Research Activities at the Information Storage Materials Laboratory

Prof. Dr. Takao Suzuki (IEEE Fellow) 

 

1.1  Overview
 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 



1.2  Research Subjects

 



1.3 Basic Studies of Magnetism and Structure of Magnetic Multilayers and Fine Particles

 


(a) Giant Magnetic Anisotropy of (FeCoNi)-Pt Alloy Thin Films

・Ordered L1o (FeCoNi)50Pt50 Magnetic Thin Films

Previously, this laboratory revealed the large magnetic anisotropy of hcp Co3Pt ordered

Alloy thin films, which is of the order of 107erg/cc at room temperature.  This hcp phase does exist in the equilibrium bulk phase diagram, and it was realized only by the fabrication of thin films.  (Yoshiyasu Yamada (PhD, 1998) his result attracted much attention in the basic magnetism, in particular in the filed of band theory.

Extending this work to (FeCoNi)50Pt50 alloy thin films, a very systematic study has been carried out to elucidate the origin of the magnetic anisotropy. Thin films of (FeCoNi)50Pt50 alloys were fabricated by electron beam deposition onto MgO single crystal substrates at elevated temperatures.

Through optimizing the fabrication conditions, one could fabricate single crystalline thin films of L10 ordered phase.  By carefully analyzing the degree of ordering, the perpendicular magnetic anisotropy constant Ku was estimated by torque magnetometer in fields up to 30 kOe. In Fig.2, the correlation between the Ku and (c/a) is given.  It is of interest to note that the Ku values strongly depend on (c/a), as in the case of hcp Co.  That is, the higher the Ku, the ratio (c/a) becomes smaller.

In Fig.3, a summary of the perpendicular magnetic anisotropy constant Ku as a function of number of valence electrons for this system is given.  Here, the Ku vales correspond to those of the order parameter S=1.  It is of great interest t find that the Ku values becomes maximum at about 50% of Fe, and follows very similar to what is known as Slater-Pauling curve for the saturation magnetizations of various alloy systems.  It is also interesting to note that the present results are in reasonable agreement with the theory based on the first principle calculation, although the theory predicted much higher magnetic anisotropy constant Ku.  This was the first finding ever reported as to the magnetic anisotropy in conjunction with the valence electrons.

A recent theoretical work reported that an addition of Mn increases Ku, which needs to be examined carefully.


 


・Giant magnetic Anisotropy of pseudo L12 ordered Fe3Pt Thin Films

As seen in Fig.1, there exists phase of L12 (fcc) for Fe3Pt alloy.  In the present study, the effort has been made to fabricate the ordered phase of Fe3Pt thin films.  Films were fabricated by electron beam deposition technique under various conditions (deposition rate, substrate deposition temperature, substrate materials, etc.).  After a careful optimization of the fabrication condition, one could obtain the ordered phase of pseudo L12 phase for Fe3Pt thin films.  The magnetic anisotropy constants of K1 and K2 were carefully examined by torque magnetometer and are shown in Fig.4, together with the order parameter S and (c/a) as a function of deposition temperature Ts.  As sown in this figure, the K1 and K2 are both very large, as compared to those of bulk materials (103erg/cc).

The magnetic anisotropy constants and the ordering parameter S are both increasing with Ts beyond 350 oC.  The maximum values of K1 and K2 are -4x106erg/cc and 2x107erg/cc, respectively.  Figure 5 shows a typical magnetization curve measured for the field direction applied at 20 degree with respect to the easy axis for magnetization <110>.  The coercivity and the double kink of the magnetization curve are well accounted for based the measured values of K1 and K2.

The finds of such giant magnetic anisotropy constants in the pseudo L12 ordered phase is of importance.  Though the origin of the magnetic anisotropy is not well understood, this finding has triggered and stimulated theoretical groups.  A recent theoretical work reported a possible model and structure for such a huge magnetic anisotropy.   The bottom line of such calculations is based on a strong hybridization between Fe and Pt d-electron bands, which gives rise to a large spin-orbit coupling.


 


・hcp (CoNi)3Pt Ordered Thin Films 

As mentioned already, the present laboratory has succeeded in realizing hcp Co3Pt ordered phase thin films, which exhibit a large magnetic anisotropy.  The work has been continued for extending the alloy compositions into the Ni side.

Figure 6 shows the perpendicular magnetic anisotropy constant Ku as a function of order parameter S in (CoNi)3Pt alloy thin films.

It is found that the Ku increases for this alloy systems, and decreases with the addition of Ni drastically.  The Ku values for Ni3Pt are smaller by nearly one order of magnitude as compared to that of Co3Pt.

As seen, the Ku values are all increasing with ordering for L10(FeCoNi)50Pt50, hcp(CoNi)3Pt, and pseudo- L12Fe3Pt alloy thin films.  Such a systematic study has not ever been found in literature.

 

 

(b) Magnetic Properties and Structure of Fine Particles fabricated by Electron Beam Lithography and IBICVD

・Magnetization Distribution of Co and Fe Fine Particles

Single crystalline films of Co and Fe were successfully fabricated by electron beam deposition technique on Al2O3 and MgO single crystal substrates at elevated temperatures of 300−400 oC during deposition, respectively.  For the case of Co single crystal films, the magnetic anisotropy of Ku1 and Ku2, together with other magnetic parameters such as saturation magnetization were carefully examined and found to be nearly the same as those of bulk hcp Co single crystals.  However, the temperature dependence of the Ku1 and Ku2 are much different from that of bulk, i.e., Ku1 does not become zero at about 230 oC but remains positive.  Fine particles of those hcp Co were successfully fabricated by electron beam lithography. 


Figure 7 shows the magnetization distribution of a fine particle (1 μm diameter, 56nm thickness) observed by magnetic force microscopy at room temperature and 200 oC.  As shown here, a conical type distribution is found at room temperature, which changes to a vortex type structure as the temperature increases.  Simulation results based on Landau-Lifshitz-Gilbert equation for both temperatures using the measured magnetic anisotropy constants do not explain the observed ones.  To obtain a conical and vortex structures, the magnetic anisotropy constants must be lowered by about 3x107erg/cc. The reason for the reduction of the magnetic anisotropy may be in the contribution of the surface magnetic anisotropy of a fine particle, which lowers the perpendicular magnetic anisotropy.

 


On the other hand, the observed and simulated results for thicker particles (220nm) are in reasonable agreement to each other. 

This finding is of great interest since the present observation revealed non-uniform distribution of the perpendicularly magnetized state.  Instead, a conical or vortex type of magnetization appears in thin, fine particles.  This means that a patterned medium for high density recording may require careful tuning of magnetic parameters for magnetization distribution, otherwise non-uniform distribution becomes a serious issue.  Such vortex and conical distribution of magnetization are the first observed in this study.

 

・Fabrication of (FeCo)-Pt Dots by Ion Beam Induced CVD (IBICVD)

This novel technique has been developed in this laboratory, which allows one to fabricate very fine particles of any size.  The principle is shown in Fig.8, where an ion beam of Ga+ decomposes molecular gases of Co2(CO)8, and so on. The detailed process of fabrication should be found in original papers.

In Fig.9, an example of a (FeCo)50Pt50 fine particle is given, where the magnetization image taken by magnetic force microscopy is found. 

The contrast of white (inside) and black (peripheral) implies a uniform distribution of magnetization pointing toward up and down, which is along the easy axis for magnetization.

This is the first demonstration ever reported about the fabrication of magnetic ternary alloy dots by this IBICVD.  This technique has a potential to fabricate sub-nano-particles provided any external disturbance and beam fluctuation are carefully avoided.

Comparing to the case of Co particles discussed above, the magnetic anisotropy constants of FeCoPt fine particles are much higher than those of Co, thus leading to this magnetization uniformly distributed, neither vortex nor conical type.

 

 

(c) Perpendicular Exchange Bias in FePt/FeMn Multilayers

Much work has been carried out on exchange bias mechanisms in (Ferromagnetic layer/Anti-ferromagnetic layer) coupled systems, where the magnetic anisotropy lies in the plane of a film.  Recent high density recording requires high sensitive magnetic head, and therefore the exchange bias in a perpendicularly magnetized system has been attracting much attention.  In this laboratory, a study of such perpendicular exchange bias mechanism already started a four years ago, before this subject became so popular.  A brief description of exchange bias is illustrated in Fig.10, where both the in-plane and perpendicular exchange biased Hysteresis loops are shown.

Multilayers of (FePt/FeMn) were fabricated by Ion beam deposition system for which both thicknesses of FePt and FeMn layers are systematically changed.  Figure 11 shows the result of exchange bias field HB and the blocking temperature TB (the temperature at which the exchange bias field vanishes).  It is of interest to find that the TB for the in-plane case is higher than that of the perpendicular case.  Also, the HE is higher for the in-plane than for the perpendicular.  This difference is qualitatively explained based on a model put forwarded by Malozemoff, where the canted magnetization distribution is assumed. (Fig.12) Such a difference in TB between the in-plane and perpendicular direction has not been reported in literature and this is the first time such results were found.  A simulation is on the way to explain more details the magnetization distribution near the interface region.


One interesting result from simulation is that the magnetization direction along an interface exhibits an oscillation, that is, it changes its angle sinusoidally along the interface.  Though a more careful analysis is needed, such a phenomenon is certainly invaluable information for the exchange bias mechanism.