Z.W. Ouyang

Griffiths-like behavior in Ge-rich magnetocaloric compounds Gd5(SixGe1−x)4

The unusual low field magnetic behavior is revealed in the dc and ac magnetic susceptibility measurements of Gd5(SixGe1−x)4 (x<∼0.5). These anomalies can be ascribed to the presence of short range ferromagnetic correlations and ferromagnetic clustering in both antiferromagnetic and paramagnetic states. As a result, a novel triangular regime is discovered in the low field magnetic and crystallographic temperature-composition (T-x) phase diagram. The nature of paramagnetic state in this regime can be understood within the framework of Griffiths-like model, which not only originates from the disorder and the modification of competing interslab and intraslab magnetic exchange interactions induced by orthorhombic-monoclinic structural distortion but also correlates with the thermal history or initial magnetic disordering.

Size-dependent training effect in exchange coupled NiFe2O4/NiO nanogranular systems

The particle size dependent training effect has been investigated on the exchange coupled NiFe2O4/NiO nanogranular systems, with average particle size (DNFO) of NiFe2O4 ranging from ∼3 nm to ∼55 nm. For all samples, analysis of the field cycles (n) dependence on exchange bias fields (HEB) suggests the existence of two distinct forms of training mechanism during training procedure. One is related to an athermal contribution leading to the abrupt single cycle training, the other is the conventional thermal activation mechanism responsible for the gradual reduction of HEB during the subsequent cycles. With the increase of particle size, the relative change of HEB and enhanced coercivity (△HC) after training display a nonmonotonic size-scaling behavior and reaches the maxima for DNFO ∼ 22 nm. In this system, this largest reduction reveals the weakest dynamic stability of the interfacial exchange coupling energy during field cycle process. Moreover, different decay rate of HEB and ΔHC with field cycles are obs...

Anomalies of inverse direct current susceptibility in spin-chain compounds Ca3Co2-xMnxO6

Various types of anomalies of inverse dc susceptibility (H/M) have been observed in spin-chain compounds Ca3Co2-xMnxO6. For x = 1.0, the H/M curve exhibits a field-dependent downturn from the paramagnetic Curie-Weiss behavior, signaling the presence of a Griffiths-like cluster phase. A slight decrease of x leads to a rapid suppression of the Griffiths-like singularity, followed by an appearance of field-independent, non-Griffiths-like behavior at x = 0.92. This feature persists until x = 0.33, at and below which the H/M curve presents an unambiguous, field-independent upturn from the Curie-Weiss law. These Mn-doping-dependent anomalies of H/M curves are associated with complex competing magnetic interactions.

Al3+ doping effects and high-field phase diagram of La0.5Sr0.5Mn1−x Al x O3

Magnetization measurements of La0.5Sr0.5Mn1−x Al x O3 (0 ≤ x ≤ 0.25) under pulsed high magnetic fields up to 50 T have been carried out, in which the Al3+ ions doping and magnetic field effects on the charge-ordering/antiferromagnetic to ferromagnetic phase transitions have been discussed. A triple-phase diagram with the critical field, doping level and temperature has been determined, in which the antiferromagntic and ferromagnetic phase boundaries were clearly defined. The change from long-range charge-ordered/antiferromagnetic phases to the robust short-range ones upon the Al3+-doping was observed. According to the experimental results, we assume that Al3+ ion doping at the Mn sites dilutes the Mn3+-O-Mn4+ network, weakens the double-exchange interaction and further suppresses the FM phase (metallic conduction), which leads to the critical magnetic fields destroying the antiferromagnetic order increase with the increase of the doping level.

Spin-glass-like freezing in spin-chain compounds Ca3Co2−xMnxO6: Effect of disorder

We have illustrated the presence of spin-glass-like freezing in the Mn-doped spin-chain compounds Ca3Co2−xMnxO6, which was not reported previously. With increasing Mn composition, the magnetic relaxation due to spin freezing is gradually suppressed. Interestingly, the magnetization in the intermediate Mn compositions relaxes more slowly (characteristic time is significantly enhanced) than those in x = 0.0 and 1.0. This could be understood within the framework of disorder with randomness or imperfections, which plays an important role in the formation of spin-glass-like feature in intermediate Mn compositions, quite different from the cases in x = 0.0 with strong spin frustration and in x ∼ 1.0 with perfect Co/Mn ionic order.

Spin reorientation and spin-flop transition in multiferroic manganites Y1–xTbxMnO3 (x = 0, 0.1, 0.2) single crystals

We investigated the structure and magnetic properties of the multiferroic hexagonal manganite Y1−xTbxMnO3 (x = 0, 0.1, 0.2) single crystals. At 23 K, a Mn spin reorientation transition, which is not reported in the parent compound YMnO3, is observed in Y0.8Tb0.2MnO3. At a lower temperature, another new transition is observed in the doping system, which is attributed to the formation of long range antiferromagntic order of the doped Tb3+ moments. Based on the experimental results, we suggest that the effect of Tb doping is to bring about the increase of the Mn-O-Mn bond angle and the relief of the magnetic frustration. With increasing the doping level, for x = 0.2, when a magnetic field is applied parallel to the c axis, the field induced spin-flop transition is appeared, which indicates the reorientation of the Mn3+ moments along with the field-induced ferromagnetic ordering of the Tb3+ moments. These results suggest that the possibility of the Tb doping can change the magnetic structure and ferroelectric...

Effect of nonmagnetic Sc ion on the intrachain coupling in spin-chain compounds Ca3Co2−xScxO6

We have demonstrated that in Ca3Co2−xScxO6, substitution of nonmagnetic Sc3+ (S = 0) for high-spin Cotrig, even up to the solid solubility limit of x = 0.5−0.6, does not dramatically modify the intrachain ferromagnetic (FM) interaction. This is quite different from the cases of Ir- and Rh-doping, which much enhance the intrachain FM interaction, and the cases of Mn-, Fe-, and Cr-doping, which gradually reduce the intrachain FM interaction, simultaneously introducing antiferromagnetic interaction. The results of band-structure calculations of x = 0.5 are qualitatively consistent with our magnetization data.

High magnetic-field-induced phase transitions and refined magnetic phase diagram of Gd5Ge4

We have performed magnetization measurements of Gd5Ge4 with little or no ferromagnetic (FM) impurities under a pulsed magnetic field up to 40 T. The results reveal previously undetected transitions between 85 and 105 K, that is, a distinct second-order antiferromagnetic (AFM)-paramagnetic (PM) transition, followed by a first-order PM-ferromagnetic (FM) transition. The distinct variation of the slope of the M(H) curves is observed around the AFM-PM transition. Combining our high-field data and those reported earlier, a new magnetic field-temperature phase diagram, in which the AFM, PM, and FM states are well separated, is constructed.

Magnetic field-induced metamagnetic transitions of Pr0.5Ca0.5Mn0.97Ga0.03O3

Measurements of isothermal magnetization and electrical transport on Pr0.5Ca0.5Mn0.97Ga0.03O3 have been performed in a magnetic field up to 15 T. Two distinct metamagnetic transitions, which may relate to the collapse of the pseudo-CE-type and CE-type antiferromagnetic phases (CE, a composite antiferromagnetic structure composed of a chequeboard of alternating C and E type), have been observed in the temperature region of 2.5–140 K. Different from the traditional magnetization steps, the two metamagnetic transitions are not dependent on the field sweep rate. These peculiar features are sensitive to the microstructure of the sample. A temperature-field phase diagram has been constructed and found to exhibit a minimum critical field, similar to other phase-separated systems.

Antiferromagnetic ordering in spin-chain multiferroic Gd2BaNiO5 studied by electronic spin resonance

High-field electron spin resonance (ESR) has been employed to study the antiferromagnetic (AFM) ordering state (T < TN = 55 K) of spin-chain multiferroic Gd2BaNiO5. The spin reorientation at TSR = 24 K is well characterized by the temperature-dependent ESR spectra. The magnetization data evidence a field-induced spin-flop transition at 2 K. The frequency-field relationship of the ESR data can be explained by conventional AFM resonance theory with uniaxial anisotropy, in good agreement with magnetization data. Related discussion on zero-field spin gap is presented.