Using first-principles calculations, we demonstrate that an Fe monolayer can assume very different magnetic phases on hcp ͑0001͒ and fcc ͑111͒ surfaces of 4d-and 5d-transition metals. Due to the substrates' d-band filling, the nearest-neighbor exchange coupling of Fe changes gradually from antiferromagnetic ͑AFM͒ for Fe films on Tc, Re, Ru, and Os to ferromagnetic on Rh, Ir, Pd, and Pt. In combination with the topological frustration on the triangular lattice of these surfaces the AFM coupling results in a 120°Néel structure for Fe on Re and Ru and an unexpected double-row-wise AFM structure on Rh, which is a superposition of left-and right-rotating 90°spin spirals. DOI: 10.1103/PhysRevB.79.094411 PACS number͑s͒: 75.70.Ak, 71.15.Mb Triggered by the discovery of the giant-magnetoresistance effect and the demand to realize spintronic device concepts, 1 magnetic nanostructures on surfaces have been a focus of experimental and theoretical research for more than 20 years now. In particular, there has been a tremendous effort to grow ultrathin transition-metal films on metal surfaces and to characterize and explain their magnetic properties. It is now generally believed that these structurally simple systems are well understood and more complex nanostructures such as atomic chains, clusters, or molecules on surfaces have moved into the spotlight of today's research. [2][3][4][5][6][7] Therefore, it came as a big surprise when it was experimentally shown that the prototypical ferromagnet Fe becomes a two-dimensional ͑2D͒ antiferromagnet on the W͑001͒ surface. 8 Combining spin-polarized scanning tunneling microscopy and first-principles calculations, it has been further demonstrated that complex magnetic order can be obtained even in single monolayer ͑ML͒ magnetic films on nonmagnetic substrates. For example, recently a spin-spiral state was discovered for a Mn ML on W͑110͒ ͑Ref. 9͒ and for a Mn ML on W͑001͒ ͑Ref. 10͒ and a nanoscale magnetic structure was found for an Fe ML on Ir͑111͒. 11 Surfaces of 4d-and 5d-transition metals ͑TMs͒ such as W, Re, Ru, or Ir have been particularly attractive from an experimental point of view as ultrathin 3d-TM films can often be grown pseudomorphically and without intermixing. 12-16 However, there has been controversy in the past about reports concerning dead magnetic layers and absence of magnetic order in ultrathin films on these surfaces. 14,15 The fundamental key to many unresolved puzzles may be the itinerant character of TMs resulting in competing exchange interactions beyond nearest neighbors and higher-order spin interactions beyond the Heisenberg model. The latter interactions have been proposed to play a role in transition metals; however, to our knowledge, no unambiguous proof of their importance has been given.Here, we use first-principles calculations to demonstrate that a hexagonal Fe ML can assume very different magnetic phases on a triangular lattice provided by hcp ͑0001͒ and fcc ͑111͒ surfaces of 4d-and 5d-transition metals, which are also experimentally accessib...

The design of nanoscale organic-metal hybrids with tunable magnetic properties as well as the realization of controlled magnetic coupling between them open gateways for novel molecular spintronic devices. Progress in this direction requires a combination of a clever choice of organic and thin-film materials, advanced magnetic characterization techniques with a spatial resolution down to the atomic length scale, and a thorough understanding of magnetic properties based on first-principles calculations. Here, we make use of carbon-based systems of various nanoscale size, such as single coronene molecules and islands of graphene, deposited on a skyrmion lattice of a single atomic layer of iron on an iridium substrate, in order to tune the magnetic characteristics (for example, magnetic moments, magnetic anisotropies and coercive field strengths) of the organic-metal hybrids. Moreover, we demonstrate long-range magnetic coupling between individual organic-metal hybrids via the chiral magnetic skyrmion lattice, thereby offering viable routes towards spin information transmission between magnetically stable states in nanoscale dimensions.

A generalized Irving-Kirkwood formula for the calculation of stress in molecular dynamics models J. Chem. Phys. 137, 134104 (2012) Effect of temperature, strain, and strain rate on the flow stress of aluminum under shock-wave compression J. Appl. Phys. 112, 073504 (2012) Colossal low-frequency resonant magnetomechanical and magnetoelectric effects in a three-phase ferromagnetic/elastic/piezoelectric composite Appl.The phase diagrams of magnetic shape-memory Heusler alloys, in particular, ternary Ni-Mn-Z and quarternary (Pt, Ni)-Mn-Z alloys with Z ¼ Ga, Sn, have been addressed by density functional theory and Monte Carlo simulations. Finite temperature free energy calculations show that the phonon contribution stabilizes the high-temperature austenite structure while at low temperatures magnetism and the band Jahn-Teller effect favor the modulated monoclinic 14M or the nonmodulated tetragonal structure. The substitution of Ni by Pt leads to a series of magnetic shape-memory alloys with very similar properties to Ni-Mn-Ga but with a maximal eigenstrain of 14%.

Despite the importance of martensitic transformations of Ni-Mn-Ga Heusler alloys for their magnetocaloric and shape-memory properties, the martensitic part of their phase diagrams is not well determined. Using an ab initio approach that includes the interplay of lattice and vibrational degrees of freedom we identify an intermartensitic transformation between a modulated and a nonmodulated phase as a function of excess Ni and Mn content. Based on an evaluation of the theoretical findings and experimental x-ray diffraction data for Mn-rich alloys, we are able to predict the phase diagram for Ni-rich alloys. In contrast to other mechanisms discussed for various material systems in the literature, we herewith show that the intermartensitic transformation can be understood solely using thermodynamic concepts.

Mapping the total energies obtained from first principles calculations to model Hamiltonians is a powerful technique to explore the magnetic ground state of a system. We analyze the applicability of this approach in the presence of highly polarizable substrates, e.g. for an ultrathin Fe layer on Pd(111), Rh(111), Ru(0001), or Tc(0001). We find that the traditionally accepted model Hamiltonians (Heisenberg plus nearest neighbor higher-order spin Hamiltonians) are not sufficient to capture the magnetic interactions in these systems and examine new terms that can be included to improve the description. Challenges for this technique are exemplified by the double-rowwise antiferromagnetic (AFM) ground state predicted for Fe/Rh(111). Usually, magnetic structures are explained within classical spin models like the Heisenberg model, H ¼ Àð1=2ÞTheir foundation results primarily from quantum mechanical exchange interactions of different orders and between different neighbors, and is expressed by spin operators, Ŝ. Typically, the operator is then replaced by the expectation value, S, and quantum fluctuations are neglected. The change from FM to AFM order of the Fe monolayers deposited on Ag(001) or W(001)