Qing-An Li

Superparamagnetism and transport properties of ultrafine La2/3Ca1/3MnO3 powders

Magnetic and transport properties of ultrafine La2/3Ca1/3MnO3 (LCMO) powders synthesized by mechanical alloying with a grain size of about 18 nm have been investigated. It is found that the powder sample is superparamagnetic above 95 K. The blocking temperature (TB) obtained from the characteristic peak in the zero-field-cooled magnetization decreases with the measuring applied field, and can be well fitted by TB = aln (µ0H)-b. The spontaneous magnetization of the powder sample is only 38% of that in the corresponding bulk LCMO. Unlike conventional bulk LCMO, the powder compact is insulating from 5 K to 300 K. The low-temperature magnetoresistance (MR), defined as MR = ΔR/R(0) (R(0) being the zero-field resistance), is approximately 100% under a field of 6 T, much higher than that of the conventional bulk LCMO (50-60%). When the temperature is above TB, the low-field MR associated with the spin-polarized tunnelling between grains cannot be observed; however, the high-field MR increases linearly with the applied field. The small grain size and the surface layer in which many oxygen vacancies and defects induced by high-energy milling exist are suggested to be responsible for the present observations.

Observation of a Griffiths-like phase in bilayered manganites

The authors report the observation of a Griffiths-like phase in La2−2xSr1+2xMn2O7 (x=0.30, 0.33, and 0.40) and (La0.8Eu0.2)2−2xSr1+2xMn2O7 (x=0.33) single crystals by means of electron spin resonance, magnetic susceptibility, and magnetoresistance measurements. In the temperature region TC<T<350K, the resonance signal consists of a ferromagnetic resonance line and a paramagnetic resonance line, which suggests that the system is not in a pure paramagnetic phase. The ferromagnetic resonance signal disappears and the magnetic susceptibility starts to deviate from the Curie-Weiss law at the same temperature T*≈350K, independent of doping level and anisotropy. These results indicate that a Griffiths-like phase exists at TC<T<T* in bilayered manganites, which is due to intrinsic inhomogeneity caused by quenched disorder.

Large low-field magnetoresistance of phase-separated single-crystalline Pr0.7Pb0.3MnO3

A large low-field magnetoresistance (MR) slightly above the metal–insulator transition temperature (234 K) was observed in single-crystalline Pr0.7Pb0.3MnO3. Combining the temperature dependence of magnetization, resistance, and electron spin resonance spectra, it was suggested that phase separation occurs above the Curie temperature; ferromagnetic metallic clusters imbedding in the insulating paramagnetic matrix, and spin-polarized electron tunneling between isolated ferromagnetic clusters should be responsible for the large low-field MR observed. Undoubtedly, this observation opens a window to explore large low-field MR at high temperature, which is very important for the practical application of colossal MR effect.

Mössbauer effect probe of local Jahn-Teller distortion in Fe-doped colossal magnetoresistive manganites

The local structure of the Fe-doped La1−xCaxMnO3 (x=0.00–1.00) compounds has been investigated by means of Mossbauer spectroscopy. 57Fe Mossbauer spectra provide direct evidence of Jahn–Teller distortion in these manganites. On the basis of the Mossbauer results, the Jahn–Teller coupling was estimated. It is noteworthy that the Ca-concentration dependence of the Jahn–Teller coupling strength is very consistent with the magnetic phase diagram. Our results reveal that Mossbauer spectroscopy cannot only detect the local structural distortion, but also provide a technique to investigate the Jahn–Teller coupling of Fe-doped La1−xCaxMnO3 colossal magnetoresistive perovskites.

Mössbauer effect probe of field-induced magnetic phase transitionin LaFe13−xSix intermetallic compounds

Direct evidence of a field-induced magnetic phase transition in LaFe13−xSix intermetallics with a large magneticaloric effect was provided by Fe57 Mossbauer spectra in externally applied magnetic fields. Moreover, Mossbauer spectra demonstrate that a magnetic structure collinear to the applied field is abruptly achieved in LaFe11.7Si1.3 compound once the ferromagnetic state appears, showing a metamagnetic first-order phase transition. In the case of LaFe11.0Si2.0, the Fe magnetic moments rotate continuously from a random state to the collinear state with increasing applied field, showing that a second-order phase transition is predominant. The different types of phase transformation determine the magnetocaloric effects in response to temperature and field in these two samples.

Magnetic transition and large low-field magnetoresistance near Curie temperature in polycrystalline La2/3A1/3MnO3 (A=Ca,Sr)

By means of electron spin resonance, magnetization, and resistivity measurements, phase separation and a large low-field magnetoresistance (MR) accompanied by a first-order metamagnetic transition in the vicinity of Curie temperature (TC) were observed in polycrystalline La2/3Ca1/3MnO3, but were not observed in polycrystalline La2/3Sr1/3MnO3. It is obvious that the phase separation and large low-field MR near the TC are not universal features of the colossal MR materials. By combining the magnetic field dependence of magnetization and resistance, it was suggested that the first-order metamagnetic transition from the paramagnetic to ferromagnetic state induced by a magnetic field should be responsible for the large low-field MR observed in polycrystalline La2/3Ca1/3MnO3. Undoubtedly, this observation is very important for exploring large low-field MR in high temperature, which is necessary for practical application of the colossal magnetoresistance effect.

Low-temperature magnetization step and its training effects in phase-separated La0.5Ca0.5MnO3

We observed a magnetization step accompanied by a metal-insulator transition around 75 K in La0.5Ca0.5MnO3. Repeating measurements under the same condition weaken the magnetization step and enhance the resistance at low temperature. The decayed magnetization step reappears after annealing the sample at high temperature or cooling it under a magnetic field. The low-temperature magnetization step can be attributed to the melting of the overcooled ferromagnetic fragments and its training effect may be related to structure distortions at the interfaces between the ferromagnetic and charge-ordered phases in the investigated system.

Structure, magnetic properties and phase separation of Nd0.5Ca0.5Mn1-xGaxO3 (0 ≤ x ≤ 0.1)

Abstract Structure and magnetic properties of Nd 0.5 Ca 0.5 Mn 1− x Ga x O 3 (0⩽ x ⩽0.1) were investigated systematically through X-ray diffraction, magnetization and electron spin resonance (ESR) measurements. It was found that substituting Mn with Ga reduces the charge ordering temperature ( T CO ) and induces an additional peak in magnetization–temperature curve at low temperature. Both the temperature and magnetic field dependence of the magnetization in zero-field-cooling and field-cooling modes suggest that, in the Ga-doped samples, spin-glass-like phase induced by Ga-doping and antiferromagnetic charge-orbital-ordered (COO) phase coexist at low temperature. Furthermore, the ESR spectra indicate that, when the temperature is between the Neel temperature T N and the T CO , the paramagnetic phase and antiferromagnetic COO phase coexist and the volume fraction of the former increases with temperature at the expense of the latter. In addition, magnetization evidences for the interaction between Nd and Mn ions are present.

Direct observation of phase separation in La0.45Sr0.55MnO3−δ

We provide evidence of phase separation in La0.45Sr0.55MnO3−δ using electron spin resonance, magnetic force microscopy (MFM), x-ray diffraction, and magnetic and transport measurements. The results reveal that ferromagnetic and antiferromagnetic phase coexist at low temperature and that ferromagnetic and paramagnetic phases coexist in the temperature range between the Neel and the Curie temperature. Moreover, the size and shape of ferromagnetic phase (the minority phase) in the sample were observed directly by MFM. From these results, we infer an electroneutral type phase separation, possibly resulting from a nonuniform distribution of oxygen vacancies, as opposed to charge segregation.

Low-field-induced magnetic entropy change in single-crystal Nd0.47Sr0.53MnO3

In this work, the discovery of a large low-field-induced magnetic entropy change in single-crystal Nd0.47Sr0.53MnO3 with A-type layered antiferromagnetic structure is reported. The magnetic entropy changes reach values of 11.0 and 11.5?J?kg?1?K?1 for field changes of 20?kOe along the ab-plane andc-axis, respectively. The large magnetic entropy change occurring near TN was attributed to a low-field-induced antiferromagnetic ferromagnetic phase transition. Our results provide a possibility for development of magnetic refrigerant substances that are operable with a permanent magnet rather than a superconducting one as the magnetic field source.