Insights into the magnetic dead layer in La0.7Sr0.3MnO3 thin films from temperature, magnetic field and thickness dependence of their magnetization

Abstract

Experimental investigations of the magnetic dead layer in 7.6 nm thick film of La0.7Sr0.3MnO3 (LSMO) are reported. The dc magnetization (M) measurements for a sample cooled to T = 5 K in applied field H = 0 reveal the presence of negative remanent magnetization (NRM) in the M vs. H (magnetic field) measurements as well as in the M vs. T measurements in H = 50 Oe and 100 Oe. The M vs. T data in ZFC (zero-field-cooled) and FC (field-cooled) protocols are used to determine the blocking temperature TB in different H. Isothermal hysteresis loops at different T are used to determine the temperature dependence of saturation magnetization (MS), remanence (MR) and coercivity HC. The MS vs. T data are fit to the Bloch law, MS (T) = M0 (1 – BT 3/2), showing a good fit for T < 100 K and yielding the nearest-neighbor exchange constant J/kB ≅ 18 K. The variations of TB vs. H and HC vs. T are well described by the model often used for randomly oriented magnetic nanoparticles with magnetic domain diameter ≈ 9 nm present in the dead-layer of thickness d =1.4 nm. Finally, the data available from literature on the thickness (D) variation of Curie temperature (TC) and MS of LSMO films grown under 200, 150, and 0.38 mTorr pressures of O2 are analyzed in terms of the finite-size scaling, with MS vs. D data fit to MS (D) = MS(b)(1-d/D) yielding the dead layer thickness d = 1.1 nm, 1.4 nm and 2.4 nm respectively. Brief discussion on the significance of these results is presented.

Effects of Oxygen Modification on the Structural and Magnetic Properties of Highly Epitaxial La0.7Sr0.3MnO3 (LSMO) thin films

“…The NRM effect appears in a material when the direction of M is reverse to the direction of the applied H to that material 35 . NRM has been already observed and reported by our group in LSMO thin films grown in 100 mTorr 35,39 . Lee et al have also observed NRM in LSMO films via X-ray magnetic circular dichroism (XMCD) technique 34 .…”

“…This NRM has been observed only when the measuring field is less the coercive field (H c ) since for applied H > H c only positive values of M are observed in ZFC. This observed NRM is attributed to the coexistence of magnetically inhomogeneous regions consisting of spin clusters of a lower oxidation state and the ferromagnetic order in LSMO thin films 35,39,[41][42][43] . The growth pressure dependence of NRM is plotted in Fig.…”

“…We carefully examined the magnetic behaviour of all LSMO films synthesized at different oxygen partial pressure by performing the magnetic field and temperature dependence of dc-magnetization measurements ( zero-field-cooled (ZFC) and field-cooled (FC) mode by applying static magnetic fields of 50 and 1000 Oe in a wide range of temperature (5 to 400 K). The details of measurement protocol is mentioned in our previous paper 39 . Fig.…”

“…The temperature at which magnetization vanishes and changes sign (from negative to positive in the ZFC curve and from positive to negative in the FC curve) is known as the compensation temperature (T comp ) shown in Fig. 5(c) 39 . T comp increases monotonically with an increase of growth pressure up to 150 mTorr and then decreases.…”

“…The data presented here can be interpreted by realizing that the magnetic characteristics of LSMO films are strongly influenced by the number of Mn nearest neighbors (Z). For ideal bulk LSMO (thick films), the value of Z for Mn is 6 as each Mn is surrounded by an oxygen octahedral containing 6 oxygen ions 39 . To further explore the effect of oxygen growth pressure on the effective nearest neighbors (Z eff ), we have calculated Z eff from the molecular-field based equation:…”