Field-induced ferromagnetic order and colossal magnetoresistance in La(1.2)Sr(1.8)Mn2O7: a 139La NMR study

In order to gain insights into the origin of colossal magnetoresistance (CMR) in manganese oxides, we performed a 139La NMR study in the double-layered compound La(1.2)Sr(1.8)Mn2O7. We find that above the Curie temperature T(C) = 126 K, applying a magnetic field induces a long-range ferromagnetic order that persists up to T = 330 K. The critical field at which the induced magnetic moment is saturated coincides with the field at which the CMR effect reaches a maximum. Our results therefore indicate that the CMR observed above T(C) in this compound is due to the field-induced ferromagnetism that produces a metallic state via the double exchange interaction.     

Field-induced ferromagnetic metallic state of bilayer manganite (La0.4Pr0.6)1.2Sr1.8Mn2O7: a polarized neutron diffraction study

Unpolarized and polarized neutron diffraction measurements have been carried out on the bilayer manganite (La0.4Pr0.6)1.2Sr1.8Mn2O7 which undergoes simultaneous semiconductor-metal paraferromagnetic transitions under magnetic field. Maximum entropy magnetization density reconstruction and multipole refinement on flipping ratios evidence the existence of two distinct field-induced states. The field-induced ferromagnetic state where the field is parallel to the c axis is characterized by the presence of magnetic moment on the Sr site of 0.48(2)mu(B), due to the Pr substitution. It also shows a high population of the d3(z(2)-r(2)) orbitals of Mn3+. For the field-induced state where the field is parallel to the a or b axes no magnetization density was found at the Sr site and the dx(2)(-y(2)) orbital is slightly more populated than the d3(z(2)-r(2)) one.     

Reentrant spin glass behavior in Cr-doped perovskite manganite

The magnetic and transport properties of the Cr-doped manganites La(0.46)Sr(0.54)Mn(1-y)Cr(y)O3 ( 0 < or = y < or = 0.08) with the A-type antiferromagnetic structure were investigated. Upon cooling, we find multiple magnetic phase transitions, i.e., paramagnetic, ferromagnetic (FM), antiferromagnetic (AFM), and spin glass in the y = 0.02 sample. The low temperature spin glass state is not a conventional spin glass with randomly oriented magnetic moments but has a reentrant character. The reentrant spin glass behavior accompanied by the anomalous multiple magnetic transitions might be due to the competing interactions between the FM phase and the A-type AFM matrix induced by the random Cr impurity.     

Steplike lattice deformation of single crystalline (La0.4Pr0.6)1.2Sr1.8Mn2O7 bilayered manganite

We report a steplike lattice transformation of single crystalline (La0.4Pr0.6)1.2Sr1.8Mn2O7 bilayered manganite accompanied by both magnetization and magnetoresistive jumps, and examine the ultrasharp nature of the field-induced first-order transition from a paramagnetic insulator to a ferromagnetic metal phase accompanied by a huge decrease in resistance. Our findings support that the abrupt magnetostriction is closely related to an orbital frustration existing in the inhomogeneous paramagnetic insulating phase rather than a martensitic scenario between competing two phases.     

Description of the Colossal Magnetoresistance of La 1.2 Sr 1.8 Mn 2 O 7 Based on the Spin-Polaron Conduction Mechanism in the Ferromagnetic Temperature Range

We have analyzed the resistance of La1.2Sr1.8Mn2(1 – z)O7 single crystal in magnetic fields from 0 to 90 kOe in the ferromagnetic temperature range. The observed magnetoresistance of La1.2Sr1.8Mn2O7 is described based on the spin-polaron conduction mechanism. The magnetoresistance is determined by the change in the sizes and magnetic moment directions of magnetic inhomogeneities (polarons). It is shown that the colossal magnetoresistance is ensured by an increase (along the magnetic field) of the polaron linear size. It is found using the method for separating the contributions of different conduction mechanisms to the magnetoresistance that the contribution to the magnetoresistance from the orientation mechanism at 80 K in low magnetic fields is close to 50%. With increasing magnetic field, this contribution decreases and becomes small in fields exceeding 30 kOe. The comparable contributions to the conductivity from the orientational and spin-polaron mechanisms unambiguously necessitate the inclusion of both conduction mechanisms in the magnetoresistance calculations. We have calculated the temperature variation of the polaron size (in relative units) in zero magnetic field and in a magnetic field of 90 kOe.     

Low temperature charge and orbital textures in La0.875Sr0.125MnO3

By using nuclear magnetic resonance techniques we show that for T<30 K the La0.875Sr0.125MnO3 compound displays a nonuniform charge distribution, comprised of two interconnected Mn ion subsystems with different spin, orbital, and charge couplings. The NMR results agree very well with the two spin wave stiffness constants observed at small q values in the spin wave dispersion curves [Phys. Rev. B 67, 214430 (2003)]. This picture is probably related to a yet undetermined charge and orbital superstructure occurring in the ferromagnetic insulating state of the La0.875Sr0.125MnO3 compound.     

Observation of magnetic ripple and nanowidth domains in a layered ferromagnet

We investigated ferromagnetic domain structures on nanometer to micrometer scale for single crystals of a layered ferromagnet, La(2-2x)Sr(1+2x)Mn2O7 (0.32 < or = x < or = 0.40), as functions of x and temperature by means of Lorentz electron microscopy. We have succeeded in observing the evolution of magnetic ripple structure, dynamically, related to a spin reorientation transition where the magnetization direction switches between parallel and perpendicular to the layers. Our high-resolution magnetic domain imaging revealed that the ripple state is characterized by the evolution of magnetic nanowidth domains.     

Nanomagnetic droplets and implications to orbital ordering in LA(1-x)Sr(x)CoO3

Inelastic cold-neutron scattering on LaCoO3 provided evidence for a distinct low energy excitation at 0.6 meV coincident with the thermally induced magnetic transition. Coexisting strong ferromagnetic (FM) and weaker antiferromagnetic correlations that are dynamic follow the activation to the excited state, identified as the intermediate S = 1 spin triplet. This is indicative of dynamical orbital ordering favoring the observed magnetic interactions. With hole doping as in La(1-x)Sr(x)CoO3 , the FM correlations between Co spins become static and isotropically distributed due to the formation of FM droplets. The correlation length and condensation temperature of these droplets increase rapidly with metallicity due to the double exchange mechanism.     

Intrinsic colossal magnetoresistance effect in thin-film Pr0.5Sr0.5MnO3 through dimensionality switching

A homogeneous colossal magnetoresistance (CMR) effect at low temperatures has been found in a thin-film perovskite manganite Pr0.5Sr0.5MnO3. The transition is driven not by the spin alignment as in usual CMR in bulk samples but by the localization-delocalization transition switched by the change in the effective dimensionality. Two-dimensional (x2-y2)-orbital ordering enhanced by the substrate strain is essential for the stabilization of the insulating localized state, which is on the verge of the first-order transition to the three-dimensional metallic ferromangetic state.     

Pressure-induced colossal piezoresistance effect and the collapse of the polaronic state in the bilayer manganite (La(0.4)Pr(0.6))(1.2)Sr(1.8)Mn2O7

We have investigated the effect of hydrostatic pressure as a function of temperature on the resistivity of a single crystal of the bilayer manganite (La(0.4)Pr(0.6))(1.2)Sr(1.8)Mn(2)O(7). Whereas a strong insulating behaviour is observed at all temperatures at ambient pressure, a clear transition into a metallic-like behaviour is induced when the sample is subjected to a pressure (P) of ~1.0 GPa at T < 70 K. A huge negative piezoresistance ~10(6) in the low temperature region at moderate pressures is observed. When the pressure is increased further (5.5 GPa), the high temperature polaronic state disappears and a metallic behaviour is observed. The insulator to metal transition temperature exponentially increases with pressure and the distinct peak in the resistivity that is observed at 1.0 GPa almost vanishes for P > 7.0 GPa. A modification in the orbital occupation of the e(g) electron between 3d(x(2)-y(2)) and 3d(z(2)-r(2)) states, as proposed earlier, leading to a ferromagnetic double-exchange phenomenon, can qualitatively account for our data.     

Photo-induced effect in the layered perovskite manganite La_(1.2)Sr_(1.8)Mn_(1.8)Co_(0.2)O_7

1IntroductionWiththediscoveryofcolossalmagnetoresistance(CMR)effectinmanganites,hole-dopingperovskitemanganiteswithunusualelectronictransportandmagneticpropertieshaveattractedconsiderableattention.Thesepropertiesresultfromanintrinsicinteractionbetweencharge,spin,orbitalandlatticedegreesoffreedomthatarestronglycoupledtoeachother[1—6].DoubleexchangemodelcombinedwithJohn-Tellereffectwasusedtoexplainthesepropertiespartly[7—9].InordertogetbetterunderstandingofthemechanismofCMReffect,externals…     

Persistent photoinduced magnetization in the coexisting spin-glass and ferromagnetic phases of Pr?.?Ca?.?MnO? thin film

The persistent photoinduced magnetization (PPM) in the low bandwidth material Pr(1-x)Ca(x)MnO? at the low hole doping level of x = 0.1 is reported. Upon zero-field cooling under photoexcitation, significant improvement of the ferromagnetic (FM) ordering was observed in the low temperature spin-glass phase. However, upon field cooling, the FM ordering was found to be suppressed due to weakening of the double-exchange interaction. High kinetic energy x-ray photoelectron spectroscopy measurements indicated a slight increase in the Mn3? peak under photoexcitation which clarifies the weakening of the FM interaction. The fast relaxation of the PPM is discussed in view of localization of spin polarons in sites of magnetic disorders and the results are compared with previous reports of PPM in intermediate bandwidth Pr?.?Ca?.?MnO? samples.     

Orbital domain state and finite size scaling in ferromagnetic insulating manganites

55Mn and 139La NMR measurements on a high quality single crystal of ferromagnetic (FM) La0.80Ca0.20MnO3 demonstrate the formation of localized Mn(3+,4+) states below 70 K, accompanied by a strong cooling-rate dependent increase of certain FM neutron Bragg peaks. (55,139)(1/T(1)) spin-lattice and (139)(1/T(2)) spin-spin relaxation rates are strongly enhanced on approaching this temperature from below, signaling a genuine phase transition at T(tr) approximately 70 K. The disappearance of the FM metallic signal by applying a weak external magnetic field, the different NMR radio-frequency enhancement of the FM metallic and insulating states, and the observed finite size scaling of T(tr) with Ca (hole) doping, as observed in powder La(1-x)CaxMnO3 samples, are suggestive of freezing into an inhomogeneous FM insulating and orbitally ordered state embodying "metallic" hole-rich walls.     

55Mn NMR investigation of electronic phase separation in La1-xCaxMnO3 for 0.2 < or = x < or = 0.5

55Mn NMR line shape measurements in La1-xCaxMnO3 for 0.20< or =x< or =0.50 provide experimental evidence about the existence of two distinct regions in the T-x magnetic phase diagram, where the homogeneous ferromagnetic (FM) metallic state is separated into FM metallic and FM insulating regions. These results are in agreement with recent theoretical predictions, which reveal a novel electronic phase separation in two FM states, providing orbital ordering and Jahn-Teller phonons are taken into consideration.     

Anomalous Change in the Size of a Spin Polaron in the Paramagnetic Temperature Range in La1.2Sr1.8Mn2O7

Physics of the Solid State - This study continues investigation of the magnetic and electrical properties of double perovskite La1.2Sr1.8Mn2O7, which has a colossal magnetoresistance (above 1200)...     

Effects of divalent alkaline earth ions on the magnetic and transport features of La0.65A0.35Mn0.95Fe0.05O3 (A = Ca, Sr, Pb, Ba) compounds

We have studied the magnetic and transport properties of Fe doped La_0.65A_0.35Mn_0.95Fe_0.05O₃ (A = Ca, Sr, Pb, Ba) manganites. All the compositions show ferromagnetic/metal to paramagnetic/insulator transition (T_C) except the Pb doped sample which is insulating and ferromagnetic (FM) in the entire temperature range. The magnetization and T_C are decreased by decreasing the cation size on La site. The transition temperature and magnetic moment at 77 K is a maximum for Sr doped sample and is decreasing if we increase or decrease the cation size from Sr size. The maximum value of T_C and magnetic moment for Sr based sample is most likely due to the closer ionic sizes of La and Sr as compared to the other dopants (Ca, Pb, and Ba). We observed a spin freezing type effect in the Pb doped sample below 120 K in resistivity, ac susceptibility and in magnetization. This suggests that the AFM interactions introduced by the Fe are most effective in the Pb doped composition leading to increased competition between the FM and AFM interactions. This FM and AFM interaction generates some degree of frustration leading to the appearance of spin glass like phase whose typical magnetic behavior is studied for small ion when the metallic like behavior is lost.     

Magnetic properties of the charge ordered Nd0.75Na0.25MnO3

Nd_0.75Na_0.25MnO₃ polycrystalline ceramic is prepared via sol-gel process and its magnetic properties and electron spin resonance (ESR) spectra have been investigated experimentally. As the compound is cooled from room temperature, a charge-ordered state first develops below 170 K. A high magnetic field melts the charge ordered state and stabilizes a ferromagnetic (FM) state below 170 K. A field induced transition, analogous to a spin flip transition, is observed between 40 and 170 K. The critical temperature for spin flip increases with increasing temperature. Below 130 K, the compound tends to be intrinsically inhomogeneous, i.e. FM clusters and paramagnetic domains coexist in this system at least, which is confirmed by ESR measurements. When the external magnetic field is zero, long range FM interaction is not developed in this system; however, a tendency of re-entrant FM transition is observed in this compound.     

Ferromagnetism and colossal magnetoresistance from phase competition

We report a multicomponent theory for the coexistence of charge ordering (CO), and antiferromagnetic (AFM) and ferromagnetic (FM) spin ordering. This kind of state is invoked for manganites by Moreo et al., Science 283, 2034 (1999) and observed in recent experiments. We show that doping an AFM or CO state always generates a FM component. FM, AFM, and CO necessarily coexist in a particle-hole asymmetric system. Melting of large AFM-CO orders by small magnetic fields and colossal magnetoresistance (CMR) arise whenever the CO and AFM order parameters have similar magnitude and momentum structure. Hole doping favors FM metallic states while electron doping favors AFM-CO states, as in CMR manganites.     

Half-Heusler合金Cu1-xFexMnSb的电子结构和反铁磁-铁磁相变

基于密度泛函理论（DFT）,使用局域密度近似（LDA）研究了Heusler合金Cu1-xFexMnSb的电子结构和反铁磁-铁磁相变。研究发现,两种磁状态下的合金晶格常数随掺杂浓度x变化很好地满足Vegard定理。当x>0.5时,铁磁态合金的总磁矩很好地符合SP规律,然而当x<0.5时,却发生了明显的偏离。由于整个体系存在RKKY和超交换磁耦合的竞争,因而在x=0.25时,我们观察到了独特的反铁磁—铁磁相变。进一步的态密度分析发现,Cu的掺杂浓度可以有效调整铁磁态合金的费米面位置,并且反铁磁态合金由于不同自旋方向的Mn原子的分波态密度相互补偿,总态密度形成了几乎完全对称的自旋向上带和自旋向下带。     

纳米材料中的巨磁电阻效应

纳米材料是指三维空间尺度中至少有一维处于纳米量级的材料,通常为１—１００ｎｍ,如纳米微粒,纳米线、管,纳米薄膜或其组合材料,近年来纳米材料中的巨磁电阻效应颇受人们青睐,本文将重点介绍颗粒膜,颗粒合金薄带,非连续多层膜,颗粒—薄膜混合型膜以及磁性隧道结的巨磁电阻效应。