B. Vögeli

Magnetic behavior of lithographically patterned particle arrays (invited)

This article reviews recent progress in the fabrication, characterization, and analysis of large area arrays of sub-100-nm magnetic particles made by lithographic techniques. Particles are made by electrodeposition, evaporation and liftoff, or sputtering and etching, leading to a wide range of shapes, compositions, and microstructures. The remanent states, magnetic hysteresis, and uniformity of the particles and the interparticle interactions will be discussed.

Magnetization reversal in sub-100 nm pseudo-spin-valve element arrays

The magnetization reversal exhibited by arrays of 70-nm-wide pseudo-spin-valve (PSV) elements has been investigated by measurements of minor hysteresis loops. Samples were patterned from sputtered NiFe (6 nm)/Cu (3 and 6 nm)/Co (4 nm)/Cu (4 nm) magnetic thin film stacks. The overall room temperature magnetic behavior of the arrays can be understood by considering a distribution of switching fields for both the hard (Co) and soft (NiFe) magnetic layers. Such layers interact through exchange and magnetostatic coupling. Increasing the lengths of the elements leads to narrower switching field distributions and higher mean switching fields (particularly for the hard layer). On the other hand, decreasing the thickness of the Cu spacer leads to an increase of the switching field of the hard layer. Results obtained are well described by a model that treats each PSV as a coupled pair of rectangular single-domain films and uses the values of the interaction field between layers deduced from experimental minor loops.

Magnetization switching in 70-nm-wide pseudo-spin-valve nanoelements

The magnetic domain structures and magnetization reversal of patterned 70-nm-wide pseudo-spin-valve (PSV) elements were studied by magnetic force microscopy (MFM). Both magnetically soft and hard layers form single-domain states at remanence, and can be magnetized either parallel or antiparallel to each other. The switching field of each layer, and the coupling between the layers, are quantified using MFM. Individual elements show well-defined switching fields, while the ensemble has a large switching field distribution due to variability between the PSV elements.

Patterning processes for fabricating sub-100 nm pseudo-spin valve structures

Interference lithography (IL) was used to pattern sputtered Co/Cu/NiFe layers into large-area arrays of pseudo-spin valve (PSV) elements. In order to precisely control size, aspect ratio, and shape uniformity of the elements, three methods of increasing complexity were developed. Pattern transfer was achieved by reactive-ion etching and ion milling, and was found to maintain the multilayered structure of the PSV film. The switching field of the PSV elements, and the remanent state, varied with the aspect ratio as expected. Furthermore, IL was employed to fabricate magnetic random access memory-type structures. Both sense and word lines were conductive, and the buried PSV elements had similar magnetic properties to PSV elements patterned in large-area arrays.

Magnetic switching in 100 nm patterned pseudo spin valves

Progress in developing operative high-density magnetoresistive random access memory (MRAM) devices relies critically on tailoring the magnetic switching occurring in arrays of small patterned pseudo spin valve (PSV) elements. Co/Cu/NiFe PSV films, produced by sputtering in the presence of a magnetic field, have an in-plane anisotropy and switching fields of typically 10 Oe for the soft NiFe and 40 Oe for the hard Co. These films were patterned into arrays of elliptical and circular elements with dimensions of 80 nm to 10 /spl mu/m. The layered structure in these large-area arrays of compositionally modulated PSV elements is preserved through the patterning processes. Hysteresis measurements of the dot arrays show that the switching field of the hard layer increases significantly with decreasing element size, reaching 600 Oe for the smallest elements. Additionally, in patterned elements the soft layer switches prior to field reversal due to magnetostatic coupling between the layers, leading to antiparallel alignment at remanence.

Size-dependent switching of multilayer magnetic elements

Pseudo-spin-valve NiFe∕Cu∕NiFe, Co∕Cu∕Co, NiFe∕Cu∕Co films and magnetic tunnel junction films have been patterned into arrays of rectangular elements with widths of 40–140nm and aspect ratios of 1.5–18. The switching field of the hard and soft layers and the interaction field between the layers have been measured as a function of aspect ratio. In the pseudo-spin-valve structures the behavior is dominated by magnetostatic interactions between the layers, leading to antiparallel alignment of the hard and soft layers at remanence for small aspect ratios. Patterned tunnel junction films show weaker magnetostatic effects, and the exchange bias from the antiferromagnetic layer is preserved on patterning.

Switching field trends in pseudo spin valve nanoelement arrays

Room temperature magnetic properties of arrays of NiFe 6 nm/Cu 3–6 nm Co 4 nm pseudospin valve elements with widths below 100 nm have been investigated as a function of the aspect ratio of the elements. The elements are made from sputtered film stacks patterned using interference lithography and ion milling. The separate magnetic switching of the Co and NiFe layers can be clearly distinguished. The magnetic layers interact through both exchange and magnetostatic coupling. As both the aspect ratio of the element and the Cu spacer thickness increase, the magnetostatic coupling becomes weaker and the magnetization of the layers becomes coupled parallel at remanence.

Magnetization reversal in lithographically patterned sub-200-nm Co particle arrays

A series of Co particle arrays with rectangular elements having a thickness of 10 nm, a width of 90 nm and aspect ratios of 1.3, 2.2, and 3.3, has been fabricated using interference lithography. The switching behavior of these arrays has been studied by measuring isothermal remanence measurement (IRM), dc demagnetization measurement (DCD), and hysteresis loops using magnetometry and magnetic force microscopy (MFM). The single domain structure is the only stable structure at remanence. Nonuniformity and redeposition debris from ion beam etching (IBE) cause a large reversible magnetization component. The comparison between IRM and DVD curves shows that the interactions between the dots are negligible. Both vibrating sample magnetometer (VSM) measurements and MFM images show that the dots switch over a large range of fields, which is believed due mainly to the crystallographic orientation distribution of the grams within each element.

Magnetic force microscopy and x-ray scattering study of 70×550 nm2 pseudo-spin-valve nanomagnets

The room-temperature magnetic properties of large area arrays of 70×550 nm2 nanoelements made from a NiFe 6 nm/ Cu 3 nm/ Co 4 nm multilayer stack have been investigated using magnetic force microscopy (MFM), alternating gradient magnetometry (AGM), and scattering experiments using synchrotron radiation. MFM measurements on individual elements show square major and minor loops, while the collective magnetization reversal, measured from both AGM and elementally specific hysteresis loops obtained from synchrotron scattering experiments, show a wide distribution of switching fields and interaction fields, due to the variability between the elements.

Fabrication of MRAM-type Structure and Their Magnetic Properties.

The characteristic points of MRAM are compared with those of DRAM, Flash memory, SRAM, and FeRAM. The differences in PSV-MRAM and MTJ-MRAM are discussed in terms of the directions of sense current, CIP or CPP, and connections of CMOS with MR elements.An MRAM-type structure was accomplished with three layers of PSV element, NiFe soft layer (6nm) /Cu non-magnetic layer (3-6nm) /Co hard magnetic layer (4nm) on Si-wafer. Each PSV element of 80nm X 150nm was sandwiched by a sense line and a word line at the intersection of these lines. Furthermore, switching phenomena, which were observed in magnetic hysteresis loop by using PSV thin films, are also discussed along with the size limitation of PSV dots.