Haibiao Zhou

Competition between ferromagnetic metallic and paramagnetic insulating phases in manganites

La0.67Ca0.33Mn1−xCuxO3 (x=0 and 0.15) epitaxial thin films were grown on the (100) LaAlO3 substrates, and the temperature dependence of their resistivity was measured in magnetic fields up to 12 T by a four-probe technique. We found that the competition between the ferromagnetic metallic (FM) and paramagnetic insulating (PI) phases plays an important role in the observed colossal magnetoresistance (CMR) effect. Based on a scenario that the doped manganites approximately consist of phase-separated FM and PI regions, a simple phenomenological model was proposed to describe the CMR effect. Using this model, we calculated the resistivity as functions of temperature and magnetic field. The model not only qualitatively accounts for some main features related to the CMR effect, but also quantitatively agrees with the experimental observations.

Absorption of microwaves in La1−xSrxMnO3 manganese powders over a wide bandwidth

We present the frequency dependence of microwave-absorbing properties of La1−xSrxMnO3 (x=0.4, 0.5, 0.6, and 0.7) powders at room temperature. The absorbing properties change gradually with x in the frequency range of 8–12 GHz. The optimal absorption can be achieved for a x=0.4 sample and its microwave loss peak value is about 25 dB. Further experimental results show that the absorption can be attributed to magnetic and dielectric losses and the microwave loss peak corresponds to the maximum dielectric loss tangent tanu200aδe near 10.5 GHz. Furthermore, the absorbing properties of the oxides mixed with carbonic fiber and Y-type planar hexagonal ferrite have been rudimentarily studied. Results show that these additives greatly enhance the microwave-absorbing properties of the oxides.

Ultrasonic study on charge ordering, magnetic, and structural changes in La0.25Ca0.75Mn0.93Cr0.07O3

The longitudinal and transverse ultrasonic velocities and attenuations, the transport and magnetic properties, as well as the structural changes of polycrystalline La0.25Ca0.75Mn0.93Cr0.07O3 have been studied systematically. A velocity minimum and an attenuation peak for both longitudinal and transverse waves occur around the temperature Tco of charge ordering transition accompanied by the structural change from a cubic to a tetragonal phase. Near temperature TN (where the c-axis length shows a maximum), the sharp decrease of velocities for both waves and the appearance of a longitudinal attenuation peak are attributed to the antiferromagnetic transition together with the anomalous change in c-axis length of the tetragonal phase. The simultaneous occurrence of the ultrasonic, lattice, charge ordering, magnetic, and transport anomalous characteristics indicates the strong electron–phonon and spin–phonon interactions in the system.

The effect of phase separation on charge ordering state in La1-xCaxMnO3 (x = 1/2, 2/3, and 3/4)

The magnetic phase separation characteristics are found in manganese perovskite La1/3Ca2/3MnO3 by electron spin resonance (ESR) and magnetization measurements. An extra resonance signal observed in ESR spectra just above the charge ordering (CO) temperature TCO provides strong evidence for the existence of ferromagnetic (FM) clusters near the CO state. The investigation of the resistivity of La12xCaxMnO3 (x ¼ 1/2, 2/3, and 3/4) in different magnetic fields up to 14 T shows that the effect of magnetic fields on CO state decreases with increasing x. Our results indicate that the percolative characteristics of the phase separation between FM clusters and CO state for x ¼ 1=2 and 2/3 samples are related to the magnetic field dependence of CO state. However, for x ¼ 3=4 it is assumed that there are no obvious FM clusters in the CO phase. q 2002 Published by Elsevier Science Ltd.

The Jahn-Teller effect and electron-phonon interaction in La0.25Ca0.75Mn1-xCrxO3

The ultrasonic (longitudinal and transverse) velocities and the transport and magnetic properties of polycrystalline La0.25Ca0.75Mn1-xCrxO3 (x = 0, 0.03, 0.05, and 0.07) have been studied systematically. It was found that with increasing Cr content, the resistivity increases, the charge-ordering transition temperature TCO shifts to low temperature, and the magnetic moment of the system is strengthened. From the temperature dependence of the ultrasonic velocities, one can establish that the Jahn-Teller energy and phonon exchange constant decrease with increasing Cr content.

A compact high field magnetic force microscope

We present the design and performance of a simple and compact magnetic force microscope (MFM), whose tip-sample coarse approach is implemented by the piezoelectric tube scanner (PTS) itself. In brief, a square rod shaft is axially spring-clamped on the inner wall of a metal tube which is glued inside the free end of the PTS. The shaft can thus be driven by the PTS to realize image scan and inertial stepping coarse approach. To enhance the inertial force, each of the four outer electrodes of the PTS is driven by an independent port of the controller. The MFM scan head is so compact that it can easily fit into the 52mm low temperature bore of a 20T superconducting magnet. The performance of the MFM is demonstrated by imaging a manganite thin film at low temperature and in magnetic fields up to 15T.

Evolution and control of the phase competition morphology in a manganite film

The competition among different phases in perovskite manganites is pronounced since their energies are very close under the interplay of charge, spin, orbital and lattice degrees of freedom. To reveal the roles of underlying interactions, many efforts have been devoted towards directly imaging phase transitions at microscopic scales. Here we show images of the charge-ordered insulator (COI) phase transition from a pure ferromagnetic metal with reducing field or increasing temperature in a strained phase-separated manganite film, using a home-built magnetic force microscope. Compared with the COI melting transition, this reverse transition is sharp, cooperative and martensitic-like with astonishingly unique yet diverse morphologies. The COI domains show variable-dimensional growth at different temperatures and their distribution can illustrate the delicate balance of the underlying interactions in manganites. Our findings also display how phase domain engineering is possible and how the phase competition can be tuned in a controllable manner.

Colossal anisotropic resistivity and oriented magnetic domains in strained La0.325Pr0.3Ca0.375MnO3 films

Magnetic and resistive anisotropies have been studied for the La0.325Pr0.3Ca0.375MnO3 films with different thicknesses grown on low symmetric (011)-oriented (LaAlO3)0.3(SrAl0.5Ta0.5O3)0.7 substrates. In the magnetic and electronic phase separation region, a colossal anisotropic resistivity (AR) of ∼105% and an anomalous large anisotropic magnetoresistance can be observed for 30 nm film. However, for 120 nm film, the maximum AR decreases significantly (∼2u2009×u2009103%) due to strain relaxation. The colossal AR is strongly associated with the oriented formation of magnetic domains, and the features of the strain effects are believed to be useful for the design of artificial materials and devices.

Raman spectrum and ESR of Pr0.5Ca0.4Sr0.1MnO3

Abstract The powder X-ray diffraction patterns, Raman spectra, magnetization, electron spin resonance (ESR), and resistivity for Pr 0.5 Ca 0.4 Sr 0.1 MnO 3 were studied. The Raman spectra change dramatically during the charge ordering transition, and the hardening of the 450xa0cm −1 mode of the spectra is due to the unit cell reduction of the antiferromagnetic phase. The two resonance signals in the first differentiated lines from ESR spectra suggest the coexistence of ferromagnetic and paramagnetic phases above the charge-ordered temperature, which is also confirmed by the insulator–metallic transition above the charge-ordered temperature induced by magnetic fields.

Visualization of Electronic Multiple Ordering and Its Dynamics in High Magnetic Field: Evidence of Electronic Multiple Ordering Crystals

Constituent atoms and electrons determine matter properties together, and they can form long-range ordering respectively. Distinguishing and isolating the electronic ordering out from the lattice crystal is a crucial issue in contemporary materials science. However, the intrinsic structure of a long-range electronic ordering is difficult to observe because it can be easily affected by many external factors. Here, we present the observation of electronic multiple ordering (EMO) and its dynamics at the micrometer scale in a manganite thin film. The strong internal couplings among multiple electronic degrees of freedom in the EMO make its morphology robust against external factors and visible via well-defined boundaries along specific axes and cleavage planes, which behave like a multiple-ordered electronic crystal. A strong magnetic field up to 17.6 T is needed to completely melt such EMO at 7 K, and the corresponding formation, motion, and annihilation dynamics are imaged utilizing a home-built high-field magnetic force microscope. The EMO is parasitic within the lattice crystal house, but its dynamics follows its own rules of electronic correlation, therefore becoming distinguishable and isolatable as the electronic ordering. Our work provides a microscopic foundation for the understanding and control of the electronic ordering and the designs of the corresponding devices.