Magnetic moment of Fe3O4 films with thicknesses near the unit-cell size

CNPQ - CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICOFAPEMIG - FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE MINAS GERAISFAPESP - FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULOCAPES - COORDENAÇÃO DE APERFEIÇOAMENTO DE PESSOAL DE NÍVEL SUPERIORWe perform a systematic study on the evolution of the magnetic spin moment (ms) of epitaxial [100]- and [111]-magnetite films of increasing thickness. The ultrathin films are characterized by low-energy electron diffraction, x-ray absorption spectroscopy, and x-ray magnetic circular dichroism (XMCD). By employing sum rules on the XMCD spectra we obtain ms=3.6 μB/f.u. for samples of around 35 Å. This is considered a bulk value and has been reported only for films more than 10 times thicker. Moreover, we show that even 10-Å-thick magnetite already presents a significant magnetic moment. For both grown directions the moment increases similarly with the thickness. The ferromagnetic behavior for each iron ion site (Feocta2+, Feocta3+, Fetetra3+) of Fe3O4 is measured by monitoring XMCD peaks. The deduced hysteresis curves (per ion, per site) exhibit a coercive field of 300 Oe. Our results show that both the ferrimagnetic order and the bulk moment value are preserved at room temperature around the thickness of 2 unit cells.We perform a systematic study on the evolution of the magnetic spin moment (ms) of epitaxial [100]- and [111]-magnetite films of increasing thickness. The ultrathin films are characterized by low-energy electron diffraction, x-ray absorption spectroscopy, and x-ray magnetic circular dichroism (XMCD). By employing sum rules on the XMCD spectra we obtain ms=3.6 μB/f.u. for samples of around 35 Å. This is considered a bulk value and has been reported only for films more than 10 times thicker. Moreover, we show that even 10-Å-thick magnetite already presents a significant magnetic moment. For both grown directions the moment increases similarly with the thickness. The ferromagnetic behavior for each iron ion site (Feocta2+, Feocta3+, Fetetra3+) of Fe3O4 is measured by monitoring XMCD peaks. The deduced hysteresis curves (per ion, per site) exhibit a coercive field of 300 Oe. Our results show that both the ferrimagnetic order and the bulk moment value are preserved at room temperature around the thickness of 2 unit cells.We perform a systematic study on the evolution of the magnetic spin moment (ms) of epitaxial [100]- and [111]-magnetite films of increasing thickness. The ultrathin films are characterized by low-energy electron diffraction, x-ray absorption spectroscopy, and x-ray magnetic circular dichroism (XMCD). By employing sum rules on the XMCD spectra we obtain ms=3.6 μB/f.u. for samples of around 35 Å. This is considered a bulk value and has been reported only for films more than 10 times thicker. Moreover, we show that even 10-Å-thick magnetite already presents a significant magnetic moment. For both grown directions the moment increases similarly with the thickness. The ferromagnetic behavior for each iron ion site (Feocta2+, Feocta3+, Fetetra3+) of Fe3O4 is measured by monitoring XMCD peaks. The deduced hysteresis curves (per ion, per site) exhibit a coercive field of 300 Oe. Our results show that both the ferrimagnetic order and the bulk moment value are preserved at room temperature around the thickness of 2 unit cells.901316CNPQ - CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICOFAPEMIG - FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE MINAS GERAISFAPESP - FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULOCAPES - COORDENAÇÃO DE APERFEIÇOAMENTO DE PESSOAL DE NÍVEL SUPERIORCNPQ - CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICOFAPEMIG - FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE MINAS GERAISFAPESP - FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULOCAPES - COORDENAÇÃO DE APERFEIÇOAMENTO DE PESSOAL DE NÍVEL SUPERIORSem informaçãoSem informaçãoSem informaçãoSem informaçãoOrna, J., Algarabel, P.A., Morellón, L., Pardo, J.A., De Teresa, J.M., López Antón, R., Bartolomé, F., Wildes, A., (2010) Phys. Rev. B, 81, p. 144420. , PRBMDO 1098-0121Hari Babu, V., Govind, R.K., Schindler, K.-M., Welke, M., Denecke, R., (2013) J. Appl. 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Mater., 310, p. e249. , JMMMDC 0304-8853Figueiredo, J.J.S., Basilio, R., Landers, R., Garcia, F., De Siervo, A., (2009) J. Synch. Rad., 16, p. 346. , JSYRES 0909-0495Kallmayer, M., Hild, K., Elmers, H.J., Arora, S.K., Wu, H.-C., Sofin, R.G.S., Shvets, I.V., (2008) J. Appl. Phys., 103, p. 07D715. , JAPIAU 0021-8979Huang, D.J., Chang, C.F., Jeng, H.-T., Guo, G.Y., Lin, H.-J., Wu, W.B., Ku, H.C., Chen, C.T., (2004) Phys. Rev. Lett., 93, p. 077204. , PRLTAO 0031-9007Abreu, G.J.P., Paniago, R., Pfannes, H.-D., (2014) J. Magn. Magn. Mater., 349, p. 235. , JMMMDC 0304-8853Bruns, D., Lindemann, S.R., Kuepper, K., Schemme, T., Wollschläger, J., (2013) Appl. Phys. Lett., 103, p. 052401. , APPLAB 0003-6951Fonin, M., Pentcheva, R., Dedkov, Y.S., Sperlich, M., Vyalikh, D.V., Scheffler, M., Rüdiger, U., Güntherodt, G., (2005) Phys. Rev. B, 72, p. 104436. , PRBMDO 1098-0121Ritter, M., Over, H., Weiss, W., (1997) Surf. 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ANALYSIS OF TEXTURE AND VIBRATIONAL ANISOTROPY BY MÖSSBAUER SPECTROSCOPY

L'étude des intensités des lignes dans un spectre hyperfin permet l'analyse des orientations préférentielles (textures) ou d'anisotropie des vibrations du réseau. Il est possible de développer en série des fonctions sphériques, d'une part, la texture inconnue et, d'autre part, la dépendance angulaire qui apparaît dans les intégrales de l'intensité. Ensuite les intégrales sont simplifiées par une somme limitée des fonctions sphériques avec coefficients inconnus. Quand on tient compte des relations qui existent pour les rotations des fonctions sphériques on peut calculer les coefficients par une méthode de moindre carré sur la base des intensités relatives mesurées en positions différentes de source et d'échantillon. L'application de cette méthode aux problèmes d'anisotropie de vibration permet de déterminer le paramètre d'anisotropie et de distinguer la texture de l'effet Goldanskii-Karyagin.The study of the line intensities in a hyperfine pattern makes possible an analysis of the texture or VA. The basic principles of the method are : The (unknown) texture distribution is expanded in a series of spherical harmonics and also the angular dependencies of the resonant absorbed γ-radiation which appear in the intensity integrals. These integrals thus are simplified to a finite sum of spherical harmonics with the unknown expansion coefficients of the texture function as factors. Respecting the relations for rotation of spherical harmonics a set of intensity ratio measurements for different angle positions of source (polarized) and absorber can be solved for the texture expansion coefficients by a least square fit procedure. Application of this method to the vibrational anisotropy problem makes possible the determination of the anisotropy parameter and allows to distinguish between texture and Goldanskii-Karyagin effect

TEXTURE PROBLEMS

L'orientation préférentielle d'une assemblée est appelée texture, l'assemblée pouvant comporter des cristaux, des molécules, des spins, les axes principaux du GCE à des sites spécifiques, etc. On analyse l'influence de la texture en tant qu'effet extrinsèque (ou accidentel) sur les intensités relatives d'un spectre hyperfin et compare à des effets intrinsèques (caractéristiques du matériau) telles la dépendance angulaire des spectres hyperfins et l'anisotropie des vibrations du réseau donnent lieu a l'Effet Boldanskii-Karyagin EGK. On discute la corrélation entre la texture et les paramètres Mössbauer présentant une dépendance angulaire. 1) L'orientation des spins dans un matériau magnétique ou des axes principaux du GCE peut être déterminée en corrélant les textures obtenues par RX aux intensités relatives des spectres hyperfins. 2) Inversement, on peut déterminer la texture en analysant les intensités relatives des spectres hyperfins en fonction de la direction de propagation des rayonnements γ. 3) L'attention principale est consacrée à la corrélation entre l'EGK et la texture. L'effet GK est devenu une méthode importante de détermination de l'anisotropie des vibrations du réseau ; il est cependant difficile de distinguer les deux effets en compétition. Quelques exemples démontrent l'influence sensible de la texture sur les spectres hyperfins en particulier dans des cas où une faible orientation produit sur les intensités relatives des raies un effet identique à celui d'une anisotropie relativement élevée des vibrations thermiques du réseau.Texture is the preferred orientation of an assembly. The assembly may consist of crystals, molecules, spins, principal axes of EFG's at specific sites, etc. The influence of texture as an extrinsic (accidental) effect on the relative line intensities of a hyperfine pattern will be analysed and compared with the intrinsic effects (characteristic of the material) like angular dependence of the hyperfine interaction and lattice vibrational anisotropy causing the Goldanskii-Karyagin effect, GKE. Correlation between texture and angular dependent Mössbauer parameters will be discussed : 1) The orientation of the spins in a magnetic material or the principal axes of the EFG can be determined by correlating X-ray textures with the relative line intensities of the hyperfine pattern. 2) The reverse : the texture can be determined by analysing relative line intensities, of the hyperfine pattern by varying the propagation direction of the γ-radiation. 3) The main interest is focused on the correlation of GKE vs. texture. The GKE has become an important tool to determine lattice vibrational anisotropy ; however, it is difficult to distinguish between these two competing effects. Some examples will demonstrate the sensitive influence in the hyperfine pattern by texture, particularly, cases where a relatively small deviation from randomness produces the same effect on the relative line intensities as a relatively large lattice vibrational anisotropy

ANOMALIES AT THE Sn MELTING POINT

De petites particules d'étain métallique ont été incorporées dans une matrice de BN. L'effet Mössbauer a été mesuré autour du point de fusion. Les résultats essentiels sont : 1) Au point de fusion, le signal de résonance chute à une valeur incommensurablement faible et ceci dans un intervalle de température très limité. Un élargissement de raie par diffusion n'a pas pu être détecté, confirmant Packwood et Longworth. L'observation d'un élargissement de raie par Boyle et al. a dû provenir d'impuretés. 2) La probabilité de résonance montre un phénomène prononcé d'hystérésis dû à la surfusion. L'analyse de ce phénomène, en association avec la taille des particules d'étain, a permis de déduire une énergie d'interphase liquide-solide. Cette énergie (48 ergs cm-2) est en accord raisonnable avec les résultats d'autres techniques.Small particles of metallic Sn were embedded in a BN matrix. The Mössbauer effect was measured around the melting point. The main results are : 1) At the melting point the resonance signal drops to an unmeasurable small value in a very limited temperature interval. Line broadening by diffusion could not be detected, confirming Packwood and Longworth. The observation of line broadening by Boyle et al. must have been due to impurities. 2) The resonance effect shows a pronounced hysteresis behavior due to undercooling. From the analysis of this phenomenon in conjunction with the Sn particle size an interphase energy between liquid and solid phase could be deduced. The energy (48 ergs cm-2) is in reasonable agreement with results by other techniques

STRUCTURAL AND MÖSSBAUER INVESTIGATION OF EPITAXIAL Cu/γ-Fe/Cu SANDWICH LAYERS

Nous avons étudié par LEED, par microscopie électronique (TME) et par l'effet Mössbauer des électrons de conversion (CEMS) des couches minces épitaxiques de Fe/Cu et Cu/Fe/Cu déposées sur substrats de (100) NaCl dans 1'ultravide (UHV). Des films (fcc) γ-Fe (100) paramagnétiques ont été observés. Les résultats CEMS montrent que la densité des électrons s au noyau du 57Fe décroît lorsque l'épaisseur du film de fer décroît ; ceci est probablement dû à une dilatation du réseau au voisinage de l'interface Fe/Cu.Epitaxial Fe/Cu- and Cu/Fe/Cu-thin film layers prepared on (100)NaCl substrates by vapor deposition in UHV have been investigated by LEED, transmission electron microscopy (TEM) and conversion electron Mössbauer spectroscopy (CEMS). Paramagnetic (fcc) γ-Fe (100) films have been observed. The CEMS results indicate a decrease of s-electron density at the 57Fe nucleus with decreasing Fe film thickness possibly caused by lattice expansion near the Fe/Cu interface