Magneto-Resistive Materials

Other Unique Engineering Ideas
Magnetic materials have long been interesting from both scientific and technological points of view. From magnetic storage to compasses for navigation, the magnetic properties of materials continue to drive the development of new technologies.

1. Description

2. Why

3. MR Nanowires and Arrays

4. How

5. Future Trends

6. Related Links

Description

One technologically relevant magnetic phenomenon is magnetoresistance (MR), in which the electrical transport properties are strongly affected by applied magnetic fields. Most magnetoresistive materials exhibit diminished electrical resistivity as the applied magnetic field strength increases.In some materials, MR behavior is intrinsically related to the details of the structure and electron interactions within the crystal. Other materials may not display MR behavior alone, but can be used to prepare composite materials or devices that do exhibit a magnetoresistive response.

Why

The magnetic properties of the lanthanum manganese oxide class of materials have attracted tremendous interest recently because of the dramatic increase in conductivity these systems exhibit when the magnetic moments order ferromagnetically, either by lowering the temperature or by applying a magnetic field.This huge change in the carrier mobility, which has been given the name “colossal magnetoresistance” (CMR), is both of scientific and technological interest. In particular, it is anticipated that related materials may provide the next generation of read/write heads for the magnetic data storage industry, while the “half-metallic” behavior provides fully spin polarized electrons for use in magneto-electronics applications, and for sensors in a variety of applications such as in the automotive industry. 

MR Nanowires and Arrays

Within the past decade, one-dimensional nanowires and nanowire arrays have captured the interests of many groups in a wide variety of fields, mainly due to their unusual properties and potential for integration into and miniaturization of current technologies.

  • Template-assisted electrodeposition provides a convenient, cost-effective route toward the fabrication of nanowire arrays.
  • Current theories of magnetoresistive behavior vary widely, and many are still not completely understood.
  • The reduction of magnetoresistive systems to 1-D should provide an experimental testing ground for predictions of pre-existing models and possibly give rise to new models altogether.

Moreover, MR measurements of nanowire arrays will not only provide new insights into the mechanisms of magnetoresistance, but also carry a high potential for revealing new and unexpected discoveries in the field of magnetoresistivity.In addition to measurements of nanowire arrays, a method for making electrical contact to single nanowires during electrochemical growth has recently been adopted to make measurements on single wires in an effort to separate collective array effects from the intrinsic properties of nanowires.

How

Spin dependent tunneling (SDT) wafers were deposited using dc magnetron sputtering. SDT junctions were patterned and connected with one layer of metal lines using photolithography techniques.

  • These junctions have a typical stack structure of Si(100)-Si/sub 3/N/sub 4/-Ru-CoFeB-Al/sub 2/O/sub 3/-CoFeB-Ru-FeCo-CrMnPt with the antiferromagnet CrMnPt layers for pinning at the top.

  • High-resolution transmission electron microscopy (HRTEM) reveals that the CoFeB has an amorphous structure and a smooth interface with the Al/sub 2/O/sub 3/ tunnel barrier.

  • Although it is difficult to pin the amorphous CoFeB directly from the top, the use of a synthetic antiferromagnet (SAF) pinned layer structure allows sufficient rigidity of the reference CoFeB layer.

  • The tunnel junctions were annealed at 250/spl deg/C for 1 h and tested for magneto-transport properties with tunnel magnetoresistive (TMR) values as high as 70.4% at room temperature, which is the highest value ever reported for such a sandwich structure.

  • This TMR value translates to a spin polarization of 51% for CoFeB, which is likely to be higher at lower temperatures.

  • These junctions also have a low coercivity (Hc) and a low parallel coupling field (Hcoupl).

  • The combination of a high TMR, a low Hc, and a low Hcoupl is ideal for magnetic field sensor applications.

Spin valves are widely studied due to their application as magnetoresistive material in magnetic recording heads and other magnetic field sensors. An important film property is the interlayer coupling field (called offset field Ho or ferromagnetic coupling field Hf). The Néel model for orange-peel coupling can be applied successfully to describe the interlayer coupling. The waviness associated with the developing granular structure is thereby taken as the relevant waviness.Magnetic poles created at the outer surfaces of the magnetic layers result in an anti-ferromagnetic interaction with the poles at the inner surface of the opposite layer.

  • A magnetoresistive sensor includes a magnetoresistive material, formed on a substrate, and having a first edge and a second edge.

  • A first multilayered conductive lead structure is electrically connected to the first edge.

  • And a second multilayered conductive lead structure is electrically connected to the second edge.

  • The first and second conductive lead structures are constructed of multiple layers of thin film materials

  • They alternate between at least one layer of a thin film of a refractory metal interlaid between at least two thin film layers of a highly conductive metal.

The magnetoresistive “electron”-doped materials have been investigated using powder neutron diffraction. The two materials are n-type semiconductors which exhibit antiferromagnetic ordering at, but they have different magnetic structures. The sample orders in a simple G-type antiferromagnetic structure, which is also observed in CaMnO3.The CaMnO2.89 sample, on the other hand, exhibits two magnetic features:

  • G-type reflections and,

  • A set of reflections that can be indexed on a k=(0,0,1 / 4) ordering wave vector

The model for the magnetic structure is proposed which involves Mn3+/Mn4+ charge ordering concomitant with the magnetic ordering. The presence of a set of weak, temperature independent structural reflections which can also be indexed on a k=(0,0,1 / 4) supercell suggests an oxygen vacancy ordering which may play a role in the charge ordering.

Future Trends

One of the central questions in the field of manganites concerns the lattice involvement in the mechanism of CMR. While the relation between ferromagnetism and conductivity was explained in terms of double exchange, it is now clear that a full understanding of these materials must include the lattice degrees of freedom.In particular, the formation of lattice polarons above the Curie temperature has been inferred from a variety of measurements, but direct evidence has been lacking.

Keywords

Magnetoresistive materials, Charge and magnetic ordering, magneto resistance, Spin dependent tunneling (SDT)

Related Articles

Related Links