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Introduction

Tailoring the magnetic anisotropy of thin film permalloy microstrips by combined shape and induced anisotropies By Alfredo Garcia et al. Eur. Phys. J. B (2013) 86: 136.

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Introduction

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  1. Tailoring the magnetic anisotropy of thin film permalloy microstrips by combined shape and induced anisotropies By Alfredo Garcia et al. Eur. Phys. J. B (2013) 86: 136

  2. The Giant Magnetoimpedance, GMI, effect is variation of the impedance of a metallic magnetic conductor when submmitted to the action of a dc magnetic field. Compared to GMR, GMI gives more sensitive magnetic sensors The origin is related to elecromagnetic skin effect whereby when high frequency current flows through a conductor, it is confined to a certain depth of the conductor surface. Applicable for both conductors and magnetic materials with high enough σ. The external Hdc acting along currents' direction controls the transverse permeability of the sample which magnetizes it. Introduction

  3. Introduction • A magnetically anisotropic material aligns its magnetic moments with one of the easy axis. • The atomic structure of crystal and shape of the particle determine the preferred direction of anisotropy. • If Ki and Ks are the induced during deposition and shape anisotropy of a material, then Keff = Ki – Ks. • Keff can be varied/modulated since Ks = ½ uo (Na – Nc) M, where Na and Nc are dependent on geometry of sample (i.e. aspect ratio of width and length).

  4. Objectives • Since GMI can be used in magnetic sensors and metal degradation evaluation, this study focuses on • Developing multilayer, permalloy based thin films [Fe20Ni80/Ti]2 / Fe20Ni80 patterned on Si wafer with different sizes and widths. • For the strip-shaped samples, competition between the transverse (Ki) and longitudinal (Ks) is analyzed • Kerr magnetometery is used for confirming that effective anisotropy modulates by tailoring the aspect ratio (length vs width) of the strips. • Results are important for increasing the sensitivity of thin film GMI sensors.

  5. Anisotropy of Permalloy films • dc magnetron sputtering is used for depositing permalloy on Si substrate. • Anisotropy is induced by means of magnetic field from NdFeB permanent magnets placed along width of the deposited films • Kerr hysteresis loop shows that thinner permalloy films have a uniaxial induced anisotropy along the hard axis. Thicker samples destroy anisotropy due to out of plane magnetization • In order to get thicker permalloy (522 nm) for practical applications (i.e. to save anisotropy) spacer layer of Ti (6 nm) are used between 170 nm permalloy layers

  6. [Fe20Ni80/Ti]2/Fe20Ni80 samples • [Fe20Ni80/Ti]2/Fe20Ni80 samples are prepared with thickness 522 nm • Samples of various widths and lengths are arranged on Si wafer shown below with Cu pads • Arrows on wafer is the applied H during sputtering (i.e. direction of Ki). Negative resist Photolithography is used for obtaining the pattern

  7. Kerr magnetometry of samples • Kerr effect magnetometer operating in the longitudinal mode with a spot size of 20 μm is used for obtaining Hysteresis loops • As demagnetization effects over the surface of sample may differ, all Kerr measurements are taken by positioning the spot at the center of the samples • For all the calculations Hdc is kept along the long direction of the stripe (i.e. the hard axis) • The effective anisotropy field Hk is estimated near the saturation magnetization (indicated by the arrows). • The sample size is depicted with gray rectangles

  8. Kerr Hysteresis loops Results 1- Effective anisotropy field Hk decreases monotonically as the sample with same width elongate 2- Effective anisotropy field Hk decreases monotonically as the sample with same length narrows Hk decreases as the sample gets longer and narrower 4- The tendency of Coercive field to increase can be a consequence of competition of longitudinal and transversal anisotropy values averaged at the spot positioning. 5- defective lift-off and imperfection at the borders of narrow films can also be responsible for this

  9. Variation of Hk with w and l Top → variation of Hk as a function of length of the sample Bottom → variation of Hk as a function of width of the sample Both Figures show that 1- The effective anisotropy can be controlled by aspect ration of the sample 2- Anisotropy can be tailored between 1 – 5 Oe 3- The sensitivity of the GMI to applied field (i.e. s = d (ΔZ/Z)/dH) can be increases up to 60 %.

  10. Conclusions • The competition between transverse and longitudinal anisotropy can be modulate the effective transverse anisotropy in thing film stripes. • Helps increase the sensitivity of the GMI effect • Effective anisotropy field can be varied from 5 Oe to 1 Oe, consequently the GMI sensitivity can be varied from 2.5 to 5-fold increase of sensitivity • Results on sandwiched structures ([FeNi/Ti]3/Cu/[Ti/FeN]3) also confirm that the reduction of the effective transverse anisotropy does enhance the sensitivity of the GMI effect

  11. Thanks

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