G. H. Bellesis

Media for 10 Gb/in.2 hard disk storage: Issues and status (invited)

Future 10 Gb/in.2 recording densities represent submicron trackwidths and sub‐100 nm bit lengths. This requires extremely small magnetic switching units and very high coercivities of the media to satisfy the signal‐to‐noise ratio requirements. At the same time the question of magnetic thermal stability and the lack of transducers capable of performing at these densities makes it difficult to evaluate media. An uncoupled, highly uniform magnetic grain size of about 10 nm is a compromise toward maintaining an adequately low media noise and yet maintaining magnetic stability. Here we discuss current media construction, the detrimental role of substrate roughness, the role of new media structures and alloys on microstructure and magnetic properties as well as techniques for evaluating media performance prior to the availability of the required playback heads.

Magnetostriction measurement by interferometry

A laser Doppler interferometric method using synchronous detection to measure the magnetostriction of soft magnetic films is described. The instrument measures 2 AA magnetostrictive deflections of Permalloy films on 500 mu m silicon substrates. Saturation magnetostriction of 10/sup -9/ can be measured for 1000 nm films on 100 mu m Si substrates. The ultimate resolution of the interferometer is 0.1 AA as demonstrated using piezoelectrically-driven mirrors. >

Electron paramagnetic resonance of Er3+, Dy3+, and Gd3+ in Y(CF3SO3)3⋅9H2O

The paramagnetic resonance spectra of 1% Er3+ and 1% Dy3+ substituted in hexagonal Y(CF3SO3)3⋅9H2O (YTFMS) single crystals have been measured at 4.2 K and 9.215 GHz. Resonances due to Gd3+ impurities were identified and studied at 77 K. In each case, the results strongly resemble those for the given ion in an Y(C2H5SO4)3⋅9H2O (YES) or La(C2H5SO4)3⋅9H2O (LaES) host suggesting that R3+ site symmetry in YTFMS is also C3h. For Er3+, the spin Hamiltonian parameters are found to be g∥ =1.62, g⊥ =8.67, A=0.006 cm−1, B=0.030 cm−1, and P=0.001 cm−1. For Gd3+ we find g=1.988, b02 =0.010 88 cm−1, b04 =−0.000 418 cm−1, and b06 =0.000 058 cm−1. In the case of Dy3+, resonance is observed for ions whose major g‐tensor axis coincides with the YTFMS c axis and have g∥ =10.90 and g⊥ ≊0. Another resonance, which splits into a six line pattern when the field is rotated away from the c axis, is attributed to ions whose g tensors are tipped by 20° from that axis by the perturbing effect of lattice imperfections. These have gx ...

Magnetism of rare‐earth salts R(CF3SO3)3⋅ 9H2O with R=Tb, Dy, and Ce

Magnetic susceptibilities have been measured between 0.05 and 20 K for three rare‐earth trifluoromethanesulfonate nonahydrates R(CF3SO3)3⋅ 9H2O or RTFMS whose hexagonal crystal structure is very similar to that of the rare‐earth ethylsulfate nonahydrates R(C2H5SO4)3⋅ 9H2O or RES. TbTFMS and DyTFMS order ferromagnetically at TC=0.240 K and TC=0.111 K, respectively. CeTFMS remains paramagnetic down to 0.089 K. All three compounds show extreme Ising‐type anisotropy with g⊥∼0. g∥ of each RTFMS compound compares favorably with that of the corresponding RES as expected from crystalline field theories based on a point‐charge model. In each case, the Weiss constant is close to what is expected for a system with predominantly dipolar interactions. The ferromagnetic ordering in TbTFMS and DyTFMS is essentially that of a dipolar Ising system. The splitting Δ of the ground quasi‐doublet in TbTFMS is only ∼0.5 K and the ordering appears to be largely explained by electronic models although substantial ordering of nucl...

Magnetism of hydrated rare‐earth bromates

The dc magnetic susceptibility has been measured for several rare‐earth bromates, R(BrO3)39H2O, RBR, (R=Pr, Tb, Dy, Er, Tm), using a SQUID magnetometer between ∼0.06 and 4 K. The hexagonal structure of RBR crystals is similar to that of the rare‐earth ethylsulfates, RES. Ferromagnetic transitions were observed at 0.125 K for TbBR and 0.170 K for DyBR. For other RBR no ordering was detected to the lowest temperature attained. The magnetic properties observed for all RBR studied here are very similar to those of corresponding RES but show subtle and interesting differences reflecting small differences in the crystal fields. As in the RES, short‐range exchange interaction in the RBR appears to be negligible compared with dipolar interaction.

Magnetic ordering in Ho(CF3SO3)3⋅9H2O and Ho(C2H5SO4)3⋅9H2O

The magnetic susceptibilities χ∥ and χ⊥, both ac (80 Hz) and static, of hexagonal crystals of the structurally similar Ho(C2H5SO4)3⋅9H2O, HoES, and Ho(CF3SO3)3⋅9H2O, HoTFMS, have been measured down to ∼0.1 K. Both order ferromagnetically along the c axis at Tc=0.23 K. Analysis of the susceptibilities above Tc shows g∥≊15.5 and g⊥≊0, ground doublet‐excited singlet separation 8.4 K in HoES and 15 K in HoTFMS, and dipolar interionic interactions. Cp for both salts was also measured and found to exhibit similar sharp cooperative λ peaks at 0.23 K due to ordering of the Ho3+ moments. This peak sits atop the rounded Schottky anomaly of a system of nuclear spins in a fixed hyperfine field. The independence of the electron and nuclear contributions to Cp is associated with the Ising character of the Ho3+ ground non‐Kramers doublet.

Search for high moment soft magnetic materials: FeZrN

FeN materials exhibiting high moment, low coercivity and small magnetostriction have previously been reported. Zr has been known to reduce the magnetostriction in other Fe alloys. The criteria for an ideal recording head pole material as well as shields for magnetoresistive sensors include high moment, low coercivity, high permeability, and zero magnetostriction. We present here the properties of 0.3 μm thick rf sputtered FeZrN films measured as a function of the N2 partial pressure. The films were deposited at a pressure of 3 mT using a Perkin Elmer sputtering system. The target was composed of Fe with Zr chips covering approximately 2% of the surface area. The easy axis and hard axis coercivities show minima of 1.8 and 0.6 Oe, respectively, at 7–10 % N2 partial pressure with a saturation magnetization of about 18 kG. The magnetic anisotropy field is approximately 5 Oe yielding a dc permeability of approximately 4000 along the hard axis. X‐ray data reveal a systematic change in the ratio of α–Fe and γ–Fe...

Magnetic ordering in Dy(BrO3)3·9H2O and Tb(BrO3)3·9H2O

Abstract Dipolar ferromagnetic ordering in two pseudo-hexagonal rare earth bromates (RBR) has been investigated calorimetrically above ∼ 0.09 K. In DyBR a λ-anomaly has been observed in C p at T c = 0.161 K . Ordering in non-Kramers TbBR at 0.125 K is that of an electron-nucleus system in the mixed regime.

Electron paramagnetic resonance of Ce3+ in La(CF3SO3)3.9H2O

The rare‐earth trifluoromethanesulfonates R(CF3SO3)3⋅9H2O (RTFMS) have a hexagonal structure similar to the ethylsulfates R(C2H5SO4)3⋅9H2O (RES). The lowest Kramers doublet in CeTFMS is ‖J=5/2; Jz = ±5/2〉 separated from an excited doublet ‖J=5/2; Jz= ±1/2〉 by Δ/k≂21 K. We have measured electron paramagnetic resonance on 1% and 2% Ce3+ in LaTFMS crystals at 9.214 GHz. At 4.2 K, absorption by two doublets is found yielding g∥(5/2) = 3.813 ± 0.001; g⊥(5/2) = 0.262 ± 0.002; g∥(1/2) = 0.977 ± 0.003; g⊥(1/2) = 2.151 ± 0.002. The temperature variation of intensities shows the ground doublet to be ‖±5/2〉, as in concentrated CeTFMS, but Δ/k=9.6±0.7 K. The doublet order is opposite to that for dilute Ce:LaES but the same as in CeES or Ce:YES and Ce:EuES. In 2% Ce:LaTFMS, satellite resonances due to coupled nn Ce–Ce pairs are also observed. Analysis of the pair data indicates that virtual phonon exchange, while the dominant nondipolar contribution to intrapair coupling, is smaller than for Ce pairs in CeES. This is ...

Low Temperature Properties of Rare Earth Trifluoromethanesulfonates

Hexagonal crystals of rare earth trifluoromethanesulfonate nonahydrates R(CF3SO3)39H2O, RTFMS are similar in structure to the rare earth ethylsulfate nonahydrates RES. χ|| and χ⊥ measured between 0.06 and 20K show RTFMS salts with R = Tb, Dy and Ho to be dipolar ferromagnets ordering at 0.24K, 0.111K and 0.224K, respectively, as do the corresponding RES compounds. Like CeES, CeTFMS remains paramagnetic but their Ce3+ level splittings are different. PrTFMS exhibits antiferromagnetic long-range order below TN≈0.31K while PrES shows only short- range one dimensional order. Measurements of Cp for TbTFMS and PrTFMS confirm their phase transitions.