Magnetization reversal in half metallic La_0.7Sr_0.3MnO₃ films grown onto vicinal surfaces

Abstract

We present the study of the magnetic properties of well-characterized epitaxial half metallic La 0:7 Sr 0:3 MnO 3 films grown onto vicinal SrTiO 3 (001) substrates with different miscut angles. Room temperature high resolution vectorial Kerr magnetometry measurements have been performed at different applied magnetic field directions in the whole angular range. The films present a substrate-induced uniaxial (twofold) magnetic anisotropy originated from in-plane [110]-oriented elongated structures, whereas the strength of this anisotropy increases with the miscut angle of the substrate surfaces. Our results demonstrate that we can artificially control the magnetic anisotropy of epitaxial films, up to 120 nm thick, by exploiting the substrate-induced anisotropy. We also determine in this case the minimum vicinal angle required to get well-defined uniaxial magnetic anisotropy. In cubic crystal symmetry epitaxial magnetic thin films the competition between the biaxial (fourfold) and the additional uniaxial (twofold) anisotropies can result in a magnetic reorientation, which depends on many parameters, such as substrate step density, 1,2 thickness, 3 angle of deposition, 4 and even the temperature range. 3 The biaxial and uniaxial anisotropies originate from the crystal symmetry and substrate-induced anisotropy, respectively. By resorting to the fabrication of artificial heterostructures, 5 thin films, 6-8 and superlattices, 9 we can exploit the symmetry breaking in order to control and tailor the magnetic properties of the materials, and, therefore, alter both magnetization easy and hard axes, and reversal processes. 10 Half metallic La 0:7 Sr 0:3 MnO 3 (LSMO) manganite, showing both a Curie temperature above 300 K and an almost 100% spin polarization, is of particular interest for the engineering of spintronics devices operating at room temperature (RT) such as read-heads magnetic hard disks and nonvolatile magnetic memories. 11 In this system, the substrate induces tensile or compressive strain to the film depending on the film-substrate lattice mismatch, determining in-plane or out-of-plane easy magnetization directions, respectively. 12 In particular, in LSMO/SrTiO 3 (STO)(001) an in-plane biaxial magnetic anisotropy ascribed to the substrate-induced in-plane tensile strain is generally observed. 12,13 Moreover, by using vicinal surfaces we can fabricate artificially periodic stepped surfaces with in-plane magnetic anisotropy. 14 In vicinal substrates, the surfaces are intentionally misoriented to a (near) low index surface, therefore determining surface step edges. In such a way, the high symmetry of the low index surface is broken and an additional uniaxial anisotropy is expected. 15 In-plane uniaxial magnetic anisotropy at RT already has been reported in 25-and 7-nm-thick LSMO films deposited on very low miscut STO substrates (0.13 and 0.24 ). 3 More recently, some of the authors reported on a well-defined uniaxial magnetic anisotropy in LSMO film, up to 70 nm thick, deposited on 10 vicinal STO(001) substrate....

Tailoring magnetic anisotropy in epitaxial half metallic La0.7Sr0.3MnO3 thin films

“…In addition, a nonnegligible biaxial magnetocrystalline anisotropy could not be excluded, as found in Ref. 34 in LSMO grown onto 2 mis-cut STO(001). In short, the LSMO(001) films grown on nominally flat STO(001) surfaces present a weak uniaxial anisotropy at RT.…”

Exchange coupling-induced uniaxial anisotropy in La0.7Sr0.3MnO3 thin films

“…Many emergent phenomena have not been found in the bulk state, such as abnormal magnetic anisotropy [9,10] and domain configuration control [11], but inherent within La 1-x Sr x MnO 3 thin films due to the inevitable reconstruction of the charge, spin, orbital, and lattice states at the film-substrate interface [12,13], which is thought to play an adverse role in device applications [14,15]. Thus, to tailor the magnetic properties of epitaxial La 1-x Sr x MnO 3 thin films, we need to seek a deeper understanding of the origin of substrate-induced anisotropies, which are very sensitive to a number of parameters such as the film strain imposed by the underlying substrate [11,16], substrate miscut angles [17,18], and dead layer at the film-substrate interface [9,19].…”

Direct experimental determination of the anisotropic magnetoresistive effects

“…28 For instance, by looking at the top-graphs, the angular evolution of M k -H reveal the angular-dependent behavior of the transition fields, coercive field (l 0 H C ) and switching field (l 0 H S ), i.e., yellow-bright color line, which present 180 periodicity (two-fold symmetry) characteristic of a well-defined magnetic uniaxial anisotropy system. 25 Such a symmetry emerges also in the MR-H maps for both J k K U and J ? K U in Figures 3(b) and 3(c).…”

“…These features are typical of an uniaxial magnetic anisotropy h.a. 25 Resuming, around the e.a. direction, the reversible transitions correspond to a reversal by magnetization rotation, whereas the irreversible ones correspond to propagation of magnetic domains oriented parallel to the field direction.…”