Manoj K. Srivastava

Enhanced ferromagnetic and metal insulator transition in Sm0.55Sr0.45MnO3 thin films: Role of oxygen vacancy induced quenched disorder

Effect of quenched disorder (QD) caused by oxygen vacancy (OV) and substrate induced inhomogeneous compressive strain, on the magnetic and transport properties of oriented polycrystalline Sm0.55Sr0.45MnO3 thin films is investigated. QD is related intimately to the ordering/disordering of the OVs and controls the paramagnetic-ferromagnetic/insulator-metal transition. OV ordered films show enhanced TC/TIM∼165 K, which is depressed by oxygen annealing. OV disordering realized by quenching reduces TC/TIM. The first order IM transition observed in SSMO single crystals is transformed into nonhysteretic and continuous one in the OV ordered films. QD appears to be diluted by OV disorder/annihilation and results in stronger carrier localization.

Impact of strain on metamagnetic transitions in Sm0.5Sr0.5MnO3 thin films

Sm0.5Sr0.5MnO3 thin films were deposited by DC magnetron sputtering on LaAlO3 (LAO) and SrTiO3 (STO) substrates. The film on LAO, which is under compressive strain, undergoes paramagnetic–ferromagnetic (PM–FM) transition at TC ∼116 K and shows insulator-metal transition (IMT) at TIM ∼ 115 K. The film on STO is under tensile strain and has TC ∼ 112 K; and shows IMT at TIM ∼ 110 K. Around ∼80 K, the film on STO shows a broad peak in the resistivity that could be seen as the reentrant IMT due to appearance of a metamagnetic state, the presence of which is confirmed by the discontinuous irreversible jumps in the magnetic field dependent isothermal resistivity at T < 85 K. These signatures of the metamagnetic state are not seen in the film on LAO. The occurrence and absence of metamagnetic state in films on STO and LAO, respectively, has been explained in terms of the control of the competing FM metallic and antiferromagnetic–charge ordered insulating (AFM–COI) phases by the different strain states in these films.

Carrier localization and out of plane anisotropic magnetoresistance in Nd0.55−xSmxSr0.45MnO3 thin films

The impact of carrier localization on the anisotropic magnetoresistance (AMR) has been investigated in Nd0.55−xSmxSr0.45MnO3 (x = 0.00–0.45) thin films. The substitution of smaller Sm3+ cations for larger Nd3+ reduces the average radius of the A-site of the perovskite lattice and enhances size disorder. This quenched disorder results in (1) enhanced Jahn-Teller (J-T) distortion as suggested by decrease in the out of plane lattice constant, (2) stronger carrier localization as evidenced by enhancement in the activation energy of small polaron hopping, (3) decrease in the ferromagnetic (FM) and insulator-metal transition (IMT) temperatures (TC/TIM), (4) sharpening of the IMT, (5) increase in the low field MR, and (6) large enhancement in the AMR. The AMR-T curves of all the films show a maximum just below TIM. The peak AMR measured at small magnetic field of 4.2 kOe increases from −5% for x = 0.00 to −60% for x = 0.45. The enhancement in AMR has been explained in terms of the unquenching of the orbital angu...

Comparative study of magnetic and magnetotransport properties of Sm0.55Sr0.45MnO3 thin films grown on different substrates

Highly oriented polycrystalline Sm0.55Sr0.45MnO3 thin films (thickness ∼100 nm) deposited on LaAlO3 (LAO, (001)), SrTiO3 (STO, (001)) and (La0.18Sr0.82) (Al0.59Ta0.41)O3 (LSAT, (001)) single crystal substrates by ultrasonic nebulized spray pyrolysis have been studied. The out of plane lattice parameter (OPLP) of the film on LAO is slightly larger than that of the corresponding bulk. In contrast, the OPLP of the films on STO and LSAT are slightly smaller than the corresponding bulk value. This suggests that the film on LAO is under compressive strain while LSAT and STO are under tensile strain. The films on LAO and LSAT show simultaneous paramagnetic-ferromagnetic (PM–FM) and insulator-metal transition (IMT) temperature at TC/TIM ∼ 165 K and 130 K, respectively. The PM–FM and IM transition occur at TC ∼ 120 K and TIM ∼ 105 K, respectively in the film on STO substrate. At T < TC, the zero field cooled–field cooled (ZFC–FC) magnetization of all the films shows strong bifurcation. This suggests the presence o...

Low field anisotropic colossal magnetoresistance in Sm0.53Sr0.47MnO3 thin films

SSMO5347 thin films (thicknesses ~200 nm) were deposited by on-axis dc magnetron sputtering on the single crystal LSAT (001) substrates. These films are oriented along the out of plane c-direction. The ferromagnetic and insulator-metal transition occurs at 96 K and 91 K, respectively. The magnetization easy axis is observed to lie in the plane of the film while the magnetic hard axis is found to be along the normal to this. The magnetotransport of the SSMO films, which was measured as a function of angle (theta) between the magnetic field (H) and plane of the film, shows colossal anisotropy. Magnetoresistance (MR) decreases drastically as theta increases from 0 degree (H//easy axis) to 90 degree (H//hard axis). The out-of-plane anisotropic MR (AMR) is as high as 88 % at H=3.6 kOe and 78 K. The colossal anisotropy has been explained in terms of the magnetic anisotropies at play and the magnetic domain motion in applied magnetic field.

Impact of quenched disorder on magnetotransport properties in Nd0.55-xSmxSr0.45MnO3 thin films

The effect of size mismatch induced quenched disorder on magnetotransport properties has been studied in polycrystalline Nd0.55-xSmxSr0.45MnO3 (x=0.00, 0.05, 0.15, 0.25, 0.35, and 0.45) thin films (thickness ∼300nm) prepared on LaAlO3 [LAO (001)] single crystal substrates by chemical spray pyrolysis. X-ray diffraction (XRD) data reveals that the films have oriented growth along out-of-plane (OP) direction. The OP lattice parameter decreases on substitution of Sm3+ ions for Nd3+. The insulator-metal/PM-FM transition temperatures (TIM/TC) for all the compositions decrease systematically and the transitions get sharpened as the variance σ2 that measures the quenched disorder due to size mismatch. The in-plane and out-of-plane low field magnetoresistance (LFMR) are enhanced appreciably with increasing quenched disorder (σ2). The variation in peak magnetoresistance (maximum MR around TIM) with σ2 shows drastic change above σ2 = 0.008469A2(x = 0.25). The small polaron activation energy also increases appreciabl...

First order phase transition in Sm0.53Sr0.47MnO3 films

In the present work Sm0.53Sr0.47MnO3 thin films were deposited on single crystal LAO (LaAlO3/(100)) substrates by DC magnetron sputtering. The θ-2 and ω-2θ scans show that these films are epitaxially grown and are under small compressive strain. The paramagnetic-ferromagnetic (insulator-metal) transition was found at TC∼126 K (at TIM∼128 K). The magnetic state at T<TC is akin to cluster glass, which is formed by the presence of charge ordered-antiferromagnetic clusters in the ferromagnetic matrix. The nature of the magnetic phase transition was investigated by applying the Banerjee criterion. The negative slope of the M2 – H/M curves and hysteresis in resistivity measured in the cooling and warming cycle establishes the first order nature of the phase transition.

Huge Anisotropic Magnetoresistance In Epitaxial Sm0.53Sr0.47MnO3 Thin Films

In the present work we have studied the variation of LFMR as a function of the angle (θ) between the applied magnetic field and the plane of the film. DC magnetron sputtering was used for film preparation on LSAT single crystal substrates from Sm0.55Sr0.45MnO3 targets synthesized by the solid state reaction route. X‐ray diffraction confirmed the epitaxial nature of these films. These films have simultaneous ferromagnetic and metal insulator transition at ∼91 K. These films exhibit enhanced low field colossal magnetoresistance ∼99% at 3.6 kOe at 80 K. Huge anisotropy is observed in the MR, which decreases from ∼80% to 6% as θ increases from 0 to 90°. The evolution of the resistance as a function of time and the magnetic field at a constant temperature (T≈77 K) has also been studied.

Impact of size mismatch induced quenched disorder on phase fluctuation and low field magnetotransport in polycrystalline Nd0.58−xGdxSr0.42MnO3

We report the magnetic and transport properties of polycrystalline Nd0.58−xGdxSr0.42MnO3 (x∼0.0, 0.04, 0.08, 0.12, 0.16, 0.20, 0.25, 0.30, 0.35, and 0.42). All the samples are single phase and have grain size ∼1–2 μm. As the variance σ2 increases, the paramagnetic-ferromagnetic transition shows a gradual decrease and broadening, while the decrease in insulator metal transition is sharpened. However, near equality of TC and TIM at intermediate values of σ2 could be understood in terms of the competing quenched and the grain boundary disorder. The variation in peak magnetoresistance (MR) (maximum MR around TC/TIM) with σ2 shows that maximum low field MR≈35% at H=3 kOe (68% at H=10 kOe) is centered around σ2=0.009 857 A2 (x=0.25). This shows that huge intrinsic MR can be obtained at relatively higher temperatures and lower magnetic fields in the region of enhanced phase fluctuations.