In this paper, we review some of the key concepts in ultrathin film magnetism which underpin nanomagnetism. We survey the results of recent experimental and theoretical studies of well characterized epitaxial structures based on Fe, Co and Ni to illustrate how intrinsic fundamental properties such as the magnetic exchange interactions, magnetic moment and magnetic anisotropies change markedly in ultrathin films as compared with their bulk counterparts, and to emphasize the role of atomic scale structure, strain and crystallinity in determining the magnetic properties. After introducing the key length scales in magnetism, we describe the 2D magnetic phase transition and survey studies of the thickness dependent Curie temperature and the critical exponents which characterize the paramagnetic-ferromagnetic phase transition. We next discuss recent experimental and theoretical results on the determination of the exchange constant, followed by an overview of measurements of the magnetic moment in the elemental 3d transition metal thin films in the various crystal phases that have been successfully stabilized, thereby illustrating the sensitivity of the magnetic moment to the local symmetry and to the atomic environment. Finally, we discuss briefly the magnetic anisotropies of Fe, Co and Ni in the fcc crystalline phase, to emphasize the role of structure and the details of the interface in influencing the magnetic properties. The dramatic effect that adsorbates can have on the magnetic anisotropies of thin magnetic films is also discussed. Our survey demonstrates that the fundamental properties, namely, the magnetic moment and magnetic anisotropies of ultrathin films have dramatically different behaviour compared with those of the bulk while the comparable size of the structural and magnetic contributions to the total energy of ultrathin structures results in an exquisitely sensitive dependence of the magnetic properties on the film structure.

A study of magnetic and structural properties of (CoxFe100−x)50 (0≦x≦100) alloy thin films as functions of composition and substrate deposition-temperature Ts is carried out. The intrinsic perpendicular magnetic anisotropy Ku starts to increase with deposition temperature Ts for all the films (0≦x≦100) at about TS=250–300 °C, and then becomes nearly constant. The maximum values of Ku at room temperature thus obtained are 6×107, 4×107, and 2×107 erg/cc for Fe50Pt50, (Co43Fe57)50Pt50 and Co50Pt50, respectively. Also, the Ku of Fe50Pt50 films increases with the order parameter (S), being closely related with the tetragonality c/a. These results indicate that the evolution of Ku is due to the occurrence of the ordered fct phase.

Understanding of the magnetocrystalline anisotropy in magnetic materials (the influence of different elemental components on the direction of easy magnetization) can be greatly enhanced by measuring the orbital moment anisotropy of the elemental constituents. A circular x-ray dichroism technique is presented that allows the determination of the microscopic origin of the spin reorientation transition in ultrathin single-crystalline cobalt/nickel films. The stronger anisotropy contribution of a much thinner cobalt layer redirects the easy magnetization direction of the entire film.

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We report the evolution of the magnetoresistance (MR) and magnetization behavior of Ni80Fe20 wire arrays as the width (w) is reduced from 200 to 0.3 μm. At around 1.5 μm width, the hysteretic behavior seen in continuous films shows the near reversible behavior characteristic of spin rotation processes. At low field (H<Ms), the anisotropic magnetoresistance determines the response and is strikingly size dependent. At high field (H>Ms), the linear MR response to the in-plane perpendicular hard axis field suggests a bulklike transverse MR effect.

We have studied the magnetic moment and in-plane strain in epitaxial Cu/Ni/Cu/Si͑100͒ structures by varying both the Ni and Cu buffer layer thickness. We find a sharp reduction in magnetic moment with increasing Ni lattice strain. Our structural and temperature-dependent studies exclude interdiffusion, interface roughness, and a decreased Curie temperature as possible causes of the reduced moment, but reveal a strong correlation between the strain and magnetic moment in Cu/Ni/Cu structures.

Systematic measurements of the magnetic moment per Ni atom in Cu/Ni/Cu/Si͑001͒ structures have been made using polarized neutron reflection ͑PNR͒ for Ni thicknesses in the range 30 ÅϽtϽ400 Å at room temperature. We find a dramatic reduction in the magnetic moment per atom for tϽ100 Å and near bulk values above 100 Å. These results are corroborated by alternating gradient magnetometer measurements on the same samples. A Cu/Ni-wedge/Cu/Si͑001͒ structure with 30 ÅϽtϽ150 Å was studied using magnetic circular x-ray dichroism ͑MCXD͒, polar magneto-optical Kerr effect ͑MOKE͒, and reflection high-energy electron diffraction ͑RHEED͒ in order to estimate the variation in the values of ͗L z ͘, ͗S z ͘, perpendicular anisotropy strength, and surface in-plane Ni lattice constant, respectively, during epitaxial growth. RHEED measurements show that the in-plane lattice constant falls by 1.7% in the Ni thickness range 30 ÅϽtϽ90 Å. The MCXD measurements reveal the same trend for ͗L z ͘, ͗S z ͘, and total moment per atom versus Ni thickness as found for the total moment by PNR. Polar MOKE measurements confirmed the transition from a perpendicular easy axis towards an in-plane magnetic easy axis as has already been extensively studied in the literature. Comparison of the PNR results with RHEED measurements reveal a striking correlation between the increase of in-plane strain and reduction in magnetic moment per atom with decreasing Ni thickness. While a direct strain-induced variation of the moment based on bulk phase calculations cannot account for the magnitude of the moment variations we observe, we show that the results cannot be attributed to sample contamination, interdiffusion, or a reduction of the Curie temperature with decreasing Ni thickness. Furthermore, the presence of a magnetically dead layer in the samples is not consistent with the PNR results. The strong moment variation partially explains the large thickness range for which perpendicular anisotropy is observed in this system.