Experimental Modeling of Segmented ...

Experimental Modeling of Segmented Perpendicular Magnetic Recording Media by Block Copolymer patterning Students:

Ankita Mangal

Vignesh Sundar

Advisors:

Prof. Vincent Sokalski

Prof. David Laughlin & Prof. Jimmy Zhu

2nd Year Ph.D.

5th Year Ph.D.

Motivation

In PMR media, distributions in grain size, intergranular coupling and stacking faults, lead to a distribution in crystalline anisotropy and switching fields, thus reducing SNR. We present an experimental method using block co-polymer (BCP) patterning as described in [1] to mimic the behavior of perpendicular magnetic recording media. •  Understand the impact of each magnetic layer on the switching field and Switching Field Distributions (SFD)

•  Vary layer details (structure, composition, coupling strength) while microstructural details (grain size, morphology, stacking faults and intergranular spacing) are dictated by a repeatable process using BCP •  BCP patterning uniform across sampleà perform macroscopic magnetic measurements to evaluate the switching behavior. •  High throughput of measurements as compared to more tedious MFM measurements of individual island switching. Nonetheless, the latter technique will be explored to confirm macroscopic evaluation and also identify the impact of grain size or other defects on the switching field.

Figure 2: Calculated switching field as a function of the coupling strength through the exchange breaking layer. At large J, SF is a weak function of J. At relatively weak J, the 3 curves join together. Thus SF will have large variations with J. Figure reproduced from [2]

Figure 1: Schematic of a Segmented single magnetic grain with 2 exchange breaking layers [2] By tuning Ku and J, that switching field distribution (SFD) can be reduced

Experimental Tools & Techniques BCP Patterning of CoCrPt Thin Films AGFM (Alternating Gradient MFM Force In PMR media, distributions in grain size, intra-granular coupling and stacking faults, lead(Magnetic to a distribution in Microscopy) Magnetometer) crystalline anisotropy which ultimatelyField causes a wide switching field distribution, •  Topography : Pitch and Island Size distribution •  Pitch ~35nm •  Diameter~ 24 nm

Statistical relation between Switching Field and Island size The next generation PMR media will require good control of the microstructure• with uniform data grainfor size. •  Calculate switching field •  Hysteresis loop measurement by counting #bright and dark islands after Minimizing distributions in grain size distribution and grain tofrom grainthe distances will aid in increasing SNRsaturation/reversal and reduce applying fields slope of the BCP   Annealing noise duringSelf  transitions. We plan to achieve thisasbyshown pattern transfer from self assembled C/SiN  Mask   •  Workingblock towards obtaining better magnetic resolution and SFD M-H curve below Assembly  BCP   Perpendicular  Magne5c  Layer   C/SiNx  Mask   copolymers (BCP) onto a CoCrPt magnetic layer using an approach described in Sundar, 2015, Magne5c   L ayer   Underlayers   Substrate   Forthcoming Underlayers   [1]. Substrate   Reactive Ion •  Jian-Gang and Yiming [2] in their 2011 paper showed that in segmented PMR media with exchange Etching (RIE) 4: A breaking layers, the crystalline anisotropy of each segmentFigure and Inter-segment ferromagnetic exchange a) b) c) Figure 5: MFM typical images showing a)                                 coupling can be tuned with the thickness of individual segments. By optimizing these parameters, the Pattern Perpendicular  Magne5c  Layer   hysteresis t o p o g r a p h y, b ) Transfer: switching field Underlayers   distribution can be effectively reduced. Underlayers   demagnetized Ion loop. Image Milling Substrate   Substrate   state contrast and [5]stacking fault density increases with •  Sokalski, Laughlin et al.[3] in their 2011 paper have shownsource: that the c) analysis image growth temperature, for 35nm pitch Figure 3: Patterning Steps to achieve along with an increase in the magnetic anisotropy and a decrease in the lattice patterned islands isolated magneticparameter, grains c. Mrnorm = 0.081 Combining these approaches, our aim is to study the Switching Field Distributions (SFD) in patterned segmented grain media with increasing stacking fault density. • 

x

BCP: poly(styrene-bdimethylsiloxane)(PDMS)

Characterization by FORC (First Order Reversal Curves)

Exchange Coupling Variation

•  An accurate way to determine SFD

•  Ferromagnetic Coupling •  Optimize inter-segment coupling by varying thickness of exchange breaking layer (EBL)

Figure 6: Projection of Switching Field Distribution at reversal field obtained from integrating FORC data over H. Image Source [6]

Figure 7: RKKY Interaction showing FM & AF Exchange Coupling[7]

Preliminary Results & Future Work: b)

a)

b)

1.5

Normalized Moment

a)

Unpatterned Patterned

Future Work :

1 0.5

1. 

Segmented Grain Structure in patterned media with optimized inter-segment coupling

2. 

Control Anisotropy field dependence of Switching field

3. 

Control of SFD due to Stacking Fault density

4. 

Magnetic Force Microscopy to relate island size with switching field.

5. 

FORC to determine SFD

0 −0.5 −1 −10

c)

d)

c)100

Diameter−P5a−LC−6th x mean x std

80 60 40

−5 0 5 Field (kOe)

10

Figure 10:Comparison of out of plane hysteresis loops for continuous vs. patterned CoCrPt film. The coercivity increases from ~400 Oe to 5.92 kOe

20 0 10

Figure 8: SEM images after individual patterning steps: a) as spun BCP; b) BCP after annealing; c) C pillars formed after RIE; d) Magnetic Islands after Ion Milling

20

30 40 Diameter (nm)

50

60

Figure 9: Top: MFM images showing a) Topography and b) Contrast. Topography is visible in phase image due to small lift height (~15nm). Bottom: c) Diameter distributions obtained from topography image.

References 1.  2.  3.  4.  5.  6.  7. 

Sundar, V. Templated Two-phase Growth of Magnetic Recording Media,(Forthcoming ,2015) Carnegie Mellon University, Pittsburgh, PA, USA. Jian-Gang, Z. and W. Yiming (2011). "SNR Enhancement in Segmented Perpendicular Media." Magnetics, IEEE Transactions on 47(10): 4066-4072 Sokalski, V., et al. (2011). "Magnetic anisotropy and stacking faults in Co and Co84Pt16 epitaxially grown thin films." Journal of Applied Physics 110(9): 093919. Vignesh Sundar, J. Z., David E. Laughlin, Jian-Gang (Jimmy) Zhu "Novel Scheme for Producing Nanoscale Uniform Grains Based on Templated Two-Phase Growth.” http://www.lakeshore.com/Documents/Mag%20media%20app%20note.pdf Lenormand, D. R. (2012). Characterization of Magnetic Nanostructured Materials by First Order Reversal Curve Method. Applied Physics, University of New Orleans. Worledge, D. C. (2011). Techniques for coupling in semiconductor devices, Google Patents.