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J. F. Cochran

Bio: J. F. Cochran is an academic researcher from Simon Fraser University. The author has contributed to research in topics: Ferromagnetic resonance & Magnetization. The author has an hindex of 25, co-authored 75 publications receiving 2715 citations.


Papers
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Journal ArticleDOI
TL;DR: In this article, anisotropic magnetic anisotropies and exchange interactions in ultrathin metallic magnetic films are discussed. But the authors do not consider the magnetic exchange interaction in this paper.
Abstract: (1993). Ultrathin metallic magnetic films: magnetic anisotropies and exchange interactions. Advances in Physics: Vol. 42, No. 5, pp. 523-639.

412 citations

Journal ArticleDOI
TL;DR: In this paper, a detailed discussion of the underlying physical mechanisms in FMR and SMOKE studies is presented and the temperature and thickness dependences of the perpendicular uniaxial and fourfold in-plane anisotropies are given and discussed.
Abstract: Metastable fcc structures using ultrathin layers of metastable fcc Co(001) and fcc Cu(001) were grown by molecular-beam epitaxy. The growth was studied using reflection high-energy electron-diffraction (RHEED) patterns and RHEED intensity oscillations. The magnetic properties were investigated by employing ferromagnetic-resonance (FMR) and surface magneto-optic Kerr effect (SMOKE) techniques. A detailed discussion of the underlying physical mechanisms in FMR and SMOKE studies is presented. The temperature and thickness dependences of the perpendicular uniaxial and fourfold in-plane anisotropies are given and discussed. The role of lattice strains in magnetic anisotropies is demonstrated. FMR measurements revealed that the magnetic properties of single Co layers are different from those in Co/Cu/Co trilayers becuase of a lattice-strain relaxation in the multilayer samples. The exchange coupling between fcc Co(001) layers separated by a fcc Cu(001) interlayer was studied for several structures. The exchange coupling in Co structures was found to be anisotropic. Hysteresis loops were measured by means of SMOKE. Magnetic-trilayer hysteresis loops are complex. Micromagnetic calculations were carried out to explain their main features.

242 citations

Journal ArticleDOI
TL;DR: In this article, the authors extended the theory of two-magnon scattering to disordered ferromagnetic metals and derived the formula for the rf susceptibility in the parallel (in plane) configuration by using the retarded Green's function formalism.
Abstract: The theory of two‐magnon scattering is extended to disordered ferromagnetic metals. In order to treat properly the role of two‐magnon scattering in metals we have derived the formula for the rf susceptibility in the parallel (in plane) configuration by using the retarded Green’s function formalism. The roles of the elliptical polarization, the finite values of resonance k vectors, nonresonant spin‐wave interactions, eddycurrent effects as well as the contribution of spin‐orbit interaction to the intrinsic damping are included. We have evaluated the susceptibility in the presence of magnetostatic inhomogeneities, statistical variations of local properties and fluctuating exchange interactions.

182 citations

Journal ArticleDOI
TL;DR: A new phase of Cu is grown, bcc Cu, on bcc Fe(001), and it is shown that bCC Cu(001) grows epitaxially on bCC Fe (001) and that bccfe (001), which maintains its structure for 10\char21{}11 monolayers before undergoing a structural transition.
Abstract: We have grown a new phase of Cu, bcc Cu, on bcc Fe(001). We have shown that bcc Cu(001) grows epitaxially on bcc Fe(001) and that bcc Fe(001) grows epitaxially on bcc Cu(001). bcc Cu(001) on Fe(001) maintains its structure for 10--11 monolayers before undergoing a structural transition. Ultrathin well-defined trilayers of Fe(001)/Cu(001)/Fe(001) with Cu thicknesses between 6--12 monolayers were grown. Ferromagnetic resonance, Brillouin light scattering, and surface magneto-optical Kerr effect were used to measure the coupling between the Fe layers. The coupling between the Fe layers changed from ferromagnetic to antiferromagnetic between 9--10 monolayers of Cu.

170 citations


Cited by
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TL;DR: In this paper, a detailed study of static and dynamic magnetic behavior of Fe3O4 nanoparticles with average particle sizes ranging from 5 to 150 nm is presented, and the existence of surface spin disorder can be inferred from the decrease of saturation magnetization MS at low temperatures, as the average particle size is reduced.
Abstract: We present a detailed study of static and dynamic magnetic behavior of Fe3O4 nanoparticles with average particle sizes 〈d〉 ranging from 5 to 150 nm. Bulk-like properties such as saturation magnetization, hyperfine parameters, coercive field, and Verwey transition are observed in 150 nm particles. For decreasing particle size, the Verwey temperature, TV, shifts down to ∼20 K for 〈d〉=50 nm and is no longer observable for smaller particles. The smallest particles (〈d〉=5 nm) display superparamagnetic behavior at room temperature, with transition to a blocked state at TB∼45 K, which depends on the applied field. The existence of surface spin disorder can be inferred from the decrease of saturation magnetization MS at low temperatures, as the average particle size is reduced. This disordered surface did not show effects of exchange coupling to the particle core, as observed from hysteresis loops after field cooling in a 7 T magnetic field. For particles with 〈d〉=5 nm, dynamic ac susceptibility measurements show...

1,265 citations

Journal ArticleDOI
TL;DR: Simulations confirm that voltage-controlled magnetization switching in magnetic tunnel junctions is possible using the anisotropy change demonstrated here, which could be of use in the development of low-power logic devices and non-volatile memory cells.
Abstract: A voltage-induced symmetry change in a ferromagnetic material can change its magnetization or magnetic anisotropy, but these effects are too weak to be used in memory devices. Researchers have now shown that a relatively small electric field can cause a large change in the magnetic anisotropy of a few atomic layers of iron. The results could lead to low-power logic devices and non-volatile memory cells.

1,201 citations

Journal ArticleDOI
TL;DR: In this paper, a comprehensive survey of experimental studies on the magnetic anisotropy in metallic multilayers containing Fe, Co or Ni is presented and commented on, with the help of some dedicated experimental studies.
Abstract: Ferromagnetic materials exhibit intrinsic `easy' and `hard' directions of the magnetization. This magnetic anisotropy is, from both a technological and fundamental viewpoint one of the most important properties of magnetic materials. The magnetic anisotropy in metallic magnetic multilayers forms the subject of this review article. As individual layers in a multilayer stack become thinner, the role of interfaces and surfaces may dominate that of the bulk: this is the case in many magnetic multilayers, where a perpendicular interface contribution to the magnetic anisotropy is capable of rotating the easy magnetization direction from in the film plane to perpendicular to the film plane. In this review, we show that the (in-plane) volume and (perpendicular) interface contribution to the magnetic anisotropy have been separated into terms related to mechanical stresses, crystallographic structure and the planar shape of the films. In addition, the effect of roughness, often inherent to the deposition techniques used, has been addressed theoretically. Several techniques to prepare multilayers and to characterize their growth as well as methods to determine the magnetic anisotropy are discussed. A comprehensive survey of experimental studies on the perpendicular magnetic anisotropy in metallic multilayers containing Fe, Co or Ni is presented and commented on. Two major subjects of this review are the extrinsic effects of strain, roughness and interdiffusion and the intrinsic effect of the crystallographic orientation on the magnetic anisotropy. Both effects are investigated with the help of some dedicated experimental studies. The results of the orientational dependence studies are compared with ab initio calculations. Finally, the perpendicular surface anisotropy and the in-plane step anisotropy are discussed.

1,099 citations

Journal ArticleDOI
TL;DR: In this article, the role of spin pumping in layered structures is discussed and the main body of the theory is semiclassical and based on a mean-field Stoner or spin-density functional picture, but quantum-size effects and electron-electron correlations are also discussed.
Abstract: Two complementary effects modify the GHz magnetization dynamics of nanoscale heterostructures of ferromagnetic and normal materials relative to those of the isolated magnetic constituents. On the one hand, a time-dependent ferromagnetic magnetization pumps a spin angular-momentum flow into adjacent materials and, on the other hand, spin angular momentum is transferred between ferromagnets by an applied bias, causing mutual torques on the magnetizations. These phenomena are manifestly nonlocal: they are governed by the entire spin-coherent region that is limited in size by spin-flip relaxation processes. This review presents recent progress in understanding the magnetization dynamics in ferromagnetic heterostructures from first principles, focusing on the role of spin pumping in layered structures. The main body of the theory is semiclassical and based on a mean-field Stoner or spin-density-functional picture, but quantum-size effects and the role of electron-electron correlations are also discussed. A growing number of experiments support the theoretical predictions. The formalism should be useful for understanding the physics and for engineering the characteristics of small devices such as magnetic random-access memory elements.

1,051 citations