Session X13: Focus Session: Magnetic Nanostructures-Hard Magnetic Materials and Magnetocaloric Materials

Thermal Fluctuation and Finite- Temperature Performance of Hard-Soft Composite Magnets

The demagnetization behavior of exchange-coupled hard/soft magnets was studied by finite temperature Monte Carlo simulation. Hard phase cube inclusions (Nd$_2$Fe$_{14}$B, SmCo$_5$ and Sm$_2$Fe$_{17}$N$_3$) into a soft matrix (FeCo) and hard/soft multilayer structure were studied. The easy axis of the hard and soft phase and the initial magnetization are parallel to the applied field. We found significant thermal fluctuations and lowering of the remnant magnetization with increasing soft magnet content than is anticipated from zero-temperature models, especially at higher temperatures. This greatly diminishes the expected performances of composites. For cube inclusions there is a boundary mismatch of the magnetization on the hard/soft interface. We investigated this mismatch as function of the soft phase content and temperature. The spin wave spectrum due to the mismatched dipolar interaction will be discussed.   

Control of Size and Magnetic Properties of L1$_{0}$ FePt islands by DC sputtering

FePt particles with the tetragonal L1$_{0}$ structure are attractive for high density recording media. In this study, we have fabricated well-ordered and separated L1$_{0}$ FePt islands on MgO (100) substrates by DC sputtering at temperatures varying from room temperature to 700 $^{\circ}$C. The dependence of particle size, degree of ordering and magnetic properties on sputtering time (5-20s), power (5-20W) and substrate temperature (300-1000K) were investigated. Electron diffraction patterns from TEM showed that the islands had the L1$_{0}$ tetragonal structure. TEM data revealed that a higher substrate temperature significantly increased the size of the islands from 2 to 20 nm whereas a higher sputtering power and longer sputtering times did not change the island size much but made the islands more connected with each other. Larger size islands and inter-connected islands showed a higher degree of ordering with an ordering parameter of 0.8 achieved at 600 $^{\circ}$C. The magnetic properties are currently being measured and the results will be reported.   

Nano-granular FePt thin films for thermally-assisted magnetic recording

In order to extend the data storage density in hard disk drives beyond 1 Tb/in$^2$, nano-size grains of a high crystalline anisotropy ($K_u$) material are required to obtain these high densities and maintain thermal stability. A promising approach to recording using high-$K_u$ materials is thermally-assisted magnetic recording (TAR), in which the media is locally heated above the Curie temperature while a magnetic field is applied in order to write a bit. FePt with L1$_0$ crystalline order is a potential candidate for TAR media. We deposit FePt nano-granular films with carbon as the segregant material, by co-sputtering on glass substrates at elevated temperature. Underlayer materials are selected for heat-sinking and to attain high out-of-plane L1$_0$ order. Characterization of the media by x-ray diffraction, magnetometry and transmission electron microscopy show that we can achieve properties that are promising for TAR media, such as an average grain size $<$ 7.5nm, size distribution as low as 16\%, coercivity as high as 5 tesla and $K_u >$ 4.5 x 10$^7$ erg/cm$^3$. Recording densities exceeding 600 Gb/in$^2$ have been demonstrated for our FePt granular films using a static tester.   

Confined stripe structure in periodically grooved NdCo Films with perpendicular magnetic anisotropy

Magnetic multilayers are broad research field with many interesting phenomena depending on interlayer coupling. Also, since the development of nanolithography techniques, magnetic nanowires and dots have been intensively investigated [1]. Recently, as a combination of these two fields, the concept of magnetic lateral superlattice has emerged: continuous magnetic films with a lateral modulation of their magnetic properties at submicrometric length scale [2]. In this work, we have fabricated amorphous Nd-Co films with perpendicular magnetic anisotropy and a periodic thickness modulation by e-beam lithography and ion milling. Lateral periods range from 2 $\mu$m - 500 nm and groove depths from 10 to 30 nm. MFM and Kerr magnetometry have been used for characterization. Lateral patterning modifies the interplay between magnetostatic energy, perpendicular and in plane anisotropy and exchange interaction resulting in confined magnetic stripe structures. The different regimes that appear depending on the size of the periodic thickness modulation relative to the magnetic stripe period will be discussed. [1] J.I Martin et al, JMMM, 256 (2003) 449 [2] S. P. Li et al, PRL 88 (2002) 087202; N. Martin et al, PRB 83 (2010) 174423   

Intrinsic Magnetic Properties of fct FePt Nanocubes and Rods by Cluster Beam Deposition

In this work, single crystal fct FePt nanocubes have been successfully produced by a cluster beam deposition technique without the need of post annealing. Particles have been deposited by DC magnetron sputtering using high Ar pressures (0.5 to 2 Torr) on both single crystal Si substrates and Au grids for the measurement of magnetic and structural properties, respectively. The nanocubes have a uniform size distribution with an average size of 6.5 nm. At 1 Torr, the particles have the fct structure with an order parameter of 0.5 and a RT coercivity of 2 kOe with high switching fields seen in the hysteresis loop. Particle size was controlled by changing the pressure and power and also by ex-situ annealing. In addition to these nanocubes, micron size rods (which consist of 20 nm nanoparticles) with the fct structure have been observed near the cluster gun. These particles show a room temperature coercivity of 8 kOe with an order parameter of 0.85. Intrinsic magnetic properties (Curie temperature, H$_{A}$, M$_{S}$ and magnetic viscosity) of the nanocubes and the nanoparticles (separated from the rods) have been extensively studied and the results will be reported.   

Confinement Effect on the Phase Transformation of FePt from A1 to L1

The major challenge for the application of chemically synthesized FePt nanoparticles (NPs) in magnetic storage media is the sintering problem encountered during the required high temperature annealing to obtain the high anisotropy $L$1$_{0}$ phase. In this work, we have used two methods to avoid sintering: coating the NPs with a protective layer of silica (SiO$_{2})$ and using porous aluminum oxide (Al$_{2}$O$_{3})$ as a template to hold the NPs. The NPs were synthesized via the synthesis method of Sun \textit{et al}.$^{[1]}$ The NPs were added to the Al$_{2}$O$_{3}$ by in-situ suctioning of the reaction solution into the porous Al$_{2}$O$_{3}$ template. Monodispersed FePt NPs with a size of 5.8 and 15 nm were coated with SiO$_{2}$ shells using a water-in-oil microemulsion method. High room temperature coercivities were only obtained after annealing the samples at 900\r{ }C for long times (24-48 h) under forming gas flow as compared to the usual 600-700\r{ }C. Values of 4.7 and 7.8 kOe were observed in SiO$_{2}$ and Al$_{2}$O$_{3}$ samples, respectively after annealing for 24 h at 900\r{ }C. This behavior suggests that the restricted geometry of the samples suppresses the phase transformation drastically. \\[4pt] [1] S. Sun, C. B. Murray, D. Weller, L. Folks, A. Moser\textit{ Science }\textbf{2000}, $287$, 1989.   

Towards high strength nanocomposite magnets --- Approaches from the bottom

Exchange-coupled nanocomposite magnets are regarded as the next generation of permanent magnetic materials, based on the theoretical predictions. However, many fundamental questions and technical challenges remain in understanding the inter-phase exchange interactions and in processing bulk nanocomposite magnets with enhanced energy products. We will review recent advancements in both the fundamental research and the materials processing technologies. New findings about the effects of soft-phase properties and interface conditions on the hard/soft phase exchange interactions will be presented. Particularly, the development of the bottom-up approaches in materials processing will be discussed. Novel methodology for nanoparticle synthesis including the salt-matrix annealing, surfactant-assisted ball milling and severe plastic deformation will be described. Unconventional compaction techniques including warm compaction and dynamic compaction are recommended because they can be used to retain desired nanoscale morphology for effective exchange coupling in bulk nanocomposite magnets. A perspective on fabrication of anisotropic nanocomposite magnets will be also given.   

NdCo$_{5}$ Nanoflakes and Nanoparticles Produced by Surfactant-Assisted High Energy Ball Milling

The study of size and surface effects in rare earth transition metal nanoparticles is scientifically very important. In this work our studies were focused on NdCo$_{5}$ which is interesting because of its complex magnetic ordering behavior at different temperatures. Anisotropic NdCo$_{5}$ nanoparticles have been produced by surfactant-assisted high-energy ball milling (HEBM) of nanocrystalline precursor alloys. A two-stage ball milling method has been employed to produce the NdCo$_{5}$ nanoflakes and nanoparticles. NdCo$_{5}$ flakes have a thickness below 150 nm and an aspect ratio as high as 10$^{2}$ - 10$^{3}$; the nanoparticles have an average size of 7 nm. Both the nanoparticles and nano-flakes showed high coercivities at low temperatures, with values at 50 K of 3 kOe and 3.7 kOe, respectively. The high values of coercivity observed in a planar anisotropy phase can be attributed to the large surface anisotropy of nanoparticles that leads to an effective uniaxial-type of behavior. The nanoparticles also showed spin reorientation temperatures which are lower when compared to the bulk values.   

Magnetic Studies on Nd$_{2}$Fe$_{14-x}$Mn$_{x}$B Nanoflakes and Nanoparticles Produced by Surfactant-Assisted High Energy Ball Milling

High temperature magnetic ordering studies on rare earth transition-metal nanoparticles and nanoflakes present a great challenge due to the very high reactivity of these materials. It is well known that Mn substitution for Fe in Nd$_{2}$Fe$_{14}$B compound decreases the Curie temperature to a temperature range that allows for reliable measurements to be made. In this work, we have studied the magnetic properties of Mn substituted Nd$_{2}$Fe$_{14}$B particles in the temperature range of 50-400 K. Nd$_{2}$Fe$_{14-x}$Mn$_{x}$B nanoparticles and nanoflakes have been produced by surfactant-assisted high-energy ball milling (SA-HEBM). Different size nanoparticles have been obtained by varying the milling conditions. Anisotropic Nd$_{2}$Fe$_{14-x}$Mn$_{x}$B nanoparticles have been found with a size from 13 to 25 nm. Both the nanoparticles and nano-flakes showed high coercivities at low temperatures, with values at 50 K of 2.4 kOe and 5.5 kOe, respectively. The Curie temperature was determined from the temperature dependence of magnetization. We have observed a different magnetic ordering behavior in the nanoparticles with Curie temperatures that are higher when compared to the bulk values.   

Giant Coercive Fields of 2.5 Tesla in Nanostructured Mn$_{x}$Ga Films

There is a growing interest in designing new magnetic materials that are free of rare-earth elements. The magnetism of the Heusler ferrimagnet Mn$_{x}$Ga [1] was found to be enhanced when fabricated with nanoscale structural disorder. Films of Mn$_{x}$Ga (x=2 to 3) with thicknesses of 20 to 40 nm were grown using molecular beam epitaxy at 100\r{ }C then annealed at 400\r{ }C. Disordered films were grown on lattice mismatched Si (001) substrates, then compared to epitaxially grown films on desorbed GaAs (001) substrates. While the epitaxial films have small hysteresis in the magnetization with coercive fields in the range $\mu _{o}$H$_{C}$ = 10$^{-2}$ - 10$^{-1}$ T, the disordered films exhibited surprisingly wide hysteresis with record high coercive fields as large as $\mu _{o}$H$_{C}$ = 2.5 T. These magnitudes are comparable to those of rare-earth-based magnets. This hysteresis was also present in the anomalous Hall effect. The enhanced coercive field in the disordered material arises from a combination of the exceptionally large magnetocrystalline anisotropy and nanoscale structural disorder. These results point out a new opportunity for developing rare-earth-free magnetic materials. Discovery of this unusually high coercive field is outlined and its sources discussed. [1] J. Winterlik, et al., Phys. Rev. B \textbf{77}, 054406 (2008).   

Tunable exchange length in laminate exchange coupled composite media

Exchange coupled composites - with a hard layer (HL) to anchor against thermal instabilities, and a soft layer (SL) to assist magnetization reversal - have been proposed for advanced recording applications. The reversal assist relies on a non-coherent rotation between the HL and SL, - the exchange-spring (ES) - in which the interfacial domain wall traverses the hard/soft interface, and promotes switching. Typically, the soft region is a single layer, with the emergent composite properties mainly determined by the choice of soft material. We are pursuing a more sophisticated approach, using a hard CoPtCr-SiO2 layer adjacent to a multilayer of the same material, [CoPtCr-SiO2/Pt]N, softened by lamination with Pt layers. Simulations predict that when the SL thickness exceeds a critical exchange length, a significant portion should decouple from the HL, becoming a domain wall nucleation site. Thus, ES behavior should be tunable via the Pt thickness. To test this, we have used polarized neutron reflectometry to measure the field-dependent magnetic depth profiles - and directly characterize ES formation - for a series of samples with varying Pt laminate thickness. The experimentally determined relationship between laminate thickness and ES formation will be discussed.   

Impact of Size Reduction on the Magnetocaloric Effect in Single- and Multi-Phase Manganites

Mixed-valent manganites of the form R$_{1-x}$M$_{x}$MnO$_{3}$ (R=La, Pr, Nd, Sm and M=Sr, Ca, Ba, Pb) are of interest as low-cost materials for potential application in the area of active magnetic refrigeration (AMR). An important parameter to optimize for AMR is the refrigerant capacity (RC), which depends on both the magnitude and breadth of the magnetic entropy change peak. Reducing the dimensions of a system to the nanoscale has the potential to enhance the RC by broadening a transition, but can also lead to a drop in entropy change. In this study, we contrast the impact of size reduction on the magnetic and magnetocaloric properties of single-phase La$_{0.4}$Ca$_{0.6}$MnO$_{3}$ (LCMO) and phase-separated La$_{0.35}$Pr$_{0.275}$Ca$_{0.375}$MnO$_{3}$ (LPCMO). Nanoparticles of LCMO and LPCMO were prepared by a sol-gel method; single crystals were grown in an optical floating zone furnace. XRD, SEM, and TEM were used to characterize the samples and DC magnetometry measurements were performed using a Quantum Design VSM. We find that size reduction negatively impacts both magnetization and the magnetocaloric properties in LCMO, while enhancing RC and entropy change simultaneously in LPCMO.   

Enhancement of magnetic refrigerant capacity in nanocrystalline LaMnO$_{3}$

Manganites are considered as potential magnetic refrigerants owing to their large magnetocaloric effect (MCE). However, the effective temperature span of large MCE ($\delta $T) is quite small in their bulk form resulting in small refrigerant capacity (RC). We have studied the magnetocaloric property of LaMnO$_{3 }$in its polycrystalline bulk and nanocrystaline form with particle size $\sim $50nm. MCE is quantified as the change in magnetic entropy (-$\Delta $S$_{M})$, which is calculated from the isothermal magnetization curves using Maxwell relation. The samples exhibit large -$\Delta $S$_{M}$ associated with their paramagnetic to ferromagnetic transition. Relative to the bulk sample, $\delta $T increases significantly in the nanocrystalline form giving rise to more than 63{\%} enhancement in RC. The calculated values of -$\Delta $S$_{M}$(RC) for bulk and nanocrystalline samples are $\sim $2.6J/KgK(173J/Kg) and 2.4J/KgK(282 J/Kg) respectively for magnetic field change of 50 kOe. From present study, it can be inferred that reduction of particle size to the nanoscale may be an effective way to increase $\delta $T and hence improve RC of a material.