S.Q. Guo

Tuning exchange bias in ferromagnetic/ferromagnetic/antiferromagnetic heterostructures [Pt/Co]/NiFe/NiO with in-plane and out-of-plane easy axes

In-plane exchange bias (EB) in [Pt/Co](n)/NiFe/NiO heterostructures with orthogonal easy axes is investigated. The reversible in-plane EB effect at the ferromagnetic (FM)/FM [Pt/Co](n)/NiFe interface allows one to manipulate the value and direction of the EB of the heterostructures, which can be induced by applying a magnetic field larger than the perpendicular anisotropy field of the [Pt/Co](n) multilayers. The difference between the EB of the heterostructures after field cooling and zero field cooling disappears at 120 K, which may originate from the exchange coupling at the FM/antiferromagnetic (AFM) NiFe/NiO interface. The NiFe thickness dependence of the bias field of the EB exhibits behavior similar to that in conventional FM/AFM bilayers. The EB can be maintained even at room temperature. (c) 2011 American Institute of Physics. [doi:10.1063/1.3553414]

Correlation between Tc and ns/m* in Bi2Sr2CaCu2O8+δ single crystals

Abstract The reversible magnetization measurements with applied field parallel to the c -axis were carried out for a series of Bi 2 Sr 2 CaCu 2 O 8+ δ (Bi2212) single crystals with different oxygen content varying from underdoped to overdoped states, from which the doping dependence of penetration depth and n s / m * can be obtained. It was firstly found in this system that with increasing oxygen content, the value of n s / m * increases in the underdoped region and reaches a maximum at the optimal doping, then decreases in the overdoped region. The observed deviation from the linear relationship of T c ∝ n s / m * in the underdoped region can be regarded to be due to the structural distortion and the disorder scattering, while the decrease of n s / m * in the overdoped region is suggested to be due to the phase-separation related pair breaking.

Exchange bias and its thermal stability in ferromagnetic/antiferromagnetic antidot arrays

The exchange bias (EB) effect and its thermal stability in nanoscale Co/NiO antidot arrays and sheet films have been investigated. The EB field H-E increases with increasing Co thickness (t(Co)) and reaches a maximum at t(Co) = 8 nm in the antidot arrays, whereas H-E decreases with t(Co) in the sheet films. Compared with the sheet films, H-E in the antidot arrays is either enhanced or decreased, depending on the thickness of the ferromagnetic Co layer, which is due to the three-dimensional effects in the antiferromagnetic NiO and ferromagnetic Co layers caused by the nanopores. A higher thermal stability is observed in the antidot arrays due to the out-of-plane anisotropy constant K-1 of the misaligned antiferromagnetic magnetization component

Kosterlitz-Thouless transition in an YBa2Cu3O7/PrBa2Cu3O7 superlattice

Abstract A systematic study of the electrical transport properties was carried out in a YBa2Cu3O7/PrBa2Cu3O7 (48 A/100 A) s uperlattice sample. The resistance above the temperature TKT showed an exponential inverse-square-root temperature dependence. Below the temperature TKT, the current-voltage characteristics in low-current limit obeyed a power law relationship, and the universal jump of the exponent πKR from 2 to 0 was observed around TKT. All these phenomena are consistent with the Kosterlitz-Thoul ess theory. The large KT transition parameter τ KT = (T c0 −T KT ) T KT implies an enhancement of two-dimensionality in the superlattice.

Exchange coupling in hard/soft-magnetic multilayer films with non-magnetic spacer layers

The exchange coupling in textured HM/NM/alpha-Fe/NM/HM multilayer films (HM = NdFeB or PrFeB hard magnetic layers; NM = nonmagnetic Mo, Cu, and Cr layer) is shown to be indirect and long-range. The influences of thickness of NM spacer layer and HM layer, the material of HM phase and NM spacer layers, and the texture of HM layer, on the effective critical correlation length (L-ex(eff)) and exchange-coupling between soft-magnetic (SM) and HM layers are investigated. A non-linear dependence of L-ex(eff) on the thickness of NM spacer layer is observed. Magnetostatic interaction may lead to the observed non-linear dependence

Exchange bias effect in NiO/NiFe2O4 nanocomposites

A series of (100-x)NiO/(x)NiFe(2)O(4) nanocomposites (x 0, 2.5, 5, 8.3, 12.5, 25) synthesized by a chemical coprecipitation method have been investigated. The exchange bias field H(E) of the nanocomposites reaches a maximum at x = 2.5, and then decreases with increasing x. The decrease of H(E) is attributed to the formation of isolated ferrimagnetic NiFe(2)O(4) clusters, which is confirmed by observation with the use of high resolution transmission electron microscopy. The temperature dependence of H(E) and the coercivity H(C) for pure NiO is different from those with other samples, which is due to the exchange coupling between the uncompensated antiferromagnetic core and disordered surface shell of NiO nanoparticles

Enhancing the perpendicular anisotropy of NdDyFeB films by Dy diffusion process

A large coercivity and anisotropy enhancement in perpendicular NdDyFeB (120 nm)/Dy (t(Dy)) films has been realized by a Dy grain-boundary diffusion process. The coercivity H-C and the ratio M-r/M-s reach their maxima at t(Dy) 50 nm, and the magnetic domain sizes increase with increasing t(Dy). The H-C and M-r/M-s increasing with t(Dy) is due to the enhancement of the anisotropy of (Nd, Dy)(2)Fe14B grains by Dy substitution for Nd. The coercivity mechanism is a nucleation-type mechanism. Dy and Nd elements coexist at grain boundaries, forming a (Nd, Dy)-rich phase, which may promote the nucleation of reversal domains

Magnetization behavior of lightly doped La2CuO4+δ system

Abstract We have prepared a series of single-phase La 2 CuO 4δ samples by careful heat treatment in air. The sample magnetization measurements were performed from 4.2 K to 300 K. It was found that the magnetization behavior is strongly dependent on the annealing processing. With increasing annealing time and taking proper routes, the temperature dependence of the magnetization M ( T ) changes from a significant broad antiferromagnetic (AFM) transition around 250 K followed by a paramagnetic feature to a sharp AFM transition followed by a diamagnetic effect at 34 K. A possible explanation for this phenomenon with phase separation and superconductivity is presented.

Superconductivity and ferroelectricity of Pb(Zr,Ti) O3/YBa2Cu3O7 integrated films

Abstract The superconductivity, ferroelectricity and dielectric properties of Pb(Zr,Ti)O 3 .YBa 2 Cu 3 O 7 integrated films fabricated by the DC/RF magnetron sputtering method have been investigated. The integrated films are homogeneous and epitaxially grown on the substrate. In a relatively broad band, from 120 to 1o kHz, the dielectric loss at 77 K was low. The PZT film exhibited saturation and remanence polarizations of 40 and 25 μC/cm 2 , respectively and a coercive field of 98 kV/cm. The zero resistance temperature T c0 did not decrease after an overlayer PZT was deposited onto the YBCO layer. But the T c0 was degraded when an ultrathin YBCO layer was used in the heterostructure and further degraded by ion milling.

Structural, transport and magnetic properties of PrBa2−xSrxCu3O7-δ

Abstract Structural, electrical transport and magnetic properties of polycrystalline samples of PrBa 2−x Sr x Cu 3 O 7-δ were investigated. The results were discussed based on the variation of hybridization between Pr ions and O2p of the CuO 2 planes induced by Sr doping.