Exchange anisotropy pinning of a standing spin-wave mode

Standing spin waves in a thin film are used as sensitive probes of interface pinning induced by an antiferromagnet through exchange anisotropy. Using coplanar waveguide ferromagnetic resonance, pinning of the lowest energy spin-wave thickness mode in ${\text{Ni}}_{80}{\text{Fe}}_{20}/{\text{Ir}}_{25}{\text{Mn}}_{75}$ exchange-biased bilayers was studied for a range of ${\text{Ir}}_{25}{\text{Mn}}_{75}$ thicknesses. We show that pinning of the standing mode can be used to amplify, relative to the fundamental resonance, frequency shifts associated with exchange bias. The shifts provide a unique “fingerprint” of the exchange bias and can be interpreted in terms of an effective ferromagnetic film thickness and ferromagnet-antiferromagnet interface anisotropy. Thermal effects are studied for ultrathin antiferromagnetic ${\text{Ir}}_{25}{\text{Mn}}_{75}$ thicknesses, and the onset of bias is correlated with changes in the pinning fields. The pinning strength magnitude is found to grow with cooling of the sample, while the effective ferromagnetic film thickness simultaneously decreases. These results suggest that exchange bias involves some deformation of magnetic order in the interface region.

Figure 1

A sample of data taken with a MOKE magnetometry setup focused onto the NiFe(60.5 nm)/IrMn(6 nm) sample. The vertical axis uses arbitrary units and represents the average magnetization over the laser spot focused onto the sample. The horizontal axis displays the field applied across the sample in units of oersteds. The MOKE data were gathered via a repetitive field sweeping technique with averaging over thousands of cycles. For data shown in this picture, averaging was done over two field sweeps, which resulted in a double hysteresis loop, with the difference between the two loops determined by the level of noise in the system. Also the exchange-bias shifting of the loop is shown by the dotted line and denoted by $H$${}_{\text{EB}}$.

Figure 2

(a) The experimental geometry, with the sample placed on top of the coplanar stripline. $H$ refers to the applied field direction at some angle $\theta$, $M$ refers to the magnetization direction, and $H_{\text{RF}}$ refers to the microwave rf field generated by the waveguide. The sample is rotated in place in order to change the direction of $H$ with respect to the sample’s easy axis. (b) Microwave transmission as a function of static applied field for the 0-nm IrMn sample. The values $H_f\pm$ correspond to applied resonant fields in antiparallel directions for$+$and $-$, respectively. Seen are microwave absorptions which correspond to the fundamental mode (FMR) and the first exchange mode (FEX). The microwave excitation frequency $\omega$ used was 7 GHz.

Figure 3

The exchange bias as measured from the FMR mode (empty circles, solid line), MOKE (empty diamonds, solid line), and FEX mode (empty squares, solid line), as a function of IrMn film thickness. The NiFe layer thickness is always 60.5 nm. For comparison the coercivity as measured with MOKE is shown (empty triangles, dashed line).

Figure 4

Resonant fields $H_f$ for the FMR (empty circles) and FEX (empty squares) standing spin-wave modes at different applied field angles with respect to the easy axis ($\theta$). The solid lines show fits to the data using $\text{cos}(\theta )$ and $\text{cos}(2\theta )$ components. Presented are the resonance data for different IrMn thickness capping layers: (a) IrMn $=0$ nm, (b) IrMn $=2.5$ nm, and (c) IrMn $=6$ nm.

Figure 5

(a) The calculated strengths of pinning $p(\theta =180)$ along the bias direction (empty circles, solid line) and $p(\theta =0)$ against the bias direction (empty squares, dashed line). (b) The corresponding effective magnetic thickness $t_{\text{eff}}$ of the NiFe along the bias direction (empty circles, solid line) and against the bias direction (empty squares, dashed line).

Figure 6

The effective magnetic thickness of NiFe as a function of $\theta$ with respect to the easy axis for (a) a 0-nm IrMn film and (b) a 6-nm IrMn film.

Figure 7

(a) The calculated strengths of pinning $p(\theta =180)$ along the bias direction (empty circles, dashed line) and $p(\theta =0)$ against the bias direction (empty squares, dashed line) for the IrMn 2.5-nm film cooled to the temperature indicated on the horizontal axis, in a 40-Oe field. Also shown is the complementary information on the exchange-bias shift for the FMR mode (solid triangles, solid line) and FEX mode (solid diamonds, solid line). (b) The corresponding effective magnetic thickness $t_{\text{eff}}$ of the NiFe along the bias direction (empty circles, solid line) and against the bias direction (empty squares, dashed line) for the same range of field-cooled temperatures.