Field dependent rf absorption spectra of particulate magnetic recording materials

Journal of Magnetism North-Holland

and Magnetic

Materials

104-107

(1992) 967-968

Field dependent rf absorption recording materials S.R. Hoon

‘, G.R. Lawson

spectra

‘, F. Thompson

of particulate

magnetic

‘, B. Dean ’ and R.W. Chantrell

’

‘I Depurtment of Mathematics and Physics, Manchester Polytechnic, Manchester MI 5GD, UK h Department of Physics and Astronomy, Lancashire Polytechnic. Preston PRI 2TQ, UK ’ Department of Physics, Keele Utziversity, Keele ST.5 5BG, UK Swept frequency absorption (SFA) has been studied in particulate recording materials from 0.2 MHz to 40 GHz and in fields (B) of up to 2.4 T. Line shapes have been computed as a function of damping parameter cy employing the Landau-Lifshita equation and Stoner-Wohlfarth model. Experimentally we find (Y= yB, /T,, to be dependent upon particle texture and moment alignment with field B. It

is a general

digital recording ness, MC/M, +

requirement

of

all

analogue

and

media that they exhibit high square1 and dM/dH, + x whilst the quality

of recording media may be characterized by the Switching Field Distribution (SFD) determined from the gradient of the remanant demagnetisation curve. A narrow SFD is indicative of a uniform micromagnetic environment and low media noise. The SFD will be broadened by particle interactions and surface or crystalline imperfections as these phenomena contribute to the effective anisotropy field distribution, f(Bk). As f(Bk) determines the FMR linewidth and the transverse susceptibility xt, which diverges at the switching field, FMR may be employed to investigate the SFD. Conventional FMR however employs swept dc fields (B) at constant frequency (II). Thus for broad linewidths typical of particulate materials (- 0.3 T at X-band) the magnetic state is significantly perturbed during FMR observation. This restriction is absent from the complementary Swept Frequency Absorption (SFA) technique in which B is an independent parameter. SFA consti-

05T 041 03T 02T

_

tutes a magnetically noninvasive microwave technique for determining linewidth. The FMR/SFA absorption frequency IJ,) is given by the Larmor frequency vg = -yB,rr/2~r where B,, is found from the free energy E as (VE)‘/*/M. Consideration of the limiting cases for a uniaxial anisotropy namely, (i) for B parallel to B,, B,, = (B, + B) and (ii) for B perpendicular to B,, B,, = (Bi + B’)‘/*, B B,, clearly indicate that the linewidth will reflect the distribution of easy axes. Travelling wave SFA in particulate materials has been studied from 0.2 MHz to 40 GHz and in fields up to 2.4 T employing transmission TEM coaxial cells and TE waveguide coupled to a scalar network analyser. In the TEM cells the wave vector k and B are parallel. For the TE mode, k and B are perpendicular and the absorption nonreciprocal upon reflection. This slightly broadens the TE spectra artificially. Comparative magnetic properties have been determined employing an X-band FMR spectrometer and 12 T VSM. Line shapes

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Fig. 1. SFA spectra 0312.8853/92/$05.00

for powder

35

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Frequency G Hz

magnetite

0 1992 - Elsevier

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particles

Science

(a) low frequency

Publishers

TEM absorption

B.V. All rights reserved

(b) high frequency

TE absorption.

968

S.R. Hoon et al. / Particulate magnetic recording materials

have been computed as a function of damping parameter (Y employing the Landau-Lifshitz equation and Stoner-Wohlfarth model. Fig. 1 indicates asymmetry SFA spectra for low aspect ratio partially aligned magnetite particles. Detailed analysis reveals that below 0.5 T, prior to the onset of saturation, the maximum peak height A, increases linearly with (dc) magnetisation M. Here M comprises a measure of the population difference between particle spin up and spin down states. The asymmetric FWHM linewidth W decreases linearly with A,. IV, determined from the linearised absorption, is a measure of the distribution of the local field. W approaches a minimum prior to M = M,. For random axis barium ferrite samples broad weakly field dependent line shapes are observed consistent with strong interactions between platelets. In all samples investigated relaxation times T() of between 2 and 8 x lO_ ” s may be inferred from W and are consistent for g = 2 with observed FMR linewidths of _ 0.3-0.8 T. Many of the spectra apparently display more than one component and cannot be readily characterised by W alone being neither Lorentzian nor Gaussian. The model calculations of fig. 2 indicate that this is entirely due to a distribution of uniaxial easy axes. These spectra have been calculated for random single domain particles of

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Fig. 2. Calculated susceptibility for single domain particles aspect ratio 10: 1 and M, = 350 kA m-‘.

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Fig. 3. SFA peak frequency vs B for powder (pwdr) potted (potd) materials with R = 2 line for comparison.

and

10: 1 aspect ratio and M, = 350 kA mm’ for 0.1 0.3 broadening of the peaks removes the structure seen in figs. I and 2. Fig. 3 summarises resonant peak frequency Y”(B) for a variety of dry powder and potted samples. The data generally has the form v,, = y(g’/gXB + Bk) from which Bk and effective g-values g’ may be determined. We find values of Bk typical of interacting acicular and plate-like particles and, in all cases, g ’ < 2 by up to 15%. The theoretical calculations show similar variations in g’ for (Y< 0.3. For larger cy, g’ is found to be greater than 2. The increase in W(B) with B is found to be more pronounced for large (Y. Conversely, the decrease in A(B) with increasing B diminishes for large cy. By comparison of calculated and observed SFA spectra and their field and texture dependence we conclude that experimentally CKdecreases with increasing B or particle alignment. This is an important conclusion as it enables a(B) to be directly related to micromagnetic processes. We find typical values of cy to lie between 0.1 and 0.5. SERC edged

and CAMST

support

is gratefully

acknowl-