Neutron Reflectometry of Ferromagnetic Arrays

Transcription

1 Neutron Reflectometry of Ferromagnetic Arrays Z.Y. Zhao a, P. Mani a, V.V.Krishnamurthy a, W.-T. Lee b, F. Klose b, and G.J. Mankey a a Center for Materials for Information Technology and Department of Physics and Astronomy, The University of Alabama b Argonne National Laboratory This project was funded by grants from NSF-DMR MINT Research Fall Review, 10/17/2002.

2 Introduction Applications of ordered magnetic microstructures are becoming increasingly important, especially in magnetic recording, sensors, MRAM and magnetoelectronics. Systematic studies of the magnetization reversal processes are necessary. It is crucial to achieve a good understanding of interaction effects in arrays of magnetic microelements, or between microstructures of magnetic materials and other systems. Neutron reflectometry is evolving into a powerful probe to determine magnetic structures.

3 Fabrication - Lithography Process (I) A negative tone process had been involved in the lithography procedure. AZ 5214 is a special photoresist which can be used both in positive and negative tone. Parameters involved in negative tone process : Prebake temperature and time. Spin coat speed and time. First exposure time and intensity. Reversal bake temperature and time. Flood exposure time. Developing solution strength and time.

4 Fabrication - Lithography Process (II) AZ 5214 Photoresist (1.4 micron thick) UV Radiation Opaque Metal on Mask Glass Mask Exposed Photoresist Cross links on PEB Unexposed Photoresist Unaffected by PEB Si PR Si Flood exposure UV After development Si Cross linked photoresist Previously unexposed

5 Characterization of Ni50 Fe50 Bars patterened film continuous film M/Ms Magnetic field (Oe) AFM MOKE

6 Polarized Neutron Reflectometry (PNR) Magnetic structures on flat surfaces are usually less than a few hundreds nm thick. Polarized neutron reflectometry (PNR) is well-suited to study these materials because the signal is significantly enhanced by using grazing angle scattering. In a PNR measurement, polarized neutrons with wavelength λ either parallel (+) or antiparallel (-) to the applied field H, are incident on a magnetic thin film at grazing angle and are reflected from the film. Reflected neutrons may retain or flip their polarization, resulting in 4 reflectivity curves: R++, R- -, R+-, and R-+. As a rule of thumb, for films with in-plane moments, R++ and R- - measures the magnetization component along H while R-+ and R+arise from moments perpendicular to H. spin up neutrons spin down neutrons θ i M M tot M k 0 = 2π sinθ i /λ q = 4π sinθ i /λ H θ f R -+ R ++ R -- R +-

7 PNR Diffraction from Magnetic Arrays θ f Off-Specular Polarized Incident Beam Specular Off-Specular θ i θ f Horizon Diffraction For films that are structurally and magnetically uniform across the surface, there is only specular reflections (θ i = θ f ). Lateral structures like patterned magnetic arrays or magnetic domains across the surface often gives rise to off-specular scattering (θ i θ f ). Using off-specular scattering in the measurement of lateral magnetic structures are the focus of recent developments in PNR. λ

8 Periodical Diffraction from Magnetic Arrays Polarized Incident Beam 10 µm 2 µm Saturating field H=50 Oe 2 µm a) b) µm Arrays of 2 µm x 10 µm x 100 Å permalloy bars. The bars are separated by 2 µm in either direction. Intensity map of scattering from the saturated permalloy array sample for (a) spin-up, (b) spin-down incident beam with incident angle θ i =1.08º. Scattering angle θ = θ i +θ f. The solid line at θ = 1.08º is the horizon. Dotted lines: (1) Specular reflection; (2,3) Off-specular reflection; (4) Diffraction. Dashed lines: Critical edges for total reflection from silicon and permalloy. The off-specular reflections and the diffuse scattering from interfacial roughness are enhanced at the critical edges. The difference in the intensities between (a) and (b) comes from the magnetic dependence of the scattering from the permalloy arrays.

9 MFM Images for CoNiFe Magnetic Arrays MFM images of CoNiFe elements with different applied fields. The domain structures can be clearly seen in the pictures. The dimensions of the elements are 2.6 µm x 10.9 µm x 10 nm. 60 Oe Ha 18 Oe 0 Oe -10 Oe -18 Oe -47 Oe Reference: H. Jiang, MINT Fall Review (1999).

10 Current Work: Antiferromagnetic Coupled (AFC) Co Layers Impact: Mrt eff = Mrt top Mrt bottom AFC media permits overall Mrt to be reduced and its data density increased independently of its overall thickness. Reference: AFC media can delay superparamagnetic effect in limiting future areal density increase due to instabilities. Advantages: Can be made by existing equipment without additional cost. Writing and readback characteristics are similar with conventional media.

11 Magnetic Properties of AFC Co layers Hysteresis loops of AFC Co layers, red loop is for hard axis and blue loop is for easy axis. The AFC Co/Ru/Co sample shows the characteristic behavior of antiferromagnetically coupled trilayers. For large fields the interlayer coupling is overcome and the the layers are parallel to the field. As the field is reduced the interlayer coupling overcomes the applied field and the thinner Co layer reverses at the exchange fields Hex. For each sample, the remnant state is with the adjacent magnetizations antiparallel and the thicker layer parallel to the previously applied field direction.

12 Conclusion PNR is a powerful microscopic probes to determine magnetic structures. Coercivity change between patterned magnetic arrays and continuous film has been successfully observed by PNR. PNR can be used in determination of fundamental physics of AFC magnetic films.