The possibility of half-metallic antiferromagnetism, a special case of ferrimagnetism with a compensated magnetization, in the diluted magnetic semiconductors is highlighted on the basis of the first principles electronic structure calculation. As typical examples, the electrical and magnetic properties of II-VI compound semiconductors doped with 3d transition metal ion pairs-(V, Co) and (Fe, Cr)-are discussed.PACS numbers: 71.55. Gs, 75.50.Pp Since the discovery of ferromagnetic diluted magnetic semiconductors (DMSs) [1,2,3], considerable attention has been paid to the origin and stability of their ferromagnetism [4,5,6,7,8,9]. One of the main issues has been how to realize the magnetic transition temperature that exceeds room temperature for these DMSs. This is partly due to the possible applications of such materials to spin-electronics [10]. Well-defined magnetization is certainly an indication of a new degree of freedom that can store and transfer information. However, attaining a sufficiently high magnetic transition temperature for such applications seems to be a difficult task currently [3,11,12,13,14]. Such a functionality that is expected for DMSs, however, is not restricted only to the ferromagnetic DMSs. In this Letter, we propose a new candidate for spin-electronics materials, namely, half-metallic diluted antiferromagnetic semiconductors. We expect that these materials will show significant spindependent electronic properties, and hence can be used as spin-electronics materials.The concept of half-metallic antiferromangets was first proposed by van Leuken and de Groot many years ago [15]. They pointed out that in a special case of ferrimagnetism it might be possible for a system to become half-metallic, i.e., metallic in one spin direction of electrons but insulated in the other spin direction. Obviously, this is impossible for usual antiferromagnets because of the spin-rotational symmetry, but it is not necessarily the case for ferrimagnets with a compensated magnetic moment. Van Leuken and de Groot referred to such cases as half-metallic antiferromagnetism. Later, Pickett pointed out on the basis of first-principles calculation that La 2 VCuO 6 , La 2 MnO 6 , and La 2 MnCoO 6 with a double-perovskite structure might be likely candidates for half metallic antiferromagnets [16]. However, thus far, the existence of such materials has not been verified experimentally. This indicates that intermetallic compounds or oxides may not be suitable candidates for half-metallic antiferromagnets. It could be possible that their magnetic and crystal structures are not as stable energetically as predicted by LDA (local density approximation) calculations.In contrast to the above cases where the band structure is hardly controlled by small modifications of the (a)

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The structural and magnetic properties of functional Ni-Mn-Z (Z = Ga, In, Sn) Heusler alloys are studied by first-principles and Monte Carlo methods. The ab initio calculations give a basic understanding of the underlying physics which is associated with the strong competition of ferroand antiferromagnetic interactions with increasing chemical disorder. The resulting d-electron orbital dependent magnetic ordering is the driving mechanism of magnetostructural instability which is accompanied by a drop of magnetization governing the size of the magnetocaloric effect. The thermodynamic properties are calculated by using the ab initio magnetic exchange coupling constants in finite-temperature Monte Carlo simulations, which are used to accurately reproduce the experimental entropy and adiabatic temperature changes across the magnetostructural transition. PACS numbers: 75.50.-y, 75.10.-b, 75.30.SgFollowing the concepts of Hume-Rothery the influence of composition on martensitic and magnetic transformation temperatures is commonly condensed as a dependency of electrons per atom (e/a-ratio) [1]. Experiment and first-principles calculations, however, reveal that the Z element in Ni-Mn-Z Heusler alloys (Z = Ga, In, Sn) also affects the transformation temperatures substantially [2]. Moreover, recent experiments on samples with identical composition but different heat treatment indicate that chemical disorder also plays an important role [3][4][5]. Here, we use first-principles calculations to identify the influence of chemical disorder on the magnetic exchange parameters and derive guidelines for a further systematic improvement of magnetocaloric materials [6].Besides the magnetocaloric effect (MCE) in Gd and other alloys at room temperature [7,8], the metamagnetic Ni-Mn based Heusler materials [9,10], have attracted much interest recently [11,12]. In these alloys the metamagnetic features are responsible for magnetic glass behavior and frustration due to chemical disorder [13][14][15] as well as unusual magnetization behavior under an external magnetic field such as a large jump of the magnetization ∆M (T m ) at the martensitic/magnetostructural transformation temperature T m [16]. This gives rise to the large inverse MCE of the materials [9,10,17,18]. The MCE can be influenced when Ni is substituted in part by Co: It is strongly enhanced in the case of In-based intermetallics [19,20] (with adiabatic temperature change ∆T ad = −6 K in 2 T field [20]) while in the case of Ga the MCE is turned from direct to inverse by decoupling T m and Curie temperature T C [21] (with ∆T ad = −1.6 K in 1.9 T field [22,23]).Chemical disorder in the Mn-rich Heusler alloys is responsible for competing magnetic interactions (ferromagnetic versus antiferromagnetic) because the extra Mn atoms occupy lattice sites of the Z-sublattice which interact antiferromagnetically with the Mn atoms on the Y-sublattice due to RKKY-type interactions. This competition of magnetic interactions leads to the characteristic drop of magnetization curves at T m , wh...

The g-factor of the exteremely proton-rich nucleus 23 Al(T 1/2 = 0.47 s) has been measured for the first time, applying -NMR technique on this nucleus implanted in Si. The obtained |g| = (1.58 T 0.2) suggests that the spin of the ground state of 23 Al is 5 / 2. The magnetic moment is determined as |"| = (3.95 T 0.55) " N .