Effect of Tb substitution on the magnetic properties of exchange-biased Nd2Fe14B/Fe3B
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- Hoque, S.M., Hakim, M.A., Khan, F.A. et al. J Mater Sci (2007) 42: 9415. doi:10.1007/s10853-007-1884-3
Tb-substituted (Nd,Tb)2Fe14B/Fe3B nanocomposite ribbons have been synthesized by melt spinning of Nd3Tb1Fe76Cu0.5Nb1B18.5 alloys. Tb substitution has significantly enhanced the value of coercivity and Curie temperature. Highest value of coercivity has been obtained as 4.76 kOe for the sample annealed at 953 K for 10 min. Curie temperature of Tb substituted sample, Nd3Tb1Fe76Cu0.5Nb1B18.5 is 549 K while Curie temperature of the sample without Tb, Nd4Fe76Cu0.5Nb1B18.5 is 535 K. Recoil hysteresis loops measured along the major demagnetization curve are steep having small recoil loop area. Temperature dependence of coercivity, remanent ratio and maximum energy product have been measured for the sample annealed at 893 K and 923 K for 10 min. At 5 K, coercivity and maximum energy product of the sample annealed at 893 K for 10 min are 5.2 kOe and 11.5 MGOe respectively and the sample annealed at 923 K for 10 min are 6 kOe and 13.1 MGOe respectively.
Nd-deficient Nd2Fe14B/Fe3B based nanocomposite alloys are characterized by their exchange-spring behavior resulting in remanent ratio > 0.5, which is highly desirable for permanent magnet development. Magnetic properties of exchange spring magnets are governed by the soft and hard magnetic phases that develop under appropriate annealing condition. High reduced remanence characteristic to these materials arises from exchange coupling of magnetic moments across the interface between two phases. Besides high reduced remanence such systems possess high energy product (BH)max and a reversible demagnetization curve, which has been called as exchange-spring behavior. This causes the magnetic moments of both the phases to remain in the same direction. It has been demonstrated earlier by Kneller and Hawig  that the enhancement of remanence and coercivity by this mechanism is mainly governed by the crystallite sizes of both the phases in particular the soft phase, which can be controlled by dopants and/or additives and also by controlling heat treatment. Compared to single phase Nd2Fe14B, nanocomposite Nd2Fe14B/Fe3B based alloys are economic and corrosion resistant. Various dopants and substituents have been used to enhance the value of coercivity. A partial substitution of Nd by heavy rare earth elements like Tb increases the anisotropy field, HA which enhances the coercive field, but decreases strongly the remanence due to its antiferromagnetic coupling between rare earth and the transition metal . Tb addition has previously been used to increase the coercivity of Pr2Fe14B due to higher anisotropy field of Tb2Fe14B .
Studies [4–6] of macroscopic reversible and irreversible magnetic behavior in nano-crystalline exchange coupled two-phase permanent magnetic materials demonstrate relatively steep recoil curves, which possess recoil permeabilities five times greater than those in conventional sintered magnets . By using the demagnetization remanence (DCD) technique it has been demonstrated that when a negative field (lower than the critical field for magnetization reversal of the hard phase) is applied on a previously saturated sample, a near reversible rotation of magnetization of the soft phase is obtained when the field is decreased back to zero giving rise to the high recoil permeability.
The aim of the present work is to obtain nanocomposite (NdTb)2Fe14B/Fe3B magnets with higher coercvity evolved from the composition of Nd3Tb1Fe76Cu0.5Nb1B18.5 with the variation of annealing temperature and time. In this composition, Cu and Nb controls microstructure. For the optimized annealed sample reversible and irreversible component of magnetization have been studied by recoil hysteresis and DCD technique. Temperature dependence of coercivity, remanent ratio and maximum energy product have been measured for the sample annealed at 893 K and 923 K for 10 min between 5 K and 380 K.
An ingot of composition Nd3Tb1Fe76Cu0.5Nb1B18.5 was prepared by arc melting the constituent elements in an argon atmosphere. The purity and origin of the materials were Fe (99.98%), Cu (99+%), Nb (99.8%), B (99.5%), Si (99.9%), Nd (99.9%), Tb (99.9%) from Johnson Matthey (Alfa Aesar). Amorphous ribbons were prepared from the ingot using a melt spin machine with a wheel speed of 25 m/s in an Ar atmosphere. The resulting ribbons were heat treated in an evacuated quartz tube of 10−5 mbar pressure at different temperatures and holding time to observe the effect of annealing condition on the magnetic properties. Differential Scanning Calorimetry was used to determine the crystallization temperature and x-ray diffraction (CuKα) was used to identify the phases present in the samples at different stages of the crystallization process. Magnetization measurements were performed by Quantum Design MPMSXL5 Superconducting Quantum Interference Device (SQUID) magnetometer.
Results and discussion
Hysteresis loop parameters for Nd3Tb1Fe76Cu0.5Nb1B18.5
Annealing temperature, K
Annealing time, min
In Fig. 7, the reduced quantity D(H) = [Mr − Md(H)]/2Mr = −ΔMirrev(H)/2Mr is plotted vs. reverse field H, where Md(H) is the dc field demagnetization remanence i.e. the remanence acquired after saturation in one direction and subsequent application of a dc field H in the opposite direction and Mr is the saturation remanence. The curves of Fig. 7 provide information about the stability of the reversible state and about the critical field of irreversible changes of the magnetization. For the sample annealed at 923 K, the D(H) vs. H curve is characterized by relatively sharp change of D(H) at the critical field where irreversible change in the hard phase is relatively large, which has been obtained from the derivative of the D(H) vs. H curve and found as 4,000 Oe.
In Fig. 9, temperature dependence of the coercivity, Hc, remanent ratio, Mr/Ms and maximum energy product, (BH)max are plotted. Curves for both the samples show similar behavior. Coercivity increases with the decrease of temperature up to about 150 K. Below 150 K there is a change of the slope of Hc(T). Change of slope of Hc(T) at low temperatures is related to the spin reorientation in the hard phase. This behavior of Hc(T) may be compared with that reported in Ref.  in which a stronger decay of the coercivity at low temperatures is observed. The temperature dependence of remanent ratio, Mr/M is also governed by the temperature dependence of anisotropy field. The value of Mr/Ms decreases with the increase of temperature because of easier domain wall motion due to the reduction of the anisotropy field at higher temperature. Temperature dependence of maximum energy product, (BH)max also decreases with the increase of temperature due to the reduction of anisotropy field for both samples.
A partial substitution of Nd by Tb led to the enhancement of coercive field up to a value of 4.76 kOe for the sample annealed at 953 K for 10 min. Recoil hysteresis loops are characterized by high recoil permeabilities and small recoil loop area, which indicates that the samples are exchange coupled. At low temperature, hysteresis loops are governed by the spin-reorientation of the easy axis of Nd2Fe14B. Below 200 K, field dependence of magnetization is discontinuous at low field leading to constricted hysteresis loop.
The authors acknowledge with deep sense of gratitude the support provided by Prof. Per Nordblad, Solid State Physics, Dept. of Eng. Sci., Uppsala University, Sweden. Financial support provided by the International Program for Physical Sciences, Uppsala University, Sweden is acknowledged. The authors acknowledge kind help provided by Prof. N. X. Phuc, Director Institute of Materials Science, Vietnamese Academy of Science and Technology. The authors also acknowledge the kind support provided by Dr. S. I. Bhuiyan, Chairman, Bangladesh Atomic Energy Commission and Engr. Rezaul Bari, Director, Atomic Energy Centre, Dhaka.