K. H. Chow

Magnetoresistive anisotropy and magnetoresistivity in strained La0.65Ca0.35MnO3 films near the metal-insulator transition

Magnetotransport properties of strained La0.65Ca0.35MnO3 thin films with thickness between 10 and 1000nm were investigated at temperatures near the metal-insulator transition and in magnetic fields up to 0.7Tesla. The reduction of the thin film thickness, which in turn produces an increasing epitaxial lattice strain, dramatically increases the “in-plane” magnetic anisotropy and the magnetoresistivity.

Dramatic strain induced modification of the low field anisotropic magnetoresistance in ultrathin manganite films

The authors investigated the anisotropic magnetoresistance (AMR) in ultrathin La-based manganite films grown on various substrates. It was found that depending on the strain state, the AMR in some of these systems exceeds 100% and can even change sign. These changes are very dramatic when compared to the few percent change in AMR in conventional ferromagnets. The mechanism behind these large changes in the AMR is discussed.

Local magnetic properties of a monolayer of Mn12 single molecule magnets.

The magnetic properties of a monolayer of Mn12 single molecule magnets grafted onto a silicon (Si) substrate have been investigated using depth-controlled beta-detected nuclear magnetic resonance. A low-energy beam of spin-polarized radioactive 8Li was used to probe the local static magnetic field distribution near the Mn12 monolayer in the Si substrate. The resonance line width varies strongly as a function of implantation depth as a result of the magnetic dipolar fields generated by the Mn12 electronic magnetic moments. The temperature dependence of the line width indicates that the magnetic properties of the Mn12 moments in this low-dimensional configuration differ from bulk Mn12.

Metal-insulator transition, specific heat, and grain-boundary-induced disorder in Sm0.55Sr0.45MnO3

The effects of the grain-boundary-induced lattice disorder on the resistivity, the magnetization, and the specific heat of a prototypical manganite Sm0.55Sr0.45MnO3 near half doping were investigated at temperatures near the metal-insulator transition. An increasing lattice disorder softens the magnetic phase transition from a first order phase transition into a second order transition. Furthermore, the peaks in the resistivity and specific heat are broaden and there is an increase in the charge-carrier scattering rates in the metallic and insulating states. The origin of these phenomena is discussed.

The Role of Ni-Mn Hybridization on the Martensitic Phase Transitions in Mn-rich Heusler Alloys

Room temperature x-ray diffraction, dc magnetization, and ac susceptibility measurements have been performed on a series of Mn rich Ni50Mn37-xCrxSb13 and Ni50+xMn37-xSb13 Heusler alloys. Depending on the value of x, the room temperature crystal structures of the samples are either L21 cubic or orthorhombic. It is a commonly accepted idea that the martensitic transition temperatures in Ni-Mn-Z (Z = Ga, In, Sb, Sn) based Heusler alloys decrease (increase) with decreasing (increasing) valence electron concentration, e/a. However, the present work shows that regardless of the change in e/a, the martensitic transition temperature (TM) decreases with increasing Cr or Ni concentration. These results support the model where, in the case of Mn rich Heusler alloys, it is the hybridization between the Ni atoms and the Mn atoms in the Z sites that plays the dominant role in driving the martensitic transformation.

Quench-disorder-controlled magnetoresistance in Sm0.55Sr0.45MnO3

The authors studied the low-field magnetoresistance of Sm1−xSrxMnO3 (x=0.45) which was sintered at different elevated temperatures followed by fast cooling (thermal quenching). Near the metal insulator transition the magnetoresistance is dramatically enhanced while the magnitude and width of the resistive peak are significantly reduced by increasing the sintering/quenching temperature, approaching the values observed in a single crystal of the same composition. They attribute these effects to an increase of the grain size in the system.

ANISOTROPIC MAGNETORESISTANCE IN PEROVSKITE MANGANITES

We have summarized some important results of the anisotropic transport properties of the prototypical manganite La1-xCaxMnO3 as well as a few others. The temperature dependence of anisotropic magnetoresistance in manganites exhibits a peak near the magnetic ordering temperature which differs dramatically from the ones in 3d ferromagnetic metals and alloys. Depending on the strain-driven orbital state, the AMR in manganites could be enhanced dramatically. The AMR in manganites is much larger than in ferromagnetic metals, where its magnitude is only of the order of a few percent. At low fields (below few kG) the intrinsic magnetocrystalline anisotropy becomes important and the dependence of resistance on angle changes to a more switching-like behavior. The magnitude of the switching is sufficiently large such that this phenomenon could be useful in spintronics for magnetic field sensing and nonvolatile memory applications. Moreover, at temperatures far below the metal insulator transition temperatures, the AMR in LCMO/LAO also flips and changes a sign in contrast to LCMO/STO films. Also sign flip takes place in charge and orbital ordered manganites exhibiting field induced ferromagnetism. Polycrystalline manganite samples also exhibit strong AMR. At lower temperatures the dependence of the AMR on temperature is dramatically different in polycrystalline samples than the epitaxial or single crystal samples. The behavior in the the polycrystalline samples could be governed by spin-polarized transport across the grain boundaries.

Nature of weak magnetism in SrTiO3/LaAlO3 multilayers.

We report the observation of weak magnetism in superlattices of LaAlO(3)/SrTiO(3) using β-detected nuclear magnetic resonance. The spin lattice relaxation rate of ^{8}Li in superlattices with a spacer layers of 8 and 6 unit cells of LaAlO(3) exhibits a strong peak near ~35 K, whereas no such peak is observed in a superlattice with spacer layer thickness of 3 unit cells. We attribute the observed temperature dependence to slowing down of weakly coupled electronic moments at the LaAlO(3)/SrTiO(3) interface. These results show that the magnetism at the interface depends strongly on the thickness of the spacer layer, and that a minimal thickness of ~4-6 unit cells is required for the appearance of magnetism. A simple model is used to determine that the observed relaxation is due to small fluctuating moments (~0.002μ(B)) in the two samples with a larger LaAlO(3) spacer thickness.

Near-Surface Structural Phase Transition of SrTiO~3 Studied with Zero-Field beta-Detected Nuclear Spin Relaxation and Resonance

We demonstrate that zero-field beta-detected nuclear quadrupole resonance and spin relaxation of low energy (8)Li can be used as a sensitive local probe of structural phase transitions near a surface. We find that the transition near the surface of a SrTiO(3) single crystal occurs at T(c) approximately 150K, i.e., approximately 45K higher than T(c)bulk, and that the tetragonal domains formed below T(c) are randomly oriented.

Enhancement of ferromagnetism by Cr doping in Ni-Mn-Cr-Sb Heusler alloys

A series of Mn rich Ni50Mn37−xCrxSb13 Heusler alloys have been investigated by dc magnetization and electrical resistivity measurements. Due to the weakening of the Ni-Mn hybridization, the martensitic transition shifts to lower temperatures with increasing Cr concentration, while the saturation magnetization at 5 K increases. The magnetoresistance and exchange bias properties are dramatically suppressed with increasing Cr concentration. The observed behaviors suggest that substitution of Cr for Mn in Ni50Mn37−xCrxSb13 Heusler alloys not only destabilizes the martensitic phase but also enhances ferromagnetism in the system. The possible mechanisms responsible for the observed behavior are discussed.