Photoemission study of coupled magnetic layers Z. Q. Qiu Dept. of Physics, University of California at Berkeley Outline Motivation Magnetic Phase Transition Spin Reorientation Transition Wish List

2D 0D1D Motivation GMR and Oscillatory Coupling P. Grünberg, et. al., Phys. Rev. Lett. 57, 2442 (1986). M. N. Baibich, et. al., Phys. Rev. Lett. 61, 2472 (1988). S. S. P. Parkin et. al. Phys. Rev. Lett. 64, 2304 (1990). 5ML Spectroscopy of valence electrons How does the interlayer coupling change the magnetic properties of the individual layer? Microscopy of core electrons (PEEM) Element-specificity + imaging

What is effect of the interlayer coupling on the phase transition? Single or separate phase transitions? How is the T C of the coupled sandwich compared to T C1 and T C2 ? Magnetic Phase Transition FM2 FM1 FM2 T C1 T C2 ? Co Ni T=310K380K430K Ni(3ML)/Fe(5ML)/Co(10ML)/Cu(100) Ni Co dc Ni(wedge)/Fe(5ML)/Co(10ML)/Cu(100)

Co(0.23ML)/Cu(2ML)/Ni/Cu(100) Ni 0.23ML 5.3ML5.8ML6.2ML7.1ML4.9ML Co d Ni = d Co =0.23ML Co(2.3ML)/Cu(2ML)/Ni/Cu(100) Ni 2.3ML 3.4ML4.1ML5.3ML7.5ML Co d Ni =2.5ML d Co =2.3ML Co(1.3ML)/Cu(2ML)/Ni/Cu(100) d Co = Ni 3.7ML Co d Ni = 1.3ML 4.5ML5.0ML5.4ML6.1ML 1.3ML

Co/Cu(2ML)/Ni/Cu(100) Co/Fe(5ML)/Ni/Cu(100) IIIIIIIV NiPMFMPMFM CoPM FM PM state of one layer has little effect on the Tc of the other film. FM state of one layer reduces the dc of the other layer.

d Co d Ni I 1: PM 2: PM II 1: PM 2: FM IV 1: FM 2: FM III 1: FM 2: PM J int =0 J int  0 d Co d Ni I 1: PM 2: PM II 1: PM 2: FM IV 1: FM 2: FM III 1: FM 2: PM

Spin Reorientation Transition (SRT) H = -J  S iS j – K  S iz 2 What will happen at K  0 ? K > 0 K < 0 K  0 ? Spin-orbit interaction gives -KS Z 2 if z symmetry is broken. Dipolar interaction gives 2  M 2 S Z 2. E = -KS Z 2 = -(K S /d - 2  M 2 )S Z 2 Changing d or T to tune K around zero.

Electron Microscope can be used to study stripe domains as a function of “magnetic field”. M1M1 M2M2 If  M 2 /  r  0, E = -J M 1M 2 = - M 1H with H = J H = 0 M1M1 H = J  0 equals How do the stripe domains respond to H ? Electron microscope is used for domain imaging. It is difficult to apply H in an electron microscope. SRT of M 1 in a coupled sandwich for H=0 is equivalent to the SRT of M 1 single layer within a H=J.

Sample is grown by MBE. Fe/Ni(5ML) film as the SRT layer. Cu/Co(10ML) provides the in-plane magnetic field. Cu(001) Fe/Ni H = JM Co Co Cu

2.4ML (c) 2.6ML2.8ML 50  m  // (a) (b) 10  m Fe thickness (ML)

15  m CoFe 15  m Co(10ML)/Cu/Fe/Ni(5ML)/Cu(001) The Co magnetization (in-plane field) aligns the stripe domains of the Fe/Ni layer.

Co(10ML)/Cu/Fe/Ni(5ML)/Cu(001) 6.3ML 6.9ML d Fe (ML) d Cu (ML) FC AFC 2.84ML2.44ML 50  m H K

d Fe =2.53ML 2.59ML2.66ML2.72ML d Cu =6.57ML 6.66ML6.73ML6.81ML 6.47ML6.38ML6.27ML d Cu =6.57ML (a) (b) (c)

w w L  z y x  H=J int 

Discussion of J int =0. 2.4ML (c) 2.6ML2.8ML 50  m  // (a) (b) 10  m Fe thickness (ML)

Discussion

Keep pushing the spatial resolution down to nm for laterally modulated structures (vicinal surfaces, wires, and dots …). Element-specific measurement of antiferromagnetic materials other than oxides (Mn, Cr, FeMn). Microscopy within magnetic field. Fast spin Dynamics (t<1-10ps). Improve in-situ sample preparation capability. Wish List 1.Develop techniques that are unique for certain type of problems (e.g., element-specificity). 2.Attract users who are doing outstanding research on magnetic nanostructures, but not yet using synchrotrons.

Department of Physics, University of California at Berkeley Y. Z. Wu, C. Won, and H. W. Zhao Advanced Light Source, Lawrence Berkeley National Laboratory A. Doran and A. Scholl Department of Physics, Fudan University X. F. Jin Collaborators