W.J. Gong

Exchange couplings in magnetic films

Recent advances in the study of exchange couplings in magnetic films are introduced. To provide a comprehensive understanding of exchange coupling, we have designed different bilayers, trilayers and multilayers, such as anisotropic hard-/soft-magnetic multilayer films, ferromagnetic/antiferromagnetic/ferromagnetic trilayers, [Pt/Co]/NiFe/NiO heterostructures, Co/NiO and Co/NiO/Fe trilayers on an anodic aluminum oxide (AAO) template. The exchange-coupling interaction between soft- and hard-magnetic phases, interlayer and interfacial exchange couplings and magnetic and magnetotransport properties in these magnetic films have been investigated in detail by adjusting the magnetic anisotropy of ferromagnetic layers and by changing the thickness of the spacer layer, ferromagnetic layer, and antiferromagnetic layer. Some particular physical phenomena have been observed and explained.

Broadband microwave absorption of CoNi@C nanocapsules enhanced by dual dielectric relaxation and multiple magnetic resonances

Dual dielectric relaxation of the permittivity and multiple magnetic resonances of the permeability (including one natural resonance and two exchange resonance modes) are observed in CoNi@C nanocapsules in the same 5–17 GHz frequency range which leads to a better electromagnetic-wave absorption than earlier reported for nanocomposites. A reflection loss (RL) exceeding −25 dB is obtained in a wide frequency range of 5–17 GHz when an appropriate absorber thickness between 2 and 4.8 mm is chosen. For a 2 mm absorber layer, a RL value exceeding −10 dB is achieved in the broad frequency range 12–18 GHz, which covers the whole Ku-band.

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]

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

Temperature dependence of competition between interlayer and interfacial exchange couplings in ferromagnetic/antiferromagnetic/ferromagnetic trilayers

The competition between interlayer and interfacial exchange couplings is found to be temperature dependent in Co(3 nm)/AF/Fe(10 nm) trilayers with AF equivalent to antiferromagnetic NiO, Cr(2)O(3) or Cr. The temperature dependence in trilayers with AF insulating NiO or Cr(2)O(3) spacer layer differs from that with AF metallic Cr. In the insulator case, the enhancement in the interlayer exchange coupling and the reduction in interfacial exchange coupling with increasing temperature results in dominating interlayer exchange coupling at high temperature. In the metallic spacer case, both the couplings decrease with increasing temperature, resulting in decoupling at high temperatures.

Effects of anisotropy and spin-asymmetry of ferromagnetic materials in ferromagnetic/Cr2O3/ferromagnetic trilayers

Strong effects of ferromagnetic (FM) materials on the exchange coupling are observed at different temperatures in FM(1)(3 nm)/Cr(2)O(3)(6 nm)/FM(2)(10 nm) trilayers with FM equivalent to Co, Fe, or Ni(80)Fe(20). Changes of the anisotropy of FM and spin-asymmetry of the reflection coefficients for spin-up and spin-down electrons of FM contacted the antiferromagnetic layer influence the strength of interfacial and interlayer coupling of the trilayers. Thus, the reduction of the interfacial coupling and the enhancement of the interlayer coupling with increasing temperature result in quite different magnetic behavior of different trilayers.

Effect of antiferromagnetic layer thickness on exchange bias, training effect, and magnetotransport properties in ferromagnetic/antiferromagnetic antidot arrays

The effect of antiferromagnetic (AFM) layer on exchange bias (EB), training effect, and magnetotransport properties in ferromagnetic (FM) /AFM nanoscale antidot arrays and sheet films Ag(10 nm)/Co(8 nm)/NiO(tNiO)/Ag(5 nm) at 10 K is studied. The AFM layer thickness dependence of the EB field shows a peak at tNiO = 2 nm that is explained by using the random field model. The misalignment of magnetic moments in the three-dimensional antidot arrays causes smaller decrease of EB field compared with that in the sheet films for training effect. The anomalous magnetotransport properties, in particular positive magnetoresistance (MR) for antidot arrays but negative MR for sheet films are found. The training effect and magnetotransport properties are strongly affected by the three-dimensional spin-alignment effects in the antidot arrays.

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