« PreviousContinue »
ASYMMETRIC MAGNETIZATION REVERSAL IN EXCHANGE-BIASED HETEROSTRUCTURES
Exchange bias refers to a shift of the ferromagnetic hysteresis
loop along the field axis, by an amount H, (see Fig. 1 for an example.) The bias is a consequence of an exchange interaction across the interface between dissimilarly ordered magnetic materials, e.g. a ferromagnet and an antiferromagnet (AF). This exchange interaction induces a unidirectional anisotropy as the AF material is cooled through its Néel temperature, TN [1,2]. Exchange bias is an example of a bulk property whose fundamental origin is attributed to physical processes occurring at the nanometer length-scale. This phenomenon is not simply a scientific curiosity; it underpins present-day magnetic recording technology.
Read-write heads used with magnetically stored data are based on giant magnetoresistance (GMR) sensors. These sensors consist of layers of ferromagnetic thin films separated by nonferromagnetic ones. When the the magnetizations in the ferromagnetic layers are all oriented the same way, conduction electrons pass through them relatively easily, but when the electrons must cross from films having one orientation to another they encounter more resistance through magnetic scattering. GMR arises when an external field can change the relative orientations of the magnetization in the films easily. To keep the layers from all reorienting together in the presence of an external field, some of them must be pinned. One way to accomplish pinning is exchange biasing.
Despite its technological importance, theoretical models are unable to convincingly explain observations of exchange bias (e.g. positive exchange bias), and phenomena associated with it. Even in the simplest experimental systems such as Fe on TMF, where TM = Mn or Fe, the asymmetric reversal of magnetization and the unusual temperature dependence of coercivity are not well understood.
Using polarized neutron reflectometry, we recently examined the magnetization reversal processes of a ferromagnetic Fe film exchange-coupled to twinned AF (TMF) films as a function of magnetic field . Neutron scattering measurements typical of those from a sample exhibiting large exchange bias are shown in the Figs. 2 and 3 for fields at coercivity on either side of the loop. Spin-flip (SF) scattering observed on the left hand side of the loop indicates magnetization reversal via magnetization rotation. Lack of SF scattering on the right hand side is consistent with domain nucleation (with opposite magnetization) and growth. These two fundamentally different (asymmetric) reversal processes have distinct neutron scattering signatures. The ability to discern so easily between these processes sets neutron scattering apart from magnetometry.
Comparisons of measurements like those in the Figs. 2 and
3 taken from many samples, including single crystalline and polycrystalline AF films, lead to the following picture: In the case of samples with twinned AF's, which exhibit large exchange bias, “45° exchange coupling" is energetically favorable as each AF domain independently tends to perpendicular coupling but is frustrated due to
M.R. Fitzsimmons, A. Hoffmann, and P.C. Yashar
Los Alamos National Laboratory
Los Alamos, NM 87545
C. Leighton and I. K. Schuller Department of Physics
University of California - San Diego La Jolla, CA 92093-0319
Departament de Física
Universitat Autònoma de Barcelona 08193 Bellaterra, Spain
C.F. Majkrzak and J.A. Dura
NIST Center for Neutron Research
National Institute of Standards and Technology Gaithersburg, MD 20899-8562
the twinned microstructure. Furthermore, field cooling provides an additional unidirectional asymmetry. Therefore, field reduction from positive saturation results in magnetization rotation rather than domain nucleation. This is due to the intrinsic unidirectionality that hinders formation of domains with magnetization anti-parallel to the cooling field direction. As the field is reduced from negative saturation, formation of domains with magnetization parallel to the initial cooling direction is favored. Hence reversal occurs by domain nucleation and propagation.
For the case of samples with single crystalline (untwinned) AF's, frustration is lacking; consequently, there is no anisotropy axis parallel to the cooling field with which unidirectional anisotropy can be established. In this case, magnetization rotation is always favored (as evidenced by SF scattering on both sides of the ferromagnetic hysteresis loop). We note the exchange bias for the single crystal sample is always small. A clear correlation was observed: samples with an asymmetric magnetization reversal process exhibit large exchange bias, while those with symmetric magnetization reversal process exhibit small exchange bias.
By identifying the mechanisms involved in the asymmetry favoring large exchange biasing in this system, these and related neutron reflectivity studies point out a direction for the design of next generation GMR sensors having substantial improvements in magnetic field sensitivity.
This work was supported by the U.S. Department of Energy, BES-DMS under Contract No. W-7405-Eng-36, grant DE-FG03-87ER-45332, and funds from the University of California Collaborative University and Laboratory Assisted Research.
 W.H. Meiklejohn and C.P. Bean, Phys. Rev. 105, 904 (1957).
 J. Nogués and Ivan K. Schuller, J. Magn. Magn. Mat. 192, 203 (1998).
 M. R. Fitzsimmons, P. Yashar, C. Leighton, Ivan K. Schuller, J. Nogues, C.F. Majkrzak, and J.A. Dura, Phys. Rev. Lett. 84, 3986 (2000).
THE ROLE OF THE CRP/CAMP
PROTEIN COMPLEX IN DNA TRANSCRIPTION
NA molecules direct the synthesis of specific RNA and protein molecules. In the early stages of protein synthesis, specific regions of the DNA (genes) are copied into short strands of RNA that retain all of the genetic information of the DNA sequence from which they were copied. The process by which RNA molecules are synthesized from the coding regions of DNA is known as DNA transcription. The RNA polymerase enzyme, whose function is to make a RNA copy of a DNA sequence, catalyzes the synthesis of these RNA molecules. The amount of RNA made from a particular region of DNA is controlled by gene regulatory proteins that bind to specific sites on DNA close to the coding sequences of a gene. In this highlight we describe experiments addressing how a particular gene regulatory protein controls RNA transcription from DNA.
One useful model of such a protein is the cyclic AMP receptor protein (CRP) of E. coli. Upon binding cyclic adenosine monophosphate (CAMP), CRP undergoes
a conformational change that, in
turn, promotes binding to spe
cific DNA sequences. The CRP/
Particularly powerful is the contrast variation technique  in which isotopic substitution of D for H in the solvent is routinely used to change the scattering from a macromolecule without affecting its biochemistry. In the case of a two-component complex such as CRP/CAMP/DNA (cAMP is considered to be part of the CRP component), the neutron scattering length density of CRP is quite different from that of DNA. In this case, the scattered intensity at each Q value is expressed as the sum of three terms, each of which is the product of an unknown component intensity and a known contrast term. (The contrast is the difference between the scattering length density of a component and that of the solvent.) Thus, the scattering from the complex in solution can be separated into component intensities by measuring the scattered intensity of the complex, I(Q), at a minimum of three contrasts obtained from different D2O/H2O buffer mixtures.
CAMP complex, upon binding
DNA, produces a bend in the
DNA that causes it to wrap
around the RNA polymerase to
promote DNA transcription.
A method well suited to directly study the structure of proteins and DNA in solution, where transcription takes place, is small-angle neutron scattering (SANS). The radius of gyration, R., which can be used to
NIST Center for Neutron Research
National Institute of Standards and Technology Gaithersburg, MD 20899
University of Maryland, Baltimore County
S. Wang, Y. Shi, F.P. Schwarz
Department of Chemistry Western Maryland College Westminister, MD 21157
Recent SANS measurements of CRP/DNA complexes  confirmed, in solution, the bending of the bound DNA that was observed in an early x-ray crystal structure of the complex . However, the Rg value for the complex was larger than that predicted from the same crystal structure. SANS confirmed experimentally that this value does not change with concentration. Thus, the increase in R is not due to aggregation, but it could result from an increase in the R of the CRP component upon DNA binding. Such a conformational change would be apparent in the SANS solution measurements: it was not evident in the crystal structure .
formed on CRP/DNA complexes in 0 %, 15 %, and 70 % D2O/H2O buffer solutions. The Rg values were found to be the same, (28 Å to 30 Å), for all three cases. This clearly indicates that the CRP component is the main reason that RCRP was larger than originally expected. It was found from the Q behavior of the CRP component intensity that RCRP = 28.5±0.3 Å, which is ≈ 6 Å larger than the 21.6+0.2 Å value observed in solution for CRP alone . It is also ≈ 6 Å larger than the 22.6 Å value predicted for the CRP component from an energy-minimized x-ray crystal structure of the complex by Parkinson et al. , with cAMP incorporated as in Passner and Steitz .
To model the solution structure of the CRP/DNA complex, the energy-minimized x-ray structure [5,6] was distorted in the regions thought most likely responsible for the conformational change in CRP upon DNA binding . The distance distribution function, P(r), was calculated  from the energy-minimized distorted conformation and compared to that obtained from the SANS data. As shown in Fig. 1, the P(r) function calculated from the model structure clearly fits the experimental data better than that from the x-ray crystal structure [5,6]. A molecular representation of the energy-minimized x-ray crystal structure [5,6] is shown in Fig. 2, along with the model structure that fits the SANS data.
The experimentally observed conformational change in CRP upon DNA binding may play a role in the enhancement of transcription of DNA by CRP. Perhaps this occurs through its contacts with RNA polymerase that is bound on the DNA at a site adjacent to the CRP binding site. This is the subject of further ongoing SANS studies.
 H. B. Stuhrmann and A. J. Miller, Appl. Cryst. 11, 325 (1978).
 Y. Shi, S. Wang, S. Krueger, and F. P. Schwarz, J. Biol. Chem. 274, 6946 (1999).  D. B. McKay, and T. A. Steitz, Nature 290, 744 (1981).
 S. Krueger, I. Gorshkova, J. Brown, J. Hoskins, K. H. McKenney, and F. P. Schwarz, J. Biol. Chem. 273, 20001 (1998).
 G. Parkinson, C. Wilson, A. Gunasekera, Y. W. Ebright, R. E. Ebright, and H. M. Berman, J. Mol. Biol. 260, 395 (1996).
 J. M. Passner and T. A. Steitz, Proc. Natl. Acad. Sci. USA 94, 2843 (1997).
 N. Baichoo and T. Heyduk, J. Mol. Biol. 290, 37 (1999).
POLYMER THIN FILMS UNDER SUPERCRITICAL CO2
n recent years supercritical fluids (SCFs), materials at temperatures and pressures above their critical values, are being used in both traditional industry and new advanced technical areas. The major advantage of SCFs is that their physical properties such as dielectric constant, density, and solubility parameters, can be tuned simply by adjusting the temperature and pressure. Especially, SCFs have also been shown to be effective plasticizers as well as solvents for numerous polymers. In particular, much attention has been focused on CO, since it becomes supercritical at a moderate critical temperature and pressure, T= 31.3 °C at P. 73.8 bar, and it is environmentally benign .
will advance understanding of the fundamental physics and applications of polymer thin films.
Neutron reflectivity (NR) is used for quantitative determination of the thicknesses, compositions, and interfacial structures of polymer thin films on a nanometer scale. To achieve this under in situ conditions, we have developed a temperature and pressure controlled chamber specifically for neutron reflectivity (Fig. 1). The cell is equipped with two cylindrical sapphire windows. CO, is loaded into the cell by means of a manually operated pressure generator. Pressurizing and depressurizing cycles up to 1400 bar are possible. Temperature and pressure stability of the chamber of ±0.1 °C and ±0.2 %, respectively can be achieved. Due to the high absorption of neutrons in CO2, the incident and reflected beams passed through the Si substrate with a transmission of Teflon o-ring 0.90 relative to air. It is interesting to note that the background Gasket scattering from the CO2 increases dramatically as the denThermocouple sity increases at the phase boundary. Hence the supercritical transition point can be indepen
In spite of its practical importance and numerous studies of the CO2-induced swelling in bulk polymers, fundamental questions still remain. It is important to understand the interaction of supercritical CO2 (scCO2) that can modify diffusion coefficient and the glass transition in thin polymer films. The performance of the many applications of thin films is often dependent on knowledge of the structure and dynamics of the interfaces. Therefore, research in this area
dently monitored with high accuracy.
FIGURE 1. Cross-sectional view of the supercritical CO2 chamber for neutron