Spin-valve behaviour of anti-ferromagnetic boundaries in ultrathin magnetite films

Magneto-resistance (MR) measurements on epitaxial Fe₃O₄ films grown on polished MgO have been performed. The measurements presented here are interpreted by a model that describes the MR behaviour as spin-polarised transport across anti-ferromagnetic (AF) interfaces. The Fe₃O₄ films consist of structural domains, separated by anti-phase boundaries where an AF coupling is present. These AF interfaces enhance the resistance of the films. Upon application of a magnetic field the AF-spins rotate towards each other and the resistance decreases. The AF interfaces are thus behaving as spin-valves. In agreement with the model, the observed magneto-resistance is negative and shows linear and quadratic field dependence up to the anisotropy field for fields applied parallel and perpendicular to the film plane respectively. Above the anisotropy field, the slopes of the two MR curves are expected to be equal, which is observed at 60 K. Above the Verwey transition, the shape of the normalised MR curves is independent of temperature. Below the Verwey transition the MR curve becomes more linear with decreasing temperature. A large difference between parallel and perpendicular MR is observed at the Verwey transition.     

Effect of manganese stoichiometry at B-site on magneto-transport and magnetic properties of La0.67Sr0.33MnO3manganites

To study the effect of manganese non-stoichiometry at B-site, a series of manganites with compositional formula La_0.67Sr_0.33Mn_1±xO₃ (where x = 0, 0.05 and 0.1) was synthesized by oxalate precursor method. X-ray diffraction data confirm the rhombohedral structure of La_0.67Sr_0.33Mn_1±xO₃ along with minor phases of Mn₃O₄. The average grain size is found to be 266 nm for x = 0 whereas its magnitude decreases with excess or deficiency in manganese concentration. An increase in the manganese non-stoichiometry leads to the coexistence of ferromagnetic and antiferromagnetic interactions. The effect of Mn_1±x on the magnetotransport properties could be understood on the basis of collective behaviour of magnetic spins, double exchange mechanism and ratio of Mn⁴⁺/Mn³⁺ ions. A crossover from negative to positive magnetoresistance behavior above metal-insulator transition temperature was observed for LSP-0.95 sample, whereas a positive magnetoresistance over the entire temperature region was noticed for LSP-1.10 sample.     

The effect of particle size on structural,magnetic and transport properties of La0.7Sr0.3MnO3 nanoparticles

This paper reports the synthesis of different particle size La_0.7Sr_0.3MnO₃ (LSMO) nanoparticles using non-aqueous sol gel synthesis route by calcination at temperatures 750 °C, 850 °C and 950 °C. In the present work, the effect of particle size of LSMO nanoparticles on its structural, magnetic and transport properties has been studied in detail. The X-ray diffraction analysis confirms the formation of LSMO nanoparticles having rhombohedral ($R\stackrel{?}{3}c$) structure with average particle size of 20 nm, 22.5 nm and 25.6 nm. An increase in magnetization and decrease in coercivity with increase in particle size is attributed to the magnetically disordered surface layer. The bifurcation in ZFC-FC magnetization indicates the possibility of spin glass like behavior of the LSMO nanoparticles. The effect of particle size on the resistivity and magnetoresistance were studied by using different conduction mechanism for different temperature regions. The upturn in the ρ-T curve at lower temperatures was explained by using Kondo-like transport mechanism. The maximum LFMR achieved was 32.3% at a field of 1 T at 10 K for 20 nm LSMO nanoparticle.     

How to find magneto-resistance of TVO temperature sensors in the range 0.1-10 K

An empirical method is presented to calculate magneto-resistance (MR) and a corresponding temperature shift of the well known TVO temperature sensors in the ranges from 0.1 K to approximately 10 K and up to 8 T. Features of MR, depending on the temperature and magnetic field, are discussed. The maximum positive MR-values, depending on the magnetic field, correspond to the temperature of T ≈ 0.67 K: 12.5% at 8 T. At the fixed magnetic field the zero MR-value is reached both by increasing or decreasing the temperatures with respect to 0.67 K. For example, at the magnetic field of 4 T, the zero MR-value appears at T ≈ 0.2 K and T ≈ 6 K. A constant moderate negative relative MR of 19%, which does not depend on the magnetic field from 2 to 8 T, is revealed at T ≈ 0.1 K. In the ranges from 1.5 to 4.4 K and from 0 to 9 T, in particular, the temperature shift has a linear dependence on B-value and inverse proportionality on the dimensionless sensitivity S. A brief explanation of the behavior of TVO sensors under magnetic fields is presented as well.     

Transparent, conducting, and ferromagnetic multilayer films based on ZnO/Fe3O4 by pulsed laser deposition technique

Multilayer thin films based on zinc oxide (ZnO) and iron oxide (Fe₃O₄) are fabricated using pulsed laser deposition method. The structural, electrical, and magnetic properties of these multilayer films are studied. X-ray diffraction study shows that ZnO film is highly oriented along (002) direction, while Fe₃O₄ film has preferred orientation along (222) direction. These films are transparent, conducting, and ferromagnetic at room temperature. The temperature dependence of resistance measurement shows semiconducting nature and charge transportation in these films is due to tunneling. Negative magneto-resistance of 0.28% is observed at room temperature.     

巨磁阻传感器在管道漏磁检测中的应用

磁性传感器在无损检测中已经有了很大的应用，与传统的磁性传感器相比，巨磁阻传感器有着灵敏度高、可靠性好、测量范围宽、体积小，价格低等优点。介绍了巨磁阻传感器及其在输油管道缺陷漏磁信号检测中的应用，表明采用巨磁阻传感器的检测系统可以检测到管道壁上的微小缺陷。     

On the validity of two-current model for systems with strongly spin-dependent disorder

The resistivities of the ferromagnetic alloy systems Fe-Ni and Co-Ni were studied in detail by application of the Kubo-Greenwood formalism. The electronic structure of the randomly disordered ferromagnetic alloys was computed by use of the spin-polarized Korringa-Kohn-Rostoker coherent potential approximation (KKR-CPA) method. Two sets of calculations were carried out: one fully relativistic and another one based on the two-current model. The former one will judge whether the two, not directly coupled spin-subsystems, could lead to the spontaneous magnetoresistance anisotropy, as it is supposed within the two-current model. Furthermore, all the results obtained are compared with the experimental data. We found that the two-current model calculations yield spin-resolved resistivities more polarized than could be expected from the experiments. Finally they lead to a much lower total resistivity than the relativistic calculations, showing that the scattering processes between the spin-systems are of crucial importance.     

Magneto-electrical transport in V-patterned La0.7Sr0.3MnO3 nanostructures

We investigate the electrical transport and magnetic field dependence of nano-patterned La_0.7Sr_0.3MnO₃ devices. We find that the resistivity versus temperature dependence is the same as that observed in thin films, indicating that our nano-patterning preserves the fundamental properties of the material. At temperatures below 20 K there is resistivity upturn of ~ 5 % in the smallest and thinnest device. Structures in a “V” pattern were fabricated in order to investigate domain wall resistance. We find a much smaller resistance area product as compared to previous reports observed in nanoconstrictions and also that the switching field matches that in micromagnetic simulations.     

Impact of Silver Addition on Room Temperature Magneto-Resistance in La0.7Ba0.3MnO3 (LBMO): Ag x ?( x =0.0,?0.1, 0.2, 0.3, 0.4)

La_0.7Ba_0.3MnO₃ (LBMO): Ag _x (x=0.0, 0.1, 0.2, 0.3, and 0.4) composites are synthesized by a solid-state reaction route, the final sintering temperatures are varied from 1300 °C (LBMO1300Ag) to 1400 °C (LBMO1400Ag), and their physical properties are compared as a function of temperature and Ag content. All samples are crystallized in single phase accompanied by some distortion in main structural phase peaks at higher angles with an increase in silver content. Though the lattice parameters (a,c) decrease, the b increases slightly with an increase in Ag content. The scanning electron micrographs (SEM) showed better grains morphology in terms of size and diffusion of grain boundaries with an increase in Ag content. In both LBMO1300Ag and LBMO1400Ag series, the metal insulator transition (T ^MI) and accompanied paramagnetic-ferromagnetic transition (T _C) temperatures are decreased with an increase in Ag content. The sharpness of MI transition, defined by temperature coefficient of resistance (TCR), is improved for Ag added samples. At a particular content of Ag(0.3), the T ^MI and T _C are tuned to 300 K and maximum magneto-resistance at 7 Tesla applied field (MR^{7 T}) of up to 55% is achieved at this temperature, which is more than double to that as observed for pure samples of the both 1300 and 1400 °C series at same temperature. The MR^{7 T} is further increased to above 60% for LBMOAg(0.4) samples, but is at 270 K. The MR^{7 T} is measured at varying temperatures of 5, 100, 200, 300, and 400 K in varying fields from ±7 Tesla, which exhibits U and V type shapes. Summarily, the addition of Ag in LBMO improves significantly the morphology of the grains and results in better physical properties of the parent manganite system.     

Measurement of helium temperatures by TVO-sensors under magnetic fields

Information about tests of arbitrary chosen 20 TVO resistive temperature sensors in the range from 1.5 to 5 K under magnetic fields up to 9 T is presented. Based on the obtained experimental data - more than 2000 points, the approximate and accurate methods to calculate the magneto-resistance and corresponding temperature shift due to the magnetic field are reported as well. As shown, TVO is a sensor with predictable characteristics within the investigated ranges.