Excellent Magnetic Softness Research Articles

Discontinuous spring magnet-type magnetostrictive Terfecohan/YFeCo multilayers: A novel nanostructured material principle for excellent magnetic softness

Novel physics and reversal mechanisms of the whole system switching (WS) and individual switching (IS) type are reported for hard/soft TbFeCo/YFeCo exchange-spring multilayers. The WS type usually occurs in multilayered systems, in which the magnetic anisotropy of hard TbFeCo layers is neglectable. For such a system, the ferrimagnetically coupled hard/soft multilayered state is recovered after removing applied fields from the magnetized state. At low negative fields, the magnetization switching occurs collectively for all magnetic moments in the whole system. In this case, the low-coercivity mechanism is discussed on the basis of a hard/soft interfacial point contact. This configuration is realized for TbFeCo/YFeCo discontinuous exchange-spring multilayers, in which the magnetic (Fe,Co) nanograins coexist with non-magnetic amorphous phase in the soft layers. In this state, a magnetic coercivity as small as 0.4 mT is achieved. It is considered as an excellent magnetic softness of rare-earth-based systems. Enhancing the magnetic anisotropy in the hard TbFeCo layers, the magnetization switching follows the IS type at low temperatures. Starting to decrease the applied magnetic field from the high-field state, one observes the first reversal of the magnetic moments in the soft high-magnetization YFeCo-layers in positive magnetic fields. This is the reason for the observation of the negative coercivity as well as negative-biasing phenomena.

Magnetic and microwave properties of CoFe∕PtMn∕CoFe multilayer films

CoFe ∕ PtMn ∕ CoFe films were deposited on seed layers of Ru or NiFeCr with CoFe film compositions being either Co-10at.%Fe or Co-16at.%Fe. Eight periods of the CoFe∕PtMn∕CoFe trilayers were also prepared. The magnetic properties and ferromagnetic resonance (FMR) of these films were characterized with vibrating-sample magnetometer, and field-sweep FMR system at X band (∼9.5GHz). The Ru-seeded CoFe∕PtMn∕CoFe sandwich films show excellent magnetic softness with a low hard axis coercivity of 2–4Oe, an easy axis Mr∕Ms of >98%, and a significantly enhanced in-plane anisotropy of 57–123Oe when CoFe layer thickness is above 200Å. Contrary to what was observed in the ferromagnetic/antiferromagnetic bilayer systems that have reduced FMR linewidth with the increase of film thickness, the CoFe∕PtMn∕CoFe trilayers with Ru seed layer show a minimum FMR linewidth of 45Oe at an intermediate CoFe layer thickness of 300Å at ∼9.5GHz.

Novel exchange-spring configuration for excellent magnetic and magnetostrictive softness

Magnetization and magnetostriction data are reported for discontinuous type exchange-spring Tb(Fe 0.55Co 0.45) 1.5/(Y 0.1Fe 0.9) multilayers, in which nanograins coexist with amorphous phase in soft YFe layers. This novel exchange-spring configuration exhibits an excellent magnetic and magnetostrictive softness: low magnetic coercivity ( μ 0 H C = 1 mT ) , large magnetostriction ( λ γ , 2 = 720 × 1 0 - 6 ) and large parallel magnetostrictive susceptibility ( χ λ | | = d λ | | / d μ 0 H = 29.7 × 1 0 - 2 T - 1 ) . In addition, the observed phenomena of the negative contribution to magnetostriction, the formation of the extended domain wall at the interfaces and the exchange-bias are discussed.

Magnetic properties and GMI of soft melt-extracted magnetic amorphous fibers

Co 79.6 Fe 4.6 Si 9.2 B 2.7 Mo 3.9 fibers have been produced by the modified process of melt-extraction. Hysteresis loop, magnetostriction constant, λ s and giant magneto-impedance (GMI) ratio (total impedance, real and imaginary contributions) of such fibers have been measured. Studied fibers exhibit excellent magnetic softness (coercivity about 20 A m −1 ), small positive magnetostriction constant ( λ s ≈1.13×10 −6 ) and GMI ratio of the order of 50% in as-cast state. Current annealing has been employed to tailor the magnetic properties and GMI effect of these fibers.

Domain structures of nanocrystalline Fe90Zr7B3 alloy studied by Lorentz microscopy

As-quenched Fe90Zr7B3 alloy has been crystallized at 773, 923 and 973 K, it is found that the specimens annealed at 773 and 923 K have grains with the same size, but the one annealed at 773 K has a thick amorphous matrix and a broad grain size distribution. Big domains and smooth domain walls are observed in the specimen annealed at the optimum condition (923 K), and excellent magnetic softness is obtained. On the other hand, the one annealed at 973 K has very large grains (40 nm). Very small domains with irregular walls are observed, indicating a weak intergranular exchange coupling.Through in situ Lorentz microscopy, a relaxation of internal stress in the specimen annealed at 773 K is observed at anelevated temperature (333 K). The internal stress observed is considered to be one of the important factors that degrade the soft magnetic properties. Based on the results of differential thermal analysis, a two-step annealing, where the as-quenched specimen is pre-heated at 723 K and subsequently annealed at 773 K, is utilized. The domain structure of the specimen treated by the two-step annealing is analyzed. It is found that the size of the domain is larger than that of the specimen annealed at 773 K, suggesting the possibility of control of the soft magnetic properties.

Controlling the induced anisotropy in soft magnetic films for high-frequency applications

Summary form only given. Soft magnetic materials with controllable uniaxial anisotropy are required for future ultrahigh frequency applications. Nowadays, the attention is focused on FeXN (X = Ta, Zr, Al). systems due to their high saturation magnetisation, excellent magnetic softness and good corrosion resistance. The films can be obtained in an as-deposited nanocrystalline state by sputtering Fe rich alloys in a reactive atmosphere (Ar + N/sub 2/). Such materials can be magnetically soft when the grain size becomes smaller than the ferromagnetic exchange length (as explained by the random anisotropy model). The anisotropy is created by a bias magnetic field applied during deposition or during a subsequent annealing. One limiting factor restricting the frequency range of applications is the natural ferromagnetic resonance. In order to push this limit in the GHz range, films with a uniaxial anisotropy field in excess of 20 Oe are desired. We have obtained films with excellent soft magnetic properties and controllable in plane uniaxial magnetic anisotropy by depositing Fe N films at different substrate temperatures. The induced in-plane anisotropy increases with increasing nitrogen content. Increasing the Zr/Fe ratio from 1/99 to 3/97 has little influence on the induced anisotropy.

Development of giant low-field magnetostriction in a-TerfecoHan-based single layer, multilayer and sandwich films

Giant magnetostriction in low magnetic field has been achieved with different approaches, among which single, multilayer and sandwich films with rare-earth base are considered as most promising ones. Enhancement of the 3d(Fe,Co)-magnetic moment with respect to those of 3d(Fe) and 3d(Co) strengthens the Tb–FeCo exchange energies in amorphous Tb(Fe 0.55Co 0.45) 1.5 (named as a-TerfecoHan) films. This is thought to cause diminishing of the Tb-sperimagnetic cone-angle and, thus, leading to enhancement of the magnetostriction. Indeed, a saturation magnetostriction of λ γ,2 ∼10 −3 has been obtained. After annealing, a parallel magnetostrictive susceptibility of χ λ∥ =1.8×10 −2 T −1 has been achieved at μ o H=10 mT. TbFeCo/YFe multilayers combining exchange-coupled giant magnetostrictive TerfecoHan layers and large-magnetisation Y 0.2Fe 0.8 ones exhibit an excellent magnetic and magnetostrictive softness. Subsequent annealing at 350°C leads to the relaxation of the a-TbFeCo layers and to a nanocrystallisation of the YFe layers. In this state, a magnetic coercive field μ o H C=0.3 mT and a huge magnetostrictive susceptibility, χ λ∥ =13×10 −2 T −1 have been obtained. Mössbauer studies indicate that the magnetic softness as observed is associated to the evolution of the nanocrystalline Fe-particles. Magnetostriction is also presented for Fe/TerfecoHan/Fe sandwiches. In this case, giant magnetostriction is already developed at low fields in as-deposited films. Technologically, this seems to be one of the simplest ways to prepare magnetostrictive films for microsystem applications.

Excellent magnetic softness in magnetostrictive TbFeCo/YFe multilayers

The {Tb(Fe 0.55Co 0.45) 1.5/Y 0.2Fe 0.8} 40 multilayers are investigated by X-ray diffraction and SQUID measurements. The results show that in the as-deposited multilayer, the TbFeCo and YFe layers are amorphous. An excellent magnetic softness with a coercivity of 0.3 mT was achieved after annealing at 250°C. These novel magnetic properties are associated to the evolution of a nanocrystalline structure in the YFe layers. Different temperature dependences of the coercive field are described for the amorphous and nanocrystalline multilayers.

Spontaneous Hall effect in amorphous Tb–Fe and Sm–Fe thin films

The spontaneous Hall effect in amorphous Tb–Fe and Sm–Fe thin films, which possess excellent magnetic softness, is investigated to seek a possibility of practical applications of these thin films. The resistivity of Tb–Fe thin films ranges from 180 to 250 μΩ cm as the Tb content varies from 35 to 46 at. %. Tb–Fe thin films show negative Hall resistivity ranging from −7.3 to −5.0 μΩ cm in the same composition range, giving the normalized resistivity ratio from −4.1% to −2.0%. On the other hand, the resistivity of Sm–Fe thin films ranges from 150 to 166 μΩ cm as the Sm content varies from 22 to 31 at. %. Sm–Fe thin films show positive Hall resistivity which varies from 7.1 to 2.8 μΩ cm in the same composition range, giving the normalized resistivity ratio from 4.8% to 1.7%. Between the two different sets of samples, Tb–Fe thin films with perpendicular anisotropy are considered to be more suitable for practical applications, since saturation is reached at a low magnetic field.

Magnetostriction of Tb-Fe-(B) thin films fabricated by RF magnetron sputtering

The magnetostriction of TbFe and TbFeB thin films is systematically investigated over a wide composition range from 40.2 to 68.1 at.% Tb for the B-free alloys and from 44.1 to 66.8 at.% Tb for the B containing thin films. The films were fabricated by rf magnetron sputtering. The microstructure mainly consists of an amorphous phase at low Tb contents and, at high Tb contents, a mixture of an amorphous phase and an /spl alpha/ Tb phase. Excellent magnetostrictive characteristics, particularly at low magnetic fields, are achieved in both TbFe and TbFeB thin films; for example, a magnetostriction of 138 ppm is obtained in a TbFeB thin film at a magnetic field as low as 30 Oe. These excellent magnetostrictive properties of the present thin films are supported by the excellent magnetic softness, the coercivity below 10 Oe and a typical squared-loop shape with the saturation field as low as 1 kOe. It is considered that, due to the excellent low field magnetostrictive characteristics, the present TbFe based magnetostrictive thin films are suitable for Si based microdevices.

Fe-Ta-N薄膜の軟磁性のターゲットバイアス依存性

Fe-Ta-N films (saturation magnetic flux density Bs = 1.6 T) were fabricated by the RF magnetron sputtering method at various target self-bias voltages Vdc (between －100 V and －500 V). The effects of Vdc on the magnetic properties and structures were examined. As a result, this fabrication technique was found to be useful for (1) derivation of excellent soft magnetic properties (high initial permeability μ' = 9000 at 1 MHz and coercive force Hc=0.05 Oe) within the film thickness range of 0.3～4.0 μm; (2) progress of amorphousness in an as-deposited state, and resulting homogeneous crystallization; and (3) realization of the same excellent magnetic softness as found in permalloy films.