Study on Magnetic Behavior of Nd₈Fe₈₄Ti₂B₆ Nanocomposite by FORC Diagram

Abstract

Systematic investigation was made on the magnetic property of a series of Nd₂Fe₁₄B/α-Fe nanocomposites prepared by crystallization of amorphous Nd₈Fe₈₄Ti₂B₆ ribbon at annealing temperatures in the range of 800-860 °C. Both remanence and energy product increase with increasing annealing temperature, reaching the maximum values at 850 °C. Coercivity remains around 5.5 kOe for the annealing temperatures above 800 °C. Although the smooth demagnetization curve indicates effective exchange coupling between the Nd₂Fe₁₄B/α-Fe dual phases, FORC diagram reveals the existence of α-Fe for the Nd₈Fe₈₄Ti₂B₆ ribbon annealed at 850 °C. In addition, the variation of microcoercivity at different locations indicates a distributed exchange interaction, which can be caused by the nonuniform microstructure. The magnetic property is also affected by the demagnetization effect caused by the particle shape, which is evidenced by the negative region in the first-order reversal curve (FORC) diagram.

Keywords:
nanocomposite; exchange coupling; microcoercivity

1 Introduction

Since the theoretical model for nanocomposite magnet was put forward by Coehoorn et al.¹, much work has been done in regards to this topic. Nd₂Fe₁₄B/α-Fe nanocomposite is a promising permanent magnetic material by combining advantages of Nd₂Fe₁₄B and α-Fe phases through magnetic exchange coupling between the two². However, the exchange coupling is only effective within a critical length, typically 20 nm^[3]. Hence, the microstructure plays a key role in determining the magnetic behavior. Uniform phase distribution in combination with fine grain sizes is needed for excellent magnetic performance in Nd₂Fe₁₄B /α-Fe composite⁴.

Melt spinning method is an effective way to produce Nd₂Fe₁₄B/α-Fe nanocomposite. From our available experiments we find that the processing window is pretty narrow in optimizing magnetic properties for nanocrystallites prepared directly by low-speed melt spinning. Uniformly distributed and refined microstructure can be achieved through crystallization of the amphormos matrix produced by quenching^5,6. Based on this concept, a substantial amount of work has been done to optimize the nanocomposite microstructure. Element doping, such as Zr^[7], Nb^[8], Ti^[9], W^[10] etc., or combined addition^11,12, promotes the formation of amorphous phase by increasing the whole entropy, which may effectively refine the grain size by forming grain boundaries¹³. However, it is difficult to co-regulate the microstructure of Nd₂Fe₁₄B and α-Fe phases as the crystallization behavior for the two phases differs much^14,15. It is still a pending problem to achieve an ideal microstructure in Nd₂Fe₁₄B/α-Fe nanocomposite, which is much more complex in substantial system than that defined by theoretical model². It is necessary to investigate the crystallization process as well as the corresponding exchange interaction behavior in order to gain a comprehensive understanding and improvement of the magnetic performance.

In this work the effect of microstructure on magnetic properties was investigated on stoichiometric Nd₈Fe₈₄Ti₂B₆ compound by varying annealing temperature. In order to clarify the role of exchange coupling between the Nd₂Fe₁₄B /α-Fe dual phases for a typical microstructure, first-order reversal curve (FORC) diagram method is adopted to reveal the magnetic competition in our investigated system¹⁶, which allows the characterization of the switching field distribution as well¹⁷.

2 Experimental Procedure

Stoichiometric Nd₈Fe₈₄Ti₂B₆ ingot was prepared by induction melting Nd, Fe and Fe-B under Ar atmosphere. Ribbons were obtained by ejecting the melt from a quartz nozzle with a diameter of 0.8 mm onto a spinning molybdenum wheel with the chamber pressure of 0.4 atm and the ejection pressure of 0.3 atm, respectively. Differential scanning calorimetry (DSC) curve for the as spun ribbon was obtained on Mettler Toledo TGA/DSC₁ system in a temperature range of 50-1000 °C with a heating rate of 20 °C/min. The ribbons were annealed in a series of temperatures (800 °C, 810 °C, 820 °C, 830 °C, 840 °C, 850 °C, 860 °C) for 10 minutes in a steel tube under an argon atmosphere, and water-quenching cooling was applied afterwards. Room temperature magnetic measurements were carried out on Quantum Design Versalab with a maximum magnetic field of 3 T. X-ray diffraction (XRD) was carried out on smart Rigaku X-ray diffractometer (Co Kα radiation).

3 Results and Discussion

In order to obtain amorphous Nd₈Fe₈₄Ti₂B₆ matrix, ribbons at a series of wheel velocities (14, 16, 18, 20, 22, and 24 m/s) were prepared. The XRD peak profiles indicate that amorphous phase is formed at a velocity higher than 20 m/s. Ribbons at a velocity of 22 m/s is employed in the investigation. The XRD of the ribbon is displayed at the top of Figure 1, which shows a distinct amorphous peak.

Figure 1
XRD results for the as spun ribbon and annealed samples for Nd₈Fe₈₄Ti₂B₆ at different annealing temperatures.

DSC measurement was carried out to estimate the crystallization temperature. Figure 2 displays the DSC curve for the as spun Nd₈Fe₈₄Ti₂B₆ ribbon in a temperature range of 50-1000 °C with a heating rate of 20 °C/min under argon atmosphere. Two apparent anomalies can be distinguished at 610 °C and 695 °C, which correspond to the two-step crystallizaton of Nd₂Fe₁₄B. Firstly, a metastable intermediate phase is formed as indicated by the peak at 610 °C. Secondly, the metastable phase is decomposed and transformed into Nd₂Fe₁₄B phase with increasing temperature¹⁸.

Figure 2
DSC curve for as spun Nd₈Fe₈₄Ti₂B₆ ribbon in a temperature of 50-1000 °C with a heating rate of 20 °C/min under Ar atmosphere.

Both nucleation rate and grain growth rate increase exponentially with increasing annealing temperature, so there is a optimum annealing temperature to maximize the nucleariotion rate and refinement of the grains. Short heating time and rapid quenching are aslo necessary for a refined microstructure. Based on these considerations, we design the annealing experiments from 800 °C to 860 °C with an increment of 10 °C for 10 minutes. The XRD patterns for the annealed samples are also shown in Figure 1. TbCu₇-type metastable structure is evident for sample annealed at 800 °C, for which some trace of Nd₂Fe₁₄B and ɑ-Fe phase can also be distinguished. With increasing annealing temperature, Nd₂Fe₁₄B and ɑ-Fe phases become dominant instead of the metastable phase.

Magnetic measurements were carried out to investigate the magnetic interactions for the annealed Nd₈Fe₈₄Ti₂B₆ with Nd₂Fe₁₄B/α-Fe dual phases. The remanence, coercivity, energy product of the annealed samples are shown in Figure 3. As shown in the inset of Figure 3, the demagnetization curves of all annealed samples are smooth except the one annealed at 800 °C. This is characteristic of a monophase, which means that the inter-exchange between the soft and permanent magnetic phases takes effect. The increasing remanance with annealing temperature indicates an enhanced exchange interaction between Nd₂Fe₁₄B and α-Fe phases, which reaches a maximum value of 8.1 kGs at an annealing temperature of 850 °C. Magnetic energy product shows the same trend with remanence, which reaches a maximum value of 7.6 MGOe also at an annealing temperature of 850 °C. Generally speaking, the coercivity deteriorates with increasing remanence. However, the coercivity remains steady with increasing annealing temperature, which may be attributed to the refined microstructure after annealing.

Figure 3
The remanence, coercivity and magnetic energy product for samples annealed at 800, 810, 820, 830, 840, 850, and 860 °C for 10 min under Argon atmosphere. Inset is the corresponding demagnetization curves for all the samples.

In order to get a comprehensive understanding of our investigated system, we use FORC diagram method to analyze magnetic exchange interaction and the switching field distribution for the sample annealed at a temperature of 850 °C. Each FORC is a partial recoil loop, which begins by saturating the sample under a magnetic field of 30 kOe. The magnetic field decreases to a reversal field $\mathrm{Hr}$ and a FORC is recorded with the magnetic field increasing from$\mathrm{Hr}$to 30 kOe. Figure 4 shows a batch of representative FORCs. The FORC diagram $\rho (\mathrm{Hr},H)$ was obtained by the following mixed second derivative:

Figure 4
The FORCs with different reversal field $\mathrm{Hr}$.

$$\rho (\mathrm{Hr},H)=-\frac{1}{2}\frac{\partial M^2(\mathrm{Hr},H)}{\partial \mathrm{Hr}\partial H}$$ (1)

The coordinate in the diagram is transformed from $\{\mathrm{Hr},H\}$to ^{$\{\mathrm{Hc}=(H-\mathrm{Hr})/{\mathrm{2,}}_{}\mathrm{Hb}=(H+\mathrm{Hr})/2\}$[19]}. As for each loop $H\ge \mathrm{Hr}$,$\mathrm{Hc}$ is always a positive value. So the FORC diagram is confined to the right-hand half plane. The $\mathrm{Hc}$co-ordinate is referred to as the microcoercivity. In our experiment $\mathrm{Hr}$ varies from –10 kOe to 0 kOe with an increment of 200 Oe. The calculated FORC diagram is shown in Figure 5 by using SF=3^[20], which indicates two main magnetic phases in the annealed sample. An irreversible peak is located at position around 60 kOe due to the exchange interaction between soft and hard magnetic phase, which is consistent with the demagnetization curve. The effective microcoercivity is widely dispersed, which may be due to the inhomogeneous microstructure. However, soft phase still exists as illustrated by the reversible peak around the origin. The negative region indicates the existence of demagnetization effect caused by the particle shape, which is against the magnetization of the system.

Figure 5
FORC diagram for Nd₈Fe₈₄Ti₂B₆ at an annealing temperature of 850 °C.

4 Conclusion

The magnetic performance of Nd₈Fe₈₄Ti₂B₆ is significantly affected by annealing temperature, and an optimum combination of magnetic properties is obtained with an annealing temperature of 850 °C. The FORC diagram can give more detailed information than mere demagnetization curves. From FORC diagram we find that the exchange interaction intensity is distributed across a wide range, which results in a wide range of microcoercivity. In spite of the smooth curve indicated by the demagnetization curve, soft magnetic is evident in FORC diagram because of the deficient coupling of α-Fe with hard magnetic phase. The demagnetization effect generated by particle shape also has an effect on magnetic performance of our investigated system. There is still much room for improving the magnetic performance of Nd₈Fe₈₄Ti₂B₆ nanocomposite by enhancing the exchange interactions between Nd₂Fe₁₄B and α-Fe phases through optimizing the microstructure.

Acknowledgements

This work is supported by the special fund of Rare earth Industrial upgrading in 2014 on “Key engineering preparation techniques of low-cost bonded mixed rare earth permanent magnet (MM-Fe-B)” sponsored by Industrialization and Information Department.

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Publication Dates

• Publication in this collection
  04 Dec 2015
• Date of issue
  Nov 2015

History

• Received
  27 Sept 2014
• Reviewed
  09 Oct 2015