Structural and magnetic properties of granular Co-Pt multilayers with perpendicular magnetic anisotropy

Abstract

We present a study of granular Co-Pt multilayers by means of high-resolution transmission electron microscopy (HRTEM), extended x-ray absorption fine structure (EXAFS), SQUID-based magnetic measurements, anomalous Hall effect (AHE), and x-ray magnetic circular dichroism (XMCD). We describe these granular films as composed of particles with a pure cobalt core surrounded by an alloyed Co-Pt interface, embedded in a Pt matrix. The alloy between the Co and Pt in these granular films, prepared by room temperature sputter deposition, results from interdiffusion of the atoms. The presence of this alloy gives rise to a high perpendicular magnetic anisotropy (PMA) in the granular films, as consequence of the anisotropy of the orbital moment in the Co atoms in the alloy, and comparable to that of highly-ordered CoPt L1 0 alloy films. Their magnetic properties are those of ferromagnetically coupled particles, whose coupling is strongly temperature dependent: at low temperatures, the granular sample is ferromagnetic with a high coercive field; at intermediate temperatures the granular film behaves as an amorphous asperomagnet, with a coupling between the grains mediated by the polarized Pt, and at high temperatures, the sample has a superparamagnetic behavior. The coupling/decoupling between the grains in our Co-Pt granular films can be tailored by variation of the amount of Pt in the samples.

Perpendicular magnetic anisotropy in granular multilayers of CoPd alloyed nanoparticles

“…10(b)], which will be discussed later. A similar magnetic phase diagram has been obtained for the CoPt NPs [21]. Nevertheless, the highest T 1 reported in the case of capping with Pt is ∼140 K, while for CoPd NPs the highest T 1 is ∼280 K at H dc = 0.1 kOe (Table III).…”

“…The element specific ratio (c/a) Co = 2(R Co−Pd /R Co−Co ) 2 − 1 at the Co atoms in L1 0 unit cell [39] is ≈1.08. This is larger than that measured in bulk L1 0 CoPd, c/a ∼0.97 [44], but it is similar to that found in the CoPt NP multilayers [21]. This has been explained as relaxation of the atom-atom distances because of finite size in the L1 0 structure, and DFT calculations for finite size Co-Pt relaxed clusters effectively predict a ratio c/a(EXAFS)>1 [39].…”

“…Light gray layers correspond to the amorphous alumina and darker layers to Co and Pd. The particulates are approximately spherical with diameter D ∼ 3 nm and are arranged in a hexagonal network within the layers, characteristic of Co deposited on amorphous alumina [20][21][22]. The fast Fourier transform (FFT) in the nanoparticles shows the existence of crystallized regions with definite crystal planes, alternating with amorphous regions.…”

“…In contrast, capping with the nonmagnetic elements W or Pt, which do alloy with Co, modifies the intrinsic magnetic anisotropy. Indeed, while the Co-W NPs are amorphous, albeit magnetically anisotropic [22], the Co-Pt alloyed NPs present PMA and an atomically ordered structure with short range L1 0 arrangement [21], which has been suggested as the origin of the observed PMA. Moreover, the Pt multilayer samples constitute the only case of all those mentioned above which shows ferromagnetic order within the layers.…”

“…The systems with t Co = 0.7 nm, where a non-magnetic transition metal is subsequently deposited, could be used to study magnetic ordering and anisotropy induced by a metallic spacer connecting magnetic NPs within the Co-Pd layer. In previous works our group has studied [Al 2 O 3 /Co/M] N multilayers made of layers of Co NPs capped with different nonmagnetic transition metals M = Au, Ag, Cu, W, Pt, and separated by thin amorphous 3 nm-thick Al 2 O 3 layers [19][20][21]. Capping the Co NPs with metals which do not diffuse into Co, like the noble metals Au and Ag, or Cu, results in an increase in the surface anisotropy of the Co NPs [19,20].…”