Superferromagnetism and coercivity in Co-Al2O3 granular films with perpendicular anisotropy

Abstract

Magnetic properties of nano-granular Co0.55–(Al2O3)0.45 films have been studied by vibrating sample magnetometry, ferromagnetic resonance, and magnetic force microscopy (MFM). The films possess a growth-induced perpendicular anisotropy related to the shape anisotropy of the granules. Being unpercolated physically, the films demonstrated a clear magnetic percolation behavior in the temperature dependence of the coercivity and MFM scans. The temperature dependence of the coercivity measured along the easy magnetization axis of the granules indicates the presence of a collective long-range magnetic state sustained by a ferromagnetic interparticle interaction. This interaction is supposed to form a homogeneously magnetized superferromagnetic state, however, the perpendicular anisotropy and the competition between the dipole-dipole and ferromagnetic interactions lead to the emergence of an inhomogeneous labyrinth-like magnetic stripe-domain structure with the averaged domain width being about two orders of magnitude larger than the averaged granule size. The temperature evolution of the spontaneous magnetization inside the stripe domains has been measured by the MFM up to the temperature of superferromagnetic ordering (420 K). The formerly developed mean-field-based model of the coercivity of an interacted ensemble has been tested and found to yield adequate quantitative predictions.

Tailored Magnetic and Electric States in 3d-Metal-Insulator Films: Characterization and Applications

“…In the absence of preferable crystallographic orientation of bcc α-FeCo(Zr) conrmed with XRD, the observed magnetic anisotropy is usually assigned to elongated, ellipsoidal shape of granules [27,31]. Compara- The value of eective inductive contribution to the impedance estimated basing on the correspondent equivalent circuit was found to be as high as 20 µH/µm 3 at 1 MHz that is much higher than in the case of known diodes, pnp junctions, metal semiconductor interfaces (see references in [33]).…”

Competition of the Coulomb and hopping-based exchange interactions in granular magnets

“…The exchange interaction between the grains leads to the formation of long-range magnetic order appearing below a certain temperature [3,[23][24][25], which is called the ordering temperature T ord . For Ising model [24 and 35] the ordering temperature in granular magnets with FM coupling is related to the intergrain exchange interaction as T ord = z n H ex , where z n = 6 is the coordination number for three dimensional cubic lattice.…”

“…Numerous papers were devoted to magnetic anisotropy and magneto-dipole interaction [15][16][17][18][19][20][21][22]. Much less is known about the exchange interaction between magnetic grains [23][24][25][26][27]. The influence of the intergrain exchange coupling on the magnetic state of the whole granular magnet are currently understood, however the microscopic picture of the intergrain exchange interaction is still missing.…”

Influence of the Coulomb interaction on the exchange coupling in granular magnets

“…The most complicated task is the control of the intergrain distance on the scale of 1 nm. Nonetheless, several studies observed the intergrain exchange interaction of FM type with rather large value [29,30,31].…”

“…The magneto-dipole (MD) [25,26,27,28] and the exchange interactions [29,30,31,32,33] are the main intergrain coupling mechanisms in GFM. Long range MD interaction leads to the formation of super spin glass (SSG) state in the system.…”