CoNiO x Interface Interfacial Moments In Co/NiO Origins Of Antiferromagnetic/Ferromagnetic Coupling Hendrik Ohldag, Tom Regan, Andreas Scholl, Frithjof Nolting, Elke Arenholz, Franz Ulrich Hillebrecht and Joachim Stöhr MPI Halle Co And NiO H. Ohldag, A. Scholl et al., Phys. Rev. Lett. 86(13), pp 2878 (2001). F.U. Hillebrecht, H. Ohldag et al., Phys. Rev. Lett. 86(15), pp 3419 (2001). H. Ohldag, N.B. Weber et al., Synch. Rad. News 13(6), pp. 25 (2001). T.J. Regan, H. Ohldag et al., Phys. Rev. B, in print (2001). Or on the web: http://www-als.lbl.gov/als/als_news/news_archive/vol.178_060601.html X-ray absorption shows chemically active interface PEEM reveals its polarization. Co DEPOSITION CREATES AND POLARIZES CoNiO X INTERFACE 776 778 780 0 .00 0 .05 0 .10 Co C oO Co/NiO M odel E le ctro n Y ield (a rb . u n its.) P hoton E nergy (eV ) 868 870 872 874 0 .00 0 .01 0 .02 L 3 -edge L 2 -ed g e B A Co Ni Ni NiO Co/NiO M odel Line shape in x-ray absorption used to identify metal-oxide transition: M M x O y “Real System” Co/NiO Co/CoNiOx/NiO 5m Co MCD NiO MCD NiO Co NiO Co CoNiO The Solution: Chemical and magnetic characterization of all layers and their interface. The Unsolved Origin Of: -1500 -1000 -500 0 500 1000 1500 -1.0 -0.5 0.0 0.5 1.0 M ag netization(a.u.) A p p lied Field (O e) FM 1 FM 2 AFM - Loop shift (UNIDIRECTIONAL anisotropy) - - Loop widening (UNIAXIAL anisotropy) - If a ferromagnet is coupled to an antiferromagnet Giant Magneto Resistance: (GMR)-read head for hard disks PEEM to image AFM and FM domains in Co/ NiO(001) AFM REORIENTATION UPON Co DEPOSITION PARALLEL COUPLING UNIAXIAL ANISOTROPY PEEM to image AFM domains only at NiO(001) surface. SURFACE WALLS ARE NOT MIRROR PLANES LIKE BULK WALLS References The Method: A Photoemission Electron Microscope (PEEM) magnifies the local electron yield onto a screen. Spatial resolution ~ 50-100nm 790 795 800 1 .5 2 .0 2 .5 3 .0 C o L 2 E le c tro n Y ie ld (a .u .) P h o to n E n e rg y (e V) e - e - 876 879 882 100 200 300 N iO L 2 E le c tro n Y ie ld (a .u .) P h o to n E n e rg y (e V) e - e - Absorption of monochromatic tuneable x-rays elemental and chemical contrast Absorption of circular polarized x-rays (circular dichroism) spin orientation / ferromagnetic (FM) contrast Absorption of linear polarized x-rays (linear dichroism) spin anisotropy / antiferromagnetic (AFM) contrast 500 600 700 800 900 3 6 E le c tro n Y ie ld (a .u .) P h o to n E n e rg y (e V) e - Ni Fe e - e - FeO Fe e - 0 20 40 60 704 706 708 710 P ho to n E ne rg y (e V) E le c tro n Y ie ld (a .u.) Fe Fe + O x e 30 o no Fe Fe 3+ Fe 2 + XAS and dichroism link between interface polarization and magnetic properties “FREE” CoNiO x SPINS CAUSE UNIAXIAL ANISOTROPY (EXCHANGE COUPLING) “PINNED” CoNiO x SPINS MAY CAUSE UNIDIRECTIONAL ANISOTROPY (EXCHANGE BIAS) 0 10 20 30 40 0 .0 0 .3 0 .7 1 .0 1 .3 In te rfa ce th ic kn e s s (n m) A n n e a lin g tim e a t 6 0 0 K (m ins) 0 400 800 1200 C o e rc ivity (O e) Free Interfacial moments: - Exchange coupling - Uniaxial anisotropy - Coercivity increase Pinned Interfacial moments: - Exchange bias - Unidirectional anisotropy - Loop shift -2000 -1000 0 1000 2000 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 H ysteresis o f U ncom p en sated In terfacialN isp in s in 3n m C o/N iO N iO X M C D A sym m e try [% ] A pplied Field (O e)