Magnetic field induced order–order phase transition in magnetocaloric systems Mn2−xFexAs0.5P0.5 and MnFeAsyP1−y

An analysis is presented of experimental and theoretical results of the MnFeAs_yP_1−y (0.15≤y≤0.66) and Mn_2−xFe_xAs_0.5P_0.5 (0.5≤x≤1.0) systems to identify main traits that underlie the mechanism of formation of different antiferromagnetic (AF) phases in the two systems. The discrepancy between the calculated from first principles and experimental values of the magnetic moment in the ferromagnetic phase with cation substitution in the system Mn_2−xFe_xAs_0.5P_0.5 is due to the appearance of a canted magnetic structure. In this case, the emergence of an AF phase with decreasing iron concentration precedes a significant change in the electronic d-band filling. In the model of the spiral structure in the system of itinerant electrons it is shown that the stabilization of the AF phase with decreasing arsenic concentration, while maintaining the number of d-electrons, is a consequence of changes in the shape of the density of electronic states that occur with a decrease in unit-cell volume.     

Surface antiferromagnetic domain structures of NiO (0 0 1) studied using UV photoemission electron microscope

Direct observation of the antiferro (AF) magnetic domain structures of a NiO (0 0 1) surface is found to be possible using a spectroscopy photoelectron low-energy electron microscope (SPELEEM) and a commercial UV Hg excitation light source without using any polarizers. The principle is based on the magnetic linear dichroism (MLD) effect, where different domain contrasts are produced according to the relative angle between the antiferromagnetic axis and the linearly polarized light. The observed AF magnetic domain structures are strongly affected by both bulk AF magnetic domain structures and the stresses induced during the sample cleaving process. Moreover, the AF magnetic domain structures are found to be irreversible when the sample is heated to over its Néel temperature and then cooled. The possibility of imaging AF magnetic domain structures without using synchrotron radiation or a polarizer is attractive.     

Magnetic Raman optical activity in FeF2

Raman optical activity (ROA) of magnons and phonons in antiferromagnetic FeF₂ (T_N=78 K) has been studied as a function of temperature and in applied magnetic field. For exciting light incident along the rutile-structure c-axis, ROA is observed for magnons but not phonons. In zero field, the small anisotropy-induced splitting (0.09 cm⁻¹) of the two acoustic-magnon branches is observed by light scattering for the first time. The splitting in applied magnetic field is found to reduce with increasing temperature in accordance with theory. No ROA was detected for two-magnon excitations.     

Perspectives of antiferromagnetic spintronics

Antiferromagnets are promising for future spintronic applications owing to their advantageous properties: They are magnetically ordered, but neighboring magnetic moments point in opposite directions, which results in zero net magnetization. This means antiferromagnets produce no stray fields and are insensitive to external magnetic field perturbations. Furthermore, they show intrinsic high frequency dynamics, exhibit considerable spin–orbit and magneto-transport effects. Over the past decade, it has been realized that antiferromagnets have more to offer than just being utilized as passive components in exchange bias applications. This development resulted in a paradigm shift, which opens the pathway to novel concepts using antiferromagnets for spin-based technologies and applications. This article gives a broad perspective on antiferromagnetic spintronics. In particular, the manipulation and detection of antiferromagnetic states by spintronics effects, as well as spin transport and dynamics in antiferromagnetic materials will be discussed. We will also outline current challenges and future research directions in this emerging field.     

Ab initio investigation of the magnetic states of Ca2MnO4 and Ca2MnO3.5

An analysis of the electronic and magnetic properties of Ca₂MnO₄ and Ca₂MnO_3.5 is carried out within local spin density functional theory using the augmented spherical wave method. From energy differences between the hypothetic magnetic configurations both systems are found to be insulating antiferromagnets in the ground state with a 1 eV gap. However we identify an intermediate half metallic ferromagnetic state with the Hund’s rule expected moments for Mn^IV (3 μ_B) and Mn^III (4 μ_B, high spin HS configuration), respectively. The latter result of moment magnitude finds support in recent experimental evidence of Mn^III bismuth oxide as a ferromagnet in its ground state. This is characterized by a small density of states (DOS) magnitude of itinerant states in spin (↑) channel pointing to a metallic-like behavior as it is experimentally evidenced. For both Ca₂MnO₄ and Ca₂MnO_3.5 the chemical bonding characteristics are resolved for the two spin channels. Relationship to colossal magnetoresistive compounds is proposed.     

Morphological and magnetic study of CaMnO3−x oxides obtained from different routes

The CaMnO_3−x (x=0 and 0.02) mixed oxide was synthesized from both thermal treatment of the [CaMn(C₃H₂O₄)₂(H₂O)₄] metallo-organic precursor and ceramic method. In the case of the latter method, temperatures of 1350 °C and 50 h were necessary; however, lower temperatures, 800 °C, and shorter reaction times, 15 h, were utilized in the attainment of the mixed oxide from the precursor. As a consequence, the morphology of the different products is clearly different. The samples exhibit three-dimensional antiferromagnetic ordering with T_N near 120 K, and a low-dimensional antiferromagnetic ordering at high temperatures. The presence of a ferromagnetic component above T_N was also observed in both compounds, it is slightly stronger in the phase prepared by the ceramic route.     

Crystal structures and magnetic properties of the honeycomb-lattice antiferromagnet M2(pymca)3(ClO4), (M = Fe,Co, Ni)

We report on the syntheses, crystal structures, and magnetic properties of a series of transition metal coordination polymers M₂(pymca)₃(ClO₄), (pymca = pyrimidine-2-carboxylic acid, M = Fe (1), Co (2), and Ni (3)). These compounds are found to crystallize in a trigonal crystal system, space group P31m, with the lattice constants a = 9.727 Å and c = 5.996 Å for 1, a = 9.608 Å and c = 5.996 Å for 2, and a = 9.477 Å and c = 5.958 Å for 3 at room temperature. In these compounds, each pymca ligand connects to two M²⁺ ions, forming a honeycomb network in the ab plane. The temperature dependences of magnetic susceptibilities in these compounds show broad maxima, indicating antiferromagnetic interactions within two-dimensional honeycomb layers. We also observed an antiferromagnetic phase transition at low temperatures by magnetic susceptibility and heat capacity measurements. From the crystal structures and magnetic properties, we conclude that the compounds 1, 2, and 3 are good realizations of honeycomb-lattice antiferromagnets.     

Current-induced torques in magnetic metals: Beyond spin-transfer

Current-induced torques (CITs) on ferromagnetic (FM) nanoparticles and on domain walls in FM nanowires are normally understood in terms of transfer of conserved spin angular momentum between spin-polarized currents and the magnetic condensate. In a series of recent articles, we have discussed a microscopic picture of CITs in which they are viewed as following from exchange fields produced by the misaligned spins of current carrying quasiparticles. This picture has the advantage that it can be applied to systems in which spin is not approximately conserved. More importantly, this point of view makes it clear that CITs can also act on the order parameter of an antiferromagnetic (AFM) metal, even though this quantity is not related to total spin. In this informal and intentionally provocative review we explain this picture and discuss its application to antiferromagnets.     

Spin excitations in exchange-dominated regime on (1 0 0) and (1 1 0) antiferromagnetic surfaces

Theoretical investigations in the context of Heisenberg model have been made for (1 0 0) and (1 1 0) magnetic surface dynamics for a semi-infinite antiferromagnet geometry. The calculations apply to the exchange dominated regime and are based on a spin-wave operator and matching technique within the framework of non-interacting spin-wave theory. The theoretical formalism developed here does not include either relaxation or reconstruction at the surface and no electronic effects have been considered. Dispersion curves of surface spin-waves are obtained within a single framework by matching the evanescent and travelling solutions, respectively, obtained from the secular equation and satisfying the boundary conditions brought about by the surface. The excitation spectrum of the surface spin-waves has been obtained and compared with that for bulk spin-waves. The quantized bulk modes of the same energy travelling to and away from the surface are related to one another by reflection coefficients, for which sum rules are derived. The numerical results for the evolution of acoustic and optical modes are presented for two different surface planes, namely (1 0 0) and (1 1 0). The findings reported here show that: (i) the reduced coordination number for atoms near the surface as well as the surface orientation play an obvious and crucial part in the surface spin-wave spectra; (ii) the evolutions of bulk as well as surface modes undergo significant changes as a function of the bulk-surface exchange integrals for a given direction of propagation of the spin-wave modes along the surface.