Electrical and Magnetic Properties, and Electronic Structures of Pseudo-Gap-Type Antiferromagnetic L1 0 -Type MnPt Alloys Rie Y. Umetsu 1; * 1 , Kazuaki Fukamichi 1; * 2 and Akimasa Sakuma 2 1 Department of Materials Science, Graduate School of Engineering, Tohoku University, Sendai 980-8579, Japan 2 Department of Applied Physics, Graduate School of Engineering, Tohoku University, Sendai 980-8579, Japan The electrical resistivity , magnetic susceptibility , electronic specific heat coefficient  e , the Ne ´el temperature T N and the electronic structure and the magnetocrystalline anisotropy energy (MAE) have been investigated for L1 0 -type MnPt alloy system. This alloy system exhibits the characteristic behaviors of pseudo-gap type antiferromagnets, presenting with the highest T N , the smallest values of ,  and  e in the vicinity of the equiatomic composition. The values of ,  and  e increase with deviating from the equiatomic composition. From the linear muffin-tin orbital (LMTO) band calculations, the L1 0 -type MnPt alloy system has a pseudo-gap in the electronic structure and the density of states (DOS) at the Fermi energy (E F ) is lowest at the equiatomic composition in accord with the experimental results. Furthermore, the results of the LMTO band calculations including the spin–orbit interaction make it clear that the direction of the magnetic moment of Mn in the equiatomic composition is parallel to the c-axis, consistent with the reported spin structure determined by neutron diffractions. The magnetocrystalline anisotropy constant K is about 1:39  10 6 Jm 3 which is larger in magnitude than that of L1 0 -type MnNi equiatomic alloy, though the signs are different. (Received September 1, 2005; Accepted November 28, 2005; Published January 15, 2006) Keywords: antiferromagnet, L1 0 -type manganese platinum alloy, pseudo-gap, electronic structures, the Ne ´el temperature, effective exchange constant 1. Introduction In the vicinity of the equiatomic concentration, Mn forms alloys with Ni, Pd, Pt, Rh and Ir in a wide range of concentration. 1–4) The crystal structure of these alloys is a B2 (CsCl)-type cubic phase at high temperatures and transforms into an L1 0 (CuAu-I)-type tetragonal phase with a diffusion- less martensitic transformation process at low temperatures. From neutron diffractions and magnetic data, 5–10) it has been reported that the L1 0 -type phase exhibits a collinear antiferromagnetic structure and MnNi, MnPd and MnPt equiatomic alloys have a very high Ne ´el temperature T N of about 1100, 780 and 970 K, respectively. Due to such a high stability of antiferromagnetism, especially, MnPt alloy has been investigated intensively from the viewpoint of practical applications as antiferro- magnetic pinning layers of giant magnetoresistance (GMR) 11–14) and tunnel magnetoresistance (TMR) de- vices. 15–18) In order to develop the excellent properties for GMR and TMR devices, however, the investigations on fundamental magnetic properties for these practical anti- ferromagets have been highly desired, because the exchange biasing characteristics are closely related to the spin structures, magnetocrystalline anisotropy energy (MAE) and the magnitude of T N of the antiferromagnetic materi- als. 19–21) In addition, recent interest is how to design the devices with a low electrical resistance at room temper- ature, 22,23) especially in the current perpendicular to the plane (CPP)-type spin valves. 24–27) Because the antiferromagnetic layer is relatively thick, compared with other magnetic layer in the GMR and TMR devices, the reduction of its electrical resistivity is important for such devices. Accordingly, it is meaningful to investigate the electrical properties of anti- ferromagnetic Mn alloys. Recently, it has been pointed out from the theoretical calculations that the density of states (DOS) of the L1 0 -type MnPt equiatomic alloy exhibit a pseudo-gap around the Fermi energy E F . 28) Such a characteristic electronic structure is common to the L1 0 -type MnNi, 29) MnPd 30) and MnIr 31,32) equiatomic alloys with the same crystal structure. The pseudo-gap is attributed to the antiferromagnetic staggered field due to the antiferromagnetic spin arrangements because no pseudo-gap is formed in the paramagnetic state as well as the ferromagnetic state. 29) Therefore, the high stability of the antiferromagnetism for these alloys is closely correlated to their characteristic electronic structure. In the present paper, we have carried out systematic investigations for electrical resistivity, magnetic susceptibil- ity and electronic specific heat coefficient. Especially, we added new data for low temperature region to the preliminary results 33) in order to discuss the electrical and magnetic properties as well as the theoretical results for the present alloy system. For detailed discussion of the correlation between the antiferromagnetic stability and the electronic state for the L1 0 type MnPt alloy system and their variation with the Pt composition, the electronic structures have been calculated by using the tight-binding linear muffin-tin orbital (LMTO) method with the coherent potential approximation (CPA) in the off-equiatomic composition. Furthermore, the LMTO band calculations including the spin–orbit interaction were performed in order to investigate the magnetocrystal- line anisotropy energy (MAE) because the MAE in the antiferromagnetic materials plays an important role in the exchange biasing-field in spin valves. 21) Furthermore, the theoretical calculations of the MAE give the reason why the magnetic phase diagram of the MnPt alloy system is rather complicated, compared with that of other L1 0 -type Mn alloy * 1 Present address: CREST-Japan Science and Technology Agency, Tokyo 105-6218, Japan * 2 Present address: Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai 980-8577, Japan Materials Transactions, Vol. 47, No. 1 (2006) pp. 2 to 10 Special Issue on Present Status of the Research on Ordered Alloys and the Application to Magnetic Devices #2006 The Japan Institute of Metals