T. K. Nath

Strain-dependent magnetic phase diagram of epitaxial La0.67Sr0.33MnO3 thin films

Effects of lattice strain on magnetic behavior of epitaxial La0.67Sr0.33MnO3 thin films grown by 90° off-axis sputtering have been studied. The size of epitaxial strain was varied using four different substrates, i.e., (001) LaAlO3, (001) SrTiO3, (001) La0.3Sr0.7Al0.65Ta0.35O9, and (110) NdGaO3. The observed magnetism of coherent epitaxial films grown on these substrates, particularly anisotropy and Curie temperature, exhibit strong correlations with lattice strains. Spin reorientation transitions have been observed. The dependence of Curie temperature on the bulk and Jahn–Teller strains has been determined.

Three-dimensional strain states and crystallographic domain structures of epitaxial colossal magnetoresistive La0.8Ca0.2MnO3 thin films

The evolution of three-dimensional strain states and crystallographic domain structures of epitaxial colossal magnetoresistive La0.8Ca0.2MnO3 films have been studied as a function of film thickness and lattice mismatch with two types of (001) substrates, SrTiO3 and LaAlO3. In-plane and out-of-plane lattice parameters and strain states of the films were measured directly using normal and grazing incidence x-ray diffraction techniques. The unit cell volume of the films is not conserved, and it exhibits a substrate-dependent variation with film thickness. Films grown on SrTiO3 substrates with thickness up to ∼250 A are strained coherently with a pure (001)T orientation normal to the surface. In contrast, films as thin as 100 A grown on LaAlO3 show partial relaxation with a (110)T texture. While thinner films have smoother surfaces and higher crystalline quality, strain relaxation in thicker films leads to mixed (001)T and (110)T textures, mosaic spread, and surface roughening. The magnetic and electrical transport properties, particularly Curie and peak resistivity temperatures, also show systematic variations with respect to film thickness.The evolution of three-dimensional strain states and crystallographic domain structures of epitaxial colossal magnetoresistive La0.8Ca0.2MnO3 films have been studied as a function of film thickness and lattice mismatch with two types of (001) substrates, SrTiO3 and LaAlO3. In-plane and out-of-plane lattice parameters and strain states of the films were measured directly using normal and grazing incidence x-ray diffraction techniques. The unit cell volume of the films is not conserved, and it exhibits a substrate-dependent variation with film thickness. Films grown on SrTiO3 substrates with thickness up to ∼250 A are strained coherently with a pure (001)T orientation normal to the surface. In contrast, films as thin as 100 A grown on LaAlO3 show partial relaxation with a (110)T texture. While thinner films have smoother surfaces and higher crystalline quality, strain relaxation in thicker films leads to mixed (001)T and (110)T textures, mosaic spread, and surface roughening. The magnetic and electrical tra...

Strain modification of epitaxial perovskite oxide thin films using structural transitions of ferroelectric BaTiO3 substrate

Effects of induced biaxial strain on the electrical transport and magnetic properties of epitaxial thin films of SrRuO3 and La0.67Sr0.33MnO3 by structural transitions of ferroelectric BaTiO3 substrates have been studied. Large jumps of electrical resistivity (∼5% in SrRuO3 and ∼12% in La0.67Sr0.33MnO3) and low field magnetization (∼70% in La0.67Sr0.33MnO3) have been observed in the films at the structural transition temperatures of BaTiO3 substrate. The hysteretic jumps are reproducible through many thermal cycles, and they can be attributed to strain effects induced by the substrate. The use of phase transitions of ferroelectric substrates to manipulate lattice strain of epitaxial thin film heterostructures can be a useful way to modify the properties of perovskite oxides.

Temperature dependence of solubility limits of transition metals (Co, Mn, Fe, and Ni) in ZnO nanoparticles

X-ray diffraction studies on bulk amount of chemically prepared nanocrystalline powder of Zn1−xTMxO (TM=Co, Mn, Fe, and Ni) show that the evolution of secondary phases (Co3O4, Mn3O4, Fe3O4, or NiO) along with the single phase Zn1−xTMxO strongly depend on growth temperature and doping concentration. The highest solubility limits of Co, Mn, Fe, and Ni in ZnO are 30%, 30%, 20%, and 3% (atomic weight), respectively. The magnetization measurement shows that the secondary phase formation reduces the magnetization of single phase Zn1−xTMxO, which may be the important clue that the secondary phase is not responsible for magnetism in Zn1−xTMxO.

Effect of three-dimensional strain states on magnetic anisotropy of La0.8Ca0.2MnO3 epitaxial thin films

Magnetic anisotropy of La0.8Ca0.2MnO3 (LCMO) epitaxial thin films grown on (001) SrTiO3 and LaAlO3 a substrates exhibits strong correlation with substrate-induced strain states as determined by normal and grazing incidence x-ray diffraction. In a 250 A thick LCMO (001)T film grown on SrTiO3 substrate, an in-plane biaxial magnetic anisotropy is observed, and it is accompanied by a substrate-induced in-plane biaxial tensile strain. In contrast, the observed magnetic easy axis for a 250 A (110)T film grown on LaAlO3 substrate is perpendicular to the film plane, and the corresponding in-plane strain is biaxial compressive. In both cases the magnetic easy axes are along the crystallographic directions under tensile strain, indicating the presence of a positive magnetostriction. In thicker films (∼4000 A) grown on both substrates that are nearly strain relaxed, the magnetic easy axis lies in the film plane along the [110] direction of the (001) substrate.

Microstructural and magnetic properties of ZnO:TM (TM=Co,Mn) diluted magnetic semiconducting nanoparticles

We have investigated the structural and the magnetic properties of 3d transition metal (TM) doped Zn1−xTMxO (TM=Co,Mn) diluted magnetic semiconducting nanoparticles for different doping concentrations (0⩽x⩽0.4) synthesized by chemical “pyrophoric reaction process.” From x-ray diffraction measurements the solubility limits of Co and Mn in ZnO nanoparticles are found to be strongly dependent on growth (calcinations) temperature (Tg). The highest solubility limit of both Co2+ and Mn2+ in ZnO at Tg∼300°C is found to be ∼30%. High resolution transmission electron microscopy studies show that Zn1−xTMxO particles are single crystalline of high quality with a wide particle size distribution in nanometric regime. The non-mean-field-like very strong concave nature of temperature dependent magnetization curves is observed at very low temperature in both the systems without showing any distinct magnetic transition. The magnetic behaviors of those Mn2+ and Co2+ doped ZnO semiconducting nanoparticles are observed to be...

Magnetotransport and magnetic domain structure in compressively strained colossal magnetoresistance films

We have studied the magnetoresistance (MR) of compressively strained La0.7Sr0.3MnO3 (LSMO) films in various magnetic states in order to understand the role of magnetic domain structure on magnetotransport. In thin films of LSMO on (100) LaAlO3, the perpendicular magnetic anisotropy results in perpendicularly magnetized domains with fine scale ∼200 nm domain subdivision, which we image directly at room temperature using magnetic force microscopy. The main MR effects can be understood in terms of bulk colossal MR and anisotropic MR. We also find evidence for a small domain wall contribution to the MR, which is an order of magnitude larger than expected from a double exchange model.

Effects of film thickness and lattice mismatch on strain states and magnetic properties of La0.8Ca0.2MnO3 thin films

The effects of strain relaxation on the crystallographic domain structure and on the magnetic and transport properties of epitaxial colossal magnetoresistive La0.8Ca0.2MnO3 (LCMO) thin films have been studied. LCMO films in the thickness range of 100–4000 A were grown on (001) SrTiO3 and (001) LaAlO3 substrates, which impose an in-plane tensile and an in-plane compressive biaxial stress in the films, respectively. On (001) SrTiO3 substrates, the films can be grown coherently up to a thickness ∼250 A, then strain relaxation occurs at a thickness of ∼500 A. In contrast, even the 100 A film grown on (001) LaAlO3 is partially relaxed, and the critical thickness for complete strain relaxation is ∼750 A. The very thin films (<250 A) show a pure (001)T normal orientation for growth on SrTiO3 and a pure (110)T texture for growth on LaAlO3. As thickness increases, the lattice strain relaxes, resulting in mixed (001)T and (110)T textures for growth on both substrates. Both the Curie and peak resistivity temperature...

Enhanced grain surface effect on magnetic properties of nanometric La0.7Ca0.3MnO3 manganite: Evidence of surface spin freezing of manganite nanoparticles

We have investigated the effect of nanometric grain size on magnetic properties of La0.7Ca0.3MnO3 nanoparticles having average particle size (Φ) of ∼17 nm. Temperature dependence of field-cooled (FC) and zero-FC (ZFC) dc magnetization indicate the existence of two different types of relaxation processes: a relatively high temperature regime where there is a broad maximum of the ZFC curve at T=Tmax (>40 K) and another is a relatively low temperature regime that is characterized by a sharp maximum at T=TS (≈40 K). We believe that the broad maximum at Tmax is associated with the blocking of core particle moments, whereas the sharp maximum at TS is related to the freezing of surface spins. Waiting time (tw) dependence of ZFC relaxation measurements at T=50 K show weak dependence of relaxation rate [S(t)] on tw and dM/d ln(t) following a logarithmic variation in time. These features strongly support superparamagnetic (SPM) blocking of core particle moments at Tmax. At T=20 K, S(t) attains a maximum at tw=1000 ...

Electronic structure and magnetism of the diluted magnetic semiconductor Fe-doped ZnO nanoparticles

We have studied the electronic structure of Fe-doped ZnO nanoparticles, which have been reported to show ferromagnetism at room temperature, by x-ray photoemission spectroscopy, resonant photoemission spectroscopy, x-ray absorption spectroscopy, and x-ray magnetic circular dichroism (XMCD). From the experimental and cluster-model calculation results, we find that Fe atoms are predominantly in the Fe3+ ionic state with mixture of a small amount of Fe2+ and that Fe3+ ions are dominant in the surface region of the nanoparticles. It is shown that the room temperature ferromagnetism in the Fe-doped ZnO nanoparticles primarily originated from the antiferromagnetic coupling between unequal amounts of Fe3+ ions occupying two sets of nonequivalent positions in the region of the XMCD probing depth of ∼2–3 nm.