G. J. Bowden

Giant magnetoresistance by exchange springs in DyFe2/YFe2 superlattices

Magnetization and magnetoresistance measurements are reported for antiferromagnetically coupled DyFe2/YFe2 multilayers in fields up to 23 T. It is demonstrated that the formation of short exchange springs ( ~20 A) in the magnetically soft YFe2 layers results in a giant magnetoresistance as high as 32% in the spring region. It is shown that both the magnitude of the effect and its dependence on magnetic field are in good agreement with the theory of Levy and Zhang for domain wall induced giant magnetoresistance.

Negative coercivity in epitaxially grown (110) DyFe2/YFe2 superlattices

Molecular beam epitaxial methods have been used to grow single crystal Laves phase DyFe2/YFe2 superlattice samples with a (110) growth direction. Detailed magnetization curves have been obtained for YFe2 dominated multilayer samples [wDyFe2/4wYFe2]×16 with w=45, 50, and 55 A. In particular, it is shown that the formation of magnetic exchange springs in the magnetically soft YFe2 layers, can be used to engineer multilayer samples with a negative coercivity. Further, by using asymmetric field cycling procedures, we have investigated the irreversible parts of the M–B loop, associated with the switching of the DyFe2 multilayers.

Magnetic properties of epitaxial (110) multilayer films of DyFe2 and YFe2

Laves phase DyFe2/YFe2 multilayers have been grown epitaxially on a YFe2 seed layer, with a (110) growth direction. Magnetic measurements taken in applied fields of up to 12 T, and from 5 K to room temperature, show that short period multilayers (∼100 A) behave, collectively, as a single magnetic entity. As a result, it is possible to engineer magnetic compensation points, in a digital manner, by adjusting the thicknesses of the alternate DyFe2 and YFe2 layers. Nevertheless, the magnetic response of the DyFe2/YFe2 structure and that of the YFe2 seed layer are not completely independent of one another. Because of a mismatch in the Fe–Fe magnetic exchange at the multilayer/seed interface, a 180° magnetic soliton-like domain (“magnetic twister”) is set up in the top of the YFe2 seed layer. A semiquantitative model describing the properties of the magnetic twister is presented and discussed.

Magnetic control of a meta-molecule

Metamaterials offer the prospect of new science and applications. They have been designed by shaping or changing the material of the individual meta-molecules to achieve properties not naturally attainable. Composite meta-molecules incorporating a magnetic component offer new opportunities. In this work we report on the interaction between a non-magnetic split ring resonator (SRR) and a thin film of yttrium iron garnet (YIG). Strong hybridized resonances are observed. While the SRR is characterized by a magnetic and electric resonance, in practice, it is found that the YIG couples strongly to this symmetric (electric) mode of the SRR. It is also demonstrated that the anti-crossing region provides fertile ground for the creation of elementary excitations such as backward volume magnetostatic waves.

Magnetization dynamics in an exchange-coupled NiFe/CoFe bilayer studied by x-ray detected ferromagnetic resonance

Exchange-coupled hard and soft magnetic layers find extensive use in data storage applications, for which their dynamical response has great importance. With bulk techniques, such as ferromagnetic resonance (FMR), it is difficult to access the behaviour and precise influence of each individual layer. By contrast, the synchrotron radiation-based technique of x-ray detected ferromagnetic resonance (XFMR) allows element-specific and phase-resolved FMR measurements in the frequency range 0.5–11 GHz. Here, we report the study of the magnetization dynamics of an exchange-coupled Ni0.81Fe0.19 (43.5 nm)/Co0.5Fe0.5 (30 nm) bilayer system using magnetometry and vector network analyser FMR, combined with XFMR at the Ni and Co L2 x-ray absorption edges. The epitaxially grown bilayer exhibits two principal resonances denoted as the acoustic and optical modes. FMR experiments show that the Kittel curves of the two layers cannot be taken in isolation, but that their modelling needs to account for an interlayer exchange coupling. The angular dependence of FMR indicates a collective effect for the modes of the magnetically hard CoFe and soft NiFe layer. The XFMR precessional scans show that the acoustic mode is dominated by the Ni signal with the Co and Ni magnetization precessing in phase, whereas the optical mode is dominated by the Co signal with the Co and Ni magnetization precessing in anti-phase. The response of the Co signal at the Ni resonance, and vice versa, show induced changes in both amplitude and phase, which can be ascribed to the interface exchange coupling. An interesting aspect of phase-resolved XFMR is the ability to distinguish between static and dynamic exchange coupling. The element-specific precessional scans of the NiFe/CoFe bilayer clearly have the signature of static exchange coupling, in which the effective field in one layer is aligned along the magnetization direction of the other layer.

Giant magnetic modulation of a planar, hybrid metamolecule resonance

Coupling magnetic elements to metamaterial structures creates hybrid metamolecules with new opportunities. Here we report on the magnetic control of a metamolecule resonance, by utilizing the interaction between a single split ring resonator (SRR) and a magnetic thin film of permalloy. To suppress eddy current shielding, the permalloy films are patterned into arrays of 30–500 μm diameter discs. Strong hybridized resonances were observed at the anticrossing between the split ring resonance and the ferromagnetic resonance (FMR) of the permalloy. In particular, it is possible to achieve 40 dB modulation of the electric (symmetric) mode of the SRR on sweeping the applied magnetic field through the SRR/FMR anticrossing. The results open the way to the design of planar metamaterials, with potential applications in nonlinear metamaterials, tunable metamaterials and spintronics. S Online supplementary data available from stacks.iop.org/NJP/16/063002/ mmedia

Magnetic anisotropy in the cubic Laves REFe2 intermetallic compounds

In the past, the Callen–Callen (1965 Phys. Rev. 139 A455–71; 1966 J. Phys. Chem. Solids 27 1271–85) model has been highly successful in explaining the origin and temperature dependence of the magneto-crystalline anisotropy in many magnetic compounds. Yet, despite their high ordering temperatures of ∼650 K, the Callen–Callen model has proved insufficient for the REFe2 compounds. In this paper, we show that it is possible to replicate the values of the phenomenological parameters K1, K2 ,a ndK3 given by Atzmony and Dariel (1976 Phys. Rev. B 13 4006–14), by extending the Callen– Callen model to second order in HCF .I n particular, explanations are provided for (i) the unexpected changes in sign of K1 and K2 in HoFe2 and DyFe2, respectively, and (ii) the origin and behaviour of the K3 term. In addition, it is demonstrated that higher order terms are required,and that K4 exceeds K3 at low temperatures. Revised estimates of K1, K2, K3, K4 ,a ndK5 are given. Finally, an alternative ‘multipolar’ approach to the problem of magnetic anisotropy is also provided. It is shown that the latter confers significant advantages over the older phenomenological method. In particular, all the multipolar coefficients

Discrete exchange-springs in magnetic multilayer samples

The properties of soft magnetic exchange-springs in both bilayer and multilayer samples are investigated, with particular emphasis on the discrete nature of the spring. It is shown that, in a mean-field model, a very simple relationship exists between the bending field BB, the exchange field BEX, and the number of monolayers N in the soft magnetic layer. For bilayers BB/BEX = (π/2N)2, whereas for multilayers BB/BEX = (π/N)2. In addition, it is shown that Jacobi elliptic functions, originally used by Goto et al for continuous bilayer springs, provide a surprisingly robust description of discrete bilayer and symmetric multilayer exchange-springs. Finally, the problem of soft exchange-spring penetration into neighbouring hard magnetic layers is discussed. Calculations show that this is an important effect, which leads to a reduction in the bending field BB.

Engineering coercivity in epitaxially grown (110) films of DyFe2–YFe2 superlattices

Molecular beam epitaxial methods have been used to grow single crystal Laves phase DyFe2–YFe2 superlattice samples with a ~110! growth direction. It is shown that it is possible, in principle, to engineer a desired coercivity between the limits KDyFe2<K<` . This can be achieved by adjusting the relative thickness of the individual DyFe2 and YFe2 layers, in multilayer films This novel feature is illustrated, using the superlattice films @x A DyFe2 /(100-x) A YFe2#340, with x580, 60, 50, and 45. It is found that the measured coercivity is in semiquantitative agreement with a simple theoretical expression, for the nucleation fields in both bilayer and multilayer compounds. However, in practice, exchange spring penetration into the DyFe2 layers can set a limit to the maximum coercivity that can be achieved.

Micromagnetic simulation of the magnetic exchange spring system DyFe2∕YFe2

Magnetic measurements of [110] [50ADyFe2∕200AYFe2] reveal a rich switching behavior: the formation of exchange springs in this system of alternating hard and soft layers can be observed for low temperatures (LTs). For high temperatures (HTs), the appearance of the hysteresis loop changes significantly, implying a more complicated reversal process. In this article, we reproduce hysteresis loops for net and compound-specific magnetizations by means of micromagnetic simulations and assess the quality by a direct comparison to recent x-ray magnetic circular dichroism measurements. The HT switching characteristics, showing a magnetization reversal of the hard magnetic layer before the soft magnetic layer, are investigated and understood on the basis of detailed magnetic configuration plots. The crossover of LT to HT switching patterns is explained by energy considerations, and the dependence on different parameters is outlined.