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Magnetic properties of single crystalline YbFe2O4

We have investigated the magnetic properties of a single crystal of multiferroic YbFe2O4. Ferrimagnetic ordering of Fe spins is observed at ~250 K. The magnetization measured during field cooling becomes negative below ~10 K owing to the antiparallel coupling between the Fe and Yb sublattices. The magnetization below ~10 K is flipped by changing the applied magnetic field in a positive direction, a property which may be used to the development of magnetic devices. Magnetocaloric measurements show a broad peak of the entropy change, likely owing to a multiple magnetic transition.


Introduction
YbFe 2 O 4 belongs to a class of isostructural materials RFe 2 O 4 with R=Y, Ho-Lu and In [1]. Previously we have reported that this system with R=Y, Ho-Lu becomes ferroelectric below ~330 K owing to the real-space charge ordering of Fe 2+ and Fe 3+ on triangular lattices [2]. This origin of ferroelectricity differs from the ordinary ion-displacement mechanism. In addition, RFe 2 O 4 shows ferrimagnetic ordering of Fe spins at the Néel temperature (T N ) of 230-250 K [3][4][5]. Hence, the system is multiferroic below T N . Many studies of RFe 2 O 4 have revealed their intriguing magnetic properties, such as huge coercivity at low temperatures [6], a competition between ferro-and antiferromagnetic states [7], spinglass behaviour [8], and exchange bias [9]. However, full details of the magnetic properties have not been elucidated yet.
In this study, the magnetic properties of a single-crystalline sample of YbFe 2 O 4 are investigated, as polycrystalline YbFe 2 O 4 showed the highest T N of 256 K among the RFe 2 O 4 series [3]. Studies of RFe 2 O 4 have been mainly confined to LuFe 2 O 4 , in which the R-site ion (Lu 3+ ) is nonmagnetic (4f 14 ) [3,6,7,8]. In contrast, reports on the properties of RFe 2 O 4 with magnetic R 3+ ions are somewhat rare [3]. Although a magnetic study of single-crystalline YbFe 2 O 4 (Yb 3+ ; 4f 13 ; 4.6  B ) was reported about 30 years ago [10], this work reports some new properties of this system, such as the magnetization reversal and the magnetocaloric effect, in which the magnetic moment of Yb 3+ may be important.

Experimental
A single crystal of YbFe 2 O 4 was grown by the floating zone method. X-ray diffraction measurements at room temperature showed that the crystal consisted of single-phase YbFe 2 O 4 only. The magnetic measurements were carried out between 5 K and 300 K using a superconducting quantum interference  [3,5]. Figure 1a shows the magnetization-temperature (MT) curves measured with the applied field (H) of 1 kOe. The direction of H was along the c-axis. The deviation between field-cooled (FC) and zero-fieldcooled (ZFC) curves is rooted in the ferrimagnetic ordering. The inflection of the thermoremanent magnetization (TRM) shows the Néel temperature (T N ) of ~250 K, a temperature being close to those in previous studies [3,10]. The broad peak at ~130 K suggests a complicated change of magnetic structure, as reported for LuFe 2 O 4 [11]. Figure 1b shows the MT curves measured in the FC mode with H parallel to the ab-plane. The magnetization was much smaller (typically, ~1/10) than that in Fig. 1a. This is because of the Ising properties of RFe 2 O 4 , arising from the low-dimensional crystal structure [6] and the single-ion anisotropy of Fe 2+ and Fe 3+ [12].

Results and discussion
We can see the interesting behaviour in Fig. 1b. That is, the magnetization is decreased below ~110 K and has a negative polarity below ~10 K for H lower than ~700 Oe. The same tendency is observed in the c//H condition below ~60 K ( Fig. 1) [3,10], although the FC magnetization remain positive down to 5 K. The negative magnetization is plausibly brought about by the antiparallel magnetic coupling between Yb and Fe sublattices [3]; this is a similar situation to that of perovskite chromites GdCrO 3 and La 1-x Pr x CrO 3 [13]. Figure 2 shows that the data could be fitted by the Curie-Weiss behaviour of Yb 3+ moments, which have an antiparallel coupling with the Fe sublattice [13]: Here, M Fe represents the magnetization of the Fe sublattice. C, H I and H stand for the Curie constant, the internal magnetic field at Yb 3+ , and the applied magnetic field, respectively. T and Θ mean the temperature and the Weiss temperature, respectively. M Fe was assumed to be independent of temperature (~94 emu/mol). The H I and Θ values were ~1100 Oe and ~-7 K, respectively. As the negative magnetization is not observed for the H values greater than ~700 Oe (Fig. 1b), the obtained H I seems to be roughly reasonable. The small absolute value of the Weiss temperature (~7 K) is   (1)) in the c//H condition than in ab//H, owing to the Ising nature of RFe 2 O 4 . The negative magnetization offers an interesting method of magnetization switching [14]. This is shown in Fig. 3. The sample was cooled with H=500 Oe down to 5 K. As seen in Fig. 1b, the polarity of the magnetization is negative. The magnetization measured for ~600 s was almost unchanged. Then, the applied field was increased to 1220 Oe; the magnetization was switched. This switching is repeatable by changing H between 500 and 1220 Oe. Note also that the switching is attained under H in a positive direction. This is different from the ordinary method, in which the magnetization is flipped by changing the direction of H. Hence, the present method can avoid the sample heating caused by hysteretic losses, when the magnetic field crosses zero [14]. As usual, this value was calculated from magnetization-magnetic field (MH) curves [16]. The increment of the measurement temperature was 5 K. The applied field was parallel to the c-axis. The magnetic field (H) was increased from zero to 10, 20, 30, 40 and 50 kOe. Although the MCE is usually studied from the viewpoint of refrigerant applications, the sign of S gives the information on the type of magnetic ordering. In ferromagnetic ordering, S is negative at around the magnetic  (1)). Exp and Fit stand for the experimental data and the fitting curve, respectively. The details are noted in the text.  transition temperature, whereas, the value is positive for antiferromagnetism [11]. As S was positive at around T N due to the ferrimagnetic ordering, -S is plotted in Fig. 4. A peak of -S appears near T N as reported for many systems thus far [15]. The peak value is ~1 J kg -1 K -1 for H=50 kOe, and is reasonably close to that of LuFe 2 O 4 [11]. This value is smaller than those of the large or giant MCE materials (-S ~10 J kg -1 K -1 ) [15]. Note that -S exhibits a significantly broad peak, as found for LuFe 2 O 4 [11]. Calculating the peak width, we have obtained the refrigerant capacity (RC) [15]. The temperatures of the hot and cold reservoirs in the refrigeration cycle are defined as the temperatures of the full width at half maximum of -S. The former and latter temperatures for H=50 kOe were T 1~ 153 K and T 2~2 58 K, respectively. The RC values were calculated by the integration of -S between T 1 and T 2 . The RC was ~80 J kg -1 , which is ~25-50% of those of the large or giant MCE materials [15]. However, the large difference between T 1 and T 2 (~105 K) is favourable for the so-called Ericsson cycle [17]. This property is possibly owing to either a multiple magnetic transition below T N or magnetic inhomogeneity [11], originating from a magnetic frustration on a triangular lattice. The discontinuous of -S at ~190 K may be associated with a structural distortion [6], as a sharp change of -S can be observed at a first-order transition [15].
The sign of -S becomes negative below ~130 K. This is a different result from that of LuFe 2 O 4 , in which -S is positive at all temperatures [11]. The present behaviour is rooted in a decrease in the integral term (equation (2)), because of the shrinkage of the MH curves (Fig. 4b) [9]. This shrinkage arises from the enhancement of coercivity [6,9]; we should measure the MH curves at H values greater than the highest field of the present apparatus (50 kOe). The zero value of -S at ~130 K indicates a possible application to fabricate a constant temperature bath [14].
The -S value increases again below ~70 K as seen in Fig. 4a. The true origin of this behaviour is unclear. However, paramagnetic Yb 3+ moments may play some roles, considering that the magnetic entropy is decreased since Yb 3+ spins are forced to be parallel to H by the application of large fields. A role of Yb 3+ is also implied by the fit of the MT data (Fig. 2), in which the Curie-Weiss term (equation (1)) of Yb 3+ is important below ~60 K. Further studies are currently in progress, and will be published in the near future.  We have investigated the magnetic properties of a single crystal of multiferroic YbFe 2 O 4 . Ferrimagnetic ordering of Fe spins is observed at ~250 K. The magnetization measured during field cooling becomes negative below ~10 K owing to the antiparallel coupling between the Fe and Yb sublattices. The magnetization below ~10 K is flipped by changing the applied magnetic field in a positive direction, a property which may be used to the development of magnetic devices. Magnetocaloric measurements show a broad peak of the entropy change, likely owing to a multiple magnetic transition.