Dr. Tuhin Maity & Prof. Saibal Roy Micropower Systems & Nanomagnetics :

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1 Investigation of dynamic properties of exchange coupled magnetic nanopattern by a. micromagnetic simulation (project 1) b. experimental analysis (project 2) Dr. Tuhin Maity & Prof. Saibal Roy Micropower Systems & Nanomagnetics : Saibal.roy@tyndall.ie

2 Exchange Anisotropy/Exchange Bias Magnetic interaction between an antiferromagnetic (AFM) and a ferromagnetic (FM) material. Appears when a FM/AFM system is cooled in a magnetic field through the Néel temperature of the AFM phase. First invented in 1956 by Meiklejohn and Bean and named as exchange bias in M H Schematic diagram of the spin configuration of an FM-AFM bilayer

3 Applications of Exchange bias Magnetic Recording head Magnetic Memory Cell MRAM (non-volatile RAM) Figure courtesy of J. StoK hr, IBM Almaden Research Center Nature Materials 6, (2007)

4 Hysteresis loop formation Hysteresis Loop Exchange Bias Loop Inverted Loop

5 Non-Inverted and Inverted hysteresis loop (IHL) Microstructure of the material was analyzed by SuperSTEM facility at Daresbury, UK. Electron Energy Loss Spectroscopy (EELS) analysis shows Ni reach interfaces exist all over the sample. Ni Fe SuperSTEM facility at Daresbury, UK Lamellas prepared by DualBeam FIB Atomic structure of Ni 45 Fe 55 Tuhin Maity et al Physical Review B 95 (10), (2017)

6 Non-Inverted to Inverted hysteresis loop (IHL) E = k AFM sin 2 θ AFM + k FM sin 2 θ FM M FM H cos θ FM J EB M AFM M FM cos θ AFM θ FM δe/δθafm and δe/δθfm = 0 (i) θ AFM =θ FM =0, (ii) θ AFM =π &θ FM =0, (iii) θ AFM =0 &θ FM =π and (iv) θ AFM =θ FM =π Tuhin Maity et al Physical Review 95 (10), (2017)

7 Magnetic Anisotropy-Nanopatterned Nanohole array on substrate (created at 160 C by Si stamp using 5N/m 2 pressure) (Fig.b) Gold seed layer on nanopattern (Fig.c) Electroplated Ni 45 Fe 55 on nanomodulated substrate at room temperature (Fig.d) Angle dependent remanent magnetization was measured by SHB instrument with angle accuracy 5 and M r has been plotted (Fig. c & d) Validated by Micromagnetic simulation Tuhin Maity, Shunpu Li, Lynette Keeney, Saibal Roy; Phys. Rev. B 86 (024438), 7(2012)

8 Project aim / Methodology What will happen if we combine both exchange coupling and nanopatterning?

9 Micromagnetic simulation Static micromagnetic simulations were carried out by solving Landau-Lifshitz-Gilbert (LLG) equation LL:1935 LLG 1955: M(r, t) M(r, t) H t M(r, t) t 1 (a)schematic diagram of nano modulation and (b) In plane magnetic field rotation eff M(r, t) 1 E (r, t) / MS M(r, t) H eff (r, t) t 0 M(r, t) 2 M(r, t) M(r, t) Heff (r, t) ( ) /{( 1 )M S} M(r, t) 2 t OOMMF (Object Oriented Micromagnetic Framework) software (NIST US) has been used to find out macrospin behaviour in submicron to micron scale. 7 X 7 elements*. Minimum length of cubic 6.25 nm ~ exchange length (A/2π) 1/2 M. The total number of cell ~ 10 5 in each simulations. 65% area was patterned at the middle of whole substrate to avoid edge effect. H ext was increased from 0 Oe to 400 Oe to saturate the film to ensure saturated static. Typical magnetic properties for Ni 45 Fe 55 : Saturation Magnetisation (4πM S )= 1.2x10 6 A/m, Anisotropy Energy (E A ) =6.47x10 12 J /m, Anisotropy constant (K) =600 J /m 3

10 Project aim / Methodology Exchange bias for bi-magnetic materials Dynamic Study of exchange coupled patterned nano strcuture Validation of experimental results Innovative results on dynamic behaviour

11 Project roadmap (Project 1) simulation Work plan Understanding of OOMMF micromagnetic simulation Work schedule Two weeks Hysteresis loop with metastable state for nanopatterned structure One Month

12 Project roadmap (Project 1) simulation Work plan Work schedule Hysteresis loop formation for exchange bias (FM/AFM) or exchange spring (HF/SF) systems One month Micromagnetic simulation study of dynamic behaviour of exchange coupled nanopattern Two months Report submission and presentation

13 Project roadmap (Project 2) experiment Work plan Work schedule Understanding of materials fabrication by sputtering and electroplating Two weeks Fabrication of nanopatterned film One Month

14 Project roadmap (Project 2) experiment Work plan Static magnetic measurement for exchange coupled system Work schedule Two weeks Experimental study of dynamic behaviour of exchange coupled nanopattern One Month Report submission and presentation

15 Facilities OOMMF micromagnetic simulation (Project 1) SQUID magnetometer (2 350K), 5T field (Project 2) SHB hysteresis loop tracer (Project 1 & 2) Ryowa permeability spectra (Project 1 & 2) Scanning Electron Microscopy (SEM) (Project 2) Electro platting & sputtering (Project 2)

16 Micropower Systems & Nanomagnetics Prof. Saibal Roy : Head of Group

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