Understanding the magnetization process and magnetic anisotropy in magnetic materials is fundamental to develop magnetic devices such as magnetic memories, read heads, and magnetic sensors. Low-dimensional magnetic materials, such as nanowires or thin films, possess distinctive magnetic properties t...
Understanding the magnetization process and magnetic anisotropy in magnetic materials is fundamental to develop magnetic devices such as magnetic memories, read heads, and magnetic sensors. Low-dimensional magnetic materials, such as nanowires or thin films, possess distinctive magnetic properties that are different from those of their parent bulk materials. For example, Leitao reported that NiFe nanowires had a strong shape anisotropy, and Ni nanowires displayed anomalous temperature dependences of the magnetization and magnetotransport due to the reorientation of the magnetization easy axis with the temperature. In magnetic thin films, surface morphology, roughness, and crystal orientation are important factors affecting the magnetic properties of the films. In 1962, Néel first reported on magnetostatic coupling between two magnetic layers separated by a non-magnetic layer when the roughness of the surface has a periodic pattern. As magnetic thin films do not typically have perfectly smooth or flat surfaces, the magnetic poles created by the surface roughness can be the origin of the dipole interaction between two magnetic layers. In 1970, Schlömann performed a theoretical study on the demagnetization field depending on the surface roughness in magnetic thin films. The demagnetization factor was calculated under the assumptions that the roughness was periodic and the film thickness was larger than the roughness. For a surface shape with a simple sine function, the calculated demagnetizing factor was proportional to the square of the surface roughness, whereas it inversely proportional to the film thickness.Recently, many experiments have been conducted to investigate the relationship between the in-plane magnetic anisotropy and the surface roughness or shape. The ion beam erosion process with the adjustment of the incident angle and ion beam energy has been a typical technique adopted to fabricate film samples with an artificial surface morphology. It was reported that a magnetic film having a rather irregular but rippled surface with a period of ~100 nm or shorter could be prepared by Ar+, Kr+, or Xe+ ion bombardment. Recently, Chen investigated the magnetic anisotropy and the magnetostatic energy of cobalt films with a ripple structure made by the ion beam erosion process. Interestingly, a strong uniaxial magnetic anisotropy was observed along the ion beam direction. Similar studies have been done for NiFe or Co films having different types of rippled surfaces; however, no reports have been published on magnetic films with periodically oscillating wave-like surfaces or structures.
In this work, we report on the magnetic anisotropy in triangular wave-like ferromagnetic NiFe films with different wavelengths. The surface morphology of the m-plane sapphire substrate for the deposition of the permalloy Ni80Fe20 (NiFe) film could be controlled by a thermal annealing method. Unlike for the previously mentioned ion milling methods, the relationship between the wavelength and the oscillation height in the triangular wave-like film showed a linear dependence. The average wave length of the triangular wave-like NiFe thin films was approximately in the range of 25–170 nm, and the oscillation height h was proportional to wave length. Thus, the tilting angle of the facet was roughly constant for all the samples, that is to say, the ratio of height and wavelength was maintained roughly constant at ~0.133. We measured the magnetic properties of NiFe films using a MOKE setup. The M(H) data for the triangular wave-like NiFe film showed an enhanced coercivity and a distinctive dependence on the measurement direction within the film plane. A large magnetic anisotropy energy of 80–150 kJ/m3, which is up to ten times larger than the reported values, was observed with the variation of wave length. The increasing tendency of the anisotropy energy with decreasing wave length is likely due to a change in the shape anisotropy, while the anisotropy energy generated by surface magnetic charges slightly increased with increasing wavelength.
In addition, we investigated the magnetic properties of NiFe/Fe2O3 bilayers grown on m-plane sapphire substrates with variation of surface morphology. Here, NiFe/Fe2O3 bilayer is a kind of exchange bias systems consisting of ferromagnetic and antiferromagnetic layers. The NiFe/Fe2O3 bilayers were prepared on m-plane sapphire substrate by using pulsed laser deposition(PLD) and dc- sputtering. The m-plane sapphire substrate with a ripple surface was prepared by thermal annealing and the bilayers were deposited on six different m-plane sapphire substrates with various ripple periods. The magnetic hysteresis loop M(H) data were collected by a MOKE setup with an application of magnetic field along the film plane. This structure displayed a change in magnetic hysteresis loop because of the exchange interactions at the interface. The bilayers displayed an easy axis perpendicular to the longitudinal direction of the facet and a hard axis along the longitudinal direction. This result suggested that the strong magnetic anisotropy is caused by interfacial coupling between ferromagnetic layer and antiferromagnetic layers. The NiFe/Fe2O3 bilayers showed an increase of coercivity with the increase of wavelength. The aim of this experiment was to analyze the magnetic anisotropies caused by two different origins. We concluded that the interface anisotropy caused by exchange coupling is much stronger than the surface anisotropy caused by wave-like surface.
Understanding the magnetization process and magnetic anisotropy in magnetic materials is fundamental to develop magnetic devices such as magnetic memories, read heads, and magnetic sensors. Low-dimensional magnetic materials, such as nanowires or thin films, possess distinctive magnetic properties that are different from those of their parent bulk materials. For example, Leitao reported that NiFe nanowires had a strong shape anisotropy, and Ni nanowires displayed anomalous temperature dependences of the magnetization and magnetotransport due to the reorientation of the magnetization easy axis with the temperature. In magnetic thin films, surface morphology, roughness, and crystal orientation are important factors affecting the magnetic properties of the films. In 1962, Néel first reported on magnetostatic coupling between two magnetic layers separated by a non-magnetic layer when the roughness of the surface has a periodic pattern. As magnetic thin films do not typically have perfectly smooth or flat surfaces, the magnetic poles created by the surface roughness can be the origin of the dipole interaction between two magnetic layers. In 1970, Schlömann performed a theoretical study on the demagnetization field depending on the surface roughness in magnetic thin films. The demagnetization factor was calculated under the assumptions that the roughness was periodic and the film thickness was larger than the roughness. For a surface shape with a simple sine function, the calculated demagnetizing factor was proportional to the square of the surface roughness, whereas it inversely proportional to the film thickness.Recently, many experiments have been conducted to investigate the relationship between the in-plane magnetic anisotropy and the surface roughness or shape. The ion beam erosion process with the adjustment of the incident angle and ion beam energy has been a typical technique adopted to fabricate film samples with an artificial surface morphology. It was reported that a magnetic film having a rather irregular but rippled surface with a period of ~100 nm or shorter could be prepared by Ar+, Kr+, or Xe+ ion bombardment. Recently, Chen investigated the magnetic anisotropy and the magnetostatic energy of cobalt films with a ripple structure made by the ion beam erosion process. Interestingly, a strong uniaxial magnetic anisotropy was observed along the ion beam direction. Similar studies have been done for NiFe or Co films having different types of rippled surfaces; however, no reports have been published on magnetic films with periodically oscillating wave-like surfaces or structures.
In this work, we report on the magnetic anisotropy in triangular wave-like ferromagnetic NiFe films with different wavelengths. The surface morphology of the m-plane sapphire substrate for the deposition of the permalloy Ni80Fe20 (NiFe) film could be controlled by a thermal annealing method. Unlike for the previously mentioned ion milling methods, the relationship between the wavelength and the oscillation height in the triangular wave-like film showed a linear dependence. The average wave length of the triangular wave-like NiFe thin films was approximately in the range of 25–170 nm, and the oscillation height h was proportional to wave length. Thus, the tilting angle of the facet was roughly constant for all the samples, that is to say, the ratio of height and wavelength was maintained roughly constant at ~0.133. We measured the magnetic properties of NiFe films using a MOKE setup. The M(H) data for the triangular wave-like NiFe film showed an enhanced coercivity and a distinctive dependence on the measurement direction within the film plane. A large magnetic anisotropy energy of 80–150 kJ/m3, which is up to ten times larger than the reported values, was observed with the variation of wave length. The increasing tendency of the anisotropy energy with decreasing wave length is likely due to a change in the shape anisotropy, while the anisotropy energy generated by surface magnetic charges slightly increased with increasing wavelength.
In addition, we investigated the magnetic properties of NiFe/Fe2O3 bilayers grown on m-plane sapphire substrates with variation of surface morphology. Here, NiFe/Fe2O3 bilayer is a kind of exchange bias systems consisting of ferromagnetic and antiferromagnetic layers. The NiFe/Fe2O3 bilayers were prepared on m-plane sapphire substrate by using pulsed laser deposition(PLD) and dc- sputtering. The m-plane sapphire substrate with a ripple surface was prepared by thermal annealing and the bilayers were deposited on six different m-plane sapphire substrates with various ripple periods. The magnetic hysteresis loop M(H) data were collected by a MOKE setup with an application of magnetic field along the film plane. This structure displayed a change in magnetic hysteresis loop because of the exchange interactions at the interface. The bilayers displayed an easy axis perpendicular to the longitudinal direction of the facet and a hard axis along the longitudinal direction. This result suggested that the strong magnetic anisotropy is caused by interfacial coupling between ferromagnetic layer and antiferromagnetic layers. The NiFe/Fe2O3 bilayers showed an increase of coercivity with the increase of wavelength. The aim of this experiment was to analyze the magnetic anisotropies caused by two different origins. We concluded that the interface anisotropy caused by exchange coupling is much stronger than the surface anisotropy caused by wave-like surface.
주제어
#Magnetic anisotropy
#Surface morphology
#interface
#NiFe
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