Room-temperature Coercive Field Research Articles

Electrical properties of sol-gel processed barium titanate films

The sol-gel technique has been used to prepare ferroelectric barium titanate (BaTiO3) films. The electrical properties of the films have been investigated systematically. The room temperature dielectric constant (ϵ) and loss tangent (tanδ) at 1 kHz were respectively found to be 370 and 0.012. Both ϵ and tanδ showed anomaly peaks at 125°C. The room temperature remanant polarization (Pr) and coercive field (Ec) were found to be 3.2 μC/cm2 and 30 kV/cm, respectively. The capacitance–voltage (C–V) and conductance–voltage (G–V) characteristics also showed hysteresis effect. The temperature variation of C–V and G–V characteristics also confirms the ferroelectric to paraelectric phase transition at 125°C.

Exchange coupling of NiFe/FeRh–Ir thin films

We report unusually large room temperature coercive field Hc induced in Ni0.81Fe0.19 films grown on the (001) magnetically compensated surface of antiferromagnetic single crystal CsCl-type FeRh0.95Ir0.05 alloy thin layers on MgO(001) substrates. The specimens were prepared by conventional rf sputtering in Kr atmosphere, at 450–500 °C for FeRh–Ir and 20–200 °C for NiFe. The epitaxial growth of NiFe/FeRh–Ir and atomically flat terraces of FeRh–Ir were confirmed by x-ray reflectometry and scanning tunneling microscopy. For a NiFe(2.7 nm)/FeRh–Ir(22 nm) film, a maximum Hc of 810 Oe is observed. The 1/tNiFe dependence of Hc for larger NiFe thicknesses indicates that the large coercive field originates from the magnetic interaction at NiFe/FeRh–Ir interface. The large Hc combined with high corrosion resistance make FeRh based alloys a promising candidate for application as pinning layers in spin-valve devices.

Effect of annealing on the structure and magnetic properties of graphite encapsulated nickel and cobalt nanocrystals

We report the structure and magnetic studies of carbon coated nanocrystals of nickel and cobalt synthesized in a special low carbon to metal ratio arc chamber. Powder x-ray diffraction profiles show peaks associated with single phase of fcc nickel or cobalt and major peaks of graphite with no evidence of carbides or solid solutions of carbon in the metal. Measured lattice spacing of crystalline particles and that of graphite coating from high-resolution transmission electron microscope images also confirm such findings. Magnetization measurements as a function of temperature in the range 20–900 °C give a Curie temperature equal to that of bulk metal within the experimental error. Upon heating and recooling of the particles a larger magnetization as high as 57% of bulk Co and 53% of bulk Ni was measured. Also M–H hysteresis loop of the particles have been measured at room temperature after annealing in the temperature range 20–650 °C for Ni, and 20–900 °C for Co. The dependence of room temperature saturation magnetization, remanent magnetization, and coercive field of the particles on annealing temperature is reported. These data are described by transition of particles form single domain to multidomain as a result of particle growth due to annealing. We also present the particle size distribution measurements that show log-normal behavior, and indicate substantial particle size growth due to annealing.

Magnetic behavior of metastable fcc Fe-Cu after thermal treatments.

A ferromagnetic and supersaturated fcc ${\mathrm{Fe}}_{51}$${\mathrm{Cu}}_{49}$ solid solution has been obtained by mechanical alloying. After subsequent thermal treatments the fcc phase undergoes a spinodal decomposition which finally, at 780 K, yields a mixture of fcc and bcc phases. In this work, a systematic magnetic study is carried out on samples at different decomposition states in order to determine the process of transformation into the stable phases. We observe a 20% maximum diminution on the magnetic moment with increasing temperatures of the thermal treatment. The Mo\ifmmode\ddot\else\textasciidieresis\fi{}ssbauer spectrum taken at 8 K shows that 20% of the Fe atoms are in a nonferromagnetic state. On the other hand, upon heating up to 723 K the room-temperature coercive field increases dramatically to 640 Oe, and after cooling down to 10 K it decreases to 270 Oe. Deviations from the ${\mathit{T}}^{3/2}$ law in the temperature dependence of the magnetization have been observed. This behavior is explained by fluctuations in composition due to the spinodal decomposition, which lead to fluctuations of the magnetic order parameters, i.e., magnetic moment and Curie temperature.

Magnetic properties of amorphous (Fe1-xCox)77.5Nd4B18.5 alloys before and after crystallization

The magnetic properties of amorphous (Fe1-xCox)77.5Nd4B18.5 alloys before and after crystallization were studied. The Fe atom moment mu Fe is found to increase with increasing x from 2.0 mu B for x=0 to a saturation value of 2.3 mu B for x=0.7 and then to decrease, when the Co atom moment mu Co is assumed to be constant for all x. The variation in Fe moment is in good agreement with the Co concentration dependence of the average hyperfine field. The Curie temperature TC is found first to increase and then to decrease, attaining a maximum value of 895 K at about x=0.7. This variation in TC with x is calculated using a phenomenological model described by Kouvel (1969), the exchange interaction temperatures TFe-Fe, TFe-Co and TCo-Co are calculated to be 584 K, 1010 K and 825 K, respectively. With increasing Co concentration x, the crystallization temperature is approximately constant for x<or=0.7 and then increases slightly with increasing x. The crystalline phases in crystallized samples were identified by X-ray diffraction. A high room-temperature coercive field Hc and a high-energy product (BH)max for the optimally annealed samples are obtained.

Magnetic properties and crystallization of the rapidly quenched (Fe 1- x Nd x ) 81.5 B 18.5 alloys

Magnetic properties and crystallization behaviors of amorphous (Fe1-xNdx)81.5B18.5 alloys were studied. The crystallization temperature is found to rise at first and then drop monotonically with x, having a maximum value of 976 K at x = 0.11 (9 at% Nd). The (Fe1-xNdx)81.5B18.5 alloys prepared at a quenching rate of vs = 6.6 m/s are amorphous, and exhibit good glass formability. Both the coercive field Hc and energy product (BH)max depend strongly on Nd concentration. Amorphous (Fe1-xNdx)81.5B18.5 alloys with higher Nd concentration have a high coercive field at low temperature, due to the large random uniaxial anisotropy of Nd. The room-temperature Hc and (BH)max obtained on optimal annealing conditions show two maxima as a function of Nd concentration x. The highest room-temperature coercive field Hc = 22 kOe within the Nd concentrations around x = 0.368 and the maximum energy product (BH)max = 13.3 MG Oe at x = 0.055 are observed. The hard magnetic properties of these crystallized samples are related to the presence of the hard magnetic Nd2Fe14B phase.

Magnetism of rapidly quenched (Fe 1− xNd x) 75B 25 alloys

Effects of heat treatment on hard magnetic properties of rapidly quenched (Fe 1− x Nd x ) 75B 25 (0 ⩽ x⩽ 0. 223) alloys were studied. The coercive field increases with increasing x at first, goes through a maximum value at x = 0.053 to lower value, then rises rapidly again. For x = 0.223, the room temperature coercive field is found to be 21 kOe. The as-quenched samples are amorphous. The Curie temperature and saturation magnetization are found to decrease monotonically with increasing x. The maximum of the crystallization temperature is 955 K at Nd concentration of x = 0.093. The crystalline phases were identified by X-ray diffraction and thermomagnetic measurements.

Influence of Fe Substitution for Co on the Magnetic Properties of Melt-Spun CoFeZr Alloys

Influence of Fe substitution for Co on the magnetic properties of melt-spun Co-Fe-Zr alloys are studied. The room-temperature coercive field Hc and energy product (BH)max of the melt-spun Co82−xFexZr18 alloys are found to increase with increasing the substrate velocity vs at vs ⩽ 47 m/s. The annealing temperature dependence of Hc and (BH)max show a wide maximum at about Ta = 650°C for x = 0 and Ta = 520°C for x = 8. The room-temperature magnetization σs measured in a field of 8 kOe for the Co82−xFexZr18 samples melt-spun with vs = 47 m/s increase with x from 59.9 e.m.u./g for x = 0 to 78.4 e.m.u./g for x = 10, but the coercive field decreases monotonically. The highest room-temperature coercive field Hc = 3.0 kOe and energy product (BH)max = 3.9 MGOe are obtained in optimal heat treated Co82Zr18 and melt-spun Co74Fe18Zr8 alloys, respectively. X-ray diffraction and thermomagnetic measurements on Co82−xFexZr18 show that the as-quenched sample and the sample annealed at Ta < 700°C exhibit a metastable (Co, Fe)5Zr phase coexisting with f.c.c. (Co, Fe). In the Co82−xFexZr18 alloys annealed at 950°C, the Fe atoms enter the Co lattice, forming (Co, Fe)5Zr, (Co, Fe)23Zr6, and f.c.c. (Co, Fe). The hard-magnetic properties of these materials are related to the presence of the 5: 1 phase. [Russian Text Ignored].

Magnetic properties and crystallization of amorphous Co100−x−yBxZry alloys

Abstract The magnetic properties and crystallization behavior of amorphous Co 100− x − y B x Zr y (0 ⩽ x ⩽ 25; y = 10, 16) alloys were studied. In amorphous Co 90− x B x Zr 10 alloys the average magnetic moment per cobalt atom and Curie temperature are found to decrease monotonically with increasing boron concentration x , and the crystallization temperature increases linearly. The crystallized Co 90− x B x Zr 10 alloys have a small coercive field. However, high coercive fields are observed in the crystallized Co 84− x B x Zr 16 alloys. The room-temperature coercive field H c in the optimal heat-treated samples is found to increase monotonically with increasing x from 2.2 kOe for x = 0 to 4.7 kOe for x = 16. The remanent magnetization B r (at H = 8 kOe) and the energy product ( BH ) max for the Co 84 Zr 16 alloy annealed at 660 °C for a short time are obtained to be 6.1 kG and 4.2 MGOe, respectively.

Magnetic properties in melt-spun Co 82B 2Zr 16 alloy

Influence of quenching rate and heat treatment on the magnetic properties of melt-spun Co 82B 2Zr 16 alloy were studied. Large coercive field and energy product are found to occur only over a relatively narrow quenching rate. The annealing temperature dependence of coercive field of the as-quenched samples shows a wide maximum. The highest room-temperature coercive field H c = 3.0 kOe and energy product ( BH) max = 4.5 MGOe are obtained in the sample melt-spun with v s = 26.3 m s −1 after annealing at 520°C for 2 min. X-ray diffraction and thermomagnetic measurements on the samples with high H c and ( BH) max show a two-phase structure with the majority phase Co 5Zr having a Curie temperature T c = 800 K. The hard magnetic properties of the samples are associated with the presence of the Co 5Zr phase.

Magnetic properties and phase components in amorphous (Fe1−xNdx)81.5B18.5 alloys after crystallization

Magnetic properties and phase components for amorphous (Fe1−xNdx)81.5B18.5 alloys after crystallization were studied systematically. The crystallization temperature of the amorphous alloys is found to increase first with increasing Nd concentration x, goes through a maximum at x=0.11, then decreases. The crystalline phases of the samples annealed at different temperature were identified by X-ray diffraction and thermomagnetic measurements. It is observed that the crystallized (Fe1−xNdx)81.5B18.5 alloys with 3–6 at% Nd additions consist of Nd2Fe14B, Fe3B and αFe. The concentration dependence of the intrinsic coercivity iHc and maximum energy product (BH>)max are found to have a maximum at the concentration near x=0.055. A room temperature coercive field of 2.9 kOe and an energy product of 13.3 MGOe are obtained in Fe77Nd4.5P18.5 alloy annealed at 670°C for a short time. These FeNdB alloys with fewer Nd concentration would be used as an inexpensive bonded magnets, due to its higher values of Br and (BH)max.