J.N. Feng

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

Phase evaluation, magnetic, and electric properties of Mn60+xGa40−x (x = 0–15) ribbons

Mn60+xGa40−x (x = 0, 5, 10, and 15) ribbons were prepared by the melt-spinning technique and subsequently by a heat treatment at 673 K for 1 h. The magnetic phases Mn8Ga5, Mn1.86Ga, D019-Mn3Ga, and D022-Mn3Ga are found to appear in these annealed melt-spun ribbons. The Curie temperature TC of the ribbons varies from 125 K to 185 K as x changes from 0 to 5. For x = 10, the hexagonal structure of the D019-Mn3Ga phase is distorted to an orthorhombic one below the phase transition temperature Td = 185 K. The optimized values for coercivity at room temperature are 0.13, 4.4, 8.1, and 7.7 kOe for the ribbons with x = 0, 5, 10, and 15, respectively. The resistance measurements indicate that the ribbons show a typical metallic behavior for x = 5–15.

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 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

Exchange bias effect in epitaxial La0.67Ca0.33MnO3/SrMnO3 thin film structure

Bilayers consisting of La0.67Ca0.33MnO3 (LCMO) and SrMnO3 (SMO) have been prepared by pulsed-laser deposition on SrTiO3 (001) substrates. Unconventional magnetic coupling was found after cooling in a small field. The LCMO/SMO bilayers exhibit an exchange bias field of 209 Oe, which vanishes as the temperature rises above 90 K. A small magnetization has been found above the Curie temperature of the pure LCMO thin films. Spin-cluster-like antiferromagnetic (AFM)/ferromagnetic (FM) clusters have been deduced to exist at the interface due to the competing types of magnetic order at the interface. The magnetic relaxation is found to follow a double-exponential equation and a slow relaxation process is observed due to the strong exchange coupling between AFM/FM clusters and the LCMO layer. We speculate that the short-range high-temperature FM order of the Mn3+ and Mn4+ moments above the Curie temperature at the interface gives rise to the magnetic regions that pin the FM LCMO layer as the temperature decreases.

Strong effects of magnetic anisotropy on exchange coupling and magnetotransport properties of ferromagnetic/NiO/ferromagnetic trilayers

Strong effects of the magnetic anisotropy on the exchange coupling are observed in FM(1)/NiO(6 nm)/FM(2) trilayers with ferromagnetic (FM) layers Co or Fe. Different magnetic properties are found for Co/NiO/Fe and Fe/NiO/Co trilayers with Ag sublayer and cover layer. The Ag sublayer strongly affects the magnetic anisotropy of FM/antiferromagnetic (FM/AF) bilayers and further influences the exchange coupling in FM(1)/NiO/FM(2) trilayers. In particular, the sign of the magnetoresistance changes from negative after zero-field cooling to positive after field cooling, which is due to a reversal of the Co spin polarization. Furthermore, the interfacial coupling between FM and NiO enhances the blocking temperature of NiO

Large coercivity and unconventional exchange coupling in manganese-oxide-coated manganese-gallium nanoparticles

The microstructures and magnetic properties of nanoparticles, each composed of an antiferromagnetic (AFM) manganese-oxide shell and a ferromagnetic-like core of manganese—gallium (MnGa) compounds, are studied. The core-shell structure is confirmed by transmission electron microscope (TEM). The ferromagnetic-like core contains three kinds of MnGa binary compounds, i.e., ferrimagnetic (FI) D022-type Mn3Ga, ferromagnetic (FM) Mn8Ga5, and AFM D019-type Mn3Ga, of which the first two correspond respectively to a hard magnetic phase and to a soft one. Decoupling effect between these two phases is found at low temperature, which weakens gradually with increasing temperature and disappears above 200 K. The exchange bias (EB) effect is observed simultaneously, which is caused by the exchange coupling between the AFM shell and FM-like core. A large coercivity of 6.96 kOe (1 Oe = 79.5775 Am−1) and a maximum EB value of 0.45 kOe are achieved at 300 K and 200 K respectively.

Competing magnetic orders in La 0.67 Ca 0.33 MnO 3 /SrMnO 3 multilayers film

Epitaxial films of multilayers of La0.67Ca0.33MnO3/SrMnO3 (LCMO/SMO) and the reference single layer have been deposited by pulsed laser deposition on SrTiO3 (001) substrate allowing us to perform an exhaustive study of the magnetic coupling at the interface of G-type compensated antiferromagnetic (AFM) and double-exchange ferromagnetic (FM). An unexpected exchange bias (EB) effect at the compensated AFM interface under application of an in-plane cooling has been found. The LCMO/SMO multilayers exhibit an exchange bias field of 123 Oe, which vanishes as the temperature rises above 80 K Spin-cluster-like FM /AFM clusters have been deduced to exist at the interface due to the competing types of magnetic order at the interface. A small magnetization has been found above the Curie temperature of the pure LCMO thin films when applied a small field. We speculate that the short-range high-temperature FM order of the Mn3+ and Mn4+ moments above the Curie temperature at the interface gives rise to the magnetic regions that pin the FM LCMO layer as the temperature decreases.

Magnetic properties and coercivity mechanism of MnGa films

MnGa compounds possess several fascinating and useful properties, such as a large magnetocrys-talline anisotropy Ku, a high spin polarization P, a high Curie temperature TC and a flexible magnetization M. These results suggest that MnGa compounds are of potential candidates for spin-transfer-torque and rare-earth-free permanent magnet applications. There are two most interesting tetragonal phases, D022-Mn3Ga and L10-MnGa, among magnetically ordered phases of Mn-Ga binary alloys. Compared to the bulk-MnGa materials, it is easier to achieve a high coercivity (HC) in MnGa films, which is probably attributed to the suitable grain size and orientation. It is valuable to clarify the origin of magnetic properties of MnGa films, which are affected by different experimental conditions. In the present study, the D022-type and L10-type MnGa films with different thickness (10-50 nm) and underlayers are prepared on three kinds of substrates by magnetron sputtering. The magnetic properties of MnGa films are studied and the coecivity mechanism is discussed.