Fabrication and Studies of Magnetic Nanodots

Magnetization stabilization, vortex state, exchange bias, and neutron studies

Igor V. Roshchin, C.-P. Li, Xavier Batlle, Ivan K. Schuller

Physics Department, UCSD

Present collaborators on this project:

Fabrication (during sabbatical at UCSD)
Xavier Batlle, University of Barcelona, Catalonia, Spain.
First Order Reversal Curve (FORC) measurements
Randy Dumas, Kai Liu, Physics Department, UC Davis, Davis, CA,
Neutron Reflectivity
S. Roy, S. K. Sinha, Physics Department, UCSD and Los Alamos National Laboratory, Los Alamos, NM,
S. Park, R. Pynn, M. R. Fitzsimmons, Los Alamos National Laboratory, Los Alamos, NM,
Monte Carlo simulations
A. H. Romero, CINVESTAV, Universidad de Queretaro, Qro, Mexico,
J. Mejia Lopez, PUC, Chile,
D. Altbir Drullinsky, USACH, Chile,

Former collaborators on this and related projects:

Neutron Reflectivity
M. Viret, F. Ott, C.E.A. Saclay-France,
Ferromagnetic Resonance (FMR)
Chengtao Yu, M. Pechan - Ferromagnetic Resonance (FMR), Physics Department, Miami University, Oxford, OH,
and many others.

    Magnetism at nanoscale, when the size of the structures is comparable to or smaller than both the ferromagnetic (FM) and antiferromagnetic (AF) domain size, offers a great potential for new physics. Moreover, modern technology demands techniques capable of producing nanometer-sized structures over large areas.
    Using self-assembled nanopores in anodized alumina as a shadow mask [1-3], sub-100 nm magnetic dots covering over 1 cm² area are deposited by electron-beam evaporation. This method provides a good control over dot size and separation.
    Single layer FM and bilayer FM/AF nanodots or FM nanodots on a continuous layer of AF as well as continuous FM and FM/AF films with 15-20 nm-thick FM (FM=Fe, permalloy), and 20-90 nm-thick AF (AF=FeF₂) are simultaneously grown on the same substrate, and then capped in-situ with a 5-8 nm-thick Ag or Al layer to minimize oxidation of the FM. Comparative studies of magnetic properties of these samples in a wide range of temperatures and fields are presented.
    Exchange bias (EB) for Fe nanodots due to the native oxide increases with decreasing dot size, reaching values of 170 Oe at 10 K for 31 nm dots. Improved squareness (M_H=0/M_s) of the magnetization loops for Fe nanodots is caused by EB due to the AF layer or a native oxide.

Exchange bias

Fig. 1 a) Typical magnetization loop for a ferromagnetic. b) Adjacent layer of an antiferromagnetic exchange biases the ferromagnetic, causing a shift of the magnetization loop by H_e.

Superparamagnetic limit

This condition determines how long the magnetization of a ferromagnetic structure of volume V will survive thermal fluctuations at temperature T.
Superparamgnetism

Anodization of Aluminum

Fig.2. Anodization of aluminum using electropolisher setup.

Anodization of aluminum is done using a Buehler electropolisher, controlled by a computer via a house-built relay box. The same computer is used for acquisition and real-time plotting of the anodization parameters (voltage, current, temperature). Variety of anodization conditions are used for various pore sizes: various acids (oxalic, phosphoric, sulfuric), voltages, etc.

Pore formation

Fig.3 Pore formation in anodic alumina (from Ref.4).

In the electrochemical process of anodization, aluminum turns into aluminum oxide at pore bottoms, forming columnar pores. Due to the competition of electrical and mechanical (stress) forces, these pores tend to form hexagonal, close-packed pattern (see Ref. 3,4).

Nanolithography with porous alumina

Fig.4 Fabrication of nanodot arrays using nanoporous alumina.

Using porous alumina we fabricate arrays of magnetic nanodots that cover macroscopic area (over 1 cm²)[1,2,5].

Pattern transfer

Self-supporting masks (membrane)

Fig.5 Pores and dots for membrane (self-supporting) nanoporous alumina mask.

Thin film masks on Si substrate

Pattern transfer using thin film mask on Si: nanopores and nanodots

Fig.6 Pores and Dots for thin film nanoporous alumina mask grown directly on Si substrate.

Magnetization for nanodots of various sizes

Magnetic properties of the nanodot arrays are studied using SQUID and MOKE magnetometry.

Magnetic Hysteresis loops for nanodots of various sizes

Fig.7 Magnetization loops for dots of various sizes.

Magnetization: vortex state vs. single domain state

Results of SQUID magnetometry for the Fe dots with diameter ranging from 30 nm to 100 nm are consistent with a transition from a single domain to a vortex state. Micromagnetic simulations support the experimental observations. Both dot surface roughness and dot ellipticity tend to suppress vortex state. We also investigate effect of interdot interaction on dot magnetization.

Magnetization of nanodots: vortex state vs. single domain state

Fig.8 Comparison of magnetization loops for nanodots in a) vortex state, b) single domain state. Right hand side images show a zoomed-in virgin curve for each of the samples.

Other measurements and characterizations.

Small Angle Neutron Scattering on Nanodots

Fig.9 World first polarized SANS measurements on sub-100 nm magnetic dots
a) Raw data at an incident angle of 0.35° on the 2-D detector
b) Cross section through the dashed line and comparison with sample without dots (log scale) after 5 hours of counting.

In addition to SQUID and MOKE magnetometry we study magnetic nanodots by a variety of methods with our collaborators.
We were the first to perform neutron reflectivity measurements of sub-100 nm magnetic dots in collaboration with M. Viret and F. Ott (SEA, Saclay, France). Measurements performed using a Small Angle Neutron Scattering setup with polarized neutrons (diffractometer “PAPOL” in Saclay) in reflectivity geometry, allow to determine both structural and magnetic form factors. The neutron studies are planned to continue using the beam line in the USA (LANL).

Other current and planned measurements include, High resolution AFM/MFM, and VSM (FORC).

New results are coming soon (Winter 2006).

References:

1. Chang-Peng Li, Igor V. Roshchin, Xavier Batlle, M. Viret, F. Ott, and Ivan K. Schuller, "Fabrication and structural characterization of highly ordered sub-100 nm planar magnetic nanodot arrays over 1 cm2 coverage area." Journal of Applied Physics, 100(7), October 1 (2006).
2. Kai Liu, J. Nogues, C. Leighton, H. Masuda, K. Nishio, I. V. Roshchin, and Ivan K. Schuller "Fabrication and Thermal Stability of Arrays of Fe Nanodots", Applied Physics Letters 81, 4434 (2002).
3. H. Masuda and K. Fukuda, Science 268, 1466 (1995).
4. O. Jessensky, F. Muller, U. Gosele, Appl. Phys. Lett. 72, 1173 (1998).
5. MRS Bulletin 28 (7), 530 (2003).

Back to the "SCIENCE" site (presently under construction)