Low dimensional magnetism Experiments O.Fruchart, Laboratoire - PowerPoint PPT Presentation

Low dimensional magnetism – Experiments O.Fruchart, Laboratoire Louis Néel (CNRS), Grenoble 27/08/2003 Laboratoire Louis Néel. Louis Néel. Laboratoire Louis Néel. Laboratoire http://lab http:// lab- -neel neel.grenoble. .grenoble.cnrs cnrs. .fr fr/ /themes themes/couches/ /couches/ext ext/ / http://lab-neel.grenoble.cnrs.fr/themes/couches/ext/

Low dimensional magnetism – Experiments Compared to the abstract, some paragraphs have been omitted or shrunk • No time to speak of everything in 2 hours • Some slides prepared on the spot. For missing items, see reference: URSUS: Regele berii in Romania – Bere Cluj…(Fondat 1878) Olivier Fruchart – 27/08/2003 – p.3 Laboratoire Louis Néel Louis Néel Laboratoire Laboratoire Louis Néel http://lab http:// lab- -neel neel.grenoble. .grenoble.cnrs cnrs. .fr fr/ /themes themes/couches/ /couches/ext ext/ /

Low dimensional magnetism – Experiments (1. Introduction) 2. Ferromagnetic order 3. Magnetic anisotropy 4. Layered systems: from concepts to functional building bricks 5. Superparamagnetism References Olivier Fruchart – 27/08/2003 – p.4 Laboratoire Louis Néel Louis Néel Laboratoire Louis Néel Laboratoire http://lab http:// lab- -neel neel.grenoble. .grenoble.cnrs cnrs. .fr fr/ /themes themes/couches/ /couches/ext ext/ /

2. Ferromagnetic order 2.1 Methods 2.2 Metastable phases 2.3 Magnetic order versus temperature 2.4 Surface magnetization Olivier Fruchart – 27/08/2003 – p.5 Laboratoire Louis Néel Louis Néel Laboratoire Laboratoire Louis Néel http://lab http:// lab- -neel neel.grenoble. .grenoble.cnrs cnrs. .fr fr/ /themes themes/couches/ /couches/ext ext/ /

2. Ferromagnetic order [ 2.2 Metastable phases] [Avoid structural transition] Avoid structural transition • Stabilization by: Cf lecture by Stéphane ANDRIEU • Epitaxial misfit Films, or clusters in a matrix • In-plane stress Fe/Cu(001) • Optimize growth methods and 300K growth with PLD: fcc parameters Fe/Cu(001) 300K growth with MBE: fcc>bcc P. Ohresser et al., PRB59, 3696 (2001) Olivier Fruchart – 27/08/2003 – p.7 Laboratoire Louis Néel Louis Néel Laboratoire Laboratoire Louis Néel http://lab http:// lab- -neel neel.grenoble. .grenoble.cnrs cnrs. .fr fr/ /themes themes/couches/ /couches/ext ext/ /

2. Ferromagnetic order [ 2.2 Metastable phases] [fcc γ -Fe] fcc γ -Fe Theory ( e.g. ) Thin films • fcc γ -Fe for T>1185K: non-magnetic • ‘ground-state’: sensitive on lattice parameter fcc Fe/CuAu(111) L ow S pin A nti- F erro N on- M agn. & L ow S pin H igh S pin U. Gradmann et al. , JMMM 15-18, 1109 (1980) • Agreement with theory? • What happens for thicker films? V. L. Moruzzi et al. , PRB39, 6957 (1989) See also: O.K. Andersen, Physica B 86, 249 (1977) Olivier Fruchart – 27/08/2003 – p.8 Laboratoire Louis Néel Louis Néel Laboratoire Louis Néel Laboratoire http://lab http:// lab- -neel neel.grenoble. .grenoble.cnrs cnrs. .fr fr/ /themes themes/couches/ /couches/ext ext/ /

2. Ferromagnetic order [ 2.2 Metastable phases] [fcc γ -Fe] Spin-density wave antiferromagnetism Fe/Cu(001) Ferro. SDW - AF D. Qian et al., PRL87, 227204(2001) See also V. Cros et al., Europhys. Lett. 49, 807 (2000) Olivier Fruchart – 27/08/2003 – p.9 Laboratoire Louis Néel Louis Néel Laboratoire Louis Néel Laboratoire http://lab http:// lab- -neel neel.grenoble. .grenoble.cnrs cnrs. .fr fr/ /themes themes/couches/ /couches/ext ext/ /

2. Ferromagnetic order [ 2.2 Metastable phases] [fcc Co] Stabilization of fcc Co Clusters • Stabilization by: • Epitaxial misfit Films, or DPM, CNRS, Lyon, France : LASER vaporization clusters (matrix, and inert gas condensation source • Surface stress or free) M. Jamet, V. Dupuis, M. Negrier, J. Tuaillon, A. Perez • Surface orientation. e.g. (001) • Optimize growth methods and parameters HRTEM along a [110] direction fcc - structure, faceting Cf lecture by Edgar BONET Model system : shape, magneto-crystalline anisotropy Olivier Fruchart – 27/08/2003 – p.10 Laboratoire Louis Néel Louis Néel Laboratoire Louis Néel Laboratoire http://lab http:// lab- -neel neel.grenoble. .grenoble.cnrs cnrs. .fr fr/ /themes themes/couches/ /couches/ext ext/ /

2. Ferromagnetic order [ 2.2 Metastable phases] [fcc Co] 300K MBE: stacking faults 300K PLD: fcc fcc Co films Cf lecture by Stéphane ANDRIEU Co/Cu(001) I-V LEED curves • stacking faults for MBE • pure fcc for PLD M. Zheng et al. , APL74, 425 (1999) Olivier Fruchart – 27/08/2003 – p.11 Laboratoire Louis Néel Louis Néel Laboratoire Louis Néel Laboratoire http://lab http:// lab- -neel neel.grenoble. .grenoble.cnrs cnrs. .fr fr/ /themes themes/couches/ /couches/ext ext/ /

2. Ferromagnetic order [ 2.3 Magnetic order versus temperature] [Tc] Elements of theory • Bloch (1930). No magnetic order at T>OK in 2D. (spin-waves; isotropic Heisenberg) • Onsager (1944) + Yang (1951). Magnetic anisotropy 2D Ising model: Tc>0K stabilizes ordering Experiments: Tc(t) fcc 48Ni/52Fe(111)/Cu(011) Cf lecture by Dominique GIVORD Spin Wave regime Mean field inacurate in low dimension… FIG.30a GRADMANN Tc interpreted with molecular field U. Gradmann and J. Müller, R. Bergholz and U. Gradmann, Phys. Status Solidi 27, 313 (1968) JMMM45, 389 (1984) Olivier Fruchart – 27/08/2003 – p.12 Laboratoire Louis Néel Louis Néel Laboratoire Louis Néel Laboratoire http://lab http:// lab- -neel neel.grenoble. .grenoble.cnrs cnrs. .fr fr/ /themes themes/couches/ /couches/ext ext/ /

2. Ferromagnetic order [ 2.3 Magnetic order versus temperature] [Tc] Naïve model Molecular field + J 1 Nw M m 0 0 0 N s N b = µ T . . c 0 3 J k B N neighbors − 2 ( N N ) -1 b s = − ∆ N N T ( t ) ~ t b c t Less naïve… Experiments Thickness-dependant molecular field λ - ∆ T ( t ) ~ t c λ = 1 G.A.T. Allan, PRB1, 352 (1970) U. Gradmann, Conclusion : Handbook of Magn. Mater. Vol.7, ch.1 (1993) Naïve views are roughly correct Olivier Fruchart – 27/08/2003 – p.13 Laboratoire Louis Néel Louis Néel Laboratoire Louis Néel Laboratoire http://lab http:// lab- -neel neel.grenoble. .grenoble.cnrs cnrs. .fr fr/ /themes themes/couches/ /couches/ext ext/ /

2. Ferromagnetic order [ 2.3 Magnetic order versus temperature] [Bloch law] BLOCH LAW Results Analysis [ ] Ag/Fe(110)/W(110) 3 / 2 = − B ( T ) B ( 0 ) 1 bT FIT: hf hf Ag interface Remarkable result [Fe/W(110)]: Film center [ ] t = ∞ + − ∞ b ( t ) b ( ) b ( 1 ) b ( ) / Fits: T 3/2 Bloch laws ∞ b ( 1 ) ~ 10 b ( ) ∞ b ( surface ) ~ 2 .5 b ( ) Olivier Fruchart – 27/08/2003 – p.14 Laboratoire Louis Néel Louis Néel Laboratoire Laboratoire Louis Néel http://lab http:// lab- -neel neel.grenoble. .grenoble.cnrs cnrs. .fr fr/ /themes themes/couches/ /couches/ext ext/ /

2. Ferromagnetic order [ 2.3 Magnetic order versus temperature] [Dead layers] BLOCH LAW AND DEAD LAYERS Analysis Interpretation Idea: Determine surface magnization, Reality Misinterpretation enhanced or reduced. bulk bulk Ideal measurement M = ∞ + τ ( 0 K , t ) M ( 0 K , )( t ) s s Real measurement: T >0K [ ] 3 / 2 M = M − ( T , t ) ( 0 K , t ) 1 b ( t ) T s s BACK 1 2 3 4 5 6 7 8 910 1 2 3 4 5 6 7 8 910 [ ] 3 / 2 3 / 2 M ≈ ∞ × − + τ − ( T , t ) M ( 0 K , ) t ( 1 b T ) b T ∞ s s 1 S Slope h S i f p t u r i o u s Study of surface versus bulk e f f e magnetization must be undertaken c t at low temperature Olivier Fruchart – 27/08/2003 – p.15 Laboratoire Louis Néel Louis Néel Laboratoire Louis Néel Laboratoire http://lab http:// lab- -neel neel.grenoble. .grenoble.cnrs cnrs. .fr fr/ /themes themes/couches/ /couches/ext ext/ /

2. Ferromagnetic order [ 2.3 Magnetic order versus temperature] [Dead layers] BLOCH LAW AND DEAD LAYERS: Experiments Fe(110): surface magn. Enhanced • τ apparent = +0.14 • τ SW-corrected = +0.42 Ni(111): surface magn. reduced U. Gradmann, Handbook… Olivier Fruchart – 27/08/2003 – p.16 Laboratoire Louis Néel Louis Néel Laboratoire Louis Néel Laboratoire http://lab http:// lab- -neel neel.grenoble. .grenoble.cnrs cnrs. .fr fr/ /themes themes/couches/ /couches/ext ext/ /

2. Ferromagnetic order [ 2.3 Magnetic order versus temperature] [Dipolar fields] Spin- Example: Vicinal Polarized Fe(110)/W(110) Cf lecture by Dominique GIVORD Scanning Tunneling Atomically-narrow Spectroscopy domain walls Cf lecture by André THIAVILLE Thickness = 1.25AL • In-plane magnetization imaged (reflects out-of-plane) Conclusion • Dipolar fields stabilize magnetic order M. Bode et al, Rep. Prog. Phys. 66, 523 (2003) Olivier Fruchart – 27/08/2003 – p.17 Laboratoire Louis Néel Louis Néel Laboratoire Louis Néel Laboratoire http://lab http:// lab- -neel neel.grenoble. .grenoble.cnrs cnrs. .fr fr/ /themes themes/couches/ /couches/ext ext/ /