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## Paredes de domínios em ferromagnetismo; Domain walls in ferromagnetism

Ramos, Rudnei de Oliveira
Fonte: Biblioteca Digitais de Teses e Dissertações da USP Publicador: Biblioteca Digitais de Teses e Dissertações da USP
Tipo: Dissertação de Mestrado Formato: application/pdf
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Determinamos a tensão superficial de um defeito do tipo parede de domínio, utilizando a aproximação semiclássica no contexto da teoria de campos à temperatura finita para o modelo ? (?2)2 com interação invariante por simetria O(N), obtendo em seguida a temperatura crítica Tc de transição de fase e cálculo dos expoentes críticos ? e ?, aplicando então os resultados obtidos para a descrição de um modelo fenomenológico descrevendo o ferromagnetismo.; We determine the surface tension of a domain wall defect by using the semi classical approach in the context of field theory at finite temperature for the ? (?2)2 model with O(N) invariant interaction obtaining the critical temperature Tc of phase transition and the calculation of the critical exponents ? and ? applying then the obtained results to a description of a phenomenological model describing ferromagnetism.

## Polaronic ferromagnetism in conducting polymers

Pereira, E. C.; Correa, A. A.; Bulhões, L. O S; Aleixo, P. C.; Nóbrega, J. A.; De Oliveira, A. J A; Ortiz, W. A.; Walmsley, L.
Tipo: Artigo de Revista Científica Formato: 2023-2025
ENG
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Ferromagnetic behavior at room temperature is reported in metal-free-conducting polymer samples of poly(3-methylthiophene) doped with ClO 4 -. Magnetic moments associated with spin 1/2 positive polarons are possibly interacting through a Dzialoshinski-Moriya anisotropic superexchange via the dopant anions, giving rise to weak ferromagnetism. © 2001 Elsevier Science B.V. All rights reserved.

## Evidence of weak ferromagnetism in doped plasticized polyaniline (PANI-DDoESSA)0.5 from electron spin resonance measurements

Santana, V. T.; Nascimento, O. R.; Djurado, D.; Travers, J. P.; Pron, A.; Walmsley, L.
Tipo: Artigo de Revista Científica
ENG
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X-band electron spin resonance (ESR) measurements have been performed on a conducting free-standing film of polyaniline plasticized and protonated with di-n-dodecyl ester of sulfosuccinic acid (DDoESSA). The magnetic field was applied parallel and perpendicular to the plane of the film. At around 75 K a transition is observed from Pauli susceptibility to a localized state in which the spin 1/2 polarons behave as spin 1/2 dimers. A rough estimation of the intradimer and interdimer exchange constants is obtained. Below 5 K, ESR data reveal a weak ferromagnetism with the Dzyaloshinskii-Moriya vector mainly oriented in the plane of the film. The existence of a relatively well-defined n-fold axis along the chain direction in the crystalline regions confers a symmetry compatible with such analysis. © 2013 IOP Publishing Ltd.

## Sonochemical synthesis and magnetism in co-doped ZnO nanoparticles

Arruda, Larisa B.; Leite, Douglas M. G.; Orlandi, Marcelo O.; Ortiz, Wilson A.; Lisboa Filho, Paulo Noronha
Tipo: Artigo de Revista Científica Formato: 2515-2519
ENG
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The understanding and control of ferromagnetism in diluted magnetic semiconducting oxides (DMO) is a special challenge in solid-state physics and materials science due to its impact in magneto-optical devices and spintronics. Several studies and mechanisms have been proposed to explain intrinsic ferromagnetism in DMO compounds since the theoretical prediction of room-temperature ferromagnetism. However, genuine and intrinsic ferromagnetism in 3d-transition metal-doped n-type ZnO semiconductors is still a controversial issue. Furthermore, for DMO nanoparticles, some special physical and chemical effects may also play a role. In this contribution, structural and magnetic properties of sonochemically prepared cobalt-doped ZnO nanoparticles were investigated. A set of ZnO samples was prepared varying cobalt molar concentration and time of ultrasonic exposure. The obtained results showed that single phase samples can be obtained by the sonochemical method. However, cobalt nanoclusters can be detected depending on synthesis conditions. Magnetic measurements indicated a possible ferromagnetic response, associated to defects and cobalt substitutions at the zinc site by cobalt. However, ferromagnetism is depleted at higher magnetic fields. Also...

## Electric-field controlled ferromagnetism in MnGe magnetic quantum dots

Xiu, Faxian; Wang, Yong; Zou, Jin; Wang, Kang L.
Fonte: CoAction Publishing Publicador: CoAction Publishing
Tipo: Artigo de Revista Científica
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Electric-field control of ferromagnetism in magnetic semiconductors at room temperature has been actively pursued as one of the important approaches to realize practical spintronics and non-volatile logic devices. While Mn-doped III-V semiconductors were considered as potential candidates for achieving this controllability, the search for an ideal material with high Curie temperature (Tc>300 K) and controllable ferromagnetism at room temperature has continued for nearly a decade. Among various dilute magnetic semiconductors (DMSs), materials derived from group IV elements such as Si and Ge are the ideal candidates for such materials due to their excellent compatibility with the conventional complementary metal-oxide-semiconductor (CMOS) technology. Here, we review recent reports on the development of high-Curie temperature Mn0.05Ge0.95 quantum dots (QDs) and successfully demonstrate electric-field control of ferromagnetism in the Mn0.05Ge0.95 quantum dots up to 300 K. Upon the application of gate-bias to a metal-oxide-semiconductor (MOS) capacitor, the ferromagnetism of the channel layer (i.e. the Mn0.05Ge0.95 quantum dots) was modulated as a function of the hole concentration. Finally, a theoretical model based upon the formation of magnetic polarons has been proposed to explain the observed field controlled ferromagnetism.

## Lattice effects on ferromagnetism in perovskite ruthenates

Cheng, J.-G.; Zhou, J.-S.; Goodenough, John B.
Tipo: Artigo de Revista Científica
EN
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Ferromagnetism and its evolution in the orthorhombic perovskite system Sr1–xCaxRuO3 have been widely believed to correlate with structural distortion. The recent development of high-pressure synthesis of the Ba-substituted Sr1–yBayRuO3 makes it possible to study ferromagnetism over a broader phase diagram, which includes the orthorhombic Imma and the cubic phases. However, the chemical substitutions introduce the A-site disorder effect on Tc, which complicates determination of the relationship between ferromagnetism and structural distortion. By clarifying the site disorder effect on Tc in several unique series of ruthenates in which the average bond length 〈A–O〉 remains the same but the bond-length variance varies, we are able to demonstrate a parabolic curve of Tc versus mean bond length 〈A–O〉. A much higher Tc ∼ 177 K than that found in orthorhombic SrRuO3 can be obtained from the curve at a bond length 〈A–O〉, which makes the geometric factor t = 〈A–O〉/(√2〈Ru–O〉) ∼ 1. This result reveals not only that the ferromagnetism in the ruthenates is extremely sensitive to the lattice strain, but also that it has an important implication for exploring the structure–property relationship in a broad range of oxides with perovskite or a perovskite-related structure.

## Surface effect ferromagnetism in pure and reduced strontium titanate

DesRoches, Brandon J.
Fonte: Brock University Publicador: Brock University
Tipo: Electronic Thesis or Dissertation
ENG
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A room temperature ferromagnetic hysteresis is observed in single crystal strontium titanate substrates as purchased from several manufacturers. It was found that polishing all sides of the substrates removed this observed hysteresis, suggesting that the origin of the ferromagnetic behavior resides on the surface of the substrates. X-ray diffraction and energy dispersive x-ray spectra were measured however they were unable to detect any impurity phases. In similar semiconducting oxides it was previously suggested that ferromagnetism could originate in oxygen vacancies or from disorder within the single crystal. To this end substrates were annealed in both air and vacuum in a range of temperatures (600°C to 1100°G) to both create bulk oxygen vacancies and to heal surface damage. Annealing in vacuum was found to create a measureable number of oxygen vacancies however their creation could not be correlated to the ferromagnetic signal of the substrate. Annealing in air was found to effect the remnant moment of the substrate as well as the width of the x-ray diffraction peaks on the unpolished face, weakly suggesting a relation between surface based disorder and ferromagnetism. Argon ion bombardment was employed to create a layer of surface disorder in the polished crystal...

## Carbon p Electron Ferromagnetism in Silicon Carbide

Wang, Yutian; Liu, Yu; Wang, Gang; Anwand, Wolfgang; Jenkins, Catherine A.; Arenholz, Elke; Munnik, Frans; Gordan, Ovidiu D.; Salvan, Georgeta; Zahn, Dietrich R. T.; Chen, Xiaolong; Gemming, Sibylle; Helm, Manfred; Zhou, Shengqiang
Fonte: Nature Publishing Group Publicador: Nature Publishing Group
Tipo: Artigo de Revista Científica
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Ferromagnetism can occur in wide-band gap semiconductors as well as in carbon-based materials when specific defects are introduced. It is thus desirable to establish a direct relation between the defects and the resulting ferromagnetism. Here, we contribute to revealing the origin of defect-induced ferromagnetism using SiC as a prototypical example. We show that the long-range ferromagnetic coupling can be attributed to the p electrons of the nearest-neighbor carbon atoms around the VSiVC divacancies. Thus, the ferromagnetism is traced down to its microscopic electronic origin.

## Ferromagnetism in the one-dimensional Hubbard model with orbital degeneracy: From low to high electron density

Sakamoto, Harumi; Momoi, Tsutomu; Kubo, Kenn
Tipo: Artigo de Revista Científica
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We studied ferromagnetism in the one-dimensional Hubbard model with doubly degenerate atomic orbitals by means of the density-matrix renormalization-group method and obtained the ground-state phase diagrams. It was found that ferromagnetism is stable from low to high (0< n < 1.75) electron density when the interactions are sufficiently strong. Quasi-long-range order of triplet superconductivity coexists with the ferromagnetic order for a strong Hund coupling region, where the inter-orbital interaction U'-J is attractive. At quarter-filling (n=1), the insulating ferromagnetic state appears accompanying orbital quasi-long-range order. For low densities (n<1), ferromagnetism occurs owing to the ferromagnetic exchange interaction caused by virtual hoppings of electrons, the same as in the quarter-filled system. This comes from separation of the charge and spin-orbital degrees of freedom in the strong coupling limit. This ferromagnetism is fragile against variation of band structure. For high densities (n>1), the phase diagram of the ferromagnetic phase is similar to that obtained in infinite dimensions. In this case, the double exchange mechanism is operative to stabilize the ferromagnetic order and this long-range order is robust against variation of the band-dispersion. A partially polarized state appears in the density region 1.68

## Enhancement of ferromagnetism by p-wave Cooper pairing in superconducting ferromagnets

Jian, Xiaoling; Zhang, Jingchuan; Gu, Qiang; Klemm, Richard A.
Tipo: Artigo de Revista Científica
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In superconducting ferromagnets for which the Curie temperature $T_{m}$ exceeds the superconducting transition temperature $T_{c}$, it was suggested that ferromagnetic spin fluctuations could lead to superconductivity with p-wave spin triplet Cooper pairing. Using the Stoner model of itinerant ferromagnetism, we study the feedback effect of the p-wave superconductivity on the ferromagnetism. Below $T_{c}$, the ferromagnetism is enhanced by the p-wave superconductivity. At zero temperature, the critical Stoner value for itinerant ferromagnetism is reduced by the strength of the p-wave pairing potential, and the magnetization increases correspondingly. More important, our results suggest that once Stoner ferromagnetism is established, $T_m$ is unlikely to ever be below $T_c$. For strong and weak ferromagnetism, three and two peaks in the temperature dependence of the specific heat are respectively predicted, the upper peak in the latter case corresponding to a first-order transition.; Comment: 6 pages, 6 figures, submitted to Phys. Rev. B

## Ferromagnetism in Hubbard Models: Low Density Route

Mueller-Hartmann, E.
Tipo: Artigo de Revista Científica
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Thirty years ago the Hubbard model was introduced by Gutzwiller, Hubbard and Kanamori with the main purpose of mimicking the ferromagnetism of transition metals. Soon after, Nagaoka and Thouless pointed out a basic mechanism for ferromagnetism in strongly correlated electron systems by studying the motion of a single hole in a half--filled Hubbard model. This important work was hoped to shed light onto metallic ferromagnetism from the low doping regime. Unfortunately, this low doping route towards ferromagnetism has not been successful as far as rigorous results for finite doping concentrations are concerned. In the work presented here, we start from the opposite limit of low particle concentrations. In this limit we provide the first proof of a fully polarized metallic ground state for a Hubbard model. The proof proceeds by mapping Hubbard zigzag'' chains onto a continuum model with an additional degree of freedom and local first Hund's rule coupling. For this model the maximum total spin multiplet is shown to be the unique ground state for infinite Hubbard coupling. Our proof may open a low density route towards the understanding of the ferromagnetism of Hubbard models.; Comment: 6 pages, latex, in press in J. Low Temp. Phys.

## Towards a Fully Ab-Initio Description of the Diluted Magnetic Semiconductor $Ga_{1-x}Mn_{x}As$. Ferromagnetism, Electronic Structure, and Optical Response

Craco, L.; Laad, M. S.; Müller-Hartmann, E.
Tipo: Artigo de Revista Científica
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There are two competing views of itinerant ferromagnetism, the first viewing ferromagnetism as resulting from the indirect coupling between local moments via the itinerant carrier dynamics, the so-called RKKY mechanism, while in the alternative picture, ferromagnetism results from the spin polarization of itinerant carriers by the strong atomic Hund interaction - the so-called double exchange (DE) scenario. Which view describes the ferromagnetism in diluted magnetic semiconductors, materials with promise for spintronic applications, is still unclear. Here, we describe the detailed physical response of the prototype material $Ga_{1-x}Mn_{x}As$ using a combination of first-principles bandstructure with methods based on dynamical mean field theory to incorporate strong, dynamical correlations {\it and} intrinsic as well as extrinsic disorder in one single theoretical picture. We show how ferromagnetism is driven by DE, in agreement with very recent observations, along with a good quantitative description of the details of the electronic structure, as probed by scanning tunnelling microscopy (STM) and optical conductivity. Our results show how ferromagnetism can be driven by DE even in diluted magnetic semiconductors with small carrier concentration.; Comment: 4 pages...

## From Nagaoka's ferromagnetism to flat-band ferromagnetism and beyond: An introduction to ferromagnetism in the Hubbard model

Tasaki, Hal
Tipo: Artigo de Revista Científica
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This is a self-contained review about ferromagnetism in the Hubbard model, which should be accessible to readers with various backgrounds who are new to the field. We describe Nagaoka's ferromagnetism and flat-band ferromagnetism in detail, giving all necessary backgrounds as well as complete (but elementary) mathematical proofs. By studying an intermediate model called long-range hopping model, we also demonstrate that there is indeed a deep relation between these two seemingly different approaches to ferromagnetism. We further discuss some attempts to go beyond these approaches. We briefly discuss recent rigorous example of ferromagnetism in the Hubbard model which has neither infinitely large parameters nor completely flat bands. We give preliminary discussions about possible experimental realizations of the (nearly-)flat-band ferromagnetism. Finally we focus on some theoretical attempts to understand metallic ferromagnetism. We discuss three artificial one-dimensional models in which the existence of metallic ferromagnetism can be easily proved.; Comment: LaTeX2e, 72 pages, 17 epsf figures. Many minor corrections made in March 1998. This is the final version, which will appear in Prog. Theor. Phys. 99 (invited paper)

## Interfacial Ferromagnetism in LaNiO3/CaMnO3 Superlattices

Grutter, A. J.; Yang, H.; Kirby, B. J.; Fitzsimmons, M. R.; Aguiar, J. A.; Browning, N. D.; Jenkins, C. A.; Arenholz, E.; Mehta, V. V.; Alaan, U. S.; Suzuki, Y.
Tipo: Artigo de Revista Científica
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We observe interfacial ferromagnetism in superlattices of the paramagnetic metal LaNiO3 and the antiferromagnetic insulator CaMnO3. LaNiO3 exhibits a thickness dependent metal-insulator transition and we find the emergence of ferromagnetism to be coincident with the conducting state of LaNiO3. That is, only superlattices in which the LaNiO3 layers are metallic exhibit ferromagnetism. Using several magnetic probes, we have determined that the ferromagnetism arises in a single unit cell of CaMnO3 at the interface. Together these results suggest that ferromagnetism can be attributed to a double exchange interaction among Mn ions mediated by the adjacent itinerant metal.; Comment: Accepted for publication in Physical Review Letters (http://prl.aps.org/), copyright (2013) by the American Physical Society

## Itinerant Ferromagnetism and Metamagnetism in Cr Doped Perovskite Ruthenates

Durairaj, V.; Elhami, E.; Chikara, S.; Lin, X. N.; Douglass, A.; Schlottmann, G. Cao P.; Choi, E. S.; Guertin, R. P.
Tipo: Artigo de Revista Científica
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We report results of structural, magnetic and transport properties of single crystal CaRu1-xCrxO3 (0≤x≤0.36) and SrRu1-xCrxO3 (0≤x≤0.30). Cr substitution as low as x=0.08 drives CaRu1-xCrxO3 from the paramagnetic state to an itinerant ferromagnetic state with field-driven first-order metamagnetic transitions leading to a sizeable saturation moment (~0.4B/f.u.within the ab plane). The ferromagnetism occurs abruptly and reaches as high as TC=123 K for x=0.22. The Cr-driven ferromagnetism is highly anisotropic, suggesting an important role for spin-orbit coupling. Lattice constant and magnetic measurements strongly support the valence of the Cr as tetravalent (Cr4+, 3d2 configuration). Cr substitution for Ru in SrRuO3 (TC=165 K) enhances the itinerant ferromagnetism, with TC reaching 290 K for x=0.30, consistent with Cr-induced ferromagnetism in paramagnetic CaRuO3. Preliminary pressure-dependent magnetization of CaRu0.85Cr0.15O3 shows strong enhancement of the saturation magnetization (25% for P~0.7 GPa). All results indicate a coupling of Ru 4d and Cr 3d electrons that is unexpectedly favorable for itinerant ferromagnetism which often exists delicately in the ruthenates.; Comment: 13 pages and 6 figures

## Reversible room-temperature ferromagnetism in Nb-doped SrTiO3 single crystals

Liu, Z. Q.; Lu, W. M.; Lim, S. L.; Qiu, X. P.; Bao, N. N.; Motapothula, M.; Yi, J. B.; Yang, M.; Dhar, S.; Venkatesan, T.; Ariando
Tipo: Artigo de Revista Científica
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The search for oxide-based room-temperature ferromagnetism has been one of the holy grails in condensed matter physics. Room-temperature ferromagnetism observed in Nb-doped SrTiO3 single crystals is reported in this Rapid Communication. The ferromagnetism can be eliminated by air annealing (making the samples predominantly diamagnetic) and can be recovered by subsequent vacuum annealing. The temperature dependence of magnetic moment resembles the temperature dependence of carrier density, indicating that the magnetism is closely related to the free carriers. Our results suggest that the ferromagnetism is induced by oxygen vacancies. In addition, hysteretic magnetoresistance was observed for magnetic field parallel to current, indicating that the magnetic moments are in the plane of the samples. The x-ray photoemission spectroscopy, the static time-of-flight and the dynamic secondary ion mass spectroscopy and proton induced x-ray emission measurements were performed to examine magnetic impurities, showing that the observed ferromagnetism is unlikely due to any magnetic contaminant.; Comment: 6 pages, 6 figures

## Theory of ferromagnetism in vanadium-oxide based perovskites

Dang, Hung T.; Millis, Andrew J.
Tipo: Artigo de Revista Científica
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The conditions under which ferromagnetism may occur in transition metal oxides with partially filled $t_{2g}$ shells such as vanadium-based perovskites are studied using a combination of density functional and single-site dynamical mean field methods. For reasonable values of the correlation strength, rotations of the VO$_6$ octahedra play an important role in enabling ferromagnetism, with ferromagnetism typically occurring for rotations larger than a nonzero critical value. Ferromagnetism is suppressed near the Mott insulating phase but the phase boundary is otherwise only weakly dependent on carrier concentration. Design rules are suggested for new oxide systems exhibiting ferromagnetism.; Comment: 13 pages, 11 figures

## Cationic vacancy induced room-temperature ferromagnetism in transparent conducting anatase Ti_{1-x}Ta_xO_2 (x~0.05) thin films

Rusydi, A.; Dhar, S.; Barman, A. Roy; Ariando; Qi, D. -C.; Motapothula, M.; Yi, J. B.; Santoso, I.; Feng, Y. P.; Yang, K.; Dai, Y.; Yakovlev, N. L.; Ding, J.; Wee, A. T. S.; Neuber, G.; Breese, M. B. H.; Ruebhausen, M.; Hilgenkamp, H.; Venkatesan, T.
Tipo: Artigo de Revista Científica
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We report room-temperature ferromagnetism in highly conducting transparent anatase Ti1-xTaxO2 (x~0.05) thin films grown by pulsed laser deposition on LaAlO3 substrates. Rutherford backscattering spectrometry (RBS), x-ray diffraction (XRD), proton induced x-ray emission (PIXE), x-ray absorption spectroscopy (XAS) and time-of-flight secondary ion mass spectrometry (TOF-SIMS) indicated negligible magnetic contaminants in the films. The presence of ferromagnetism with concomitant large carrier densities was determined by a combination of superconducting quantum interference device (SQUID) magnetometry, electrical transport measurements, soft x-ray magnetic circular dichroism (SXMCD), XAS, and optical magnetic circular dichroism (OMCD) and was supported by first-principle calculations. SXMCD and XAS measurements revealed a 90% contribution to ferromagnetism from the Ti ions and a 10% contribution from the O ions. RBS/channelling measurements show complete Ta substitution in the Ti sites though carrier activation was only 50% at 5% Ta concentration implying compensation by cationic defects. The role of Ti vacancy and Ti3+ was studied via XAS and x-ray photoemission spectroscopy (XPS) respectively. It was found that in films with strong ferromagnetism...

## Interface Double-Exchange Ferromagnetism in the Mn-Zn-O System: New Class of Biphase Magnetism

García, Miguel Ángel Pérez; Ruiz-González, María Luisa; Quesada, Adrián; Costa Krämer, José Luis; Fernández Lozano, José Francisco; Khatib, S. J.; Wennberg, A.; Caballero Cuesta, Amador; Martín-González, Marisol S.; Villegas, Marina; Briones F
Fonte: American Physical Society Publicador: American Physical Society
Tipo: Artículo Formato: 646928 bytes; application/pdf
ENG
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In this Letter, we experimentally show that the room temperature ferromagnetism in the Mn-Zn-O system recently observed is associated with the coexistence of Mn3+ and Mn4+ via a double-exchange mechanism. The presence of the ZnO around MnO2 modifies the kinetics of MnO2→Mn2O3 reduction and favors the coexistence of both Mn oxidation states. The ferromagnetic phase is associated with the interface formed at the Zn diffusion front into Mn oxide, corroborated by preparing thin film multilayers that exhibit saturation magnetization 2 orders of magnitude higher than bulk samples.; This work has been partially supported by the University Complutense Project No. PR1/05-13325. Partial support from the EU Network of Excellence SANDIE is also acknowledged.; Peer reviewed

## Selective generation of local ferromagnetism in austenitic stainless steel using nanoindentation

Sort Viñas, Jordi; Suriñach, Santiago