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.

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 nuclear spins is indicated in our preliminary heat capacity data.

Magnetic ordering of Er + ions in ErBa2Cu30"with 6.12~x~6.91 has been studied for both orthorhombic and tetragonal phases by means of magnetic-susceptibility and heat-capacity measurements. Superconductivity in the orthorhombic phase samples was confirmed by the onset of a diamagnetic susceptibility.The superconducting transition temperature T, was observed to decrease from 92 to 55 K as x was reduced from 6.91 to 6.31. The tetragonal samples with x=6.23 and 6.12 remained paramagnetic down to 1.2 K and showed no overt trace of diamagnetism. The states of Er'+ ions in ErBa2Cu306 and ErBa2Cu307 were calculated by means of crystalline electric field (CEF) theory using both a point-charge model and scaled inelastic neutron scattering results for HoBa2Cu307. The calculated susceptibility is in excellent agreement with data on the tetragonal specimens for 1.2 K~T~160 K. For the orthorhombic samples, sharp heat-capacity peaks which resemble those for a two-dimensional Ising system and evidently correspond to the magnetic ordering of Er + moments are found. The transition temperature T decreases smoothly from 0.604 to 0.545 K as x is varied from 6.91 to 6.31. In contrast, tetragonal samples show only a broad rounded heat-capacity anomaly having its maximum at 0.5 K and closely resembling that for a one-dimensional XY model. The change of Er'+ ion anisotropy from Ising-like to XYlike is explained by the CEF calculation which shows the ground state to be planar for the tetragonal phase but with the b axis slightly favored in the orthorhombic structure. Randomness in the coupling of Er + moments caused by structural disorder may eA'ectively reduce the dimensionality of magnetic ordering as has been found in the case of random dilution. These results imply that the dominant magnetic interactions of Er'+ ions in these materials are of short range.

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 =3.79, gy =2.41, and gz =10.84 and are believed to be relatively few in number. Similar tipping is seen for Dy3+ in YES.

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.