• Journal of the Korean Vacuum Society
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• v.12 no.S1
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• pp.104-106
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• 2003

In this study, the electronic structures and physical properties of Ni$_2$MnGa alloy films and their dependence on the order-disorder structural transitions were investigated. The results show that the ordered films behave nearly the same as the bulk $Ni_2$MnGa alloy, including the martensitic transformation at 200 K. Unexpectedly, the disordering in $Ni_2$MnGa alloy films does not lead to any appreciable magnetic ordering down to 4 K. An annealing of the disordered films restores the ordered structure with an almost full recovery of the magnetic and the transport properties of the ordered $Ni_2$MnGa alloy films. A possible explanation of the disappearance of magnetic moment in the disordered film is given by using the ab initio first-principles electronic-structure calculations.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 2003.05a
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• pp.105-107
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• 2003

강자성 형상기억합금은 기존의 압전재료 및 열적 형상기억합금을 이용한 전기-열-기계적 거동의 액츄에이터 재료를 대신할 수 있는 새로운 고성능 액츄에이터 재료로서 각광을 받고 있다. 그러한 강자성 형상기억합금들 중의 한 종류로서 단결정 및 다결정 $Ni_{2}MnGa$ 합금을 이용하여 자장인가시 변형을 관찰하였다. 거대 자장 유기 변형률을 설명하기 위하여 두 모델이 제안되었다. 변태 온도보다 낮은 온도에서는 마르텐사이트 상의 재배열에 의하여 변형이 일어났으며, 그 변태온도보다 높은 온도에서는 상변태에 의한 변형이 일어났다. 미세구조 관찰을 통하여 인가 자장의 방향에 따라 우선적으로 형성되는 마르텐사이트상을 관찰하였다.

• Proceedings of the Korean Vacuum Society Conference
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• 2003.08a
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• pp.101-101
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• 2003

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 2001.07a
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• pp.323-326
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• 2001

NI$_2$MnGa-based ferromagnetic shape memory alloys (FSMA) are hoped to be used as robust actuators with high performance and power density, as a replacement of other actuation materials such as thermo-mechanical SMAs and mechanical-electric piezoelectrics. Recently, we have observed significant shape changes under magnetic field application when single- and poly-crystalline forms are used. In the present study, two mechanisms have been proposed to predict the magnetic field-induced shape change as a function of external magnetic field at temperatures below Mr and above Ar. In the case of the field-induced shape change at temperature below M$_{f}$, paired martensite variants are assumed to form by application of magnetic field. The direction of magnetization in martensites formed in austenite matrix is assumed to be parallel to the applied magnetic field in the case of shape change by application at temperature above Af. Various energies has been considered in the shape change under two mechanisms.s.

• Journal of Magnetics
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• v.9 no.2
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• pp.37-39
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• 2004

Polycrystalline Ni-Mn-Ga films have been deposited onto mica substrates held at 720 K by flash-evaporation method. At room temperature the films have a tetragonal structure with a = b = 0.598 and c = 0.576 nm typical for bulk $Ni_2MnGa$ below a martensitic transformation. Temperature measurements of ferromagnetic resonance reveal a martensitic phase transformation at 310 K. The transformation brings about a substantial decrease in the effective magnetization and a drastic increase in the ferromagnetic resonance linewidth due to a strong increase in the magnetic anisotropy in the martensitic phase.

• Journal of the Korean Vacuum Society
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• v.12 no.S1
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• pp.116-119
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• 2003

$Ni_2$MnGa films, deposited on mica and glass substrates, were studied by ferromagnetic resonance (FMR) technology. The temperature-dependent resonance field was measured and a martensitic phase transformation (MT) was found between 310 and 340 K, exhibiting an abnormality on the curve. The easy axis is found to be in the film plane. The line width increases as a whole with decreasing temperature, which is discussed in terms of the motional narrowing mechanism. The resonance field was also measured as a function of orientation and the results were fitted, exhibiting a good consistence.

• Journal of Korean Vacuum Science & Technology
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• v.7 no.1
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• pp.13-17
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• 2003

The influence of structural transition on the resistance and impedance behavior of Ni$_2$MnGa alloy was investigated. The temperature-dependent resistance and impedance were measured in a temperature range of 4 - 350 K and 185 - 300 K, respectively. The dependence of temperature coefficient of resistivity on temperature shows a kink at 220 K, which is related to the structural transition. The change in dominant scattering mechanism results in the observed kink. Significant increases were also observed around the transition temperature for both real and imaginary parts of impedance. It is thought that this phenomenon originates from disappearance of the martensite twin boundaries during the structural transformation.

• Journal of the Korean Electrochemical Society
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• v.8 no.4
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• pp.172-176
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• 2005

Recently, many researches on the high-voltage 5 V class cathode material have focused on $LiNi_{0.5}Mn_{1.5}O_4$, where $Mn^{3+}$ in the existing $LiMn_2O_4 (Li[Mn^{3+}][Mn^{4+}]O_4)$ is replaced by $Ni^{2+}(Li[Ni^{2+}]_{0.5}[Mn^{4+}]_{1.5}O_4)$ in order to utilize $Ni^{2+}/Ni^{4+}$ redox reaction in the 5V region. The partial substitution of Mn in $LiMn_2O_4$ for other transition metal element, $LiM_yMn_{1-y}O_4$(M=Cr, Al, Ni, Fe, Co, Cu, Ga etc) is known as a good solution to overcome the problems associated with $LiMn_2O_4$ like the gradual capacity fading. In this study, we synthesized $LiNi_{0.5}Mn_{1.5}O_4$ through a mechanochemical process and investigated its morphological, crystallographic and electrochemical characteristics. The results showed that 4 V peaks had been found in the cyclic volammograms of the synthesized powders due to the existence of $Mn^{3+}$ from the incomplete substitution of $Ni^{2+}$ for $Mn^{3+}$ implying that the mechanochemical activation alone was not good enough to synthesize an exact stoichiometric compound of $LiNi_{0.5}Mn_{1.5}O_4$. The synthetic condition of mechanochemical process, such as type of starting materials, ball-mill and calcination condition was optimized for the best electrochemical performance.

• Proceedings of the Korean Vacuum Society Conference
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• 2003.08a
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• pp.80-81
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• 2003

• Proceedings of the Korean Magnestics Society Conference
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• 2003.12a
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• pp.241-241
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• 2003