Homogeneous and heterogeneous magnetism in (Zn,Co)O: From a random antiferromagnet to a dipolar superferromagnet by changing the growth temperature

A series of (Zn,Co)O layers with Co contents $x$ up to 40$\%$ grown by atomic layer deposition have been investigated. All structures deposited at 160${}^{\circ }$C show magnetic properties specific to II-VI dilute magnetic semiconductors with localized spins $S=3/2$ coupled by strong but short-range antiferromagnetic interactions resulting in low-temperature spin-glass freezing for $x=0.16$ and 0.4. At higher growth temperature (200${}^{\circ }$C) metallic Co nanocrystals precipitate in two locations giving rise to two different magnetic responses: (i) a superparamagnetic contribution coming from volume disperse nanocrystals; (ii) a ferromagneticlike behavior brought about by nanocrystals residing at the (Zn,Co)O/substrate interface. It is shown that the dipolar coupling within the interfacial two-dimensional dense dispersion of nanocrystals is responsible for the ferromagneticlike behavior.

Figure 1

(a) XRD pattern of layer PM-3, (b) XRD pattern of four (Zn,Co)O layers: two highly textured ones (PM-1 and PM-S1) and two others without any dominant orientation of crystallites (PM-2 and PM-4). The corresponding orientation of the wurtzite $c$ axis with respect to the sample plane are indicated at the top of the figure. (c) A result of 24-h x-ray exposure around 45${}^{\circ }$ for the thickest layer ($t\simeq 1200$ nm) showing a broad and weak maxima at positions corresponding to the strongest reflexes for the cubic and hcp cobalt (marked and labeled in bolt), as well as to a weaker 10.1 reflection of the hcp structure expected at 44.76${}^{\circ }$.

Figure 2

Experimental and computed XANES spectra of the investigated samples. Spectra of the metallic Co (calculated and measured) are shown for comparison. Experimental and simulated spectra are shifted vertically for clarity.

Figure 3

Magnitude of the Fourier transform of the EXAFS spectra and fitting results for the samples. Spectra are shifted vertically for clarity.

Figure 4

XPS sputter depth profiles of the (a) ferromagnetic and (b) paramagnetic (Zn,Co)O films.

Figure 5

Co2p core level XPS spectra measured during Ar${}^{+}$ sputter profiling after removing of 15 nm of the (Zn,Co)O film (bottom) and at the (Zn,Co)O/Si interface region (top). (Left) Ferromagnetic (sample FM-3); (right) paramagnetic (sample PM-4) (Zn,Co)O film. Deconvoluted spectra indicate different chemical states of Co compounds.

Figure 6

High-resolution transmission electron microscopy images of (a) PM-4 sample with (Zn,Co)O wurtzite particle projected in the $\langle$01.1$\rangle$ direction, and (b) FM-3 sample with Co fcc nanocrystal projected in the $\langle$011$\rangle$ direction.

Figure 7

L-edge x-ray absorption spectra versus photon energy for (Zn,Co)O from the interface region (region A, in the crater) and from the surface region (region B of the film); the Co L-edge data are taken at normal x-ray incidence (a 90${}^{\circ }$ angle between the x-ray propagation direction and the film surface plane) in the total electron yield mode and normalized to the atomic continuum at high photon energies.

Figure 8

XAS (left scale) and XMCD (right scale) spectra in the total electron yield mode versus photon energy. Measurements are performed at room temperature under an applied magnetic field of $H=350$ Oe. The XAS spectra are obtained with nearly fully circularly polarized light (a light helicity of 0.85); a 40${}^{\circ }$ x-ray incidence angle is employed. The insert shows an enlarged view of the L${}_3$ edge of the XAS and XMCD spectra.

Figure 9

Room temperature $M\left(H\right)$ for selected paramagnetic (Zn,Co)O layers. All data show a good linearity in $H$, except the low field region where a (negative) residual sigmoidal signal mars the otherwise paramagnetic $M\left(H\right)$. This spurious nonlinearity near $H=0$ results from a spread of magnitudes of the nonlinear components to $m$ exhibited by the Si substrates we use (see Fig. 1 in Supplemental Material). The arrow indicates the direction of the growth of the Co content in these layers.

Figure 10

Temperature dependence of magnetic susceptibility. The data sets are labeled by the Co concentration $x$, established for digital alloy layers and by their periods for two superlattices. The dashed line indicates the Curie law $\chi \left(T\right)\propto 1/T$. Solid and dotted lines indicate that $\chi \left(T\right)\propto T^{-\alpha }$, where $\alpha <1$, the dependence specific for a random antiferromagnet. The values of $\alpha$ established in this way for the upper bunch of curves are plotted vs $x$ in the inset. The dashed black line shows $\alpha \propto x^{-1/3}$, and serves as a guide for the eye. This trend is not obeyed for the samples grown in a superlattice fashion, for which the magnitude of $\chi$ is low ($x\approx 0.7$$\%$), whereas $\alpha$ (marked as dotted line in the inset) corresponds to $x\approx 6$$\%$.

Figure 11

The inverse of the magnetic susceptibility $\chi$ as a function of temperature for paramagnetic (Zn,Co)O layers. The main panel groups data for layers in which CoO layers were introduced in a digital way, whereas the inset presents data for the two layers grown in a superlatticelike fashion. Note the substantially different values of ${\chi }^{-1}(T=0)$ for the two types of samples.

Figure 12

Magnetization for a series of samples measured at 2, 5, 15, and 300 K [from top to bottom, indicated in panel (d)] for two orientations of the magnetic field: in-plane (solid squares) and perpendicular to the sample plane (open circles). Lines show modeling of the data by theory outlined in Sec. 7a1. The solid and dashed lines in panels (a) and (b) are calculated for the mixed anisotropy case considering the in-plane and perpendicular magnetic field, respectively. The fraction $y$ of grains with the $c$ axis perpendicular to the film plane and the effective Co concentration $x_{\text{eff}}\left(T\right)$ are the fitting parameters, whose values are displayed in panels (a) and (b) and in Table , respectively. The dotted lines in panels (c) and (d) represent the isotropic (randomized with respect to the $c$ axis) values of magnetization in polycrystalline films. The orange dashed-dotted lines represent a considerably improved fit with an effective temperature $T_{\text{AF}}$ added to the calculations performed at the lowest temperature and displayed in Table .

Figure 13

(Main figure) A spin-glass-like behavior of two paramagnetic layers with the highest Co content. (Darker color) PM-4, $x\cong 16$$\%$; lighter one, PM-6, $x\cong 42$$\%$. Zero field cooled magnetization (ZFCM) and field cooled magnetization (FCM) measurements performed at 1 kOe, are indicated by arrows and marked by open triangles pointing towards increasing or decreasing temperature, respectively. Solid circles mark thermoremanent magnetization (TRM) (magnified 5 times for better presentation). Inset shows $T_f$ dependence on the transition metal content: $x_{\text{Mn}}$ in (Zn,Mn)O, open square from Ref. 69; $x_{\text{Co}}$ in (Zn,Co)S, solid hexagons from Ref. 70; and in (Zn,Co)O, solid triangle from Ref. 14 and bullets from this study.

Figure 14

A representative (volume) magnetization for a layer grown at 200${}^{\circ }$C (FM-2) measured at 2, 5, 15, and 300 K. Solid squares are obtained with the magnetic field applied parallel to the sample plane. For clarity the data obtained in perpendicular configuration are shown only for 300 K (open circles). Dotted lines indicate results of the modeling which assumes a presence of three independent magnetic contributions: PM, SP, and FM (planar superspin glass) one, as detailed in the text. The dashed line represents the magnitude and room temperature curvature of the superparamagnetic component, whereas the two solid lines represent (derived from sample FM-3) the ferromagneticlike component (with its anisotropy) (see Sec. 7b3).

Figure 15

Blocking phenomenon in a superparamagnetic sample (SP-1). (Main part) Temperature dependence of the zero field cooled (ZFCM), field cooled (FCM), and thermoremnant (TRM) magnetization. For a monodispersed ensemble $T_{\text{m}}$ corresponds to mean blocking temperature. Here, $T_{\text{Ir}}$ corresponds to the maximum volume of statistically relevant nanogranules. (Inset) Time evolution of the magnetization in these three magnetic states measured at 5 K (below $T_{\text{m}}$).

Figure 16

Magnetization of layer FM-3. (Main part) $M_{\text{A}}$, the magnetic moment per unit area at 300 K. Solid squares are obtained with the magnetic field applied in the sample plane; open circles, in perpendicular configuration. The dashed horizontal line indicates the estimated saturation level of the near-the-interface part of the layer which we use in conjunction with the low field data ($\left|H\right|<10$ kOe) to construct the standard data sets for this magnetic contribution $M_I\left(H\right)$, which has been employed in the modeling of the $M(H,T)$ presented in Fig. 14. The inset presents the low field in-plane zero field cooled (ZFCM) and field cooled (FCM) magnetization. The volume magnetization was obtained assuming the thickness of the Co-rich interface zone $t_I=4$ nm.