Magnetoresistance of magnetite thin films grown by pulsed laser deposition on GaAs(1 0 0) and Al2O3(0 0 0 1)

Magnetotransport properties of magnetite thin films deposited on gallium arsenide and sapphire substrates at growth temperatures between 473 and 673 K are presented. The films were grown by UV pulsed laser ablation in reactive atmospheres of O₂ and Ar, at working pressure of 8 × 10⁻² Pa. Film stoichiometry was determined in the range from Fe_2.95O₄ to Fe_2.97O₄. Randomly oriented polycrystalline thin films were grown on GaAs(1 0 0) while for the Al₂O₃(0 0 0 1) substrates the films developed a (1 1 1) preferred orientation. Interfacial Fe³⁺ diffusion was found for both substrates affecting the magnetic behaviour. The temperature dependence of the resistance and magnetoresistance of the films were measured for fields up to 6 T. Negative magnetoresistance values of ∼5% at room temperature and ∼10% at 90 K were obtained for the as-deposited magnetite films either on GaAs(1 0 0) or Al₂O₃(0 0 0 1).     

Large magnetoresistance induced by surface ferromagnetism in A-type antiferromagnetic La0.4Sr0.6MnO3 nanoparticles

Grain size effects on magnetic and transport properties for heavily Sr-doped A-type antiferromagnetic La_0.4Sr_0.6MnO₃ ceramics were studied. It was observed that with decrease in grain size, surface ferromagnetism could be introduced due to bond-breaking at surfaces. With decrease in grain size, the surface ferromagnetism was enhanced, and the phase transition order distinguished from the Arrott plot was a second one. The surface-induced ferromagnetism was insulating as judged from transport properties. With decrease in grain size, magnetoresistance was largely improved for both high magnetic and low magnetic fields. Under a 500 Oe magnetic field, the magnetoresistance is improved from 0.2%, 0.1%, 0.03% and 0.02% for the sample with grain size of 150 nm at 10, 100, 200 and 300 K, respectively, to 3%, 2.3%, 0.43% and 0.12% for the sample with grain size of 20 nm at 10, 100, 200 and 300 K. It was interesting to find that large magnetoresistance could be induced due to the surface ferromagnetism in A-type antiferromagnetic La_0.4Sr_0.6MnO₃ nanoparticles, which suggested that it was possible to search for manganites with relatively high low-field magnetoresistance in nanostructured A-type antiferromagnetic materials.     

Giant magnetoresistance in Co–Al2O3 granular films prepared by self-organized growth

Co–Al₂O₃ granular films with a narrow distribution in cluster size of Co clusters embedded in Al₂O₃ matrix were prepared by sequential deposition based on self-organized growth. Resistivity dependence of giant magnetoresistance (GMR) was studied. The GMR takes a maximum of 5.2% at room temperature and 9.4% at 13 K and 5700 Gs when the resistivity of the sample is 4×10⁵–7×10⁵ μΩ cm. The temperature dependence of resistivities and GMR were discussed especially. A temperature dependence of conductance ρ∼exp[T₁/(T+T₀)] was found, which indicates the dominant conduction mechanism is fluctuation-induced tunneling. A linear relationship of GMR versus T was observed, GMR=a–kT, in applied magnetic field 5700 Gs. The remarkable character of temperature dependence of GMR should be due to the special microstructure that the clusters are monodispersed in the films.     

Electrical transport and magnetic properties of perovskite-type electron-doped La–Pr–Mn–O epitaxial films

La_1−xPr_xMnO₃ (LPrMO) thin films have been epitaxially grown on (1 0 0)SrTiO₃ single-crystal substrates by pulsed-laser deposition. The films have a perovskite structure and give rise to the colossal magnetoresistance effect with the maximum magnetoresistance ratio of 10³% (at 240 K and 5 T). The electrical transport and magnetic properties have been investigated for the La_0.8Pr_0.2MnO₃ film with thickness 3000 Å. The results indicate that the films have quite a distinctive magnetotransport behavior compared to the bulk. The analysis of X-ray photoemission spectroscopy suggests that the valence state of Pr is 4+ in LPrMO film. Therefore, the epitaxial film is most likely an electron-doped colossal magnetoresistance system.     

Microfabrication and magnetoelectric properties of amorphous magnetic-tunnel-junctions with Co–Fe–B/Al–O/Co–Fe–B hardcore structure

Both single-barrier magnetic tunnel junctions (SBMTJs) and double-barrier magnetic tunnel junctions (DBMTJs) with an amorphous hardcore structure of Co₆₀Fe₂₀B₂₀/Al–O/Co₆₀Fe₂₀B₂₀ were microfabricated. A high TMR ratio of 102.2% at 4.2 K was observed in the SBMTJs after annealing at 265 °C for 1 h. High TMR ratio of 56.2%, low junction resistance-area product RS of 4.6 kΩ μm², small coercivity H_C=25 Oe, and relatively large bias-voltage-at-half-maximum TMR with the value V_1/2 greater than 500 mV at room temperature (RT) had been achieved in such Co–Fe–B SBMTJs. Whereas, high TMR ratio of 60% at RT and 89% at 30 K, low junction resistance-area product RS of 7.8 kΩ μm² at RT and 8.3 kΩ μm² at 30 K, low coercivity H_C=8.5 Oe at RT and H_C=14 Oe at 30 K, and relatively large bias-voltage-at-half-maximum TMR with the value V_1/2 greater than 1150 mV at RT had been achieved in the Co–Fe–B DBMTJs. Temperature dependence of the TMR ratio, resistance, and coercivity from 4.2 K to RT, and applied voltage dependence of the TMR ratio and resistance at RT for such amorphous MTJs were also investigated.     

Microstructure and magnetic properties of FePt:Ag nanocomposite films on SiO2/Si(1 0 0)

FePt:Ag nanocomposite films were prepared by pulsed filtered vacuum arc deposition system and subsequent rapid thermal annealing on SiO₂/Si(1 0 0) substrates. The microstructure and magnetic properties were investigated. A strong dependence of coercivity and ordering of the face-central tetragonal structure on both Ag concentration and annealing temperature was observed. With Ag concentration of 22% in atomic ratio, the coercivity got to 6.0 kOe with a grain size of 6.7 nm when annealing temperature was 400 °C.     

Low temperature magnetoresistance in La1.32Sr1.68Mn2O7 layered manganite under hydrostatic pressure

The La_1.32Sr_1.68Mn₂O₇ layered manganite system has been studied by the low temperature electrical resistance and magnetoresistance under hydrostatic pressure up to 25 kbar. We have observe both, a Curie temperature (T_C) and a metal-insulator transition (T_MI) at 118 K in the ambient pressure. The applied pressure shifts the T_MI to higher temperature values and induces a second metal-insulator transition (T²_MI) at 90 K, in the temperature dependence of resistivity measurements. Also, the pressure suppresses the peak resistance abruptly at T_C. When an external field of 5 T is applied, we have observed a large negative magnetoresistance of 300% at the transition temperature and a 128% at 4.5 K. However, the increased pressure decreases the magnetoresistance ratio gradually. When the pressure reaches its maximum available value of 25 kbar, the magnetoresistance ratio decreases at a rate of 1.3%/kbar. From our experimental results, the decrease of magnetoresistance ratio with pressure is explained by the pressure induced canted spin state which is not favor for the spin polarized intergrain tunneling in layered manganites.     

Magnetic and transport property studies of nanocrystalline ZnxFe3−xO4

The magnetic and transport properties of nanocrystalline Zn_xFe_3−xO₄ with x=0.0, 0.2, 0.4, 0.5, 0.6, 0.8 and 1.0, respectively, fabricated by the sol-gel method have been investigated. Large magnetoresistance (MR) was observed and found to be originated both from the tunneling of the spin-polarized electrons across the adjacent ferromagnetic grains and the scattering by the canted spins at the grain surface near the grain boundaries. It has been revealed that the MR for the Zn_xFe_3−xO₄ samples (x=0, 0.5 and 1.0) increases with the temperature decreasing from room temperature until a maximum is reached at around 55 K. Then a sharp drop occurs with the further decrease in temperature, regarded as a spin (cluster) glass transition. For the samples studied, a biggest low field (0.5 T) MR value of about 20% for x=0 at 55 K has been obtained. The mechanism of the MR behavior of the materials was discussed.     

3000% high-field magnetoresistance in super-lattices of CoFe nanoparticles

We report on magnetotransport measurements on millimeter-large super-lattices of CoFe nanoparticles surrounded by an organic layer. Electrical properties are typical of Coulomb blockade in three-dimensional arrays of nanoparticles. A large high-field magnetoresistance, reaching up to 3000%, is measured between 1.8 and 10 K. This exceeds by two orders of magnitude magnetoresistance values generally measured in arrays of 3d transition metal ferromagnetic nanoparticles. The magnetoresistance amplitude scales with the magnetic field/temperature ratio and displays an unusual exponential dependency with the applied voltage. The magnetoresistance abruptly disappears below 1.8 K. We propose that the magnetoresistance is due to some individual paramagnetic moments localized between the metallic cores of the nanoparticles, the origin of which is discussed.     

Enhanced magnetoresistance in La0.82Sr0.18MnO3-π-conjugated semiconducting polymer heterostructure

We report a large enhancement (∼90%) in magnetoresistance in La_0.82Sr_0.18MnO₃ (LSMO) layers by incorporating a π-conjugated semiconducting polymer layer in between them. The epitaxial LSMO layers were deposited by DC magnetron sputtering on SrTiO₃ single crystal substrates and have FM transition temperature (T_C)∼310 K. A semiconducting polymer poly(3-octylthiophene) (P3OT) layer was deposited over the epitaxial LSMO layer by solution dip coating technique and with subsequent deposition of another epitaxial LSMO layer, forming a LSMO-P3OT-LSMO heterostructure. The effect of P3OT incorporation on magnetotransport properties of this heterostructure has been examined in the temperature range 77-350 K. Large MR enhancement observed near room temperature in the FM regime is explained in terms of efficient magnetic field dependent carrier injection at LSMO/P3OT interface.     

Enhancement of magnetoresistances at room temperature in YSZ doping La0.67Sr0.33MnO3 system

The temperature dependence of the resistance of composite samples (1−x)La_0.67Sr_0.33MnO₃+xYSZ with different YSZ doping level x was investigated at magnetic fields 0-3 T, where YSZ represents yttria-stabilized zirconia. Results show that the YSZ dopant does not only adjust the metal-insulator transition temperature, but also increases the magnetoresistance effect. With increase of YSZ doping level for the range of x<2%, the metal-insulator transition temperature values T_P of the composites decrease, but T_P increases with increase of x further for the range of x>2%. Meanwhile, in the YSZ-doped composites, a broad metal-insulator transition temperature region was found at zero and low magnetic field, which results in an obvious enhanced magnetoresistance in the temperature range 10-350 K. Specially, a larger magnetoresistance value was observed at room temperature at 3 T, which is encouraging with regard to the potential application of magnetoresistance materials.     

Preparation and magnetoresistance of thin CrO2 films

Highly textured chromium dioxide (CrO₂) films have been deposited on Al₂O₃ single-crystal substrates by atmospheric pressure chemical vapor deposition method (CVD). X-ray diffraction patterns show that the CrO₂ films are (1 0 0)-oriented on Al₂O₃ (0 0 1) substrates, and are (1 0 1)-oriented on Al₂O₃ (0 1 2) substrates. Scanning electron microscopy images indicate that the (1 0 0)-oriented CrO₂ films grown on Al₂O₃ (0 0 1) substrates have smoother surface and better qualities than that grown on Al₂O₃ (0 1 2) substrate. At room temperature, the magnetoresistance of the (1 0 0)- and (1 0 1)-oriented CrO₂ films are nearly same, and both show a linear dependence on applied magnetic field. While at 80 K, the (1 0 1)-oriented CrO₂ films show a much larger magnetoresistance compared with the (1 0 0)-oriented CrO₂ films. The reasons are briefly discussed.     

Magnetic properties and structure of FePt/FeMn multilayers

A systematic study of the magnetic properties by ion beam sputter-deposition system, was conducted in conjunction with the structure of FePt/FeMn multilayers fabricated onto MgO(0 0 1) substrates. Both parallel and perpendicular exchange biases were observed in the multilayers and were found to decrease drastically, as the deposition temperature is higher than 350 °C, which is evidently due to the interdiffusion at the interface. The thickness dependence study shows that the perpendicular magnetic anisotropy observed in the multilayers originates from surface anisotropy, being consistent with the decrease of perpendicular magnetic anisotropy as the deposition temperature is increased. The difference between parallel and perpendicular blocking temperatures that was clearly observed, is possibly due to the spin canting out of plane at the interface.     

Seebeck and magnetoresistive effects of Ga-doped ZnO thin films prepared by RF magnetron sputtering

In this paper, Ga-doped ZnO (GZO) films were deposited on glass substrates at different substrate temperatures by RF magnetron sputtering. The effect of substrate temperature on the structural, surface morphological properties, Seebeck and magnetoresistive effects of GZO films was investigated. It is found that the GZO films are polycrystalline and preferentially in the [0 0 2] orientation, and the film deposited at 300 °C has an optimal crystal quality. Seebeck and magnetoresistive effects are apparently observed in GZO films. The thermoelectromotive forces are negative. Decreasing substrate temperature and annealing in N₂ flow can decrease carrier concentration. The absolute value of the Seebeck coefficient increases with decreasing carrier concentration. The maximal absolute value of Seebeck coefficient is 101.54 μV/K for the annealed samples deposited at the substrate temperature of 200 °C. The transverse magnetoresistance of GZO films is related to both the magnetic field intensity and the Hall mobility. The magnetoresistance increases almost linearly with magnetic field intensity, and the films deposited at higher substrate temperature have a stronger magnetoresistance under the same magnetic field, due to the larger Hall mobility.     

Exchange bias in Co nanoparticles embedded in an Mn matrix

Magnetic properties of Co nanoparticles of 1.8 nm diameter embedded in Mn and Ag matrices have been studied as a function of the volume fraction (VFF). While the Co nanoparticles in the Ag matrix show superparamagnetic behavior with T_B=9.5 K (1.5% VFF) and T_B=18.5 K (8.9% VFF), the Co nanoparticles in the antiferromagnetic Mn matrix show a transition peak at ∼65 K in the ZFC/FC susceptibility measurements, and an increase of the coercive fields at low temperature with respect to the Ag matrix. Exchange bias due to the interface exchange coupling between Co particles and the antiferromagnetic Mn matrix has also been studied. The exchange bias field (H_eb), observed for all Co/Mn samples below 40 K, decreases with decreasing volume fraction and with increasing temperature and depends on the field of cooling (H_fc). Exchange bias is accompanied by an increase of coercivity.     

Magnetic field dependence of conductance and conduction noise of an artificial grain boundary junction in a La0.67Ba0.33MnO3 epitaxial thin film

A single artificial grain boundary in La_0.67Ba_0.33MnO₃ (LBMO) thin film has been prepared by depositing the film on a bicrystal substrate using laser ablation technique. We investigated the magnetic field dependence of magnetoresistance and conductance-voltage characteristics of the grain boundary at 77 K. A decrease of nonlinearity of current-voltage characteristics was observed upon application of magnetic field. The results are explained by assuming the presence of two different types of parallel conducting channels (metallic and highly resistive) across the grain boundary. The analysis of the results reveals that the application of magnetic field suppresses magnetic disorders at the grain boundary region and increases metallic conduction channels across the grain boundary. The temperature dependence of the conduction noise of the bicrystal grain boundary was measured at 0 and 1.5 kG magnetic field and compared with a microbridge on the LBMO film having no grain boundary. The presence of the grain boundary was found to enhance noise by one order of magnitude. The noise of a bicrystal grain boundary showed a decrease in the presence of 1.5 kG magnetic field for T<210 K. This decrease of noise confirms that the application of a magnetic field induces more metallic channels across the grain boundary.     

Coercivity and squareness enhancement in ball-milled FeCo–MnO nanocomposites

FeCo–MnO nanocomposites were prepared by ball milling mixtures of Fe, Co and Mn, where MnO was obtained from the oxidation of Mn after or during the milling process. The coercivity and squareness of FeCo–MnO nanocomposites increase with increasing milling time. After annealing treatment, the coercivity and squareness increase and exhibit a maximum at 120 h milling time. The temperature dependence of the magnetic properties of FeCo–MnO was also studied and there is a distinct boundary at 120 K. The exchange-bias field and the coercivity decrease quickly with increasing temperature from 30 to 120 K. However, there are no shifted hysteresis loops and the coercivity decreases slightly with increasing temperature from 120 K to room temperature. The enhancement of the coercivity and squareness is mainly attributed to the exchange-bias effects and the reduced magnetic interactions between the FeCo particles by the efficient isolation in MnO matrix.     

Influence of heat treatment on the structure and magnetic properties of the Co/Zr and Co/Hf multilayer amorphous films

The influence of annealing on the structure and magnetic properties of amorphous Co/Zr and Co/Hf multilayer films was studied with particular attention to the dependence of the magnetic properties, thermal stability and crystallization process on layer composition and thickness. The temperature at which crystallization commences increases from 400 to 460 °C as the layer thickness d_Zr or d_Hf increases from 6 to 18 Å, and decreases from 450 to 400 °C as d_Co increases from 12 to 18 Å. Multilayers containing 19–60 at% Zr were studied. The specific magnetization was found to increase even below the temperature at which crystallization commences. Our data are compared with non-multilayer Co–Zr amorphous films and rapidly quenched metallic glasses.     

Bias voltage dependence of inverted magnetoresistance on the annealing temperature in MgO-based magnetic tunnel junctions

MgO-based magnetic tunnel junctions (MTJs) with a layer sequence Ir₂₂Mn₇₈ or Fe₅₀Mn₅₀ (10 nm)/CoFe (2 nm)/Ru (0.85 nm)/CoFeB (0.5?t<2 nm)/MgO (2.5 nm)/CoFeB (3 nm) have been fabricated. The bias voltage dependence of tunneling magnetoresistance (TMR) is given as a function of the annealing temperature for these MTJs, which shows the TMR ratio changes its sign from inverted to normal at a critical bias voltage (V_C) when an unbalanced synthetic antiferromagnetic stack CoFe/Ru/CoFeB is used. V_Cs change with the thickness of the pinned CoFeB and annealing temperature, which implies one can achieve different V_Cs by artificial control. The asymmetric V_C values suggest that a strong density-of-states modification occurs at bottom oxide/ferromagnet interface.     

CoPt alloy grown on the WSe2(0 0 0 1) van der Waals surface

The structural and magnetic properties of 3-nm-thick CoPt alloys grown on WSe₂(0 0 0 1) at various temperature are investigated. Deposition at room temperature leads to the formation of a chemically disordered fcc CoPt alloy with [1 1 1] orientation. Growth at elevated temperatures induces L1₀ chemical order starting at 470 K accompanied with an increase in grain size and a change in grain morphology. As a consequence of the [1 1 1] growth direction, the CoPt grains can adopt one of the three possible variants of the L1₀ phase with tetragonal c-axis tilted from the normal to the film plane direction at 54°. The average long-range order parameter is found to be 0.35(±0.05) and does not change with the increase in the deposition temperature from 570 to 730 K. This behavior might be related to Se segregation towards the growing facets and surface disorder effects promoted by a high surface-to-volume ratio. Magnetic studies reveal a superparamagnetic behavior for the films grown at 570 and 730 K in agreement with the film morphology and degree of chemical order. The measurements at 10 K reveal the orientation of the easy axis of the magnetization lying basically in the film plane.