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Training Effects and the Microscopic Magnetic Structure of Exchange
Argonne National Laboratory, Materials Science Division, Bldg. 223, 9700 South Cass Ave.,
Argonne, IL, 60439, US
B.K. Hill and E. Dan Dahlberg
University of Minnesota, Department of Physics, 116 Church St. SE, Minneapolis, MN,
55455, US
Abstract
Exchange bias of a partially oxidized thin film of ferromagnetic Co was studied by
magnetization measurements and polarized neutron reflectivity (PNR). The magnetization
curve shows strong effects of training with cycling of the magnetic field. Reflectivity
measurements with the field parallel to the cooling field showed the onset of spin-dependent
diffbse scattering -off the specular reflection- after a training cycle. Such scattering, of the
Yoneda type, is due to misaligned Co domains possibly close to the Co/CoO interface.
Subjecting the field cooled Co/CoO pair to a field perpendicular to the cooling field causes a
rotation of the magnetization. PNR measurements confkrned earlier susceptibility studies by
indicating that the rotation of the magnetization is reversible in fields up to 400 Oe. The
rotation of the magnetization of Co is uniform across the film thickness.
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Exchange bias was first discovered in 1956 by Meiklejok’ and Bean in CO-COO
particle systems [1]. It refers to the occurrence of a unidirectional magnetic anisotropy that
manifests itself in shifted hysteresis loops as well as an increase in coercivity for coupled
ferromagnet (F)-antiferromagnet (M) systems cooled through the Ndel temperature in the
presence of a magnetic field [1]. Initially exchange biasing was interpreted as the result of the
exchange interaction at AF/F interfaces: the magnitude of the exchange-bias field is given by
balancing the gain in Zeeman energy with the energy cost of interracial exchange when the
ferromagnet reverses its magnetization.
In the earliest model [2] both F and AF s~in
structures were assumed to be a rigid sequence of ferromagnetic planes, with an antiparallel
sequence for the AF component; the AF/F interface was
Unfortunately such an intuitive picture grossly overestimates
fields. Subsequent models [3-7] attempted to address this
taken to be atomically flat.
the size of the exchange-bias
difficulty by invoking some
roughness at the interface and/or some breakdown in domains of the AF structure.
While traditionally the magnitude of the exchange bias effect has been derived from
the shift in the hysteresis curve, it has been argued [7-9] that this method does not evaluate
correctly the unidirectional anisotropy. An alternative approach is to obtain the unidirectional
anisotropy by means of reversibly rotating the applied field with respect to the exchange bias
direction, which is determined by the cooling field
HCOO1.From measurements of this type a
value for the exchange anisotropy was derived several times larger than that obtained by the
conventional method [8]. In this paper, we conilrm with polarized neutron reflectivity (PNR)
the reversibility of the rotation of the magnetization of Co (in a Co/COO thin film) for H L
H.OOland the rate of rotation away fi-omthe bias direction. We find also that such rotation is
uniform over the film thickness.
The sample was fabricated by magnetron sputtering of Co on a Si substrate with a
nominal thickness of 120 & By allowing the sample to oxidize in air a COO layer was
Y
,
formed. The Co and COOlayer thicknesses were determined with X-ray reflectivity, using the
CuKct radiation from arotatingapode and were equal to 139~and 33 & respectively.
Magnetization measurements were made with a dc magnetometer at 5 K, after cooling from
room temperature in a magnetic field
H = 5 kOe. The field, applied along the cooling field,
was cycled five times within +/- 2 kOe. The training effect is indicated (Fig. 1) by the
asymmetry of the fust cycle and the difference between the fust cycle and subsequent cycles.
Cooling in different fields (from 1.5 to 30 kOe) yielded similar results. The symmetric shape
of the second cycle permits an estimate of the relevant parameters: exchange bias
HE= 145 *
5 C)e; coercive field
H. = 325 + 5 Oe; saturation magnetization M= 1700 emu/cm3 or close to
that of bulk Co. The training effect implies that during the reversal of the magnetization, the
ferromagnet does not reverse homogeneously, nor reversibly.
Polarized neutron reflectivi~ was measured at the POSY I instrument of the Intense
Pulsed Neuron Source at Argonne National Laboratory. In order to determine the magnetic
profiles of the virgin and trained curves, three measurements at the same field (H = -65 Oe)
were made during the first half (I) and the second half (11)of the first hysteresis cyc~e and
during the first half of the second cycle (III), (see Fig.1). The measurements were taken at 4.5
K, after cooling in a field of 5 kOe. While the specular reflectivity appears to be fairly similar
for all cases, spin-dependent diffuse scattering appears in the case of the trained
magnetization. The diffuse scattering is best visible in a contour plot of the intensity as a
fuction of the wavelength A and the angle of incidence 02and exit (+. The contour plot
presented in Fig.2 is for neutrons polarized parallel to the applied field for the trained
magnetization state (III). For this spin state the reflectivity is higher than for neutrons with
opposite polarization (Fig,3) and the diflise scattering is visible just for this spin state. The
diffuse scattering has the characteristics of the
“Yoneda scattering” [10-13]. It is
distinguished by a ridge of maxima at a critical value of the exit angie, linearly extrapolating
.