Field-induced ferromagnetic metallic state in the bilayer manganite (La0.4Pr0.6)1.2Sr1.8Mn2O7, probed by neutron scattering

The bilayer manganite La1.2Sr1.8Mn2O7 exhibits a phase transition from a paramagnetic insulating (PI) to a ferromagnetic metallic (FM) state with a colossal magnetoresistance (CMR) effect. Upon 60% Pr substitution, magnetic order and PI to FM transition are suppressed. Application of a moderate magnetic field restores an FM state with a CMR effect. Neutron scattering by a single crystal of (La0.4Pr0.6)1.2Sr1.8Mn2O7, under a magnetic field of 5 T, has revealed a long-range and homogeneous ferromagnetic order. In the PI phase, under zero field, correlated lattice polarons have been detected. At 28 K, under 5 T, the spin wave dispersion curve determines an in-plane isotropic spin wave stiffness constant of 146 meV A(2). So the magnetic field not only generates a homogeneous ferromagnetic ground state, but also restores a magnetic coupling characteristic of FM CMR manganites.     

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.     

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.     

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…     

Pressure tuning of magnetic interactions in layered (La0.6Nd0.4)1. 2Sr1.8Mn2O7 manganite

The effect of pressure of up to 9 GPa on the magnetic and structural properties of layered (La0.6Nd0.4)1.2Sr1.8Mn2O7 manganite has been investigated. Pressure shifts the Mn atoms from the center of each bilayer towards the adjacent bilayer. The observed shift of the Mn atoms involves mainly a charge redistribution within the d(3z(2)-r(2)) axial orbital. The decrease in the interbilayer spacing and the shift of the Mn atoms away from the center of each bilayer result in increased magnetic and electronic coupling between the adjacent bilayers along the c axis.     

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.     

Investigation of magnetic transitions through ultrasonic measurements in double-layered CMR manganite La1.2Sr1.8Mn2O7

A polycrystalline, double-layered, colossal magnetoresistive manganite La_1.2Sr_1.8Mn₂O₇ is synthesized by sol-gel process and its magnetic and ultrasonic properties were investigated in the temperature range 80–300 K. The sample has Curie temperature at 124 K, where the sample exhibits a transition from paramagnetic insulator to ferromagnetic metallic state. The longitudinal sound velocity measurements show a significant hardening of sound velocity below T_C, which may be attributed to the coupling between ferromagnetic spins and longitudinal acoustic phonons. The magnetization and ultrasonic studies reveal the presence of secondary transition at ≈ 260 K in this sample. The present sound velocity measurement results confirm the reliability of ultrasonic investigations as an independent tool to probe magnetic transitions in manganites.     

Highly degenerate canted spin structure in bilayer manganite La1.1Sr1.9Mn2O7

We have observed magnetic diffuse signals of the bilayer manganite La1.1Sr1.9Mn2O7 by neutron scattering. Even at the low temperature, 20 K, there is a diffuse signal, which is much stronger than that observed in the high temperatures, 144 and 288 K. This signal cannot be characterized by a bilayer structure as observed at 144 K nor by a monolayer structure as at 288 K, but by a high degeneracy of a canted spin structure. Thus, an observation of this diffuse signal is the direct evidence for the establishment of the canted spin structure in La1.1Sr1.9Mn2O7.     

过渡金属离子掺杂对La1.2Ca1.8Mn2O7输运性质的影响

研究了过渡金属元素掺杂对层状钙钛矿结构锰氧化物La1.2Ca1.8Mn2O7体系输运性质的影响.实验结果表明不同过渡金属离子掺杂均导致体系的金属-绝缘转变温度降低,电阻率与最大磁电阻值(Cr除外)增加.当掺杂量较高时,磁性Cr、Fe、Co、Ni离子对体系性质的影响与其磁性相互作用密切相关.     

Room temperature magnetocaloric effect in Pr1.75Sr1.25Mn2O7 double-layered perovskite manganite system

Abstract

In this work, we have studied on double-layered perovskite (Ruddlesden–Popper) manganite structure in Pr_1.75Sr_1.25Mn₂O₇ synthesised by sol–gel method. The crystal structure of the double-layered perovskite is found as tetragonal from the X-ray diffraction analysis with I4/mmm space group. A high Curie temperature, T_C = 305 K is observed from the temperature dependence of magnetisation measurement. The isothermal magnetisation curves showed that magnetic phase transition is second order due to the positive slope of the Arrott plots. Maximum magnetic entropy change (ΔS_M) and adiabatic temperature change (ΔT_ad) values are calculated as 3.99 J kg^?1 K^?1 and 2.1 K under external magnetic field of 70 kOe, respectively. Since our double-layered perovskite manganite sample has desired T_C value and relatively high ΔS_M, it can be a potential candidate as a magnetocaloric material for room temperature magnetic cooling systems.     

Nonlinear optical spectroscopy of (La0.6Pr0.4)0.7Ca0.3MnO3 manganite

The electronic structure of doped orthorhombic (La_0.6Pr_0.4)_0.7Ca_0.3MnO₃ manganite is studied using the second optical harmonic generation method. The analysis of the frequency and polarization dependence of the intensity related to the second harmonic demonstrates that the applied magnetic field stimulates a transition of spin-polarized electrons from the d orbitals of Mn ions to the p orbitals of O₂ ligands. The manifestation of such transition in the orthorhombic RMnO₃ manganite with the distorted crystal lattice is attributed to the dipole-forbidden electronic transitions arising in the applied magnetic field. The mechanism of magnetooptical activity induced by the charge transfer between Mn and O ions is discussed.     

NMR on 55Mn in the ferromagnetic Ni-substituted manganite La0.6Pb0.4Mn0.86Ni0.14O3

NMR on ⁵⁵Mn in the single-crystal manganite La_0.6Pb_0.4Mn_0.86Ni_0.14O₃ (T _C =242 K), which exhibits metallic conductivity below T _C , is investigated in the temperature range 61–215 K. At low temperatures, together with a line corresponding to the averaged hyperfine field at the ⁵⁵Mn nuclei (the averaging is due to the motion of electronic holes along Mn sites), the NMR spectrum also contains two lines corresponding to localized states Mn⁴⁺ and Mn³⁺. In the temperature range 100–200 K it is found that the complicated NMR spectrum is transformed into a single line on account of a delocalization of the holes in the e _g orbitals of manganese. A comparison of the NMR data with the temperature dependence of the resistivity suggsets that a wide distribution of charge-carrier mobilities exists in the crystal. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 8, 522–527 (25 October 1999)     

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)...     

类钙钛矿化合物Ca（Mn2Cu1）Mn4O12的磁性与磁电阻效应

利用固相反应法制备了名义成分为Ca(Mn2Cu1)Mn4O12的类钙钛矿锰氧化物.x射线衍射表明，为了获得较为致密的样品和减小杂相含量，可以采用高温烧结再在1073K长时间空气中退火的制备方法.样品在低温下同时存在铁磁相和反铁磁相，由于反铁磁相的存在导致样品在4.5K时的磁化强度显著降低，并在8T的高磁场下仍未达到饱和.样品呈半导体导电性质，在85K和6T磁场下磁电阻比的最大值可达-46%. 关键词： ［AC3］(B4)O12类钙钛矿锰氧化物 庞磁电阻效应 铁磁性 反铁磁性     

类钙钛矿化合物Ca(Mn2 Cu1)Mn4 O12的磁性与磁电阻效应

利用固相反应法制备了名义成分为Ca(Mn2 Cu1 )Mn4 O1 2 的类钙钛矿锰氧化物 .x射线衍射表明 ,为了获得较为致密的样品和减小杂相含量 ,可以采用高温烧结再在 10 73K长时间空气中退火的制备方法 .样品在低温下同时存在铁磁相和反铁磁相 ,由于反铁磁相的存在导致样品在 4 5K时的磁化强度显著降低 ,并在 8T的高磁场下仍未达到饱和 .样品呈半导体导电性质 ,在 85K和 6T磁场下磁电阻比的最大值可达 - 4 6 % .     

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.     

Enhanced charge gap in the bilayer manganite La2-2xSr1+2xMn2O7 near x = 0.4

We have investigated the optical conductivity spectra of La2-2xSr1+2xMn2O7 (0.3相似文献   

Study of the e(g) orbitals in the bilayer manganite La(2--2x)Sr(1+2x)Mn(2)O(7) by using magnetic Compton-profile measurement.

From the magnetic Compton-profile (MCP) measurement, we have directly differentiated for the first time the populations in two e(g)-type orbitals ( x(2) - y(2) and 3z(2) - r(2)) in a manganite. The experimental MCP's along the [001] direction for La(2--2x)Sr(1+2x)Mn(2)O(7) at x = 0.35 and 0.42 are fitted by the theoretical profiles obtained from the (MnO(6))(8-) ab initio calculations. The calculation confirms that the MCP clearly detects the oxygen hybridization in the e(g) orbitals. The e(g) state is dominated by the x(2) - y(2)-type orbital with almost constant population, while the population in the 3z(2) - r(2)-type orbital decreases with increasing the hole concentration x.     

双层钙钛矿锰氧化物La1.2-xDyxSr1.8Mn2O7(x=0.05)多晶的磁性研究

多晶样品La1.2-xDyxSr1.8Mn2O7(x=0.05)采用传统固相反应法制备,通过测量样品的磁化强度与温度变化曲线(M-T)以及不同温度下磁化强度与外加磁场的变化曲线(M-H)对样品的磁性进行了研究。研究发现,在15 K-400 K的测量温度范围内,在类Griffiths相温度(TG≈360 K)以上,样品处于纯顺磁态;在奈尔温度(TN≈183 K)- TG范围内,系统处于反铁磁-铁磁共存态,存在类Griffiths相,随温度的降低,样品的铁磁性逐渐增强;在TN以下,随温度的降低,铁磁性逐渐减弱,反铁磁性增强,样品呈现出了团簇玻璃行为。另外,在居里温度Tc≈210 K附近,系统发生了二级相变,样品在7 T外加磁场下的最大磁熵变绝对值为0.6 J/(kg·K)。