Michael J. Pechan

Dipolar induced, spatially localized resonance in magnetic antidot arrays

Dipole induced, spatially localized ferromagnetic resonances (at 35 GHz) are observed in micron-sized antidot arrays in permalloy films fabricated with photolithography. All square (3 μm×3 μm) and rectangular (3 μm×4, 5, and 7 μm) array samples exhibit double resonances, with each resonance possessing uniaxial in-plane anisotropy. Interestingly, the easy axes of the two resonances are orthogonal in all cases. The magnitude of the induced dipolar anisotropy decreases with increasing rectangular aspect ratio for one of the resonances, but remains essentially constant for the other. Micromagnetic simulations reveal that the two resonance peaks are the consequence of a dipole field distribution producing two areas with distinctly different demagnetizing field patterns.

Direct measurement of spatially localized ferromagnetic-resonance modes in an antidot lattice (invited)

Recent ferromagnetic-resonance (FMR) measurements and related simulations on antidot structures suggested the existence of spatially localized modes. In this report we confirm the existence of these modes using time-resolved Kerr microscopy (TRKM) as a local probe of the magnetodynamics. FMR measurements on an antidot array (a 40-nm-thick permalloy film with a hole size of 1.5μm and a hole lattice spacing of 3μm×5μm) at frequencies between 10 and 35GHz reveal two main resonances, whose relative amplitudes and orthogonal uniaxial in-plane anisotropies suggest the existence of modes localized between holes along each of the principal axes. TRKM measurements in applied fields ranging from 100to600Oe show explicitly the existence of these two modes—one at low frequency between the holes along the short axis and one at higher frequency between the holes along the long axis. TRKM also reveals additional mode structure, most notably a low-frequency mode localized along the edges of the antidots, similar to the e...

Coercivity enhancement above the Néel temperature of an antiferromagnet/ ferromagnet bilayer

Single-crystal thin films of the antiferromagnet FeF2 have been used to exchange bias overlayers of Fe. An unexpected coercivity enhancement is observed at temperatures above the Neel temperature of the FeF2. This coercivity reaches a peak value of over 600 Oe close to the Neel temperature and persists to above 300 K. The coercivity is correlated with the growth of an anisotropy in the ferromagnet, the increase of the antiferromagnetic susceptibility and the increase of the ferromagnetic resonance linewidth. We argue that the growth of spin fluctuations in the antiferromagnet leads to an enhanced ferromagnetic anisotropy, and therefore coercivity, above the Neel temperature.

Anisotropy determination in epitaxial Sm–Co/Fe exchange springs

We report in-plane anisotropy in epitaxial Sm–Co(x)/Fe(y) bilayers as determined by ferromagnetic resonance (FMR). Four samples, (x,y)=(35,30) and (20, 20) nm each on MgO (110) and (100) substrates, have been prepared via magnetron sputtering. The two substrate orientations result in twofold and fourfold Sm–Co symmetry respectively, with the Sm–Co c-axis in-plane. Magnetization curves indicate elastic exchange spring Fe behavior in reversing fields up to the Sm–Co switching fields (6 and 8 kG at room temperature in the (35, 30) and (20, 20) nm films, respectively). 35 GHz in-plane FMR measurements were made in order to map the crystalline anisotropy of the Fe layer as well as the induced anisotropy from the exchange coupling to the Sm–Co layer. The twofold Sm–Co samples exhibit a clear superposition of the near fourfold Fe crystal field anisotropy (530 Oe) and the unidirectional exchange-bias anisotropy (≈650 Oe) arising from the Fe/Sm–Co interface. The crystalline Fe anisotropy in the fourfold Sm–Co samp...

Magnetic structure of holmium

The magnetic structure of high purity single crystals of holmium has been studied by neutron diffraction techniques. Although the general characteristics of the magnetic structure have been found to agree with earlier measurements, some discrepancies have been resolved and new features have been observed. The magnetic form factor has been measured and compared with relativistic atomic calculations. The low temperature structure (T<20 K) is that of a conical ferromagnet with wave vector (1/6)(2π/c) along the c axis. The basal plane moment is 9.7 μB and the c‐axis ferromagnetic component is 1.6 μB at T=6 K. Bunching of the basal plane moments around the easy hexagonal direction has been observed below T=50 K. Evidence for asphericity in the magnetization density is presented and discussed. The wave vector of the basal plane modulation decreases monotonically with temperature in general accordance with the Elliott–Wedgewood theory. Several inflection points were observed, however, which correspond to commens...

A simple vibrating sample magnetometer for use in a materials physics course

An inexpensive vibrating sample magnetometer (VSM) has been developed for use in a materials physics course. An exercise using the VSM allows students to measure the magnetic properties of various materials and thus gain experience applicable to contemporary research on magnetic materials. This paper describes specific aspects of the construction of a VSM and presents measurements for two 5-mm-long Ni wires of different diameters and for floppy disk media. A 178-μm-diam Ni wire served as a calibration sample for the system; the results from a 51-μm-diam Ni wire set the limit of precision for this system at approximately 5×10−3 emu.

Magnetization reversal and nanostructure refinement in magnetically annealed Nd2Fe14B∕α-Fe-type nanocomposites

Nanostructure refinement, magnetic anisotropy and hard magnetic property enhancement have been observed in melt-spun Nd2.4Pr5.6Dy1Fe84Mo1B6 nanocomposites annealed in an in-plane or out-of-plane field of 1.2T. The magnetic annealing results in an enhancement of an out-of-plane (110) crystal texture of α-Fe and an in-plane uniaxial magnetic anisotropy of the 2:14:1 phase. Magnetic annealing also introduces finer, less angular and more homogeneously distributed soft and hard nanograins. Field dependent torque measurement indicates a complex magnetization reversal mechanism in these nanocomposites. Compared with the sample annealed without a field, there is a noticeable improvement in the hard magnetic properties for the magnetically annealed samples. Especially, the energy product (BH)max was enhanced by 26.6% (from 94to119kJ∕m3). The improvement in the magnetic properties is a result of the enhanced crystallographic texture, nanostructure refinement, and in-plane uniaxial magnetic anisotropy enhancement.

Lateral standing spin waves in permalloy antidot arrays

Spin wave modes in permalloy antidot arrays have been investigated with ferromagnetic resonance at 9.7 GHz. In contrast to a quadratic dispersion expected for exchange standing spin waves, nearly linear relationship exists between the resonance field and mode index, and it can be approximately described by the dipole-dipole and exchange theory. Time-dependent micromagnetic simulations show that the spin wave modes are a result of lateral confinement from the vacant holes. Furthermore, the simulations visually reveal the existence of localized spin waves due to localized boundary conditions in antidots arrays.

Disorder-tuning of hysteresis-loop properties in Co/CoO-film structures

We have studied the effect of magnetic disorder on the magnetization reversal process in thin Co/CoO-films. The antiferromagnetic CoO layer allows a reversible tuning of the magnetic disorder by simple temperature variation, which was experimentally verified by measuring the ferromagnetic resonance linewidth as a function of temperature. The Co/CoO-films exhibit a discontinuous magnetization reversal process for temperatures above a critical temperature Tc, whereas smooth magnetization loops occur for T Tc.

Temperature dependence of interface anisotropy in Ni/Mo multilayers

Equal layer thickness Ni/Mo multilayer samples ranging in Ni layer thickness d from 14 to 340 A have been investigated by x‐band ferromagnetic resonance at temperatures between 4 and 293 K. The dependence of anisotropy (less demagnetization) Ha upon d clearly indicates the interface anisotropy plays a significant role in these samples. Interface (Ks) and volume (Kv) contributions to the anisotropy are extracted at each temperature using an approximate form of Ha=2(Kv+2Ks/d)/M. The Ks temperature dependence is shown to be inconsistent with a simple mean field model and indicates that interface strain due to the Ni‐Mo lattice mismatch is largely responsible for the origin of Ks.