Magnetic anisotropy changes induced by strains in FePt/BaTiO3

A. ROMÁN; A. LÓPEZ PEDROSO; L. NEÑER; M. AGUIRRE; A. BUTERA; J. E. GÓMEZ; MARTÍN SIRENA; L. B. STEREN

Dublin

Conferencia; IEEE International Magnetics Conference, INTERMAG Europe 2017; 2017

Intermag

Magnetic anisotropy changes induced by strains in FePt/BaTiO3A. Roman, A. LopezPedroso, L. Neñer, M. H. Aguirre*, A. Butera, J. E. Gómez, M. Sirena, L.B.SterenInstituto de Nanociencia y Nanotecnología y Depto. deMateria Condensada, Centro Atómico Constituyentes, San Martin (Argentina)Facultad de Cs. Exactas y Naturales, Universidad deBuenos Aires, Buenos Aires (Argentina)*Instituto de Nanocienciade Aragón, Laboratorio de Microscopías Avanzadas, Universidad de Zaragoza,& Dpto Física de la Materia Condensada, Universidad de Zaragoza (Spain)Centro Atómico Bariloche e Instituto Balseiro, S.C. deBariloche (Argentina) Nowadays, the design andfabrication of artificial multiferroics (MF) is one of the main challengesfor the development of oxide spintronics. Tuning the magnetic state ofnanostructures by electric field or strain appears to be the key for low-energydevices. Magneto-electric and magneto-elastic coupling at interfaces betweenferromagnetic (FM) and ferroelectric (FE) layers are at the origin of thesephenomena [1]. Both, the magnetic order and anisotropy of the FM component canbe affected by the strain induced by the FM/FE lattice mismatch, while chargetransfer between both compounds can depend on the ferroelectric polarization. Avariety of FM and FE has been combined into artificial MF, being the magneticcompounds generally oxides or transition metal alloys. In particular,FePt/BaTiO3 seems to be a very promising system. First-principlecalculations recently revealed that important changes of magnetic anisotropy shouldbe observed in the FePt overlayer when the BaTiO3 is poled[2].  Moreover, FePt magnetic properties have shown to be stronglysensitive to strain [3].  In this work, we studieda series of FePt/BaTiO3 (FEPT/BTO) bilayers grown by sputtering onSrTiO3(100) substrates. The ferromagnetic layer thickness was variedbetween 20nm and 60 nm keeping the BaTiO3 thickness fixed at 60 nm.  The quality of the films was probed by X-raydiffraction, High-resolution Scanning Transmission Electron Microscopy withHigh Angular Annular Dark Field detector (HR-STEM-HAADF) and Atomic Force Microscopymeasurements. STEM-HAADF images show that the BTO grows epitaxially on STO substrates while FePt is polycrystalline with astrong (111) texture. The BTO c-axis lattice parameter is 4.6% larger than the bulkvalue due to in-plane biaxial compressive strain. In consequence, thetetragonal distortion associated to ferroelectric properties is c/a=1.08, higherthan bulk BTO tetragonal distortion [4]. The magneticcharacterization of the samples was performed by magnetic force microscopy(MFM) and magnetometry experiments, complemented with micromagnetic simulations.   Sampleswith FePt layers thicker than a critical thickness, tc, show the presence of stripe-like domains togetherwith an anomalous temperature dependence of the coercive field, Hc. Thisbehaviour was already found on FePt thin films deposited onto siliconsubstrates [5]. The effect of the BaTiO3-induced strain is notably appreciatedin the slope change of the FePt coercivity close to BaTiO3 structuralphase transitions (Fig.1) [6].Micromagneticsimulations, calculated using MUMAX [7], were made to better understand thephysics underneath the magnetic domains structures for different film thicknesses. The parameters used in the simulationswere:  Ms=1100 emu/cc, deduced frommagnetometry measurements and the stiffnes constant A=0.95x10-6 erg/ccextracted from Ref. 9. Considering that the magnetic easy-axis in the FePt fccstructure is oriented along the (111) direction and coincides with the filmstexture, we introduced an out-of-plane uniaxial anisotropy. The MFM images weresuccessfully reproduced using an anisotropy constant Kp=2.25 Merg/cc [10] (Fig2). Moreover, simulations also allow us to calculate the coercive field, thatagrees with the measured values.              Themagnetic behaviour observed in our system is discussed in the frame of themagnetic-elastic coupling mechanisms. [1] C. A. Vaz, J. Phys.: Condens. Mat. 24, 333201(2012).[2] M. Lee, H. Choi, Y. C. Chung, J. Appl. Phys. 113, 17C729 (2013). [3] N. R. Álvarez, J. E. Gómez, A.E. Moya Riffo, M.A. Vicente Álvarez, A.Butera, J. Appl. Phys.  119 1 (2016).[4] H.D. Megaw, Nature (London), 155,484, (1945)[5] J. M. Guzmán, N. Álvarez, H. R. Salva, M. V. Mansilla, J. E. Gómezand A. Butera, J. Magn. Magn. Mater.,347 61 (2013).[6] M. K. Lee, T. K. Nath, C.B. Eom, M. C. Smoak and F. Tsui, Appl.Phys. Lett. 77,3547 (2000).[7] A. Vansteenkiste, J. Leliaert, M. Dvornik, M. Helsen, F. Garcia-Sanchez and B. Van Waeyenberge, AIPAdvances 4, 107133 (2014) [8] H.Kanazawa, G. Lauhoff1 and T. Suzuki,J. Appl. Phys. 87, 6143 (2000)[9] S. Okamoto,N. Kikuchi, O. Kitakami, T. Miyazaki, Y. Shimada, and K. Fukamichi, Phys. Rev.B 66, 024413 (2002)[10] E. Sallica Leva, R. C. Valente, F. Martínez Tabares, M. VásquezMansilla, S. Roshdestwensky, and A. Butera, Phys. Rev. B 82, 144410 (2010) Fig1:  Hc vs. T for FePt(40nm)/BaTiO3(60nm).The  crystalline structures of the BaTiO3  at different temperatures are indicated as R(rhombohedral), O (orthorhombical) and T (tetragonal), respectively.Fig2: MFM images  for FePt(40nm)/BaTiO3(60nm).(a) measured and (b)  simulated, respectively.