Ryota Goto

Large Tunneling Magnetoresistance at Room Temperature Using a Heusler Alloy with the B2 Structure

A Co2Cr0.6Fe0.4Al Heusler alloy film exhibited a B2 structure, which was deposited using a magnetron sputtering system on a thermally oxidized Si substrate at room temperature without any buffer layers. The film exhibited the magnetic moment of 2.04µB per formula unit, nearly the integer number of Bohr magnetons, suggesting a localized nature of ferromagnetism similar to that of many Heusler compounds, which is a necessary condition for half metallicity. A spin- valve-type tunneling junction with a Co2(Cr, Fe)Al Heusler alloy film was fabricated using metal masks, which consists of Co2Cr0.6Fe0.4Al(10 nm)/AlOx (1.8 nm)/CoFe (3 nm)/NiFe (5 nm)/IrMn (15 nm)/Ta (5 nm), deposited on a thermally oxidized Si substrate without a buffer layer. The junction demonstrated large tunneling magnetoresistances of 16% at room temperature and 26.5% at 5 K.

Structural, magnetic, and transport properties of full-Heusler alloy Co2(Cr1−xFex)Al thin films

The structural, magnetic, and transport properties are investigated for full-Heusler alloy Co2(Cr1−xFex)Al (CCFA) thin films sputtered on thermally oxidized Si substrates at room temperature (RT). X-ray diffraction reveals that the films possess the B2 structure for x=0, decrease the atomic site ordering by substituting Fe for Cr(0.4≦x≦0.6), and form the A2 structure for x=1. Both the magnetic moment and the Curie temperature of the films increase with increasing Fe content (x), although the moment for x<1 is significantly smaller than that of the calculated value for the L21 structure. Magnetic tunnel junctions (MTJs) with CCFA films as either upper or bottom ferromagnetic layers are also fabricated by using metal masks. The maximum tunneling magnetoresistance (TMR) at RT for the MTJ was observed to be 19.1% for x=0.4 with the CCFA film as an upper ferromagnetic layer, despite the atomic disorder of the CCFA film. This is consistent with our previous TMR observation with the CCFA film as a bottom ferroma...

Interfacial state and magnetic properties of Nd-Fe-B/Nd thin films

Nd-Fe-B/Nd(-O) interfaces were prepared by thin film technique for the investigation of relationship between interfacial state and coercivity. A Nd-Fe-B layer was deposited by UHV magnetron sputtering and the film was oxidized under low vacuum conditions. On the oxidized Nd-Fe-B layer, a Nd layer was deposited and the film was annealed at 250–650 °C. The coercivity of Nd-Fe-B layer decreased after the oxidation. However, it recovered after the deposition of Nd layer and the annealing at 350 °C. High-resolution transmission electron microscopy (HRTEM) observations revealed that an fcc NdO phase forms after the oxidation and an amorphous phase exists along the interface between Nd2Fe14B and Nd2O3 phases after annealing at 350 °C. Therefore, the recovery of coercivity is related to the amorphous phase, which is considered to be formed by the transformation from NdO to Nd2O3 and α-Nd, and the release of strain at the interface.

Microstructure evaluation for Dy-free Nd-Fe-B sintered magnets with high coercivity

Nd-Fe-B sintered magnets are used for motors of hybrid or electric vehicles due to their high energy products. Dy is added to Nd-Fe-B sintered magnets to work in a high temperature environment. Although the addition of Dy decreases the magnetization of Nd-Fe-B magnets, it increases coercivity; a decrease in the amount of Dy is strongly required. Recently, Nd-Fe-B sintered magnets with a grain size of 1 μm achieved high coercivity of ∼20 kOe without the addition of Dy or other heavy rare earth elements. In this paper, the microstructure of their magnets was observed and compared to magnets with a grain size of ∼3 μm. The coercivity of magnets consisting of larger particles was 17 kOe. Microstructures were observed by the scanning electron microscope and the shapes of grains and the distribution of the Nd-rich phase were evaluated. The observation was promoted in two directions. One direction is the plane perpendicular to the magnetically aligned direction (c plane side) and the other is the side parallel t...

Wettability and Interfacial Microstructure Between

For achievement of high coercivity in Dy-free or -lean Nd-Fe-B permanent magnet system, understanding of reaction process between Nd-Fe-B main and Nd-rich phases is required. In this work, the wettability between Nd2Fe14B and Nd-rich phases was investigated by the measurement of contact angles under isothermal heating. The activation energy for wetting was estimated by the time when the contact angle became constant and the activation energies for ternary and Cu-added Nd-rich ingots were 196.8 and 162.6 kJ/mol, respectively. Interfacial microstructure drastically changed at the temperature lower than the melting point of the Nd-rich ingots and the interface of Cu-added Nd-rich ingots became rough at lower temperature than ternary ones. It is concluded that Cu-addition is effective for the reduction of the activation energy of wetting and improves wetting behavior of Nd-rich phase.

{\hbox {Nd}}_{2}{\hbox {Fe}}_{14}{\hbox {B}}

Low coercivity and its large temperature dependence of a Nd2Fe14B magnet with respect to its magnetic anisotropy field have been addressed as the coercivity problem. To elucidate the physical origin of this problem, we have investigated the temperature dependence of the magnetization reversal behavior in the Nd-Fe-B hot-deformed magnet. Based on the analysis of the energy barrier evaluated from magnetic viscosity measurements, the coercivity problem is discussed in terms of the following three aspects: magnetization reversal process, intrinsic coercivity without thermal demagnetization effect, and energy barrier height. The analyses lead us to conclude that domain wall pinning is dominant in the magnetization reversal in the Nd-Fe-B hot-deformed magnet. The temperature dependences of the intrinsic coercivity and the energy barrier height are explained by the grain boundary model with an intermediate layer. These analyses would be utilized to discuss the detailed structure and magnetic properties of the grain boundary, which gives a new insight to overcome the coercivity problem.

and Nd-Rich Phases in Nd–Fe–B Alloys

This study provides the effect of Cu addition on coercivity (HcJ) and interfacial microstructure in Nd-Fe-B/Nd-rich thin films. All films were deposited by using ultra high vacuum (UHV) magnetron sputtering, and the Nd-Fe-B layer was oxidized under several atmospheres with different oxygen content. Then, the films were annealed at 250–550 °C under UHV. The films oxidized in low vacuum (10−2−10−5 Pa) (under low oxygen state) exhibited the recovery of HcJ by the annealing at 450 °C. On the contrary, the HcJ of the films oxidized in Ar (under high oxygen state) decreased with increasing annealing temperature. However, the HcJ increased drastically at the temperatures above 550 °C. In addition, the Cu added films, which were annealed at temperatures above 350 °C, showed higher coercivities than the films without Cu addition. The XRD analysis suggested the existence of C-Nd2O3 phase in the Cu added films annealed at 550 °C. It can be considered that the Cu addition decreases the eutectic temperature of Nd-rich phase and influences the interfacial state between Nd2Fe14B and Nd-rich phase.

Temperature-dependent magnetization reversal process and coercivity mechanism in Nd-Fe-B hot-deformed magnets

This study provides the influence of microstructural change of the interface between Nd2Fe14B and Nd-O phases on coercivity of Nd-Fe-B thin films during annealing at low temperature (~350 °C). All films were prepared by using ultra high vacuum (UHV) magnetron sputtering, and the Nd-Fe-B layer was oxidized under Ar gas atmosphere (O2 content; ~2 Vol.ppm). Then, the films were annealed at 250–350 °C under UHV condition. After oxidation, the coercivity of Nd-Fe-B film decreased to around 40% of the coercivity of as-deposited Nd-Fe-B film. The Nd-rich phase changed from α-Nd to amorphous Nd(-O), and the interface of Nd2Fe14B/Nd(−O) became rough. In the Nd-Fe-B films oxidized and subsequent annealed at 350 °C, the coercivity decreased to around 20%. In the films, poly crystalline hcp Nd2O3 phase crystallized in Nd-rich phase, and there were some steps at the surface of Nd2Fe14B phase contacting with hcp Nd2O3 phase. Regardless of crystal orientation of Nd2Fe14B, the microstructural changes of the interface described above were observed.

Effect of Cu addition on coercivity and interfacial state of Nd-Fe-B/Nd-rich thin films

The magnetization reversal mechanism of a permanent magnet has long been a controversial issue, which is closely related to the so-called coercivity problem. It is well known that the energy barrier for magnetization reversal contains essential information on reversal process. In this study, we propose a method to analyze the energy barrier function for the magnetization reversal. Preferentially (001) oriented Nd-Fe-B films with and without a Nd overlayer are used as model magnets. By combining the magnetic viscosity and time dependent coercivity measurements, the barrier function has been successfully evaluated. As a result, although the Nd-Fe-B films with and without Nd overlayer exhibit different magnetic behaviors, the power indices for their energy barrier are almost the same, suggesting that the magnetization reversal proceeds in a similar mode.

Influence of microstructural change of the interface between Nd2Fe14B and Nd-O phases on coercivity of Nd-Fe-B films by oxidation and subsequent low-temperature annealing

Nd-Fe-B sintered magnets are required to achieve high coercivity for improvement of their thermal stability. Dy is added to increase coercivity, however, this element decrease magnetization and energy products. Therefore, Dy-lean Nd-Fe-B sintered magnets with high coercivity are strongly demanded. To increase coercivity, it is necessary that microstructure of sintered magnets is consisted of both fine main phase particles and homogeneously distributed Nd-rich phases around the main phase. To meet those requirements, Nd-Fe-B atomized powders were applied to the fabrication process of sintered magnets. Comparing with the case of using strip casting (SC) alloys, jet-milled powders from atomized powders show homogeneous distribution of Nd-rich phase. After optimized thermal treatment, coercivities of sintered magnets from atomized powders and SC alloys reach 1050 kAm−1 and 1220 kAm−1, respectively. This difference in coercivity was due to initial oxygen concentration of starting materials. Consequently, Nd-rich phases became oxides with high melting points, and did not melt and spread during sintering and annealing.