Distribution Of Blocking Temperatures Research Articles

Extraction of blocking temperature distribution from zero-field-cooled and field-cooled magnetization curves

Superparamagnetic fluctuation of magnetic nanoparticles is studied. We examine the validity of the simplified approach using the difference between zero-field-cooled magnetization and field-cooled one. The results show that it is valuable for estimating the blocking temperature distribution of polydispersive particles even in magnetic fields, if both surface effects and inter-particle interactions are negligible.

Blocking temperature in exchange coupled MnPt∕CoFe bilayers and synthetic antiferromagnets

Exchange coupled Mn51.5Pt48.5∕Co90Fe10 bottom-pinned bilayers and synthetic antiferromagnets (SAFs) were prepared by magnetron sputtering. Exchange bias, coercivity, and blocking temperature were determined as a function of the MnPt thickness. Interfacial exchange energy was found to be Jex=0.4erg∕cm2 for a Mn51.5Pt48.5(t⩾20nm)∕Co90Fe10 (5nm) bilayer. Thermal stability of the bilayers was analyzed by separation of the blocking temperature distribution in (a) width ΔTB and (b) center temperature TB,center of the distribution. For thick MnPt films ΔTB is constant but steadily increases for thin films. SAF structures show higher exchange coupling and higher blocking temperatures at thin layer thickness (tMnPt=8.5nm).

Distribution of blocking temperature in exchange-coupled Fe3O4–CrMnPt system

The distribution of local blocking temperatures (TBi) in an exchange-coupled half-metallic-Fe3O4–antiferromagnetic-Cr45.5Mn45.5Pt9 system (in atomic %) was investigated, with the aim of confirming thermal stability of unidirectional anisotropy induced in Fe3O4 film, which has hard magnetic properties. The distribution of TBi was mostly at or around 265°C. This distribution in the Fe3O4–CrMnPt system was equivalent or superior to that found in a previously studied soft magnetic-Ni81Fe19–antiferromagnetic-Pt50Mn50 system, where PtMn had been used in a giant magnetoresistance device as the standard antiferromagnetic material for the exchange-biased layer. These results make it reasonable to conclude that the exchange coupled Fe3O4–CrMnPt system has excellent thermal stability.

Magnetic properties of Co–Cu nanoparticles dispersed in silica matrix

The magnetic properties of Co–Cu/silica nanocomposites prepared by sequential ion implantation have been investigated. The nanostructure is formed of 4nm average size particles dispersed in silica matrix and with mainly FCC structure. The hysteresis loops at 3K indicate that the nanoparticles have uniaxial anisotropy with values smaller than that of single Cobalt implants. The samples are characterized by a superparamagnetic behavior with blocking temperatures that depend on the Co/Cu implanted ratio, and the blocking temperature distributions are narrower than those obtained by taking into account only the particle size distribution. Results are discussed considering size effects and that the nanoparticles are formed of a Co–Cu solid solution or only of Co.

Improved thermal stability of exchange bias of Mn–Ir/Co–Fe bilayers by novel in situ thermal annealing procedure

The effect of in situ thermal annealing of Mn–Ir films under ultra-high vacuum on the metallurgical microstructure and on the thermal stability of the unidirectional anisotropy constant, JK, was investigated for Mn73Ir27/Co70Fe30 bilayers. As a result, we found that the average lateral grain size of Mn–Ir increases with increasing the in situ annealing temperature, TIR and that the shape of JK–T curve changes to convex with increasing the TIR, implying a narrowing distribution of local blocking temperature. We conclude that ultra-high vacuum annealing is effective in improving the thermal stability of the exchange anisotropy of Mn–Ir/Co–Fe bilayers through the change of lateral grain size distribution of the Mn–Ir layer.

Structural and magnetic properties of MnAs nanoclusters formed by Mn ion implantation in GaAs

Ferromagnetic (FM) nanostructures embedded in semiconductors are attracting interest because their physical properties could be used in new devices such as memories, sensors or more generally involving “spintronics”. In this work, we present results on the synthesis and the characterization of nanosized MnAs ferromagnets buried in GaAs. These nanocrystals can be formed either by single Mn implantation or by Mn+As co-implantation at room temperature into GaAs wafers followed by thermal annealing. High-resolution transmission electron microscopy and diffraction analysis show that the MnAs precipitates exhibit the regular hexagonal structure with a 3 m orientation relationship with respect to the matrix. The mean diameters of the nanocrystals range from 9 to 13 nm depending on the annealing conditions. Magnetization measurements by superconducting quantum interference device magnetometry reveal an FM state. Upon warming, a progressive transition to a superparamagnetic state is evidenced, probably due to the distribution of individual blocking temperatures, resulting both from the size distribution of the nanocrystals and from dipolar interactions. Curie temperatures in the range of 360 K are measured, significantly higher than for bulk MnAs.

Blocking temperature distribution in implanted Co–Ni nanoparticles obtained by magneto-optical measurements

Three samples of Co–Ni alloy nanoparticles with different compositions were prepared by sequential ion implantation in silica slides. Transmission electron microscopy (TEM) showed the presence of spherical nanoparticles dispersed in the matrix. Magneto-optical Kerr effect analysis identified two magnetic components attributed to superparamagnetic particles in unblocked and blocked states, respectively. Magnetic field loops were measured as a function of temperature. Blocking temperature distributions were obtained; and their comparison with the size distributions derived from TEM provided the average magnetic anisotropy of the particles.

Size and shape effects on exchange field of patterned NiO/NiFe films

The exchange field and domain configurations were investigated in patterned polycrystalline and (100) NiO/NiFe films. The exchange field was enhanced in the polycrystalline patterns with the aspect ratio larger than one (the long edge parallel to the original easy axis). Exchange field of patterned (100) NiO/NiFe on MgO, in contrast, showed little dependence on the aspect ratio, but strong dependence on the pattern size. The exchange field increased from 8.6 Oe in the sheet film to 48.4 Oe in the 2 μm patterns. The different dependence of the exchange field of polycrystalline and (100) NiO/NiFe on the pattern size can be plausibly explained by the difference of the domain size in NiO, and the variation of the blocking temperature distribution. Magnetic force microscopy images of patterned polycrystalline NiO/NiFe showed two kinds of domain configurations (single-domain state and multidomain state) in the sample with the pattern size of 2 μm, which may be the direct observation of various exchange paths at the interface of NiO/NiFe.

Thermally stable spin valve films with synthetic antiferromagnet pinned by NiMn for recording heads beyond 20 Gbit/in.2

The thermal stability of spin valve films with synthetic antiferromagnet (SAF) pinned by antiferromagnetic IrMn, NiO, and NiMn layers were studied. The SAF layer enhances the thermal stability in general; however, the blocking temperature (and the blocking temperature distribution) of the antiferromagnet is still important for the magnetic rigidity of the pinned layer. Once the temperature reaches the blocking temperature the SAF layer can go into either the spin flip or flop state, depending upon the magnetic moment ratio of the reference layer and pinned layers. The GMR linear head response can be distorted for nonlinearity. The NiMn pinned SAF structure shows magnetic and thermal stability which makes it practical for the real products. A high GMR of 11% can be obtained in both bottom and top NiMn SAF spin valves by advanced processes. Recording heads were built using such stacks which demonstrated recording areal density of 20 Gbit/in.2 and beyond.

Correlation between lifetime and blocking temperature distribution in spin-valve structures

The blocking temperature distribution Tb(T) and the failure activation energy (as defined by a 10% drop in the magnetoresistance amplitude in a reverse field equivalent to the self-demagnetizing field of a micron size stripe height device) have been determined in spin-valve sheet films with FeMn, IrMn, PtMn, NiMn, and CrPdMn antiferromagnetic exchange biasing layers. We find a clear correlation between the expected lifetime and the fraction of loose (e.g., unblocked) antiferromagnetic grains, which we believe is due to pinned layer rotation being the main failure mechanism in these systems. For CrPdMn structures, a good agreement is found between the stability of sheet films and of finished sliders. From these data, only NiMn and PtMn appear to be suitable for disk-drive applications.

Thermal stability of NiMn spin valve heads

The thermal stability of NiMn-based spin valves was studied both at the coupon level and in full read/write heads. In the coupons, the blocking temperature distribution showed no component below 150/spl deg/C. As a result, the exchange field at elevated temperature was found to increase with time (because of improved chemical ordering of the NiMn layer), and irreversible losses in the /spl Delta/R/R response were observed only above 250/spl deg/C (because of layer interdiffusion). If a 10% drop in /spl Delta/R/R amplitude is used as a criterion to calculate time to failure, the NiMn heads should have several hundred years of lifetime at 150/spl deg/C operating temperature. Full read/write heads showed linear response with read-back amplitudes above 2 mV//spl mu/m at 6-mA operating current. In contrast to most other giant magnetoresistance heads, the output of the head remains the same after heating with a high bias current and degrades only with the shorting of the stripe. All these properties make NiMn superior to other antiferromagnets for spin valve head applications.

Exchange biasing and thermal stability of CoFe/PtPdMn films

Two sets of PtPdMn exchange biased films Ta 50 Å/CoFe 100 Å/PtPdMn (tAF) Å/Ta 50 Å, with PtPdMn thickness, tAF=350, 600 Å, were deposited on Si substrates by dc magnetron sputtering techniques. After magnetic annealing, these two sets of films exhibited values of exchange bias field, Hex=229 and 254 Oe, respectively. The PtPdMn layer was then thinned to various thicknesses from 600 down to 50 Å by ion beam etching. Hex does not retain its original value. It decreases with decreasing tAF and becomes zero at tAF∼75 Å. In addition, we have observed that the training effect or the anomalous hysteresis loss becomes more pronounced with decreasing tAF. This confirms that not only face-centered-tetragonal phase but, more critically, tAF plays role in determining exchange biasing and its thermal stability. The blocking temperature, TB, appears unaffected by the thinning of the PtPdMn layer, and no apparent change occurs in the local blocking temperature distribution, as suggested by the finite size effect.

Memory effect in standard spin valve structures

Memory effect has been observed in both standard top and bottom spin valves. The change of the magnetization state in the pinned FM layer, below the blocking temperature, reverses the direction of the exchange bias and destroys the magnetoresistance properties. This reversed exchange bias is much weaker, causing severe consequences in SV applications. This behavior can be explained in terms of blocking temperature distribution in the AFM layer perhaps due to the structural randomness. By varying cooling procedures, the exchange coupling in regions with different blocking temperatures can be separated. It is found that the maximum exchange bias is very close to the sum of the exchange biases in different regions. The domain wall energy in the FM layer has to be taken into account in order to explain the behavior of the reversed bias. The insertion of a synthetic antiferromagnetic subsystem (Co/Ru/Co) stabilizes the magnetization state in the pinned layer because of the additional interlayer coupling through the Ru layer. This suppresses the memory effect.

Antiparallel pinned NiO spin valve sensor for GMR head application (invited)

NiO antiferromagnetic material possesses certain advantages for spin valve applications and has attracted considerable attention. Some of the key advantages are its insulating properties, very high corrosion resistance, less sensitivity to composition, and its low reset temperature. This material, however, has a low blocking temperature which prevents its application to simple spin valve designs. The use of this material in spin valve structures required significant improvements in thermal stability, blocking temperature, and the spin valve design. In the present study, the blocking temperature and the blocking temperature distribution of the NiO films have been improved by depositing the films reactively using ion beam sputtering. A number of improvements in the processing method and deposition system had to be made to allow full NiO spin valve deposition for mass production. Another critical part was the use of antiparallel pinned design in place of the simple design to improve the thermal stability of the NiO spin valves as read elements at disk drive temperatures. The selection of the ferromagnetic pinned layers and the Ru spacer thickness in AP-pinned spin valves has significant impact on the behavior of the devices. These spin valves are all bottom type, NiO/PL1/Ru/PL2/Cu/Co/NiFe/Ta, where the metallic portion of the spin valve is deposited on top of the NiO AF layer. The PL1 and PL2 are ferromagnetic layers comprising NiFe and Co layers. Read elements have been made using these spin valves that delivered areal densities of 12 Gbit/in. These topics and other improvements which resulted in successful use of NiO spin valves as GMR heads in hard disk drives will be discussed.

Blocking temperature distribution and long-term stability of spin-valve structures with Mn-based antiferromagnets

We have determined the blocking temperature distribution Tb(T) in spin-valve sheet films with FeMn, IrMn, PtMn, NiMn and CrPdMn antiferromagnetic layers (AFM). We find a clear dependence of Tb(T) on the field applied during the measurement, which we link to the reversal state of the pinned layer through the torque applied on the AFM. Using fields large enough to fully reverse the pinned layer, NiMn and PtMn show little or no components of the blocking temperature below 150 °C, whereas both IrMn and CrPdMn (the latter in a “synthetic” AFM design) exhibit important low-temperature trailing edges of the distribution. Accelerated annealing experiments in a low reversed field equivalent to the self-demagnetizing field in a micron-size head allows us to access the time to failure and the failure activation energy from which the expected lifetime can be assessed. We find a general correlation between the expected lifetime and the fraction of loose (e.g., unblocked) AFM spins at any given temperature. Accordingly, only NiMn and PtMn are found to exhibit a sufficient long-term stability for disk-drive operations.

Thermal relaxation of interacting fine magnetic particles – field-cooled and zero-field-cooled magnetization variation

Dipole interaction makes average energy barrier of magnetic fine particles for thermal relaxation reduced while the corresponding blocking temperature distribution is a function of both dipolar interaction strength and particle size distribution of the system. Flatness and fast drop-off of the λ- shape behavior of the field-cooled and zero-field-cooled magnetization varies with both dipolar intereaction strength and field level applied.

Inverted spin valves with IrMn biasing layer

Inverted IrMn-based spin valves (ISV) were fabricated with the aim of low thickness/high output dual spin valve applications. Optimized structures exhibit the following characteristics: H/sub exch/=750 Oe, R/sub sq./=18.5 /spl Omega//sq., /spl Delta/R/R=7.5% and T/sub b/=250/spl deg/C with IrMn thickness as low as 75 A. The blocking temperature distribution, however, exhibits nonzero components from 150/spl deg/C onwards. Accordingly, accelerated sheet films lifetime experiments and current heating on full read/write heads show that partial rotations of the pinned layer occur well below T/sub b/. These structures therefore appear to be insufficiently stable for disk-drive applications.

Exchange coupling and stability of PtMn-based spin valves

The exchange coupling and stability of spin valves with antiferromagnetic (AF) PtMn films were investigated through the correlation between the magnetic properties and their microstructures. The exchange coupling field (H/sub ex/) decreases during iterative MR measurements and this decrease associates closely with the changes in the rotational hysteresis loss (W/sub r/), exchange anisotropy energy (K/sub e/) and blocking temperature (T/sub b/) distribution. The ordering of PtMn films, featuring an antiphase configuration, is found to play a significant role in determining the exchange coupling properties and stability of these spin valves.

Memory effect and temperature behavior in spin valves with and without antiferromagnetic subsystems

Temperature behavior and memory effect in standard spin valves (SV) and SVs with synthetic antiferromagnetic (Co/Ru/Co) (SV-SAF) subsystems have been studied. SV-SAFs show much better temperature stability. Memory effect refers to the phenomenon that the exchange bias can be altered at temperatures (TR’s) much lower than the blocking temperature (TB), and these temperatures (TR’s) are imprinted into SVs. The memory effect greatly deteriorates the magnetoresistance behaviors in SV. Our results suggest that the memory effect is caused by a distribution of local blocking temperatures (Tb’s). The magnetization state in the pinned layer is critical in determining the temperature behavior of HE and magnetoresistance. By partially reversing the magnetization in the pinned ferromagnetic (FM) layers, we are able to separate the temperature dependencies of the local exchange bias (He) associated with regions consisting of different Tb’s. Two features have been observed: (1) the local exchange bias (He) with a narrow Tb distribution has a weak temperature dependence; (2) the simple algebraic sum of local He’s nearly reproduce the total HE with the difference between these two quantities representing the domain wall energy in the FM layer. On the other hand, SV-SAFs show strong resistance to memory effects because of two factors; the strong exchange coupling through the Ru layer, and the net magnetic moment of Co/Ru/Co layers in SV-SAF being close to zero. The former makes the two SV-SAF FM layers behave coherently, while the latter makes the interaction between the SV-SAF and the external field negligibly small.

Spin-Valve GMR Films Based on Antiferromagnetic NiMn

ABSTRACTIn this work we have investigated the magnetic and structural properties of spin-valve films exchange biased by CoFe/NiMn compared to CoFe/lrMn spin valves. A spin-valve film of structure Ta/NiFe/CoFe/Cu/CoFe/NiMnITa showed a giant-magnetoresistance of 5.5% with a Hex∼850 Oe after annealing. Structural ordering of the NiMn in an fct phase after annealing has been observed using x-ray diffraction, with the data exhibiting pure fct(111) texture. The thermal stability of both the exchange pinning and magnetoresistance of NiMn spin-valves was evaluated. The blocking temperature (Tb) of this film is 400°C, compared to 225°C for IrMn based spin-valve films (IrMn thickness ∼50 Å). Blocking temperature distribution and remnant blocking temperature measurements also suggest that NiMn based spin-valves have far superior pinning stability with temperature than IrMn based spin-valves. Magnetoresistance measurements show a linear decrease with temperature due to electron-phonon scattering with the ∆R/R being reduced by 33% at 150°C (as compared to a 50% decrease for IrMn based spinvalves). Extended annealing at 150°C did not effect spin-valve performance, with ∆R/R and R being constant over 60 hours, suggesting that no appreciable interdiffusion is occurring.