Relationship between granularity of an antiferromagnet and exchange bias: Measurements of CoO doped with Pt

We studied exchange bias in magnetic multilayers incorporating antiferromagent CoO doped with up to 35 atomic percent of Pt. The exchange bias increased with doping in epitaxial films, but did not significantly change in polycrystalline films at the lowest measured temperature of 5 K, and decreased at higher temperatures. We explain our results by the increased granularity of the doped antiferromagnetic films, resulting in simultaneous enhancement of the uncompensated spin density and reduction of the magnetic stability of antiferromagnetic grains.

Figure 1

(a) $\theta -2\theta$ x-ray scans of epitaxial samples as labeled. Arrows show CoOPt(200) peaks. (b) CoO(200) rocking curve for $s0$ (symbols), fitted by a Lorenzian with FWHM of $0.15°$ (solid curve). (c) Same for $s35$, fitted by a sum of two Lorenzians with FWHM of $0.08°$ and $1.35°$, as shown by three curves.

Figure 2

Hysteresis loops acquired immediately after cooling from RT to 5 K in field 500 Oe: (a) sample $p0$, (b) $p35$, (c) $s0$, (d) $s35$. Solid curves are for the first loop and dashed curves are for the second loop. Arrows show scan directions at the reversal points of Co(8). The field is in the Co[010] direction for epitaxial samples.

Figure 3

(a), (b) Temperature dependence of the exchange bias field $H_E$ (a), and coercivity $H_c$ (b), for polycrystalline samples. (c), (d) Same as (a), (b) for epitaxial samples. Symbol types distinguish samples with different Pt concentration: circles for $x=0$, triangles for $x=15$, crosses for $x=25$, and stars for $x=35$. The field is in the Co[010] direction for epitaxial samples.