebala bi dovesti do nižih vrijednosti gustoća kritičnih struja, što bi otvorilo mogućnosti primjena (MRAM).

We have measured the magnetic susceptibility, resistivity, magnetoresistivity and Hall effect of nonstoichiometric cuprous selenide between 5 K and 350 K. Our results show that below 170 K Cu_(2-x)Se is a mixture of diamagnetic Cu_1.995Se and paramagnetic Cu_3Se_2. The phase diagram of the Cu-Se system, in which 170 K represents the eutectic isotherm, governs the relative content of the two phases. For the Cu_3Se_2 phase a transition to an antiferromagnetic state is observed at about 50 K, with the corresponding Weiss temperature 120 K. On heating above 170 K Cu_(2-x)Se becomes completely diamagnetic, but the transformation is slow and strongly time dependent. The complicated magnetic behaviour is ascribed to a broad temperature hysteresis of the process.

The magnetic moment of a ferromagnetic layer can be reversed by spin transfer from a spin-polarized current (i.e. without applying a magnetic field), as predicted first by Slonczewski in 1996 and then in other contexts. This effect has been conrmed by recent experiments on nano-pillars at Cornell's University and at Orsay. We will present our latest experiments obtained on Co/Cu/Co nano-pillars in which we find that a positive current switches the magnetic conguration from AP to P whereas a negative one switches from P to AP. This asymmetry is the signature of reversal by spin injection. The required current density is about 10^7 A/cm . We will also show the variation of the critical current with the applied magnetic field. In fact, this study is of interest to decide between the different theoretical approaches as they predict quite different field dependence of the switching current. Our interpretation is based on the dynamical approach of Slonczewski with a calculation of the current spin polarization in a standard model of the CPP-GMR. We show how the spin asymmetry involved in Slonczewski's equations is infuenced by the thickness of the layer and the spin diffusion lengths in the magnetic and the nonmagnetic layers. Optimizing the parameters of the samples, the critical currents can certainly be reduced and be of interest for application to the switching of submicronic magnetic devices.

Organski vodiči su niskodimenzionalni sustavi, no još je uvijek upitno u kojoj su mjeri međulančane korelacije odgovorne za njihova fizikalna svojstva na visokim temperaturama. Teoretski se očekuje da uobičajeni “Fermi-liquid” opis fizikalnog stanja, koji se uspješno koristi za 3D metale, prestaje vrijediti u slučaju elektron-elektron interakcija u 1D sustavu. Novo, “non-Fermi-liquid” (ili tzv. “Luttinger-liquid”) stanje, koje je predloženo i za organske vodiče, karakterizirano je činjenicom da elementarna pobuđenja više nisu kvazičestice već kolektivne fluktuacije naboja i spina bozonskog karaktera. Fizikalna svojstva bi tada trebala slijediti određene karakteristične ovisnosti o temperaturi i frekvenciji. Pitanje je u kojoj mjeri se ovakav opis može koristiti za organske vodiče, budući da su rezultati dva nedavna nezavisna istraživanja Hall efekta organskog vodiča (TMTSF)_2PF_6 na visokim temperaturama interpretirani na dva različita načina. Kako bismo pokušali naći odgovor na ovo važno pitanje izvšili smo detaljna mjerenja vodljivosti (u sva tri glavna kristalografska smjera) te Hall efekta i magnetootpora izostrukturnog spoja (TMTSF)_2ReO_4, koji zbog nešto većeg volumena aniona ima i izraženiju anizotropiju. Spoj karakterizira metal-izolator prijelaz na T_MI ≈ 180 K. Rezultati mjerenja u metalnom području (T_MI < T < 300 K), pokazuju vrlo anizotropnu vodljivost (ponašanje metalnog tipa samo u smjeru najveće vodljivosti), mali i pozitivni magnetootpor (u smjeru najmanje vodljivosti) te pozitivni Hall efekt (koji ne pokazuje znatnu ovisnost o temperaturi). Dobivene rezultate ćemo usporediti s onima za izostrukturni spoj (TMTSF)_2PF_6 u istom području temperatura i pokušati dati odgovor na aktualno pitanje.

Le renversement de l'aimantation d'une couche magnétique par injection d'un courant polarisé en spin à partir d'une autre couche magnétique, jouant le rôle de polariseur de spin, est un nouveau phénomène proposé par Slonczewski. Nos observations expérimentales dans le Co(15nm)/Cu(10nm)/Co(2.5nm) piliers montrent la possibilité d'utiliser ce phénomène pour la commande de tout dispositif micro magnétique ou d'électronique de spin. Du point de vue de la physique fondamentale, le renversement d'aimantation par injection de courant en spin est exactement l'inverse de la GMR. Plus précisément, le renversement d'aimantation par injection de courant résulte d'effets d'accumulation de spin directement impliqués dans la GMR en courant perpendiculaire.

U ovom radu bit će prikazana mjerenja kutne i temperaturne ovisnosti magnetootpora organskog vodiča (TMTSF)_2PF_6 u osnovnom stanju vala gustoće spina (VGS). Istraživanja su vršena za struje J u smjeru glavnih kristalografskih osi (a, b' i c*), te magnetskim poljima B < 9 T u ravninama okomitim i/ili paralelnim smjeru struje: (1) JIIa, B u (b',c*) ravnini ? magnetootpor je anizotropan s maksimumom za Bllc*. Anizotropija (tj. omjer vrijednosti magnetootpora u maksimumu i minimumu) na 4.2 K jednaka je 24 u slaganju s vrijednostima drugih autora, dok je vrijednost na 2.2 K nešto veća. Temperaturna ovisnost magnetotpora slijedi aktivaciono ponašanje s energetskim procjepom 21 K. (2) Jllb', B u (a,c*) ravnini ? magnetootpor je izrazito anizotropan s maksimumom za BIIc*.Anizotropije magnetootpora na 4.2 K i 2.2 K jednake su 140, tj. 104. Kutna ovisnost je slična onoj za (TMTSF)_2NO_3. Aktivaciono ponašanje otpora i magnetootpora u VGS stanju pokazuje jasnu razliku u ponašanju, iznad i ispod 4 K. (3) JIIc*, B u (a,b') ravnini ? opažen je pozitivan magnetootpor iznosa oko 45 %, što je potvrđeno na nekoliko uzoraka. Anizotropija magnoetootpora je reda veličine 1. Izmjerena je neobična ovisnost kutne ovisnosti magnetootpora: maksimum i minimum na 4.2 K međusobno izmjenjuju mjesta na 2.2 K, što takoder pokazuje i temperaturna ovisnost magnetootpora. (4) JIIc*, B u (b',c*) ravnini ? opažen je longitudinalni magnetootpor iznosa oko 300%, na 4.2 K i na 2.2 K. Na 4.2 K magnetootpor je praktički neovisan o smjeru magnetskog polja, a na 2.2 K pokazuje tipičnu cos^n (theta) ovisnost, s maksimumom za BIIc*. Rezultati ukazuju na mogućnost postojanja dva (ili više) osnovnih podstanja unutar VGS osnovnog stanja, iznad i ispod temperature od 4 K.

Izvršena su detaljna istraživanja Hallovog efekta u osnovnom stanju nesumjerljivog vala gustoće spina (VGS), u području linearne električne vodljivosti, na temperaturama do 4.2 K i u magnetskom polju do 9T, koja su pokazala da je osnovno stanje karakterizirano malim brojem nosioca naboja ovisnim o temperaturi. Istraživanja u sumjerljivom lokaliziranom VGSu pokazala su postojanje izotropnog, negativnog i temperaturno ovisnog magnetootpora. Dobiveno ponašanje se može razumjeti kao posljedica reduciranog raspršenja elektrona na antiferomagnetskim fluktuacijama. Detaljna istraživanja anizotropije u magnetootporu za sva tri smjera vodljivosti u osnovnom stanju nesumjerljivog VGS pokazala su da je efektivna aktivaciona energija veoma anizotropna i ovisna o magnetskom polju za temperature iznad 4K. Rezultati mjerenja promjene magnetootpora s temperaturom, za struju paralelnu smjeru srednje vodljivosti, ukazuju na promjenu aktivacione energije s temperaturom i magnetskim poljem te time i činjenicu da postoje dva različita mehanizma raspršenja unutar VGS faze. Po prvi puta je eksperimentalno dobiven dielektrični odgovor vala gustoće naboja koji je zapet na pozadinsku rešetku sumjerljivim potencijalom. Dobiveno ponašanje je a skladu sa teorijskim predviđanjima.Nadalje, izvršena su mjerenja dielektrične funkcije u lokaliziranom limitu sumjerljivog VGS, tj. u antiferomagnetskoj fazi 2D organskog materijala (slabo feromagnetska faza) te u osnovnom stanju nesumjerljivog VGS u 1D organskom materijalu. Vrijednost izmjerene dielektrične konstante je u prvom slučaju reda veličine 10^3, što je nekoliko redova veličine manja vrijednost od vrijednosti karakteristične za nesumjerljivi VGS koja iznosi 10^7do 10^9. Dobiveno ponašanje se može razumjeti ako se pretpostavi postojanje domenskih zidova naboja u nasumičnoj domenskoj strukturi feromagnetske faze. Dobiveno ponašanje nesumjerljivog VGS ukazuje da je stupanj kompleksne strukture VGSa detektiran u pojedinom eksperimentu vrlo osjetljiv o stupnju čistoće mjerenog monokristala.

We have studied the current-voltage characteristics of the (TMTSF)2PF6 in the spin density state (SDW), and in zero and finite external magnetic field. For the oscillating part of the nonlinear voltage response to the applied DC electric field, the fundamental frequency distribution (as a function of this field) and a nonlinear relation between the frequency and the SDW current reveal the growth of parallel conduction channels characterized by lower velocities and larger cross-sections. The number of fundamental frequencies, their amplitude and the level of low-frequency noise as well as the depinning behaviour provide a consistent indication of the sample inhomogeneities and associated local field variations, and might be well understood within the framework of the phase slippage model. The increase of the threshold electric field with the applied magnetic filed can be explained by the Bjeli-Maki theory, if the imperfect nesting is taken into account. Finally, the electric-field dependence of the Hall resistivity is consistent with the sliding mechanism of the SDW conduction., Proučavali smo karakteristike napon – struja materijala (TMTSF)2PF6 u stanju valova spinske gustoće (SDW). Za oscilatorni dio nelinearnog naponskog odziva na istosmjerno električno polje, osnovna frekventna raspodjela (kao funkcija tog polja) i nelinearan odnos frekvencije i SDW struje pokazuju rast usporednih kanala vođenja označenih manjim brzinama i većim udarnim presjecima. Broj osnovnih frekvencija, njihove amplitude i razina niskofrekventnog šuma, kao i otkočno ponašanje sustavno pokazuju na nehomogenosti uzorka i pridružene varijacije lokalnog polja, i mogu se shvatiti u okviru modela klizne faze. Rast praga električnog polja s magnetskim poljem može se protumačiti Bjeliš-Makijevom teorijom, ako se uzme u obzir ugniježdenje. Konačno, nalazimo da je ovisnost Hallovog otpora o električnom polju u skladu s kliznim mehanizmom SDW vođenja struje.

We describe an attempt to find the key parameter for the optimization of recently discovered large magnetoresistance in non-stoichiometric silver chalcogenides. Our measurements of the resistivity, magnetoresistance and the Hall effect of Ag_2Se and their simple analysis lead us to the conclusion that the most important optimization variable is the Hall mobility of the charge carriers. We suggest that the largest magnetoresistance may be expected in the samples with the Hall mobility equal to about 0.1 m2/Vs.

The angular dependence of the magnetoresistance of quasi-1D organic compound (TMTSF)_2PF_6 has been studied for currents parallel to a, b' and c* axes, for 2K