Strain-induced perpendicular magnetic anisotropy inLa2CoMnO6−ɛthin films and its dependence on film thickness

Abstract

Ferromagnetic insulating La 2 CoMnO 6- (LCMO) epitaxial thin films grown on top of SrTiO 3 (001) substrates present a strong magnetic anisotropy favoring the out of plane orientation of the magnetization with a large anisotropy field (70 kOe for film thickness of about 15 nm). Diminishing oxygen off-stoichiometry of the film enhances the anisotropy. We attribute this to the concomitant shrinkage of the out of plane cell parameter and to the increasing of the tensile strain of the films. Consistently, LCMO films grown on (LaAlO 3 ) 0.3 (Sr 2 AlTaO 6 ) 0.7 and LaAlO 3 substrates (with a larger out-ofplane lattice parameter and compressive stress) display in-plane magnetic anisotropy. Thus, we link the strong magnetic anisotropy observed in LCMO to the film stress: tensile strain favors perpendicular anisotropy and compressive stress favors in plane anisotropy. We also report on the thickness dependence of the magnetic properties. Perpendicular anisotropy, saturation magnetization and Curie temperature are maintained over a large range of film thickness. PACS: 75.30.Gw,75.50.Dd, 75.47.Lx, 75.70.Ak I IntroductionSpintronic devices rely on the use of the spin degree of freedom of the electrons as control variable and are presently based on the generation and control of highly spinpolarized currents in ferromagnetic metals [1] or, more recently, on controlling pure spin currents [2]. In this context, ferromagnetic insulators may play an important role as spin sources or spin conductors [3]. In addition, they also have emerged as potential candidates for magnetically active barriers or spin filters [1]. Ferromagnetic insulating materials are scarce since in many cases ferromagnetic interactions are of exchangetype and driven by carriers. Among these rare materials, La 2 CoMnO 6 and La 2 NiMnO 6 double perovskites have been reported to be ferromagnetic insulators when cationic ordering (Co,Ni/Mn) is achieved [4,5]. Ferromagnetic ordering in these materials relies on the Goodenough-Kanamori-Anderson rules that predict a ferromagnetic interaction between Co 2+ or Ni 2+ (t 2g 5 e g 2 and t 2g 6 e g 2 respectively) and Mn 4+ (t 2g 3 ) cations when they are ordered in a fully alternating way in the lattice [6,7,8]. In addition, they have been intensively investigated recently because of claims of magnetodielectric response 2 [9,10], which could be highly interesting for the implementation of new devices including tunable filters, magnetic sensors, and spin-charge transducers [11].Much interest is added when the magnetization of these materials is found to be out-ofplane, as the possibility of controlling perpendicular magnetic anisotropy (PMA) opens the door to the implementation of high density magnetic memory devices [12,13]. Actually, in some cases, magnetic anisotropy appears to be sensitive to lattice distortion and point towards promising spintronic applications based on the ability to control magnetic properties through modification of strain conditions. It has been proved in several material systems that t...

Anisotropic sensor and memory device with a ferromagnetic tunnel barrier as the only magnetic element

“…S1). As reported in our previous work, films grown on SrTiO 3 substrates are under tensile strain and exhibit strong PMA 21 . Magnetic properties of the barrier are not clearly determined via SQUID magnetometry because magnetic signal of such a thin film is very low.…”

“…u ., which suggest a high degree of Co/Mn cationic ordering 19,20 . We have also shown that LCMO grown under tensile strain (on STO) exhibits a strong perpendicular magnetic anisotropy (PMA), which has a magnetocrystalline origin due to the spin-orbit coupling (SOC) in Co 2+ ions 20,21 . This suggests that LCMO is a good candidate to exhibit tunneling anisotropic magnetoresistance (TAMR).…”

Spontaneous cationic ordering in chemical-solution-grown La2CoMnO6 double perovskite thin films

“…When the magnetic field is applied perpendicular to the sample plane (the OP configuration), saturation is reached at~30 kOe, while when the field is applied parallel to the sample plane (the IP configuration), saturation is not reached even at 80 kOe. Thus, the perpendicular-to-plane direction is the easy magnetization direction, as previously observed in samples grown by RF magnetron sputtering 23,35 . Our previous results for samples prepared by RF sputtering indicate that the Co 2+ valence state is preferentially stabilized irrespective of the structural strain even in nonstoichiometric samples 36 .…”

“…b High-resolution XRD pattern of the (002) diffraction peak. The oscillations around the main diffraction peak are an indication of the high structural quality of the epitaxial LCMO films contribution to the magnetic moment of Co 2+ occurs and generates a strong perpendicular magnetic anisotropy 35,36 , as in the present case. On the other hand, M(H) loops in Fig.…”

“…In addition, it is important to remark that the conclusion regarding the higher degree of cationic ordering in samples prepared on (111)-STO substrates was derived by comparing magnetization values measured with the magnetic field applied parallel to the sample plane and that, in the case of the (001)-STO samples, corresponds to the hard magnetization direction. Therefore, the lower values of the magnetization obtained in the (001)-STO samples do not necessarily imply a lower cationic ordering but result from the strong perpendicular magnetic anisotropy present in the (001)oriented LCMO thin films 35 .…”