S. Wirth

Magnetism of nanometer-scale iron particles arrays (invited)

The magnetization behavior in arrays of small ferromagnetic iron particles is investigated. Arrays were fabricated by a combination of chemical vapor deposition and scanning tunneling microscopy. This method allows a variety of particle arrays to be grown differing in particle height, diameter, or arrangement. Moreover, the arrays can be grown directly onto different materials such as Au or permalloy. Magnetic measurements were conducted by Hall magnetometery up to 100 K and compared to switching field measurements by means of magnetic force microscopy at room temperature. The magnetization reversal mechanisms were studied from magnetization curves measured for an arbitrary angle ϑ of the applied field with respect to the long axis of the particles. By analyzing the reversible rotation, the particles’ magnetic core diameter and the shape anisotropy could be determined. A phenomenological model based on thermally activated magnetization reversal was introduced and compared to experimental switching field d...

HIGH MAGNETIC FIELD STUDIES OF 3D AND 4F MAGNETISM IN (R0.7A0.3)MNO3: R=LA3+, PR3+, ND3+, A=CA2+, SR2+, BA2+, PB2+

Magnetization curves up to 25 T at 4.2 K are reported for mixed-valence manganites (R0.7A0.3)MnO3. All the R=La compounds show an extrapolated ferromagnetic moment of ∼3.4 μB/f.u. The A=Sr and Ca compounds show essentially no high-field slope, dM/dB<0.001 μB/T whereas the Pb and Ba compounds show slopes of 0.0047 and 0.0025 μB/T, suggesting that electrons of both spins are present at the Fermi level. All Pr compounds show much greater slopes, dM/dB≈0.02 μB/T associated with a singlet ground state for Pr3+. All except the Ca compound have extrapolated moments of 3.6±0.3 μB/f.u. but (Pr0.7Ca0.3)MnO3 exhibits two first-order magnetization processes, at 5 and 6 T, due respectively to melting of charge order as the compound becomes ferromagnetic and a level-crossing transition for the Pr3+ ion. All the Nd compounds, except for the charge ordered, Ca containing compound, have a magnetization which is ∼1 μB greater than their La counterparts, due to ferromagnetic alignment of Nd and Mn moments.

Hall cross size scaling and its application to measurements on nanometer-size iron particle arrays

Hall crosses were used to measure the magnetic properties of arrays of ferromagnetic, nanometer-scale iron particles. The arrays typically consist of several hundred particles of 9–20 nm in diameter. It is shown that the sensitivity of the measurements can be improved by matching the areas of the Hall cross and the array grown onto it by at least an order of magnitude. We predict that single particles of diameter as small as 10 nm can be measured if grown onto Hall crosses of appropriate size.

Design optimization for a SmCo-biased colossal magnetoresistive thin film device

A device structure composed primarily of a film made from colossal magnetoresistive (CMR) material and a permanent magnet (PM) film used for biasing the CMR film is proposed as a sensor for magnetic objects. The PM film provides a magnetic field through the CMR film, which is substantially decreased when a magnetic object approaches the device, as part of the magnetic flux is now diverted through the object. To test the feasibility of this concept, we performed simulations of the magnetic field distribution with a three-dimensional finite boundary program. Our vertical model structure consists of a strip of CMR material covered by a PM strip with magnetic in-plane texture and magnetized along its width. The maximum possible biasing field through the CMR material is mainly controlled by the thickness to width ratio of the PM material and cannot be improved by the use of a soft magnetic cladding. The same effect is observed on the flux reduction by approaching a magnetic object. Preliminary results on an ex...

Effect of ion-irradiation-induced disorder on the low-field magnetoresistance of La0.67A0.33MnO3 (A=Sr, Ca)

We report on a systematic study of the correlation between the disorder produced by ion irradiation and the magnetoresistance (MR) behavior of two doped manganite films: a highly textured La0.67Sr0.33MnO3 film on a (100) LaAlO3 substrate and a film of La0.67Ca0.33MnO3 on a YSZ substrate with a high degree of large-angle grain boundary disorder. The films were grown via metal organic chemical vapor deposition and exposed to irradiation of 200 keV Ne ions. The temperature and field dependence of the resistivity were measured on the same samples at different levels of damage. For the Sr-doped film, the bulk magnetic properties were observed to degrade faster with cumulative damage than transport. For both films at low temperature, a low-field hysteretic MR was either induced or enhanced by ion damage. These effects are discussed in terms of diffusive transport through “granular” magnetically disordered regions.

GRAIN BOUNDARY EFFECTS ON TRANSPORT IN METALORGANIC CHEMICAL VAPOR DEPOSITION-GROWN, CA-DOPED LANTHANUM MANGANITES

The transport properties of metalorganic chemical vapor deposited films of lanthanum manganite grown on various substrates are investigated. The more disordered films show a magnetoresistance that is both large and relatively temperature independent over a wide temperature range. At low magnetic fields, a linear field dependence is observed and is attributed to spin-polarized intergrain tunneling. In addition, at low fields a hysteretic dependence of resistivity on the magnetic field has been observed. This effect has been attributed to the scattering of spin-polarized carriers at the grain boundary.

Grain Boundary Effets On Transport In MOCVD-Grown, Ca-Doped Lanthanum Manganites

THURSDAY AFTERNOON, JANUARY 8,1998 259 BAYVIEW, 2:OO TO 5:OO Session FD CMR MATERIALS AND THEIR PROPERTIES R. B. Van Dover, Cochair Bell Labs, ID-154, 600 Mountain Road, Murray Hill, NJ 07974 A. Gupta, Cochair IBM Research Center, P.O. Box 218, Yorktown Heights, NY 10598